Self-Assembly of a Carboxyl -Bipyridinium Zwitterion into a High-Performance Photoswitchable NLO Crystal

Efficient photoswitchable second-order nonlinear optical (NLO) materials are highly attractive for optical signal processing. However, achieving high-performance switching in molecular crystals remains challenging, requiring simultaneous optimization of response speed (<120 s saturation), modulation depth (∼90%), and operational durability. Here, we report an asymmetric, purely organic carboxylate-bipyridinium salt crystal that satisfies these criteria: peak ON/OFF NLO contrast high up to 95%, saturation within 120 s, and stable cyclability. Upon irradiation, intramolecular electron transfer generates carboxylate-bipyridinium radicals that are molecularly localized yet electronically coupled through cofacial π−π stacking along extended columns. This radical−π coupling broadens visible absorption and enables efficient, reversible switching. The synergy between structurally permitted photoinduced electron transfer and intermolecular radical coupling establishes a design principle for organic photonic switches combining rapid response with high contrast.