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The interfacial charge transfer still limits the photoactivity of artificial Z-scheme photocatalysts although they showed complementary light absorption and a strong photoredox ability. In this study, layered metallene is designed as an efficient electron mediator for constructing a C3N4/bismuthene/BiOCl 2D/2D/2D Z-scheme system. This bismuthene serves as a bridge processing superior charge conductibility, abundant metal-semiconductor contact sites, and the shortened charge diffusion distance, enhancing the photocatalytic CO2 reduction reaction activity and stability. Density functional theory calculations show that the bismuthene creates a built-in electric field and congregates interfacial electrons, which is confirmed by the stable and consistent emission of the ultrafast transient absorption spectra. This work gives new insight into the interface design of Z-scheme photocatalysts by selecting a novel metallene electron mediator.

The twisted intramolecular charge transfer (TICT) state plays an important role in determining the performance of optoelectronic devices. However, for some nonfluorescent TICT molecules, the "invisible" TICT state could only be visualized by modifying the molecular structure. Here, we introduce a new facile pressure-induced approach to light up the TICT state through the use of a pressure-related liquid-solid phase transition of the surrounding solvent. Combining ultrafast spectroscopy and quantum chemical calculations, it reveals that the "invisible" TICT state can emit fluorescence when the rotation of a donor group is restricted by the frozen acetonitrile solution. Furthermore, the TICT process can even be effectively regulated by the external pressure. Our study offers a unique strategy to achieve dual fluorescence behavior in charge transfer molecules and is of significance for optoelectronic and biomedical applications.

Density functional theory and time-dependent density functional theory methods were carried out to investigate the excited-state intramolecular charge transfer process (ICT) of coumarin 340 (C340) in tetrahydrofuran (THF) solvent for the first time. The plotted electrostatic potential and optimized geometric structures vividly indicated that the intermolecular hydrogen bond can be formed between the N–H group of C340 monomer and the oxygen atom of THF solvent. The analysis of frontier molecular orbitals and the differences in electronegativity confirmed the occurrence of ICT process in hydrogen-bonded complex C340-THF upon photoexcitation. The results of calculated bond lengths and bond energies revealed that the intermolecular hydrogen bond in C340-THF is significantly strengthened in the first excited state. Furthermore, the calculated electronic spectra, non-covalent interactions and IR spectra provided strong evidence for the hydrogen bond reinforce. Based on the theoretical calculations, we demonstrated that the ICT process in C340-THF can be facilitated effectively by the excited-state intermolecular hydrogen bond strengthening.

The excited-state double proton transfer (ESDPT) process of dipyrido[2,3-a:3',2'-i]carbazole (DPC) in ethanol (EtOH) solvent is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The computational results provide convincing evidence that proton transfer did not occur spontaneously for the DPC monomer due to the lack of hydrogen bonds. Interestingly, after adding EtOH to DPC, two intermolecular hydrogen bonds were formed in the ground-state, and the intermolecular hydrogen bonds were strengthened in the excited-state, as confirmed by comparing the changes in the primary bond parameters. In addition, the charge transfer was observed in the DPC-EtOH complex compared with the DPC monomer. In particular, a reliable ESDPT process occurs within the system upon photoexcitation, which was monitored by the formation and disappearance of characteristic peaks in the IR spectrum. All results adequately proved that the participation of EtOH exerts a remarkable effect on the ESDPT process. Overall, our work not only comprehensively elaborated the simultaneous mechanism of ESDPT but can also pave the way towards the design and synthesis of novel molecules.

Harnessing ingenious modification of molecular structure to regulate excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) characteristics holds great promise in fluorescence sensing and imaging. Based on the 3-hydroxyflavone (3HF) molecule, 2-(2-benzo[b]furanyl)-3-hydroxychromone (3HB) and 2-(6-diethylamino-benzo[b]furan-2-yl)-3-hydroxychromone (3HBN) were designed by the extension of the furan heterocycle and the introduction of a diethylamino group. The analysis of important hydrogen bond length, frontier molecular orbitals, infrared spectra, and potential curves have cross-validated our results. The results indicate that proper site furan heterocycle extension and diethylamino donor group substitution not only shift the absorption and emission spectra to the red but also effectively modulate the excited-state dynamic behaviors. Strengthened ICT characteristics from 3HF to 3HB and to 3HBN make the occurrence of ESIPT increasingly difficult due to the higher energy barriers, which indicates that the ESIPT and ICT processes are competitive mechanisms. We envision that our work would open new windows for improving molecular properties and developing more fluorescent probes and organic radiation scintillators.

