title: Photoconversion of β-Lapachone to α-Lapachone via a Protonation-Assisted Singlet Excited State Pathway in Aqueous Solution: A Time-Resolved Spectroscopic Study.
abstract: The photophysical and photochemical reactions of β-lapachone were studied using femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy techniques and density functional theory calculations. In acetonitrile, β-lapachone underwent an efficient intersystem crossing to form the triplet state of β-lapachone. However, in water-rich solutions, the singlet state of β-lapachone was predominantly quenched by the photoinduced protonation of the carbonyl group at the β position (O9). After protonation, a series of fast reaction steps occurred to eventually generate the triplet state α-lapachone intermediate. This triplet state of α-lapachone then underwent intersystem crossing to produce the ground singlet state of α-lapachone as the final product. 1,2-Naphthoquinone is examined in acetonitrile and water solutions in order to elucidate the important roles that water and the pyran ring play during the photoconversion from β-lapachone to α-lapachone. β-Lapachone can also be converted to α-lapachone in the ground state when a strong acid is added to an aqueous solution. Our investigation indicates that β-lapachone can be converted to α-lapachone by photoconversion in aqueous solutions by a protonation-assisted singlet excited state reaction or by an acid-assisted ground state reaction.
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Lili
authors.initials: L
authors.lastname: Du
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Ming-De
authors.initials: MD
authors.lastname: Li
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Yanfeng
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: ‡Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China.
authors.forename: Jiadan
authors.initials: J
authors.lastname: Xue
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Xiting
authors.initials: X
authors.lastname: Zhang
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Ruixue
authors.initials: R
authors.lastname: Zhu
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Shun Cheung
authors.initials: SC
authors.lastname: Cheng
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: Xuechen
authors.initials: X
authors.lastname: Li
authors.affiliation: †Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R., China.
authors.forename: David Lee
authors.initials: DL
authors.lastname: Phillips
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D009285:Naphthoquinones / Q000737:chemistry*; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory; D012996:Solutions / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D000097:Acetonitriles; D009285:Naphthoquinones; D011522:Protons; D012996:Solutions; C014638:beta-lapachone; C040756:1,2-naphthoquinone; C415043:alpha-lapachone; C032159:acetonitrile