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Citation: dos Reis, J.R.T.; Leite, F.F.;
Sharma, K.; Ribeiro, G.A.S.; Silva,
W.H.N.; Batista, A.A.; Paschoal, A.R.;
Paraguassu, W.; Mazzoni, M.; Barbosa
Neto, N.M.; et al. Raman
Spectroscopy on Free-Base Meso-
tetra(4-pyridyl) Porphyrin under
Conditions of Low Temperature and
High Hydrostatic Pressure. Molecules
2024,29, 2362. https://doi.org/
10.3390/molecules29102362
Academic Editor: Vincent Boudon
Received: 9 February 2024
Revised: 1 April 2024
Accepted: 2 April 2024
Published: 17 May 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
molecules
Article
Raman Spectroscopy on Free-Base Meso-tetra(4-pyridyl)
Porphyrin under Conditions of Low Temperature and High
Hydrostatic Pressure
Jhon Rewllyson Torres dos Reis 1, Fabio Furtado Leite 1,2 , Keshav Sharma 3, Guilherme Almeida Silva Ribeiro 4,
Welesson Henrique Natanal Silva 4, Alzir Azevedo Batista 5, Alexandre Rocha Paschoal 6, Waldeci Paraguassu 1,
Mario Mazzoni 4, Newton Martins Barbosa Neto 1, * and Paulo Trindade Araujo 3, *
1
Graduate Program in Physics, Institute of Natural Sciences, Federal University of Pará, Belém 66075-110, PA,
Brazil; jhon.rewllyson@gmail.com (J.R.T.d.R.); fabioleite@unifap.br (F.F.L.); paraguassu@ufpa.br (W.P.)
2
Department of Exact and Technological Sciences, Federal University of Amapá, Macapá68903-419, AP, Brazil
3Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA;
ksharma1@crimson.ua.edu
4Department of Physics, Federal University of Minas Gerais, Belo Horizonte 31270-901, MG, Brazil;
almeida.guilherme13@gmail.com (G.A.S.R.); welessonhenrique@gmail.com (W.H.N.S.);
mmazzonibh@gmail.com (M.M.)
5
Departament of Chemistry, Federal University of São Carlos, São Carlos 13565-905, SP, Brazil; daab@ufscar.br
6Department of Physics, Federal University of Ceara, Fortaleza 60455-760, CE, Brazil; paschoal@fisica.ufc.br
*Correspondence: barbosaneto@ufpa.br (N.M.B.N.); paulo.t.araujo@ua.edu (P.T.A.)
Abstract: We present a Raman spectroscopy study of the vibrational properties of free-base meso-
tetra(4-pyridyl) porphyrin polycrystals under various temperature and hydrostatic pressure condi-
tions. The combination of experimental results and Density Functional Theory (DFT) calculations
allows us to assign most of the observed Raman bands. The modifications in the Raman spectra when
excited with 488
nm
and 532
nm
laser lights indicate that a resonance effect in the
Qy
band is taking
place. The pressure-dependent results show that the resonance conditions change with increasing
pressure, probably due to the shift of the electronic transitions. The temperature-dependent results
show that the relative intensities of the Raman modes change at low temperatures, while no frequency
shifts are observed. The experimental and theoretical analysis presented here suggest that these
molecules are well represented by the C2v point symmetry group.
Keywords: porphyrin; resonance Raman spectroscopy; hydrostatic pressure; low temperature
1. Introduction
Over the past few decades, porphyrin molecules have attracted a great deal of atten-
tion given their central role in numerous fundamental natural processes [
1
]. The interplay
between the structural and spectroscopic properties of these molecules enables the opti-
mization of their electronic characteristics aiming at specific applications [2,3].
The structure of porphyrins consists of a macrocycle formed by four pyrrolic rings
interconnected with methyne bridges, and this arrangement is upheld by the insertion of
either two hydrogen atoms (free-base porphyrins) or a metal ion (in metalloporphyrins) at
the center of the macrocycle [
1
,
4
]; see Figure 1. The study of the optical properties as-
sociated with distinct porphyrins is driven by their cyclic conjugation, which leads to
a pronounced absorption of near-ultraviolet and visible light as well as a red emission
that is readily observable with the naked eye [
1
,
4
]. Additionally, these molecules present
intriguing nonlinear optical traits [
5
,
6
]. Their absorption spectra are primarily composed of
two characteristic bands known as the
B
-bands (or Soret bands), localized in the blue region
of the spectrum, and the
Q
-bands, found in the green-red portion of the spectrum [
1
,
4
,
7
,
8
].
These spectroscopic responses are related to porphyrin’s electronic and vibronic properties,
Molecules 2024,29, 2362. https://doi.org/10.3390/molecules29102362 https://www.mdpi.com/journal/molecules
Molecules 2024,29, 2362 2 of 22
tuned through the modification of its structure, such as the substitution of the central ion
and the addition of outlying and axial groups [
3
,
6
–
10
]. Those are very desirable possibili-
ties since they create opportunities to employ porphyrin derivatives in many applications
such as (i) photovoltaic cells [
11
–
13
], (ii) sensors [
8
,
14
], (iii) cancer treatment [
15
–
17
], and
(iv) fluorescence imaging [18,19], among others.
Dissolved in organic solvents, free-base tetrapyridyl porphyrin (H
2
TPyP), as shown
in Figure 1, depicts a complex
Q
-band with multiple electronic transitions and their corre-
sponding vibronic progressions [20].
Molecules 2024, 29, x FOR PEER REVIEW 2 of 26
of two characteristic bands known as the B-bands (or Soret bands), localized in the blue
region of the spectrum, and the Q-bands, found in the green-red portion of the spectrum
[1,4,7,8]. These spectroscopic responses are related to porphyrin’s electronic and vibronic
properties, tuned through the modification of its structure, such as the substitution of the
central ion and the addition of outlying and axial groups [3,6–10]. Those are very desirable
possibilities since they create opportunities to employ porphyrin derivatives in many
applications such as (i) photovoltaic cells [11–13], (ii) sensors [8,14], (iii) cancer treatment
[15–17], and (iv) fluorescence imaging [18,19], among others.
Dissolved in organic solvents, free-base tetrapyridyl porphyrin (H
2
TPyP), as shown
in Figure 1, depicts a complex Q -band with multiple electronic transitions and their
corresponding vibronic progressions [20].
Figure 1. Schematic representation of free-base tetrapyridyl porphyrin (H
2
TPyP). Within the
macrocycle’s plane, two distinct directions are defined: (i) the x-direction containing only nitrogen
atoms, and (ii) the y-direction containing nitrogen atoms bonded with hydrogen. The indices α and
β give the carbon positions in the pyrrolic rings, and m indicates the carbon position in the
methynic bridge. The carbon atoms occupying these positions are labeled as follows: C
, linked to
the central nitrogen atoms; C
, located at the outer edge of the macrocycle; and C
(meso-carbon),
connecting the pyrrolic rings.
While the optical properties of tetrapyridyl porphyrins have been extensively studied
[6,10,20,21], their vibrational properties, especially in their crystalline form, remain poorly
explored. Although the vibrational modes of other porphyrins have been investigated [22–
25], their behaviors are substantially different from H
2
TPyP’s vibrational modes. In
addition, the few studies of H
2
TPyP modes lack proper assignments and detailed
descriptions of their symmetries [26,27], which are intimately connected with porphyrin’s
vibronic transitions [20]. In this context, Raman spectroscopy emerges as a non-invasive,
fast, and reproducible method to study the properties of these vibrational modes under
different thermodynamic conditions, e.g., low temperatures and high pressures [28–33].
In the present work, we combine Raman spectroscopy measurements with first-
principle calculations to provide assignments for the Raman modes in poly-crystals of
free-base tetrapyridyl porphyrin or C-H
2
TPyP (see Figure S1 in Supplementary Materials).
The evolution of the assigned modes in C-H
2
TPyP under high pressures, low
temperatures, and different excitation energies is addressed. In addition, we elucidate the
modifications in porphyrin´s resonance conditions under high pressures, along with
possible symmetry changes occurring at both high pressures and low temperatures.
Figure 1. Schematic representation of free-base tetrapyridyl porphyrin (H
2
TPyP). Within the macro-
cycle’s plane, two distinct directions are defined: (i) the x-direction containing only nitrogen atoms,
and (ii) the y-direction containing nitrogen atoms bonded with hydrogen. The indices
α
and
β
give the carbon positions in the pyrrolic rings, and
m
indicates the carbon position in the methynic
bridge. The carbon atoms occupying these positions are labeled as follows:
Cα
, linked to the central
nitrogen atoms;
Cβ
, located at the outer edge of the macrocycle; and
Cm
(meso-carbon), connecting
the pyrrolic rings.
While the optical properties of tetrapyridyl porphyrins have been extensively stud-
ied [
6
,
10
,
20
,
21
], their vibrational properties, especially in their crystalline form, remain
poorly explored. Although the vibrational modes of other porphyrins have been investi-
gated [
22
–
25
], their behaviors are substantially different from H
2
TPyP’s vibrational modes.
In addition, the few studies of H
2
TPyP modes lack proper assignments and detailed de-
scriptions of their symmetries [
26
,
27
], which are intimately connected with porphyrin’s
vibronic transitions [
20
]. In this context, Raman spectroscopy emerges as a non-invasive,
fast, and reproducible method to study the properties of these vibrational modes under
different thermodynamic conditions, e.g., low temperatures and high pressures [28–33].
In the present work, we combine Raman spectroscopy measurements with first-
principle calculations to provide assignments for the Raman modes in poly-crystals of
free-base tetrapyridyl porphyrin or C-H
2
TPyP (see Figure S1 in Supplementary Materials).
The evolution of the assigned modes in C-H
2
TPyP under high pressures, low temperatures,
and different excitation energies is addressed. In addition, we elucidate the modifications
in porphyrin’s resonance conditions under high pressures, along with possible symmetry
changes occurring at both high pressures and low temperatures.
2. Results and Discussion
2.1. Raman Bands Assignments
The Raman spectra of C-H
2
TPyP show a rich distribution of bands, ranging from
150 cm−1to 1650 cm−1; see Figure 2.
Molecules 2024,29, 2362 3 of 22
Molecules 2024, 29, x FOR PEER REVIEW 3 of 26
2. Results and Discussion
2.1. Raman Bands Assignments
The Raman spectra of C-H2TPyP show a rich distribution of bands, ranging from
150 cm to 1650 cm; see Figure 2.
200 400 600 800 1000 1200 1400 1600
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 2. Raman spectra of C−H2TPyP experimentally obtained with excitation centered at 488 nm
(represented by a blue solid line) and 532 nm (green solid line), and the DFT−calculated Raman
spectrum for the H2TPyP molecule (gray solid line). In the theoretical spectrum, Raman intensity (in
A4/amu) refers to the Raman activity (scaering factor).
