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 chromop hores. 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 fluores cence spectroscopy and laser flash transient absorption spectroscopy. Evidence is presented for charge transfer quenching of pyrazoline S2 i n all compounds and for enhanced intersystem crossing from pyrazoline S1 specifically in II. Significant fluorescence is observed from III o nly 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.