Ultralong Room-Temperature Phosphorescence Induced by H-Bonding and π–π Interactions in Halogen-Free Carbazole Derivatives

Materials having room-temperature phosphorescence (RTP) are in limelight due to their advantageous long luminescence lifespan for applications in optoelectronics, bioimaging, and security purpose devices. Development of metal- and halogen-free organics for persistent luminescence and RTP has been envisaged through aggregation, heavy atom substitution, host–guest interactions, etc. In this article, we present experimental evidence for the effect of H-bonding and other nonbonding interactions on types of emission in five carbazole derivatives. An unsubstituted carbazole showed only fluorescence in powder form, while the N-aryl-substituted carbazole having carbonyl and nitrile groups (Benzal-CN-Cbz, Acph-CN-Cbz, and Bnph-CN-Cbz) showed dual emission consisting of fluorescence and phosphorescence in powder form under ambient conditions. Acph-CN-Cbz showed the longest excited-state lifetime exceeding 1.2 ms in powder form. RTP in Benzal-CN-Cbz, Acph-CN-Cbz, and Bnph-CN-Cbz is correlated to H-bonding and nonbonding interactions evident from single-crystal analysis. H-bonding involving the carbonyl group and extensive π–π interactions caused close packing in the solid state and is associated with the long-lived triplet excited state and RTP. Further, persistent luminescence having a lifetime of up to 1.5 s at ambient temperature in air was displayed in drop-casted PVA films. To the best of our knowledge, only a few organics, without halogen and heavy atoms, are available with such long lifetimes.