Unusual Near Infrared (NIR) Fluorescent Palladium(II) Macrocyclic Complexes Containing M-C Bond with the Bioimaging Capability
NIR luminescent metal complexes are promising probes in bioimaging and biosensing, however generally suffered from the oxygen interference arising from heavy metal effect. We designed new tetradentate macrocyclic benzitripyrrin (C^N^N^N) ligands by combination of M-C bond formation and reducing the π-conjugation to achieve NIR fluorescent Pd complexes (700-1000 nm) with quantum yields up to 14%. To understand the origin of NIR fluorescence, detailed analyses by DFT/TDDFT calculations together with femtosecond and nanosecond transient absorption spectroscopes suggest that the M-C bond formation indeed leads to destabilization of the d–d excited state and less effective quenching of emission; and importantly small spin-orbital coupling (SOC) and large singlet-triplet energy gap are the primary causes of the non-population of triplet states. Comparison of PdII and PtII analogues shows that non-radiative channel of out-plane vibration of tripyrrin plane effectively quenches the fluorescence of PtII complex but not the PdII congener. We also demonstrate the proof-of-concept applications of PdII complexes (Pd-1 and Pd-3) encapsulated in silica nanoparticles, in both in vitro and in vivo bioimaging experiments without oxygen interference. Moreover, pH-induced reversibly switching NIR fluorescence can be achieved even intracellularly using the Pd complex (Pd-2), which are potential to further develop perspective stimuli-responsive NIR materials.