Ab initio calculation of the excited states of nitropyrenes

The ubiquitous presence of nitro-derivatives of polycyclic aromatic hydrocarbons like pyrene in the environment is a source of preoccupation given the fact that many of them have been shown to be toxic, mutagenic and/or carcinogenic. An understanding of their photophysics and photochemistry can provide insight into the potential for their sunlight-induced photodegradation in the environment. In the present work, ab initio quantum chemical methods (MP2/def2-TZVP and ADC(2)/def2-TZVP) were employed to calculate the geometries of the three mononitro pyrenes, three of the dinitropyrenes and a trinitropyrene in the ground state and in the lowest excited singlet and triplet states. Absorption spectra predicted from the vertical excitation energies and oscillator strengths of the first 10 excited singlet states (ADC(2)/def2-TZVP/COSMO acetonitrile) compare favorably with the experimental spectra in acetonitrile and adiabatic triplet energies with values derived from phosphorescence spectra. Except for 2-nitropyrene, which was predicted to be planar in the ground (S0) and lowest excited singlet (S1) and triplet (T1) states, the nitro groups of the other compounds were not in the plane of the ring in S0 or T1, but one of the nitro groups was predicted to become coplanar with the ring in the optimized geometry of S1. The theoretical results are discussed in the context of their overall consistency with the experimentally observed photophysical properties of these compounds.