Time-resolved resonance Raman spectroscopic studies on photoinduced transients

Abstract

Transient absorption spectroscopy has been applied to find the relative yield of the radical cations for PVCz and PDBVCz. It has been found that the ratio of the relative yield of the radical cation for monomer to polymer depends on the structure of the polymer. NMR spectroscopy has been used to study the structure of these polymers qualitatively. The vibrational frequencies of the neutral unsubstituted carbazole and carbazole derivatives have been assigned. Further, the vibrational frequencies of radical cations of monomer, dimer and that of PVCz have been assigned. The structure of the monomer and dimer radical cation and radical cation in PVCz has been inferred from the Raman spectra. The monomer radical cation is shown to have a structure with a contribution from the electron-deficient ring, whereas the dimer radical cation has been shown to contain quinonoidal and benzenoidal structure. The evidence for the partial exchange of the odd electron in the dimer radical cation in the polymer chain with the neutral species has been shown from our Raman study. TR studies on the photogenerated radical cations of PVCz and its model compound NEC have been carried out. Density functional theoretical calculations have been carried out for theoretical calculation of vibrational frequencies. The vibrational modes of the radical cation and the neutral species have been assigned from the calculated PEDs. A comparison of the TR³ spectra of NEC radical cation with that of PVCz radical cation shows that the interaction of the radical cation with the neutral species in PVCz results in the shift in the vibrational frequency towards lower wavenumber in comparison with the model compound NEC. The higher reactivity of perfluorinated derivative of DFBP over BP has been addressed by TR³ spectroscopic investigation. The analyses of the results of time-resolved resonance Raman spectroscopic studies on triplet state and ketyl radical of DFBP show that the unpaired electron of the triplet state is localized on the carbonyl group. This localization of the spin is due to spin-destabilizing nature of the fluorine. It is concluded that the higher reactivity of DFBP over BP can be attributed to: (a) non-planar structure of the triplet excited state of DFBP than BP, thus increasing the accessibility to the CO group, and (b) highly polarized and electrophilic nature of the CO group in the triplet excited state.