Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Syntheses, structural, photophysical and theoretical studies of heteroleptic cycloplatinated guanidinate(1-) complexes bearing acetylacetonate and picolinate ancillary ligands

Cycloplatination of symmetrical N,N',N''-triarylguanidines, (ArNH)2C[double bond, length as m-dash]NAr with cis-[Pt(TFA)2(S(O)Me2)2] in toluene afforded cis-[Pt(TAG)(TFA)(S(O)Me2)] (TAG = triarylguanidinate(1-)-κC,κN; TFA = OC(O)CF3; 6-9) in 75-82% yields. The reactions of 6-9 and the previously known cis-[Pt(TAG)X(S(O)Me2)] (X = Cl (1) and TFA (2-5)) with acetylacetone (acacH) or 2-picolinic acid (picH) in the presence of a base afforded [Pt(TAG)(acac)] (acac = acetylacetonate-κ2O,O'; 10-18) and [Pt(TAG)(pic)] (pic = 2-picolinate-κN,κO; 19) in high yields. The new complexes were characterised by analytical, IR and multinuclear NMR spectroscopies. Further, molecular structures of 11, 12, 13·0.5 toluene and 14-19 were determined by single crystal X-ray diffraction. Absorption spectra of 10-19 in solution and their emission spectra in crystalline form were measured. Platinacycles 10-19 are bluish green light emitter in the crystalline form, and emit in the λPL = 488-529 nm range (11 and 13-19) while 12 emits at λPL = 570 nm. Unlike other platinacycles, the emission band of 12 is broad, red shifted, and this pattern is ascribed to the presence of an intermolecular N-H⋯Pt interaction involving the endocyclic amino unit of the six-membered [Pt(TAG)] ring and the Pt(ii) atom in the adjacent molecule in an asymmetric unit of the crystal lattice. Lifetime measurements were carried out for all platinacycles in crystalline form, which revealed lifetime in the order of nanoseconds. The origin of absorption and emission properties of 11, 15, 18 and 19 were studied by TD-DFT calculations.

Cyclometalated Platinum(II) Metallomesogens Based on Half-Disc-Shaped β-Diketonate Ligands with Hexacatenar: Crystal Structures, Mesophase Properties, and Semiconductor Devices

A series of new half-disc-shaped platinum(II) complexes [Pt(ppy)(ALⁿ-6OC_nH_2n+1)] (Pt-An), [Pt(ppyF)(ALⁿ-6OC_nH_2n+1)] (Pt-Bn), and [Pt(ppyCF₃)(ALⁿ-6OC_nH_2n+1)] (Pt-Cn) (ALⁿ-6OC_nH_2n+1 = 1,3-bis(3,4,5-trialkoxyphenyl)propane-1,3-dionato; n = 1, 6, 12) with concise structures have been designed and synthesized, in which 2-phenylpyridine (ppy) derivatives were used as cyclometalated ligands and hexacatenar β-diketonate derivatives ALⁿ-6OC_nH_2n+1 as auxiliary ligands. The single-crystal data of the methoxy diketonate analogues Pt-A1, Pt-B1, and Pt-C1 indicate that they all display excellent square planarity. These platinum(II) complexes show a certain emission tunability (ranging from λ = 506-535 nm) by the introduction of fluorine or trifluoromethyl into ppy. Thermal studies reveal that the fluorine-substituted complexes are liquid crystals but the trifluoromethyl-substituted complexes are not. The platinum(II) complexes Pt-A12, Pt-B6, and Pt-B12 can form a hexagonal columnar mesophase via intermolecular π-π interactions. In addition, compared to the reported platinum(II) metallomesogens, Pt-A12 and Pt-B12 exhibit improved ambipolar carrier mobility behaviors in semiconductor devices at the liquid crystal states.

