Theoretical investigation of excited state charge and proton transfer mechanism for the novel 10‐methyl‐indolo[2,3‐a]‐indolo[2,3‐a′]acridone molecule

Abstract

A novel acridone derivative, 10‐methyl‐indolo[2,3‐a]‐indolo[2,3‐a′]acridone (13), was reported by Fang and coworkers [J. Org. Chem. 2019, 84, 3832] in a previous paper. Spectroscopic studies on excited‐state intramolecular proton transfer (ESIPT) of 13 were mentioned, while the mechanism of ESIPT for 13 is deficiency. In this present work, based on the time dependent density functional theory (TDDFT), we investigated the ESIPT mechanism of 13 theoretically. The primary bond lengths, bond angles and the infrared (IR) vibrational spectra involved in the formation of hydrogen bonds verified that the intramolecular hydrogen bond should be strengthened, which manifests the tendency of excited state proton transfer. The redistribution of charge resulting from photoexcitation process reveals the driving force for ESIPT process. According to the results of theoretical potential energy curves along N‐H coordinate, an about 14.825 kcal/mol barrier has been found in the S0 state. However, a low barrier of 6.708 kcal/mol was calculated in the S1 state, which demonstrates that the proton transfer process is more likely to occur in the excited state. In other words, the proton transfer was facilitated by photoexcitation. Particularly, the study about ESIPT mechanism of 13 should be helpful for further understanding property of acridone derivatives.

Sensing of fluoride anion based on desilylation and intramolecular charge transfer of 2‐[2‐(tert‐butyl‐diphenyl‐silanyloxy)‐phenyl]‐4,5‐diphenyl‐1H‐imidazole

“…The constructing potential energy curve of S 0 state is based on the shortening of F and Si bond length from 5.00 to 1.40 Å with fixing the step of 0.10 Å, which is a good manner to deal with the studies of fluorescence sensor. [ 37–45 ] It can be seen clearly that the formation of the intermediate (i.e., the complex between TBDI and fluoride anion) is endergonic about 7.613 kcal/mol in the ground state. And the fluoride anion interacts with TBDI sensor via hydrogen bonding interaction in the intermediate complex.…”

Efficient excited‐state intramolecular proton transfer in acridone derivatives—A case study of Paratrimerin C

“…Later, the dual fluorescence of the indole‐fused acridone was spectroscopically studied, showing keto and enol emissions at about 450 and 650 nm, respectively [69]. A theoretical investigation of the same system by TD‐DFT was reported by Yang et al one year later [70]. By studying the potential energy curves (PECs) along the NH coordinate, the authors concluded that the intramolecular PT (from NH indole to CO of the acridone) occurs at the excited state with a barrier of 6.7 kcal/mol.…”