| dc.description.abstract | Chemical reactions can be stimulated by the light absorbed by molecules. One of the simple examples is the cis-trans photoisomerization of stilbene. We have studied the photochemistry of trans-stilbene and its derivatives, p-trans-aminostilbene, m-trans-aminostilbene and o-trans-aminostilbene, using first principle density functional theory and time-dependent density functional theory. Molecular geometries of ground state (So) and first excited singlet state (S1), UV-vis spectra, fluorescence spectra as well as the potential energy surfaces of photoisomerization were calculated.
The molecular geometries of S1 state are differential from these of S0 state, in particular, the ethylene and amine groups. The ethylene groups own the characteristics of alternated single-double bonds at So state; however, at S1 state, these bonds have obvious characteristics of partial double bonds. The pyramidalization of amine group at So state decreases or even completely vanishes at S1 state. In general, the π-electrons on trans-stilbene and trans-aminostilbene at S1 state are more delocalization than at So state.
The calculated UV-vis spectra are good agreement with experimental observations; moreover, the calculated UV-vis spectra are similar for m-trans-aminostilbene and o-trans-aminostilbene rotamers. Similar results are observed for the fluorescence spectra of m-trans-aminostilbene rotamers; furthermore, they also own similar activation energies (Ea) of cis-trans isomerization. More interestingly, the difference of emission wavelengths of o-trans-aminostilbene rotamers is up to 57 nm with one of them close to the experimental result. In addition, the activation energies of cis-trans isomerization for o-trans-aminostilbene rotamers are discrete, one with lower Ea can have fast isomerization and have weak fluorescencing and vice versa. Taking these results together, the single fluorescence of m-trans-aminostilbene from experiments is contributed from its two rotatmers simultaneously; in contrast, only one of m-trans-aminostilbene rotamers with stronger fluorescencing dominates the experimental observations. | en_US |