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Three cyanostilbene isomers DCSB-In , DCSB-IO and DCSB-Ou with different cyano positions have been designed and synthesized. The –CN regio-isomerization in this system results in distinct aggregation-induced optical properties, mechano- and piezochromic luminescent properties. By shifting –CN unit from β to α position, molecules convert from a typical ACQphore (DCSB-Ou) to AIEEgen (DCSB-In). It is noteworthy that DCSB-Ou with β-CN groups exhibits an obvious mechanochromic (MC) property, whereas no photoluminescent (PL) shift is observed from DCSB-In upon grinding though accompanied with the crystalline-amorphous phase transition like most reported MC materials. Moreover, all isomers show piezochromic (PC) properties, and the PL of both DCSB-In and DCSB-IO red-shifts consistently and almost linearly dependent on the increased hydrostatic pressures, whereas DCSB-Ou experiences two stages of fluorescent change with more than 200 nm of wavelength shift. With only regio-isomerization but varied MC and PC performances, these cyanostilbene analogues as the model molecules provide a platform to explore the distinct MC and PC mechanisms. Photophysical properties, structural analysis, theoretical calculations and in-situ tests were comparatively studies to establish the origin of MC and PC phenomenon. The subtle modulation of –CN position could significantly influence the intermolecular interaction, thus control the π-stacking mode in solid-state, and endow molecules with different stimuli dependent fluorescent properties. • Three isomers DCSB-In , DCSB-IO , and DCSB-Ou were designed and synthesized. • By shifting –CN unit position, the molecule converts from an ACQphore (DCSB-Ou) to an AIEgen (DCSB-In). • DCSB-In and DCSB-IO show almost linearly piezochromic (PC) property under increased hydrostatic pressures. • DCSB-Ou exhibits both obvious mechanochromism (MC) upon grinding and two stages of PC capacity. [ABSTRACT FROM AUTHOR]

The absorption and fluorescence spectra of some biologically active thymol based Schiff bases have been studied at room temperature in a series of solvents of varying polarity. The solvent effect on the spectral properties of Schiff bases has been analyzed using Reichardt and Kawski equations and Kamlet-Taft and Catálan multi-parametric solvent polarity scales. The substituents and the solvents strongly influence the excited state behavior of these molecules owing to different electronic arrangements. The excited state dipole moments ( μ e /μ g and Δμ) of these molecules have been calculated using solvatochromic shift methods based on bulk solvent polarity functions f (ε,n) and φ (ε,n) and microscopic solvent polarity parameter E T N , respectively. The increase in excited state dipole moments (μ e ) indicates a more polar excited state and intramolecular charge transfer (ICT) characteristics of these molecules. The molecular orbitals (HOMO-1, HOMO, LUMO and LUMO+ 1) and molecular electrostatic potential (MEP) surfaces have been generated from their optimized geometries using semi-empirical (PM3) calculations to account for electronic excitations and changes in the overall charge distribution of Schiff bases. The Taft and Catalán multi-parametric equations used in the interpretation of the specific solute-solvent interactions reveal that the most important contribution to the solute-solvent interaction in the excited state comes from the hydrogen-bond acceptor capacity of the solvent. [ABSTRACT FROM AUTHOR]

In this work, the time-dependent density functional theory (TDDFT) method was performed to investigate the electronic spectra and charge transfer in excited states of firefly luciferin anion (NRO - ) in aqueous solution. A novel electronic spectral blue-shift (usually red-shift) phenomenon of NRO - was observed due to the significant influence of intermolecular hydrogen bonds between firefly luciferin anion and water molecules. We theoretically demonstrated that the excited-state intermolecular hydrogen bond weakening is confirmed by the increase of hydrogen bond length and decrease of hydrogen bond binding energy from our optimized ground-state and excited-state geometric conformations of the hydrogen-bonded NRO - –H 2 O complex. The hydrogen bond weakening behavior could also be indicated by photoinduced charge transfer from the carboxyl group to the planar part of NRO - in the excited state. Furthermore, we pointed out that the electronic spectral blue-shift should be ascribed to the intermolecular hydrogen bond weakening in this electronic excited state. The weakened hydrogen bonding interaction in the excited state would induce smaller downshift for the excited-state energy level than that for ground-state energy level. As a result, the energy gap between the excited state and ground state becomes larger, which means that the spectral peak corresponding to the electronic excited state will shift to the blue induced by the excited-state hydrogen bond weakening dynamics. [ABSTRACT FROM AUTHOR]

The intrinsic photophysical properties of the resorcylic acid moiety of zearalenone offer a convenient label free method to determine zearalenone levels in contaminated agricultural products. Steady-state fluorescence and density functional methods were applied to investigate the role of structural chemistry on zearalenone detection. Geometry optimization calculations using the B3LYP density functional identified a tautomeric form of zearalenone. Excited state geometries for zearalenone and a tautomeric form were obtained by MNDO semi-empirical optimizations. Steady-state fluorescence studies suggest that fluorescence quenching at neutral pH is associated with water interactions. Time-dependent density functional and ground state calculations indicate that the anionic and dianionic forms of zearalenone possess lower band gaps, excitation energies, and the lowest unoccupied molecular orbitals are positioned over the non-fluorophoric portion of zearalenone. These results suggest that deprotonation of one or more of the phenolic hydroxyls diminishes the intensity of the fluorescence emission of zearalenone. [ABSTRACT FROM AUTHOR]

