A multi-state photoswitch regulated by higher excited states.

Integrating multiple photoswitching units into a single molecule is a challenging way to achieve the manipulation of molecule geometry by selectively activating individual photoresponsive moieties. A lack of understanding from a mechanism viewpoint obstructs the advancement and realization of such systems. Herein, we construct a new hybrid ionic photoswitch (PZ-SP-MeSO4) that combines spiropyran (SP) and the ionic arylazopyrazolium (PZ) counterpart. Independent modulation of PZ (E/Z) and SP (closed/open) moieties via specific irradiation wavelengths has been achieved. The photostationary state composition (>66% Z-isomer content), fatigue resistance (>15 cycles), thermal stability of the Z-isomer (t 1/2 = 717 days in water) and pH -dependent behavior enabled PZ-SP-MeSO4 to act as a potential hybrid photoswitch for molecular logic gate application, in addition to excellent water solubility (18.4 mM). More importantly, we elucidated the wavelength-gated regulating mechanisms for PZ-SP-MeSO4: 450 nm (S1) excites E → Z isomerization primarily and 365 nm (S3) triggers the SP → MC transformation. Uniquely, upon 420 nm excitation (S2), soon after the E → Z isomerization, the relay SP → MC transformation occurs from the hot ground state species due to excess vibrational energy. The understanding of the multi-state photoswitch regulated by higher excited states in this work establishes a blueprint for designing advanced light-responsive molecules.