Time-Dependent Density Functional Theoretical Investigation of Photoinduced Excited-State Intramolecular Dual Proton Transfer in Diformyl Dipyrromethanes

Abstract

In recent research [ Chem. Commun. 2014 , 50 , 8667 ], it was found that photoinduced enolization occurred in 1,9-diformyl-5,5-diaryldipyrromethane (DA) by excited-state dual proton transfer resulting in a red-shifted absorption, a phenomena not observed in 1,9-diformyl-5,5-dimethyldipyrromethane (DM) and 1,9-diformyl-5-aryldipyrromethane (MA). The observation was supported by preliminary density functional theoretical (DFT) calculations. In the work reported here, a detailed and systematic study was undertaken considering four molecules, 1,9-diformyldipyrromethane (DH), DM, MA, and DA and their rotational isomers using DFT methods. Different processes, namely, cis-trans isomerization and single and double proton transfer processes, and their mechanistic details were investigated in the ground and excited states. From the simulation studies, it was seen that the presence of different substituents at the meso carbon does not affect the λ values during cis → trans isomerization. However, enolization by proton transfer processes were found to be influenced by the substituents, as seen in the experiments. Enolization was observed to follow a stepwise mechanism, that is, diketo → monoenol → dienol. While monoenols showed negligible substituent effects on the λ values, a large red shift in λ was seen only in DA, in agreement with the experimental findings. This observation can be attributed to the lowering of the keto → enol activation barrier, stabilization of DA in the S state, and the charge transfer nature of the transitions involved in DA.

Time-Dependent Density Functional Theory Investigation of Excited State Intramolecular Proton Transfer in Tris(2-hydroxyphenyl)triazasumanene

“…34−41 Pradhan et al reported the TDDFT calculations of the mechanistic excited state dual PT via enolization of diformyl-dipyrromethanes through a stepwise process influenced by the substituents. 35 Another TDDFT calculation from Zhao et al revealed that the excited state double-PT mechanism of pigment alkannin involved a stepwise process, where a polar aprotic solvent affected the first PT step. 36 We have been interested in the chemistry of "buckybowls", which are curved aromatic compounds comprising fullerene fragments, and their heteroatom-doped derivatives.…”

“…However, ESIPT is quite a susceptible process, highly dependent on the external conditions such as the solvents and temperature, as well as the intrinsic natures of the compounds including the strength of the hydrogen bond. − In particular, the ESIPT in the solid and the solution states are different, so that the comprehensive computational study is highly demanded for the effective design of ESIPT molecules. ,,, Time-dependent density functional theory (TDDFT) is a suitable computational method for more in-depth understanding of the mechanistic detail and electronic effect of the ESIPT process. , Recently, the mechanistic details and electronic effects of multiple excited PT process by TDDFT were highly studied. Nevertheless, reports on the intramolecular dynamic processes are still few. − Pradhan et al reported the TDDFT calculations of the mechanistic excited state dual PT via enolization of diformyl-dipyrromethanes through a stepwise process influenced by the substituents . Another TDDFT calculation from Zhao et al revealed that the excited state double-PT mechanism of pigment alkannin involved a stepwise process, where a polar aprotic solvent affected the first PT step …”

Photoinduced Irreversible Intramolecular Proton Transfer of Arnebinones B, D, and E: The Case of Photoenolization at the p-Benzoquinone-CH2/CH-π System

“…The two reported classical photoinduced keto–enol tautomerism reactions are excited-state intramolecular proton transfer − (ESIPT, Scheme A) and photoenolization , (Scheme B). The occurrence of ESIPT requires the assistance of an intramolecular hydrogen bond, − and photoenolization also involves a spatially adjacent proton donor and proton acceptor. However, the proton transfers in these two types of photoreactions are reversible cyclic processes, and the ketone and enol forms cannot be separated due to the ultrafast time scale in their tautomeric equilibrium. − Exploration of the photoinduced irreversible intramolecular proton transfer between the ketone and enol that produces thermally stable and isolable products is a fascinating task.…”

The sensing mechanism studies of the fluorescent probes with electronically excited state calculations

“…It is particularly prevalent when water is used as the solvent, in order to consider the hydrogen bonding or coordination interactions between the probe and water molecules. In some studies, probe–solvent or analyte–solvent interactions have been investigated by including a few discrete solvent molecules . Furthermore, a hybrid quantum mechanics/molecular mechanics (QM/MM) approach has also been used to describe probe–analyte–solvent systems, whereby the probe or probe–analyte complex is treated by QM and the solvent is treated by MM.…”

“…In some studies, probe-solvent or analyte-solvent interactions have been investigated by including a few discrete solvent molecules. 87 Furthermore, a hybrid quantum mechanics/molecular mechanics (QM/MM) approach has also been used to describe probe-analyte-solvent systems, 43,79,88 whereby the probe or probe-analyte complex is treated by QM and the solvent is treated by MM.…”