At present, the safety and environmental compatibility of commercial sunscreens are becoming unprotected. Most of the commercial sunscreens lack absorption and protection in high-energy UVA area. In this work, it has been proved that biflavonoids have sunscreen properties for the first time, especially, Amentoflavone (AME) covers almost the entire UV region and has high absorption. Different substituents have a great influence on the UV absorption of bisflavonoids. Therefore, we can adjust the substituents to obtain a broad protection region for sunscreens. Depending on the two-proton transfer donor and acceptor sites, it is theoretically proved that there is a reasonable competitive relationship between the two structures, and only one part of the proton transfer can be carried out in the molecule, which proved to be the proton donation of H 1 –O 1 –H 2. When UV excited, biflavonoids will form intramolecular hydrogen bonds through proton transfer and emit energy in an ultra-fast pathway. Biflavonoid precursors could be used in sunscreen field and then the mechanism was proved, and it is a breakthrough in the research of expanding the field of plant filter. AME performs better than the current mature commercial sunscreens, and this work provided theoretical support to design a new biflavonoids for UV protection in future. [Display omitted] • Plant-derived flavonoids Amentoflavone and Isoginkgentin have potential UVA and UVB sun protection properties. • Intramolecular proton transfer (ESIPT) takes place in the excited state for both compounds. • The broad-spectrum sunscreens could be realized by adjusting the position and number of substituents. [ABSTRACT FROM AUTHOR]

An explicit method for calculating the spatial distribution of excited species in the reaction cell is presented. This method is used to analyze the relation between experimental conditions (excitation wavelength, ground state concentration, and laser intensity) and the observed luminescence decays (and the corresponding reaction kinetics) in both transient emission and transient absorption pulse laser photolysis experiments. Subsequently, the analysis was applied to the experimental conditions of previously extensively studied pyrene excimer formation in cyclohexane (J.B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, London, 1970) and significantly different data were obtained in the redesigned experiment; the excimer formation and decay rate constants at 23.5 °C were found to be (4.77±0.04)×10 9 M −1 s −1 and (2.07±0.01)×10 7 s −1 , respectively. Most importantly, the excimer dissociation into a singlet excited pyrene and ground state pyrene was found to be negligible, which is an important result that crucially affects the data processing in many previous and current applications of excimer formation. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we report the spectral signatures of photoinduced tautomerism of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) in DMF–water mixtures with varying compositions. Excited state intramolecular proton transfer (ESIPT) reaction of CMOH has been observed in bulk DMF, indicated by dual fluorescence from its normal and tautomeric forms while only a single emission peak is observed in water from its anionic species. Binary mixture of a polar aprotic (DMF) and a polar protic (water) solvent gives rise to a competition between intramolecular and intermolecular hydrogen bonding (with media) processes of the probe. This competition is found to be largely dependent on the proton affinity of the media and also on the excitation energy. Solvent separated ion pair and intermolecularly H-bonded CMOH–Solvent complex have been detected in the excited state at specific solvent compositions that are converted to the anionic form due to the change in excitation wavelengths. The formation of hydrogen bonded 1:1 molecular clusters of different rotamers of CMOH with DMF and water in the ground state has been investigated using quantum chemical calculations. A combined experimental and theoretical analysis indicates that the HOMO to LUMO transitions dictate the electronic absorption profiles of the CMOH–DMF and CMOH–water clusters. These findings are expected to shed light on the mechanism of acid–base reactions of several hydrogen bonded systems that are part of many biologically relevant processes. [Copyright &y& Elsevier]

Abstract: The solute–solvent interactions of hydrogen-bonded phenol–(H2O) n (n=3–5) clusters in electronic excited states were investigated by means of the time-dependent density functional theory (TDDFT) method. The geometric structures and IR spectra in ground state, S1 state, and T1 state of the clusters, were calculated using the density functional theory (DFT) and TDDFT methods. Only the ring form isomer, the most stable one of the cluster, was considered in this study. Four, five and six intermolecular hydrogen bonds were formed in phenol–(H2O)3, phenol–(H2O)4, and phenol–(H2O)5 clusters, respectively. Based on the analysis of IR spectra, it is revealed that the “window region” between unshifted and shifted absorption bands in both S1 and T1 state becomes broader compared with that in ground state for the corresponding clusters. Furthermore, two interesting phenomenon were observed: (1) with the anticlockwise order of the ring formed by the intermolecular hydrogen bonds in the H-bonded phenol–(H2O) n (n=3–5) clusters, the strengths of the intermolecular hydrogen bonds decrease in all the S0, S1 and T1 states; (2) upon electronic excitation, the smaller the distance between phenol and water is, the larger the change of intermolecular hydrogen bonds strength is. Moreover, the intermolecular hydrogen bond (phenolic OH is the H donor) is strengthened in excited state compared with that in ground state. But the intermolecular hydrogen bond (phenolic OH is the H acceptor) is weakened in excited state. [Copyright &y& Elsevier]

