A DFT/MRCI study on the excited state charge transfer states of N-pyrrolobenzene, N-pyrrolobenzonitrile and 4-N,N-dimethylaminobenzonitrile

Abstract

The ground and excited state properties for the electron donor–acceptor systems N-pyrrolobenzene (PB), N-pyrrolobenzonitrile (PBN) and 4-N,N-dimethylaminobenzonitrile (DMABN) are calculated using the DFT formalism for the ground state and a DFT/MRCI approach for the excited states. Solvent effects are roughly approximated by the Onsager model. A good agreement with experimental data is obtained for the ground state geometry, rotation barriers, and vertical excitation energies. PB fluoresces from an intramolecular charge transfer (ICT) state only in polar solvents whereas PBN shows ICT emission even in an apolar environment. A charge separated state can be formed in BP and BPN for all benzene–pyrrolo rotation angles due to a node at the contributing pyrrolo nitrogen molecular orbital (MO). This ICT state is stabilized for larger twist angles in polar solvent and for PB also in the gas phase. Thus, the red-shifted ICT band of all systems is assigned to emission out of a twisted ICT state. The normal fluorescence of locally excited (LE) character originates from the S₁ minimum at planar geometry. The dual fluorescence of DMABN in slightly polar solvents can be explained by geometrical relaxation with increasing rotation but also the formation of a planar ICT state cannot be ruled out.