Modulation of Thermally Activated Delayed Fluorescence in Waterborne Polyurethanes via Charge-Transfer Effect.

Graphical Mind the gap : Charge-transfer states and a receiver state, 3 (n–π)*, can jointly promote reverse intersystem crossing to produce thermally activated delayed fluorescence even though there is a large apparent energy gap. The design principle is very promising for display technology. Here, we designed several waterborne polyurethanes (WPUs) with efficient thermally activated delayed fluorescence (TADF) via serving charge-transfer (CT) states as a mediate bridge between singlet and triplet states to boost reverse intersystem crossing (RISC). By tuning substituents of diphenyl sulfone (DS), we found that O,O′- and S,S′-substituted DS covalently incorporated in WPUs solely show typical fluorescence emission with lifetimes in the nanosecond range. Interestingly, TADF appears by replacing the substituent with the nitrogen atom, of which lifetimes are up to ≈10 microseconds and ≈1 millisecond in air and vacuum, respectively, even though the energy gap between singlet and triplet states (Δ E ST ) is still large for generating TADF. To explain this phenomenon, an energy level mode based on CT states and an 3 (n-π*) receiver state was proposed. By the rational modulation of CT states, it is possible to tune the Δ E ST to render TADF-based materials suitable for versatile applications.