Dynamics of trans-cis photoisomerization of hetarylazo dyes

Dynamics of trans-cis photoisomerization of novel hetarylazo dyes containing hydrogenated quinoline and triazole or tetrazole moieties has been studied by femtosecond laser photolysis with spectrophotometric detection. For all the dyes under study, the absorbance dynamics after photoexcitation in the long-wavelength absorption band (λ_pump = 550 nm) is described by three fast processes with characteristic times of 0.07–0.27, 0.4–1.0, and 3–7 ps. The effect of the solvent and the azo dye structure on the dynamics of transient species has been investigated.     

Stimulated Raman Scattering in Metal-Dielectric Nanocomposites with Spectrally Degenerate Dielectric Constant

JETP Letters - The effects of enhancement of stimulated Raman scattering in titanium oxynitride (TiON) nanofilms are considered. The mechanism of giant amplification of a Stokes wave is based on,...     

Long-lived coherent oscillations of the femtosecond transients in cyanobacterial photosystem I

The pulsed excitation of electronic levels coupled to specific nuclear modes by a 26 fs laser pulse at 706 nm creates a wavepacket in the nuclear space of photopystem I (PS I) of Synechocystis sp. strain PCC 6803 both in the ground state and in the one-exciton manifold. Fourier transform of transient decay curves shows several low frequency peaks. The most prominent Power Spectral Density (PSD) peaks are at omega = 49 cm(-1) and omega = 88 cm(-1). The peculiarity of the coherent wavepacket in the PS I of S. sp. strain PCC 6803 is the unique, long-lived 49 cm(-1) and 88 cm(-1) oscillations with decay times up to 10 ps. It was suggested that such a long-lived coherence is determined by a contribution of the ground state wavepacket. The dependence of these two PSD peaks on the probe wavelength resembles the profile of the transient absorption spectra of PS I. The pump-probe signal in the Soret region reflects the dynamics of the ground state wavepacket created by pulsed excitation of the Q(y)-band. It was shown that the multimode Brownian oscillator model allows a quantitative fit of the oscillatory patterns of the pump-probe signal to be obtained.     

Femtosecond dynamics of primary processes in visual pigment rhodopsin

The dynamics of the coherent photoisomerization of the 11-cis-retinal in bovine rhodopsin is studied by femtosecond time-resolved laser absorption spectroscopy with a resolution of 30 fs. Rhodopsin is excited with 500-, 535-, and 560-nm femtosecond pulses to produce different initial Franck-Condon states with different vibrational energies of the molecule in its electronically excited state. The time evolution of the photoinduced differential absorption spectra of rhodopsin is measured upon excitation with a femtosecond pulse in a spectral range from 400 to 720 nm. Oscillations in the time-resolved absorption of the rhodopsin photoproducts, such as photorhodopsin with a vibrationally excited all-trans-retinal and in its initial-state rhodopsin with a vibrationally excited 11-cis-retinal, are examined. It is demonstrated that these oscillations reflect the dynamics of coherent vibrational wavepackets. The Fourier transform of these oscillatory components yields frequencies, amplitudes, and initial phases of various vibrational modes involved in the motion the wavepackets in both photoproducts. The main vibrational modes manifest themselves at frequencies of 62 and 160 cm^?1 for photorhodopsin and 44 and 142 cm^?1 for initial-state rhodopsin. It is shown that these vibrational modes are directly involved in the coherent reaction under the study, with their amplitudes in the power spectrum produced by the Fourier transform of the kinetic curves being dependent on the wavelength of rhodopsin excitation.     

Spectral and kinetic parameters of transient species in the photolysis of naphthylmethylideneiminospironaphthopyran by excitation at different wavelengths: Nano- and femtosecond laser photolysis

Intramolecular processes occurring in a photobifunctional compound (PBC) comprising the spironaphthopyran and hydroxyazomethine moieties have been studied in methanol solutions by femtosecond laser photolysis using light with wavelengths of 340 and 490 nm. At the excitation wavelength of 490 nm, the cis-trans photoisomerization in the azomethine moiety occurs in the S₁ state. In the case of PBC photolysis with 340-nm light, the opening of the spiro bond of the spiropyran moiety (formation of the X form) also takes place during relaxation of the S_n state to the S₁ state followed by isomerization to the merocyanine form. The spectral and kinetic characteristics of different electronically excited have been were determined. The data have been compared with those of nanosecond laser photolysis.     

