Synthesis,Characterization And Photoelectrical Properties Of Deep Red And Near-infrared Donor-Acceptor Fluorescence Molecules With Pyridine Thiadiazoles As Construction Primitives

Although organic electromechanical luminescent devices（OLED）have been widely used,it is still the theme to reduce the cost of materials and expand the luminous band（color field）.However,there is still a shortage of luminescent materials in the deep red to the near infrared region when compared with visible light,which has important applications in such fields as energy,communications,bioimaging,sensing and advanced optoelectronics.Among them,near-infrared imaging technology has great application space in the military and civilian fields,such as target search and tracking,remote temperature control and weather forecasting and so on.Almost 50%of the solar energy belongs to the NIR spectral region,which requires the study of NIR absorbing dyes and photovoltaic materials for higher solar energy conversion efficiency.Typical organic near-infrared materials fall into several broad categories such as metal complexes,ionic dyes,π-conjugated chromophores and charge-transfer chromophores.In recent years,research has focused on phosphorescent near-infrared materials and small-molecule near-infrared luminescent materials.In order to obtain higher luminous efficiency,organic near-infrared phosphorescent electroluminescent devices（PhOLEDs）based on Os,Ir or Pt complexes are attracting much attention due to the utilization of non-radiative triplet energies.Among them,the phosphorescent light-emitting devices based on Pt complexes achieved maximum external quantum efficiency（EQEs）of 24%in the light color range of 700-900 nm.The disadvantage is that the external quantum efficiency of the phosphorescent device decreases rapidly with the increase of the current density,which may be mainly attributed to the square planar structure in which the platinum complex easily aggregates and the long-lived triplet exciton annihilation.In addition,the use of expensive rare metals also limits the industrial application of near-infrared organic phosphorescent devices.Therefore,the D-A typeπ-conjugated NIR luminescent material has obvious advantages due to its cost advantage and the adjustable molecular structure.However,the D-A type material still has its own problems,most near-infrared fluorescent materials have relatively low fluorescence quantum yield and exciton utilization,so most of the near-infrared organic fluorescent devices（FOLEDs）exhibit considerably lower external quantum efficiency,greatly limiting the development and application of organic near-infrared fluorescent devices.The basic design principle of near-infrared materials is to reduce the bandgap,that is to reduce the energy gap between the frontier molecular orbital（LUMO and HOMO）.Among them,the D-A type material intermolecular charge transfer state（ICT）is a special excited state,electrons and holes are located in different parts of the molecule.This type of molecule possesses a relatively small energy gap,which makes it easier to obtain deep red and near-infrared emission.However,the luminous efficiency of ICT states is generally low,because the overlap between electron orbitals is very small,which affects the radiation transition efficiency.So when we using ICT states to achieve ideal near-infrared emissions,there is an urgent need to find effective ways to improve the efficiency of CT-based luminescence.Based on the research of CT materials,our group proposed the design concept of"Hybrid Local Emission and Charge Transfer"（HLCT）.The idea controls the composition of the excited state through the rational design of molecular structure,allowing the molecule not only obtain a narrow bandgap emission,but also achieve high material luminescence efficiency and exciton utilization.This is of great significance for the development and further application of organic near infrared materials.In this paper,A series of D-A fluorescence materials from deep red to near infrared luminescence were synthesized with pyridadithiadiazole as the basic receptor building unit and common donor groups.The effects of molecular design,molecular structure and aggregation on the molecular excited states and fluorescence luminescence efficiency were studied in detail.The specific research results are as follows:1.In this part,two isomers（p-TPA-PT and d-TPA-PT）were synthesized based on the difference of 4,7-position reactivity in pyridine ring.The fitting results of the excited-state dipole moment of molecule are linear relationship of two different slopes,and the fluorescence decay curve of the material is in line with an exponential fitting,indicating that this type of excited state is a completely new excited state,that is,Local and Charge transfer effective mixed excited state（HLCT）.The lack of steric hindrance of hydrogen atoms between donors and acceptor of the molecule p-TPA-PT,reduces the twist angle and increases the freedom of rotation of single bonds.In d-TPA-PT,the torsional angle increases due to steric hindrance of hydrogen atoms,and at the same time,the molecular structure becomes more rigid due to the limited