Optical Nonlinearity Of Organic Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons(PAHs)Shows great potential in organic light emitting diodes(OLEDS)and field-effect transistors(FETs)due to their excellent capability of electron delocalization and fluorescence.However,the nonlinear optical(NLO)properties of PAHs are rarely studied systematically.It is of great importance to investigate the NLO properties of PAHs of different chemical construction not only for deepening the knowledge and understanding of NLO but also for providing theoretical and experimental support for the development of new optical nonlinear materials.In this dissertation,two isomeric pyrene based PAHs were studied.Linear characterization was performed by ultraviolet absorption spectrum and fluorescence emission spectrum.Optimized structures along with frontier molecular orbital distributions was obtained by the quantum chemical calculation of density functional theory(DFT).By means of z-scanning technique,the molecules were systematically studied by using a tunable femtosecond laser at different energies and wavelengths.Both molecules exhibit wide band reverse saturation absorption and are caused only by two photon absorption(TPA).On this basis,the corresponding physical parameters were obtained by numerical fitting.The effects of planar properties of molecules on their nonlinear properties are preliminarily analyzed.Finally,the femtosecond transient absorption technique is used to study the internal excited states and the transition law of energy levels.The energy level model and rate equation of molecules were determined and the energy level lifetime were fitted and given.The difference on NLO properties of the two molecules is caused by the effect of intermediate state resonance enhancement(ISRE)and the related energy level model is given.This not only shows the excellent NLO properties of PAHs,but also provides a reference for the further optimization of organic materials.