Organoplatinum(II) Cruciform: A Versatile Building Block to Fabricate 2D Microcrystals with Full-Color and White Phosphorescence and Anisotropic Photon Transport

Conventional square-planar platinum complexes typically form one-dimensional assemblies as a result of unidirectional metallophilic and/or π⋅⋅⋅π intermolecular interactions. Organoplatinum(II) complexes with a cruciform shape are presented herein to construct two-dimensional (2D) microcrystals with full-color and white phosphorescence. These 2D crystals show unique monocomponent π⋅⋅⋅π stacking, from either the cyclometalating or noncyclometalating ligand, and the bicomponent alternate π⋅⋅⋅π stacking from both ligands along different facet directions. Anisotropic tri-directional waveguiding is further implemented on a single hexagonal microcrystal. These results demonstrate the great capability of the organoplatinum(II) cruciform as a general platform to fabricate 2D phosphorescent micro-/nanocrystals for advanced photonic applications.

Efficiently luminescent heteroleptic neutral platinum(II) complexes based on N^O and N^P benzimidazole ligands

A series of new luminescent cycloplatinated(II) complexes (5a-8a and 5b-8b) with formulas Pt(bt)(N^O) and Pt(bt)(N^P) have been synthesized [bt = phenylbenzothiazole, N^O = (2-(1H-benzimidazole)-phenyl)diphenylphosphine oxide derivatives for 1a-4a and N^P = (2-(1H-benzimidazole)-phenyl)diphenylphosphine derivatives for 1b-4b]. The crystal structures of the complexes show distorted square planar geometries around the platinum centers. There are no obvious π-π and Pt-Pt intermolecular interactions in the crystal lattice due to the presence of sterically bulky ancillary ligands. Consequently, these complexes exhibit structured monomeric emissions in the range of 527-540 nm in CH₂Cl₂ solution. The photoluminescent quantum yields of Pt(bt)(N^O) (5a-8a) in CH₂Cl₂ solution at room temperature are higher than those of Pt(bt)(N^P) (5b-8b). The above result is well consistent with the crystal structural characteristics of the complexes. The structured emission with microsecond radiative lifetimes and the result of TD-DFT calculations indicate that the emissions of these complexes are mainly attributed to a mixed ³LC-MLCT state.

Dipyrrolyldiketone PtII Complexes: Ion-Pairing π-Electronic Systems with Various Anion-Binding Modes

A variety of π-electronic ion-pairing assemblies can be constructed by combining anion complexes of π-electronic systems and countercations. In this study, a series of anion-responsive π-electronic molecules, dipyrrolyldiketone Pt^II complexes containing a phenylpyridine ligand, were synthesized. The resulting Pt^II complexes exhibited phosphorescence emission, with higher emission quantum yields (0.30-0.42) and microsecond-order lifetimes, and solution-state anion binding, as revealed by our spectroscopic analyses. These Pt^II complexes displayed solid-state ion-pairing assemblies, exhibiting various anion-binding modes, which derived from pyrrole-inverted and pyrrole-non-inverted conformations, and packing structures, with the contribution of charge-by-charge assemblies, which were dependent on the substituents in the Pt^II complexes and the geometries and electronic states of their countercations.

Effect of the Fluoro-Substituent Position on the Crystal Structure and Photoluminescence of Microcrystals of Platinum β-Diketonate Complexes

Molecular packing has an important effect on the photophysical properties of crystalline materials. We demonstrate in this work the modulation of molecular packing and emission properties of microcrystals by minor molecular structural variations. Four platinum β-diketonate complexes, with two fluoro substituents (1) or one fluoro atom substituted on different positions of the auxiliary phenylpyridine ligand (2-4) have been synthesized. These complexes were used to prepare one-dimensional microcrystals with well-defined shapes and uniform sizes. Although 1-4 display similar emission spectra in the solution state, the corresponding microcrystals display different emission colors from green to yellow and orange. In addition, different temperature-responsive (80-298 K) emission spectral changes have been observed from these microcrystals, including the intensity variation of the locally excited (LE) emission without obvious wavelength shifts, competition between the LE and metal-metal-to-ligand charge-transfer emissions, and the sole wavelength shift of the π-π excimer emissions. These differences in emission properties are rationalized by different molecular packings of these materials, as revealed by single-crystal X-ray analyses.

