title: pH Dependent Photodeprotection of Formaldehyde: Homolytic C-C Scission in Acidic Aqueous Solution versus Heterolytic C-C Scission in Basic Aqueous Solution.
abstract: The photodeprotection of formaldehyde was investigated for 3-(1-hydroxypropan-2-yl)benzophenone (3-HPBP) with ultrafast time-resolved spectroscopy. The femtosecond transient absorption results indicated the singlet excited state of 3-HPBP transformed efficiently into its triplet state by a fast intersystem crossing. In acidic (pH = 0) and basic (pH = 12.5) aqueous solutions, the triplet intermediate was a key precursor for the deprotection of formaldehyde via two different pathways. However, little photodeprotection was observed in neutral (pH = 7) aqueous solution where the triplet intermediate appeared to undergo a proton coupled electron transfer process to form a ketyl radical transient. The important benzylic biradical intermediates seen in the acidic and basic aqueous solutions were identified by time-resolved resonance Raman spectra whose vibrational frequency patterns were consistent with DFT calculation results for the benzylic biradical intermediate. The results here indicate that the β-carbon alcohol group of the triplet state 3-HPBP is deprotonated in basic aqueous solutions and this leads to a heterolytic C-C bond cleavage to deprotect formaldehyde and produce the benzylic carbanion triplet state species, whereas protonation of the carbonyl moiety of the triplet state 3-HPBP leads to direct generation of a benzylic biradical intermediate and the deprotection of formaldehyde in acidic aqueous solutions via a homolytic C-C bond cleavage.
authors.affiliation: Department of Chemistry, The University of Hong Kong , Pokfulam Road, Hong Kong S.A.R., P. R. China.
authors.forename: Mingyue
authors.initials: M
authors.lastname: Liu
authors.affiliation: School of Pharmacy, Anhui Medical University , Hefei 230001, P. R. China.
authors.forename: Wenjian
authors.initials: W
authors.lastname: Tang
authors.affiliation: Department of Chemistry, The University of Hong Kong , Pokfulam Road, Hong Kong S.A.R., P. R. China.|Department of Chemistry, Shantou University , Guangdong 515063, P. R. China.
authors.forename: Ming-De
authors.initials: MD
authors.lastname: Li
authors.affiliation: Department of Chemistry, The University of Hong Kong , Pokfulam Road, Hong Kong S.A.R., P. R. China.
authors.forename: David Lee
authors.identifier: 0000-0002-8606-8780
authors.initials: DL
authors.lastname: Phillips