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The excited-state intramolecular proton transfer (ESIPT) reaction of two phenol-quinoline molecules (namely PQ-1 and PQ-2) were investigated using time-dependent density functional theory. The five-(six-) membered-ring carbocycle between the phenol and quinolone moieties in PQ-1 (PQ-2) actually causes a relatively loose (tight) hydrogen bond, which results in a small-barrier (barrier-less) on an excited-state potential energy surface with a slow (fast) ESIPT process with (without) involving the skeletal deformation motion up to the electronic excitation. The skeletal deformation motion that is induced from the largest vibronic excitation with low frequency can assist in decreasing the donor-acceptor distance and lowering the reaction barrier in the excited-state potential energy surface, and thus effectively enhance the ESIPT reaction for PQ-1. The Franck-Condon simulation indicated that the low-frequency mode with vibronic excitation 0 → 1' is an original source of the skeletal deformation vibration. The present simulation presents physical insights for phenol-quinoline molecules in which relatively tight or loose hydrogen bonds can influence the ESIPT reaction process with and without the assistance of the skeletal deformation motion.

The absorption and fluorescence spectra of 4-(3-methoxybenzylidene)-2-methyl-oxazalone (m-MeOBDI) dissolved in neutral, acidic, and basic solvent environments have been investigated and assigned by using Franck–Condon (FC) simulations at the quantum TDDFT level. Four different structures of m-MeOBDI in the ground and excited states are optimized and are found to be responsible for the observed absorption and fluorescence spectra. The (absorption) fluorescence of m-MeOBDI in pure methanol and neutral/basic methanol/water (1/9 vol) mixed solvent is found to arise from the (ground neutral N-I) excited neutral N-I* and cationic C-III* species, respectively. In acidic solvent, the absorption is found to arise from ground acidic C-II species, and the excited divalent cation DC-IV* is found to be formed in its excited state due to the excess H+ in the solution, and then it emits ∼560 nm fluorescence. FC simulations have also been employed to confirm our assignments as well as interpret the vibronic band profiles. As expected, the simulated emission spectrum of the divalent cationic species is in good agreement with the experimental observation. Therefore, within the present FC simulation, the observed absorption and fluorescence spectra have been reasonably interpreted and novel fluorescence mechanisms of m-MeOBDI in various pH solvent environments have been proposed.

The electronic structure, optical absorption and charge transfer performances of Zn-porphyrin (ZnPP) dye molecule adsorbed on anatase (101) and rutile (110) TiO2 surfaces have been investigated systematically using spin-polarized density functional theory calculations. The calculated results indicate that the strong interactions of ZnPP dye molecule and TiO2 surfaces cause an obvious variation of the conduction band minimum and valence band maximum for ZnPP dye molecule adsorbed on TiO2 surfaces, and the valence band of adsorption systems has a large bandwidth compared with that of the TiO2 bulk and surfaces, which induces a red-shift of the optical absorption edge and improves greatly the optical absorption of adsorption systems in the ultraviolet and visible light regions. Moreover, the change of charge density indicates that the photoelectrons transfer from the ZnPP dye molecule to TiO2 across the interface, and a built-in electric field is formed at the interface, which effectively enhances the separation of photogenerated electron-hole pairs. These results suggest that the sensitized actions of ZnPP dye molecule on TiO2 surfaces can generate an excellent photovoltaic performance of TiO2 photoanode in DSSCs.

We consider the overdamped motion of a Brownian particle in an asymmetric spatially periodic potential which fluctuates periodically in time, under assumption of finite duration of the relaxation response of the system on deterministic dichotomous fluctuations. It is assumed that the period of these fluctuations is much larger than the characteristic diffusion time and the potential barrier height is small as compared to the thermal energy (an adiabatic high-temperature flashing ratchet). We derive an analytical expression for the average particle velocity, which is concretized for a saw-tooth potential profile. It is revealed the different, linear and quadratic, asymptotic behavior of the average velocity as a function of the relaxation time for extremely and not extremely asymmetric potential profiles, respectively. The result is interpreted in terms of the self-similar representation.

