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Rotationally resolved UV spectroscopy of the rotamers of indole-4-carboxylic acid: Evidence for charge transfer quenching.

ABSTRACT: Rotationally resolved electronic spectra of two conformational isomers of jet-cooled indole-4-carboxylic acid (I4CA) and the deuterated forms of the acid (-COOD) and amide (-ND) groups have been obtained using a UV laser/molecular beam spectrometer. The in-plane orientation of the acid group defines the two lowest energy rotamers of I4CA. The S₁ ← S₀ origin bands of the two rotamers and four isotopologues have been fit to asymmetric rotor Hamiltonians in both electronic states. From the best-fit parameters, the positions of the H-atoms in the principal axis frames of each conformer have been determined and serve to unambiguously identify the syn forms (i.e., COH⋯O) of the cis and trans rotamers. The experimental S₀ and S₁ inertial parameters, hydrogen atom positions, and transition dipole moment (TDM) orientations are compared with the results of theoretical calculations. The TDM orientation indicates that the S₁ state is the ¹L_a state in contrast to most substituted indoles. The molecular orbital properties and natural charges are investigated to better understand the ¹L_a/¹L_b state reversal and the extent of photoinduced intramolecular charge transfer that impacts the rotamer-dependent fluorescence lifetimes.

SUBMITTER: Yi JT

PROVIDER: S-EPMC8230874 | biostudies-literature |

REPOSITORIES: biostudies-literature

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