Abstract:

The gas-phase spectroscopic properties of 6-cyanoindole (6-CNI) were investigated using mass-selected one-color resonant two-photon ionization (R2PI), UV–UV hole-burning, and IR-dip spectroscopy to elucidate its excited-state characteristics. The observed spectra were analyzed with ab initio and density functional theory (DFT) calculations and compared with previously reported results for 5-cyanoindole (5-CNI), providing insight into the structural and electronic variations induced by cyano substitution. The R2PI spectrum exhibited sharp vibronic features in the low-frequency region and pronounced spectral congestion at higher frequencies, indicative of closely spaced excited electronic states. Time-dependent DFT calculations reproduced these spectral trends, confirming the dominant π–π* character of the lowest electronic transitions. A smaller S₁–S₂ energy gap was obtained for 6-CNI (~0.085 eV) compared with that of 5-CNI (~0.11 eV), accounting for  the earlier onset of vibronic congestion in 6-CNI. These findings reveal how the position of the cyano substituent modulates the electronic distribution and excited-state dynamics of indole, establishing a foundation for understanding substitution effects in indole-based chromophores.