In this study, the redox behaviors of N,N′-disubstituted dihydrophenazine and dihydrodibenzo­[a,c]­phenazine and the distinct properties of their corresponding radical cation and dication species were systematically investigated and compared. A structure–activity relationship investigation revealed that the dication of dihydrophenazine was very reactive and unstable. Nevertheless, the unprecedented crystallization and single-crystal structures of their radical cation and dication species, especially the dication of dihydrophenazine DPPZ-B 2+ , were successfully achieved. X-ray crystallographic analysis unveiled the unique changes in the molecular geometry and electronic structure of dihydrophenazine and dihydrodibenzo­[a,c]­phenazine before and after oxidation. Moreover, the geometry planarization in the dihydrodibenzo­[a,c]­phenazine radical cation (DPAC •+ ) and dication (DPAC 2+ ) was demonstrated for the first time, which could contribute to the understanding of its intriguing conformation-adaptive behavior. We expect that this study will provide new insight into the understanding of the structure–activity and structure–conformation relationship of dihydrophenazines, which can be helpful to the design and application of dihydrophenazine-based materials in the future.

Organic diradicaloids usually display an open-shell singlet ground state with significant singlet diradical character (y0) which endow them with intriguing physiochemical properties and wide applications. In this study, we present the design of an open-shell nitrogen-centered diradicaloid which can reversibly respond to multiple stimuli and display the tunable diradical character and chemo-physical properties. 1a was successfully synthesized through a simple and high-yielding two-step synthetic strategy. Both experimental and calculated results indicated that 1a displayed an open-shell singlet ground state with small singlet-triplet energy gap (ΔES−T = −2.311 kcal mol−1) and a modest diradical character (y0 = 0.60). Interestingly, 1a was demonstrated to undergo reversible Lewis acid-base reaction to form acid-base adducts, which was proven to effectively tune the ground-state electronic structures of 1a as well as its diradical character and spin density distributions. Based on this, we succeeded in devising a photoresponsive system based on 1a and a commercially available photoacid merocyanine (MEH). We believe that our studies including the molecular design methodology and the stimuli-responsive organic diradicaloid system will open up a new way to develop organic diradicaloids with tunable properties and even intelligent-responsive diradicaloid-based materials.

In this study, we present the de novo design of a new open-shell nitrogen-centered diradicaloid which can reversibly respond to multiple stimuli and display the tunable diradical character and chemo-physical properties. 1a was successfully synthesized through a simple and high-yielding two-step synthetic strategy. Both experimental and calculated results indicated that 1a displayed an open-shell singlet ground state with small singlet-triplet energy gap (ΔES−T = -2.311 kcal mol− 1) and a modest diradical character (y0 = 0.55). Interestingly, 1a was demonstrated to undergo reversible Lewis acid-base reaction with both Lewis acid and Brønsted acid to form acid-base adducts due to its intrinsic Lewis basicity. The formation of Lewis acid-base adduct was proven to effectively tune the ground-state electronic structures of 1a as well as its diradical character and spin density distributions. Based on this, we succeeded in devising a photoresponsive system based on 1a and a commercially available photoacid merocyanine (MEH). We believe that our studies including the entirely new molecular design methodology and the newly found stimuli-responsive organic diradicaloid system will open up a new way to develop new diradicaloids with tunable properties and even intelligent-responsive diradicaloid-based materials.