Effect of the excitation wavelength and the structure of nitrated 1,2-dyhydroquinolines on dynamics of primary photophysical and photochemical processes

Dynamics of transformations of excited states and active transient species generated in the photolysis of nitrated 1,2-dihydroquinolines (N-DHQ) has been studied by femto- and nanosecond laser pulse photolysis. Spectral and kinetic parameters of primary photophysical and photochemical processes have been determined, and their dependence on the substituent position at the aromatic ring of 1,2-dihydroquinoline (DHQ) and on the wavelength of excitation light has been established. The lifetime of the excited singlet state S₁ in N-DHQ is ca. 100 and 500 fs for 8- and 6-nitro-substituted DHQ, respectively, which is shorter in comparison with DHQ without the nitro group by a factor of 10⁴ and more. The major decay channel of the S₁ state is the successive formation of three transient species with lifetimes of 0.5 to 16 ps. A triplet state is generated only upon excitation of the short-wavelength band by UV light. The quantum yield of the triplet state depends on the structure of N-DHQ.     

Femtopicosecond relaxation of zinc porphyrinate trimer linked by the triazole bridge

The femtosecond dynamics of excitation relaxation of the zinc porphyrinate trimer was studied by the pumping—scanning method. The obtained results were examined using semi-empirical quantum chemical calculations. The dynamics of excitation relaxation shows that the application of similar systems as models for studying photosystems and development of artificial analogs of natural photosynthetic centers is promising. An explanation for the observed coherent dynamics of exciton bands was proposed.     

Relaxation Kinetics of Excitonic States in ZnSe Quantum Dots: A Femtosecond Laser Spectroscopy Study

Relaxation processes in ZnSe quantum dots upon excitation by a 30-fs pulse at a wavelength of 360 nm have been studied by broadband femtosecond absorption spectroscopy. The diameter of ZnSe nanoparticles was 3.7 ± 0.6 nm. A colloidal solution of ZnSe in cyclohexane was used. In the differential spectra, a bleaching band at the edge of the excitonic absorption band of ZnSe, an absorption band of the biexcitonic transition with a peak at about 420 nm, and a broad structureless absorption band in the region from 440 to 750 nm have been revealed. From the analysis of the absorption and luminescence spectra, the shift of the excitonic luminescence band δ_XX = 127 meV has been measured. From the femtosecond photolysis data, an estimate of the biexcitonic interaction Δ_XX ≈ 75 meV has been obtained. It has been shown that the relaxation kinetics of the differential spectra is described by three-exponential dependences with time constants and corresponding amplitude contributions of 1 ps (42%), 13 ps (22%), and 91 ps (17%). The kinetic component of 1 ps (42%) is presumably due to hole transport to surface traps. The kinetic components of 13 ps (22%) and 91 ps (17%) apparently describe the processes of electron transport to shallow and deep traps.     

Femtosecond dynamics of intramolecular photoinduced proton transfer in <Emphasis Type="Italic">N</Emphasis>-substituted 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones

Femtosecond dynamics of processes in the excited state of 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones has been studied by femtosecond absorption spectroscopy. The rate constants of intramolecular photoinduced proton transfer (IPPT) have been determined for the N-substituted derivatives (0.7–11 ps⁻¹). The IPPT rate constant depends on the inductive constant of the substituent and the potential barrier height, which was calculated by a quantum-chemical method (TDDFT). The multiexponential character of the kinetics of photoinduced absorption by the compounds with a low inductive constant of the N-substituent in the spectral region of the S₁ → S_N absorption and the stimulated emission of the IPPT product is explained by the rapid (∼10 ps⁻¹) relaxation process preceding the IPPT.