要旨
The nature of the lowest energy triplet state (T1) ofp-nitroaniline (PNA), N,N-dimethyl-p-nitroaniline (DMPNA) and nitrobenzene (NB) is reexamined using the semiempirical CNDO/S-CI method with selected parameter options. The present results indicate that in the case of the unperturbed molecules the short-axis polarized π*←n(π) triplet largely localized at the acceptor end of the molecule may lie lower in energy than the triplet manifold counterpart of the intense intramolecular charge-transfer D+→A- singlet excitation. Computations suggest, however, that polar solvents strongly stabilize the PNA and DMPNA π*←π charge-transfer triplet relative to other excitations, whereas specific solvent hydrogen-bonded interactions stabilize the π*←n(σ) triplet of NB below those of π*←π character. These assignments allow a rationalization of phosphorescence lifetime data, Tn ←T1 absorption measurements and relative photochemical behavior.
