Use of Dimeric Excited States of the Donors in D₄-A Systems for Accessing White Light Emission, Afterglow, and Invisible Security Ink

Abstract

Pure organic white light emitters with an afterglow (AG) feature have unique advantages and various potential applications. However, the studies of single-component organic white light emitters with AG and visible light excitation (VLE)-dependent efficient room-temperature phosphorescence (RTP) features remain a challenging area of research in photophysics. Herein, we synthesize three terephthalonitriles in which 2,3,5,6-positions are covalently attached to 2-fluoro-phenoxy (TOF), 2-chloro-phenoxy (TOC), and 2-methoxy-phenoxy groups (TOM) to give twisted geometries. We observed that powder samples of both TOC and TOM show white light emission with CIE coordinates of (0.32, 0.38) and (0.26, 0.33), respectively, while TOM gives VLE-dependent efficient RTP under ambient conditions. In addition, both TOC and TOF exhibit a dim AG feature. Spectroscopic studies reveal that dual emission of these chromophores originates via radiative decay of monomeric excited states (singlet, triplet), dimer-like excited states (DLES) (singlet, triplet), and aggregated triplet states. Detailed spectroscopic and X-ray analyses reveal the signature of DLES that is formed via conformational reorganization of the phenoxy donors in the excited states. Single-crystal X-ray diffraction analysis shows that the multiple lp(O)···π(C≡N)/C═C, Cl/F···π, and hydrogen-bonding interactions in the X-ray structures play a significant role in facilitating intersystem crossing, stabilizing multiple triplet states, and suppressing nonradiative decay, thereby triggering dim afterglow under ambient conditions. We found that TOC and TOF exhibit persistent RTP (PRTP) with lifetimes of 139 and 736 ms, respectively, when they are embedded in the polyvinyl alcohol matrix. Given the PRTP feature, invisible security ink application is developed. These results provide a guidance to design white light-emitting materials with afterglow and visible light-activated efficient RTP features.