Theoretical Studies On Luminescence Process Of Tetradentate Pt(?) Complexes

In recent years,organic photoelectric materials have been widely used in the preparation of electronic devices.With the excellent photoelectric properties,organic light-emitting diodes(OLED)show the great potential in a wide range of optical applications and make great progress.As the core of OLED,the work performance of emissive layer will determine device efficiency directly.To improve the device performance,a thorough consideration of the photophysical mechanism of central materials must be conducted.Our studies successfully explored the luminescence character of several tetradentate Pt(II)complexes from the perspective of theoretical and computational chemistry.The relationship between electronic structure and molecular spectra of organometallic complexes were considered to investigate specific photochemical phenomena.On the other hand,the structure–property relationships behind intramolecular excited state decay and intermolecular excitation energy transfer were explored,which can offer a more comprehensive understanding of exciton utilization.We hope our systematic studies can offer feasible idea for future design of new electroluminescent materials.The main contents and results are described as follows: 1.Understanding the mechanisms of white light emission from a tetradentate Pt complex and its work properties in various surrounding environmentsIn this chapter,based on the fact that the calculated energy gap of two main emission peaks is close to the experimental result,we confirmed that the white emission with dual emission peaks of studied Pt(II)complex can be formed with the 0–0 peak and the low-energy emission peak,where the low-energy peak is constituted by the high-frequency normal modes with larger Huang–Rhys factors.Indeed,according to Kasha's rule,it is rare to detect emission not from the first excited state.Moreover,the emission spectra in solution,crystal phase and amorphous phase were calculated to understand the work performance in different molecular environments.Here,one of our conclusions that the decreased reorganization energy of ground state causes the red-shift in emission is different from many previous works.That is because the maximum emission of this complex corresponds to the 0-0 peak rather than the vertical emission energy.Additionally,it is found that the complex would have lower intersystem crossing rate in molecular packing surroundings with stronger intermolecular interactions;it can be attributed to the restricted intramolecular vibrational motions.2.What accounts for the color purity of tetradentate Pt complexes ? A computational analysisIn this chapter,we confirmed the nature underlying an interesting experimental phenomenon.The fact that the color purity of studied Pt(II)complexes will improve obviously by introducing a tert-butyl(tbu)to the 4-position of pyridine ring can be attributed to the restrained structural deformation between the first triplet excited state and the ground state,where the structural deformation is associated with Huang-Rhys factor.What is more,the shape of the emission peak are determined by Huang-Rhys factor;the height and bandwidth of intermediate and high-frequency regions associated with pyridine ring would be diminished caused by the lower Huang-Rhys factor.Meantime,the better color purity obtained by adding an electron-donating group to pyridine ring can be interpreted as the result of unit conversion from wavelength to wavenumber.In addition,the weaker intersystem crossing rate can also be attributed to the lower Huang-Rhys factor.3.The study of intramolecular decay and intermolecular energy transfer for phosphorescent organic light-emitting devicesIn this chapter,theoretical calculations of intramolecular excited state decay and intermolecular excitation energy transfer(EET)were conducted to investigate the difference in EQE between two studied devices.The greater EQE of PtON7 based device is mainly governed by the stronger energy transfer efficiency,with a secondary role played by the higher photoluminescence quantum yield of emitter.Our calculations indicate that the different triplet EET(TEET)rate have the main contribution to the difference in energy transfer efficiency between two studied devices.In view of the fact that the electronic structure would affect the intramolecular excited state decay and intermolecular excitation energy transfer obviously,we confirmed that the description of electronic structure are accurate through the consideration of environmental effects,where the smaller emission peak width of emitters in condensed phase can be interpreted as the result of the lower Huang–Rhys factors.