Elucidating the Mechanism of Efficient Eu(III) and Yb(III) Sensitisation from a Re(I) Tetrazolato Triangular Assembly

The reaction of Re(CO)5Br with deprotonated 1H-(5-(2,2':6',2''-terpyridine)pyrid-2-yl)tetrazole yields a triangular assembly formed by tricarbonyl Re(I) vertices. Photophysical measurements reveal blue-green emission with a maximum at 520 nm, 32 % quantum yield, and 2430 ns long-lived excited state decay lifetime in deaerated dichloromethane solution. Coordination of lanthanoid ions to the terpyridine units red-shifts the emission to 570 nm and also reveals efficient (90 %) and fast sensitisation of both Eu(III) and Yb(III) at room temperature, with a similar rate constant kET on the order of 107 s-1. Efficient sensitisation of Eu(III) from Re(I) is unprecedented, especially when considering the close proximity in energy between the donor and acceptor excited states. On the other hand, comparative measurements at 77 K reveal that energy transfer to Yb(III) is two orders of magnitude slower than that to Eu(III). A two-step mechanism of sensitisation is therefore proposed, whereby the rate-determining step is a thermally activated energy transfer step between the Re(I) centre and the terpyridine functionality, followed by rapid energy transfer to the respective Ln(III) excited states. At 77 K, the direct Re(I) to Eu(III) energy transfer seems to proceed via a ligand-mediated superexchange Dexter-type mechanism.

SHG-active luminescent thermometers based on chiral cyclometalated dicyanidoiridate(iii) complexes

Multifunctional optical materials can be realized by combining stimuli-responsive photoluminescence (PL), e.g., optical thermometry, with non-linear optical (NLO) effects, such as second-harmonic generation (SHG). We report a novel approach towards SHG-active luminescent thermometers achieved by constructing unique iridium(iii) complexes, cis-[IrIII(CN)2(R,R-pinppy)2]- (R,R-pinppy = (R,R)-2-phenyl-4,5-pinenopyridine), bearing both a chiral 2-phenylpyridine derivative and cyanido ligands, the latter enabling the formation of a series of molecular materials: (TBA)[IrIII(CN)2(R,R-pinppy)2]·2MeCN (1) (TBA+ = tetrabutylammonium) and (nBu-DABCO)2[IrIII(CN)2(R,R-pinppy)2](i)·MeCN (2) (nBu-DABCO+ = 1-(n-butyl)-1,4-diazabicyclo-[2.2.2]octan-1-ium) hybrid salts, (TBA)2{[LaIII(NO3)3(H2O)0.5]2[IrIII(CN)2(R,R-pinppy)2]2} (3) square molecules, and {[LaIII(NO3)2(dmf)3][IrIII(CN)2(R,R-pinppy)2]}·MeCN (4) coordination chains. Thanks to the chiral pinene group, 1-4 crystallize in non-centrosymmetric space groups leading to SHG activity, while the N,C-coordination of ppy-type ligands to Ir(iii) centers generates visible charge-transfer (CT) photoluminescence. The PL characteristics are distinctly temperature-dependent which was utilized in achieving ratiometric optical thermometry below 220 K. The PL phenomena were rationalized by DFT/TD-DFT calculations indicating an MLCT-type of the emission in obtained Ir(iii) complexes with the rich vibronic structure providing a few emission bands that variously depend on temperature due to the role of thermally activated vibrations. As these crucial vibrational modes depend on the crystal lattice, the thermometry performance differs within 1-4 being the most efficient in 4 while the SHG is by far the best also for 4. This proves that pinene-functionalized cyclometalated dicyanidoiridates(iii) are great prerequisites for tunable PL-NLO conjunction with the most effective multifunctionality ensured by the insertion of these anions into bimetallic frameworks.

