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.

Time-Resolved Spectroscopic Study of N,N-Di(4-bromo)nitrenium Ions in Selected Solutions

“…The fs-TA and ns-TA experiments were performed by following the same experimental apparatuses and methods that have been detailed previously [19,20,21]. In detail, fs-TA measurements were done using a femtosecond regenerative amplified Ti:sapphire laser system (Sarasota, FL, USA), in which the amplifier was seeded with the 120 fs laser pulses from an oscillator laser system.…”

“…A Lorentzian function with a 10 cm −1 band width for the vibrational frequencies and a scaling factor of 0.975 were employed for comparison with experimental results [23,24]. More details could refer to our previous works [19,20]. The Gaussian 09 program suite [25] installed in HKU Research Computing facilities was employed.…”

Time-Resolved Spectroscopic Study of N,N–Di(4–bromo)nitrenium Ions in Acidic Aqueous Solution

“…As discussed above, both the radical cation intermediate 3 and the dication intermediate 4 are possible intermediates after photoexcitation of 1 in the acidic mixed aqueous solution. Therefore, DFT computations were employed to predict the normal Raman spectra of the radical cation 3 and the dication 4, and these predicted spectra were compared with the experimental TR 3 spectrum observed at 50 µs in Figure 3 [22,23,24,25,26,27]. Examination of Figure 3 shows that the calculated Raman spectrum of the dication species 4 (Figure 3B) exhibits significantly better agreement with the experimental TR 3 spectrum (Figure 3A) in terms of the vibrational frequency patterns of both spectra.…”

Time-Resolved Spectroscopic and Density Functional Theory Investigation of the Photogeneration of a Bifunctional Quinone Methide in Neutral and Basic Aqueous Solutions

“…The experiments here used the same experimental setups and methods detailed in previous work [24,29].…”