The spectroscopic properties of 3-Aminophthalimide (3AP) molecule were investigated [Chem. Phys. 2002, 283, 249, New J. Chem. 2018, 42, 1181]. The result was that the 3AP molecule was exhibiting excited-state intramolecular proton transfer (ESIPT). In the research, we revised previous result using time-dependent density functional theory (TDDFT) method. The fluorescence spectrum shows that the only fluorescence peak is from initial enol form, which is different from the traditional case of ESIPT. The red shift of characteristic peaks in infrared vibration spectra is not induced by ESIPT process. The change in the vibration mode of the amino group causes the red shift of characteristic peak in the infrared spectrum. Energy curves indicate that the barrier (19.71 kcal/mol) is anomalously high in the first excited state. In addition, there are not stable points to lead the ESIPT to form a keto isomer. Together, these results demonstrate that there is not an ESIPT process happening of 3AP molecule.

Excited-state deactivation dynamics of Thioflavin-T (ThT) in gradual viscosity solvents were investigated. Femtosecond transient absorption spectra and dynamic decay curves both present significant distinction of ThT in different volume ratios binary mixtures solvents. Dynamics fitting lifetime of twisted intramolecular charge transfer (TICT) state is strongly dependent on solvents viscosity. Compared to rotation corresponding time of ThT in low viscosity solvent (0.6 cp) experimentally coincident well with Stokes-Einstein-Debye (SED) equation, the relation between rotation corresponding time and relatively high viscosity (5.9 cp to 1091.2 cp) is more consistent with fractional SED equation. Combined with optimized geometric structures of ThT by density functional theory and time-dependent density functional theory, further understand TICT state lifetime increases with increasing solvents viscosity. Our work provides a comprehensive understanding of fluorescence molecular rotor (FMR) deactivation process in different viscosity solvents and is helpful to design new FMR.

Studies of excited state multiple proton transfer (ESMPT) are paramount importance for both biological systems and chemical reactions while its mechanism in a controllable manner has been long sought. For this purpose, we conducted a theoretical study on the ESMPT mechanisms of 1-H-pyrrolo[3,2-h]quinoline (PQ) connecting different number methanol (MeOH) molecules (PQ-(MeOH)n=1,2). The results show that the ESMPT process can occur for PQ-(MeOH)n=1,2 complexes and MeOH molecules can effectively regulate the proton transfer barriers. The non-fluorescence of PQ-MeOH was explained by the combination of rapid ESMPT and following intersystem crossing (ISC) processes. The observed fluorescence of PQ-(MeOH)2 was owing to the contribution of increased excited-state barriers to emission is greater than the consumption of ISC process. These exciting findings revealed the cooperation mechanism between ESMPT and ISC processes for PQ-(MeOH), whereas the competitive mechanism for PQ-(MeOH)2. Our work firstly implemented the bidirectional regulation of the ESMPT mechanism, which will open new avenues for improving the luminescence properties and characterizing the cluster structure.

In this work, density functional theory and time-dependent density functional theory were performed to investigate intermolecular hydrogen bond (H-bond) effect on Coumarin-1 (C1). The optimized geometric structures of C1 in acetonitrile and methanol (MeOH) revealed that C1 exhibits planar configuration in ground state and locally excited (LE) state. Due to intermolecular H-bond interaction between C1 and MeOH, twisted intramolecular charge transfer (TICT) state with twist configuration exists in excited state for C1–MeOH. The apparent variation of characteristic group frequencies and intensities for IR spectra is ascribed to dramatic charge density changes. Compared with molecular orbitals of LE state, the low charge coupling degree of C1–MeOH in TICT state is induced by twist configuration in TICT state. Indeed, such low charge coupling degree induced TICT state is nonluminous that responsible for low fluorescence quantum yield of C1–MeOH. We not only provide a comprehensive understanding of TICT state deactivation process but also benefits to design new environment-sensitive probe.

