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Ultrafast Dynamics of the Excited States of 1-(p-Nitrophenyl)-2-(hydroxymethyl)pyrrolidine

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journal contribution
posted on 08.03.2012, 00:00 by Rajib Ghosh, Dipak K. Palit
The dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)­pyrrolidine (p-NPP) has been investigated using the subpicosecond transient absorption spectroscopic technique in different kinds of solvents. Following photoexcitation using 400 nm light, conformational relaxation via twisting of the nitro group, internal conversion (IC) and the intersystem crossing (ISC) processes have been established to be the three major relaxation pathways responsible for the ultrafast deactivation of the excited singlet (S1) state. Although the nitro-twisting process has been observed in all kinds of solvents, the relative probability of the occurrence of the other two processes has been found to be extremely sensitive to solvent polarity, because of alteration of the relative energies of the S1 and the triplet (Tn) states. In the solvents of lower polarity, the ISC is predominant over the IC process, because of near isoenergeticity of the S1(ππ*) and T3(nπ*) states. On the other hand, in the solvents of very large polarity, the energy of the S1(ππ*) state becomes lower than those of both the T3(nπ*) and T2(nπ*/ππ*) states, but those of the T1(ππ*) state and the IC process to the ground electronic (S0) state are predominant over the ISC, and hence the triplet yield is nearly negligible. However, in the solvents of medium polarity, the S1 and T2 states become isoenergetic and the deactivation of the S1 state is directed to both the IC and ISC channels. In the solvents of low and medium polarity, following the ISC process, the excited states undergo IC, vibrational relaxation, and solvation in the triplet manifold. On the other hand, following the IC process in the Franck–Condon region of the S0 state, the vibrationally hot molecules with the twisted nitro group subsequently undergo the reverse nitro-twisting process via dissipation of the excess vibrational energy to the solvent or vibrational cooling.