Nandita Biswas

Early time dynamics of trans-azobenzene isomerization in solution from resonance Raman intensity analysis

Resonance Raman spectra have been recorded for trans- azobenzene in carbon tetrachloride using 16 excitation wavelengths in the region from 355–600 nm. It has been observed that for many totally symmetric fundamentals viz. C–N, N5N stretch, etc., the resonance Raman intensities decrease near the maxima of the resonant electronic

Wavepacket dynamical studies on trans-azobenzene: absorption spectrum and resonance Raman excitation profiles of the n−π∗ transition

(2 ^1A_g \leftarrow1 ^1A_g)

Partitioning of the total reorganization energy into its vibrational and solvent (inertial and reorientational) contributions using resonance Raman intensities

transition. This is attributed to interference due to preresonant scattering from the strongly allowed

SIMPLE APPROACH TO DETERMINING ABSOLUTE RAMAN CROSS SECTIONS USING AN OPTICAL PARAMETRIC OSCILLATOR

(1 ^1A_u \leftarrow1 ^1A_g)

Resonance de-enhancement in the 2 1 A g state of trans-azobenzene

electronic transition. The Raman excitation profiles (REPs) for the ten Franck–Condon active fundamentals have been successfully modeled using Heller’s time-dependent approach with the inclusion of interference effect from higher electronic state. The short time isomerization dynamics is then examined from a priori knowledge of ground-state normal mode descriptions to convert the wave packet motion in dimensionless normal coordinates to internal coordinates. It is observed that within 5–30 fs of photoexcitation, the major changes experienced by trans-azobenzene are on N = N and C–N stretching vibrations, while N = N suffers reduction, C–N bond elongates, and with time the ring C atoms distort relatively out of the plane.

Density functional calculations of structures, vibrational frequencies, and normal modes of trans- and cis-azobenzene

Abstract Time-dependent wavepacket propagation techniques have been used to calculate the absorption spectrum and the resonance Raman excitation profiles of the n −π ∗ transition in azobenzene. A comparison of both the calculated absorption spectrum and excitation profiles with experiment has been made. From an analysis of the data, it is concluded that the Raman intensities are mainly due to resonance from the n −π ∗ transition and not from the pre-resonance of the π−π ∗ transition, as reported earlier. We find that the isomerization pathway is through the inversion mechanism rather than by rotation. This is the first direct spectroscopic evidence for the isomerization pathway in trans-azobenzene.

Structures, vibrational frequencies, and normal modes of substituted azo dyes: infrared, Raman, and density functional calculations

The concept and determination of inertial solvent reorganization energy for an intramolecular charge-transfer process are presented in this Letter. Resonance Raman RR spectra have been recorded for 4-nitro,4 -dimethylamino-azobenzene DA within the charge-transfer transition, in acetonitrile and benzonitrile. Mode-specific and solvent reorganization energies are determined by simulating the absorption and RR spectra. The results indicate that the vibrational reorganization energy for DA is comparable in the two solvents and the difference is observed mainly in the solvent reorganization energy that has been partitioned into inertial and reorientational components. The inertial response has greater contribution than its reorientational counterpart in acetonitrile, as expected, whereas the reverse trend is observed in benzonitrile.

Resonance Raman study of the solvent dynamics for ultrafast charge transfer transition in 4-nitro-4'-dimethylamino-azobenzene

In the present investigation, it is shown that use of commercial optical parametric oscillators (OPOs) has allowed for a simple method of determining the absolute Raman cross sections utilizing the measurement of integrated intensity ratios. Standard curves for the absolute Raman cross sections of the commonly used internal standards (viz., acetonitrile 918 cm−1 peak and nitrate 1045 cm−1 band) have been developed from the relative Raman intensities measured in the wavelength range 450–650 nm. The absolute intensities are fitted to the A-term frequency dependence and found to be consistent with the available literature data. We have demonstrated that, using an OPO, one can develop a standard curve for any internal standard, in order to accurately determine absolute Raman cross sections for a given system. This approach, in general, avoids the use of precalibrated lamps or external standard methods which are cumbersome, in comparison to measuring the integrated intensity changes with respect to an internal standard.

Study of solvent effects on the molecular structure and the reorganization energies of 4-nitro-4′-dimethylaminoazobenzene using resonance Raman intensities

We analyze the origin of de-enhancement for a number of vibrational modes in the 21Ag excited state oftrans-azobenzene. We have used the time-dependent wave packet analysis of the RR intensities by including the multimode damping effects in the calculation. This avoids the use of unrealistically large values for the damping parameter. It is concluded that the de-enhancement is caused by the interference between the two uncoupled electronic states, and that the intensities observed under the so-called symmetry forbidden 21Ag ← 11Ag transition are purely due to resonance excitation. It is also observed that the use of the time-dependent approach to study the de-enhancement effects caused by multiple electronic states on the RR intensities is not necessarily useful if one is interested in the structural dynamics.