rotation of the single bond.In addition,the superimposed D-A effect in the molecular structure allows the molecule DTPA-PT to have a higher radiation transition rate（k_r）and a lower non-radiative transition rate(k_nr)than the other two isomers,making the molecule DTPA-PT more High fluorescence quantum yield.At the same time,the double-triphenylamine substitution is helpful for the injection and transport of holes.Therefore,the DTPA-PT electroluminescent device achieves the maximum external quantum efficiency（3.87%）and the maximum luminance of 12000 cd m^-2.These results suggest that the use of pyridine thiadiazole（PT）as a receptor for the construction of D-A-type molecules has great potential applications in organic electroluminescent.2.The p-TPA-PT with small torsion angle was used as the prototype,further introduced the electron-absorbing into the molecular structure,the molecules p-TPA-PT-F and p-TPA-PT-CN were synthesized.The theoretical calculation shows that these molecules have small torsion angles,and their frontier orbits are partially separated and overlapped,so they all have a large S₁ state oscillator strength.Fluorine and cyano groups are easy to form intermolecular interaction,their introduction to a certain extent,affect the molecular configuration structure.The crystal structure of the molecule p-TPA-PT-F is similar to that of p-TPA-PT,while the crystal structure of p-TPA-PT-CN varies greatly.Its crystal molecules become very planar and the moleculars packing close.The optimized molecular structure of p-TPA-PT-CN also has very small torsion angle,very good molecular planarity and sufficient orbital overlap.The strength of excited state oscillator is also significantly higher than that of p-TPA-PT and p-TPA-PT-F.In addition,the introduction of fluorine atom and cyano group also brought about the red shift of molecular absorption and emission spectra,and the emission of the three materials gradually red-shifted to the near infrared region.However,the fluorescence quantum yield of the three molecular states are basically Maintain at a high level of about 0.3.In the non-doped electroluminescent device,the maximum external quantum efficiency of the molecule p-TPA-PT-F increased from 1.55%to 2.00%,although the electroluminescence spectrum of the molecule p-TPA-PT-F showed no significant spectral redshift relative to p-TPA-PT.For the molecule p-TPA-PT-CN,the external quantum efficiency of device（1.45%）is comparable to that of the molecule p-TPA-PT but its electroluminescence shifts from 638 nm to 700 nm.The small decrease in external quantum efficiency may be due to the high non-radiative transition rate caused by narrow bandgap.3.On the basis of study of the symmetric molecule DTPA-PT,it has a high fluorescence quantum efficiency,but its color is located in the deep red region.In order to further reduce the band gap to infrared region,thiophene groups were introduced into the molecule as a bridging structure between the donor and the acceptor,and two contrast units,diphenylamine and carbazole,were chosen as the donors in the molecule.Based on this,three near-infrared light-emitting materials DTPS-PT,DTPS-ffBT and DCZS-PT were synthesized.Crystallographic analysis revealed that the molecular DCZS-PT possessed the smallest twist angle and the closest crystal packing structure.The twisting angle of the molecular DTPS-ffBT is relatively large（23.3°）,and its crystal packing is the most loose.The molecular DTPS-PT torsion angle between the size of the two,the molecule also showed a relatively suitable packing density.In addition,due to the change of donor and acceptor strength,the intramolecular charge transfers gradually weakened from DTPS-PT,DTPS-ffBT to DCZS-PT.Solvation model fitting to three excited states dipole moments were 11.1D,12.3D and 13.3D.Molecular excited state dipole moment between the typical LE and CT molecules,showing the properties of LE and CT hybrid excited state.The fluorescence quantum yield of the material is affected by the polarity of the solvent,but the molecular quantum yield of the fluorescence in the solid state and in moderately polar solvents can be maintained at relatively high levels.The main emission peaks of the three molecules all achieved near-infrared emission over 700nm.Among them,DTPS-PT 40%doped molecular devices emit near 840nm,EQE of 0.183%,the maximum irradiation power of 2202 mW Sr^-1m^-2.However,the DTPS-PT non-dopant device only obtained 0.081%of EQE at a peak of818 nm with a maximum irradiance of 590 m W Sr^-1m^-2.The dope increases the external quantum efficiency of the DTPS-PT light-emitting device while also maintaining and improving the color purity of the material.In addition,the emission and external quantum efficiencies of DTPS-ffBT 10%doped devices are also significantly improved compared to100%pure light-emitting devices.The EQE is increased by about 100%and the emission peak is red-shifted by 8 nm.However,although the doping improves the luminescent efficiency of the electroluminescent device of DCZS-PT,it has a great influence on the color of the light,and the emission of the device is blue-shifted below 700 nm under a concentration of 10%.