These spectra were acquired by exciting the sample at 488 nm (resonant with the
Q(0,2) absorption band), and at 532 nm (resonant with the intersection between the
Q (0,0) and Q(0,0) bands, referred to herein as the Q(0,0) band) [20]. These
vibronic progressions arise from the coupling of the electronic absorption band Q(0,0)
with the vibrational modes centered around 1245 cm (Q(0,2) band) [20]. To provide
a clearer depiction of the investigated modes, we present and discuss the results by
zooming into the specific spectral regions, as depicted in Figures 3–11. The experimental
spectra were deconvoluted using Lorenian functions. This constitutes a conventional
approach in Raman spectroscopy, stemming from the intrinsic properties of Raman
scaering (RS). The semi-classical interpretation of RS relies on the forced damped
oscillator model, which follows the Lorenian function. In the quantum mechanical
framework, each vibration exhibits a Lorenian probability of light scaering [34]. The
fiing procedure enables us to identify the spectral band positions with a deviation of
~0.2 cm . The corresponding center-peak wavenumbers ( κ ) of the Raman bands,
obtained with 488 nm and 532 nm, are listed in Table 1. Illustrations with an overview
of the identified vibration paerns are provided in Table S2 in Supplementary Materials.
Figure 2. Raman spectra of C-H
2
TPyP experimentally obtained with excitation centered at 488 nm
(represented by a blue solid line) and 532 nm (green solid line), and the DFT–calculated Raman
spectrum for the H
2
TPyP molecule (gray solid line). In the theoretical spectrum, Raman intensity (in
A4/amu) refers to the Raman activity (scattering factor).
These spectra were acquired by exciting the sample at 488
nm
(resonant with the
Qy1(
0, 2
)
absorption band), and at 532
nm
(resonant with the intersection between the
Qy1(
0, 0
)
and
Qy2(
0, 0
)
bands, referred to herein as the
Qy(
0, 0
)
band) [
20
]. These vibronic
progressions arise from the coupling of the electronic absorption band
Qy(
0, 0
)
with the
vibrational modes centered around 1245
cm−1
(
Qy1(
0, 2
)
band) [
20
]. To provide a clearer
depiction of the investigated modes, we present and discuss the results by zooming into
the specific spectral regions, as depicted in Figures 3–11. The experimental spectra were
deconvoluted using Lorentzian functions. This constitutes a conventional approach in
Raman spectroscopy, stemming from the intrinsic properties of Raman scattering (RS). The
semi-classical interpretation of RS relies on the forced damped oscillator model, which
follows the Lorentzian function. In the quantum mechanical framework, each vibration
exhibits a Lorentzian probability of light scattering [
34
]. The fitting procedure enables us
to identify the spectral band positions with a deviation of
∼
0.2
cm−1
. The corresponding
center-peak wavenumbers (
κ
) of the Raman bands, obtained with 488
nm
and 532
nm
, are
listed in Table 1. Illustrations with an overview of the identified vibration patterns are
provided in Table S2 in Supplementary Materials.
Table 1. H
2
TPyP experimental and DFT–calculated Raman modes. In the table,
ν
stands for stretching;
δ
for bending; and
τ
for twist modes, respectively. The index “Pyr” identifies Raman modes related to
the pyridyl ring. The indexes “IP” and “OP” stand for in-plane and out-of-plane modes, respectively.
The indexes “x” and “y” indicate vibrations only in the respective direction.
Raman Mode Symmetry
(C2v)Calculated
(cm−1)
Experimental (cm−1)
488 nm
Qy(0,2)
532 nm
Qy(0,0)
δIP(Cm−Pyrrole)A1163 164 160
Out-of-phase (XY) bending of the angles
between the pyrrole groups.
τOP(Pyrrole)A1199 199 195 Out-of-phase twist of the pyrrole
groups.
τ(Pyridyl)A1213 223 218 In-phase twist of the pyridyl groups.
δIP(Cm−Pyrrole)x+τ(Pyridyl)B1233 - 239 Bending of the angles between the Cm
and the X pyrrole groups and
out-of-phase twist of the pyridyl groups.
Molecules 2024,29, 2362 4 of 22
Table 1. Cont.
Raman Mode Symmetry
(C2v)Calculated
(cm−1)
Experimental (cm−1)
488 nm
Qy(0,2)
532 nm
Qy(0,0)
PBM
δIP(Cm−Pyrrole)A1327 321 317 Porphyrin Breathing Mode (PBM):
In-phase bending of the angles between
the pyrrole groups.
δ(C−C)Pyr A2367 357 354 In-phase bending of the C−C bonds in
pyridyl groups.
τIP(Pyrrole)A2427 - 426 Twist of the Y pyrrole groups.
δ(C−C)Pyr +δ(C−N)Pyr A1501 - 511 Out-of-phase bending of the C −C and
C−N bonds in the pyridyl groups.
δOP(Cm−Cα−N)A1557 - 561 In-phase bending of the angles between
the Cm−Cαand Cα−N bonds.
δOP(N−H)+δOP (Cβ−H)A2630 636 633
Out-of-phase bending of the
N−H
and
Cβ−H bonds.
δOP(N−Cα−Cβ)A2672 - 666 In-phase bending of angles between the
N−Cαand Cα−Cβbonds.
δOP(N−H)+δOP (Cβ−H)A1719 - 710 In-phase bending of the N −H and
Cβ−H bonds.
δOP(Cm−Cα−Cβ)A2739 - 730
Out-of-phase bending of angles between
the Cm−Cαand Cα−Cβbonds.
δ(C−H)Pyr A2752 - 744
In-phase bending of the
C−H
bonds in
the pyridyl groups.
δOP(Cβ−H)A1772 - 786 Out-of-phase bending of the
Cβ−H bonds.
δ(C−H)Pyr A2789 - 797
In-phase bending of the
C−H
bonds in
the pyridyl groups.
δ(C−H)Pyr A2859 - 844
In-phase bending of the
C−H
bonds in
the pyridyl groups.
δ(C−H)Pyr A1864 - 855 Out-of-phase bending of the C −H
bonds in the pyridyl groups.
δOPCβ−H)yA2884 - 871 Bending of the Cβ−H in the Y
pyrrole groups.
δIP(Cm−Cα−N)A1887 - 892 In-phase bending of the angles between
the Cm−Cαand N −Cαbonds.
δ(C−H)Pyr A1966 967 966 Out-of-phase bending of the C−H
bonds in the pyridyl groups.
δ(C−N)Pyr +δ(C−C)Pyr A1980 991 989 In-phase bending of the C−N and
C−C bonds.
δIP(Cβ−H)x+ν(N−Cα−Cβ)xA21003 1001 1000 Bending of the Cβ−H bonds and
stretching of the N −Cα−Cβbonds in
the X pyrrole groups.
ν(Cα−Cβ)A11006 1017 1014 Out-of-phase stretching of the
Cα−Cβbonds.
δIP(Cβ−H)A11065 1063 1063 In-phase bending of the Cβ−H bonds.
δIP(Cβ−H)A11069 1086 1085 Out-of-phase bending of the
Cβ−H bonds.
δIP(N−H)A21122 - 1142 Bending of the N −H bonds.
δ(C−H)Pyr A11206 1211 1213 Out-of-phase bending of the C−H
bonds in the pyridyl groups.
ν(Cm−Pyridyl)+δ(C−H)Pyr A11235 1241 1241 In-phase stretching of the Cm−Pyridyl
bonds and bending of the C −H bonds
in the pyridyl groups.
δIP(N−Cα)x+ν(Cα−Cβ)x+ν(N−Cα)yA11289 1287 -
Bending of the N −Cαbonds and
stretching of the Cα−Cβbonds in the
X pyrrole groups. Stretching of the
N−Cαbonds in the Y pyrrole groups.
δ(C−H)Pyr A21310 1324 1314 Out-of-phase bending of the C−H
bonds in the pyridyl groups.
Molecules 2024,29, 2362 5 of 22
Table 1. Cont.
Raman Mode Symmetry
(C2v)Calculated
(cm−1)
Experimental (cm−1)
488 nm
Qy(0,2)
532 nm
Qy(0,0)
δIP(Cβ−H)+ν(Cα−Cβ)xA21316 1324 1330 In-phase bending of the Cβ−H bonds.
Stretching of the Cα−Cβin the X
pyrrole groups
δIP(N−Cα)+ν(Cα−Cβ)A11356 1357 1357 In-phase bending of the angles between
the N −Cαbonds and stretching of the
Cα−Cβbonds.
δIP(Cβ−H)+ν(N−Cα−Cβ)yA21366 1373 1373 Bending of the Cβ−H bonds.
Stretching of the N −Cαand Cα−Cβ
bonds in the Y pyrrole groups.
ν(Cβ−Cβ)+ν(Cm−Cα−N)A11438 1436 1434 In-phase stretching of the Cβ−Cβ,
Cm−Cα, and N −Cαbonds.
ν(Cm−Cα)x+ν(Cα−Cβ)xA21448 1454 1451
Stretching of the
Cm−Cα
and
Cα−Cβ
bonds in the X pyrrole groups.
δ(C−H)Pyr A21474 1470 - Out-of-phase bending of the C−H
bonds in the pyridyl groups.
ν(Cβ−Cβ)+ν(Cm−Cα−N)yA11499 1489 1495 Out-of-phase stretching of the Cβ−Cβ.
Stretching of the Cm−Cαand N −Cα
bonds in the Y pyrrole groups.
ν(Cβ−Cβ)+ν(Cm−Cα−N)xA11545 1538 - In-phase stretching of the Cβ−Cβ
bonds. Stretching of the Cm−Cαand
N−Cαbonds in the X pyrrole groups.
δIP(N−Cα)y+ν(Cβ−Cβ)x+ν(Cm−Cα)A11554 1553 1549
Bending of the angles between the
N−Cαbonds in the Y pyrrole groups.
Stretching of the Cβ−Cβbonds in the
X pyrrole groups. Out-of-phase
stretching of the Cm−Cαbonds.
ν(C−C)Pyr A11581 1589 1580 In-phase stretching of the C−C bonds
in the pyridyl groups.
In Figure 3, the spectral region of 100
cm−1<κ<
400
cm−1
is displayed. Five Raman
bands are observed for both excitations and are located at 164
cm−1
(161
cm−1
), 199
cm−1
(195
cm−1
), 223
cm−1
(221
cm−1
), 321
cm−1
(317
cm−1
), and 357
cm−1
(354
cm−1
) when ex-
cited at 488
nm
(532
nm
). A Raman band at 239
cm−1
is observed in the spectrum obtained
with 532
nm
excitation, presenting no corresponding band in the spectrum obtained with
488
nm
. Within this same spectral region, theoretical calculations predict nine Raman-active
vibrational modes for the H
2
TPyP molecule. These modes are assigned to the following
vibrations (
OP
stands for out-of-plane, and
IP
stands for in-plane):
δIP(Cm−Pyrrole)
at
163
cm−1
;
ν(Cm−Pyridyl)
at 189
cm−1
;
τOP(Pyrrole)
at 199
cm−1
;
τ(Pyridyl)
at 213
cm−1
;
δIP(Cm−Pyrrole)x+τ(Pyridyl)
at 233
cm−1
;
τ(Pyrrole)
at 284
cm−1
;
δIP(Cm−Pyrrole)
at 327
cm−1
;
τ(Pyrrole)
at 352
cm−1
; and
δ(C−C)Pyr
at 367
cm−1
. Despite the shifts when
compared to experimental results, the calculations indicate that the vibrations at 163
cm−1
,
199
cm−1
, 213
cm−1
, 233
cm−1
, 327
cm−1
, and 367
cm−1
correspond to the six observed
Raman bands, as shown in Figure 3and summarized in Table 1and Table S2 in SI (illus-
trations 1–6). No Raman bands were observed below 150
cm−1
under ambient conditions
or either excitation wavelengths. We note that the in-phase
δIP(Cm−Pyrrole)
mode at
327
cm−1
, from now on designated as “Pophyrin’s Breathing Mode (PBM)”, represents the
breathing of porphyrin’s central ring.