Extremely solvent-enhanced absorption and fluorescence spectra of carbazole were investigated by performing a generalized multi-set damped Franck-Condon spectral simulation. Experimental absorption and fluorescence spectra of carbazole in the gas phase were first well reproduced by performing an un-damped Franck-Condon simulation, but a one-set scaling damped Franck-Condon simulation severely underestimated the intensities of the peaks of experimental absorption and fluorescence spectra of carbazole in n-hexane. Then, a multi-set scaling damped Franck-Condon simulation was proposed and carried out for simulating the extremely solvent-enhanced absorbance and fluorescence, and here, the simulated spectra agreed well with the experimental ones. Five (four) representative solvent-enhanced normal modes corresponding to the combination of ring stretching and ring breathing vibrational motions were determined to be responsible for enhanced absorbance (fluorescence) in n-hexane solution. Furthermore, different scalings were applied to the ground and first-excited states, resulting in different enhancement of absorbance and fluorescence, and this analysis revealed atoms in the carbazole interacting with n-hexane solvent molecules and, hence, leading to different normal-mode vibrational vector patterns in the ground and first-excited states, respectively. Basically, the same conclusion was drawn from a simulation with HF-CIS and the three functionals (TD)B3LYP, (TD)B3LYP-35, and (TD)BHandHLYP. The present multi-set scaling damped Franck-Condon simulation scheme was demonstrated to successfully interpret extremely solvent-enhanced absorbance and fluorescence of carbazole in n-hexane-solvent.

We consider a Brownian particle moving in a slightly fluctuating potential. Using the perturbation theory on small potential fluctuations, we derive a general analytical expression for the average particle velocity valid for both flashing and rocking ratchets with arbitrary, stochastic or deterministic, time dependence of potential energy fluctuations. The result is determined by the Green’s function for diffusion in the time-independent part of the potential and by the features of correlations in the fluctuating part of the potential. The generality of the result allows describing complex ratchet systems with competing characteristic times; these systems are exemplified by the model of a Brownian photomotor with relaxation processes of finite duration.

Externally stimulated fluctuations of the size of a particle in an asymmetric environment result in its directional motion. This is demonstrated by considering the particle drift in a periodic symmetric channel, which arises due to fluctuations of the particle size. The emergence of the drift is caused by the time variation of the particle entropy, whereas fluctuations of the diffusion coefficient accompanying fluctuations of the size enhance the drift. The effect of the geometry of the channel and the amplitude and frequency of fluctuations on the drift velocity is studied by the Brownian dynamics method. A coarse-grained analytical description of the process is proposed whose predictions are in satisfactory agreement with the results of simulation.

Coherent dynamics of localised π-electron rotations in an aromatic ring molecule may play an important role in designing next generation photo-induced organic molecular devices. In this paper, we present a theoretical study of laser control of localised π-electron rotations in a low symmetry aromatic ring molecule, which are created by the coherence of two or more electronic excited states. The control scenario is to manipulate energy spacings between two electronic states by utilising AC Stark shift induced by strong non-resonant ultraviolet (UV) laser. We set three targets for laser manipulation of localised electron rotational dynamics for (P)-2,2′-biphenol with two aromatic rings. The first target is flipping of the direction of the localised π-electron rotation on the same ring, the second one is the transfer of the π-electron rotation localisation from one aromatic ring to the other, and the third one is freezing the unidirectional π-electron rotation localised on one aromatic ring. We propo...

Sum-frequency generation spectroscopy is a powerful tool for analysing molecular species and orientation configurations on a surface or an interface. In addition to conventional vibrational resonant techniques, the electronic resonant (electronic sum-frequency generation [ESFG]) ones have been developed recently. We have established the theoretical formulation of ESFG spectroscopy based on the susceptibility approach, considering resonances with both symmetry-allowed and symmetry-forbidden electronic excited states; derivatives of transition dipole moments and vibronic couplings beyond the Condon approximation were included. With the aid of density functional theory computations, simulated ESFG spectra in resonances with both types of excited states of acetone are demonstrated.

