Photochemical isomerization in the absence of a potential barrier: origin of wavelength-dependent ground-state recovery kinetics

Anharmonic vibrational effects in linear and two-dimensional electronic spectra

Anharmonic modulation of electronic gap is manifested in changing magnitudes of cross peaks of 2D electronic spectrum.

Excited-state dynamics of dGMP measured by steady-state and femtosecond fluorescence spectroscopy

The room-temperature fluorescence of 2'-deoxyguanosine 5'-monophosphate (dGMP) in aqueous solution is studied by steady-state and time-resolved fluorescence spectroscopy. The steady-state fluorescence spectrum of dGMP shows one band centered at 334 nm but has an extraordinary long red tail, extending beyond 700 nm. Both the fluorescence quantum yield and the relative weight of the 334 nm peak increase with the excitation wavelength. The initial fluorescence anisotropy after excitation at 267 nm is lower than 0.2 for all emission wavelengths, indicating an ultrafast S(2) --> S(1) internal conversion. The fluorescence decays depend strongly on the emission wavelength, getting longer with the wavelength. A rise time of 100-150 fs was observed for wavelengths longer than 450 nm, in accordance with a gradual red shift of the time-resolved spectra. The results are discussed in terms of a relaxation occurring mainly on the lowest excited (1)pi pi*-state surface toward a conical intersection with the ground state, in line with recent theoretical predictions. Our results show that the excited-state population undergoes a substantial "spreading out" before reaching the CI, explaining the complex dynamics observed.

Tracking Ultrafast Excited-State Bond-Twisting Motion in Solution Close to the Franck−Condon Point

Applying optimal control to photoinduced trans-cis isomerization in condensed phase, the dynamics of bond-twisting motion of 1,1'-diethyl-4,4'-cyanine in methanol and propanol is revealed. The shape of the optimized pulse resulting from minimization of the photoisomer formation can be directly related to the initial excited-state dynamics in close proximity to the Franck-Condon point. The solvent viscosity-dependent ultrafast wavepacket motion is reflected in the prominent down-chirp of the optimized pulses and reveals a detailed picture of the control mechanism: The reduction of the isomer production is achieved by most efficient dumping of excited population back to the trans ground state. In the higher-viscosity solvent, propanol, wavelength-dependent oscillatory features are superimposed to the overall chirp structure pointing to the importance of excited-state vibrational coherences for the dumping process.

Femtosecond study on the isomerization dynamics of NK88. I. Ground-state dynamics after photoexcitation

Recently, optimal control of a photoisomerization reaction in the liquid phase was demonstrated for the first time on the system 3,3(')-diethyl-2,2(')-thiacyanine (NK88). Additionally, the class of cyanines to which the molecule NK88 belongs draws a lot of attention in different recent theoretical publications. Therefore, a better understanding of the molecular dynamics of this molecular system is of special interest. Experiments using the femtosecond pump-supercontinuum probe technique with an excitation wavelength of 400 nm and a spectral range from 370 to 620 nm for the probe beam have been performed. In order to analyze the dynamics properly the time window has been chosen to comprise the characteristic times of the contributing processes, additionally we have employed two solvents, methanol and ethylene glycol, and have conducted anisotropy measurements. The spectroscopic data have been assigned to different molecular states with the help of density functional theory and second-order Moller-Plesset perturbation theory calculations. The analysis of the data has revealed in the most likely model that three different isomers exist with different lifetimes. On the basis of experimental and theoretical data, a conclusive scheme of the isomerization reaction is presented.

Femtosecond study on the isomerization dynamics of NK88. II. Excited-state dynamics

The molecule 3,3(')-diethyl-2,2(')-thiacyanine isomerizes after irradiation with light of the proper wavelength. After excitation, it undergoes a transition, in which one or more conical intersections are involved, back to the ground state to form different product photoisomers. The dynamics before and directly after the transition back to the ground state is investigated by transient absorption spectroscopy in a wavelength region of 360-950 nm, as well as by fluorescence upconversion. It is shown that the excited-state dynamics are governed by two time scales: a short one with a decay time of less than 2 ps and a long one with about 9 ps. A thorough comparison of the experimental results with those of configuration interaction singles and time-dependent density functional theory calculations suggests that these dynamics are related to two competing pathways differing in the molecular twisting on the excited surface after photoexcitation. From the experimental point of view this picture arises taking into account the time scales for ground-state bleach, excited-state absorption, stimulated emission, fluorescence, and assumed hot ground-state absorption both in the solvent methanol and ethylene glycol.

Effect of TiO2 Colloids on the Fluorescence Behavior of Two Cyanine Dyes

The photophysical properties of two typical cyanine dyes [3,3'-diethyl-9-methyl-thiacarbocyanine iodide (dye A) and anhydro-3,3'-disulfopropyl-5,5'-diphenyl-9-ethyloxacarbocyanine hydroxide (dye B)] in the absence and presence of TiO(2) colloids have been investigated by UV-visible spectroscopy, (1)H-NMR spectroscopy, fluorescence spectroscopy, fluorescence lifetime measurements, and ESR measurements. It was found from the absorption spectra and NMR results that there are two isomers in the ground state of these dyes. Steady-state fluorescence spectra show that the fluorescence intensities of dye A and dye B are enhanced and quenched by TiO(2) colloids, respectively. Time-resolved fluorescence lifetime measurements indicate that the lifetimes of dye A and dye B in the presence of TiO(2) colloids are longer and shorter than those obtained in the absence of TiO(2) colloids, respectively. ESR measurements demonstrate that the electron transfer efficiency from (1)dye B* to the conduction band of TiO(2) is much larger than that from (1)dye A* to the conduction band of TiO(2). The different fluorescence behavior of dye A and dye B can be intepreted in terms of whether phi(Tr,nr)(0)-phi(Tr,nr) (the reduction of the quantum yield for radiationless transition in the excited singlet state (1)dye* caused by the TiO(2) colloids) is larger or smaller than phi(ET) (the quantum yield of electron transfer from (1)dye* to the conduction band of TiO(2) colloids).