Vibrational spectroscopic characterization of the photochromic molecule α-dinitrobenzylpyridine bypolarization-resolved linear and nonlinear Raman scattering

Abstract

This thesis is about the vibrational spectroscopic characterization of a famous photochromic molecule, alpha-dinitrobenzylpyridine (alpha-DNBP), in its ground state. For this purpose, spontaneous Raman spectroscopy and the combination of time-resolved and polarization-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopies were employed to acquire vibrational spectra in fingerprint region. The vibrational modes assignment was carried out using results from the density-functional theory (DFT) calculations. During this study, the comparison between the DFT calculated and the experimental Raman spectra presented a discrepancy. The calculated spectrum predicted the two nitro modes at 1360cm-1 and 1379cm-1 as distinct peaks, but the spontaneous Raman and the conventional CARS spectroscopies were unable to resolve them despite the availability of sufficient instrumental spectral resolution. In order to experimentally verify the existence of the two modes, the difference in their DFT-calculated depolarization ratios was used as reference. Furthermore, when the depolarization ratio values for the spontaneous Raman spectra, acquired for two orthogonal polarization detection, were calculated around the spectral region of the two modes, they gave a clear hint for the existence of the two distinct modes at roughly 1347cm-1 and 1363cm-1. However, polarization-resolved spontaneous Raman spectroscopy could not resolve them. In this thesis, the freedom of polarization control offered by the CARS configuration was then utilized for this purpose. A novel polarization-controlled CARS spectroscopy scheme, which is slightly different from the conventional polarization-resolved CARS spectroscopy, was employed. In this scheme, the angel of the polarization of the analyzer can be varied to suppress the CARS peak of one mode over the other. This thesis successfully demonstrates the fully resolved two nitro peaks at 1347cm-1 and 1363cm-1 in the acquired polarization-controlled CARS spectra and presents the vibrational characterization of alpha-DNBP in the ground state. First transient absorption measurements on thioxanthone were preformed to initiate the change from ground state studies to studies of the ultrafast transitions in excite states.