Europium(III) Photoluminescence Governed by d8-d10 Heterometallophilic Interactions in Trimetallic Cyanido-Bridged Coordination Frameworks

We report an efficient pathway toward sensitization of red room temperature Eu^III emission by the charge-transfer (CT) states related to d⁸-d¹⁰ heterometallophilic interactions achieved by the simultaneous application of tetracyanidometallates of Pt^II/Pd^II and dicyanidometallates of Au^I/Ag^I in the construction of a trimetallic d-d-f assembly. The combination of Eu³⁺, [M^II(CN)₄]^2- (M = Pt, Pd), and [M^I(CN)₂]^- (M = Au, Ag) ions along with 4,4'-bipyridine N,N'-dioxide (4,4'-bpdo) results in four novel isostructural 2D {[Eu^III(4,4'-bpdo)(H₂O)₂][M^II(CN)₄]}·[M^I(CN)₂]·H₂O (M^II/M^I = Pt/Au, 1; Pt/Ag, 2; Pd/Au, 3; Pd/Ag, 4) coordination networks. They are built of hybrid coordination layers, based on cyanido-bridged {Eu^III[M^II(CN)₄]}_n square grids coexisting with metal-organic {Eu^III(4,4'-bpdo)}_n chains, with the further attachment of [M^I(CN)₂]^- ions through metallophilic {M^II-M^I} interactions. This results in dinuclear {M^IIM^I} units generating an orange emissive metal-to-metal-to-ligand charge-transfer (MMLCT) state, whose energy is tuned by the applied d⁸-d¹⁰ metal centers. Thanks to these CT states, 1-4 exhibit room temperature red Eu^III photoluminescence enhanced by energy transfer from {M^IIM^I} units, with the additional role of 4,4'-bpdo also transferring the energy to lanthanides. These donor CT states lying in the visible range successfully broaden the available efficient excitation range up to 500 nm. The overall emission quantum yield ranges from 8(1)% for 4 to 15(2)% for 1, with the intermediate values for 2 and 3 relatively high among the reported Eu^III-based compounds with tetracyanido- and dicyanidometallates. We found that the sensitization efficiency is equally high for all compounds because of the similar energies of the CT states, while the main differences are related to the observed emission lifetimes ranging from ca. 80 μs for 4 to 120-130 μs for 2 and 3 to ca. 180 μs for 1. This phenomenon was correlated with the energies of the vibrational states, e.g., cyanide stretching vibrations, responsible for nonradiative deactivation of Eu^III excited states, which are the highest for the Pd/Ag pair of 4 and the lowest for the Pt/Au pair in 1. Thus, the heaviest pair of Pt^II/Au^I cyanide metal complexes is proven to be the best candidate for the sensitization of room temperature Eu^III luminescence.

Synthesis and Characterization of Strongly Solvatochromic Molybdenum(III) Complexes

A series of seven molybdenum(III) complexes with the general formula of [Mo(diimine)Cl₄]^- were synthesized and characterized by X-ray diffraction, IR, cyclic voltammetry (CV), and UV-vis. The complexes were discovered to be highly solvatochromic, showing shifts in λ_max between ∼120 and 170 nm in solvents ranging from water to acetone. Varying the substituents on the diimine ligand influenced the absorption energy such that electron-withdrawing groups induced a red shift while electron-donating groups exhibited the opposite effect. The complexes were surprisingly stable in both acidic and basic solutions, and in the case where carboxylic acid substituents were present, additional shifts in the absorption maxima were observed, corresponding to the state of protonation of these groups. Both the Mo^IV/III and Mo^III/II redox couples were observed in CV experiments and were complemented with density functional theory (DFT) calculations.

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.

Lanthanide Photoluminescence in Heterometallic Polycyanidometallate-Based Coordination Networks

Solid-state functional luminescent materials arouse an enormous scientific interest due to their diverse applications in lighting, display devices, photonics, optical communication, low energy scintillation, optical storage, light conversion, or photovoltaics. Among all types of solid luminophors, the emissive coordination polymers, especially those based on luminescent trivalent lanthanide ions, exhibit a particularly large scope of light-emitting functionalities, fruitfully investigated in the aspects of chemical sensing, display devices, and bioimaging. Here, we present the complete overview of one of the promising families of photoluminescent coordination compounds, that are heterometallic d–f cyanido-bridged networks composed of lanthanide(3+) ions connected through cyanide bridges with polycyanidometallates of d-block metal ions. We are showing that the combination of cationic lanthanide complexes of selected inorganic and organic ligands with anionic homoligand [M(CN)_x]ⁿ⁻ (x = 2, 4, 6 and 8) or heteroligand [M(L)(CN)₄]²⁻ (L = bidentate organic ligand, M = transition metal ions) anions is the efficient route towards the emissive coordination networks revealing important optical properties, including 4f-metal-centred visible and near-infrared emission sensitized through metal-to-metal and/or ligand-to-metal energy transfer processes, and multi-coloured photoluminescence switchable by external stimuli such as excitation wavelength, temperature, or pressure.