Excited-state intramolecular proton transfer (ESIPT) and twist intramolecular charge transfer (TICT) are the two most fundamental dynamic processes, ubiquitous in biological and chemical reactions. The excited-state properties of (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one (CDPA) in various solvents with different polarities were investigated by using steady-state and femtosecond transient absorption spectroscopy combined with DFT/TDDFT calculations. The results demonstrated that CDPA exhibited low fluorescence in polar acetonitrile (ACN) due to ESIPT but high fluorescence in nonpolar n-Hexane was attributed to intramolecular rotation blocking ESIPT. TDDFT calculations confirmed that the dramatic phenyl group torsional of CDPA in Hexane, whereas a near planar conformation in ACN solvent. The ESIPT barrier decreases regularly with the increase of solvent polarity from n-Hexane, tetrahydrofuran to ACN solvent. These results demonstrated that the ESIPT and TICT processes of CDPA are competitive mechanisms. Our work revealed the effect solvent polarity on the emission behavior and excited-state deactivation mechanism of CDPA, which could help to design and develop new polarity probe in the microenvironments.

Judiciously tuning the forward-backward excited state intramolecular proton transfer (ESIPT) barriers can effectively influence the generation of white light. Harnessing the 1-hydroxy-9H-fluoren-9-one (HHF) ESIPT characteristics as the prototype, two electron-withdrawing substituents (nitro group and formyl group) were introduced. The influence of electron-withdrawing group on the energy barrier of the forward-backward ESIPT processes has been studied by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The frontier molecular orbitals, infrared spectra, non-covalent interactions, electrostatic potential, electronic spectra, molecular structure have cross-validated our results. These results indicate that the electron-withdrawing groups dramatically promote the occurrence of the forward ESIPT process and hinder the backward ESIPT. The potential curves revealed that the forward ESIPT energy barrier was obviously reduced to 5.73 kcal/mol (HHF-CHO) and 4.55 kcal/mol (HHF-NO2) compared with HHF (7.64 kcal/mol). Meanwhile, the backward ESIPT energy barrier is availably improved to 3.57 kcal/mol (HHF-CHO) and 3.84 kcal/mol (HHF-NO2) compared with HHF (2.51 kcal/mol). Through the introduction of the electron-withdrawing groups, the energy barrier of the forward-backward ESIPT is efficaciously modulated. Furthermore, the more stable and better white fluorescence has been achieved. Our work will be conducive to improve molecular ESIPT properties and exploit white luminescent molecules.

The relationships between the excited state intramolecular proton transfer (ESIPT) behaviors and antioxidant activities of three hydroxyanthraquinone (HAQ) compounds containing 2,6-DHAQ, 1,5-DHAQ and 1,5-DAAQ were systematically explored via the experimental and theoretical methods. The measured femtosecond transient absorption indicates that 1,5-DAAQ exhibits the slower ESIPT reaction than 1,5-DHAQ due to its larger S1-state energy barrier in the scanned potential energy surfaces, which means that the transformation of 1,5-DAAQ from enol form to keto form is more difficult. Based on this, the antioxidant activities of the compounds in enol- and keto-form were studied. Concretely, the antioxidant activities of 1,5-DHAQ and 1,5-DAAQ in the keto form are stronger than that in the enol form, which is estimated from the calculated ionization potential (IP) values. In addition, compared with that at the S0 state, the involved three compounds in the enol and keto forms have the lower IPs at the S1 state, revealing that their antioxidant activities will be enhanced upon the photo-excitation. Derived from the difficulty of ESIPT reactions and antioxidant activities of the three compounds in the experiments and theory, it can be concluded that antioxidant activity of the compound can be regulated through the transformation of enol- and keto-form in the ESIPT reaction. This new discovery can help to design and synthesize more efficient ESIPT-based antioxidants in experiments.

Excited-state charge-carrier dynamics governs the performance of organometal trihalide perovskites (OTPs) and is strongly influenced by the crystal structure. Characterizing the excited-state charge-carrier dynamics in OTPs under high pressure is imperative for providing crucial insights into structure-property relations. Here, we conduct in situ high-pressure femtosecond transient absorption spectroscopy experiments to study the excited-state carrier dynamics of CH3NH3PbBr3 (MAPbBr3) under hydrostatic pressure. The results indicate that compression is an effective approach to modulate the carrier dynamics of MAPbBr3. Across each pressure-induced phase, carrier relaxation, phonon scattering, and Auger recombination present different pressure-dependent properties under compression. Responsiveness is attributed to the pressure-induced variation in the lattice structure, which also changes the electronic band structure. Specifically, simultaneous prolongation of carrier relaxation and Auger recombination is achieved in the ambient phase, which is very valuable for excess energy harvesting. Our discussion provides clues for optimizing the photovoltaic performance of OTPs.