Theoretical Study On Two-photon Absorption Properties Of Aggregation-induced Emission Materials Based On Tetraphenylethylene

Two-photon absorption(TPA)is a third-order nonlinear optical phenomenon,which has broad applications in the fields of biology,chemistry,and materials science.Organic molecular materials with strong TPA intensity are widely used in the fields of optical limiting,frequency up-conversion lasing,two-photon photodynamic therapy and two-photon fluorescence microscopy.Therefore,exploring the potential and applications of organic TPA materials has become a hot research field.In recent years,various types of organic TPA molecules have been synthesized in experiments.At the same time,the relationship between the structure and property has been extensively studied in theory.However,many materials with large TPA cross-sections have reduced fluorescence quantum yield and fluorescence quenching in the aggregate state,which limits the TPA applications.Recently,aggregation-induced emission(AIE)materials and their applications have been widely investigated.Some molecular materials with TPA and AIE properties have been synthesized in experiments.In these molecules,the tetraphenylvinyl(TPE)or triphenylamine groups are often included.And also,some donors are attached on the TPE unit in order to enhance TPA intensity.For TPE group,there are several possible substitution positions and numbers of donor.Therefore,the effects of the donor position and number on TPA should be considered and the structure and property relationships should be analyzed.Based on the study,the TPA and AIE materials can be designed in theory.In this thesis,using the hybrid density functional theory,the TPA properties of a series of organic molecules containing TPE groups were studied theoretically.The effects of donor strength,position and number on TPA properties were explored.By changing the strength of the donor,the planarity and conjugated length of the molecules,several structures with TPA and AIE properties were designed and the structure and TPA properties was discussed.The main work of this paper can be divided into the following two parts: 1.Two-photon absorption properties of D-A-D type chromophoresThe TPA properties of a series of D-A-D type chromophores containing TPE and triphenylamine moieties have been investigated employing the density functional response theory in combination with the polarisable continuum model.Special emphases are placed on the effects of the strength,position and number of donors on TPA.Our calculations demonstrate that the TPA intensity enhances with the increase of the electron-donating ability of the substituents on the triphenylamine unit,which is consistent with experimental observations.It is shown that the position of donors has an important effect on TPA.The donor on the side position of TPE unit decreases the TPA cross section,thereby more donors could not necessarily produce stronger TPA.The cis-trans isomerism effect is also explored and it is found that all the cis structures have lower TPA intensities than those of corresponding trans forms.By considering a different number of donors on TPE and triphenylamine unit respectively,it is interesting to find that the structure with one terminal donor on TPE and two donors on triphenylamine unit has the largest TPA cross section.The few-state model and the natural bond orbital charge analysis are used to explore the reason behind it.2.Two-photon absorption properties of D-?-A type chromophoresThe two-photon absorption properties of a series of D-?-A organic molecules containing tetraphenylethylene and cyano groups were studied employing the density functional response theory in combination with the polarizable continuum model.The effect of donor position and number on TPA properties was investigated.In addition,by increasing the planarity and conjugated length of the molecule,as well as enhancing the strength of the electron donor,we designed three molecular structures and calculated their TPA properties.The results show that the position of the donor has an important influence in the intensity of TPA.The methoxy donor at the end of the molecule can increase the TPA intensity effectively.As the number of substituents increases,the positions of the TPA peaks shift red.The effect of adding electron donor groups on different side positions has a significant difference in the TPA properties.Compared with the experimental molecules,the TPA cross sections of the designed molecules are greatly enhanced.When the TPE group is replaced by the triphenylamine group,the TPA peak is greatly red-shifted,and the TPA intensity is significantly increased.Since these molecules contain TPE or triphenylamine groups,they could have AIE properties.