Photoconversion of β-Lapachone to α-Lapachone via a Protonation-Assisted Singlet Excited State Pathway in Aqueous Solution: A Time-Resolved Spectroscopic Study

The photophysical and photochemical reactions of beta-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, beta-lapachone underwent an efficient intersystem crossing to form the triplet state of beta-lapachone. However, in water-rich solutions, the singlet state of beta-lapachone was predominantly quenched by the photoinduced protonation of the carbonyl group at the beta position (O-9). After protonation, a series of fast reaction steps occurred to eventually generate the triplet state alpha-lapachone intermediate. This triplet state of alpha-lapachone then underwent intersystem crossing to produce the ground singlet state of alpha-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 beta-lapachone to alpha-lapachone. beta-Lapachone can also be converted to alpha-lapachone in the ground state when a strong acid is added to an aqueous solution. Our investigation indicates that beta-lapachone can be converted to alpha-lapachone by photoconversion in aqueous solutions by a protonation-assisted singlet excited state reaction or by an acid-assisted ground state reaction.