Navigation

Recent Publications

More...

Home

Direct Spectroscopic Observation Photogenertaion Binol Quinone Methide via Water Assisted Singlet Excited State Proton Transfer

ailleen - Posted on 18 September 2015

Project Description: 

femtosecond and nanosecond transient absorption (fs-TA and ns-TA) have been used to investigate the early events taking place for p-binol quinone methide precursors (Scheme 1, BQMP-a and BQMP-b) after the photoexcitation. As far as we know, this is the first time that fs-TA and ns-TR3 experiments were performed on BQMP-a and BQMP-b in MeCN and aqueous solutions in order to investigate the excited states, intermediates and dynamics of reaction intermediates, which are involved in the photochemistry of BQMPs. In order to distinguish the important role of the bi-naphthol ring in the photochemistry, the photoreaction of the parent compound QMP-a (scheme 1) that does not contain another naphthol ring was also studied by time-resolved spectroscopy methods in both MeCN and aqueous solutions and compared to the results found for BQMP-a here. To help determine the geometries and vibrational spectra of the intermediate species, and to make assignment of the experimental vibrational bands, density functional theory (DFT) calculations were done using the B3LYP methods with a 6-311G** basis set for all of the species examined here. Our time-resolved spectroscopy results provide the direct evidence that water assisted singlet ESIPT to generate the binaphthoquinone methides from bi-naphthol derivatives in aqueous solutions.

Research Project Details
Project Duration: 
02/2013-05/08/2016
Project Significance: 
Considering the importance of quinone methides mentioned above, numerous investigations on photogeneration of p-quinone methide were described by Wan, Kresge Fereccero and Richard in the past decade. Unlike the o- quinone methide precusors, the excited state intramolecular proton transfer (ESIPT) processes of p-quinone methide precusors could hardly take place for the large distance between the phenol and hydroxyl group. Therefore, excited state proton transfer (ESPT) processes were proposed to explain the photogeneration of p-quinone methides. 1 It has been shown that the quaternary ammonium salts of phenollic Mannich bases (Scheme 1, -NMe3+I̵̶) and alcohol analogues (Scheme 1, -OH) are the efficient leaving groups for quinine methide precursors, among which the former are better and preferable. Although lots of studies have been reported on the generation of photogeneration of p-quinone methide, few works provide the direct evidence on the proton transfer process to the best of our knowledge.
Results Achieved: 
The present work reports fluorescence, fs-TA, ns-TA, ps-IR and ns-TR3 spectroscopic investigation of the photophysical and photochemical reactions of BQMPs in MeCN and aqueous solutions. For comparison, the photoreaction(s) of the parent compound QMP-a are also studied by fs-TA in both MeCN and aqueous solutions in order to help better understand the role of water and binaphthol ring in the photochemistry of BQMPs in aqueous solutions. ISC processes are observed for QMP-a in both MeCN and 1:1 MeCN:H2O mixed solutions with time constants of about 300 ps. In pure MeCN, after excited the precursor to the S1, only one obvious transition process is achieved. The IC process of BQMPs from S1 to S0 takes place in MeCN 369 ps for BQMP-a and 526 ps for BQMP-b by fluorescence and the low efficiency ISC process also take place to produce the triplet species (Scheme 4). In contrast, as displayed in Scheme 4, ESIPT takes place for the singlet excited state of BQMPs with the assistance of water in aqueous solutions. The water molecules allow a proton to leave the hydroxyl group (O1) and then attach a proton to the carbon of another naphthol ring (C3) with in 125 ps for a and 7.4 ps for b. This solvent mediated ESIPT will then lead to structural rearrangement and formation of 2 almost simultaneously. Then, the HX leaves also followed with structural rearrangement to form BQMs. Furthermore, the -NMe3+ group on BQMP-b also provide higher leaving rate (within 220 ps) than -OH for BQMQ-a (with in 906 ps). Therefore, BQMP-b is thought to be a better precursor for the generation of BQM to alkylate the DNA bases to shut down the DNA replication process, which will play an important role in the field of anti-cancer treatment in biology system.