As depicted in Figure 3, it is evident that the spectrum acquired with excitation at
532
nm
exhibits greater resolution compared to the spectrum acquired with excitation at
488
nm
. This observation aligns with the fact that the absorbance at 532
nm
is approximately
twice that at 488 nm [20], potentially resulting in a stronger resonance effect.
Within the spectral range of 400
cm−1<κ<
600
cm−1
, no Raman bands were de-
tected in the spectrum at 488
nm
, as shown in Figure 4. Nevertheless, at 532
nm
, three
distinct Raman bands emerge at 426
cm−1
, 511
cm−1
, and 561
cm−1
, indicating the reso-
Molecules 2024,29, 2362 6 of 22
nance of these modes with
Qy(0, 0)
electronic transition. The observed Raman bands are
assigned to the vibrations
τIP(Pyrrole)
at 427
cm−1
,
δ(C−C)Pyr +δ(C−N)Pyr
at 501
cm−1
,
and
δOP(Cm−Cα−N)
at 557
cm−1
in the calculated spectrum, respectively; see Table 1
and Table S2 in SI (illustrations 7–9).
Molecules 2024, 29, x FOR PEER REVIEW 6 of 26
100 150 200 250 300 350 400
𝛅
𝐈𝐏
𝐂
𝐦
− 𝐏𝐲𝐫𝐫𝐨𝐥𝐞
𝐱
+𝛕 𝐏𝐲𝐫𝐢𝐝𝐲𝐥
𝛕𝐏𝐲𝐫𝐢𝐝𝐲𝐥
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 3. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 100 cm < κ < 400 cm . The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
As depicted in Figure 3, it is evident that the spectrum acquired with excitation at
532 nm exhibits greater resolution compared to the spectrum acquired with excitation at
488 nm. This observation aligns with the fact that the absorbance at 532 nm is approxi-
mately twice that at 488 nm [20], potentially resulting in a stronger resonance effect.
Within the spectral range of 400 cm <κ<600 cm
, no Raman bands were de-
tected in the spectrum at 488 nm, as shown in Figure 4. Nevertheless, at 532 nm, three
distinct Raman bands emerge at 426 cm, 511 cm, and 561 cm, indicating the res-
onance of these modes with Q(0,0) electronic transition. The observed Raman bands are
assigned to the vibrations τ(Pyrrole) at 427 cm , δ(C−C
) +δ
(C−N
) at
501 cm, and δ(C−C
−N
) at 557 cm in the calculated spectrum, respectively;
see Tables 1 and S2 in SI (illustrations 7–9).
Figure 3. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
range of 100
cm−1<κ<
400
cm−1
. The experimental spectra were obtained by exciting the sample
at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and green (532
nm
) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
Molecules 2024, 29, x FOR PEER REVIEW 7 of 26
400 450 500 550 600
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 4. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 400 cm < κ < 600 cm . The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green (532
nm) solid curves represent the fiings obtained through the deconvolution process using Lorenian
functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers to the
Raman activity (scaering factor).
Figure 5 presents the Raman spectra within the range of 600 cm <κ<830 cm.
Several Raman bands are resonant when porphyrins are excited under 532 nm
(633 cm , 666 cm , 710 cm , 730 cm , 744 cm , 786 cm , and 797 cm ),
whereas the spectrum obtained with 488 nm excitation exhibits only one Raman band at
636 cm (corresponding to 633 cm at 532 nm ). According to DFT calculations (see
Tables 1 and S2 in SI (illustrations 10–16)), these Raman bands are assigned to the follow-
ing vibrations: δ(N−H
)+δ
C−H at 630 cm , δ N − C−C
at 672 cm ,
δ(N−H
)+δ
C−H at 719 cm , δ C−C
−C
at 739 cm , δ(C−H
)
at 752 cm, δC−H at 772 cm, and δ(C−H
) at 789 cm.
Figure 4. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
Molecules 2024,29, 2362 7 of 22
range of 400
cm−1<κ<
600
cm−1
. The experimental spectra were obtained by exciting the sample
at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green (532 nm) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
Figure 5presents the Raman spectra within the range of 600
cm−1<κ<
830
cm−1
.
Several Raman bands are resonant when porphyrins are excited under 532
nm
(633
cm−1
,
666
cm−1
, 710
cm−1
, 730
cm−1
, 744
cm−1
, 786
cm−1
, and 797
cm−1
), whereas the spec-
trum obtained with 488
nm
excitation exhibits only one Raman band at 636
cm−1
(cor-
responding to 633
cm−1
at 532
nm
). According to DFT calculations (see Table 1and
Table S2 in SI (illustrations 10–16)), these Raman bands are assigned to the following
vibrations:
δOP(N−H)+δOP (Cβ−H)
at 630
cm−1
,
δOP(N−Cα−Cβ)
at 672
cm−1
,
δOP(N−H)+δOP (Cβ−H)
at 719
cm−1
,
δOP(Cm−Cα−Cβ)
at 739
cm−1
,
δ(C−H)Pyr
at 752 cm−1,δOP(Cβ−H)at 772 cm−1, and δ(C−H)Pyr at 789 cm−1.
Molecules 2024, 29, x FOR PEER REVIEW 8 of 26
600 650 700 750 800
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 5. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 600 cm < κ < 830 cm . The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
The spectral region of 830 cm < κ < 1040 cm (Figure 6) displays eight resonant
Raman bands ( 844 cm , 855 cm , 871 cm , 892 cm , 966 cm , 989 cm ,
1000 cm , and 1014 cm ) under 532 nm excitation. However, when excited under
488 nm , only the higher energy bands at 967 cm , 991 cm , 1001 cm , and
1017 cm are resonant. The DFT calculations (see Tables 1 and S2 in SI (illustrations 17–
24)) suggest the following assignments to these bands: two δ(C−H
) at 859 cm and
864 cm , δ(C−H)
at 884 cm , δ(C−C
−N
) at 887 cm , δ(C−H
) at
966 cm , δ(C−N
) +δ
(C−C
) at 980 cm , δC−H
+νN−C
−C
at
1003 cm, and νC−C
at 1006 cm.
Figure 5. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the
spectral range of 600
cm−1<κ<
830
cm−1
. The experimental spectra were obtained by exciting
the sample at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and
green (532
nm
) solid curves represent the fittings obtained through the deconvolution process using
Lorentzian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers
to the Raman activity (scattering factor).
The spectral region of 830
cm−1<κ<
1040
cm−1
(Figure 6) displays eight resonant
Raman bands (844
cm−1
, 855
cm−1
, 871
cm−1
, 892
cm−1
, 966
cm−1
, 989
cm−1
, 1000
cm−1
,
and 1014
cm−1
) under 532
nm
excitation. However, when excited under 488
nm
, only
the higher energy bands at 967
cm−1
, 991
cm−1
, 1001
cm−1
, and 1017
cm−1
are reso-
nant. The DFT calculations (see Table 1and Table S2 in SI (illustrations 17–24)) suggest
the following assignments to these bands: two
δ(C−H)Pyr
at 859
cm−1
and 864
cm−1
,
δOPCβ−H)y
at 884
cm−1
,
δIP(Cm−Cα−N)
at 887
cm−1
,
δ(C−H)Pyr
at 966
cm−1
,
δ(C−N)Pyr +δ(C−C)Pyr
at 980
cm−1
,
δIP(Cβ−H)x+ν(N−Cα−Cβ)x
at 1003
cm−1
,
and ν(Cα−Cβ)at 1006 cm−1.
Molecules 2024,29, 2362 8 of 22
Molecules 2024, 29, x FOR PEER REVIEW 9 of 26
850 900 950 1000
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 6. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 830 cm < κ < 1040 cm. The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
As shown in Figure 7, the spectral region of 1040 cm < κ < 1180 cm exhibits
three resonant Raman bands under 532 nm excitation: 1068 cm , 1085 cm , and
1142 cm. The spectrum acquired with 488 nm displays two bands at 1068 cm and
1086 cm (the same bands observed at 532 nm ). These bands are assigned to the
δC−H vibrations at 1065 cm and 1069 cm , and the δ (N−H
) vibration at
1122 cm, respectively; see Tables 1 and S2 in SI (illustrations 25–27).
Figure 6. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the
spectral range of 830
cm−1<κ<
1040
cm−1
. The experimental spectra were obtained by exciting
the sample at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and
green (532
nm
) solid curves represent the fittings obtained through the deconvolution process using
Lorentzian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers
to the Raman activity (scattering factor).
As shown in Figure 7, the spectral region of 1040
cm−1<κ<
1180
cm−1
exhibits three
resonant Raman bands under 532
nm
excitation: 1068
cm−1
, 1085
cm−1
, and 1142
cm−1
.
The spectrum acquired with 488
nm
displays two bands at 1068
cm−1
and 1086
cm−1
(the
same bands observed at 532 nm). These bands are assigned to the δIP(Cβ−H)vibrations
at 1065
cm−1
and 1069
cm−1
, and the
δIP(N−H)
vibration at 1122
cm−1
, respectively; see
Table 1and Table S2 in SI (illustrations 25–27).
Molecules 2024, 29, x FOR PEER REVIEW 10 of 26
1050 1100 1150
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 7. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 1040 cm < κ < 1180 cm. The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
Figure 8 shows the spectral range of 1180 cm < κ < 1320 cm. In this ra nge, three
Raman bands are resonant under both 532 nm (1213 cm, 1241 cm, and 1314 cm),
and 488 nm ( 1211 cm , 1241 cm , and 1287 cm ) excitations. The lower energy
bands 1213 cm and 1241 cm at 532 nm ( 1211 cm and 1241 cm at 488 nm )
are assigned to the calculated vibrations δ(C−H
) at 1206 cm , and ν(C−
Pyridyl)+ δ
(C−H
) at 1235 cm, respectively. The theoretical mode δ(N−C
)+
νC−C
+ν
(N−C
) at 1289 cm is assigned to the 1287 cm band at 488 nm ,
while the δ(C−H
) mode at 1310 cm is assigned to the 1314 cm band at
532 nm; see Tables 1 and S2 in SI (illustrations 28–31).
