Diimine Bis Research Articles

Dicationic Diimine Pt(II) Bis(N-heterocyclic allenylidene) Complexes: Extended Pt···Pt Chains, NIR Phosphorescence, and Chromonics.

Although square-planar Pt(II) complexes are well-known to self-assemble into supramolecules via noncovalent intermolecular Pt···Pt and/or π-π interactions, the self-assembly of dicationic Pt(II) complexes was scarce due to the electrostatic repulsive force. Herein, a series of dicationic diimine bis(N-heterocyclic allenylidene) Pt(II) complexes were synthesized and characterized. Close Pt···Pt and/or π-π contacts are observed in the crystals of these complexes. In particular, complexes 1·2PF6 and 2·2PF6 exhibit one-dimensional packing with extended Pt···Pt contacts of 3.302 and 3.240 Å, respectively. The photophysical properties of these complexes in the solution and solid state were investigated. NIR emission was recorded for complexes 1·2PF6 (λmax = 950 nm) and 2·2PF6 (λmax = 855 nm) in the solid state at 298 K. To explore the aggregate behaviors of these complexes, the counteranion PF6- was exchanged to the large lipophilic anion 2,3,4-tris(dodecyloxy)benzene sulfonate (LA-) and the hydrophilic anion Cl-. Complexes 1·2LA and 2·2LA or 1·2Cl and 2·2Cl could self-assemble with Pt···Pt and/or π-π interactions in the nonpolar or aqueous solutions as well. Further increasing the concentration of 1·2Cl and 2·2Cl in aqueous solution, chromonic mesophases with NIR emission (λmax = 988 nm) were obtained. DFT and TD-DFT calculations were performed to gain deep insight into the dication-dication packings and photophysical properties of the complexes. The σ-donating as well as π-accepting character of the N-heterocyclic allenylidene ligand endows complexes with rigid and electron-delocalized coplanar features, which are conducive to achieving the self-assembling processes associated with Pt···Pt and/or π-π interactions.

1-D "Platinum Wire" Stacking Structure Built of Platinum(II) Diimine Bis(σ-acetylide) Units with Luminescence in the NIR Region.

A square-planar platinum(II) complex, Pt(DiBrbpy)(C≡CC6H4Et-4)2 (1) (DiBrbpy = 4,4-dibromo-2,2'-bipyridine), and crystals of its three solvated forms, namely, 1·DMSO, 1·1/2(CH3CN), and 1·1/8(CH2Cl2), were developed and characterized. 1·DMSO and 1·1/2(CH3CN) contain quasi-dimeric and dimeric structures with luminescence in the visible range, whereas 1·1/8(CH2Cl2) exhibits NIR luminescence at 1022 nm due to its intrinsic 1-D "platinum wire" stacking structure with strong Pt-Pt interactions. 1·1/8(CH2Cl2) represents the first compound based on platinum(II) diimine bis(σ-acetylide) molecular units with the NIR luminescence beyond 1000 nm. 1 selectively responds to DMSO and CH3CN by changing its color and luminescence property and the three solvated forms can be reversibly converted to each other upon exposure to corresponding solvent vapors. Their desolvated forms, namely 1a, 1b, and 1c, obtained after heating 1·DMSO, 1·1/2(CH3CN), and 1·1/8(CH2Cl2), respectively, can also be restored to the original solvated forms upon exposure to corresponding solvent vapors. 1a and 1b emit NIR luminescence peaked at 998 and 1018 nm respectively, suggesting indirect synthetic methods as powerful alternatives to achieve NIR luminescence with long wavelength. In contrast, 1c exhibits a red luminescence with a broad unstructured emission band centered at 667 nm. All the responses to organic solvent vapors and heating are due to the structural transformations which result in the conversion of the lowest energy excited states between 3MLCT/3LLCT and 3MMLCT in solid-state as supported by time-dependent density functional theory (TD-DFT) calculations.

Chromophoric Dyads for the Light-Driven Generation of Hydrogen: Investigation of Factors in the Design of Multicomponent Photosensitizers for Proton Reduction.

