Excited state structure of isolated 4-cyanoindole from a combined Franck-Condon and rotational constants analysis†

Photoenhancement of the C≡N Stretching Vibration Intensity of Aromatic Nitriles

“…These results can be ascribed to photoexcitation-induced changes in the electronic structure of the molecule, especially changes in the property of the CN bond. It is has been shown that excitation to the lowest excited electronic state of 4-CNI results in the partial transfer of an electron from the pyrrole ring to the phenyl ring, which effectively increases the level of electronic conjugation between the nitrile group and the indole ring (hereafter termed CN– indole conjugation), leading to a lengthening of the CN bond . Because the intensity (rate) of a vibration transition is proportional to (dμ/d Q ) 2 , an increase in the CN bond length would increase the molar absorptivity of its stretching vibration, as observed in the study presented here.…”

“…This hypothesis is based on the notion that upon electronic excitation, the cyano moiety, which is a strong electron-withdrawing group (EWG), tends to facilitate (partial) charge migration within the aromatic ring(s), leading to lengthening of the CN bond. For example, Schmitt and co-workers − have shown, for several cyanoindoles, including 2-cyanoindole (2-CNI), 4-cyanoindole (4-CNI), and 5-cyanoindole (5-CNI), that the CN bond in the S 1 state is longer than that in the S 0 state and the C–C bond connecting the cyano group and the indole ring is shortened. Such a photoexcitation-induced electronic structure change is expected not only to shift ν CN to lower wavenumbers but also to significantly increase the magnitude of ε CN .…”

Tryptophan as a Template for Development of Visible Fluorescent Amino Acids

“…For example, Matsika and Abou-Hatab et al have shown, using high level quantum mechanical calculations, that the brighter (fluorescent) state of 4CN-indole is the 1 L b state in aqueous solution, whereas in the gas phase the 1 L a state is responsible for both the absorption and emission of 4CN-indole. Similarly, Schmitt and co-workers , have studied the electronic transition properties of 4CN-indole, using both experimental and theoretical methods, and concluded that unlike most other indole derivatives, the lowest excited singlet (S 1 ) state of 4CN-indole in the gas phase is mainly 1 L a -character and the corresponding transition dipole moment runs through positions 1 and 4 (Figure ). A similar conclusion was also reached for indole-4-carboxylic acid (4CO 2 H-indole) by Plusquellic and co-workers .…”

“…Transition dipole moment for excitation to the lowest excited singlet state of 4CN-indole. Reprinted with permission from ref . Copyright 2021 Elsevier.…”