Figure 7. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
Molecules 2024,29, 2362 9 of 22
range of 1040
cm−1<κ<
1180
cm−1
. The experimental spectra were obtained by exciting the sample
at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and green (532
nm
) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
Figure 8shows the spectral range of 1180
cm−1<κ<
1320
cm−1
. In this range, three
Raman bands are resonant under both 532
nm
(1213
cm−1
, 1241
cm−1
, and 1314
cm−1
),
and 488
nm
(1211
cm−1
, 1241
cm−1
, and 1287
cm−1
) excitations. The lower energy bands
1213
cm−1
and 1241
cm−1
at 532
nm
(1211
cm−1
and 1241
cm−1
at 488
nm
) are assigned to
the calculated vibrations
δ(C−H)Pyr
at 1206
cm−1
, and
ν(Cm−Pyridyl)+δ(C−H)Pyr
at
1235
cm−1
, respectively. The theoretical mode
δIP(N−Cα)x+ν(Cα−Cβ)x+ν(N−Cα)y
at 1289
cm−1
is assigned to the 1287
cm−1
band at 488
nm
, while the
δ(C−H)Pyr
mode
at 1310
cm−1
is assigned to the 1314
cm−1
band at 532
nm
; see Table 1and Table S2 in SI
(illustrations 28–31).
Molecules 2024, 29, x FOR PEER REVIEW 11 of 26
1200 1250 1300
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
x10
Figure 8. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 1180 cm < κ < 1320 cm. The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
The spectral range of 1290 cm < κ < 1410 cm exhibits the same four resonant
Raman bands under both 532 nm and 488 nm excitations (see Figure 9) centered at
1314 cm (the same 1314 cm band discussed above in Figure 8), 1330 cm ,
1357 cm, and 1373 cm. The main difference between the two spectra lay on the in-
tensity (i.e., resonant effects) of the bands: the bands at 488 nm appear less structured
when compared with the band at 532 nm. According to the DFT calculations, these bands
are assigned to the following vibrations: δ(C−H
) at 1310 cm , δC−H+
νC−C
at 1316 cm , δ(N−C
)+νC
−C
at 1356 cm , and δ C−H+
νN − C−C
at 1366 cm; see Tables 1 and S2 in SI (illustrations 31–34).
Figure 8. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
range of 1180
cm−1<κ<
1320
cm−1
. The experimental spectra were obtained by exciting the sample
at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and green (532
nm
) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
The spectral range of 1290
cm−1<κ<
1410
cm−1
exhibits the same four resonant Ra-
man bands under both 532
nm
and 488
nm
excitations (see Figure 9) centered at 1314
cm−1
(the
same 1314
cm−1
band discussed above in Figure 8), 1330
cm−1
, 1357
cm−1
, and 1373
cm−1
.
The main difference between the two spectra lay on the intensity (i.e., resonant effects) of
the bands: the bands at 488
nm
appear less structured when compared with the band at
532
nm
. According to the DFT calculations, these bands are assigned to the following vibra-
tions:
δ(C−H)Pyr
at 1310
cm−1
,
δIP(Cβ−H)+ν(Cα−Cβ)x
at 1316
cm−1
,
δIP(N−Cα)+
ν(Cα−Cβ)
at 1356
cm−1
, and
δIP(Cβ−H)+ν(N−Cα−Cβ)y
at 1366
cm−1
; see Table 1
and Table S2 in SI (illustrations 31–34).
Molecules 2024,29, 2362 10 of 22
Molecules 2024, 29, x FOR PEER REVIEW 12 of 26
1300 1350 1400
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 9. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 1290 cm < κ < 1410 cm. The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
The spectral range 1400 cm < κ < 1530 cm, shown in Figure 10, exhibits three
common Raman bands for each excitation. At 532 nm (488 nm), these bands are centered
at 1434 cm ( 1436 cm ), 1451 cm ( 1454 cm ), and 1495 cm ( 1489 cm ). The
DFT calculations indicate that these bands correspond to the following vibrations:
νC−C
+ν
(C−C
−N
) at 1438 cm , ν(C−C
)+νC
−C
at 1448 cm ,
and νC−C
+ν
(C−C
−N
) at 1499 cm; see Tables 1 and S2 in SI (illustrations
35, 36 and 38). The vibration δ(C−H
) at 1474 cm is only resonant at 488 nm (see
Tables 1 and S2 in SI (illustrations 35–38)).
Figure 9. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
range of 1290
cm−1<κ<
1410
cm−1
. The experimental spectra were obtained by exciting the sample
at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and green (532
nm
) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
The spectral range 1400
cm−1<κ<
1530
cm−1
, shown in Figure 10, exhibits three
common Raman bands for each excitation. At 532
nm
(488
nm
), these bands are centered
at 1434
cm−1
(1436
cm−1
), 1451
cm−1
(1454
cm−1
), and 1495
cm−1
(1489
cm−1
). The DFT
calculations indicate that these bands correspond to the following vibrations:
ν(Cβ−Cβ)+
ν(Cm−Cα−N)
at 1438
cm−1
,
ν(Cm−Cα)x+ν(Cα−Cβ)x
at 1448
cm−1
, and
ν(Cβ−Cβ)
+ν(Cm−Cα−N)y
at 1499
cm−1
; see Table 1and Table S2 in SI (illustrations 35, 36 and 38).
The vibration
δ(C−H)Pyr
at 1474
cm−1
is only resonant at 488
nm
(see Table 1and Table S2
in SI (illustrations 35–38)).
As shown in Figure 11, the spectral range 1500
cm−1<κ<
1620
cm−1
exhibits three
resonant Raman bands centered at 1538
cm−1
, 1553
cm−1
, and 1589
cm−1
in the 488
nm
spectrum. The lower energy Raman band (1538
cm−1
) is absent in the 532
nm
spectrum,
while the other two are also resonant, with their centers (1549
cm−1
and 1594
cm−1
) slightly
redshifted. These bands are assigned to the vibrations
ν(Cβ−Cβ)+ν(Cm−Cα−N)x
at
1545
cm−1
,
δIP(N−Cα)y+ν(Cβ−Cβ)x+ν(Cm−Cα)
at 1554
cm−1
, and
ν(C−C)Pyr
at
1581 cm−1; see Table 1and Table S2 in SI (illustrations 39–41).
Table 1provides assignments for a total of forty-one vibrational modes, comprising
fifteen that resonate exclusively with
Qy(
0, 0
)
electronic transition, three resonating only
with
Qy(
0, 2
)
vibronic progression, and twenty-three that resonate with both. It is note-
worthy that reference [
20
] elucidated, via the deconvolution of the absorbance UV-Vis
spectrum, that the
Qy(
0, 2
)
vibronic progression arises from the coupling between
Qy(
0, 0
)
electronic transition and a vibrational mode centered at 1245
cm−1
. This mode closely
aligns in energy with
δ(Cm−Pyridyl)+δ(C−H)pyr
(1241
cm−1
) which resonates with
both 488 nm (Qy(0, 2)) and 532 nm (Qy(0, 0)); see Figure 8and Table 1.
Molecules 2024,29, 2362 11 of 22
Molecules 2024, 29, x FOR PEER REVIEW 13 of 26
1400 1450 1500
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 10. Experimental Raman spectra (top and middle) measured for C-H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 1400 cm < κ < 1530 cm. The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
As shown in Figure 11, the spectral range 1500 cm < κ < 1620 cm exhibits three
resonant Raman bands centered at 1538 cm , 1553 cm , and 1589 cm in the
488 nm spectrum. The lower energy Raman band (1538 cm) is absent in the 532 nm
spectrum, while the other two are also resonant, with their centers (1549 cm and
1594 cm) slightly redshifted. These bands are assigned to the vibrations νC−C
+
ν(C−C
−N
) at 1545 cm , δ(N−C
)+νC
−C
+ν
(C−C
) at 1554 cm ,
and ν(C−C
) at 1581 cm; see Tables 1 and S2 in SI (illustrations 39–41).
Figure 10. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
range of 1400
cm−1<κ<
1530
cm−1
. The experimental spectra were obtained by exciting the sample
at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and green (532
nm
) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
Molecules 2024, 29, x FOR PEER REVIEW 14 of 26
1500 1550 1600
532 nm
488 nm
Calculated spectrum
Raman shift (cm
-1
)
Raman intensity
Figure 11. Experimental Raman spectra (top and middle) measured for C−H2TPyP (black dots) and
calculated (boom) for the H2TPyP molecule (gray solid line) under ambient conditions in the spec-
tral range of 1500 cm < κ < 1620 cm. The experimental spectra were obtained by exciting the
sample at 488 nm (middle spectrum) and 532 nm (top spectrum). The blue (488 nm) and green
(532 nm) solid curves represent the fiings obtained through the deconvolution process using Lo-
renian functions (red solid lines). In the theoretical spectrum, Raman intensity (in A4/amu) refers
to the Raman activity (scaering factor).
Table 1 provides assignments for a total of forty-one vibrational modes, comprising
fifteen that resonate exclusively with Q(0,0) electronic transition, three resonating only
with Q(0,2) vibronic progression, and twenty-three that resonate with both. It is note-
worthy that reference [20] elucidated, via the deconvolution of the absorbance UV-Vis
spectrum, that the Q(0,2) vibronic progression arises from the coupling between
Q(0,0) electronic transition and a vibrational mode centered at 1245 cm. This mode
closely aligns in energy with δ(C−Pyridyl
)+δ
(C−H
) (1241 cm) which resonates
with both 488nm (Q(0,2)) and 532nm (Q(0,0)); see Figure 8 and Table 1.
2.2. Hydrostatic Pressure Experiments
To explore the structural stability of C-H2TPyP, studies were conducted under high-
pressure conditions, from 0.1 GPa to 7.7 GPa. In Figure 12, the Raman spectra acquired
from samples under ambient conditions and submied to various loads are presented. To
facilitate a more comprehensive discussion, the Raman spectra are divided into three dis-
tinct wavenumber regions: 80– 680 cm , 960– 1250 cm , and 1435– 1650 cm . It is
worth noting that even at a very low pressure (0.1 GPa), some modes that were not visible
under ambient conditions become apparent. The lack of theoretical prediction for some of
these modes as vibrational modes of the H2TPyP molecule suggests that the bands in the
low-wavenumber region (100–150 cm) are associated with the laice vibration of C-
H2TpyP, i.e., librations and the torsion of porphyrin’s ring.
Figure 11. Experimental Raman spectra (top and middle) measured for C-H
2
TPyP (black dots) and
calculated (bottom) for the H
2
TPyP molecule (gray solid line) under ambient conditions in the spectral
range of 1500
cm−1<κ<
1620
cm−1
. The experimental spectra were obtained by exciting the sample
at 488
nm
(middle spectrum) and 532
nm
(top spectrum). The blue (488
nm
) and green (532
nm
) solid
curves represent the fittings obtained through the deconvolution process using Lorentzian functions
(red solid lines). In the theoretical spectrum, Raman intensity (in A
4
/amu) refers to the Raman
activity (scattering factor).