Abstract: The steady-state absorption and emission as well as the time-resolved fluorescence spectra of coumarin 102 (C102) in both aprotic and alcoholic solvents have been used to study the effect of excited-state hydrogen bond on the dynamic fluorescence of C102 chromophore in various solutions. The dual fluorescence of C102 in alcohols, which is dependent on the hydrogen-bonded donation ability of the solvent, has been assigned to the distribution of free C102 and a hydrogen-bonded complex. Furthermore, a shift of the fluorescence spectra induced by excited-state hydrogen bond has been demonstrated to take place within hundreds of picoseconds by the performance of the time-resolved fluorescence spectra with the time-correlated single-photon-counting (TCSPC) technique. Moreover, the time-dependent density functional theory (TDDFT) has been used to calculate the hydrogen-bonded equilibrium constant pK HB in different electronic states. It has been demonstrated for the first time that the hydrogen bond strengthening in electronic excited states could decrease the free energy of the hydrogen-bonded complex due to its stronger binding energy. Therefore, the hydrogen-bonded equilibrium will become markedly in favor of the hydrogen-bonded forms in electronic excited states by comparison with the case in the ground state. [Copyright &y& Elsevier]

Abstract: Er3+-doped KCaY(VO4)2 microcrystalline samples were synthesized using a high temperature solid-state reaction technique. Spectroscopic properties of Er3+: KCaY(VO4)2 are studied and the nature of emissions is discussed. A strong green and infrared luminescence were observed under excitation at 314nm in the O2−→V5+ charge-transfer transitions and direct excitation of Er3+ ions at 435nm. A strong emission lines in the blue region are due to the transitions of VO4 3− ions have been observed at 77K. The Judd–Ofelt parametrization scheme has been applied to the analysis of the room temperature absorption spectra in order to evaluate the intensity parameters, the branching ratios and the radiative lifetimes of the 4I13/2, 4I11/2, 4F9/2 and 4S3/2 emitting levels. The effective cross-section has been calculated for the 4I13/2→4I15/2 transition, indicating that the title compounds is a promising active medium for application in the three-level laser system. The up-conversion emission in Er3+: KCaY(VO4)2 was investigated at 300K. The decay profiles of the Stokes and anti-Stokes emissions were measured and the mechanism of up-conversion luminescence is discussed. [Copyright &y& Elsevier]

VO2 thin films deposited on fused-quartz substrates were prepared by a pulsed laser deposition technique. Vanadium dioxide shows an ultrafast, passive phase transition (PT) from a monoclinic semiconductor phase to a metallic tetragonal rutile structure when the sample temperature is above 68°C. The fast PT can also be optically induced by laser excitation. In this paper, we report the optical properties of prepared thin films by absorption and photoluminescence (PL) measurements. In addition, a degenerate-four-wave-mixing measurement was also conducted using a 30 ps YAG pulsed laser operated at 532 nm. It is the first time, to our knowledge, that a large polarizability and relatively slow nanosecond excited state dynamical processes have been identified in VO2. The polarizability is found to be ∼300 times greater than the χ(3) value of a CS2 reference. The mechanism for slow process is believed to be associated with the structural change. [Copyright &y& Elsevier]

The fluorescent behaviors and nonadiabatic dynamics of salicylideneanilines and its two derivatives have been theoretical studied. The enol form transfers to cis -keto after photoexcitation to the first single (S 1) excited state without any energy barrier, and the derivatives with substituent of chlorine and hydroxyl have similar dynamics progress. The excited state intramolecular proton transfer occurs in 46 fs by the results of nonadiabatic dynamics together with photoexcitation progress, which is also demonstrated by the results of geometric structure and infrared spectra. The potential curves scan indicates the energy difference before and after the proton transfer reaction of Enol (11.12 kcal/mol) is larger than that of Enol-CL (8.97 kcal/mol), and the Stokes shift of Enol (257 nm) is larger than that of Enol-CL (200 nm) and Enol-OH (198 nm). This comparison confirms the substitute of chlorine and hydroxyl hinder the decrease in aromaticity after photoexcitation and energy difference between the ground (S 0) and the S 1 state, thereafter induce smaller Stokes shift. The energy barrier between Enol and Cis -keto (5.96 kcal/mol) and energy difference between Cis -keto and Trans -keto (10.27 kcal/mol) are so small that there is a decay pathway of Enol (S 0)→proton transferred Cis -keto (S 1)→fluorescent Cis -keto (S 0)→Enol (S 0) or Cis -keto (S 1) → Trans -keto (S 1)→fluorescence of Trans -keto (S 0)→Enol (S 0). Image 1 •. The proton transfer of SA is demonstrated by steady and transient configuration. •. Substituent induces smaller Stokes shift by hindering the decrease in aromaticity. •. Two main decay pathways of SA are proposed with their fluorescent dynamics. [ABSTRACT FROM AUTHOR]