Theoretical Study On Two-photon Absorption Properties Of Organic Molecules

In recent years, a lot of studies in both theoretical and experimental ways have focusedon materials that have strong two-photon absorption (TPA) properties. The materials withlarger TPA cross sections have good potential applications in three-dimensional optical datastorage, two-photon fluorescence microscopy, micro-fabrication, up-converted lasing, opticallimiting and optical biology fields. The present studies aim to find and synthesize materialswith strong TPA property.The organic conjugated molecules have attracted one's great attention, becausethey have the multiformity of structure and can be easily cut out. For push-pullcompounds, results indicate that TPA cross sections of organic conjugated molecules arerelated to not only the molecule length, the property of the πcenter, and the strength of thesubstitutions, but also the substitutions' symmetry, molecular dimension, and so on.According to properties of the push-pull electrons, the substitutions are partitioned into thetwo kinds of donor (D) and acceptor (A). A symmetrical molecule means one in which thetwo substitutions at the two ends are either donors or acceptors. An asymmetrical moleculemeans one that has a donor and an acceptor. This work aims to theoretically study the effectsof the molecular structures on the optical properties, and to investigate thestructure-property-relation of molecules.First, using few-state model and DFT method, we calculate the nonlinear opticalproperties of three Benzothiazolyl-derivatives newly synthesized. The three molecules havetypical A-π-A structure. We use density function theory to optimize geometry structure of theorganic molecules and calculate the molecular transition dipole moments and the energy ofexcitation state. The finite field method is used to calculate difference of dipole moment of theground state and excited state. The charge distribution density of ground state and CT state iscalculated. The total work is done on GAUSSIAN-98 program package. The basis sets weused is 6-31G.. The charger-transferred processes for the charge-transferred states aredisplayed by MOLEKEL. The calculation results show that when delocalization of the πelectrons in the organic molecules is increased, the maximum TPA peaks take place inred-shift way. When the conjugated chain is relative long, the increasing size of conjugatedchain has larger contribution on TPA cross sections than the changing of pull-electron base.This kind of molecule has a better TPA characteristic.The nonlinear optical properties of newly synthesized organic compounds have beeninvestigated by use of the response field method at density functional theory level. The twomolecules have typical D-π-D structure. We use the response field method at densityfunctional theory level to investigate their two-photon absorption properties. The calculatedresult of TPA cross sections with response theory at density functional theory level is differentfrom that at HF level, which shows that the electron correlated energy is important for theaccuracy of computing. This type of molecules is shown to have strong TPA cross-sections.The molecule that has stronger donor possesses smaller two-photon absorption cross section,indicating that the two-photon absorption of molecules is also related to the molecular planebesides the strength of donor and acceptor for pulling and pushing electrons. It is furtherindicated that the molecular plane has an important influence on TPA cross section.