π-Hole···dz2[PtII] Interactions with Electron-Deficient Arenes Enhance the Phosphorescence of PtII-Based Luminophores

Two phosphorescent Pt^II-based cyclometalated complexes were co-crystallized with perfluorinated arenes to give 1:1 co-crystals. The X-ray study revealed that each of the complexes is embraced by arenes^F to give infinite reverse sandwich structures. In four out of six structures, a d_z² orbital of Pt^II is directed to the arenes^F ring via π-hole···d_z²[Pt^II] interactions, whereas in the other two structures, the filled d_z² orbital is directed toward the arene C atoms. Computed molecular electrostatic potential surfaces of the arenes^F and the complexes, noncovalent interaction indexes for the co-crystals, and natural bond orbital calculations indicate that π-hole···d_z²[Pt^II] contacts (and, generally, the stacking) are of electrostatic origin. The solid-state photophysical study revealed up to 3.5-fold luminescence quantum yield and 15-fold lifetime enhancements in the co-crystals. This increase is associated with the strength of the π-hole···d_z²[Pt^II] contact that is dependent on the π-acidity of the arene^F and its spatial characteristics.

Photoluminescent Anisotropy Amplification in Polymorphic Organic Nanocrystals by Light-Harvesting Energy Transfer

Polymorphism and anisotropy are fundamental phenomena of crystalline materials. However, the structure-dependent photoluminescent (PL) anisotropy in polymorphic organic crystals has remained unexplored. Herein, two polymorphic nanocrystals, green-emitting nanorods (PtD-g) and yellow-emitting nanoplates (PtD-y), were obtained from a platinum(II)-β-diketonate complex. The PtD-y crystals display remarkable PL anisotropy with an anisotropy ratio of up to 0.87 whereas the emission of the PtD-g crystals is nearly unpolarized. The polarization properties are rationalized on the different molecular packing of these crystals. By light-harvesting energy transfer, the PtD-y crystals are successfully used to amplify the emission polarization of a red-emitting platinum acceptor (PtA) doped into the donor crystalline matrix, which is otherwise weakly polarized as pure crystals.

An efficient room-temperature synthesis of highly phosphorescent styrenic Pt(ii) complexes and their polymerization by ATRP

Cyclometalated Pt(ii) complexes are among the most efficient phosphorescent materials, yet their incorporation into polymers remains rare.

Protein staining agents from low toxic platinum(ii) complexes with bidentate ligands

Low toxic platinum(ii) complexes with bidentate ligands have been developed as novel protein staining agents.

Influence of ancillary ligands on the photophysical properties of cyclometalated organoplatinum(ii) complexes

The photophysical properties of three series of cyclometalated organoplatinum(ii) complexes were studied and the influence of changing the ancillary ligands and substituents on their luminescence properties were investigated.

Luminescent Cyclometalated Platinum Complexes with π-Bonded Catecholate Organometallic Ligands

A series of cyclometalated platinum(II) complexes of the type [(ppy)Pt(L_M)]ⁿ⁺ (n = 0, 1) with π-bonded catecholates acting as organometallic ligands (L_M) have been prepared and characterized by analytical techniques. In addition, the structures of two complexes of the series were determined by single-crystal X-ray diffraction. The packing shows the formation of a 1D supramolecular assembly generated by d_Pt-π_Cp* interactions among individual units. All complexes are luminescent in the solid state and in solution media. The results of photophysics have been rationalized by means of density functional theory (DFT) and time-dependent DFT investigations.

The phosphorescence properties of a series of diarylethene-containing platinum complexes: the effect of ligand photoisomerization

Benefitting from isomerization in the ancillary-ligand diarylethene, a control of phosphorescence efficiency becomes available.

HOMO-LUMO energy gap control in platinum(II) biphenyl complexes containing 2,2'-bipyridine ligands

A series of platinum(ii) biphenyl 2,2'-bipyridine complexes containing electron-donating and electron-withdrawing moieties on the 4 and 4' positions of the bipyridine ligand exhibit emission from excited states in the 600 nm region of the spectrum upon excitation in the metal-to-ligand charge transfer transition located near 450 nm. These complexes are distorted from planarity based on both single crystal structure determinations and density functional theory (DFT) calculations of isolated molecules in acetonitrile. The DFT also reveals the geometry of the lowest-lying triplet state (LLTS) of each complex that is important for emission behavior. The LLTS are assigned based on the electron spin density distributions and correlated with the singlet excited states to understand the mechanism of electronic excitation and relaxation. Time-dependent DFT calculations are performed to compute the singlet excited state energies of these complexes so as to help interpret their UV-Vis absorption spectra. Computational and experimental results, including absorption and emission energy maxima, electrochemical reduction potentials, LLTS, singlet excited states, and LUMO and HOMO energies, exhibit linear correlations with the Hammett constants for para-substituents σp. These correlations are employed to screen complexes that have not yet been synthesized. The correlation analysis indicates that the electronic structure and the HOMO-LUMO energy gap in Pt(ii) complexes can be effectively controlled using electron-donating and electron-withdrawing moieties covalently bonded to the ligands. The information presented in this paper provides a better understanding of the fundamental electronic and thermodynamic behavior of these complexes and could be used to design systems with specific applications.