Two new dyads have been synthesized and studied as photosensitizers for the light-driven generation of H2 from aqueous protons. One of the dyads, Dy-1, consists of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine benzenedithiolate (bdt) charge transfer (CT) chromophore, denoted as PtN2S2. The two components are connected through an amide linkage on the bdt side of the PtN2S2 complex. The second dyad, Dy-2, contains a diketopyrrolopyrrole dye that is linked directly to the acetylide ligands of a Pt diimine bis(arylacetylide) CT chromophore. The two dyads, as well as the Pt diimine bis(arylacetylide) CT chromophore, were attached to platinized TiO2 via phosphonate groups on the diimine through sonication of the corresponding esters, and each system was examined for photosensitizer effectiveness in photochemical generation of H2 from aqueous protons and electrons supplied by ascorbic acid. Of the three photosensitizers, Dy-1 is the most active under 530 nm radiation with an initial turnover frequency of 260 h(-1) and a total of 6770 turnovers over 60 h of irradiation. When a "white" LED light source is used, samples with Dy-2 and the Pt diimine bis(arylacetylide) chromophore, while not as effective as Dy-1, perform relatively better. A key conclusion is that the presence of a strongly absorbing organic dye increases dyad photosensitizer effectiveness only if the energy of the CT excited state lies below that of the organic dye's lowest excited state; if not, the organic dye does not improve the effectiveness of the CT chromophore for promoting electron transfer and the light-driven generation of H2. The nature of the spacer between the organic dye and the charge transfer chromophore also plays a role in the effectiveness of using dyads to improve light-driven energy-storing reactions.

Synthesis, Photophysics, and Reverse Saturable Absorption of Bipyridyl Platinum(II) Bis(acetylide) Complexes Bearing Aromatic Electron-Withdrawing Substituents on the Acetylide Ligands

Three platinum(II) bipyridyl bis((7-R-fluoren-2-yl)acetylide) complexes (R = benzoyl (Pt-1), 2-(N-phenylbenzimidazoly) (Pt-2), or 2-(3-phenylquinoxalinyl) (Pt-3)) are synthesized and characterized. Their photophysical properties and reverse saturable absorption are systematically investigated via UV-vis absorption, emission, transient absorption, and nonlinear transmission spectroscopy/technique. All three complexes possess ligand-centered (1)π,π* transitions below 400 nm, and a broad, featureless (1)MLCT/(1)LLCT absorption band in the region of 400-550 nm in CH2Cl2 solutions. They are emissive in a variety of fluid solutions at room temperature and at 77 K glassy matrix. On the basis of the emission lifetime, solvatochromic effect, and thermally induced Stokes shift, the emitting states are tentatively ascribed to (3)π,π*/(3)MLCT/(3)LLCT states for Pt-1, and predominantly (3)MLCT/(3)LLCT states for Pt-2 and Pt-3 in polar solvents like CH3CN. Pt-1-Pt-3 also exhibit broad triplet excited-state absorption, i.e., 425-800 nm for Pt-1 and Pt-3, and 425-725 nm for Pt-2, from the same excited states that emit. Strong reverse saturable absorption (RSA) occurs at 532 nm for ns laser pulses from all of the complexes due to the stronger triplet excited-state absorption at this wavelength, suggesting that the π-conjugated aromatic electron-withdrawing substituents at the fluorenylacetylide ligands enhance the RSA of the Pt(II) diimine bis(acetylide) complexes.

Counteranion and donor disposition effects on the topology of luminescent cadmium coordination polymers incorporating bis(pyridylmethyl)piperazine isomers

