Charge transfer-triggered reversible spin-state switching

Triggering the spin-state switching (SSS) in solutions at room temperature (RT) has attracted attention in the field of molecular switches. Herein, we report the preparation and switching properties of a ferrous complex Fe(H2Bpz2)2(L-C2H5) (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, L-C2H5 = (4,15-bis[4-(N,N-diethylamino)phenyl][1,2,5]thiadiazolo-[3,4-i]dipyrido[a,c]phenazine)). Upon near-infrared (NIR) laser on, the ferrous complex formed H-aggregation in the solution with enhanced ligand-to-ligand charge transfer, which thus achieved a high-spin state. Upon switching the NIR laser off, the ferrous complex changed to J-aggregation with enhanced intramolecular charge transfer, which resulted in the low-spin state; as a consequence, charge transfer triggered reversible SSS in the solution at RT. More interestingly, further confinement into the solid-state three-dimensional polymer sponge did not affect the SSS performance of the ferrous complex, suggesting the high potential of Fe(H2Bpz2)2(L-C2H5) for practical applications in future molecular electronic and spintronic devices. Charge-transfer triggered reversible spin-state switching on ferrous complex aggregates, both in solution and in the sponge state with alternating laser on and off, has been achieved, providing new avenues for future molecular electronic devices.