Research On Regulation Of Nonlinear Absorption Of Organic Conjugated Small Molecules By Local Excitation

The optical nonlinearity of organic conjugated materials is strongly related to microstructure and electronic excitation.Investigation the influence of material microscopic properties on ultrafast nonlinear optical properties not only contribute to understand the interaction process between laser and materials but also contribute to design high nonlinearity materials through nonlinear optical experiments and theoretical calculations.Polycyclic aromatic hydrocarbons(PAHs)derivatives and sulfur-containing heterocyclic compounds are typical organic conjugated materials with abundant π-electrons and excellent charge transport properties,they have promising applications in the fields of optical limiting,all-optical switching and two-photon fluorescence imaging.However,there is still a significant space for improvement in the optoelectronic performance of these materials,especially the ultrafast broadband nonlinear absorption need further optimization.This thesis investigates the regulation of the electronic excitation and configuration on the ultrafast nonlinear absorption and photophysical processes of several PAHs and sulfur-containing heterocyclic small molecules,with using Z-scan technology,optical limiting devices and timeresolved transient absorption spectra(TAS),combined with the quantum chemical calculations,provide a guidance for the development and design of novel organic optoelectronic materials.The main research includes the following sections:Two pyrene chalcone derivatives B1(pyrene as the donor)and B2(pyrene as the acceptor)with opposite charge transfer are designed and synthesized by using the push-pull electronic effect of peripheral substituents to modulate the electronic excitation,investigate the regulation of the broadband nonlinear absorption by local excitation and charge transfer.Femtosecond Z-scan experiments and theoretical calculations show that local excitation on the pyrene contributes to the enhancement of the two-photon absorption(TPA)of B1 at 600 nm and 650 nm,and that charge transfer to the pyrene group contributes to the enhancement of the two-photon resonance of B2 at 700-900 nm.TAS investigate the excited state properties of two pyrene derivatives and reveal the relaxation dynamics from the locally excited state to the charge transfer state.Optical limiting experiments indicate that they are excellent ultrafast broadband optical limiting materials.In order to extend the mechanism of local excitation enhancing TPA found in pyrene molecules to similar systems,investigate the ultrafast optical nonlinearity of two novel naphthanthryl chalcone derivatives NAn-1 and NAn-2 with different local excitation strength.The results of femtosecond Z-scan experiments and theoretical calculations suggest that NAn-2 has a larger transition dipole moment and spatial delocalization,and the strong local excitation on the anthracene group is the main reason for its TPA cross section increasement about three times than NAn-1.TAS show that NAn-2 has a shorter charge transfer process(1 ps)and excited state lifetime(20 ps),which is related to the modulation of the charge transport properties by the terminal substituent.Finally,a strategy for the construction of two-photon absorption enhancement of PAHs chalcone small molecules is proposed.The push-pull effect can be used to flexibly adjust the local excitation on PAHs groups to achieve the modulation of the nonlinear absorption of the material.Two novel benzothiazole hydrazone derivatives BTH-1 and BTH-2,are designed and synthesized,BTH-1 connected with twisted triphenylamine to enhanced charge transfer and BTH-2 connected with planar trithiophene to enhanced local excitation.Femtosecond Z-scan experiments reveal that the TPA cross section of planarized BTH-2 is approximately 2-3 times larger than that of torsional BTH-1 at 600-900 nm.Theoretical analysis suggests that the local excitation promoted by planar trithiophenes is an effective mechanism for enhancing the nonlinear absorption.The physical process of intensity-dependent nonlinear absorption transition of BTH-2 under different pulse width(fs,ps,ns)is discussed and the saturable intensity and the reverse saturable absorption absorption coefficient are obtained by numerical fitting.The broadband excited state absorption properties of benzothiazole hydrazone derivatives are investigated using TAS at 450-1075 nm.It is found that planarized BTH-2 has significantly enhanced of excited state absorption under near infrared band and twisted BTH-1 has a faster intramolecular charge transfer process.The research on the benzothiazole hydrazone derivatives indicates that the electronic excitation properties and nonlinear optical response are influenced by molecular configuration,the large structural differences could introduce other nonlinear variables.A set of trithiophene chalcone positional isomers with various torsion angles(I1-6°,I2-25°,I3-37°,I4-60°)are designed and synthesised,in order to clarify the effect of π system torsion on microscopic electronic and macroscopic optical nonlinearity.Self-defocusing effects and conformationdependent reverse saturable absorption of the four isomers at 600 and 700 nm are investigate used open and closed-aperture Z-scan experiments,and the nonlinear absorption coefficient of the near-planar I1 molecule is 2-3 times larger than that of I4.Theoretical analysis shows that manipulation of molecular torsion by heterotopic substitution can modulate molecular spatial delocalization and prompt nonlinear absorption.Combining TAS and rigid potential energy surface curves investigate the effect of ground state configuration on the excited state dynamics of isomers,the results show that the planarized structure contributes to accelerate the relaxation of excited particles.