Photoinduced Intramolecular Charge Transfer Processes Research Articles

Disentangling Time Scales of Vibrational Cooling, Solvation, and Hydrogen Bond Reorganization Dynamics Using Ultrafast Transient Infrared Spectroscopy of Formylperylene.

Unraveling dynamics of solvation and hydrogen bond (H-bond) reorganization between a solute and solvent is crucial to understand the importance of specific and nonspecific interactions in a solution-phase chemical reaction. Ultrafast time-resolved infrared (TRIR) spectroscopy provides direct opportunity to monitor site-specific intermolecular dynamics on a real-time scale by probing vibrational marker bands in the excited state of a solute. Herein, we report the real-time dynamics of vibrational cooling, solvation, and hydrogen bond reorganization of formylperylene (FPe) through TRIR spectroscopy of carbonyl (C═O) stretching mode in nonpolar, polar aprotic, and polar protic solvents. High sensitivity of the C═O stretch frequency (υ̅C═O) to photoinduced intramolecular charge transfer processes induced by specific and nonspecific solvent interactions led us to monitor the dynamics of dipolar solvation, site-specific H-bond formation, and reorganization processes by the TRIR method. In nonpolar cyclohexane, the υ̅C═O stretch band appears at 1610 cm-1 and exhibits negligible spectral shift over several tens of picoseconds. In acetonitrile, the υ̅C═O peak shifts to 1594 cm-1 and exhibits a further temporal red shift of about 5 cm-1 with a characteristic solvation time scale of acetonitrile (τ ∼ 0.5 ps). In methanol, υ̅C═O exhibits two bands corresponding to free and H-bonded FPe in early time scale. The free FPe population converts to the hydrogen-bonded population with a lifetime of about 10 ps. Vibrational cooling (τvc ∼ 12-20 ps) in the excited electronic state of FPe could independently be monitored from the temporal dynamics of the ring vibration mode, which is less sensitive to solvation and hydrogen bonding. The present study provides insight into the specific and nonspecific solvation-controlled charge transfer dynamics in aprotic and protic solvents using FPe as a probe.

Synthesis and investigation on liquid crystal and optical properties of dyads based on triphenylene and perylene

Mesogenic dyads consisting of a triphenylene (donor) and a perylene (acceptor) showed photoinduced intramolecular charge transfer process and fluorescent quenching.

Photoinduced intramolecular charge transfer process of betaine pyridinium: A theoretical spectroscopic study

Using Time-Dependent Density Functional Theory and taking into account bulk solvent effects, we investigate the absorption and emission spectra of a betaine pyridinium molecule, the 2-(1-pyridinio) benzimidazolate (SBPa). This molecule exhibits strong photoinduced intramolecular charge transfer (ICT). We have identified two different electronic states involved, respectively, in the strong bathochromic ICT absorption band (S 2) and in the moderate emission band (S 1). The ICT process is analyzed in terms of charge distribution and dipole moment evolutions upon photoexcitation. These results are compared with steady-state spectroscopic measurements.

Photoinduced intramolecular charge transfer phenomena in 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid

A donor acceptor substituted aromatic system 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid (DMAPPDA) has been synthesized and its spectral properties have been explored on the basis of steady state absorption and fluorescence spectroscopy. Spectral features point largely towards a possible occurrence of photoinduced intramolecular charge transfer process from the donor NMe2 group to the acceptor acid group. Solvent dependency of the large Stokes’ shifted emission band and the calculated large excited state dipole moment support the polar character of the charge transfer excited state. Quantum yield calculations and effect of addition of acid and base on the steady state spectra were also performed to further scrutinize the excited state CT character.

Geometry, Electronic Structure, and Related Properties of Dye Sensitizer: 3,4-bis[1-(carboxymethyl)-3-indolyl]-1H-pyrrole-2,5-dione

The geometry, electronic structure, polarizability and hyperpolarizability of dye sensitizer 3,4-bis[1-(carboxymethyl)-3-indolyl]-1H-pyrrole-2,5-dione (BIMCOOH) were studied using density functional theory (DFT) with hybrid functional B3LYP, and the electronic absorption spectra were investigated using semi-empirical quantum chemical method ZINDO-1 and time-dependent DFT (TDDFT). The results of natural bond orbital suggest that the natural charges of the dione, indole, and acetic groups are about 0.15e, 0.29e, and 0.44e, respectively. The calculated isotropic polarizability, polarizability anisotropy invariant and hyperpolarizability are 305.4, 188.3, and 1155.4 a.u., respectively. The electronic absorption spectral features in visible and near-UV region were assigned to the * transition due to the qualitative agreement between the experiment and the TDDFT calculations, and the transitions of the excited states 911 related to photoinduced intramolecular charge transfer processes. The analysis of electronic structure and UV-Vis absorption indicates that the indole groups primarily contributed sensitization of photo-to-currency conversion processes, and the interfacial electron transfer between semiconductor TiO2 electrode and dye sensitizer BIMCOOH are electron injection processes from excited states of the dyes to the semiconductor conduction band.

Subpicosecond Transient Absorption of Donor−Acceptor Biphenyls. Intramolecular Control of the Excited State Charge Transfer Processes by a Weak Electronic Coupling

The photoinduced intramolecular charge transfer processes of three differently twisted 4-(dimethylamino)-4‘-cyanobiphenyl derivatives (I−III) have been investigated using time-resolved transient ab...

Theoretical model of photoinduced intramolecular charge transfer processes

Molecular systems which undergo upon photoexcitation intramolecular charge transfer processes accompanied by conformational changes are known to exhibit complex spectroscopic and dynamical behaviour. Typical spectroscopic effects are dual fluorescence and pronounced solvatochromism. Dynamical features are revealed by the temperature dependence of fluorescence emission intensities and shifts under stationary conditions; the precursor-successor behaviour for the observed decay of the high-frequency band and the rise of the low-frequency band, after pulse excitation in correspondence of the high-frequency absorption; the fluorescence depolarization effects; the dynamic Stokes shifts measured in time-dependent experiments. All these effects can be rationalized by a stochastic model in which coupling of the fluorescent probe with the polar environment is explicitly taken in account. Ingredients of the model are the potential energy surface for the probe internal coordinates, and hydrodynamic and dielectric properties of the solvent. The model reduces to the simple kinetic scheme based on two-state interconversion process only in the limiting case of a relatively high energy barrier separating the two emitting states, and rapid equilibration of the polar solvent with respect to the instantaneous probe dipole moment.