Environmental photochemistry of nitro-PAHs: direct observation of ultrafast intersystem crossing in 1-nitropyrene

Femtosecond broadband transient absorption experiments of 1-nitropyrene, a nitro-polycyclic aromatic hydrocarbon of environmental concern are presented in cyclohexane and hexane solutions. The transient absorption spectra show the presence of three species that are assigned to the Franck-Condon excited lowest singlet (S1) state, the structurally relaxed S1 state, and the lowest excited triplet state. The spectral changes at early times are interpreted in terms of conformational dynamics; primarily due to an ultrafast rotation of the nitro group in the S1 state. This excited state relaxation is followed by intersystem crossing with a time constant of 7 ps. CIS/6-31G(d,p) calculations predict planarization of the nitro-aromatic torsional angle as the major nuclear relaxation coordinate, from 32.8 degrees at the HF/6-31G(d,p) level of theory in the ground state (27.46 degrees at B3LYP/6-31++G(d,p)) to 0.07 degrees in the S1 state. Vertical excitation energies at the TDDFT/6-31++G(d,p) and TDDFT/IEFPCM/6-31++G(d,p) levels of theory predict a small energy gap (<0.12 eV) between the S1(pipi*) state and the third excited triplet state T3(npi*) in the gas phase and in cyclohexane, respectively. The small energy gap suggests a large spin-orbit coupling between the S1(pipi*) and T3(npi*) states, which explains the ultrafast intersystem crossing of 1-nitropyrene in nonpolar solvents.