IISER researchers come up with NIR light-emitting organic molecules

Bhopal: Researchers at the Indian Institute of Science Education and Research (IISER) Bhopal have created a new family of organic molecules that emit light in the near-infrared (NIR) range, opening possibilities for OLEDs for various applications.

Light-emitting diodes or LEDs are tiny light-emitting devices that are commonly used in applications such as television screens, gadget displays, etc. They are different from traditional filament bulbs in that while bulbs emit light when heated, LEDs emit light when electricity (in the form of electrons) passes through them.

OLEDs are a form of LEDs where the light emitting materials are organic molecules -- chemicals made largely of carbon and hydrogen.

Although visible light-emitting OLEDs have already found mass application in displays for gadgets, televisions, and lighting, producing NIR-emitting OLEDs is challenging due to their unique molecular energy structure.

Light-emitting molecules generate light when electrons fall from a high energy state (HOMO) to a low energy state (LUMO) inside the molecule, and the colour of the emitted light depends on the energy difference between the two states. The energy difference in organic molecules corresponds to visible light, making visible light-emitting OLEDs easier to design.

The team at IISER created new organic molecules that have a very low-energy LUMO. This means that the energy difference between LUMO and HOMO is smaller, resulting in near-infrared light.“We have achieved a ni tro group containing polycyclic aromatic molecules that exhibit fluorescence (light emission) in the near-IR wavelengths,” said Prof. Jeyaraman Sankar, IISER Bhopal, in a statement.

The team’s new approach to obtaining stable electron-deficient molecules with NIR emission using nitration as a strategy is unique and has been published in the journal of the Royal Society of Chemistry - Chemical Science. The innovation lay in fine-tuning the structure of the nitro-group-containing molecules that typically do not fluoresce and are therefore usually non-light-emitting. Another challenge is to position the emission in the near-IR region. The team engineered the structure of the molecules such that they emit near-IR light. In addition, the synthesised molecules show strong electron affinity values, excellent stability, and structural rigidity.

The fluorescence quantum yields (the amount of near-IR light emitted) are among the best for any known near-IR light-emitting electron-deficient materials, the team said.

Apart from their use in OLED applications, the strong electron affinity, NIR emissive nature, high stability, and redox robustness of these molecules make them excellent candidates for catalysts.

These molecules can also be used for near-IR devices that help in side effect alleviation, after selected chemotherapy procedures.

The potential applications for near-IR-emitting OLEDs are vast, including biomedical sensors, short-range communication, night vision, and security.

The discovery could potentially extend the use of organic light-emitting devices beyond visible light, with implications in a range of applicationsAfrom medical equipment to advanced security gadgets, the researchers said.