A theoretical study on water-assisted excited state double proton transfer process in substituted 2,7-diazaindole-H2O complex

Abstract

The substituted effect on the first excited-state proton transfer (ESPT) process in 2,7-diazaindole-H 2 O (2,7-DAI-H 2 O) complex in water was studied in detail at the TD-M06-2X/6-311 + G(d, p) level. The frontier molecular orbital, geometries, reaction mechanism and energies of ESPT process with different substituent have been analyzed. ESPT process in the title complex occurred concertedly but highly asynchronously no matter of the electronic nature of substituent. The absorption and fluorescence peaks, H-bond distances, asynchronicity of ESPT and barrier height were affected by the substituent. The Hammett's substituent constant had linear correlation with the difference between the sum of N 1 −O 11 and O 11 −N 7 distances in the reactant and that in the TS and Mulliken charge of H 3 O + .

Theoretical Insights into Excited-State Intermolecular Proton Transfers of 2,7-Diazaindole in Water Using a Microsolvation Approach

“…Frontier molecular orbitals (MOs) for each species [normal form, intermediary structure (IS), and tautomer form] along the PT coordinates and their simulated normal absorption and tautomer emission spectra of all complexes were analyzed to show the nature of the charge distribution upon photoexcitation. Moreover, potential energy curves (PECs) along the PT coordinates of the N form in S 0 and S 1 states for all 2,7-DAI­(H 2 O) n complexes were scanned by fixing the covalent bond of N1–H at varying values with a step of 0.05 Å to give PT barriers, and this approach has been used in similar systems. ,, The reaction energy (Δ E ) of all 2,7-DAI complexes was computed to give the thermodynamic information. Furthermore, the strength of two intermolecular H-bonds corresponding to the ESInterPT process, including the N1–H bond of 2,7-DAI and the O–H bond of water nearest to N7– of 2,7-DAI­(H 2 O) n complexes (one hydration shell), was compared to those of two hydration shells using their corresponding calculated infrared (IR) vibrational spectra in S 0 and S 1 states.…”

“…Moreover, potential energy curves (PECs) along the PT coordinates of the N form in S 0 and S 1 states for all 2,7-DAI(H 2 O) n complexes were scanned by fixing the covalent bond of N1−H at varying values with a step of 0.05 Å to give PT barriers, and this approach has been used in similar systems. 39,40,65 The reaction energy (ΔE) of all 2,7-DAI complexes was computed to give the thermodynamic ) n complexes (one hydration shell), was compared to those of two hydration shells using their corresponding calculated infrared (IR) vibrational spectra in S 0 and S 1 states. These two intermolecular H-bond strengths were further supported by their topology analysis at bond critical points (BCP), which can be calculated by using Espinosa's equation…”