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Characterization of anti-Kasha mechanism in open-form versus Kasha mechanism in closed-form a Chemical structure of DCM-IFC in its open form. b Excitation spectra (dotted line, monitored at λem = 520 and 710 nm) and emission spectra (solid line, excited at λex = 480 and 560 nm) of open-form DCM-IFC. c Electron–hole analysis involved during the photoexcitation of the open form, based on the molecular structure at the Franck–Condon state. Femtosecond time-resolved transient absorption spectra (d) and kinetics (e) of open-form DCM-IFC (excited at λex = 480 nm). f Jablonski diagram illustrating the anti-Kasha mechanism. g Chemical structure of DCM-IFC in its closed form. h Excitation spectra (dotted line, monitored at λem = 710 nm) and emission spectra (solid line, excited at λex = 480 and 560 nm) of closed-form DCM-IFC. i Electron–hole analysis involved during the photoexcitation of the closed form, based on the molecular structure at the Franck–Condon state. Femtosecond time-resolved transient absorption spectra (j) and kinetics (k) of closed-form DCM-IFC (excited at λex = 480 nm). Note: open-form DCM-IFC was investigated at pH 11.3 and closed-form DCM-IFC was investigated at pH 2.0. l Two structurally related reference compounds that are locked into open form or closed form. Note: all fluorescence spectra were measured in a mixture solution of acetonitrile (MeCN)/Britton–Robinson buffer.
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Fluorescence-based technologies have revolutionized in vivo monitoring of biomolecules. However, significant technical hurdles in both probe chemistry and complex cellular environments have limited the accuracy of quantifying these biomolecules. Herein, we report a generalizable engineering strategy for dual-emission anti-Kasha-active fluorophores,...
Citations
... Different pathways for radiative deactivation in organic luminogens can be envisioned to attain multi-color emission. One example involves deviating from Kasha's rules [21], in which a two-color emission can be observed from the first and second excited states [22][23][24][25]. Multiple fluorescence emissions also can be observed when a fluorophore exists in the ground or excited state in more than one form (e.g. ...
Lophine-benzothiazole dyads were synthesized via the amine-isocyanate reaction involving N-alkylamino lophine and 2-(5′-isothiocyanate-2-hydroxyphenyl)benzothiazole. The resultant coupling generated a thiourea linkage, connecting the two primary fluorophores with varying chain lengths (2 and 8 methylene groups). These newly formed dyads exhibit distinct combinations of fluorescence processes, modulated by both the excitation wavelength and the solvent. Varying the excitation wavelength allowed for the observation of emissions from both lophine and benzothiazole (enol species) fluorophores. Notably, a tautomeric equilibrium in the excited state was also identified, resulting in emissions from both enol and keto forms, transitioning to the anionic species with ICT emission. Finally, multiple color emissions from orange to blue can be obtained using a binary DCM:DMSO mixture via simultaneous solvatochromic effect and ratiometric emission from excited-state intra-molecular proton transfer (ESIPT) lophine-benzothiazole dyads. Theoretical calculations corroborate the proposed emission mechanism. The results indicate that the ESIPT dyads are promising for developing sensitive fluorescent probes, allowing their fluorescence modulation in solution.
... Generally, compared to the appearance or decrease of a single emission signal, ET based probes typically have two emission peaks, which make internal referencing of the signal possible. 38 And as one of the most typical ET process, FRET usually requires the following four conditions to achieve a satisfying signal: (i) a donor with a high quantum yield; (ii) a spectral overlap of the donor emission with the acceptor absorption; (iii) an appropriate relative orientation of the transition dipoles and (iv) a close enough donor−acceptor distance (typically less than 10 nm). 27 These critical requirements severely hindered the exploration of new functionalized QDs based on the energy transfer mechanism and thus are not favorable for the development of highly efficient sensing methodology. ...
The exploration of emerging functionalized quantum dots (QDs) through modulating the effective interaction between the sensing element and target analyte is of great significance for high-performance trace sensing. Here, the chromone-based ligand grafted QDs (QDs-Chromone) were initiated to realize the electronic energy transfer (EET) driven specifically by ethylenediamine (EDA) in the absence of spectral overlap. The fluorescent and colorimetric dual-mode responses (from red to blue and from colorless to yellow, respectively) resulting from the expanded conjugated ligands reinforced the analytical selectivity, endowing an ultrasensitive and specific response to submicromolar-liquid of EDA. In addition, a QDs-Chromone-based sensing chip was constructed to achieve the ultrasensitive recognition of EDA vapor with a naked-eye observed response at a concentration as low as 10 ppm, as well as a robust anti-interfering ability in complicated scenarios monitoring. We expect the proposed EET strategy in shaping functionalized QDs for high-performance sensing will shine light on both rational probe design methodology and deep sensing mechanism exploration.
