Room Temperature In Fluid Solution Research Articles

Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes.

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts due to the lack of deactivating d-d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula Cp2Ti(C2Ar)2·MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp2Ti(C2Ph)2. TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium 3LMCT in all cases except Cp2Ti(C2Ph)2·CuBr, which is dominated by CuBr-to-Ti charge transfer. In room-temperature fluid solution, only Cp2Ti(C2Ph)2 and Cp2Ti(C2Ph)2·AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10-4. The parent complexes photodecompose in room-temperature solution with quantum yields, Φrxn, between 0.25 and 0.99. The coordination of MX decreases Φrxn by two to three orders of magnitude. There is a clear trend that Φrxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the 3LMCT state and the singlet transition state for decomposition.

Cover Picture: Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex (Angew. Chem. Int. Ed. 50/2019)

Inversion symmetry increases the excited-state lifetime of a molecular ruby at room temperature in fluid solution to 4.5 ms while the photoluminescence quantum yield remains as high as 8.2 %, as shown by K. Heinze and co-workers in their Research Article on page 18075. The chromium(III) complex [Cr(tpe)2]3+ is furthermore competent in light-induced electron and energy transfer processes, and thus resembles classical noble metal complexes.

Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

AbstractPhotoactive metal complexes employing Earth‐abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non‐innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (τ=4500 μs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2]3+ are redox non‐innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2]3+ surpass those of the classical photosensitizer [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n‐butyl)amine).

Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

Photoactive metal complexes employing Earth‐abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non‐innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (τ=4500 μs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2]3+ are redox non‐innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2]3+ surpass those of the classical photosensitizer [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n‐butyl)amine).

Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)2]2+

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1 crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2 state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2 state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2 state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1 ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.

A comparison of the metal-ligand interactions of the pentafluorophenylethynyl and trifluoropropynyl ligands in transition metal cyclam complexes

The pentafluorophenylethynyl ligand, C2C6F5, is a commonly used electron withdrawing ligand. In the context of developing complexes with blue-shifted emission for organic light emitting diode applications, it is often necessary to have electron poor ligands that are also relatively high on the spectrochemical series. The trifluoropropynyl ligand, C2CF3, is also electron withdrawing and has never been directly compared to C2C6F5. Herein complexes of the type trans-[M(cyclam)(C2R)2]OTf, where M = CoIII and CrIII and C2R = C2C6F5, are prepared and characterized by X-ray crystallography, cyclic voltammetry, and UV-Vis, IR, and Raman spectroscopy. Emission spectroscopy for the CrIII complex is also reported. These C2C6F5 complexes are compared to the corresponding complexes where C2R = C2CF3, and C2C6H5. For trans-[Co(cyclam)(C2R)2]OTf, the reversible CoIII/II reduction for the C2C6F5 complex occurs 120 mV more cathodic than that for the C2CF3 complex, indicating that C2CF3 is more electron withdrawing, a conclusion that is supported by analysis of CC stretching frequencies. The lowest energy metal-centered (MC) 1A1g → 1T2g absorption band for the C2CF3 and C2C6F5 complexes in CH3CN appear at 425 and 438 nm, respectively, suggesting that C2CF3 is a stronger field ligand. In the region of the LMCT transitions, the absorption spectra for the trans-[Cr(cyclam)(C2R)2]OTf complexes, where C2R = C2C6H5 and C2C6F5, show significant vibronic structure that is also superimposed on the MC transitions, suggesting some degree of charge transfer character in the latter. The CrIII complexes are all emissive in room-temperature fluid solution with the C2CF3 complex showing significant fine structure and the C2C6H5 and C2C6F5 complex emission being broad, structureless, and redshifted by comparison. This suggests that emission in the C2CF3 complex is of 2Eg (Oh) origin and emission for the C2C6H5 and C2C6F5 complexes is of 2T1g (Oh) origin. Emission for all CrIII complexes in 77 K glass shows significant fine structure. The presence of a dominant 0-0′ emission band in the C2C6H5 and C2C6F5 complexes suggests a non-centrosymmetric ground state. This is supported by the crystal structure for trans-[Cr(cyclam)(C2C6F5)2]OTf that shows an angle of 77.8° between the planes defined by the aryl rings.

Understanding the Excited State Behavior of Cyclometalated Bis(tridentate)ruthenium(II) Complexes: A Combined Experimental and Theoretical Study.

