In the condensed phase, polyatomic molecules with emission energy gaps in the near-infrared (NIR) region are expected to yield much lower emission intensities. The main non-radiative deactivation pathway that quenches NIR emission has long been recognized as the ‘energy gap law’. This law specifies that, in the absence of crossing between two potential energy surfaces, which is the most commonly encountered transition between the lowest electronically excited singlet (S1) or triplet (T1) state and the ground state (S0), the relaxation of S1 or T1 to S0 can be facilitated through the overlap of the wavefunctions between the zero vibration level of S1 or T1 and the higher isoenergetic vibration levels of the S0 state, followed by fast vibrational relaxation to the S0 state via heat. This vibrational relaxation accelerates when the emission gap is shifted towards the deep red and NIR regions, in which only a few vibrational ladders (in S0) are required to reach the zeroth vibration level of the S1 (or T1) state of polyatomic molecules. This fast non-radiative deactivation pathway drastically reduces the emission intensity and hence the exciton diffusion length, hampering the optoelectronic applications of these molecules, as evidenced by the lack of high-performance NIR organic light-emitting diodes (OLEDs) and NIR-harvesting organic photovoltaics (OPVs). The latter application is of prime importance because ∼50% of solar radiation is in the NIR region of 700–1,300 nm. Therefore, organic materials exhibiting intense NIR emission are in great demand.
Recently, the research group of Prof. Liang-sheng Liao, collaborated with Prof. Yun Chi (City University of Hong Kong) and Pi-Tai Chou (National Taiwan University), reported a unified semi-empirical approach by incorporating exciton theory into the formula for the energy gap law. They demonstrated that partitioning the reorganization energy by increasing the exciton delocalization length significantly enhanced the PLQY in the NIR region, thereby overcoming the luminescent boundary set by the energy gap law. A proof of concept was provided experimentally by the strategic design and syntheses of a new series of Pt(‖) complexes, which, on assembling, produced intense NIR emission maximized at 866–960 nm with an unprecedented PLQY of 5–12% and superior performance in NIR OLEDs.
One of the co-first authors, Dr. Yun Hu, is from FUNSOM, Soochow University.
Link to Paper: https://www.nature.com/articles/s41566-020-0653-6
Link to Prof. Liao’s Group: http://funsom.suda.edu.cn/funsomen/c4/00/c3002a50176/page.htm
Editor: Danting Xiang