Oxygen-Insensitive Aggregates of Pt(II) Complexes as Phosphorescent Labels of Proteins with Luminescence Lifetime-Based Readouts

Abstract

The synthesis and photophysical properties of a tailored Pt(II) complex are presented. The phosphorescence of its monomeric species in homogeneous solutions is quenched by interaction with the solvent and therefore absent even upon deoxygenation. However, aggregation-induced shielding from the environment and suppression of rotovibrational degrees of freedom trigger a phosphorescence turn-on that is not suppressed by molecular oxygen, despite possessing an excited-state lifetime ranging in the microsecond scale. Thus, the photoinduced production of reactive oxygen species is avoided by the suppression of diffusion-controlled Dexter-type energy transfer to triplet molecular oxygen. These aggregates emit with the characteristic green luminescence profile of monomeric complexes, indicating that Pt-Pt or excimeric interactions are negligible. Herein, we show that these aggregates can be used to label a model biomolecule (bovine serum albumin) with a microsecond-range luminescence. The protein stabilizes the aggregates, acting as a carrier in aqueous environments. Despite spectral overlaps, the green phosphorescence can be separated by time-gated detection from the dominant autofluorescence of the protein arising from a covalently bound green fluorophore that emits in the nanosecond range. Interestingly, the aggregates also acted as energy donors able to sensitize the emission of a fraction of the fluorophores bound to the protein. This resulted in a microsecond-range luminescence of the fluorescent acceptors and a shortening of the excited-state lifetime of the phosphorescent aggregates. The process that can be traced by a 1000-fold increase in the acceptor's lifetime mirrors the donor's triplet character. The implications for phosphorescence lifetime imaging are discussed.

Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

“…This is in agreement with previous observations reported in the bibliographic literature, where different Pt(II) and Pd(II) complexes showed emission as monomers, dimers or aggregates. [19][20][21]32,35,50 Φ L(air) ± 2 (%)…”

“…Notably, Pt(II) complexes can form aggregates both in solution and in the solid state. [18][19][20][21][22] Such interactions involve the d 8 -configured metal center and facilitate the intermolecular coupling between d z 2 orbitals protruding out of the coordination plane. Hence, aggregation leads to a red-shifted emission originating from an excimer or an metal-metal-to-ligand charge-transfer ( 3 MMLCT, d-d-) state, which is insensitive to quenching by 3 O 2 .…”

“…Hence, aggregation leads to a red-shifted emission originating from an excimer or an metal-metal-to-ligand charge-transfer ( 3 MMLCT, d-d-) state, which is insensitive to quenching by 3 O 2 . 18,20,21,23 While there is a vast toolbox of luminescent Pt(II) complexes available, luminescent Pd(II) complexes are relatively rare, mainly due to the significantly lower ligand-field splitting related to such coordination compounds. Hence, the metal-centered (MC, d-d*) states of Pd(II) complexes are thermally accessible, and the antibonding d x 2 -y 2 orbital can be populated, favoring a nonradiative deactivation by excited state distortion and conical intersection with the ground state.…”

Platinum-Doped Dendritic Structure as a Phosphorescent Label for Bacteria in Two-Photon Excitation Microscopy

“…While these clusters can even act as donors in energy transfer processes, the free monomeric species in solution are nonemissive. 13 Moreover, we have shown that self-assembled micelles based on tailored copolymers doped with Pt(II) complexes can be tracked by optical and electron microscopy, due to their oxygen-insensitive luminescence and contrast enabled by the high atomic number. The resulting dual readout was employed to track their uptake and localization in eukaryotic cells with high spatiotemporal resolution.…”

Recent progress and future challenges in the supramolecular polymerization of metal-containing monomers

“…Likewise, Strassert, Viappiani and co-workers have exploited self-assembled Pt II complexes as labels for other biomolecules, such as bovine serum albumin (BSA, Figure 18). 143…”