Near-infrared (NIR)-responsive activation of Ru-benziphthalocyanine complexes via singlet-triplet transition.

Near-infrared (NIR) light-responsive molecules are important tools in life sciences due to high tissue transparency and low phototoxicity in this wavelength region. However, conventional compounds are generally activated via S0-S1 transition, which requires relatively high-energy excitation. Here, in order to extend the excitation wavelength into the NIR region, we designed and synthesized Ru π-arene complexes of 6π/18π tautomeric benziphthalocyanines (BPcs), aiming to utilize the low-energy singlet-triplet transition of these complexes for activation. Experimental studies revealed that selective π-coordination of Ru to the benzene ring of BPcs disrupts the strongly NIR-absorptive, 18π-electron aromatic structure of BPcs. Nevertheless, the Ru-BPc complexes still exhibit weak and broad absorption in the NIR region, derived from singlet-triplet transition with a metal-to-ligand charge-transfer character, as indicated by theoretical calculations. NIR irradiation of Ru-BPcs at >800 nm led to efficient dissociation of Ru, releasing BPcs with strong NIR absorption and fluorescence. Thus, these complexes exhibit OFF/ON-type activation via NIR-induced Ru release due to S0-T1 transition.