Theoretical Study On Molecular Luminescence Of Organic Room Temperature Phosphorescent Materials

So far,traditional efficient room temperature phosphorescent?RTP?systems are noble metal-containing inorganic and organometallic compounds with the drawbacks of cytotoxicity and instability.Thus,it is urgent to alternatively develop pure organic RTP materials with low environmental load.Pure organic RTP molecule can make full use of 75%triplet excitons in the excited states produced by light excitation to achieve high luminescence efficiency.The high-performance RTP organic light-emitting diode?OLED?with high quantum efficiency and high energy/charge transfer is obtained by designing and synthesizing organic molecules to tune the luminescent properties of the molecules and the luminescent colors of the materials.The packing modes and intermolecular interaction in solid state play subtle and complex effects on the photophysical behaviors of pure organic RTP molecules.Moreover,the quantitative relationship between molecular geometric changes and optical properties is crucial for designing new molecular structures.Therefore,we choose two RTP molecules with different spatial structures,using density functional theory?DFT?and time-dependent density functional theory?TD-DFT?,to explore the effects of different dimer conformations of crystalline pure organic RTP materials on the optical properties by varying the extent of movement or rotational angles based on the T₁structure.This provides a new theoretical guidance for the design and development of novel pure organic RTP molecules.The results are shown as follows:We employed TD-DFT to explore the optical properties of a pure organic phosphorescent of 4,6-diphenyl-2-oxazole-1,3,5-triazine?DPhCzT?containing N atom in monomeric and dimeric forms.The calculation results suggested that the bulk RTP emission spectrum of DPhCzT is mainly determined by the dimer rather than the monomer.To deeply probe the effect of T₁ state dimer with different conformations on the emission spectra,the optical properties of various dimers were investigated by varying the extent of movement or rotational angles based on the T₁ structure for the first time.Furthermore,we found that the shortest intermolecular N···N distance(d_N···N)has a significant effect on the emission spectrum of the vertical-and tilting-movement dimers,because the variation of E_g results in the emission wavelength strongly dependent on d_N···N.The maximum red shift of the tilting-movement compressed dimer is larger than that of the vertical case.Additionally,the packing degree of face to face affects greatly the maximum emission spectrum(?_em)of vertical-and tilting-movement dimmers.It is mainly ascribed to the C–H···?,n···?,and?···?non-covalent interactions in dimers.Hence,our results shown that the different conformations of crystalline dimers have great influence on the photophysical behaviors of large planar?systems.And the intermolecular C–H···?and?···?non-covalent interactions should be considered,which can effectively inhibit the non-radiative decay of molecules.Furthermore,the packing style of the compact face to face favors intermolecular coupling,which stabilizes the triplet excitons and accelerates the intersystem crossing process to achieve higher intensity RTP.These might provide a new insight and theoretical guidance for designing pure organic RTP materials with longer lifetime and high intensity.We studied the optoelectronic properties of a series of benzoyl dibenzothiophene derivatives?BDBT,BrBDBT,and ClBDBT?in monomeric and dimeric forms using TD-BMK/6-31G?d,p?and TD-M06HF/6-31G?d,p?,respectively.The predicted?_em of dimers and monomers of these three RTP molecules are close to the experimental values,which is consistent with the experimental single-molecule luminescence.However,the intense phosphorescence emission is originated from the crystalline state at room temperature in experiment.Therefore,we further explored the influence of dimers with different conformations on the optical properties by varying the extent of movement or rotational angles based on the T₁ structure.We found the following phenomena:?i?the change of the shortest intermolecular distance?d_S···_C or d_C···_C?leading to E_g has a significant effect on the emission spectra of vertical-and tilting-movement BDBT and ClBDBT dimmers.?ii?The maximum red shift of the vertical-movement compressed dimer of BDBT is larger than that of the tilting case,while it's opposite for ClBDBT.And the optical properties of BrBDBT dimer conformation are almost remain changed.?iii?Unlike the large?-conjugated system,the parallel-and tilting-movement have almost no effect on photophysical properties of BDBT and ClBDBT.?iv?The heavy halogen atomic effect in ClBDBT and BrBDBT enhances the spin-orbit coupling of monomer molecules relative to BDBT,resulting in?em red shift.However,we found that?em,energy level difference?Eg?,and transition energy?E?of all BDBT and ClBDBT dimer conformations showed the same trend,and the value of?_em is almost identical?866.42 vs 866.85 nm?.Therefore,our calculations suggested that the effect of dimer conformation in solid state on the optical properties mainly determined by the overlap degree of the intermolecular face to face stacking style.Furthermore,the different conformations of the crystalline dimers have a great influence on the photophysical behavior of the RTP material in vertical and tilting directions.