Capturing the Progressive Conformational Evolutions of Sterically-Congested Dihydrophenazines via Crystallization

To gain a deeper understanding of the sequential multistep excited-state structural evolutions of N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) luminophores, we strategically freeze distinct conformations by crystallization, allowing to capture the progressive conformational transformations within a DPAC-based framework by utilizing single-crystal X-ray diffractometry. Our focus lies in the innovative modification of DPAC via the synthesis of cyano (CN)-substituted derivatives DPAC-nCN (n=1-4, with n indicating the number of CN groups). The incorporation of electron-withdrawing CN groups modulates electron delocalization and lowers energy barriers, facilitating access to conformational polymorphism within the crystals. Unlike the limited diversity observed in the crystallization behaviour of DPAC, the DPAC-2CN to DPAC-4CN derivatives exhibit distinct crystalline forms, with conformational diversity increasing in tandem with the number of CN substituents. Notably, the single DPAC-4CN molecule features multi-colored crystals transitioning from blue to red, with the folding angle of the polycyclic dihydrodibenzo[a,c]phenazine ring progressive varying from ~130° to ~172°. Additionally, DPAC-4CN's red crystals with high-energy planar conformation (~172°) can experience a sudden jumping when subjected to stimuli. This study not only advances the understanding of conformational dynamics in dihydrophenazines but also paves a new way for the development of dynamic crystal materials.