Tuning intramolecular charge transfer and suppressing rotations in thianthrene derivatives for enhancement of room-temperature phosphorescence.

Polymer-based organic room-temperature phosphorescent (ORTP) materials have advantages such as low cost, abundant resources and ease of processing, rendering them highly suitable for real-world applications. However, the trade-off between the phosphorescence quantum yield (Φ phos.) and phosphorescence lifetime (τ phos.) highlights the challenge for the development of efficient ORTP materials. Here, a synergistic strategy was proposed to promote n-π* transitions, provide charge-transfer (CT) intermediate and inhibit intramolecular motions to achieve efficient ORTP. A thianthrene (TA) unit was attached to planar and rigid polyaromatic hydrocarbons (PAHs), and the resulting luminogens were used as guest molecules and embedded into melamine-formaldehyde (MF) polymers, respectively. The TA chromophore promotes n-π* transitions, and the PAH units generate CT intermediates and inhibit rotations, which open intersystem crossing (ISC) channels and facilitate ISC processes. It was found that the τ phos. values of TA-Na@MF and TA-Phen@MF were about 30-fold that of TA@MF. The phenanthryl group was larger than the naphthyl group, which created steric hindrance and limited rotations. As a result, TA-Phen@MF demonstrated the best RTP performance with an ultralong τ phos. of 1006.45 ms and a high Φ phos. of 50.31%. To the best of our knowledge, the observed RTP represents the longest persistence luminescence among TA derivatives. Thanks to its efficient RTP properties and processability, TA-Phen@MF was blended with ethylene vinyl acetate (EVA) to produce stretchable and recyclable persistent RTP elastomers. This work provides helpful guidance for achieving high-performance polymer-based ORTP materials.