EXCITED-STATE PROTON-TRANSFER IN 1-NAPHTHOL AMMONIA CLUSTERS

Excited-state proton transfer dynamics are reported for the 1-naphthol(NH3)n cluster system for n = 3 and 4. Picosecond time- and mass-resolved pump (S1 <-- S0)-probe (I <-- S1) experiments demonstrate the following results: (1) excited-state proton transfer occurs for n = 3 and 4 clusters only; (2) for n = 5 clusters the proton is transferred in the ground state and for n = 2 clusters no proton transfer can be observed; (3) the proton transfer time in the n = 3 cluster at the 0(0)0 transition is ca. 60 ps; (4) this time is reduced to ca. 40 ps and ca. 10 ps for 800 and 1400 cm-1 of vibrational energy in S1, respectively; (5) for the n = 4 clusters these times are approximately 70, 70, and 30 ps, for 0, 800, and 1400 cm-1 of vibrational energy in S1, respectively; (6) both n = 3 and 4 clusters exhibit a second low-amplitude decay component, which is about an order of magnitude slower than the initial decay; and (7) 1-naphthol-d1(ND3)n clusters have a greatly reduced rate constant for the excited-state proton transfer dynamics. These observations are well fit and explained by a simple statistical/barrier penetration model involving proton tunneling and the effect of van der Waals vibrations on the height and width of the barrier to proton transfer.