Molecules 2024,29, 2362 12 of 22
2.2. Hydrostatic Pressure Experiments
To explore the structural stability of C-H
2
TPyP, studies were conducted under high-
pressure conditions, from 0.1
GPa
to 7.7
GPa
. In Figure 12, the Raman spectra acquired
from samples under ambient conditions and submitted to various loads are presented.
To facilitate a more comprehensive discussion, the Raman spectra are divided into three
distinct wavenumber regions: 80
–
680
cm−1
, 960
–
1250
cm−1
, and 1435
–
1650
cm−1
. It is
worth noting that even at a very low pressure (0.1
GPa
), some modes that were not visible
under ambient conditions become apparent. The lack of theoretical prediction for some
of these modes as vibrational modes of the H
2
TPyP molecule suggests that the bands in
the low-wavenumber region (100
–
150
cm−1
) are associated with the lattice vibration of
C-H2TpyP, i.e., librations and the torsion of porphyrin’s ring.
Molecules 2024, 29, x FOR PEER REVIEW 15 of 26
150 300 450 600
(a)
Raman intensity
1440 1530 1620
Wavenumber
(
cm
-1
)
0.0
7.7
7.3
6.8
6.4
6.1
5.6
5.3
5
4.8
4.5
4.2
4
3.7
3.3
2.9
2.8
2.5
2
1.6
1.5
1.1
0.8
0.7
0.4
0.3
0.1
(c)
(GPa)
990 1100 1210
Wavenumber (cm
-1
)
Wavenumber (cm
-1
)
(b)
Figure 12. Raman spectrum of C-H2TpyP excited at 488 nm with pressures ranging from 0.1 Gpa
to 7.7 Gpa for three distinct wavenumber regions: (a) 80– 680 cm, (b) 960–1250 cm, and (c)
1435– 1650 cm . The spectrum at 0.0 GPA was acquired at ambient conditions and it is being
shown as a reference.
Figure 13 shows the evolution of the Raman band frequencies with increasing pres-
sure, presenting distinct rates dω/dP, as summarized in Table 2. Notably, at pressures of
0.8 GPa, 1.5 GPa, 2.5 GPa, and 5.6 GPa, changes in the wavenumber displacement are ev-
ident for some Raman bands, as indicated in Table 2.
Figure 13. Evolutions of the frequencies of the Raman bands with increasing pressure. The evolution
of the Raman bands with pressure is analyzed for three distinct wavenumber regions: (a)
50– 400 cm , (b) 600– 1300 cm , and (c) 1435–1625 cm , to facilitate a more comprehensive
discussion.
Figure 12. Raman spectrum of C-H
2
TpyP excited at 488
nm
with pressures ranging from 0.1
Gpa
to 7.7
Gpa
for three distinct wavenumber regions: (a) 80
–
680
cm−1
, (b) 960
–
1250
cm−1
, and
(c) 1435
–
1650
cm−1
. The spectrum at 0.0 GPA was acquired at ambient conditions and it is being
shown as a reference.
Figure 13 shows the evolution of the Raman band frequencies with increasing pressure,
presenting distinct rates
dω/dP
, as summarized in Table 2. Notably, at pressures of 0.8
GPa
,
1.5
GPa
, 2.5
GPa
, and 5.6
GPa
, changes in the wavenumber displacement are evident for
some Raman bands, as indicated in Table 2.
The crystal lattice frequency vibrations undergo high blueshift rates (10.1
–
25
cm−1GPa−1
).
The vibrations initially at 98
cm−1
and 133
cm−1
disappear at 0.8
GPa
, and the vibra-
tion initially at 117
cm−1
disappears at 1.5
GPa
. The remaining lattice vibrations at
103
cm−1
and 81
cm−1
have their shift rates decreased at 1.5
GPa
and at 2.5
GPa
, re-
spectively. Moreover, the
τOP(Pyrrole)
vibration (199
cm−1
) undergoes a blueshift at a
rate of 3.8
cm−1GPa−1
, manifesting a gradual decrease in intensity, followed by an in-
crease in its Full Width at Half Maximum (FWHM). Both
PBM
(323
cm−1
) and
δ(C−C)Pyr
(359
cm−1
) vibrations display a blueshift at comparable rates of 2.7
cm−1GPa−1
and
3.1
cm−1GPa−1
, with
δ(C−C)Pyr
observed until 4.5
GPa
, while
PBM
remains up to
7.7
GPa
. The
δIP(Cm−Pyrrole)+τ(Pyridyl)
(242
cm−1
) vibration undergoes a high
blueshift rate (10.1
cm−1GPa−1
). In contrast, a minor displacement rate is identified
Molecules 2024,29, 2362 13 of 22
for the
δOP(N−H)+δOP (Cβ−H)
(636
cm−1
) mode (1.3
cm−1GPa−1
) throughout the
entire process. This rate is notably lower when compared to the displacement rates of other
vibrational modes within the range of 100 to 400
cm−1
. Furthermore, the disappearance of
some lattice modes beyond 0.8
GPa
and the observed increase in the FWHM bands suggest
the initiation of an amorphization process.
Molecules 2024, 29, x FOR PEER REVIEW 15 of 26
150 300 450 600
(a)
Raman intensity
1440 1530 1620
Wavenumber
(
cm
-1
)
0.0
7.7
7.3
6.8
6.4
6.1
5.6
5.3
5
4.8
4.5
4.2
4
3.7
3.3
2.9
2.8
2.5
2
1.6
1.5
1.1
0.8
0.7
0.4
0.3
0.1
(c)
(GPa)
990 1100 1210
Wavenumber (cm
-1
)
Wavenumber (cm
-1
)
(b)
Figure 12. Raman spectrum of C-H2TpyP excited at 488 nm with pressures ranging from 0.1 Gpa
to 7.7 Gpa for three distinct wavenumber regions: (a) 80– 680 cm, (b) 960–1250 cm, and (c)
1435– 1650 cm . The spectrum at 0.0 GPA was acquired at ambient conditions and it is being
shown as a reference.
Figure 13 shows the evolution of the Raman band frequencies with increasing pres-
sure, presenting distinct rates dω/dP, as summarized in Table 2. Notably, at pressures of
0.8 GPa, 1.5 GPa, 2.5 GPa, and 5.6 GPa, changes in the wavenumber displacement are ev-
ident for some Raman bands, as indicated in Table 2.
Figure 13. Evolutions of the frequencies of the Raman bands with increasing pressure. The evolution
of the Raman bands with pressure is analyzed for three distinct wavenumber regions: (a)
50– 400 cm , (b) 600– 1300 cm , and (c) 1435–1625 cm , to facilitate a more comprehensive
discussion.
Figure 13. Evolutions of the frequencies of the Raman bands with increasing pressure. The evolution
of the Raman bands with pressure is analyzed for three distinct wavenumber regions: (a) 50
–
400
cm−1
,
(b) 600–1300 cm−1, and (c) 1435–1625 cm−1, to facilitate a more comprehensive discussion.
Table 2. Experimental
dω/dP
rates for the observed Raman bands. CLV stands for Crystal Lattice
Vibration. Some Raman modes present two slopes with pressure; their intercept and
dω/dP
at such
pressures are indicated as follows:
#
0.8 GPa, ** 1.5
Gpa
, * 2.5
Gpa
, and
$
5.6
GPa
. The numbers in
brackets are the errors in the intercept and dω/dP rates obtained from fitting.
Raman Mode Intercept Position at
0.1 GPa (cm−1)
dω/dP
(cm−1/GPa)
CLV 81.2 (3.6); 100.4 (2.6) * 10.1 (2.0); 1.6 (0.8) *
CLV 98.5 (1.4) 10.1 (2.7)
CLV 102.6 (0.9); 117.6 (1.4) ** 14.6 (0.9); 5.6 (0.3) **
CLV 116.6 (1.0) 14.7 (1.1)
CLV 133.2 (2.2) 25.0 (4.3)
δIP(Cm−Pyrrole)162.9 (1.1) 8.1 (2.1)
τOP(Pyrrole)203.1 (0.8) 3.8 (0.2)
τ(Pyridyl)221.4 (0.4) 2.1 (0.6)
δIP(Cm−Pyrrole)+τ(Pyridyl)242.0 (1.7) 10.1 (3.2)
PBM 322.8 (0.4) 2.7 (0.1)
δ(C−C)Pyr 358.9 (0.5) 3.1 (0.2)
δOP(N−H)+δOP Cβ−H636.1 (0.5) 1.3 (0.1)
δN−Cα−Cβ642.0 (1.2); 670.0 (0.8) * 3.3 (1.1); 1.3 (0.2) *
δ(C−H)Pyr 970.4 (0.4); 1017.6 (5.0) $2.3 (0.1); −1.1 (0.7) $
δ(C−N)Pyr +δ(C−C)Pyr 993.8 (0.7) 2.3 (0.2)
δIPCβ−Hx+νN−Cα−Cβx1004.1 (0.7) 2.9 (0.2)
δIPCβ−H1066.4 (0.8) 3.3 (0.2)
δIPCβ−H1088.0 (0.5) 3.5 (0.1)
δIP(N−H)1145.9 (0.8); 1142.7 (0.8) ** −8.8 (1.2); 2.5 (0.2) **
Molecules 2024,29, 2362 14 of 22
Table 2. Cont.
Raman Mode Intercept Position at
0.1 GPa (cm−1)
dω/dP
(cm−1/GPa)
δIP(N−H)1167.5 (1.0) * 3.0 (0.2) *
δ(C−H)Pyr 1216.8 (0.5) 2.5 (0.1)
ν(Cm−Pyridyl)+δ(C−H)Pyr 1250.5 (0.9) 4.2 (0.2)
νCβ−Cβ+ν(Cm−Cα−N)1438.7 (0.4) 2.6 (0.1)
ν(Cm−Cα)x+νCα−Cβx1455.8 (0.5) 4.0 (0.1)
νCβ−Cβ+ν(Cm−Cα−N)y1488.6 (1.1) 11.7 (2.1)
νCβ−Cβ+ν(Cm−Cα−N)x1537.4 (0.3); 1550.8 (0.4) #11.4 (0.7); 2.9 (0.1) #
δIP(N−Cα)y+νCβ−Cβx+
ν(Cm−Cα)1555.5 (0.7); 1563.5 (0.6) #7.5 (1.6); 4.5(0.1) #
ν(C−C)Pyr 1602.9 (0.8) 2.5 (0.2)
In the region ranging from 960
cm−1
and 1250
cm−1
, most of the Raman bands un-
dergo a gradual blueshift, except for the band centered around 1144
cm−1
, which initially
displays a redshift at a rate of
−
8.8
cm−1GPa−1
(see Table 2and Figure 13b). In the
region within 1440
cm−1
and 1650
cm−1
, the vibrations
ν(Cβ−Cβ)+ν(Cm−Cα−N)
(1439
cm−1
),
ν(Cm−Cα)x+ν(Cα−Cβ)x
(1456
cm−1
),
δIP(N−Cα)y+ν(Cβ−Cβ)x+
ν(Cm−Cα)
(1555
cm−1
), and
ν(C−C)Pyr
(1603
cm−1
) undergo a blueshift, presenting
rates from 2.5
cm−1/ GPa
to 4.0
cm−1/ GPa
. The vibrations
ν(Cβ−Cβ)+ν(Cm−Cα−N)y
(1489
cm−1
) and
ν(Cβ−Cβ)+ν(Cm−Cα−N)x
(1537
cm−1
) undergo an initial blueshift,
with rates around 11
cm−1/ GPa
. The former Raman band disappears at 0.8
GPa
, and the
latter has its shift rate greatly decreased at the same pressure.
