Excited state structure of isolated 2-cyanoindole and the binary 2-cyanoindole-(H2O)1 cluster from a combined Franck-Condon and rotational constants fit

Laser induced fluorescence and fluorescence emission spectra of 2-cyanoindole and the binary 2-cyanoindole-(H2O)(1) cluster in a molecular beam were analyzed using a combination of Franck-Condon (FC) fits of the emission spectra and ab initio calculations. The structural changes upon electronic excitation to the lowest excited singlet state have been elucidated by a combined rotational constants/vibronic intensities fit. The experimentally determined geometry changes and the ab initio calculations point to an L-b-like excited state for the monomer. FC fits of emission spectra of the binary water cluster and quantum chemical calculations show that the emission of the water cluster also takes place from an L-b-like excited state structure. However, for 2-cyanoindole-(H2O)(1) the second highest occupied molecular orbital mixes strongly with the highest one. Therefore, the difference between L-a and L-b characteristics are smaller than for other substituted indoles, including the other positional conformers of the n-cyanoindoles. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The laser induced fluorescence and fluorescence emission spectra of 2-cyanoindole and its binary cluster with water were analyzed through a combination of experimental fits and computational calculations. It was found that both the monomer and water cluster emission occur from an L-b-like excited state structure. However, for 2-cyanoindole-(H2O)(1), the mixing of molecular orbitals results in smaller differences between L-a and L-b characteristics compared to other substituted indoles.