... The ratiometric methods have some advantages in comparison to a single emission wavelength such as eliminating any interference from the environment or instrument, the fluctuation of the excitation source, and the concentration of probe [61][62][63]. Our as-synthesized PIM-1/PFTBT Pdots have dual emission bands in the green and red regions. ...
... One effective strategy to address the challenge is to regulate the luminescence switch from Kasha's rule to anti-Kasha's rule and realize dual-emission from the lowest energy-excited state (S1) to ground state (S0) and higher excited state (Sn, n>1) to S0 at the same time. 43 As a classical photophysical principle, Kasha's rule indicates that luminophores generally give rise to one emission profile from S1 to S0 without excitation energydependence. 44 While, anti-Kasha emission can originate from the upper excited state by making full use of excess electronic energy. ...
Despite the rapid development of probes for targeting single organelle, construction of robust dual-organelle targeting probes with multicolor emission was rarely reported. Towards this end and in consideration of the unique advantages of aggregation-induced emission luminogens (AIEgens), two dual emissive AIEgens with donor-π-acceptor structures were designed and syn-thesized, namely QT-1 and QF-2. Both two AIEgens exhibited excitation wavelength-dependence defying the Kasha’s rule. The invariant near-infrared emission was originated from S1 of AIEgens to S0. However, its anti-Kasha luminescence at a short wavelength from the S2 state can be tuned by changing the excitation wavelength. Given the two-color emission of AIEgens, QT-1 and QF-2 could stain lipid droplets (LDs) and mitochondria in a blue and red fluorescence, respectively. Moreover, thanks to the near-infrared emission and abundant ROS generation efficiency of QT-1, it was chosen as a photodynamic thera-py agent to selectively kill cancer cells from normal cells. Upon light irradiation, obvious decrease of mitochondrial membrane potential (MMP) and serious change of mitochondrial shape in cells were observed, which corresponded to the efficient inhibi-tion of tumor growth in vivo. This work afforded a promising strategy for the construction of multicolor emission by tuning anti-Kasha behaviors and to expand their application in dual-organelle targeting-based phototheranostics.
... The decreased energy level of (π,π*) state also resulted in the redshift of the enhanced emission. Interestingly, according to previous work, this should be an anti-Kasha's emission from the higher excited state due to the thermal equilibrium between two coupled states and a much larger f value (>0.21) of (π,π*) state [67][68][69] . However, for SQ, the energy level of (π,π*) state was lower than (n,π*) state, which changed from a closely coupled feature to the (π,π*) dominant lowest state with the increased solvent polarity. ...
Proximity effect, which refers to the low-lying (n,π*) and (π,π*) states with close energy levels, usually plays a negative role in the luminescent behaviors of heterocyclic luminogens. However, no systematic study attempts to reveal and manipulate proximity effect on luminescent properties. Here, we report a series of methylquinoxaline derivatives with different electron-donating groups, which show different photophysical properties and aggregation-induced emission behaviors. Experimental results and theoretical calculation reveal the gradually changed energy levels and different coupling effects of the closely related (n,π*) and (π,π*) states, which intrinsically regulate proximity effect and aggregation-induced emission behaviors of these luminogens. With the intrinsic nature of heterocycle-containing compounds, they are utilized for sensors and information encryption with dynamic responses to acid/base stimuli. This work reveals both positive and negative impacts of proximity effect in heterocyclic aggregation-induced emission systems and provides a perspective to develop functional and responsive luminogens with aggregation-induced emission properties.
... 31,[45][46][47][48] Furthermore, several D-A systems could exhibit higher excitation states depending on the surrounding media, leading to exciting features like anti-Kasha emission. [49][50][51][52] Introducing heterocyclic scaffolds into the molecular backbone of D-p-A-p-D compounds has rendered compounds with strong ICT character. [53][54][55][56] Ibrahim et al. designed a new and intriguing variety of D-A molecules based on benzofuran-cyanovinyl-pentafluorophenyl derivatives. ...
... Surprisingly, the oscillator strength (f) values for S 1 , S 2 , and S 3 states for both the emitters are found to be zero (f = 0) ( Figure 3A(ii),D(ii)), indicating that the S 1 state is nonemissive, which is in contrast to the conventional Kasha's formulation. 54 As mentioned earlier, few other functionals such as BPV86, PBE0, and M062X were also attempted. However, f value was still found to be zero for S 1 , including the large difference in energies between S 1 and >S 1 states, indicating that S 1 is indeed a dark state. ...
