Time-resolved spectroscopic and density functional theory investigation of the influence of the leaving group on the generation of a binol quinone methide

Quinone methides are important reactive intermediates in photobiology and photochemistry, with little known regarding the photo-induces generation of reactive quinone methides species from relevant precursors in aqueous solutions. Here, the time-resolved spectroscopy methods including the transient absorption experiments and time-resolved Resonance Raman were employed to directly study the photophysics and photochemical processes of 6,6′-Bis(hydroxymethyl)-1,1′-binaphthyl-2,2′-diol (BQMP-a) in neat MeCN and 1:1 MeCN:H2O aqueous solutions. In aqueous solution, the intramolecular excited state proton transfer (ESIPT) process was observed where the water molecules help deliver the proton on the hydroxyl group to the naphthol ring within 125 ps in the singlet excited state of BQMP-a. This solvent mediated ESIPT will then lead to the leaving of the H2O group in 906 ps followed by the generation of the binol quinone methide (BQM-a) intermediate with a lifetime of 1.7 μs. As a result, the BQMP-a provides a different production rate of quinone methides compared with the 1,1′-(2,2′-Dihydroxy-1,1′-binaphthyl-6,6′-diyl) bis(N,N,N-trimethylmethanaminium) bromide system, which indicates the leaving group is an important factor in the generation of the reactive DNA alkylated intermediates.