Synthesis, photophysical and electrochemical study of diisocyano-bridged homodinuclear rhenium(I) diimine complexes

The diverse functions of isocyanides in phosphorescent metal complexes

In this Perspective, we highlight many examples of photoluminescent metal complexes supported by isocyanides, with an emphasis on recent developments including several from our own group. Work in this field has shown that the isocyanide can play important structural roles, both as a terminal ligand and as a bridging ligand for polynuclear structures, and can influence the excited-state character and excited-state dynamics. In addition, there are many examples of isocyanide-supported complexes where the isocyanide serves as a chromophoric ligand, meaning the low-energy excited states that are important in the photochemistry are partially or completely localized on the isocyanide. Finally, an emerging trend in the design of luminescent compounds is to use the isocyanide as an electrophilic precursor, converted to an acyclic carbene by nucleophilic addition which imparts certain photophysical advantages. This Perspective aims to show the diverse roles played by isocyanides in the design of luminescent compounds, showcasing the recent developments that have led to a substantial growth in fundamental knowledge, function, and applications related to photoluminescence.

Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans-[Re( N N )(CO)2 (L)2 ]n+ Complexes Containing N-Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies

The combined action of ultraviolet irradiation and microwave heating onto acetonitrile solution of [Re( N N )(CO)₃ (NCMe)]OTf ( N N =phenantroline and neocuproine) afforded cis,trans-Re( N N )(CO)₂ (NCMe)₂ ]⁺ acetonitrile derivatives. Substitution of relatively labile NCMe with a series of aromatic N-donor ligands (pyridine, pyrazine, 4,4'-bipyridine, N-methyl-4,4'-bipyridine) gave a novel family of the diimine cis,trans-[Re( N N )(CO)₂ (L)₂ ]⁺ complexes. Photophysical studies of the obtained compounds in solution revealed unusually high absorption across the visible region and NIR phosphorescence with emission band maxima ranging from 711 to 805 nm. The nature of emissive excited states was studied using DFT calculations to show dominant contribution of ³ MLCT (dπ(Re)→π*( N N )) character. Electrochemical (CV and DPV) studies of the monocationic diimine complexes revealed one reduction and one oxidation wave assigned to reduction of the diimine moiety and oxidation of the rhenium center, respectively.

Supramolecular Construction of Cyanide-Bridged ReI Diimine Multichromophores

The reactions of labile [Re(diimine)(CO)₃(H₂O)]⁺ precursors (diimine = 2,2'-bipyridine, bpy; 1,10-phenanthroline, phen) with dicyanoargentate anion produce the dirhenium cyanide-bridged compounds [{Re(diimine)(CO)₃}₂CN)]⁺ (1 and 2). Substitution of the axial carbonyl ligands in 2 for triphenylphosphine gives the derivative [{Re(phen)(CO)₂(PPh₃)}₂CN]⁺ (3), while the employment of a neutral metalloligand [Au(PPh₃)(CN)] affords heterobimetallic complex [{Re(phen)(CO)₃}NCAu(PPh₃)]⁺ (4). Furthermore, the utilization of [Au(CN)₂]^-, [Pt(CN)₄]^2-, and [Fe(CN)₆]^4-/3- cyanometallates leads to the higher nuclearity aggregates [{Re(diimine)(CO)₃NC} _xM] ^m+ (M = Au, x = 2, 5 and 6; Pt, x = 4, 7 and 8; Fe, x = 6, 9 and 10). All novel compounds were characterized crystallographically. Assemblies 1-8 are phosphorescent both in solution and in the solid state; according to the DFT analysis, the optical properties are mainly associated with charge transfer from Re tricarbonyl motif to the diimine fragment. The energy of this process can be substantially modified by the properties of the ancillary ligands that allows to attain near-IR emission for 3 (λ_em = 737 nm in CH₂Cl₂). The Re-Fe^II/III complexes 9 and 10 are not luminescent but exhibit low energy absorptions, reaching 846 nm (10) due to Re^I → Fe^III transition.

The binuclear dual emitter [Br(CO)3Re(PN)(NP)Re(CO)3Br] (PN): 3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine, a new bridging P,N-bidentate ligand resulting from the ring opening of tetrahydrofuran

Lithium diphenylphosphide unexpectedly provokes the ring-opening of tetrahydrofuran (THF) and by reaction with 3,6-dichloropyridazine leads to the formation of the ligand 3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine (P⋯N), which was isolated. The reaction of this ligand with the (Re(CO)₃(THF)Br)₂ dimer yields the novel complex [Br(CO)₃Re(μ-3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine)₂Re(CO)₃Br] (BrRe(P⋯N)(N⋯P)ReBr), which was crystallized in the form of a chloroform solvate, (C₄₆H₄₀Br₂Cl₂N₄O₈P₂Re₂)·(CHCl₃). The monoclinic crystal (P2₁/n) displays a bimetallic cage structure with a symmetry inversion centre in the middle of the rhenium to rhenium line. The molecule shows two oxidation signals occurring at +1.50 V and +1.76 V which were assigned to the Re^I/Re^II and Re^II/Re^III metal-centered couples, respectively, while signals observed at -1.38 V and -1.68 V were assigned to ligand centered reductions. Experimental and DFT/TDDFT results indicate that the UV-Vis absorption maximum of BrRe(P⋯N)(N⋯P)ReBr occurring near 380 nm displays a metal to ligand charge transfer (MLCT) character, which is consistent with CV results. Upon excitation at this wavelength, a weak emission (Φ_em < 1 × 10^-3) is observed around 580 nm (in dichloromethane) which decays with two distinct lifetimes τ₁ and τ₂ of 24 and 4.7 ns, respectively. The prevalence of non-radiative deactivation pathways is consistent with efficient internal conversion induced by the high conformational flexibility of the P⋯N ligand's long carbon chain. Measurements in a frozen solvent at 77 K, where vibrational deactivation is hindered, show intense emission associated with the ³MLCT state. These results demonstrate that BrRe(P⋯N)(N⋯P)ReBr preserves the dual emitting nature previously reported for the mononuclear complex RePNBr, with emission associated with and states.

Fluorescence probes for prokaryotic and eukaryotic cells using Re(CO)3+complexes with an electron withdrawing ancillary ligand

Re(CO)₃⁺complexes with an ancillary ligand present an electron withdrawing effect suitable for cell imaging.