| The photochemical behavior of isolated heterocyclic molecules in the gas phase provides an ideal model system for us to understand the complicated excited-state relaxation kinetics of some important molecules,such as imidazole、pyrrole and other molecules,which are the light-absorbing groups of some pigments,aromatic amino acids,and DNA bases.The study of photoprotection mechanism of these macromolecules under ultraviolet light has been a hot topic for decades,that due to containing N heterocyclic molecular is widespread in nature,these individual molecules are often seen as manageable models for experimental and theoretical research,by studying these small molecular models,to promote the development of step by step toward larger molecules.2-hydroxypyridine is a medium-sized N-heterocyclic molecule withπand n electron systems.Its isomer is 2-pyridone.The tautomerization is achieved by intramolecular proton transfer between oxygen and nitrogen atoms in the molecule.The UV-induced kinetics of 2-hydroxypyridine provides a model system that can be used to help us study more complex biomolecules.In this paper,we investigate the ultrafast dynamics of the excited states of 2-hydropyridine and 2-pyridone using femtosecond time-resolved photoelectron velocity imaging combined with a pump/probe scheme.Eight ultraviolet wavelengths were selected as the pump light in the wavelength range of 250-276.9 nm,and the detection light wavelength was fixed at 240.1 nm.The excited state of 2-hydroxypyridine has been experimentally measured to have three lifetime components,the component1of the short lifetime of picosecond scale can be attributed to the time that wave packet from 1ππ*characteristics of the potential energy surface evolves to 1nπ*characteristics at the Franck-Condon region of the vertical excitation,the wave packet along a lower barrier left Franck-Condon region.The rate of decay does not vary significantly with wavelength.Based on the attribution ofτ1,we attributeτ2 to the time of the wave packet wave packet from single state S1of 1nπ*characteristics still transition to a lower triplet state T1（3ππ*）in the potential energy surface,with the increase of excitation energy lead to specific vibration modes overlap to improve the efficiency of ISC.Third lifetime component3 from the 180 ps under270 nm to 10 ps under 250 nm,it is attributable to that population in S2 state after wave packet movement to S1/S2 conical intersection region,and with the increase of excitation energy enhances the decay rate of S2 state.In addition,the dynamics of the excited states of2-pyridone molecules were investigated at the excitation wavelengths of 334.1 nm and 335.2nm.At the shorter excitation wavelength,an interesting quantum beat was superimposed on the transient signal of the lifetime component.We believe that this phenomenon may be related to the excitation of specific vibration modes.The work of this paper reveals the excited state relaxation kinetics of 2-hydroxypyridine and its tautomer.The trajectory of the wave packet on the multi-dimensional potential energy surface is analyzed by real-time observation,it is strongly affected by the conical intersection.From the initial excited Franck-Condon region finally relaxed back to the ground state,2-hydroxypyridine molecules have three different lifetimes during the evolution of the excited state S1over time.Analysis of the kinetic processes provides an understanding of the photochemical behavior in the complex dynamics of radiationless decay,which may be universal in some larger biomolecules. |
