Intersystem crossing (ISC) in DMABN and DMABA: Laser induced optoacoustic spectroscopy and semi-empirical (AM1) calculations

The individual contribution of the intersystem crossing (ISC) process in the total non-radiative deactivation ofthe excited fluorophores, p-N,N-dimethylaminobenzonitrile (DMABN) and p-N,N-dimethylaminobenzaldehyde(DMABA) was determined by laser-induced optoacoustic spectroscopy (LIOAS). It was noted that, for DMABN, the ISC yield is almost insensitive to the solvent polarity, while, it is highly solvent dependent for DMABA. Semi-empirical (AM1) calculations have been performed to optimize the ground state geometry of both the fluorophores. The AM1-SCI (singly excited configuration interaction) method has been adopted to get the energies and the dipole moments of their excited states. From the calculated oscillator strengths for the optical transitions and also from absorption studies in solvents of different polarity it is revealed that DMABN is excited to the S-2 state and DMABA to the S-3 state selectively with a laser beam of 320 nm.Theexperimental results for the large variation of the ISC yield (phi(ISC)) in DMABA has been ascribed to the reordering of the n pi* and pi pi* states with a change in the solvent polarity.