Tuning of the excitation wavelength in Eu3+-aminophenyl based polyfluorinated β-diketonate complexes: a red-emitting Eu3+-complex encapsulated in a silica/polymer hybrid material excited by blue light

A series of new antenna complexes of Eu³⁺ based on aminophenyl β-diketonate ligands was designed and their photophysical properties were evaluated.

A unique class of neutral cyclometalated platinum(ii) complexes with π-bonded benzenedithiolate: synthesis, molecular structures and tuning of luminescence properties

A unique class of luminescent neutral cyclometalated platinum(ii) complexes 2–4 with π-bonded benzenedithiolate are reported including two X-ray molecular structures.

Rare observation of ‘aggregation induced emission’ in cyclometalated platinum(ii) complexes and their biological activities

Three strong solid state emissive cyclometalated platinum(ii) complexes showing AIE property were synthesized and the feasibility of one of the dyes for cell imaging was reported.

When self-assembly meets biology: luminescent platinum complexes for imaging applications

Self-assembled luminescent structures based on platinum complexes. A new tool for bioimaging?

Photophysical properties of an unusual bichromophoric species constructed from a cyclometalated Pt(ii) chromophore and a blue Bodipy-acetylacetonate species

Complex 1 exhibits a formally forbidden energy transfer from the metal-based triplet to the fluorescent Bodipy singlet state.

Photophysical properties of cyclometalated Pt(II) complexes: counterintuitive blue shift in emission with an expanded ligand π system

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C(^)N)Pt(O(^)O) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (C(^)N = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); O(^)O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C(^)N ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (E(0-0) = 479 nm, 1; E(0-0) = 495 nm, 2; E(0-0) = 470 nm, 3) with disparate radiative rate constants (kr = 1.4 × 10(5) s(-1), 1; kr = 0.10 × 10(5) s(-1), 2; kr = 2.6 × 10(5) s(-1), 3). Zero-field splittings of the triplet states (ΔE(III-I) = 11.5 cm(-1), 1'; ΔE(III-I) < 2 cm(-1), 2; ΔE(III-I) = 46.5 cm(-1), 3) were determined using high resolution spectra recorded in Shpol'skii matrices. The fact that the E0-0 energies do not correspond to the extent of π-conjugation in the aromatic C(^)N ligand is rationalized on the basis of structural distortions that occur upon cyclometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the C(^)N ligands in the triplet state alter the spin-orbit coupling in the complexes.

Synthesis and photophysical properties of difluoroboron complexes of curcuminoid derivatives bearing different terminal aromatic units and a meso-aryl ring

The synthesis of nine curcuminoids and their difluoroboron complexes is described, with seven of them containing a meso-phenyl ring. Dynamic (19)F NMR confirmed the fact that rotation of that meso-aryl fragment is restricted in the latter systems at room temperature and become allowed at higher temperature (>45 °C). The molecular structure of a meso-substituted derivative in the solid state showed that the phenyl ring lies in a highly twisted plane with respect to the mean curcuminoid plane. The photophysical properties of the nine compounds were investigated in solvents of different polarity. Meso-substitution with a phenyl ring has little influence on fluorescence emission properties in solution, radiative and nonradiative kinetic constants being similar for meso- and nonsubstituted compounds, which is in contrast to the case of BODIPY derivatives. However, introduction of an electron donor p-methoxy group at the meso-phenyl ring leads to small perturbation of the curcuminoid π-system fluorescence emission. We also report the influence of the meso-phenyl group on the emission properties of the aggregated solids.