Theoretical study of phosphorescence in dye doped light emitting diodes

Abstract

Phosphorescence of platinum͑II͒ octaethyl porphyrin ͑PtOEP͒, which has been used in organic light emitting diodes to overcome the efficiency limit imposed by the formation of triplet excitons, is studied by time-dependent ͑TD͒ density functional theory ͑DFT͒. The spin-orbit coupling ͑SOC͒ effects and the phosphorescence radiative lifetime ͑ p r ͒, calculated by the TD DFT method with the quadratic response technique, are analyzed for a series of porphyrins in order to elucidate the internal heavy atom effect on p r . While the significance of the d orbital admixture into the lowest unoccupied molecular orbital e g ͑ * ͒, proposed by Gouterman et al. ͓J. Chem. Phys. 56, 4073 ͑1972͔͒, is supported by our SOC calculations, we find that the charge-transfer ͑CT͒ mechanism is more important; the CT state of the 3 A 2g symmetry provides effective SOC mixing with the ground state, and a large 3 A 2g -3 E u transition dipole moment gives the main contribution to the radiative phosphorescence rate constant. The IR and Raman spectra in the ground state and first excited triplet state ͑T 1 ͒ are studied for proper assignment of vibronic patterns. An orbital angular momentum of the T 1 state is not quenched completely by the Jahn-Teller effect. A large zero-field splitting is predicted for PtP and PtOEP which results from a competition between the SOC and Jahn-Teller effects. A strong vibronic activity is found for the e g mode at 230 cm −1 in PtP phosphorescence which is shifted to 260 cm −1 in PtOEP. This out-of-plane vibration of the Pt atom produces considerable change of the SOC mixing. The role of charge-transfer state of d * type is stressed for the explanation of the electroluminescent properties of the dye doped light emitting diode.

Theoretical Study of Phosphorescence of Iridium Complexes with Fluorine‐Substituted Phenylpyridine Ligands

“…Since the charge pairs are nongeminate, they have random spin orientation; thus the singlet and triplet colliding pairs are equally probable. [8] In general, the triplet state has three spin projections (M S = 0, Ϯ1) and the singlet spin state has only one microstate (M S = 0). The singlet excitons in PPV and in other organic conjugated polymers provide intense luminescence.…”

Organometallic Materials for Electroluminescent and Photovoltaic Devices

“…This notion can also be applied to the electron-hole injection, migration and recombination processes, and electron transfer in DSSCs (Minaev et al, 2009b). Since ISC is a spin-forbidden T-S quantum transition, its rate constant also depends on SOC, and on the relative positions of nearby electronic and vibronic states of different symmetry and spin-vibronic interactions (Minaev & Ågren, 2006). Calculations of SOC and radiative rate constants are very important for understanding the function of modern OLEDs.…”

Homoleptic tetraazaphenanthrene-based copper(I) complexes: Synthesis, spectroscopic characterization, crystal structures and computational studies

“…According to the previous theoretical calculations (DFT) [56], the MLCT excited state flattening from the pseudo D 2d to D 2 symmetry effectively splits the nearly degenerate Cu 3d xz and 3d yz HOMO orbitals. These changes in 3d MO energy levels translate into an increased energy splitting between the two observed 1 MLCT transitions that involve them, leading to a red-shift of the low-energy shoulder and a blue-shift of the main MLCT absorption peak. Fig.…”

“…Owing to the significant similarities in absorption spectra and photophysical behavior there have been recent attempt to replace Ru(II) with Cu(I) diimine complexes in dye-sensitized solar cells (DSSCs) [16][17][18][19][20]. However, the efficiencies and stabilities of the devices based on [Cu I (N^N) 2 ] + dyes, to date, are still inferior when compared to those of the Ru(II) dye-based solar cells because the MLCT states of Cu(I) diimine complexes undergo pseudo Jahn-Teller distortions, with significant structural reorganization, that are solvent-dependent and so the 1 MLCT state lifetime can be influenced by the coordination geometry around Cu(I) center [20]. It is well known that the coordination chemistry of Cu(I) complexes is largely dependent on the electronic and steric effects of the coordinated ligands.…”

“…There has been much interest over the past two decades in emissive coordination complexes of the transition metal elements with a focus on their applications as photosensitizers in solarenergy conversion [1,2], in display devices [3], in phototherapy [4], as photo-catalysts [5], and as dopants to increase the efficiency of organic light emitting diodes (OLED) [6][7][8]. In this regard, first row transition metal complexes are potentially useful, being considered inexpensive, being less toxic, displaying a greater ease of synthesis, and are more abundant when compared to the thirdrow noble metal complexes.…”