FLUORESCENCE LIFETIMES OF JET-COOLED CARBONYL-SUBSTITUTED INDOLES - EVIDENCE OF INTRAMOLECULAR CHARGE-TRANSFER QUENCHING

Fluorescence lifetime measurements were made for the S1 origins of a number of jet-cooled indole chromophores having carbonyl-containing substituents bonded directly to the chromophore with no spacing aliphatic carbons. The lifetimes are significantly shorter than those of compounds having carbonyl substituents spaced farther from the chromophore. 4-indole carboxylic acid and 5-indole carboxylic acid each exhibit two S1 conformer origins that have decidedly different lifetimes. These observations are consistent with the occurrence of fluorescence quenching due to charge transfer to the carbonyl. Further lifetime measurements with the foregoing compounds deuterated at the acidic sites rule out excited-state proton transfer from these sites as an important quenching mechanism.