With the generous support of the National Science and Technology Council Academic Summit Program, the research team led by Professor Pi-Tai Chou of National Taiwan University made a leap to break the world record of organic strong light emission held by the team itself from 840 nanometers to 1000 nanometers. This outstanding research achievement was officially published in the top international optoelectronics journal "Nature Photonics" on October 10 this year. The success is expected to be applied to real-time biomedical imaging and biochemical sensing analysis, making feasible the wearable biomedical apps in the future. Also, the corresponding long wavelength can reduce the loss of information transmission, improving the efficiency of optical fiber technology. Thus, this breakthrough sets an indispensable cornerstone for the future development of cutting-edge biomedicine and communication technology.
Near-infrared wavelengths in the range of 1000-1700 nm are commonly known as NIR(II) light. In the field of biomedicine, NIR(II) emission can pass through skin tissue and blood vessels for deeper imaging; It is also an important wavelength range for optical fiber technology in information transmission. At present, the light-emitting materials in this area mainly rely on semiconductor substrates and lanthanide metal-related phosphors, but they are subject to slow response time, scarce resource and bio-incompatibility, which are not conducive to biomedical and optoelectronic applications. If there is to be a breakthrough in NIR(II) light-emitting materials for more general applicability, diverse organic materials are the top choice. However, this type of organic materials is very rare.
Organic molecules, after being excited from the ground state to the electronic excited state by light or electrical energy, can return to the original ground state in the form of light or heat. As early as 1970, it has been described in the "energy gap law", when the energy difference between the excited and the ground states, the so-called "energy gap", is smaller, the exciton-vibration coupling becomes stronger, causing the excitons to turn into heat without emission. The energy gap of the NIR(II) region is much lower than that of the visible light region, so the organic molecular material in NIR(II) region commonly has negligible emission. How to make strong NIR(II) emission for organic molecules has long been regarded as a mission-impossible task.
To make the impossible possible, since 2017, Chou’s team initiated a research project by probing its theoretical basis, mulling about whether the exciton-vibration coupling strength of organic material can be effectively reduced via other means if coupling is an inevitable law. This in turn reduces the efficiency of heat dissipation. Through the cooperation with Professor Chi Yun (National Tsing Hua University), Professor Hung Wen-Yi (National Ocean University), and Dr. Chuang Wei-Tsung (National Synchrotron Radiation Research Center), the paper published in the journal "Nature Photonics" on 10/10, 2022 is based on the self-assembly of platinum metal complexes incorporating more -conjugated, planarized ligand and the replacement of hydrogen atom with deuterium, so-called deuteration. The results of intense 1000 nm emission broke the world record 840 nm published by Chou’s group (Nature Photonics, 2020), making outstanding international contributions.
After more than a year of hard work, Professor Chou’s team has successfully deuterated the entire molecule, further reducing the heat dissipation rate due to the reduction of the exciton-vibration coupling. The combination of self-assembly and deuterium effect gives rise to intense NIR(II) emission. Upon fabrication, the corresponding OLEDs has reached an emission peak around 1000 nm with an internal quantum yield of as high as 21% and an external quantum efficiency of 4.2%, all of which are world records. Quoted by Professor Chou as saying, in the future, this team will challenge the emission at 1700-2000 nm region, which is an unexplored territory. Also, they are planning to commercialize the existing technology and hope those who are interested can join the event.
