Ultrafast Excited State Dynamics Of Heteroatom Substituted Polycyclic Compound

Excited state dynamics of molecules can reveal the photophysical and photochemical processes after the molecules were excited by absorbing a photon.Usually,excited state processes include vibration relaxation,internal conversion,intersystem crossing,charge transfer,proton transfer and so on.These processes cab relate with important function in the field of chemistry,material science and biology.Therefore,it is important to understand those reactions in depth and to reveal the relationship between heteroatoms and its excited state dynamics.In this dissertation,we study the excited state dynamics of 8-azaguanine（8-AG）,xanthones derivatives（CY-B）and spiro-conjugated molecules（CY-1）by time-resolved spectroscopy technique in order to understand the key factors that determine the excited state dynamics in three kinds of heteroatom substituted polycyclic compound system.At the same time,the relationship between the molecular structure of ketones and excited state dynamics was analyzed.Firstly,we studied the excited state relaxation mechanism of the two-ring heterocyclic molecule 8-azaguanine（8-AG）in solution and explained the unique fluorescence emission of8-AG.In neutral 9H-8-AG,there are two excited state relaxation pathways to reach the conical intersection between the excited state（S₁）and the ground state（S₀）and this leads to the ultrafast excited state lifetimes.For 8H-8-AG,these two pathways are suppressed and it returns to the ground state mainly through radiation transition.For the first time,we show that the larger Stokes shift in 8H-8-AG fluorescence is originated from the 8H-8-AG water complex.Next,we studied the excited state dynamics of the three-ring anthrone derivative CY-B.Although the structural of CY-B is similar to anthrone,its absorption wavelength is close to the ultraviolet region.With the help of femtosecond transient absorption spectroscopy,effective intersystem crossing is observed in CY-B and the excited state relaxation mechanism is proposed.Finally,excited state dynamics of the spiro-conjugated molecule CY-1 composed of two polycyclic aromatic ketones were studied.This molecule have two aromatic ketone units which are connected by a sp³ hybrid spiro carbon atom.This structure guarantee the perpendicular geometry of the upper and lower halves in the molecule,which leads to small coupling between them.Through the steady-state and time-resolved spectra of CY-1 in different solvents,it is confirmed that there are two emissive states in this compound.One is a locally excited state and the other is the charge transfer state.At the same time,intersystem crossing can also take place in CY-1,which can be used as a potential triplet sensitizer.In summary,this paper studied the excited state dynamics of heteroatom-substituted polycyclic compounds,expounds the mechanism of the influence of heteroatoms on the photophysical and photochemical properties of polycyclic compounds.At the same time,it can be observed that as the molecular conjugate system gradually increases,the absorption spectrum of the molecule gradually shifts to the visible light region,which is beneficial to the application of the molecule as a photosensitizer in more fields.