Activating efficient phosphorescence from purely organic materials by crystal design

Journal: Nature Chemistry

Authors: Onas Bolton, Kangwon Lee, Hyong-Jun Kim, Kevin Y. Lin, and Jinsang Kim

Affiliation: University of Michigan, Ann Arbor

Phosphorescent materials, those which emit light long after excitation, have found increasing use in organic electronics as well as chemical and biological sensing. Perhaps the most well-known phosphors are the greenish glow-in-the-dark materials used in toys, among other things, which are metal oxides and metal chelates. Purely organic molecules have only shown detectable phosphorescent behavior at low temperature and under inert atmosphere, which are impractical conditions for most applications. This article demonstrates high quantum yield (=ratio of light emitted to light absorbed) phosphorescent all-organic materials based on a few molecular design principles.

Phosphorescence is the light emission that results from electronic relaxation from an excited triplet state (T1) to the singlet ground state (S0). Singlet and triplet states are defined by the relationship between the spins of an electron pair; a singlet state means the electrons have opposite spins while a triplet state means the electrons have parallel spins. The transition from a triplet state to singlet ground state is a quantum-mechanically forbidden transition because it would mean two electrons with parallel spin would occupy a single orbital, therefore violating the Pauli exclusion principle. Therefore, the time scale for phosphorescence is very long (on the order of milliseconds, compared to  nanoseconds for quantum-mechanically allowed fluorescence emission) because it requires a spin-flip before phosphorescent emission can occur. Highly phosphorescent materials therefore require 1) facile population of the triplet excited state, and 2) light emission from T1 rather than non-radiative decay paths such as molecular vibrations for relaxation from the triplet state.

In order to make highly phosphorescent organic materials, the authors utilize three molecular design principles known facilitate intersystem crossing from an excited singlet state to the lower energy triplet state to promote phosphorescence. Firstly, the organic molecules used are aromatic carbonyls (e.g. benzaldehyde derivatives) because they have significant spin-orbit coupling that eases the transition between excited singlet and triplet states. Secondly, proximity of heavy atoms such as bromine increase orbital mixing between singlet and triplet states, which also helps promote intersystem crossing. Finally, the bromine of a neighboring molecule allows halogen bonding with the carbonyl, a non-covalent interaction where the bromine is pushed up next to the carbonyl in the crystal state. This serves as a sort of directed heavy atom effect because the proximity of the halogen allows electron delocalization from the carbonyl onto the bromine, further promoting intersystem crossing to the triplet state.

In order to maximize phosphorescence quantum yield, the molecules should be tightly packed into a crystalline solid to maximize halogen bonding (Figure 1b) which promotes intersystem crossing to the triplet state rather that fluorescence emission from the singlet state observed in solution (Figure 1a). However, the authors found that, in a pure crystal, the halogenated benzaldehydes tended to quench each other, meaning that interactions with other molecules allow non-radiative decay of the excited state (Figure 1c, “Ex” = excimer state, a dimeric excited state). The authors solve this problem by embedding the active phosphorescent molecule into a matrix of dibromobenzene in order to maximize the halogen bonding with the carbonyls of the active molecule but prevent alternative relaxation processes such as excimer formation when the active molecules can interact with each other, thus maximizing phosphorescence quantum yield (Figure 1d). The authors were also able to further substitute the rings with different groups in order to change the molecular orbital levels and thus tune the phosphorescence emission color.

This article provides a proof-of-concept showing that purely organic crystals can be used to make color-tunable phosphorescent materials with quantum yields up to 55% thanks principally to the directed halogen bonding that increases population of the T1 state.

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