Theoretical Investigation On Photoelectric Properties Of Selected Polycyclic Conjugated Systems

Polycyclic conjugated systems have attracted considerable attention due to their potential applications in organic light emitting diodes(OLEDs),biological imaging and chemical sensors.A thorough understanding of electronic absorptions,emissions,and luminescence mechanisms of these polycyclic conjugated systems is highly required for the rational design of novel functional materials.Furthermore,the photoelectric properties of these polycyclic conjugated systems can be tuned through the structural and chemical modification.The discovery of thermally activated delayed fluorescence(TADF)and aggregation-induced emission(AIE)materials has greatly improved the luminous efficiency of fluorescent materials,and facilitated the practical application of these optical functional materials to undoped optoelectronic devices and biological imaging.Here,several kinds of polycyclic conjugated systems have been explored by extensive calculations.The main research contents include:(ⅰ)DFT and TD-DFT calculations have been used to investigate photoelectric properties of coronene(C24)and its chalcogenide analogues(POC,PSC,and PSeC),effects of epoxy substitutions,surface migration of oxygen-containing groups,and the excited-state nonadiabatic kinetics of C24 epoxides;(ⅱ)Based on QM/MM calculations,the packing effects on the light emission of naphthyridine-based luminophor and its aggregation-induced emission mechanisms,as well as the mechanisms of the aggregation-induced emission of two triphenylamine salicylaldehyde derivatives combained excited-state intramolecular transfer(ESIPT)and dimer emission have been discussed.The main results are as follows:1.Structural and spectroscopic properties,electronic transition characteristics,electron and hole transport properties of C24,persulfurated coronene(PSC)and its chalcogenide analogues(POC and PSeC)were studied by using DFT and TD-DFT methods.The computational results show that the electronic structure and spectral properties of C24 can be significantly affected by the chalcogenide substitution.Different from the delocalized π-type LUMO of C24,LUMO orbitals of PSC and PSeC are basically σ*-type orbitals,which may be ascribed to S-S and Se-Se bonds at the periphery.In addition,the low-lying states(S1～S15)of PSC cannot be populated through light excitation,and the peripheral S-S bonds may facilitate the internal conversion from Si to S0.Compared to the C24 dimer,the dimer of POC has larger effective electron transfer integrals,indicating that the oxygen atom substitution can effectively improve the electron transport performance of C24.The PSeC exihibits similar electronic structure and transition characteristics with PSC,while its dimer was predicted to have higher hole mobility.2.Electronic structures of C24 epoxides and PSC oxides,oxygen migration on the basal plane,absorption and emission properties,and nonadiabatic dynamics of the excited C24 epoxides have been systematically explored theoretically.The present results show that the edge-epoxide compounds were more stable energetically,and the applied external electric field can significantly reduce the diffusion barrier from center to edge for oxygen migration on the surface of the C24 and PSC oxides.More importantly,electronic absorptions and emissions of C24 epoxides strongly depend on the bonding position of the epoxy group and its density.Besides,the introduction of the epoxy group on the basal plane of C24 can activate the optically forbidden S1→S0 transition.The surface hopping dynamics simulations reveal that the non-radiative decay of the oxygen-centered C24a3 in the S1 state is much faster than that of the rimepoxide C24d3,which is caused by the relatively weak C-O bond in C24a3.In contrast to C24 epoxides,the spectroscopic properties of the PSC oxides are less influenced by the oxygen-containing group.3.The fluorescence quenching and aggregation induced delayed fluorescence of 2,7-di(9,9-dimethylacridin-10(9H)-yl)-1,8-naphthyridine(DMAC-ND),have been explored by using QM and QM/MM methods.Based on the different packing modes of DMAC-ND molecule in solution,crystal and amorphous phases,the morphology dependence on the luminescent properties has been studied.The calculated results show that the effective ISC can only occur in the aggregation configuration.The predicted krisc values indicate obvious morphology and temperature dependences,and aggregation and the increase of temperature can promote RISC.In addition,the RISC process in the crystal is more effective than that in the amorphous phase.Based on the predicted intermolecular interactions and Huang-Rhys factors,such remarkable packing effects on the luminescent properties of DMAC-ND can be ascribed into the strong intermolecular interactions and the restriction of low-frequency vibrations in the crystal and amorphous phases.4.Plausible mechanisms for the emission quenching in solution and AIE enhancement in the solid state of two triphenylamine salicylaldehyde derivatives,denoted as TS and TS-OMe,have been explored by QM and QM/MM calculations and MD simulations.The computational results indicate that the TS and TS-OMe monomers may undergo the ESIPT process,differing from the experimental speculation that the weak emission in solution only comes from the E*isomer.In addition,multiple possible nonradiative decay channels combined with the lower energy barriers access to the S1/S0 conical intersections for TS and TS-OMe are responsible for their emission quenching in acetonitrile solution.On the contrary,the access to the S1/S0 conical intersection is suppressed in crystal,owing to the steric restriction and intermolecular interaction.The dimer emission of the E*isomer for the TS and the ESIPT process to induce the K*isomer for the TS-OMe are responsible for the emission in their aggregation states.