Unveiling the mechanisms of organic room-temperature phosphorescence in various surrounding environments: a computational study.

Room-temperature phosphorescence (RTP) from pure organic materials has been promising in next-generation OLEDs. Understanding the photophysical properties of RTP molecules is attractive but challenging. In this study, through a combined quantum mechanics and molecular mechanics (QM/MM) method taking 2-(3,4-dimethoxybenzyl)isoindoline-1,3-dione (complex b) as an example, we comparatively investigate the photophysical properties of complex b in diverse environments (solution, crystal, and amorphous). From solution to amorphous to crystal phase, the excited-state decay rates for the molecule indicate that the AIE phenomenon of complex b is mainly induced by the increased phosphorescence rates. However, the increased nonradiative decay rate knr of T1 → S0 from the solution to the crystal phase could be attributed to the different electron coupling in the crystal phase. Moreover, the theoretical results also show that the small energy gap between the lowest singlet excited state (S1) and triplet excited state (T1) and low reorganization energy can help enhance intersystem crossing to facilitate a more competitive radiative process from the T1 state to ground state (S0). Additionally, the stronger intermolecular π-π interaction can cause high phosphorescence quantum efficiency in the crystalline phase. Our study presents a rational explanation for aggregation-induced RTP, which is beneficial for the design of new organic RTP materials in the future.

The study of intramolecular decay and intermolecular energy transfer for phosphorescent organic light-emitting devices.

Due to the huge potential of organic light-emitting diodes (OLEDs) in optical display devices, the exciton utilization of devices should be elucidated comprehensively to achieve a high external quantum efficiency (EQE). In this study, theoretical calculations of intramolecular excited state decay and intermolecular excitation energy transfer (EET) were conducted to investigate the differences in EQE between the two studied systems. Compared to the PtOO7-based system (using PtOO7 as the guest and 26mCPy as the host), the greater EQE of the PtON7-based system (using PtON7 as the guest and 26mCPy as the host) was mainly governed by the stronger energy transfer efficiency, with a secondary role being played by the higher photoluminescence quantum yield of the emitter. We confirmed that the different triplet EET (TEET) rates mainly contribute to the difference in the energy transfer efficiency between two studied systems, where the higher TEET rate from 26mCPy to PtON7 can be attributed to the restrained structural deformation of PtON7 and the desirable energy gap in the energy transfer process. Our calculations indicated that the electronic structure can evidently affect the intramolecular excited state decay and intermolecular excitation energy transfer. In addition, considering the environmental effects on the emission spectra of the emitters, the simulated spectra were consistent with the experimental measurements, which indicated that our descriptions of electronic structures are accurate; furthermore, an effective description of the molecular environment should be obtained. Our computational protocol successfully explored the relationship between the electronic structures, intramolecular excited state decay, and intermolecular excitation energy transfer, which can provide a deep understanding for the design and development of high-quality OLEDs from a molecular perspective.

Theoretical insights into the effect of ligands on platinum(ii) complexes with a bidentate bis(o-carborane) ligand structure.

Carboranes feature a wealth of unique structures and properties in phosphorescent transition-metal complexes (PTMCs). Herein, we identify the influence between the electronic structure in carboranes and the main ligand based on the density functional theory (DFT) and time-dependent density functional theory (TD-DFT), which affects the phosphorescence properties of carborane-containing Pt compounds. Furthermore, the mechanism, including singlet-triplet splitting energies ΔE(Sn - T1), transition dipole moment for S0 - Sn transitions, the zero-field splitting (ZFS), the radiative decay rate constant (kr), the Huang-Rhys factor (S), and the spin-orbit coupling (SOC) matrix elements have been carefully investigated. The results presented here reveal the functional action 1,1'-bis(o-carborane) contributes to the emission process owing to the manipulation of main ligand dtb-bpy and complex 1a shows promising prospects for achieving highly efficient phosphorescence via engineering the conjugation of the main ligand dtb-bpy.