MOLECULAR PHOSPHORESCENCE ENHANCEMENT VIA TUNNELING THROUGH PROTON-TRANSFER POTENTIALS

A set of four substituted aminosalicylates, whose respective structures permit normal fluorescence, twisted intramolecular charge-transfer fluorescence, and proton-transfer fluorescence, in various combinations, were studied at 77 K to observe the effect on intersystem crossing. It is shown that the molecular structures which are capable of intramolecular proton transfer exhibit greatly enhanced normal molecule phosphorescence. The excitation mechanism involves proton-tunneling S1 /\/\/\>→ S1′(PT) after primary excitation, enhanced intersystem crossing from the tautomer excited state S1′(PT) /\/\/\>→ T1′(PT), followed by reverse proton-transfer tunneling via the triplet potential T1′(PT) /\/\/\>→ T1. The competitive singlet-state excitation processes are delineated by picosecond transient absorption spectroscopy. The mechanism described requires the specfic electronic state energy ordering S0 < S0′ < T1 < T1′ < S1′ < S1 ... where the primes represent the states of the tautomer form of the molecule. © 1990 American Chemical Society.