Excited-State Dynamics of Nitrated Push-Pull Molecules: The Importance of the Relative Energy of the Singlet and Triplet Manifolds

We present a study of the dynamics following photoexcitation in the first electronic band of NO2-para-substituted nitronaphthalenes. Our main goal was to determine the interplay between the nitro group, electron-donating substituents, and the solvent in defining the relative excited-state energies and their photoinduced pathways. We studied 4-nitro-1-naphthylaminc and 1-methoxy-4-nitronaphthalene in solution samples through femtosecond fluorescence up-conversion and transient absorption techniques. In all solvents, both compounds have ultrafast fluorescence decays, showing that, similarly to the parent compound 1-nitronaphthalene, these molecules have highly efficient S-1 decay channels. The evolution of the transient absorption signals in the visible region reveals that for the methoxy-substituted Compound, independently of solvent polarity, the photophysical pathways are the same as in 1-nitronaphthalene, namely, ultrafast intersystem crossing to an upper triplet state (receiver T-n state) followed by relaxation into the lowest energy phosphorescent triplet T-1. In contrast, for the amino-substituted nitronaphthalene, the excited-state evolution shows a strong solvent dependence: In nonpolar solvents, the same type of intersystem crossing through an upper receiver triplet state dictates the photochemistry. However, in methanol, where the first singlet excited state shows an important solvent-induced stabilization, we observed typical signals of the repopulation of the electronic ground state in the time scale of less than 1 ps followed by vibrational cooling within S-0. Excited-state calculations at the time-dependent density functional level with the PBEO functional give an approximate characterization of the states involved and appear to correlate well with the experimental results as they show that the S-1 state of the amino compound is stabilized with respect to upper triplet states only in the polar solvent. These findings sustain and illustrate the recent view that the intersystem crossing channel so prevalent in nitroaromatic compounds is related to an energy coincidence between the pi-pi* first singlet excited state and upper triplet states with n-pi* character. Our results indicate through direct observations that if the S-1 state is sufficiently stabilized, other rapid decay channels like internal conversion to the ground state will minimize the transfer of population to the triplet manifold.