(A) Electron configurations of a singlet ground state (S 0 ), a singlet excited state (S 1 ), and a triplet excited state (T 1 ). (B) Jablonski diagram for fluorescence (F), intersystem crossing (ISC), and phosphorescence (P) emissions. hν designates photoexcitation. Reproduced from Ref. [4] with permission. Copyright 2018, The Chemical Society of Japan.

Contexts in source publication

Context 1

... some materials spontaneously emit light under irradiation of proper wavelengths of light (generally UV or even higher energy light), which is generally called photoluminescence. The well-known phenomena of photoluminescence are fluorescence and phosphorescence (Figure 1) [4]. Both include radiative decay processes, during which molecules give off photons after receiving the excitation energies under irradiation. ...

Context 2

... crystal structure of 1 is shown in Figure 9 [13]. The asymmetric unit comprises two [Au(dmit) 2 ] δ− anions and one BPY 2δ+ cation ( Figure 10) [13]. For simplicity, the formal charge on the BPY cation will be denoted as 2+ below, and similarly [Au(dmit) 2 ] δ− as [Au(dmit) 2 ] − , unless this designation is misleading. ...

Context 3

... simplicity, the formal charge on the BPY cation will be denoted as 2+ below, and similarly [Au(dmit) 2 ] δ− as [Au(dmit) 2 ] − , unless this designation is misleading. The short molecular axes of two crystallographically independent Au(dmit) 2 anions are arranged almost perpendicularly to each other ( Figures 10 and 11) [13]. Such orthogonal arrangement of metal(M)-dithiolene(L) complex molecules (M = Zn, Ni, Pd, Pt, Cu, Au, etc) along their short molecular axes has been scarcely observed in a great number of related complexes reported thus far [19][20][21][22][23][24][25]38,[40][41][42][43][44][45][46] [34]. ...

Context 4

... of the Au(dmit) 2 anions faces a BPY cation, and they together form mixed-stacking columns. Only the Au(dmit) 2 anions in the mixed-stacking columns contain Au atoms (Au2) that adopt both planar (P, Au2A) and non-planar (NP, Au2B) geometries in a disordered manner ( Figure 10) [13]. The remaining Au atoms (Au1) are not disordered and adopt the P geometry. ...

Context 5

... distances from Au2B to the nearest C atoms on BPY (3.037-3.46 Å; Au2B-C17 and Au2B-C18; for atomic numbering scheme, see Figure 12) are comparable or shorter than the sum of the corresponding van der Waals radii (Au-C; d vdW (Au-C) = 3.36 Å) at all temperatures examined (Figure 12) [13]. In particular, one of the Au-C distances (Au2B-C18), which is greater than d vdW (Au-C) above 280 K, rapidly decreases to be less than d vdW (Au-C) below ~280 K. ...

Context 6

... distances from Au2B to the nearest C atoms on BPY (3.037-3.46 Å; Au2B-C17 and Au2B-C18; for atomic numbering scheme, see Figure 12) are comparable or shorter than the sum of the corresponding van der Waals radii (Au-C; d vdW (Au-C) = 3.36 Å) at all temperatures examined (Figure 12) [13]. In particular, one of the Au-C distances (Au2B-C18), which is greater than d vdW (Au-C) above 280 K, rapidly decreases to be less than d vdW (Au-C) below ~280 K. ...

Context 7

... order to clarify the differences in electronic states between P and NP geometries, we carried out a quantum chemistry calculation based on the molecular structure observed at 100 K ( Figure 13) [13]. The results show that there are not so many differences between the two geometries in the spin distribution and molecular orbitals. ...

Context 8

... Au2A and Au2B refer to the P and NP Au atoms in the complex anions that face the BPY cations in the crystal, respectively. Figure 13a-g red and green lobes designate the opposite atomic charge distributions to each other (Mulliken charge; red = −0.753, green = +0.753), ...

Context 9

... yellow and grey (parts of) spheres are (parts of) gold/sulphur and carbon atoms without significant electron densities. Similarly in Figure 13d,h, the blue and green lobes designate the opposite electron densities to each other (blue = positive, green = negative). In Figure 13, note that the HOMO is based on the monoanion's electron configuration, and thus corresponds to the LUMO of the neutral complex molecule. ...

Context 10

... in Figure 13d,h, the blue and green lobes designate the opposite electron densities to each other (blue = positive, green = negative). In Figure 13, note that the HOMO is based on the monoanion's electron configuration, and thus corresponds to the LUMO of the neutral complex molecule. Since HOMO and LUMO are named/defined on the basis of the neutral molecules in the field of molecular conductors and magnets, care must be taken when readers compare the figure above with those in other references. ...

Context 11

... a thermodynamic transition at a similar temperature to that observed in the dark occurs under continuous UV irradiation. However, the phase behavior is totally vice versa compared with that under the dark condition ( Figure 14) [13]. The UV irradiation promotes the system to a highly excited state that is thermally inaccessible. ...

