Transition Metal-Based NIR Material with Heteroleptic Structure for Bio-Medical Application

Abstract

Among the near-infrared (NIR) ranges, the area between 700 nm and 1000 nm, called the optical tissue window, is widely used in the bio-medical field because it has less scatter with skin cells and can penetrate deep into the skin to promote cell metabolism. However, it is very challenging to develop a low-gap organic material that emits light in the NIR region due to the increased proportion of non-radiative decay rate by the energy gap law. In order to overcome this hurdle, various types of development methods are being studied such as the use of thermally activated delayed fluoresce (TADF) and transition metal-based phosphorescent materials. In this study, a novel Ir(III) complex was synthesized and designed using a thienothiophene moiety. This material is composed of a fused thiophene structure, and thus is exceedingly rigid and has an extended construction length, thereby reducing the energy gap. In addition, since it has a high electron density structure, the covalent bond between the ligand and iridium can be strengthened to provide stability to the device. Moreover, dipole moments are more likely to be horizontally oriented in heteroleptic structures than in homogeneous structures, thus boosting the performance of the device. Notably, it has a lower non-radiative decay rate than other reported Ir(III)-based NIR emitters. The radiative lifetime is 660 ns, allowing for efficient luminescence with a narrow energy gap. To demonstrate the properties of this novel material, a NIR OLED was fabricated with an emission peak of 700 nm. This optoelectronic device has a very high radiant emittance and a stable operation lifetime with a Lambertian light distribution. It exhibits excellent electrical and optical properties compared to previously reported Ir(III)-based NIR OLEDs. Furthermore, significant proliferation results were verified compared to the control group in fibroblast cell experiments.