Theoretical Study On Charge Transfer Process Of Two Photon Excited States

Two-photon absorption(TPA)is a third-order nonlinear optical process in which a molecule absorb two photons at the same time,and undergo an intermediate state from ground state to excited state.Compared with traditional one-photon absorption(OPA),the main difference between OPA and TPA is that the intensity of TPA is proportional to the square of the incident light intensity,having the advantages of deeper penetration and higher spatial resolution.These advantages of TPA molecules promote their wide application in physics,chemistry,biology,medicine,materials and other fields,including two-photon fluorescence microscopy,three-dimensional(3D)optical data storage,non-destructive biological tissue imaging,photodynamic therapy and dye-sensitised solar cells,etc.These applications often feature large two-photon absorption cross-section,therefore,it is very important to design two-photon absorption molecules with large two-photon absorption cross-section.The molecular structure is a critical factor that influences the charge transfer in the two-photon absorption process,therefore,a clear understanding of the influence of molecular structure on the charge transfer represents the basis for obtaining large two-photon absorption cross-sections.We utilized density functional theory(DFT)to study the properties one-and two-photon absorption of A-?-D-?-A dye molecules and two chiral molecular systems.The influence of the charge transfer in one-and two-photon transitions was studied by varying the strength of the electron donor and the orientation of chromophores in the molecules.In order to study the characteristics of intramolecular charge transfer in the two-photon absorption process in depth,based on the three state model,the intermediate state hidden in the two-photon absorption process is determined by calculating the integral value of the transition dipole moment in the two-photon transition process,and then the charge transfer in the two-photon transition process was analyzed in detail.This paper mainly introduces the following two works:1.One-and two-photon absorption properties of quadrupolar A-?-D-?-A dyes with donors of varying strengthsBased on density functional theory and time-dependent density functional theory(TD-DFT),the one-and two-photon absorption properties of A-?-D-?-A dye molecules with different donors were studied.Carbazole and triphenylamine were selected as the electron donors while diphenylsulfone was used as the electron acceptor because of its strong electron affinity.The primary outcomes of our investigation on those two quadrupolar A-?-D-?-A dyes result in three significant conclusions: 1)enhancing the electron-donating ability of the central donor can significantly promote the redshift of OPA and TPA peaks;2)The intramolecular charge transfer increases along with the electron-donating ability of the central donor;3)increasing the ability of the push-pull electrons of the central substituents thus increases the intramolecular charge transfer and is a more efficient method to enhance the molecular.2D and 3D visualization methods of the electronic transition between excited states is a valuable tool to study charge transport properties in the two-photon transition process,which can deepen a understanding of charge transport properties in two-photon transition process.2.Unraveling the detailed mechanism of TPA and ECD about two novel brominated azaphilones: Bromophilones A and BUnder the SMD solvent model,the one-and two-photon absorption characteristics of two novel brominated azaphilones molecules(bromophilone A and bromophilone B)in DMSO solvent were calculated,and the effects of molecular chirality on one and two-photon absorption were analyzed in detail.Through the analysis of the electronic transition process in the two-photon transition of bromophilone A and B,we can find that for the same two-photon absorption state,the conversion of molecular chirality can change the mode of charge transition.The electronic circular dichroism(ECD)spectra of bromophilone A and B measured in the previous report are opposite at 431 nm.The ECD spectra of bromophilone A and B derived from our theoretical calculation and simulation are in good agreement with the experiments.The electric-magnetic interaction of the corresponding excited states was studied using visualization method.Finally,we propose the underlying mechanism responsible for the opposite ECD spectrum orientation at 431 nm.