Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives: Direct evidence for the ^1* state

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The hydride stretch infrared spectra of indole, indole-H₂O, 3-methyl indole, 3-methyl indole-H₂O, the main conformer of tryptamine (TRA), two conformers of N-acetyl tryptophan amide (NATA), and three conformers of N-acetyl tryptophan methyl amide (NATMA), have been recorded in the electronically excited singlet states using excited-state fluorescence-dip infrared spectroscopy. NATA and NATMA are methyl-capped dipeptides of tryptophan that have conformational flexibility and exhibit sensitivity in their electronic spectra to the conformation of the dipeptide backbone. In the indole monomer, the indole NH stretch fundamental at the S₁ origin is shifted from its ground-state value (3525 cm^–1) to 3478 cm^–1. The corresponding band in the indole-H₂O complex appears at 3387 cm^–1, shifted by a similar amount from its ground-state position (3436 cm^–1). Higher vibronic levels within 1500 cm^–1 of the S₁ origin, which have been identified previously [B. J. Fender et al., Chem. Phys. Lett. 239, 31 (1995)] as being ¹L_b or ¹L_a in character, all show similar excited state indole NH stretch absorptions. The corresponding spectra in 3-methyl indole, 3-methyl indole-H₂O, TRA, and in the C5 conformers of NATA and NATMA all are missing the indole NH stretch absorption. In its place, a broad background absorption appears, spread over the entire 2800-3800 cm^–1 region. In these molecules, other CH stretch or amide NH stretch absorptions remain sharp, appearing in their expected frequency ranges. Finally, the C7 conformations of NATA and NATMA, which possess an intramolecular hydrogen bond in the dipeptide backbone, have all infrared transitions washed out, replaced by a stronger broad background absorption. The entire data set can be explained by the presence of an excited ^1* state which is dissociative along the indole NH stretch coordinate, as recently predicted by Sobolewski and Domcke [Chem. Phys. Lett. 315, 293 (1999)]. In the weak coupling case (indole, indole-H₂O), the gap between the ^1* state and the S₁ origin is too large to be reached by infrared excitation. The selective loss of the indole NH stretch in the intermediate coupling case reflects the strong coupling of the ¹L_b state NH stretch (v = 1) level to the ^1* state, which is dissociative along the NH stretch coordinate. In the NATA and NATMA C7 conformers, an inversion of ordering of the electronic states occurs, pushing the ¹L_a state below the ¹L_b origin, and strengthening the coupling of all hydride stretch vibrational levels to the ^1* dissociative continuum. These results highlight the important influence of the conformation of the polypeptide backbone on the photophysics of tryptophan in polypeptides. ©2003 American Institute of Physics.