UC Santa Barbara

Excited state intramolecular proton transfer (ESIPT). To elucidate the mechanism of ESIPT and reaction barrier of tunneling mechanism in indigo, a combination of laser desorption, jet cooling, resonance enhanced multiphoton ionization (REMPI), time-of-flight mass spectrometry is employed to produce vibronically resolved spectra of deuterium substituted indigo. The picosecond two color REMPI spectrum of indigo displayed a red shift and peak splitting upon deuterium substitution on and above the barrier, as well as no shift or blue shift below the barrier, where the barrier corresponds to the separation of the proton transfer and hydrogen transfer forms of the keto-enol tautomer in the S1 state. In pump probe measurements, deuterium substitution induces a longer excited state lifetime with a kh/kd ratio of 2.5, indicating that tunnel effect takes place in the ESIPT of the non-deuterated system.