J. Phys. Chem. 1986, 90, 6808-681 1
6808
Proton Transfer in Rare Gas Matrices and Ethanol Solutions of 2-Hydroxy-4,5-benzotropone Du-Jeon Jang,? Gerard0 A. Brucker, and David F. Kelley*f Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: June 27, 1986)
Excited-state proton transfer in 2-hydroxy-4,5-benzotropone(HBT) has been studied by steady-state and time-resolved emission spectroscopies. Time-resolved and line-width measurements indicate that the matrix-isolated HBT proton-transfer time is between 25 fs and 10 ps. The static HBT/ethanol spectra indicate the presence of different hydrogen-bonded conformations in solution. The time-resolved results indicate that these different conformations exhibit vastly different proton-transfer rates.
The rates and mechanisms of excited-state intramolecular proton transfer (ESIPT) have recently been subjects of great interest. Several molecular systems have been studied under a variety of conditions. These studies have shown that in many cases where ESIPT occurs, the reaction has little or no energetic bamer. For example, studies of methyl salicylate in molecular jets' and N e matrices2 indicate that the isolated molecule has no barrier to ESIPT. Several studies have also shown that the rates of ESIPT, and even the Occurrence of ESIPT, depend critically upon the nature of the solvation. For example, Itoh et aL3 have shown that 7hydroxyquinoline will undergo ESIPT only when complexed to two alcohols: presumably one hydrogen bonded to the nitrogen and the other hydrogen bonded to the alcohol. McMorrow and Kasha4 have recently shown that the nature of the hydrogenbonding environment plays a crucial role in the proton-transfer dynamics of 3-hydroxyflavone. From static spectroscopic studies, these authors infer the existence of several solventsolute hydrogen-bonding structures, with widely varying proton-transfer rates. Time-resolved and static spectroscopic studies have also shown that the strength of the solventsolute hydrogen bonding and the acidities and basicities of the alcohol and ketone moieties Timeresolved studies have large effects on the rates of indicate that the unsolvated, intramolecularly hydrogen-bonded ~ reform exhibits very rapid proton transfer (
