Fluorescent molecules offer another route to efficient organic LEDs
The promise of organic LEDs comes in part from their flexibility, both figurative and literal. Organic chemical space is vast, and molecules can be designed to emit in any part of the visible spectrum. The colors can be combined to give good quality white light, or they can be used on their own in electronic displays that are thin, lightweight, and even transparent and pliable. But not every organic molecule makes for a good OLED. The electrically generated excitations have random spins, so excited-state molecules are about three times as likely to end up in a spin triplet state, from which phosphorescence is quantum mechanically forbidden and thus unobservably slow, as in a spin singlet state that readily fluoresces. Quantum efficiency is thus capped at an impractically low 25%.