Molecular Symmetry Effect On 1,3,4-oxadiazole Based Intramolecular Charge-transfer Chromophores

Intramolecular charge transfer(ICT)is a process by which charge is transferred between different groups within a molecule.Organic small molecules with intramolecular charge transfer characteristic have wide applications in many fields,such as organic light-emitting diodes,dye-sensitized solar cells,organic fluorescence sensors and nonlinear optical materials.The main factor restricting the development of ICT compounds is that the fluorescence efficiency is relatively low.In order to meet the commercialization requirements of devices,people have been continuously exploring and developing organic conjugated small molecular materials with excellent ICT properties.As we know,the ICT characteristic and fluorescence quantum efficiency are highly dependent on the molecular structures.Therefore,it is necessary to study the molecular structure-property relationship for obtaining high-performance organic ICT conjugated small molecules.We have designed and synthesized three methoxyphenyl substituted 1,3,4-oxadiazole derivatives in different symmetry.The influence of the different molecular geometry on the ICT properties is investigated by varying molecular symmetry and the distribution of the push-pull electron groups.The result could provide guidance in the development of high performance organic luminescent materials.The main conclusions are as follows:1.Photophysical properties: In different polar solvents,the maximum absorption band of L-OXD is centered at 320 nm,for B-OXD is 290 nm,for T-OXD is 300 nm.All the absorption spectra show little red-shift on the increasing solvent polarity,indicating that the electronic and structural nature of the ground and Franck–Condon excited states does not change much.In contrast to the absorption spectra,a large solvatochromism of fluorescence spectra with the increasing polarity of solvents is observed,which can reflect the stronger polarity of the charge transfer excited state than the Franck–Condon excited state.The fluorescence spectra of L-OXD and B-OXD display 30~40 nm red-shift from CHEX to ACN.The significant fluorescence red-shift of T-OXD is ~100 nm from CHEX to ACN,even larger in DMSO(~133 nm),which is more obvious than that in B-OXD and L-OXD.Compared to the counterparts,the coplanar tri-branched structure of T-OXD leads to more pronounced solvatochromism effect on fluorescence spectra.2.Theoretical Study: Theoretical calculations at the density functional theory(DFT)and time-dependent density functional theory(TD-DFT)levels are carried out on these molecules.Firstly,the molecular geometry in the ground state and excited states are optimized,respectively.In the gas phase,S0 to S1 has the highest oscillator strength(f=1.74)in L-OXD and mainly involves HOMO-L+1 and H-1-LUMO orbital transitions.As for B-OXD,the mainly transition is also S0-S1(f=0.76),including HOMO?L+1 and H-1?LUMO orbital transitions.For T-OXD,the oscillator strength of S0-S1 is 0,while S0-S2 and S0-S3 are both 1.30,which means S0-S2 and S0-S3 are the highly allowed transitions for T-OXD.The allowed transitions involving multiple component orbital: H-2?LUMO,H-1?LUMO,HOMO?LUMO,HOMO?L+1 and HOMO?L+2.And the maps of difference in electron density upon vertical transition between the ground and excited states are calculated.In all of these transitions,the side methoxyphenyl group shows more negative part than the positive part,while the 1,3,4-oxadiazole ring and center benzene ring have more positive part than the negative part,indicating that the electron are transferred from the side methoxyphenyl group(Donor)to the central 1,3,4-oxadiazole and the benzene ring(Acceptor).The atomic dipole moment corrected Hirshfeld(ADCH)population method model is also used to describe the exhaustive charge transfer of the mainly allowed transitions for the chromophores.The charge variation sum of the transition between the ground state and the first excited state for L-OXD is 0.0821 |e|,and the charge variation sum of S0-S1 for B-OXD is 0.0810 |e|.As for T-OXD,the S2 and S3 transitions transfer more charge(0.1379|e|),corresponding to the result of the previously obtained spectrum-more pronounced solvatochromic effects.