The synthesis and characterization of the donor-acceptor substituted cyclometalated ruthenium(II) polypyridine complex isomers [Ru(dpb-NHCOMe)(tpy-COOEt)](PF6) 1(PF6) and [Ru(dpb-COOEt)(tpy-NHCOMe)](PF6) 2(PF6) (dpbH = 1,3-dipyridin-2-ylbenzene, tpy = 2,2';6,2"-terpyridine) with inverted functional group pattern are described. A combination of resonance Raman spectroscopic and computational techniques shows that all intense visible range absorption bands arise from mixed Ru → tpy/Ru → dpb metal-to-ligand charge transfer (MLCT) excitations. 2(PF6) is weakly phosphorescent at room temperature in fluid solution and strongly emissive at 77 K in solid butyronitrile matrix, which is typical for ruthenium(II) polypyridine complexes. Density functional theory calculations revealed that the weak emission of 2(PF6) arises from a (3)MLCT state that is efficiently thermally depopulated via metal-centered ((3)MC) excited states. The activation barrier for the deactivation process was estimated experimentally from variable-temperature emission spectroscopic measurements as 11 kJ mol(-1). In contrast, 1(PF6) is nonemissive at room temperature in fluid solution and at 77 K in solid butyronitrile matrix. Examination of the electronic excited states of 1(PF6) revealed a ligand-to-ligand charge-transfer ((3)LL'CT) as lowest-energy triplet state due to the very strong push-pull effect across the metal center. Because of the orthogonality of the participating ligands, emission from the (3)LL'CT is symmetry-forbidden. Hence, in this type of complex a stronger push-pull effect does not increase the phosphorescence quantum yields but completely quenches the emission.

Ruthenium Bis-diimine Complexes with a Chelating Thioether Ligand: Delineating 1,10-Phenanthrolinyl and 2,2′-Bipyridyl Ligand Substituent Effects

Despite the high π-acidity of thioether donors, ruthenium(II) complexes with a bidentate 1,2-bis(phenylthio)ethane (dpte) ligand and two chelating diimine ligands (i.e., Ru(diimine)2(dpte)(2+)) exhibit room-temperature fluid solution emission originating from a lowest MLCT excited state (diimine = 2,2'-bipyridine, 5,5'-dimethyl-2,2'-bipyridine 4,4'-di-tert-butyl-2,2'-bipyridine, 1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 5-chloro-1,10-phenanthroline, 5-bromo-1,10-phenanthroline, 5-nitro-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, and 3,4,7,8-tetramethyl-1,10-phenanthroline). Crystal structures show that the complexes form 2 of the 12 possible conformational/configurational isomers, as well as nonstatistical distributions of geometric isomers; there also are short intramolecular π-π interactions between the diimine ligands and dpte phenyl groups. The photoinduced solvolysis product, [Ru(diimine)2(CH3CN)2](PF6)2, for one complex in acetonitrile also was characterized by single-crystal X-ray diffraction. Variations in the MLCT energies and Ru(III/II) redox couple, E°'(Ru(3+/2+)), can be understood in terms of the influence of the donor properties of the ligands on the mainly metal-based HOMO and mainly diimine ligand-based LUMO. E°'(Ru(3+/2+)) also is quantitatively described using a summative Hammett parameter (σT), as well as using Lever's electrochemical parameters (EL). Recommended parametrizations for substituted 2,2'-bipyridyl and 1,10-phenanthrolinyl ligands were derived from analysis of correlations of E°'(Ru(3+/2+)) for 99 homo- and heteroleptic ruthenium(II) tris-diimine complexes. This analysis reveals that variations in E°'(Ru(3+/2+)) due to substituents at the 4- and 4'-positions of bipyridyl ligands and 4- and 7-positions of phenanthrolinyl ligands are significantly more strongly correlated with σp(+) than either σm or σp. Substituents at the 5- and 6-positions of phenanthrolinyl ligands are best described by σm and have effects comparable to those of substituents at the 3- and 8-positions. Correlations of EL with σT for 1,10-phenanthrolinyl and 2,2'-bipyridyl ligands show similar results, except that σp and σp(+) are almost equally effective in describing the influence of substituents at the 4- and 4'-positions of bipyridyl ligands. MLCT energies and d(5)/d(6)-electron redox couples of the complexes with 5-substituted 1,10-phenanthroline exhibit correlations with values for other d(6)-electron metal complexes that can be rationalized in terms of the relative number of diimine ligands and substituents.

Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Thermal and Photoinduced Electron Transfer in Directional Bis(terpyridine)ruthenium(II)–(Bipyridine)platinum(II) Complexes

AbstractMetalloligands L1 and L2 consisting of directional bis(terpyridine)ruthenium(II) units and bipyridine moieties were constructed by amide formation. From these metalloligands two Ru–Pt heterobimetallic complexes 1 and 2 were derived by a building‐block method by means of platination with [PtCl2(dmso)2]. Both bimetallic complexes 1 and 2 feature metal‐to‐ligand charge transfer (MLCT) absorptions, and emission occurs at room temperature in fluid solution from 3MLCT(Ru) states in all cases. Energy transfer from platinum to ruthenium is observed in 2 but not in 1 (light harvesting). The one‐electron‐reduced species [1]– and [2]– were prepared by reduction of 1 and 2 with decamethylcobaltocene. EPR spectra and DFT calculations reveal that the spin density is localized at the tpy–CO/Ru (tpy = terpyridine) site in [1]–, whereas it is centered at bpy–CO/Pt (bpy = 2,2′‐bipyridine) in [2]–. Efficient photoinduced electron transfer from triethanolamine to 1 and 2 is feasible by excitation at 500 nm [MLCT(Ru)].

