Unusual near infrared (NIR) fluorescent palladium(ii) macrocyclic complexes containing M–C bonds with bioimaging capability

Near infrared (NIR) luminescent metal complexes are promising probes in bioimaging and biosensing, however they generally suffer from oxygen interference arising from heavy metal effects. 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 density functional theory/time-dependent density functional theory (DFT/TDDFT) calculations together with femtosecond and nanosecond transient absorption spectroscopies suggest that 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 the large singlet-triplet energy gap are the primary causes of the non-population of triplet states. Comparison of Pd(II) and Pt(II) analogues shows that the non-radiative channel of the out-plane vibration of the tripyrrin plane effectively quenches the fluorescence of the Pt(II) complex but not the Pd(II) congener. We also demonstrate the proof-of-concept applications of Pd(II) 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 reversible switching of NIR fluorescence was achieved even intracellularly using the Pd complex (Pd-2), which shows the potential to further develop perspective stimuli-responsive NIR materials.