Figure 14 shows representative C-H
2
TPyP Raman spectra for selected hydrostatic
pressures. When compared with the spectrum at 0.1
GPa
(the lowest pressure in the
experiment), the spectrum at 2.5
GPa
shows three new Raman modes centered at 673
cm−1
,
1150
cm−1
, and 1175
cm−1
; see Figure 14a. In addition, the intensity of the Raman mode at
1017
cm−1
(see illustration 24 in Table S2 in Supplementary Materials) increases relative to
the intensity of the mode at 1001
cm−1
(see illustration 23 in Table S2 in Supplementary
Materials), making both modes (see Table 1), which are associated with a distinct stretching
of the carbons βand α, distinguishable.
Molecules 2024, 29, x FOR PEER REVIEW 17 of 26
at 2.5 GPa , respectively. Moreover, the τ(Pyrrole) vibration (199 cm ) undergoes a
blueshift at a rate of 3.8 cm GPa, manifesting a gradual decrease in intensity, followed
by an increase in its Full Width at Half Maximum (FWHM). Both PBM (323 cm) and
δ(C−C
) ( 359 cm ) vibrations display a blueshift at comparable rates of
2.7 cm GPa and 3.1 cm GPa , with δ(C−C
) observed until 4.5 GPa , while
PBM remains up to 7.7 GPa. The δ(C−Pyrrole
)+τ
(Pyridyl) (242 cm) vibration un-
dergoes a high blueshift rate (10.1 cmGPa). In contrast, a minor displacement rate is
identified for the δ(N − H) + δ (C−H) (636 cm) mode (1.3 cm GPa) through-
out the entire process. This rate is notably lower when compared to the displacement rates
of other vibrational modes within the range of 100 to 400 cm. Furthermore, the disap-
pearance of some laice modes beyond 0.8 GPa and the observed increase in the FWHM
bands suggest the initiation of an amorphization process.
In the region ranging from 960 cm and 1250 cm, most of the Raman bands un-
dergo a gradual blueshift, except for the band centered around 1144 cm, which initially
displays a redshift at a rate of −8.8 cmGPa (see Table 2 and Figure 13b). In the region
within 1440 cm and 1650 cm , the vibrations νC−C
+ν
(C−C
−N
)
(1439 cm ), ν(C−C
)+νC
−C
( 1456 cm ), δ(N−C
)+νC
−C
+
ν(C−C
) ( 1555 cm ), and ν(C−C
) ( 1603 cm ) undergo a blueshift, presenting
rates from 2.5 cm/GPa to 4.0 cm/GPa. The vibrations νC−C
+ν
(C−C
−N
)
(1489 cm) and νC−C
+ν
(C−C
−N
) (1537 cm) undergo an initial blueshift,
with rates around 11 cm/GPa. The former Raman band disappears at 0.8 GPa, and the
laer has its shift rate greatly decreased at the same pressure.
Figure 14 shows representative C-H2TPyP Raman spectra for selected hydrostatic
pressures. When compared with the spectrum at 0.1 GPa (the lowest pressure in the ex-
periment), the spectrum at 2.5 GPa shows three new Raman modes centered at 673 cm,
1150 cm, and 1175 cm; see Figure 14a. In addition, the intensity of the Raman mode
at 1017 cm (see illustration 24 in Table S2 in Supplementary Materials) increases rela-
tive to the intensity of the mode at 1001 cm (see illustration 23 in Table S2 in Supple-
mentary Materials), making both modes (see Table 1), which are associated with a distinct
stretching of the carbons β and α, distinguishable.
400 600 800 1000 120
0
Raman intensity
Raman shift (cm
-1
)
4.0 GPa
2.5 GPa
3.3 GPa
0.1 GPa
(a)
673
1017
1150
1175
1077
1086
1500 1600
1553
0.4 GPa
(b)
6.4 GPa
2.0 GPa
1.5 GPa
0.8 GPa
0.1 GPa
1604
Figure 14. Raman spectra ranging from (a) 300 cm to 1200 cm , and (b) from 1500 cm to
1800 cm. The spectra were excited at 488 nm and acquired under different hydrostatic pressures.
Figure 14. Raman spectra ranging from (a) 300
cm−1
to 1200
cm−1
, and (b) from 1500
cm−1
to
1800 cm−1. The spectra were excited at 488 nm and acquired under different hydrostatic pressures.
Molecules 2024,29, 2362 15 of 22
The Raman band initially at 1086
cm−1
(out-of-phase bending of the
Cβ−H
pair; see
illustration 26 in Table S2 in Supplementary Materials) undergoes a frequency upshift and
an intensity decrease, favoring the observation of the lower energy band at 1077
cm−1
(in-phase bending of the
Cβ−H
pair; see illustration 25 in Table S2 in Supplementary
Materials), whose main change is connected to its intensity increase. These two bands
start fading and lose resolution when the pressure is further increased to 3.3
GPa
. It is
important to comment that the in-phase bending of the
Cβ−H
pair at 1077
cm−1
appears
at 1063
cm−1
when measured at ambient conditions. With increasing pressure, the inactive
vibration at ambient conditions
δIP(N−H)
centered at 1171
cm−1
(see Figure 15) becomes
active with the frequency slightly upshifted to 1175
cm−1
. It is worth mentioning that
the modes centered at 673
cm−1
and 1150
cm−1
only undergo a slight enhancement of
their intensities.
Molecules 2024, 29, x FOR PEER REVIEW 18 of 26
The Raman band initially at 1086 cm (out-of-phase bending of the C−H pair;
see illustration 26 in Table S2 in Supplementary Materials) undergoes a frequency upshift
and an intensity decrease, favoring the observation of the lower energy band at
1077 cm (in-phase bending of the C−H pair; see illustration 25 in Table S2 in Supple-
mentary Materials), whose main change is connected to its intensity increase. These two
bands start fading and lose resolution when the pressure is further increased to 3.3 GPa.
It is important to comment that the in-phase bending of the C−H pair at 1077 cm
appears at 1063 cm when measured at ambient conditions. With increasing pressure,
the inactive vibration at ambient conditions δ(N − H) centered at 1171 cm (see Fig-
ure 15) becomes active with the frequency slightly upshifted to 1175 cm. It is worth
mentioning that the modes centered at 673 cm and 1150 cm only undergo a slight
enhancement of their intensities.
Figure 15. Schematic illustrations of the Raman vibration activated at higher pressures: bending of
the N−H bonds; IP stands for in-plane.
From Figures 12 and 14b, it is noteworthy that the modes centered at 1537 cm
(νC−C
+ν
(C−C
−N
); see illustration 39 in Table S2 in Supplementary Materials)
and at 1555 cm (δ(N−C
)+νC
−C
+ν
(C−C
); see illustration 40 in Table S2
in Supplementary Materials) gradually upshift in frequency for pressures up to 7.7 GPa.
Initially, νC−C
+ν
(C−C
−N
) displays an higher upshift rate of
11.4 cmGPa compared to the 7.5 cm GPa observed for δ(N−C
)+νC
−
C+ν
(C−C
). However, beyond 0.8 Gpa, both rates decrease to 2.9 cm GPa and
to 4.5 cm GPa , respectively. This implies that after 0.8 GPa , δ(N−C
)+νC
−
C+ν
(C−C
) upshifts more than one and a half times when compared with
νC−C
+ν
(C−C
−N
). This observation explains the observed spliing in the Ra-
man bands with increasing pressure. Their relative intensities present an interesting be-
havior: the intensity of the mode νC−C
+ν
(C−C
−N
) is first enhanced and then
suppressed with increasing pressure, while the intensity of the mode δ(N−C
)+
νC−C
+ν
(C−C
) is continuously suppressed. Finally, the Raman-active vibra-
tion ν(C−C
) theoretically centered at 1581 cm (see illustration 41 in Table S2 in
Supplementary Materials) undergoes both a frequency upshift to 1604 cm and a sub-
stantial enhancement of its intensity with increasing pressure.
In addition to structural modifications observed, our findings indicate the influence
of pressure load on resonance conditions of C-H
2
TPyP, probably due to modifications in
the Q(0,2) and Q(0,0) bands’ energy gap. Indeed, as mentioned in Section 2.1, some
of the Raman bands observed at 532 nm (488 nm) do not have a correspondent in the
spectra at 488 nm ( 532 nm ), evidencing the resonance effect [31,33,34], which occurs
when the different regions of the optical absorption spectrum (i.e., the Q(0, 2) and the
Q(0,0) bands) are excited [34]. It is also known that the resonance conditions of
Figure 15. Schematic illustrations of the Raman vibration activated at higher pressures: bending of
the N −H bonds; IP stands for in-plane.
From Figures 12 and 14b, it is noteworthy that the modes centered at 1537
cm−1
(
ν(Cβ−Cβ)+ν(Cm−Cα−N)x
; see illustration 39 in Table S2 in Supplementary Mate-
rials) and at 1555
cm−1
(
δIP(N−Cα)y+ν(Cβ−Cβ)x+ν(Cm−Cα)
; see illustration 40
in Table S2 in Supplementary Materials) gradually upshift in frequency for pressures
up to 7.7
GPa
. Initially,
ν(Cβ−Cβ)+ν(Cm−Cα−N)x
displays an higher upshift
rate of 11.4
cm−1GPa−1
compared to the 7.5
cm−1GPa−1
observed for
δIP(N−Cα)y+
ν(Cβ−Cβ)x+ν(Cm−Cα)
. However, beyond 0.8
Gpa
, both rates decrease to 2.9
cm−1
GPa−1
and to 4.5
cm−1GPa−1
, respectively. This implies that after 0.8
GPa
,
δIP(N−Cα)y+
ν(Cβ−Cβ)x+ν(Cm−Cα)
upshifts more than one and a half times when compared
with
ν(Cβ−Cβ)+ν(Cm−Cα−N)x
. This observation explains the observed split-
ting in the Raman bands with increasing pressure. Their relative intensities present
an interesting behavior: the intensity of the mode
ν(Cβ−Cβ)+ν(Cm−Cα−N)x
is
first enhanced and then suppressed with increasing pressure, while the intensity of the
mode
δIP(N−Cα)y+ν(Cβ−Cβ)x+ν(Cm−Cα)
is continuously suppressed. Finally,
the Raman-active vibration
ν(C−C)Pyr
theoretically centered at 1581
cm−1
(see illustra-
tion 41 in Table S2 in Supplementary Materials) undergoes both a frequency upshift to
1604 cm−1and a substantial enhancement of its intensity with increasing pressure.
