Abstract
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 S{sub 1}{yields}S{sub 1}{prime}(PT) after primary excitation, enhanced intersystem crossing from the tautomer excited state S{sub 1}{prime}(PT){yields}T{sub 1}{prime}(PT), followed by reverse proton-transfer tunneling via triplet potential T{sub 1}{prime}(PT){yields}T{sub 1}. The competitive singlet-state excitation processes are delineated by picosecond transient absorption spectrometry. The mechanism described requires the specific electronic state energy ordering S{sub O} < S{sub O}{prime} < T{sub 1} < T{sub 1}{prime} < S{sub 1}{prime} < S{sub 1} {hor ellipsis} where the primes represent the states of the tautomer form of the molecule.
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