Experimental and theoretical study of inhomogeneous electron transfer in betaine: comparisons of measured and predicted spectral dynamics

Ultrafast intermolecular electron transfer (ET) from orthomethoxyaniline (orthoanisidine, ANS) to a number of excited (S1) 4-trifluoromethyl-1,2-benzopyrones (coumarins) having differently substituted 7-amino group has been investigated by femtosecond fluorescence up-conversion technique. The ET dynamics in the present systems are nonsingle-exponential and occur faster than the diffusive solvation dynamics. The ET rates are largely dependent on the nature of the substituents at the 7-amino group of the coumarins. This dependence is well correlated with the free energy changes (ΔG0) for the ET reactions. The ET dynamics become slower on using deuterated ANS as the donor, where the amino group hydrogens of ANS are substituted by deuterium. The deuterium isotope effect, however, gradually reduces as the ET dynamics becomes faster. Conventional ET theories can not explain all the observations. The results are explained on the basis of the two-dimensional ET model, which considers the solvent coordinate and the intramolecular coordinate separately to depict the ET process. It is seen that in coumarin-ANS systems the ET occurs much faster than the coumarin-aniline systems. It is indicated that the electronic coupling matrix element, a parameter which determines the extent of interaction between the reactant and the product states in the ET process, is much larger in the present systems than for the coumarin-aniline systems. The deuterium isotope effect on the ET dynamics is explained in terms of the changes in the ΔG0 values on isotopic substitution of the solvent donors.

Section: Introductionmentioning

confidence: 93%

Ultrafast intermolecular electron transfer from orthomethoxyaniline to excited coumarin dyes

Pal

Shirota

Tominaga

et al. 1999

“…B-ET times determined from transient absorption measurements 3,5 are given for comparison in the first column of Table II.…”

Section: B Time-resolved Anti-stokes Raman Measurementsmentioning

confidence: 99%

Mode-specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer

Hogiu

Werncke

Pfeiffer

et al. 2000

Vibrational relaxation in the electronic ground state initiated by intramolecular back-electron transfer (b-ET) of betaine-30 (B-30) is studied by picosecond time-resolved anti-Stokes Raman spectroscopy. Measurements were carried out with B-30 dissolved in slowly as well as in rapidly relaxing solvents. We observed a risetime of the Raman band with the highest frequency near 1600 cm−1 which is close to the b-ET time τb-ET of B-30. For B-30 dissolved in propylene carbonate (τb-ET∼1 ps), the population of this mode exhibits a rise time of 1 ps whereas vibrational populations between 400 and 1400 cm−1 increase substantially slower. In contrast, in glycerol triacetin (τb-ET∼3.5 ps) and in ethanol (τb-ET∼6 ps) rise times of all modes are close to the respective b-ET times. Within the first few picoseconds, direct vibrational excitation through b-ET is favored for modes with the highest frequencies and high Franck–Condon factors. Later on, indirect channels of population due to vibrational energy redistribution (IVR) become effective. Thermal equilibrium populations of the Raman active modes are established within 10 to 15 ps after optical excitation.

“…28,29 Alternatively, the quantum description of vibrational motion is employed instead of classical one to study numerically the reverse relaxation of excited state in inverted region. 30,31 Another simulation procedure was developed for the similar goal in Refs.…”

Section: ͑26͒mentioning

confidence: 99%

Pump-probe spectroscopy of H-bonds as the level-crossing problem

Burshteǐn

Chernobrod

Sivachenko

1998