... It has also been argued that IC from the S 2 state in Fl derivatives may be relatively slow because of the large S 1 −S 2 gap (∼0.7 eV) and the large differences in the localization of the electron density in both excited states. 26,42 Therefore, given the observation of non-Kasha's rule electron loss, it seems likely that tunnelling through RCB 2 occurs on a similar time scale to IC from S 2 to S 1 and that both processes are slower than tunnelling via RCB 1 (>1 ps). ...
... Finally, it should be noted that anti-Kasha's fluorescence in the condensed phase has been observed previously. 42,43 The overall picture of a single electronic RCB in a polyanion is clearly invalid, and the energetic landscape is not only spatially anisotropic, but it is also highly complex in energy with an RCB for each (ro)vibrational level of each electronic state. Additionally, not every electronic state in the dianion will correlate to the same electronic state in the final anion following removal of an electron, although a Koopmans' picture seems to hold for electron tunnelling. ...
The long-range electronic structure of polyanions is defined by the repulsive Coulomb barrier (RCB). Excited states can decay by resonant electron tunnelling through RCBs, but such decay has not been observed for electronically excited states other than the first excited state, suggesting a Kasha-type rule for resonant electron tunnelling. Using action spectroscopy, photoelectron imaging, and computational chemistry, we show that the fluorescein dianion, Fl2-, partially decays through electron tunnelling from the S2 excited state, thus demonstrating anti-Kasha behavior, and that resonant electron tunnelling adheres to Koopmans' correlations, thus disentangling different channels.
... In recent years, anti-Kasha emissions that violate this rule have attracted attention from both fundamental and applied viewpoints because they highlight the possibility of utilizing the otherwise dissipated excess energy for other functions. [2][3][4][5][6] Anti-Kasha emissions are known to occur in the gas phase, 7 where a molecule is isolated in vacuum, and the energy relaxation rates become much longer than those in solid or solution systems. Thus, an excited molecule in the gas phase does not have sufficient time to relax to its vibrational ground state before making a radiative transition, which gives rise to anti-Kasha emissions from higher-lying levels. ...
Kasha's rule generally holds true for solid-state molecular systems, where the rates of internal conversion and vibrational relaxation are sufficiently higher than the luminescence rate. In contrast, in systems where plasmons and matter interact strongly, the luminescence rate is significantly enhanced, leading to the emergence of luminescence that does not obey Kasha's rule. In this work, we investigate the anti-Kasha emissions of single molecules, free-base and magnesium naphthalocyanine (H 2 Nc and MgNc), in a plasmonic nanocavity formed between the tip of a scanning tunneling microscope (STM) and metal substrate. A narrow-line tunable laser was employed to precisely reveal the excited-state levels of a single molecule located under the tip and to selectively excite it into a specific excited state, followed by obtaining STM-photoluminescence (STM-PL) spectrum to reveal energy relaxation from the state. The excitation to higher-lying states of H 2 Nc caused various changes in the emission spectrum, such as broadening and the appearance of new peaks, implying the breakdown of Kasha's rule. These observations indicate emissions from the vibrationally excited states in the first singlet excited state ( S 1 ) and second singlet excited state ( S 2 ), as well as internal conversion from S 2 to S 1 . Moreover, we obtained direct evidence of electronic and vibronic transitions from the vibrationally excited states, from the STM-PL measurements of MgNc. The results obtained herein shed light on the energy dynamics of molecular systems under a plasmonic field and highlight the possibility of obtaining various energy-converting functions using anti-Kasha processes.
... According to the Kasha's rule, this peak may not show as it is not the lowest excited level. However, the Kasha's rule may be overcome [42,43]. The last section ranges from 550 to 750 nm. ...
Circularly polarized (CP) light has shown great potential in quantum computing, optical communications, and three-dimensional displays. It is still a challenge to produce high-efficiency and high-purity CP light. Herein, we proposed a strategy to design chiral organic small molecules for CP light generation. These kinds of chiral molecules are formed by achiral light-emitting groups and achiral alkyl chains through conformational lock, which indicates that chirality can also be introduced into achiral light-emitting groups through rational molecular design. The chirality of these molecules can be further tuned by changing the length of the alkyl chains connecting the diketopyrrolopyrrole unit. The chiroptical properties of these molecules are confirmed by calculated electronic circular dichroism and chiral emission spectra, and further confirmed in experiments. The strategy developed in this work will greatly enlarge the candidate library of chiral luminescent materials.
