Room-Temperature Phosphorescence of Thiophene Boronate Ester-Cross Linked Polyvinyl Alcohol; A Triplet-to-Singlet FRET-Induced Multi-Color Afterglow Luminescence with Sulforhodamine B

A thiophene-derived boronate ester-cross-linked polyvinyl alcohol, 1-PVA, has been prepared, which was found to exhibit room-temperature phosphorescence (RTP) at 475 nm upon UV irradiation (lambda(em) = 254 nm). The quantum yield (Phi(p)) was determined to be 6.3% with a phosphorescence lifetime of 256 ms, the cyanish afterglow being visually detected after switching the UV lamp off. The emission properties were rationalized by El-Sayed's rule, where introduction of a sulfur atom brings about n -> pi* transition in the T-2 state to promote an ISC channel. Given that the phosphorescence spectrum at 77K was consistent with that in the PVA film, the rigid environment of PVA-based matrix may stabilize the triplet state. In addition, the delayed emissive color was tuned by doping various amounts of sulforhodamine B (SRhoB) in the film; a triplet-to-singlet Forster-type resonance energy transfer process occurred from cross-linked thiophene boronate to SRhoB acceptor which modified the emission to a white-light afterglow. Given watersensitivity of the resultant film, high-contrast water writing on the film was achieved, which would be profitable for a low-cost security film.