Four divalent cadmium coordination polymers, incorporating isomers of the hydrogen bonding capable diimine bis(pyridylmethyl)piperazine, have been prepared via solvent diffusion methods. Single crystal X-ray diffraction studies revealed both nitrogen donor disposition and anion dependence on dimensionality and topology. {[Cd(H 2O) 4(4-bpmp)](ClO 4) 2 · (4-bpmp) · 4H 2O} n ( 1, 4-bpmp = bis(4-pyridylmethyl)piperazine) exhibits cationic 1-D [ Cd ( H 2 O ) 4 ( 4 -bpmp ) ] n 2 n + chains that aggregate via extensive hydrogen bonding. {[Cd(3-bpmp) 3(H 2O) 2](ClO 4) 2·5H 2O} n ( 2, 3-bpmp = bis(3-pyridylmethyl)piperazine) manifests 1-D coordination polymer chains with curled, pendant monodentate 3-bpmp ligands. {[Cd(H 2O) 4(4-bpmp)] 2[Cd(H 2O) 3(SO 4)(4-bpmp)][Cd(H 2O) 2(SO 4) 2(4-bpmp)]·10H 2O} n ( 3) displays sets of cationic [ Cd ( H 2 O ) 4 ( 4 -bpmp ) ] n 2 n + chains, neutral [Cd(H 2O) 3(SO 4)(4-bpmp)] n chains and anionic [ Cd ( H 2 O ) 2 ( SO 4 ) 2 ( 4 -bpmp ) ] n 2 n - chains linked into a pseudo 3-D lattice via numerous hydrogen bonding interactions. {CdSO 4(H 2O) 3(3-bpmp) 0.5] n ( 4) possesses a 2-D layered structure in which cadmium ions are connected by bridging 3-bpmp tethers and ligated sulfate ions. All four coordination polymers undergo blue–violet luminescence upon exposure to ultraviolet light.

Syntheses, Luminescence Switching, and Electrochemical Studies of Photochromic Dithienyl-1,10-phenanthroline Zinc(II) Bis(thiolate) Complexes

A series of zinc(II) diimine bis(thiolate) complexes with photochromic diarylethene-containing phenanthroline ligands was synthesized, and their photophysical and photochromic properties were studied. The X-ray crystal structures of two of these complexes have been characterized. All complexes exhibit strong 3LLCT phosphorescence at 510-620 nm in the solid state at 77 and 298 K and in EtOH-MeOH glass at 77 K. Detailed studies revealed that the absorption, emission, and electrochemical properties of the complexes could be readily switched via the photochromic ring-closing and ring-opening reactions.

Cyclometalated Iridium(III) Diimine Bis(biotin) Complexes as the First Luminescent Biotin-Based Cross-Linkers for Avidin

Four luminescent cyclometalated iridium(III) diimine complexes [Ir(N-C)2(N-N)](PF6) (HN-C = 2-(4-(N-((2-biotinamido)ethyl)aminomethyl)phenyl)pyridine, Hppy-4-CH2NHC2NH-biotin, N-N = 3,4,7,8-tetramethyl-1,10-phenanthroline, Me4-phen (1a); N-N = 4,7-diphenyl-1,10-phenanthroline, Ph2-phen (2a); HN-C = 2-(4-(N-((6-biotinamido)hexyl)aminomethyl)phenyl)pyridine, Hppy-4-CH2NHC6NH-biotin, N-N = Me4-phen (1b); N-N = Ph2-phen (2b)), each containing two biotin units, have been synthesized and characterized. The photophysical and electrochemical properties of these complexes have been investigated. Photoexcitation of these iridium(III) diimine bis(biotin) complexes in fluid solutions at 298 K and in alcohol glass at 77 K resulted in intense and long-lived luminescence. The emission is assigned to a triplet metal-to-ligand charge-transfer (3MLCT) (d pi(Ir) --> pi*(N-N)) excited state. The emissive states of complexes 1a,b are probably mixed with some 3IL (pi --> pi*) (Me4-phen) character. The interactions of these iridium(III) diimine bis(biotin) complexes with avidin have been studied by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays and emission titrations. The potential for these complexes to act as cross-linkers for avidin has been examined by resonance-energy transfer- (RET-) based emission quenching experiments, microscopy studies using avidin-conjugated microspheres, and HPLC analysis.

A Photochromic Platinum(II) Bis(alkynyl) Complex Containing a Versatile 5,6-Dithienyl-1,10-phenanthroline

A luminescent diarylethene-functionalized platinum(II) diimine bis(alkynyl) complex displays interesting photochromic behavior with successful photosensitization by excitation into the 3MLCT/LLCT excited state of this complex to trigger photocyclization.

