Speed tunability of the excited-state intramolecular proton transfer process based on seven-membered ring pyrrole-indole H-bond systems

Abstract

A speed tunable excited-state intramolecular proton transfer (ESIPT) system featuring a seven-membered ring, -NH- type hydrogen bonding pyrrole-indole moiety based on ‘naked’ diazaborepins (NDABs) was exhibited. In this paper, the speed of ESIPT process can be tuned by the substituents on indole ring, including electron donating and withdrawing group for NDAB-H, NDAB-OMe, NDAB-COOEt, NDAB-NEt2, NDAB-CF3. It has explained by DFT and TD-DFT calculation in nonpolar toluene. The energy gaps between NS1 and TS1 (△E = ET-S1 − EN-S1) of most NDABs are slightly positive, except for the energy gap of NDAB-NEt2 with strong donating group on indole ring is negative. Meanwhile, the S1 state potential energy curve of NDABs with a small energy barrier is in the order NDAB-NEt2 < NDAB-OMe ~ NDAB-H < NDAB-COOEt < NDAB-CF3. Therefore, it indicates that ESIPT reaction could happen on the NDABs system. Strong withdrawing groups on indole ring of NDABs could restrain the ESIPT process and strong donating groups could promote the ESIPT process, which is in good agreement with the corresponding Stokes shift between absorption and emission spectra in N and/or T tautomer experimentally. The ESIPT process of NDABs in polar aprotic DMSO by the same calculation method also has been certified.