Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational study.

Previous studies have established that the major pathway for the first singlet excited state of 1-nitronaphthalene is intersystem crossing to the triplet manifold. In this contribution we present determinations of the decay of the S1 state of this compound in several solvents to establish the time scale of the multiplicity change as a function of the polarity and hydrogen-bonding ability of the solvent environment. The measurements were made with the femtosecond frequency up-conversion technique to follow the weak spontaneous molecular emission which precedes triplet formation. Our results show that in all environments the S1 lifetime is 100 fs or less, making 1-nitronaphthalene the organic compound with the fastest multiplicity change ever measured. We also show that the bathochromic shifts observed for the first absorption band imply changes in the relative energies of the singlet and triplet manifolds, which in turn manifest in a 2-fold increase of the fluorescence lifetime in cyclohexane compared with the polar solvents. Additionally, we performed excited-state calculations at the TD-DFT/ PBE0/6-311++G(d,p) level of theory with the PCM model for solvation. The TD-DFT theory identifies the presence of upper triplet states which can act as receiver states in this highly efficient photophysical pathway. Together, the experimental and theoretical results show that the dynamics of the S1 state in 1-nitronaphthalene represent an extreme manifestation of El-Sayed's rules due to a partial (n-pi*) character in the receiver triplets which are nearly isoenergetic with S1, determining a change in the molecular spin state within 100 fs.