In addition to structural modifications observed, our findings indicate the influence
of pressure load on resonance conditions of C-H
2
TPyP, probably due to modifications in
the
Qy(
0, 2
)
and
Qy(
0, 0
)
bands’ energy gap. Indeed, as mentioned in Section 2.1, some
of the Raman bands observed at 532
nm
(488
nm
) do not have a correspondent in the
Molecules 2024,29, 2362 16 of 22
spectra at 488
nm
(532
nm
), evidencing the resonance effect [
31
,
33
,
34
], which occurs when
the different regions of the optical absorption spectrum (i.e., the
Qy1(
0, 2
)
and the
Qy(
0, 0
)
bands) are excited [
34
]. It is also known that the resonance conditions of vibrational modes
are often affected by external stimuli (e.g., temperature and pressure) that perturb the
molecular geometry [35,36].
A new Raman mode at 242
cm−1
is observed at 0.1
GPa
with excitation at 488
nm
,
as shown in Figure 16a, and its intensity increases with compression, up to 0.8
GPa
. Fur-
thermore, as shown in Figure 16b, the Raman-active vibration centered at 673
cm−1
(not
present in the spectrum at 0.1
GPa
) has emerged in the spectrum acquired at 2.5
GPa
with
the excitation at 488
nm
. These bands are assigned to the
δIP(Cm−Pyrrole)x+τ(Pyridyl)
and
δOP(N−Cα−Cβ)
vibrations, respectively, as seen in Table 1and Table S2 in SI (il-
lustrations 4 and 11). Although not present when the sample is excited at 488
nm
, these
bands are resonant with the 532
nm
excitation at ambient conditions. The Raman features
centered at 1007
cm−1
, 1150
cm−1
, and 1604
cm−1
(Figure 16c,d), which are assigned to the
ν(Cα−Cβ)
,
δIP(N−H)
, and
ν(C−C)Pyr
vibrations (Table 1and Table S2 in SI (illustra-
tions 24, 27, and 41)), present the same behavior: they appear in the spectrum obtained
at 4.5
GPa
with excitation at 488
nm
, but they are not present when the pressure is set to
0.1
GPa
. In addition, these bands are also resonant with the 532
nm
excitation at ambient
conditions. These results suggest that the resonance conditions of the porphyrin molecules
are changing with changing pressure. In other words, the increase in pressure seems
to result in an increased energy separation between electronic levels. Therefore, bands
which are resonant at 532
nm
(ambient conditions) become resonant at 488
nm
for higher
pressures.
It is also important to note that the results associated with the molecules’ decom-
pression show that the frequency shifts are reversible for most bands, but the vibrations
between 970
cm−1
and 1003
cm−1
present signs of irreversibility (see Figure S3 in Supple-
mentary Materials).
Molecules 2024, 29, x FOR PEER REVIEW 19 of 26
vibrational modes are often affected by external stimuli (e.g., temperature and pressure)
that perturb the molecular geometry [35,36].
A new Raman mode at 242 cm is observed at 0.1 GPa with excitation at 488 nm,
as shown in Figure 16a, and its intensity increases with compression, up to 0.8 GPa. Fur-
thermore, as shown in Figure 16b, the Raman-active vibration centered at 673 cm (not
present in the spectrum at 0.1 GPa ) has emerged in the spectrum acquired at 2.5 GPa
with the excitation at 488 nm . These bands are assigned to the δ(C−Pyrrole
)+
τ(Pyridyl) and δN − C−C
vibrations, respectively, as seen in Tables 1 and S2 in SI
(illustrations 4 and 11). Although not present when the sample is excited at 488 nm, these
bands are resonant with the 532 nm excitation at ambient conditions. The Raman fea-
tures centered at 1007 cm , 1150 cm , and 1604 cm (Figure 16c,d), which are as-
signed to the νC−C
, δ(N−H
) , and ν(C−C
) vibrations (Tables 1 and S2 in SI
(illustrations 24, 27, and 41)), present the same behavior: they appear in the spectrum ob-
tained at 4.5 GPa with excitation at 488 nm, but they are not present when the pressure
is set to 0.1 GPa. In addition, these bands are also resonant with the 532 nm excitation at
ambient conditions. These results suggest that the resonance conditions of the porphyrin
molecules are changing with changing pressure. In other words, the increase in pressure
seems to result in an increased energy separation between electronic levels. Therefore,
bands which are resonant at 532 nm (ambient conditions) become resonant at 488 nm
for higher pressures.
100 150 200 250
𝛅𝐈𝐏 𝐂𝐦−𝐏𝐲𝐫𝐫𝐨𝐥𝐞 𝐱
+𝛕 𝐏𝐲𝐫𝐢𝐝𝐲𝐥
0.8 GPa
488 nm
Ambient pressure
488 nm
Raman shift (cm
-1
)
Raman intensity
0.1 GPa
488 nm
Ambient pressure
532 nm
(a)
Figure 16. Cont.
Molecules 2024,29, 2362 17 of 22
Molecules 2024, 29, x FOR PEER REVIEW 20 of 26
800 900 1000 1100 1200
Raman shift (cm
-1
)
Raman intensity
(c)
1175
Figure 16. Raman spectra ranging from (a) 100 cm
to 300 cm
, (b) 300 cm
to 950 cm
, (c)
800 cm
to 1200 cm
, and (d) from 1400 cm
to 1700 cm
. The spectra were acquired under
different hydrostatic pressures and excited at both 488 nm and 532 nm (ambient conditions).
It is also important to note that the results associated with the molecules’ decompres-
sion show that the frequency shifts are reversible for most bands, but the vibrations be-
tween 970 cm and 1003 cm present signs of irreversibility (see Figure S3 in Supple-
mentary Materials).
Figure 16. Raman spectra ranging from (a) 100
cm−1
to 300
cm−1
, (b) 300
cm−1
to 950
cm−1
,
(c) 800
cm−1
to 1200
cm−1
, and (d) from 1400
cm−1
to 1700
cm−1
. The spectra were acquired under
different hydrostatic pressures and excited at both 488 nm and 532 nm (ambient conditions).
2.3. Low-Temperature Experiments
Temperature-dependent Raman spectroscopy has also been performed to complement
the understand of porphyrin’s vibrational properties. The C-H
2
TPyP molecules were
submitted to temperatures ranging from 78
K
to 299
K
. Differently from the behavior
presented at variable pressures, no shifts in the Raman band centers or broadenings of the
bands’ linewidths were detected in this range of temperatures, as shown in Figure 17.
Molecules 2024,29, 2362 18 of 22
Molecules 2024, 29, x FOR PEER REVIEW 21 of 26
2.3. Low-Temperature Experiments
Temperature-dependent Raman spectroscopy has also been performed to comple-
ment the understand of porphyrin’s vibrational properties. The C-H2TPyP molecules were
submied to temperatures ranging from 78 K to 299 K . Differently from the behavior
presented at variable pressures, no shifts in the Raman band centers or broadenings of the
bands’ linewidths were detected in this range of temperatures, as shown in Figure 17.
200 400 600 800 1000 1200 1400 1600
78 K
100 K
140 K
180 K
220 K
260 K
Raman intensity
Raman shift (cm
-1
)
299 K
Figure 17. Raman spectrum of C-H2TPyP excited at 532 nm with temperatures ranging from 78 K
to 299 K.
The analysis of the relative intensities of the Raman bands with respect to the PBM
(321 cm) intensity at 299 K shows that the intensities of most of the Raman modes re-
main essentially unchanged. However, some vibrations have their intensities greatly al-
tered at lower temperatures, such as the τ(Pyrrole) vibration at 429 cm, whose rela-
tive intensity has an initial value of 0.3 at room temperature (299 K) that is increased to
0.7 at 180 K (see Figure 18a), and the δ(C−H
) mode at 801 cm, whose relative in-
tensity goes from 0.3 to 0.9 when the temperature is lowered from 299 K to 180 K; see
Figure 18b.
Figure 17. Raman spectrum of C-H
2
TPyP excited at 532
nm
with temperatures ranging from 78
K
to
299 K.
The analysis of the relative intensities of the Raman bands with respect to the PBM
(
321 cm−1
intensity at 299
K
shows that the intensities of most of the Raman modes remain
essentially unchanged. However, some vibrations have their intensities greatly altered at
lower temperatures, such as the
τIP(Pyrrole)
vibration at 429
cm−1
, whose relative intensity
has an initial value of 0.3 at room temperature (299
K
) that is increased to 0.7 at 180
K
(see
Figure 18a), and the
δ(C−H)Pyr
mode at 801
cm−1
, whose relative intensity goes from 0.3
to 0.9 when the temperature is lowered from 299 K to 180 K; see Figure 18b.
Molecules 2024, 29, x FOR PEER REVIEW 22 of 26
300 250 200 150 100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
I
429
/I
321
Temperature (K)
(a)
300 250 200 150 100
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
I
802
/I
321
Temperature (K)
(b)
Figure 18. (a) τ(Pyrrole) (at 429 cm) and (b) δ(C−H
) (at 802 cm) Raman modes’ rela-
tive intensities as a function of temperature. The relative intensities ploed here are the ratio of the
modes’ absolute intensities with relation to the absolute intensity of the PBM mode (321 cm) at
299 K. The errors associated with the measurements of the relative intensities are lower than 0.1%
for all acquired spectra.
Overall, the effects observed in the relative intensities of the Raman bands are indic-
ative of perturbations in the molecular symmetry at lower temperatures, which are likely
altering the Raman activities of the modes. The graphs containing the temperature-de-
pendent relative intensities for each assigned mode are available in Supplementary Mate-
rials; see Table S3.
Finally, the literature reports that isolated H2TPyP belongs to the D point group
[37]. For this symmetry, it is expected that antisymmetric vibrational modes with respect
to the molecule inversion center, called odd modes, will not be Raman-active [38,39]. De-
spite that, some vibrations observed in this work, at ambient conditions, higher pressures,
and lower temperatures, are odd modes (for instance, τ (Pyrrole) ( 199 cm ) and
δ(C−C
) (367 cm)). The observation of such modes in the Raman spectra strongly
indicates that a reduction in the planarity of the molecule, and consequently a change in
its symmetry, is taking place. We hypothesize that this planarity reduction could be asso-
ciated with a saddle-shaped conformation (already observed in porphyrins [40]), due to
local fields in the porphyrin crystal. Our theoretical predictions could only describe our
experimental results after considering such symmetry change, predicting that H2TPyP as-
sumes the C point group. In addition, the new vibration observed at 1175 cm at
higher pressures is also an odd mode and possesses an antisymmetric vibration in the YZ
plane. Its appearance indicates a further planarity modification with pressure, without
further symmetry changes. Lastly, the changes in the relative intensities observed both at
high pressures and low temperatures also indicate changes in the molecular symmetry.
