Theoretical Investigation of the Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of Cresol Derivatives

Abstract

The novel photodeamination process of cresol derivatives 1 and 3 has been reported experimentally ( J. Org. Chem . 2015 , 80 , 10817 ). However, a full theoretical interpretation of the mechanism is still lacking. In the present study, we aim to provide insight into the factors that promote the deamination reaction through density functional theory (DFT) and time-dependent DFT methods. Calculated absorption and emission spectra are in good agreement with the experimental results. Hydrogen-bond strengthening in the excited state has been verified by analyzing relevant bond parameters and vibrational frequencies as well as frontier molecular orbitals (FMOs), implying that hydrogen-bond interaction acts as the important parameter for the excited-state intramolecular proton-transfer (ESIPT) reaction. The proton-transfer and deamination reactions have been qualitatively analyzed through Gibbs free-energy reaction profiles in different electronic states. It can be concluded that the ESIPT and photodeamination reactions occur in the excited state. To further illustrate the photodeamination mechanism, the constructed 2D potential-energy surface indicates that the photodeamination reaction is infeasible without the ESIPT reaction. This work provides the first theoretical rationale for ESIPT-induced photodeamination occurring spontaneously because of protonation of a basic nitrogen atom.

Formation of Quinone Methides by Ultrafast Photodeamination: A Spectroscopic and Computational Study

“…Li et al recently computed the potential energy surfaces for the formation of QM 9 from 5a at the PCM­(water)/TD-O3LYP/6-311+G­(2d,p) level of theory . The reaction pathway involved the relaxed 5a (S 1 ) and zwitterionic intermediate 7 .…”

“…It should be stressed that amines 5a and 6a and the corresponding salts 5 and 6 react quite differently, and this finding was explained by excited state intramolecular proton transfer (ESIPT) that takes place exclusively in amines . Furthermore, computations performed by Li et al indicate that ESIPT takes place first, giving zwitterion 7 , which then undergoes deamination . We previously proposed plausible pathways for the photodeamination from 5a and 5 and suggested the involvement of the zwitterionic intermediate 7 and the cation 8 , respectively .…”

“…This higher reaction efficiency has implications for practical QM applications; photodeamination has previously been successfully utilized in biological systems. , In the continued effort to gain more understanding of the mechanistic pathways responsible for QM formation, we experimentally studied the deamination reactions of simple cresol derivatives 5 , 5a , 6 , and 6a (Scheme ). Additionally, Li et al recently conducted a theoretical study also aiming to elucidate the deamination mechanism for amines 5a and 6a . In our approach, we chose cresol as a substrate primarily due to synthetic reasons, since the methyl group prevents the substitution by the phenolic OH at the para -position.…”

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

“…15 The most commonly used photochemical method to generate QMs in biological systems is photodeamination of suitably substituted phenols. 16 The mechanism of photodeamination has therefore been investigated by a variety of approaches, including nanosecond laser flash photolysis (LFP), 17 computations, 18 and femtosecond transient absorption (fs-TA) spectroscopy. 19 The major discovery was that photodeamination takes place in an adiabatic ultrafast photochemical reaction on the S 1 potential energy surface, delivering QMs in the excited state and without any detectable intermediate en route.…”

A TD‐DFT investigation of the photo‐induced excited state intramolecular proton transfer dynamics for the novel 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) system

“…Because of the four‐level cycle, ESIPT systems usually own the large Stoke shift, which generally could be as large as 6000 to 12 000 cm −1 . Also, this large Stoke shift can also diminish self‐absorption, and they are expected to become potentially intrinsic luminescent materials, chemical and biological as well as cell sensing, optoelectronics, molecular switches, and so on …”