Infrared spectroscopy of jet-cooled, electronically excited clusters of Coumarin 151: excited-state interactions and conformational relaxation

Infrared-optical double-resonance measurements have been made for electronically excited, hydrogen-bonded clusters involving Coumarin 151 molecules under jet-cooled conditions. Two main differences are reported between the ground and excited states: 1. Infrared resonances attributed to "donor"-OH and -NH bonds, as well as the symmetric-stretch modes of non-hydrogen-bonded NH2 tend to downshift in energy and are usually intensified in the excited state. Also, "donor"-NH groups tend to develop an additional strong infrared resonance in the excited state, which is observed for clusters of C151 with methanol, ethanol, di-isopropyl ether, and for the "B" conformers of the water n = 1, 2 and acetone n = 1 clusters. The effect evidently requires no activation energy, since it is observed following electronic-origin excitation. A similar effect is also observed in the doubly-hydrogen-bonded dimers of some aminophthalimide molecules. This result is discussed in terms of possible excited-state proton transfer and Fermi resonance interactions. 2. Several clusters have shown changes in the infrared spectra consistent with vibronically-induced conformational rearrangement. The driving force is the relative stabilization of the NH2 proton-donor site in the excited state and the formation of a strong NH...O hydrogen bond. The cases noted here are C151/(H2O)(2), C120/(H2O)(2) and C151/((CH3)(2)CO)(1). One surprising observation here is that the activation energy for the conformational relaxation C151/(H2O)(2) is measured to be only 60cm(-1), and yet the displacement of the water dimer within the cluster exceeds 7Angstrom. (C) 2002 Elsevier Science B.V. All rights reserved.