Tetraphenylpyrazine-based Aiegens And Their Structure-property Relationship

In 2001,our group termed a special photophysical phenomenon that the silole derivative is weakly emission in solution but induced to emit intensely upon aggregation as aggregation-induced emission(AIE).After more than ten years study,large amount of novel AIE systems have been developed.However,according to incomplete statistics,more than 80% of the work related to AIE is based on silole and tetraphenylethere(TPE)systems,and these two systems also suffer from some disadvantages.In 2015,we developed a new AIE system of tetraphenylpyrazine(TPP),which could be facilely prepared,easily modified and possess good thermo and photo stability.TPP is a kind of potential AIE “star” molecular system.Firstly,as to the low reaction yield of TPP derivatives reported before,the synthetic route is optimized,which greatly improve the yield of TPP derivatives.Based on this optimized route,we have obtained TPP-T,TPP-2T,T-TPP-T and TPP-4T by changing the number and linking positions of thiophene substituents to the TPP core,and all of them feature the aggregation-enhanced emission(AEE)characteristics,which embodies the universality of TPP as an AIE luminogen(AIEgen).TPP emit at a short wavelength,by attaching carbazole derivatives to the TPP core,we parpared TPP-C,TPP-PhC,TPP-2C,TPP-2PhC and TPP-Ph2 C,and they are all deep blue emiters.Their nondoped OLED devices were fabricated,among which the maximum external quantum efficiency and the CIE of TPP-C is 1.49% and(0.16,0.11)respectively,presenting the broad prospects of TPP derivatives to prepare deep blue AIE luminescent materials.Analysising the single crystals,we found that the pyrazine ring of TPP is a curved plane.Combined with the results of reorganization energy calculation,we can understand the reason why the TPP derivatives are relatively weak emission in solid state and their AIE properties are not that obvious: There are large numbers of high frequency bond stretching modes in the TPP derivatives both in solution and solid states,which account for a large proportion of the reorganization energy.In solution state,the low frequency rotation of each group and the high frequency stretching motions in the molecular comsume the excited state energy,making them weak emission.In solid state,the rotation modes were suppressed due to the physical constraint of the interaction of the moleculars,and the radiationless pathway were blocked,leading to the AEE property.However,the high frequency stretching motions cannot be blocked by the packing of the moleculars,leading to the existence of a large number of nonradiative transition channels in the aggregation state all the time,which making the weak AIE results.Based on the optimized structures in the ground and excited states of these TPP carbazole derivatives,conbined with their photophysical properties,we have obtained the luminescence mechanism of TPP derivatives: The luminescence of single substituted TPP derivatives originates from the groups which along to the axis of the substituent,and the groups that deviate from the axis have little influence on the luminescence(Mainly inhibit the excessive accumulation in the molecules).The luminescence of double substituted TPP derivatives can be regarded as the superposition of two single substituents.Finally,we realized the full color emission moleculars based on TPP for the first time.By adjusting the intramolecular D-A strength and molecular conjugated length,the benzene ring,diazosulfide,thiophene,thiophene dioxide groups were connected between two TPP molecules,and DTPP,DTPP-Ph,DTPP-BT,DTPP-T,DTPP-TO,DTPP-TBTT were obtained.Among these TPP derivatives,DTPP-BT has the mechanochromic property,and it can be used for the sensor to character the hole blocking capability of some materials in OLEDs.