Theoretical Design And Investigation On D-A Organic Molecules With Highly Thermally Activated Delayed Fluorescence Property

Recently,thermally activated delayed fluorescence(TADF)materials based on organic molecules have attracted extensive attentions.The effective reverse intersystem crossing(RISC)from triplet excited state(T1)to singlet excited state(S1)for TADF can be realized when the singlet-triplet energy gap(?EST)is as possible as small,realizing the highest internal quantum efficiency of 100% in theory.The majority of TADF molecules reported in experiment are donor-acceptor models(D-A).However,the varied D and A units will produce large difference in material properties.For example,the change of emitting colors,a noticeable improvement in device efficiency,opposite properties and so on.It is important for synthesizing high-performance TADF materials to unveal the structure-property relationship.In this paper,based on the reported TADF molecules,we have designed a series of molecules by modifying structure and predicted their properties.In addition,two reported TADF molecules with similar structures and opposite propertieis have been investigated to find the main factors deciding TADF property.Specially,the tuning range functional is used to calculate the properties of TADF molecules.The whole paper is as followed:1.The majority of TADF molecules are in D-A modes.Numerous researches focuse on varying the D or A unit and try the best to find a best combination of D and A units with better TADF performances.Two TADF molecules based on three dimensional D-A triptycene have been synthesized and reported,which attracts our interest.On the basis of reported molecule,we have designed a series of novel TADF molecules by introducing different D units and increasing conjugation length.We have analyzed their structure,HOMO and LUMO distributions,UV-vis absorption spectra,?Est and fluorescence decay rate,electron and hole reorganization energy.The results indicate that increasing appropriately the conjugation or selecting an appropriate combination of electron-donating and electron-accepting units is a valid method to decrease ?Est.2.In 2016,two pure blue TADF molecules have been reported.Different from traditional TADF material,these two molecules realize their HOMO and LUMO separation by the multiple resonance effect between boron and nitrogen atoms.Based on triphenylboron derivative TADF molecule(DABNA-1),we have designed compounds 1-4 by introducing substituents at the para-position of boron atom.We intend to improve the TADF performance by introducing substituents,while maintaining the emitting wavelength varying in a small range.By using density functional theory,the calculated results indicate that designed compounds 1-4 have lower ?Est and larger oscillator strength than these of DABNA-1.Subsequently,the fluorescence decay rates of compounds 1-4 are greater than that of DABNA-1 with emitting wavelengths varying in a small range.Therefore,our designed molecules are predicted to be better TADF performances.3.The investigation on TADF design is always focused on varying the D and A units all the time.Some researches indicate that the efficiency is increased noticeably when the phenyl ring is introduced between D and A units.The phenyl ring serves as a bridge which connects D and A units.However,for TADF materials,the study on the bridge units is seldom.Therefore,in this section,we have designed a series of TADF molecules by changing the number and linkages of phenyl rings.In 2015,one TADF molecule DCBPy containing phenyl(pyridin-4-yl)methanone acceptor and carbazole donor has been reported.This DCBPy molecule displays high external quantum efficiency of up to 24% and low efficiency roll-off.The DCBPy molecule contains one phenyl(pyridine-4-yl)methanone acceptor and two carbazolyl donors are introduced ortho and meta into the phenyl ring.Based on DCBPy molecule,compounds 2-5 have been designed.By comparing their geometric structures between ground state and the lowest singlet excited state,analyzing reorganization energy,?Est,fluorescence decay rate and spin-orbital coupling strength,the results indicates that the introduction of connected bridge noticeably enhance their fluorescence decay rate and spin-orbital coupling strength.4.All the time,the rule of designing TADF materials emphasizes better HOMO and LUMO separation.Besides,the other factors influencing TADF characteristic are not clear.As a result,in this section,we have chosen two TADF molecules with 10H-phenoxaborin acceptor unit and different donating units.These two molecules have better HOMO and LUMO separation,but one molecule has TADF property and the other is not.Therefore,effective HOMO and LUMO separation can not decide the TADF property of molecule.By comparing their property difference,we intend to search for the conclusive factor on the TADF characteristic.The calculated results indicate that small ?Est and strong SOC are important for TADF property.In addition,the energy difference between S1 and T1 and the structure are also important for RISC process.Combining the nature transition orbital(NTO)and energy of higher excited states and SOC values,the potential RISC channels have been predicted.