Room-Temperature Phosphorescence Invoked Through Norbornyl-Driven Intermolecular Interaction Intensification with Anomalous Reversible Solid-State Photochromism

Herein, norbornyl (NB), a bulky annular nonconjugated spacer, is melded into π systems to construct two groups of ladder-type room-temperature phosphorescence (RTP) luminogens. The effect of the NB on π-π interactions, packing modes and RTP performance is explored systematically. The experimental and computational results demonstrate the versatility of NB in reducing π-π distances and synergistically intensifying the intermolecular interactions, which not only induces intersystem crossing from S₁ to T_n but also diminishes the nonradiative decay of triplet excitons. Impressively, 1800-fold phosphorescence lifetime enhancement is achieved in comparison with the reference compounds without NB. The molecular packing and RTP performance can be further modulated by the length of the backbones and terminal end-groups. It is quite peculiar that NB-annulated phthalic acid exhibits reversible photochromism in the solid state, likely due to the formation of persistent radical pairs. Our study paves an ingenious avenue towards enhancing intermolecular interactions and provides significant implications for a better comprehensive understanding of the origin of their RTP and the inherent photophysical mechanism.