Context 12

... is also a surprising aspect of the phase transition. Figure 14. Temperature-dependence of the Au2B site occupancy (Occ (%)) under dark (black circles) and UV-irradiated (red triangles) conditions. ...

Context 13

... general substances, the structural transitions are of the first order, where the volumes suddenly change at the transition temperatures in a discontinuous manner. However, 1 does not exhibit any discontinuous change in the unit cell volume V at ~280 K whether under the dark or UV-irradiation, as V is independent of Occ ( Figure 15). Thirdly, the phase transition is also unique from the viewpoint of Ehrenfest classification. ...

Context 14

... general substances, the structural transitions are of the first order, where the volumes suddenly change at the transition temperatures in a discontinuous manner. However, 1 does not exhibit any discontinuous change in the unit cell volume V at ~280 K whether under the dark or UV-irradiation, as V is independent of Occ ( Figure 15). Such structural uniqueness is closely associated with charge and spin degrees of freedom. ...

Context 15

... structural uniqueness is closely associated with charge and spin degrees of freedom. Based on the magnetic susceptibility and crystal structure data, the phase transition is characterized by a change in the amount of CT (q) between [Au(dmit)2] (1−q)− and BPY 2(1−q)+ ; q ~ 0 (HT) ⟷ q = 0.01 (LT) and diamagnetic (HT) ⟷ paramagnetic (LT) (Figure 16) [13]. In such an electronic phase transition (between closed-shell and open-shell molecular species) the charge and spin usually lose their degrees of freedom in the LT phase, which is the opposite behavior to that observed in the present case. ...

Context 16

... data, the phase transition is characterized mit)2] (1−q)− and BPY 2(1−q)+ ; q ~ 0 (HT) ⟷ q = 0.01 (LT) re 16) [13]. In such an electronic phase transition species) the charge and spin usually lose their pposite behavior to that observed in the present reedom, the electronic system in 1 behaves like a paramagnetic (LT) (Figure 16) [13]. In such an electronic phase transition (between closed-shell and open-shell molecular species) the charge and spin usually lose their degrees of freedom in the LT phase, which is the opposite behavior to that observed in the present case. ...

Context 17

... UV-irradiation for 30 min at 298 K, the UV lamp was turned off. Then, the ESR spectra under the dark condition were repeatedly measured for the same single crystal at 298 K. Along with the ESR spectra under UV irradiation around T C (Figure 17a,b) [13], the result is shown in Figure 17c as the ESR intensity vs. elapsed time (h) [13]. From the curvature of the decay curve, it is apparent that there should be at least two different relaxation times, τ i (i = 1, 2). Figure 17. ...

Context 18

... UV-irradiation for 30 min at 298 K, the UV lamp was turned off. Then, the ESR spectra under the dark condition were repeatedly measured for the same single crystal at 298 K. Along with the ESR spectra under UV irradiation around T C (Figure 17a,b) [13], the result is shown in Figure 17c as the ESR intensity vs. elapsed time (h) [13]. From the curvature of the decay curve, it is apparent that there should be at least two different relaxation times, τ i (i = 1, 2). Figure 17. ...

Context 19

... the curvature of the decay curve, it is apparent that there should be at least two different relaxation times, τ i (i = 1, 2). Figure 17. The X-band (9.3 GHz) electron spin resonance (ESR) measured using the single crystal of 1. ...

Context 20

... at 298 K, it took several days (~140 h) to recover the original intensity before irradiation, during which time the energy received from the photons was stored in 1 and gradually released as heat, suggesting a complicated mechanism involving structural changes. Meanwhile, the molecular and crystal structures remain unchanged under UV-radiation, with the exception of an obvious change in Occ upon UV-irradiation, from ~8% (dark) to ~14% (UV) at 300 K, for example (Figure 14) [13]. Considering the general scheme of photoluminescence (Figure 1) [4] as well as the results of the X-ray structural analyses (Figures 12 and 14) [13], the magnetic susceptibility ( Figure 16) [13], and the ESR (Figure 17) [13], an excitation from S 0 to S 1 and successive ISC from S 1 to T 1 are considered to occur in 1 under UV irradiation. ...

Context 21

... the molecular and crystal structures remain unchanged under UV-radiation, with the exception of an obvious change in Occ upon UV-irradiation, from ~8% (dark) to ~14% (UV) at 300 K, for example (Figure 14) [13]. Considering the general scheme of photoluminescence (Figure 1) [4] as well as the results of the X-ray structural analyses (Figures 12 and 14) [13], the magnetic susceptibility ( Figure 16) [13], and the ESR (Figure 17) [13], an excitation from S 0 to S 1 and successive ISC from S 1 to T 1 are considered to occur in 1 under UV irradiation. However, 1 does not emit light after UV irradiation. ...