Platinum Chromophore-Based Systems for Photoinduced Charge Separation: A Molecular Design Approach for Artificial Photosynthesis

Photoinduced charge separation is a fundamental step in photochemical energy conversion. In the design of molecularly based systems for light-to-chemical energy conversion, this step is studied through the construction of two- and three-component systems (dyads and triads) having suitable electron donor and acceptor moieties placed at specific positions on a charge-transfer chromophore. The most extensively studied chromophores in this regard are ruthenium(II) tris(diimine) systems with a common 3MLCT excited state, as well as related ruthenium(II) bis(terpyridyl) systems. This Forum contribution focuses on dyads and triads of an alternative chromophore, namely, platinum(II) di- and triimine systems having acetylide ligands. These d8 chromophores all possess a 3MLCT excited state in which the lowest unoccupied molecular orbital is a pi orbital on the heterocyclic aromatic ligand. The excited-state energies of these Pt(II) chromophores are generally higher than those found for the ruthenium(II) tris(diimine) systems, and the directionality of the charge transfer is more certain. The first platinum diimine bis(arylacetylide) triad, constructed by attaching phenothiazene donors to the arylacetylide ligands and a nitrophenyl acceptor to 5-ethynylphenanthroline of the chromophore, exhibited a charge-separated state of 75-ns duration. The first Pt(tpy)(arylacetylide)+-based triad contains a trimethoxybenzamide donor and a pyridinium acceptor and has been structurally characterized. The triad has an edge-to-edge separation between donor and acceptor fragments of 27.95 Angstroms. However, while quenching of the emission is complete for this system, transient absorption (TA) studies reveal that charge transfer does not move onto the pyridinium acceptor. A new set of triads described in detail here and having the formula [Pt(NO2phtpy)(p-C triple-bond C-C6H4CH2(PTZ-R)](PF6), where NO2phtpy = 4'-{4-[2-(4-nitrophenyl)vinyl]phenyl}-2,2';6',2''-terpyridine and PTZ = phenothiazine with R = H, OMe, possess an unsaturated linkage between the chromophore and a nitrophenyl acceptor. While the parent chromophore [Pt(ttpy)(C triple-bond CC6H5)]PF6 is brightly luminescent in a fluid solution at 298 K, the triads exhibit complete quenching of the emission, as do the related donor-chromophore (D-C) dyads. Electrochemically, the triads and D-C dyads exhibit a quasi-reversible oxidation wave corresponding to the PTZ ligand, while the R = H triad and related C-A dyad display a facile quasi-reversible reduction assignable to the acceptor. TA spectroscopy shows that one of the triads possesses a long-lived charge-separated state of approximately 230 ns.

Platinum-Functionalized Chiral Molecular Squares as Light-Emitting Materials

ABSTRACTA family of new chiral metallocycles based on Pt(II) diimine metallocornors and bis(acetylene) bridging ligands have been synthesized, and characterized by a variety of techniques including 1H and 13C NMR, UV-visible, luminescence, infrared, and circular dichroism (CD) spectroscopies, and mass spectrometry. All metallocycles exhibit very strong phosphorescence with quantum yields of 8.3 to 15.7%. Chiral Pt(II)-based molecular squares were used as the light-emitting layer in multiplayer devices, and a maximum brightness of 5470 cd/m2 with a maximum luminous efficiency of 0.93 cd/A was achieved.

Dyads for photoinduced charge separation based on platinum diimine bis(acetylide) chromophores: synthesis, luminescence and transient absorption studies.