Linking ReI and PtII Chromophores with Aminopyridines: A Simple Route to Achieve a Complicated Photophysical Behavior

The bifunctional aminopyridine ligands H₂ N-(CH₂ )_n -4-C₅ H₄ N (n=0, L1; 1, L2; 2, L3) have been utilized for the preparation of the rhenium complexes [Re(phen)(CO)₃ (L1-L3)]⁺ (1-3; phen=phenanthroline). Complexes 2 and 3 with NH₂ -coordinated L2 and L3, respectively, were coupled with cycloplatinated motifs {Pt(ppy)Cl} and {Pt(dpyb)}⁺ (ppy=2-phenylpyridine, dpyb=dipyridylbenzene) to give the bimetallic species [Re(phen)(CO)₃ (μ-L2/L3)Pt(ppy)Cl]⁺ (4, 6) and [Re(phen)(CO)₃ (μ-L2/L3)Pt(dpyb)]²⁺ (5, 7). In solution, complexes 4 and 6 show ³ MLCT {Re}-based emission at 298 K, which changes to the ³ IL(ppy) state at 77 K. The photophysical properties of compounds 5 and 7 display a pronounced concentration dependence, presumably due to the formation of bimolecular aggregates. Analysis of the spectroscopic data, combined with TD-DFT simulations, suggest that unconventional heteroleptic {Re(phen)}⋅⋅⋅{Pt(dpyb)} π-π stacking operates as the driving force for ground-state association. The latter, together with intra- and intermolecular energy-transfer processes, determines the appearance of multiple emission bands and results in nonlinear relaxation kinetics of the excited states.

Synthesis, characterization, crystal structure, electrochemical, solvatochromic and biological investigation of novel N4 and N3 type Cu(ii) Schiff base complexes

In the present study, three novel Cu(ii) Schiff base complexes were synthesized and characterized using infrared spectroscopy, elemental analysis, conductivity measurements and X-ray crystallography.

Phosphorescent platinum(ii) alkynyls end-capped with benzothiazole units

Phosphorescent platinum(ii) alkynyls. Phosphorescent trans-bis(alkynyl) bis(phosphine) or bis(cyanide) Pt^II derivatives (1–4) based on the 2-phenylbenzothiazole unit have been prepared. The negative solvatochromic behavior of 4 has been analyzed by the Kamlet–Taft solvent scale and the Gutmann's acceptor numbers. The optical properties were addressed by TD and DFT calculations on 2 and 4.

Fully Ir(iii) tetrazolate soft salts: the road to white-emitting ion pairs

The first examples of anionic Ir(iii) bis-tetrazolate complexes and their combination with a cationic Ir(iii)tetrazole derivative forming “fully tetrazolate” Ir(iii) based soft salts as O₂-sensitive white emitters are described herein.

Water-soluble Ir(iii) complexes of deprotonated N-methylbipyridinium ligands: fluorine-free blue emitters

Using the cation 1-methyl-3-(2′-pyridyl)pyridinium to produce cyclometalating ligands gives novel Ir^III complex salts that are water-soluble and emit efficiently blue or blue-green light.

Exploring energy transfer in luminescent heterometallic ruthenium-iridium ion pairs

Here, we report the synthesis of a luminescent ion pair with the formula [Ru(dtBubpy)3][Ir(ppy)2(CN)2]2. The crystal structure of this three component, heterometallic assembly is described, along with the luminescence properties of the salt. The modulation of the energy transfer between the blue-green-emitting iridium complex and the red-emitting ruthenium complex is also discussed as a function of both medium and concentration.

A new class of highly solvatochromic dicyano rhenate(I) diimine complexes--synthesis, photophysics and photocatalysis

A new class of dicarbonyl dicyano rhenate(I) diimine complexes, cis,trans-[Re(CO)(2)(CN)(2)(N-N)](-), with highly environmentally sensitive MLCT absorption and emission properties was synthesised and characterised. Preliminary experiments revealed that these complexes are active photocatalysts for CO(2) reduction.