Context 22

... the molecular and crystal structures remain unchanged under UV-radiation, with the exception of an obvious change in Occ upon UV-irradiation, from ~8% (dark) to ~14% (UV) at 300 K, for example (Figure 14) [13]. Considering the general scheme of photoluminescence (Figure 1) [4] as well as the results of the X-ray structural analyses (Figures 12 and 14) [13], the magnetic susceptibility ( Figure 16) [13], and the ESR (Figure 17) [13], an excitation from S 0 to S 1 and successive ISC from S 1 to T 1 are considered to occur in 1 under UV irradiation. However, 1 does not emit light after UV irradiation. ...

Context 23

... the molecular and crystal structures remain unchanged under UV-radiation, with the exception of an obvious change in Occ upon UV-irradiation, from ~8% (dark) to ~14% (UV) at 300 K, for example (Figure 14) [13]. Considering the general scheme of photoluminescence (Figure 1) [4] as well as the results of the X-ray structural analyses (Figures 12 and 14) [13], the magnetic susceptibility ( Figure 16) [13], and the ESR (Figure 17) [13], an excitation from S 0 to S 1 and successive ISC from S 1 to T 1 are considered to occur in 1 under UV irradiation. However, 1 does not emit light after UV irradiation. ...

Context 24

... the molecular and crystal structures remain unchanged under UV-radiation, with the exception of an obvious change in Occ upon UV-irradiation, from ~8% (dark) to ~14% (UV) at 300 K, for example (Figure 14) [13]. Considering the general scheme of photoluminescence (Figure 1) [4] as well as the results of the X-ray structural analyses (Figures 12 and 14) [13], the magnetic susceptibility ( Figure 16) [13], and the ESR (Figure 17) [13], an excitation from S 0 to S 1 and successive ISC from S 1 to T 1 are considered to occur in 1 under UV irradiation. However, 1 does not emit light after UV irradiation. ...

Context 25

... the band gap (~1.2 eV) between the valence (HOMO) and conduction (LUMO) bands in 1 (under the dark condition) is clearly less than the UV-vis photon energies (~1.6-5 eV), and the band widths are almost negligible (~0.01 eV). These features make it difficult to apply the PersL mechanism (Figure 2) directly to 1. On the other hand, if we should interpret the observed relaxation in 1 (Figure 17c) [13] on the basis of the LPL scheme ( Figure 3) [11], the extremely slow decay process should contain repetitive exchange between excited states and intermediate states. In fact, the intermediate states are considered to include a partially charge separated state formed by a pair of radical cation and anion dissociated from photo-generated exciplex [11]. ...

Context 26

... the LPL scheme requires a power-law decay, while a standard phosphorescence exhibits an exponential decay. The observed decay (Figure 17c), though it does not involve an emission process, is reproduced by an exponential function [13]. Thus, the relaxation mechanism of 1 could be different from any of known or proposed mechanisms. ...

Context 27

... discussed thus far, the CT interaction between the cations and [Au(dmit) 2 ] − anions is indispensable for producing NP coordinate Au atoms in the Au(dmit) 2 anion salts. The selected cations (Figure 18) have closely related molecular structures to that of BPY 2+ , still systematically differ from each other in such aspects as electric charge, symmetry, structural flexibility, and redox reactivity under the dark and/or under UV irradiation. Some of them have also similar molecular structures to biphenyl, which is well known as a photoluminescent compound under UV radiation [96][97][98]. ...

Context 28

... we carried out single crystal X-ray structural analysis of all the obtained salts both under the dark and under the UV-irradiated conditions. Selected structures are shown in Figures 19 and 20. Then we examined the intermolecular interactions between the cations and anions by calculation [95]. ...

Context 29

... we will discuss possible key factors for the photoresponsive NP-Au(dmit) 2 anions in the crystalline CT salts. The difference in molecular geometries (isomers) and the flexibility of the molecular structures (co-planarity) did not result in systematic variation in the cation-anion CT interactions (Figures 19 and 20). Since the energy scale of the transfer integrals in this kind of molecular crystal (~0.1 eV) is clearly different from that of UV photons (~3-5 eV), the interactions between the frontier and higher energy orbitals could be important under UV excitation. ...

Context 30

... T 1 and T 2 are the initial and final temperatures of the transition, and T C ~250 K is the mid-point estimated based on the occupancy (Occ (%)) in Figure 14 [13]. On the other hand, ∆S can be also calculated as the statistical entropy as follows: ...

Context 31

... agreement between Equations (2) and (5) suggests that the calculated energy levels should be consistent with each other. The same calculation semi-quantitatively accounts for the slight lowering of the observed T C under UV irradiation (Figure 14) [13], where the Occ changes between ~17% and 8%. ...