3. Materials and Methods
C-H2TPyP was synthesized following the procedures described in reference [41], and
the spectrometric analysis of the resulting crystals are in good agreement with the litera-
ture [42].
The vibrational properties of C-H2TPyP were investigated via Raman spectroscopy
using a T64000 spectrometer from Horiba Jobin Yvon (Lille Country, France). The scat-
tered light was collected using a 20 × magnification objective lens in a backscaering
configuration. The spectral resolution was ± 2 cm. The measurements were conducted
under both ambient conditions and high pressures. C-H2TPyP was excited using two dif-
ferent laser lines: 488 nm for both ambient and high-pressure conditions, and 532 nm
for ambient conditions only. No fluorescence background was observed upon sample ex-
citation at 488 nm. However, the issue of fluorescence background arises when the sample
is excited with 532 nm. To address this problem, a baseline correction of the spectrum is
Figure 18. (a)
τIP(Pyrrole)
(at 429
cm−1
) and (b)
δ(C−H)Pyr
(at 802
cm−1
) Raman modes’ relative
intensities as a function of temperature. The relative intensities plotted here are the ratio of the modes’
absolute intensities with relation to the absolute intensity of the PBM mode (321
cm−1
) at 299
K
.
The errors associated with the measurements of the relative intensities are lower than 0.1% for all
acquired spectra.
Overall, the effects observed in the relative intensities of the Raman bands are in-
dicative of perturbations in the molecular symmetry at lower temperatures, which are
likely altering the Raman activities of the modes. The graphs containing the temperature-
Molecules 2024,29, 2362 19 of 22
dependent relative intensities for each assigned mode are available in Supplementary
Materials; see Table S3.
Finally, the literature reports that isolated H
2
TPyP belongs to the
D2h
point group [
37
].
For this symmetry, it is expected that antisymmetric vibrational modes with respect to the
molecule inversion center, called odd modes, will not be Raman-active [
38
,
39
]. Despite
that, some vibrations observed in this work, at ambient conditions, higher pressures, and
lower temperatures, are odd modes (for instance,
τOP(Pyrrole)
(199
cm−1
) and
δ(C−C)Pyr
(367
cm−1
)). The observation of such modes in the Raman spectra strongly indicates that
a reduction in the planarity of the molecule, and consequently a change in its symmetry,
is taking place. We hypothesize that this planarity reduction could be associated with a
saddle-shaped conformation (already observed in porphyrins [
40
]), due to local fields in the
porphyrin crystal. Our theoretical predictions could only describe our experimental results
after considering such symmetry change, predicting that H
2
TPyP assumes the
C2v
point
group. In addition, the new vibration observed at 1175
cm−1
at higher pressures is also
an odd mode and possesses an antisymmetric vibration in the
YZ
plane. Its appearance
indicates a further planarity modification with pressure, without further symmetry changes.
Lastly, the changes in the relative intensities observed both at high pressures and low
temperatures also indicate changes in the molecular symmetry.
3. Materials and Methods
C-H
2
TPyP was synthesized following the procedures described in reference [
41
],
and the spectrometric analysis of the resulting crystals are in good agreement with the
literature [42].
The vibrational properties of C-H
2
TPyP were investigated via Raman spectroscopy
using a T64000 spectrometer from Horiba Jobin Yvon (Lille Country, France). The scattered
light was collected using a 20
×
magnification objective lens in a backscattering configu-
ration. The spectral resolution was
±
2
cm−1
. The measurements were conducted under
both ambient conditions and high pressures. C-H
2
TPyP was excited using two different
laser lines: 488
nm
for both ambient and high-pressure conditions, and 532
nm
for ambient
conditions only. No fluorescence background was observed upon sample excitation at
488 nm. However, the issue of fluorescence background arises when the sample is excited
with 532 nm. To address this problem, a baseline correction of the spectrum is performed.
The baseline determination proceeds as follows: First, a numerical derivative of the ex-
perimental data is calculated. Since the fluorescence bands are generally much broader
profiles compared to Raman bands, the first derivative is used to distinguish them. In the
derivative spectrum, each Raman band appears as two symmetric bands around zero and
the fluorescence signal grows as a background curve with the increase in the wavenumber.
This fluorescence curve is then adjusted using a multiparametric function, integrated, and
subsequently employed as the baseline for the original dataset.
The high-pressure Raman spectra were measured using a diamond anvil cell (
µ
-scope
DAC HT(S)) from Almax easyLab (Diksmuider, Belgium). A mineral oil, specifically Nujol,
was used as the pressure-transmitting medium [
43
]. The sample was loaded into a 100
µm
hole drilled in a stainless-steel gasket (thickness of 200
µm
), using an electric discharge
machine from Almax easyLab. Pressure measurements were calibrated by monitoring
the shifts in the ruby fluorescence lines [
44
,
45
]. The increase in fluorescence background
originating from C-H
2
TPyP upon its insertion into the pressure cell makes it impractical to
acquire the Raman signal using 532 nm excitation in high-pressure experiments.
The low-temperature Raman spectra measurements were performed with the Janis
ST-500 cryostat from Lake shore Cryotornics (Westerville, OH, USA) and the samples, after
being properly accommodated in the cryostat, were excited with a 532
nm
(2.33
eV
) CW
laser using 40
×
objective lens with numerical aperture 0.60. The scattered light was
acquired in a backscattering configuration, using an Andor SR303i spectrometer operating
with a 1200 L/mm grating, coupled to an iDUS CCD camera.
Molecules 2024,29, 2362 20 of 22
Our theoretical approach was based on the Density Functional Theory (DFT) for-
malism as implemented in the ORCA code [
46
], considering isolated molecules. We
employed a polarized triple-zeta basis set (def2-TZVP) and the Generalized Gradient
Approximation (GGA) within the Perdew–Burke–Ernzerhof (PBE) parametrization for
the exchange–correlation functional. The calculated main bond lengths and angles are
provided in supporting information (see Table S1 in Supplementary Materials) and were
computed with both GGA (PBE) and META-GGA (M06-L). In Figure S2 in Supplementary
Materials, the structure of H
2
TPyP computed with GGA (PBE) is used as reference to
the analysis of Table S1 in Supplementary Materials. We found an excellent agreement
between the two functionals, with the largest absolute difference in bond lengths being
only 0.012 Å. Following the complete geometry optimization, the Raman spectra were
determined numerically, with the best approximation to the experimental data achieved
using the PBE functional, which is consistent with previous studies [47].
4. Conclusions
Raman-active vibrations in H
2
TPyP have been poorly explored and, in this work,
through the combination of experiments and DFT calculations, we provide a thorough
discussion of such vibrations. Every measured Raman-active vibration within 100
cm−1
to 1700
cm−1
that is resonant with either 532
nm
or 488
nm
is now assigned, with their
symmetries and resonance properties properly addressed. In addition, the results show
that the resonance conditions of active vibrations are tunable under hydrostatic pressure.
In other words, bands which, under ambient conditions, are only active under 532
nm
excitation become readily active at 488
nm
with increasing pressure. Finally, H
2
TPyP has
been reported to possess the point symmetry
D2h
, but the experimental results presented
here, combined with DFT calculations, suggest that these molecules are better described
under the
C2v
symmetry. The pressure- and temperature-dependent results indicate that
molecular planarity is being further perturbed at lower temperatures and higher pressures.
Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/molecules29102362/s1, Figure S1: X-ray diffractogram of C-
H
2
TPyP. The peaks reveal a crystalline structure of the investigated sample. Figure S2: Structure
of H2TPyP molecule calculated with GGA (PBE) and used as reference to Table S2 in SI. Figure
S3: Raman shift of the C-H
2
TPyP bands under compression (
▲
) and decompression (
∇
). Table S1:
Main bond lengths determined by DFT calculations employing two choices of exchange-correlation
functionals, GGA (PBE) and META-GGA (M06-L). The bonds are indicated in Figure S2, which
presents the structure of optimized porphyrin. The two functionals are in excellent agreement with
each other: the largest absolute difference is only 0.012 Å, while the largest percentage difference is
0.9%. Table S2: Schematic illustrations of the Raman modes observed in this work. The black and red
lines indicate out-of-plane opposite bonds. The arrows are in-plane vibrations, and the symbols
⊙
and
⊗
are out-of-plane opposite vibrations. Green and purple arrows represent out-of-phase modes.
The indexes “IP” and “OP” stand for in-plane and out-of-plane modes, respectively. The indexes
“x” and “y” indicate vibrations only in the respective direction. Table S3: Raman relative intensity
(Y-axis) of C-H
2
TPyP bands, relative to the PBM mode at 321 cm
−1
, as function of temperature
(X-axis), ranging from 299 K to 76 K. The inserted values indicate the ratio between the intensity at
the analyzed Raman band frequency (in cm−1) and the reference (PBM).
Author Contributions: Conceptualization, N.M.B.N., W.P. and P.T.A.; methodology, N.M.B.N., A.A.B.,
W.P., J.R.T.d.R., W.H.N.S., G.A.S.R., K.S., F.F.L., P.T.A., A.R.P. and M.M.; software, M.M., G.A.S.R. and
W.H.N.S.; validation, N.M.B.N., J.R.T.d.R., P.T.A., W.P., A.A.B. and M.M.; formal analysis, J.R.T.d.R.,
N.M.B.N., W.P., F.F.L., A.R.P. and P.T.A.; investigation, J.R.T.d.R., F.F.L., W.H.N.S., G.A.S.R. and K.S.;
resources, N.M.B.N., P.T.A., A.A.B., M.M., A.R.P. and W.P.; data curation, J.R.T.d.R.; writing—original
draft preparation, N.M.B.N., J.R.T.d.R. and P.T.A.; writing—review and editing, N.M.B.N., J.R.T.d.R.,
F.F.L., M.M. and P.T.A.; visualization, J.R.T.d.R., N.M.B.N., P.T.A. and M.M.; supervision, N.M.B.N.,
M.M. and W.P.; project administration, N.M.B.N., P.T.A., A.A.B. and W.P.; funding acquisition,
N.M.B.N., P.T.A., A.A.B. and W.P. All authors have read and agreed to the published version of
the manuscript.
Molecules 2024,29, 2362 21 of 22
Funding: The authors are indebted to Brazilian agencies CNPq under Grant No. [306147/2020-3],
FAPESPA, and CAPES under Grant No. [Finance Code 001; AUXPE 88881.159129/2017-01]. NMBN
acknowledges the support from the Brazilian National Council for Scientific and Technological
Development (CNPq) (process number: 465572/2014-6), and the São Paulo Research Foundation.
PTA is grateful to the National Science Foundation (NSF) under Grant No. [1848418] for the finan-
cial support. NMBN is especially grateful to the Fulbright Foundation for the Visiting Professor
Scholarship Award.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article and Supplementary Materials.
Conflicts of Interest: The authors declare no conflict of interest.
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