ROLE OF THE 5-ARYL MOIETY IN THE PHOTOPHYSICS OF TRIARYL-2-PYRAZOLINES

Computational studies on 1,3,5-triaryl-2-pyrazolines demonstrated the perpendicular orientation of the planes of the two independent chromophores. We investigated the photophysics of bichromophoric 1,3-diphenyl-5-aryl-2-pyrazolines (aryl substituent: phenyl (I), beta-naphthyl (II) and 9-anthracenyl (III) using steady state and time-resolved fluorescence spectroscopy and laser flash transient absorption spectroscopy. Evidence is presented for charge transfer quenching of pyrazoline S2 in all compounds and for enhanced intersystem crossing from pyrazoline S1 specifically in II. Significant fluorescence is observed from III only when the anthracene chromophore is excited directly. Quenching of pyrazoline excitation by the anthracene moiety leads to the anthracene triplet; we infer the intermediacy of a phantom intramolecular charge transfer (ICT) state.