Infrared-Enhanced Fluorescence-Gain Spectroscopy: Conformation-Specific Excited-State Infrared Spectra of Alkylbenzenes

摘要

An ultraviolet infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S-1 origin levels of different conformational isomers. The shorter lifetimes of these IR-excited molecules, combined with their red-shifted emission, provides a way to discriminate the fluorescence due to the infrared-excited molecules from the S, origin fluorescence, resulting in spectra with high signal-to-noise ratios. Spectra for a series of phenylalkanes and a capped phenylalanine derivative (AcPhe-NHMe) demonstrate the potential of the method. The excited-state spectrum in the alkyl CH stretch region of ethylbenzene is well-fit by an anharmonic model developed for the ground electronic state, which explicitly takes into account stretch bend Fermi resonance.