Beyond bent-to-planar transformation: ICT and TICT dynamics in a novel disubstituted N,N'-dihydrophenazine

Large electronic and structural reorganization upon photoexcitation endows organic π-conjugated molecules with complex and unconventional photophysical behavior. Herein, we report the synthesis, structural characterization, and excited-state dynamics of a new disubstituted N,N'-dihydrophenazine derivative (DPP-(CHO)₂) functionalized with formyl (-CHO) groups. Single-crystal X-ray diffraction and theoretical calculations reveal a planar ground-state conformation. The introduction of -CHO substituents induces a strong intramolecular charge transfer (ICT) character, resulting in a pronounced solvent-dependent emission, with fluorescence intensity maxima shifting from 523 nm in non-polar solvents to 610 nm in polar protic media. Femtosecond-time-resolved spectroscopy shows an ultrafast ICT process (∼250 fs in DMF and ∼550 fs in MeOH), followed by the phenyls twisting that leads to a twisted ICT (TICT) state. The timescale of this torsional relaxation strongly depends on solvent hydrogen-bonding ability and viscosity, ranging from a few picoseconds in DMF to hundreds of picoseconds in alcohols and triacetin. The emission decays of ICT and TICT states govern the observed photobehavior, with lifetimes modulated by solvent polarity. Above 140 K, non-radiative decay becomes thermally activated, with an energy barrier of 1.2 and 1.9 kcal/mol associated with the ICT and TICT emitters, respectively. In addition, the solid-state behavior is examined in both powder and dispersed in a PMMA film, revealing spectroscopy and photodynamics features dictated by the formation of H- and J-aggregates. These results highlight the key role of substituent effects and solvent interactions in controlling the excited-state dynamics of N,N'-dihydrophenazine derivatives.