Theoretical perspective for substitution effect on luminescent properties of through space charge transfer-based thermally activated delayed fluorescence molecules

Designing thermally activated delayed fluorescence emitters with through-space charge transfer: a theoretical study

Recently, thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) features have been widely applied in developing organic light-emitting diodes with high luminescence efficiencies. The performance of TSCT-TADF molecules depends highly on their molecular structures. Therefore, theoretical investigation plays a significant role in designing novel highly efficient TSCT-TADF molecules. Herein, we theoretically investigate two recently reported TSCT-TADF molecules, 1'-(2,12-di-t-butyl[1,4]benzoxaborinino[2,3,4-kl]phenoxaborinin-7-yl)-10-phenyl-10H-spiro[acridine-9,9'-fluorene] (AC-BO) and 1-(2,12-di-t-butyl[1,4]benzoxaborinino[2,3,4-kl]phenoxaborinin-7-yl)-9',9'-dimethyl-9'H-spiro [fluorene-9,5'-quinolino[3,2,1-de]acridine](QAC-BO). The calculated photophysical properties (e.g. excited state energy levels and luminescence properties) for these two compounds are in good agreement with experimental data. Based on the systematic analysis of structure-performance relationships, we design three novel TSCT-TADF molecules with high molecular rigidity and evident TSCT features, i.e., DQAC-DBO, DQAC-SBO, and DQAC-NBO. They exhibit deep-blue light emissions and fast reverse intersystem crossing rates (KRISCs). Our calculations demonstrate that the nearly coplanar orientation of the donor and acceptor is critical to achieve remarkable KRISCs and fluorescence efficiencies in TSCT-TADF molecules.

A molecular descriptor of a shallow potential energy surface for the ground state to achieve narrowband thermally activated delayed fluorescence emission

Narrowband thermally activated delayed fluorescence (TADF) molecules have extensive applications in optoelectronics, biomedicine, and energy. The full-width at half-maximum (FWHM) holds significant importance in assessing the luminescence efficiency and color purity of TADF molecules. The goal is to achieve efficient and stable TADF emissions by regulating and optimizing the FWHM. However, a bridge from the basic physical parameters (such as geometric structure and reorganization energy) to the macroscopic properties (delayed fluorescence, efficiency, and color purity) is needed and it is highly necessary and urgent to explore the internal mechanisms that influence FWHM. Herein, first-principles calculations coupled with the thermal vibration correlation function (TVCF) theory were performed to study the energy consumption processes of the excited states for the three TADF molecules (2,3-POA, 2,3-DPA, and 2,3-CZ) with different donors; inner physical parameters affecting the FWHM were detected. By analyzing the basic geometric and electronic structures as well as the transition properties and reorganization energies, three main findings in modulating FWHM were obtained, namely a large local excitation (LE) proportion in the first singlet excited state is advantageous in reducing FWHM, a donor group with weak electron-donating ability is beneficial for achieving narrowband emission, and small reorganization energies for the ground state are favorable for reducing FWHM. Thus, wise molecular design strategies to achieve efficient narrowband TADF emission are theoretically proven and proposed. We hope that these results will promote an in-depth understanding of FWHM and accelerate the development of high color purity TADF emitters.

Achieving narrowband emissions with tunable colors for multiple resonance-thermally activated delayed fluorescence materials: effect of boron/nitrogen number and position

The boron/nitrogen (B/N)-based multiple resonance-thermally activated delayed fluorescence (MR-TADF) materials with tunable colors have attracted widespread attention owing to their great potential in next-generation display, white lighting, and imaging applications. Numerous MR-TADF emitters with different B/N number and position have been reported to realize full-color narrowband emissions. To gain a better understanding of the effect of B/N number and position on the photo-electronic properties, geometric and electronic properties, Huang-Rhys factors and reorganization energies, charger transfer and absorption/emission properties were analyzed in detail to determine the structure-property relationship for the investigated molecules. The calculated results show that the molecules with para-atoms having the same electronic characteristics (para-B-π-B/para-N-π-N) exhibited smaller structural relaxations upon excitation, and the molecules with increased B/N atoms showed more obvious short-range charge transfer (SRCT) properties. Besides, the para-B-π-N and para-B-π-B/para-N-π-N substructures could reduce and enhance the donor and acceptor strengths, respectively, leading to tunable HOMO-LUMO gaps and emission colors. Such theoretical insights well rationalize the experimental results, revealing that the small reorganization energy and dominant SRCT property should be two key factors in realizing narrowband emissions of MR-TADF materials. These findings and understandings could give an in-depth insight into the structure-property relationship, providing molecular design strategies for the exploration of narrowband MR-TADF materials with tunable emission colors.

Role of halogen effects and cyclic imide groups in constructing red and near-infrared room temperature phosphorescence molecules: theoretical perspective and molecular design

Organic room temperature phosphorescence (RTP) has been widely investigated to realize long-lifetime luminescent materials and improvement in their efficiency is a key focus of research, especially for red and near-infrared (NIR) RTP molecules. However, due to the lack of systematic studies on the relationship between basic molecular structures and luminescence properties, both the species and amounts of red and NIR RTP molecules remain far from meeting the requirements of practical applications. Herein, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations, the photophysical properties of seven red and NIR RTP molecules in tetrahydrofuran (THF) and in the solid phase were theoretically studied. The excited state dynamic processes were investigated by calculating the intersystem crossing and reverse intersystem crossing rates considering the surrounding environmental effects in THF and in the solid phase using a polarizable continuum model (PCM) and quantum mechanics and molecular mechanics (QM/MM) method, respectively. The basic geometric and electronic data were obtained, Huang-Rhys factors and reorganization energies were analyzed, and natural atomic orbital was used to calculate the orbital information of the excited states. Simultaneously, the electrostatic potential distribution on molecular surfaces was analyzed. Further, intermolecular interactions were visualized using the molecular planarity binding independent gradient model based on Hirshfeld partition (IGMH). The results showed that the unique molecular configuration has the potential to achieve red and NIR RTP emission. Not only did the substitutions of halogen and sulfur make the emission wavelength red-shifted, but also linking the two cyclic imide groups could further make the emission wavelength longer. Moreover, we found that the emission characteristics of molecules in THF had a similar trend as in the solid phase. Based on this point, two new RTP molecules with long emission wavelengths (645 nm and 816 nm) are theoretically proposed and their photophysical properties are fully analyzed. Our investigation provides a wise strategy to design efficient and long-emission RTP molecules with an unconventional luminescence group.