The platinum diimine bis(acetylide) chromophore was utilized to explore photoinduced intramolecular reductive quenching with phenothiazine donors in chromophore-donor dyad complexes. Compounds of the general formula Pt(X(2)-bpy)(C triple bond C-p-C(6)H(4)CH(2)(D))(2) (where D = phenothiazine (PTZ) or trifluromethylphenothiazine (TPZ) and X = (t)Bu or CO(2)Et) were synthesized from the corresponding Pt(X(2)-bpy)Cl(2) and aryl acetylene by a CuI-catalyzed coupling reaction. Solvent dependence was explored for the system with X = (t)Bu in MeCN, CH(2)Cl(2), EtOAc, and toluene. Electron transfer quenching of the (3)MLCT excited state of the platinum diimine bis(acetylide) takes place in MeCN leaving no intrinsic emission from the excited state, but in toluene both the PTZ and TPZ dyad complexes exhibit no emission quenching. Picosecond pump-probe transient absorption (TA) experiments were used to monitor decay of the (3)MLCT excited state and electron transfer to form the charge-separated (CS) state. Electrochemical measurements were used to estimate the driving force for charge recombination (CR), with deltaE(CR) based on the reduction potential corresponding to Pt(X(2)-bpy)(C triple bond C-Ar)(2) --> Pt(X(2)-bpy(*)(-))(C triple bond C-Ar)(2) and the oxidation corresponding to donor --> donor(*)(+). Kinetic information from the TA measurements was used to correlate rate and driving force with the electron transfer reactions. Concomitant with the decay of the (3)MLCT excited state was the observation of a transient absorption at ca. 500 nm due to formation of the PTZ or TPZ radical cation in the CS state, with the rate of charge separation, k(CS), being 1.8 x 10(9) to 2 x 10(10) s(-1) for the three dyads explored in MeCN and 1:9 CH(2)Cl(2)/MeCN. The fastest rate of CR occurs for X = CO(2)Et and D = PTZ, the compound with smallest deltaE(CR) = 1.71 V. The rate of CR for dyads with X = (t)Bu and D = PTZ or TPZ was estimated to be 1.7-2.0 x 10(8) s(-1) in MeCN. The slower rate corresponds to a greater driving force for CR, deltaE(CR) = 2.18 and 2.36 V for D = PTZ and TPZ, respectively, suggesting that the driving force for charge recombination places it in the Marcus inverted region.

Synthesis and characterization of platinum diimine bis(acetylide) complexes containing easily derivatizable aryl acetylide ligands.

New Pt(II) diimine bis(acetylide) complexes where the diimine is a substituted bipyridine or phenanthroline and the arylacetylide is 4-ethynylbenzaldehyde have been prepared in good to excellent yields. Spectroscopic characterization supports a square planar coordination geometry with cis-alkynyl ligands, and the crystal structure of one of the complexes, Pt(phen)(Ctbd1;CC(6)H(4)CHO)(2) (1), confirms the assignment. The new diimine bis(acetylide) complexes exhibit an absorption band ca. 400 nm that corresponds to a Pt(d) --> pi diimine charge transfer transition and are brightly emissive in fluid solution, with excited state lifetimes in the range 100-800 ns. Correlation of diimine substituent with lambda(max) for the 400 nm absorption band gives strong support to the MLCT assignment. Complex 1 undergoes electron transfer quenching, showing good Stern-Volmer behavior with a variety of oxidative and reductive quenchers. Quenching studies conducted with DNA nucleosides (A, T, C, G) were also investigated. Silyl-protected adenosine and guanosine were found to quench the luminescence of 1 better than similarly protected cytidine or thymidine. Since the former are the more easily oxidized bases, the results suggest that the Pt(II) diimine bis(acetylide) complexes are more powerful photooxidants than photoreductants with regard to electron transfer to DNA bases.

Toward a molecular photochemical device: a triad for photoinduced charge separation based on a platinum diimine bis(acetylide) chromophore.

A donor−chromophore−acceptor triad (D−C−A) in which the chromophore is a Pt diimine bis(acetylide) moiety (red), the donor is phenothiazine (blue), and the acceptor is nitrophenyl (green) has been synthesized and studied spectroscopically. Irradiation leads to a charge-separated state that has a 70 ns lifetime as measured by transient absorption spectroscopy.

Synthesis and Characterization of (Di-tert-butylbipyridine)bis(pyridine-4-thiolato)platinum(II), Pt(dbbpy)(S-4-py)(2): A Synthon for Supramolecular Systems Containing the Platinum Diimine Bis(Thiolate) Chromophore.

A new diimine bis(thiolate) complex of Pt(II) that is capable of functioning as a synthon for extended and branched structures has been synthesized and characterized. The complex possesses a solvat...