The electronic and optical properties of Fe or/and Ni (co)doped anatase and rutile TiO2 were investigated using the spin-polarized density functional theory. Our calculated results indicate that the synergistic effect of (Fe + Ni) codoping can lead to a band gap narrowing and the hybridized states of Fe 3d and Ni 3d appearing in the forbidden gap, which enhances greatly the optical absorption of TiO2 nanomaterials from the ultraviolet-light to the infrared-light region and reduces the recombination of photogenerated electron–hole pairs. In particular, (Fe + Ni) codoping can improve greatly the infrared-light absorption of TiO2 nanomaterials. Furthermore, the researches of electronegativity show that (Fe + Ni)-codoped TiO2 system has a stronger redox power for hydrogen generation by photocatalytic water splitting compared with pure, Fe-doped, and Ni-doped TiO2 systems. These results lead to an outstanding solar energy photocatalytic water splitting for hydrogen generation in (Fe + Ni)-codoped TiO2 photocatalyst.

We present the results of a theoretical study on dynamic Stark-induced coherent π-electron rotations in a chiral aromatic ring molecule. This is an extension of our previous papers, which have been published in Mineo , H. [ Phys. Chem. Chem. Phys. 2016 , 18 , 26786 - 26795 ] and Mineo , H. [ J. Phys. Chem. Lett. 2018 , 9 , 5521 - 5526 ]. In those papers, the time-dependent Schrodinger equation was solved under a restricted condition in which a degenerate excited state should be formed at the center of the two relevant excited states by dynamic Stark effects. The dynamic Stark-induced degenerate state (DSIDS) is essential to create unidirectional π-electron rotations. In the present theoretical treatment, the above restriction is relaxed and the DSIDS is set to be at any energy position between the two excited states. This indicates a wide applicability of the dynamic Stark effects to coherent control of photophysical properties in aromatic molecules, such as coherent ring currents and current-induced magnetic fluxes of low-symmetric aromatic molecules. Analytical expressions for the coherent π-electron angular momentum are derived within a three-electronic-state model by using the Laplace transform method. The validity of the developed theoretical procedure is demonstrated by carrying out simulations of the coherent angular momentum of l-phenylalanine. Effects of varying the DSIDS on the time-dependent coherent angular momentum and the populations in the three electronic states are examined, and the results are analyzed using approximate expressions for the time-dependent coherent angular momentum and the populations. Modulations in the time-dependent coherent angular momentum appear when the DSIDS is set at an energy position between the two excited states, while there are no beating modulations when the DSIDS is set at the center position. Such differences originate from whether interferences between the two dressed states take place or not.

In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CH

Quantum chemistry calculations at the level of (TD)-DFT plus PCM solvent models are employed for analyzing potential energy surfaces and as a result two local minima with D2, two local minima with C2H, and one second-order transition state with D2H group symmetry are found in both ground S0 and excited-state S1 potential energy surfaces. Simulated vibronic coupling distributions indicate that only second-order transition states with D2H group symmetry are responsible for observed absorption and fluorescence spectra of rubrene and vibrational normal-motions related with atoms on the aromatic backbone are active for vibronic spectra. The Stokes shift 1120 cm−1 (820 cm−1) and vibronic-band peak positions in both absorption and fluorescence spectra in non-polar benzene (polar cyclohexane) solvent are well reproduced within the conventional Franck–Condon simulation. By adding damped oscillator correction to Franck–Condon simulation, solvent enhanced vibronic-band intensities and shapes are well reproduced. Four (three) normal modes with vibration frequency around 1550 cm−1 (1350 cm−1) related to ring wagging plus CC stretching and CH bend motions on the backbone are actually interpreted for solvent enhanced absorption (fluorescence) spectra of rubrene in benzene and cyclohexane solutions.