On the possible role of a mixed valence–Rydberg state in the fluorescence of indoles

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Murrell’s localized orbital model is applied to indole, considered as composed of three fragments: benzene, ethylene, and a ≳NH group. In addition to the usual locally excited and charge-transfer states, locally excited 3s and 3pπ Rydberg states of the ≳NH group have been introduced and the mixed valence–Rydberg (3dπ) character of the locally excited state of the ethylene fragment considered. The model predicts a pure Rydberg 3s←1A transition at 46 kK, polarized perpendicular to the molecular plane, which may explain the disappearance, in condensed media, of a shoulder (at 42.4 kK) present in the vapor phase absorption spectrum of indole. The 3dπ orbital confers a long range character to the 1La excited state. Experiment unambiguously shows that ’’exciplex’’ formation of indoles with polar solvents occurs only if the emission originates from this state. Change, in polar media, from the Franck–Condon to a solvated Rydberg state, is therefore postulated.