UV wavelength-dependent photoionization quantum yields for the dark 1nπ* state of aqueous thymidine

Abstract

Despite the important role of the dark ¹nπ* state in the photostability of thymidine in aqueous solution, no detailed ultraviolet (UV) wavelength-dependent investigation of the ¹nπ* quantum yield (QY) in aqueous thymidine has been experimentally performed. Here, we investigate the wavelength-dependent photoemission spectra of aqueous thymidine from 266.7 to 240 nm using liquid-microjet photoelectron spectroscopy. Two observed ionization channels are assigned to resonant ionizations from ¹ππ* to the cationic ground state D₀ (π⁻¹) and ¹nπ* to the cationic excited state D₁ (n⁻¹). The weak ¹nπ* → D₁ ionization channel appears due to ultrafast ¹ππ* → ¹nπ* internal conversion within the pulse duration of ∼180 fs. The obtained ¹nπ* quantum yields exhibit a strong wavelength dependence, ranging from 0 to 0.27 ± 0.01, suggesting a hitherto uncharacterized ¹nπ* feature. The corresponding vertical ionization energies (VIEs) of D₀ and D₁ of aqueous thymidine are experimentally determined to be 8.47 ± 0.12 eV and 9.22 ± 0.29 eV, respectively. Our UV wavelength-dependent QYs might indicate that different structural critical points to connect the multidimensional ¹ππ*/¹nπ* conical intersection seam onto the multidimensional potential energy surface of the ¹ππ* state might exist and determine the relaxation processes of aqueous thymidine upon UV excitation.