Photophysics of <i>meta</i>- and <i>para</i>-nitrophenolate. I. The role of configuration, microsolvation, and solvation on the charge transfer transition

The structure of the first five singlet excited electronic states of meta-nitrophenolate (m-NP) and para-nitrophenolate (p-NP) is investigated here in the gas phase, micro-solvent, and bulk solvent environments. The first excited state (S1) of m-NP was found to be a charge transfer state in the gas phase, whereas that of p-NP is a ππ* state. A stronger coupling between the donor and acceptor molecular orbitals in p-NP leads to a higher excitation energy of the S1 state as compared to m-NP. An examination of the topography of the potential energy surfaces reveals low energy conical intersections of the S1 state with the S2 and S3 states in p-NP. Such conical intersections in m-NP are located at higher energies relative to the S1 state minimum. Repulsion between the S1(1A') and S2(1A″) electronic states via the out-of-plane -NO2 torsional mode leads to a symmetrical double well topography of the S1 state of m-NP. Present results establish that while a single solvent molecule like water, methanol, and acetonitrile leads to a blue shift in the first absorption band maxima of m-NP, the electronic localization on the donor molecular orbital due to a single water molecule yields a red shift in the first absorption band of p-NP. However, the attachment of a single water molecule to the nitro site of p-NP facilitates electronic delocalization, leading to a blue shift in the S1 state excitation energy. As a result, in an aqueous medium, the first absorption band maximum was observed very near to that found in bare p-NP.