What causes double emission in TADF molecules?

TADF is a phenomenon that allows excited electrons that would otherwise dissipate energy as heat to emit light in visible frequencies. When electrical energy is applied to materials a distribution of singlet and triplet electrons are created. Singlet electrons can fluoresce triplets can't. In TADF triplets can absorb surrounding heat and transition to singlet leading to an emission. Now a few things are clear from this

1. In a TADF material there will be two sets of emission. First from the normal singlet electrons and then from the triplets that converted to singlet.

2. There will be a delay in the second emission. Because it takes some time for triplet electrons to absorb energy and transition to a state where emission is possible. This time is in nanosecond range so it can still be called fluorescence. But because of the slight delay its called delayed fluorescence.

3. The frequency of the delayed fluorescence can either be the same as normal singlet fluorescence or it can be slightly lesser. Usually delayed fluorescence occurs at a lesser frequency. So a single TADF molecule can emit two colors at a time. A higher energy color and a lower energy color.

This double emission in TADF has some exciting possibilities of its own. Since each molecule can emit atleast two frequencies it can be used to create efficient , simpler "phosphor free" integrated white light devices. Because the second emission is delayed TADF can be used to design high throughput communication systems that carry data on visible light.

#engineering #light

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Notes by Akshat Jiwan Sharma

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