Reaction of 4-(2,2-dimethylhydrazino)dimethylhydrazone-3(Z)-penten-2-one with BrRe(CO)5 and fac-BrRe(CO)3(THF)2: Synthesis, structural characterization, and DFT examination of the β-diketimine-substituted compound fac-BrRe(CO)3[(Me2NNCMe)2CH2

Thermolysis of BrRe(CO)5 with 4-(2,2-dimethylhydrazino)dimethylhydrazone-3(Z)-penten-2-one in toluene at 70 °C furnishes the new β-diketimine-substituted complex fac-BrRe(CO)3[(Me2NNCMe)2CH2] (1) in 50–70% isolated yield. Product 1 is also obtained in comparable yield when the same reactants are irradiated at 366 nm at room temperature in fluid solution. Treatment of the parent ligand with the “lightly stabilized” rhenium compound fac-BrRe(CO)3(THF)2 affords 1 as the sole observable rhenium product. Complex 1 has been characterized in solution by IR and 1H NMR spectroscopy, and the molecular structure has been determined by single-crystal X-ray diffraction analysis. The solid-state structure confirms that the ancillary nitrogen ligand in 1 coordinates to the rhenium center as a chelating β-diketimine moiety, making this the first structurally characterized example of a β-diketimine derivative containing the parent ligand. Complex 1 has been examined by DFT analysis, and the ground-state structure computed for 1 is in agreement with the X-ray diffraction structure. Treatment of 1 with the phosphines Ph2PH and 1,2-(Z)-Ph2PCHCHPPh2 (dppen) leads to loss of the β-diketimine ligand and formation of fac-BrRe(CO)3(Ph2PH)2 (2) and fac-BrRe(CO)3(dppen) (3), respectively. Both of these products have been isolated, and the X-ray diffraction structure of the Ph2PH-substituted derivative determined. The relative energies of the different tautomeric forms of the 4-(2,2-dimethylhydrazino)dimethylhydrazone-3(Z)-penten-2-one ligand and 4-(2,2-dimethylhydrazino)-3(Z)-penten-2-one, the precursor to the parent ligand, have been investigated by DFT analyses. The most stable tautomer for each compound contains a six-membered ring and exhibits an intramolecular N–H bond to the imine and keto moiety, respectively.

Correction to “Ligand Initiated Self-Assembly of Pt and Ir Nanoparticles about Ru(II) Diimines in Room Temperature Fluid Solution”

Correction to “Ligand Initiated Self-Assembly of Pt and Ir Nanoparticles about Ru(II) Diimines in Room Temperature Fluid Solution”

Ligand Initiated Self-Assembly of Pt and Ir Nanoparticles about Ru(II) Diimines in Room Temperature Fluid Solution

The addition of Pt or Ir nanoparticles (np’s) capped with poly(vinylpyrrolidone), designated M(PVP)n, to aqueous–ethanol solutions of (bpy)2Ruppz2+, bis(2,2′-bipyridine)(4,7-phenanthrolino-5:6,5′6′-pyrazine)ruthenium(II), and (bpy)2Rudpp2+, bis(2,2′-bipyridine)(2,3-bis(2-pyridyl)pyrazine)ruthenium(II), leads to the spontaneous aggregation of the nanoparticles about the complex. A comparison of the aggregation about different Ru(II) diimines indicates aggregation initiates at the heteroleptic ligand. Although initiating at the ppz ligand, continued aggregation of the np’s about the complex dilutes the specificity of the initial interaction, leading to larger aggregates of differing shape. TEM analyses of the aggregates indicate the volume occupied by the individual np’s is a small fraction of the total volume of the aggregate, suggesting a somewhat open structure interlaced with the solvent. Correlating TEM analyses of the aggregation with the electronic spectra of the solutions reveals a new absorption as...

Effects of Sequence, Connectivity, and Counter Ions in New Amide-Linked Ru(tpy)2–Re(bpy) Chromophores on Redox Chemistry and Photophysics

New cationic metallo ligands L1-L3 based on bis(terpyridine) ruthenium(II) complexes decorated with differently substituted 2,2'-bipyridines attached via amide groups (5-NHCO-bpy, 4-CONH-bpy, 5-CONH-bpy) were prepared. Coordination of Re(I)Cl(CO)(3) fragments to the bpy unit gives the corresponding bimetallic Ru~Re complexes 1-3. Hydrogen bonds of the bridging amide groups to [PF(6)](-) counterions or to water molecules are observed both in the solid state and in solution. The impact of the amide orientation, the connecting site, and the coordination of counterions on redox and photophysical properties is explored. Both the metallo ligands L1-L3 and the bimetallic complexes 1-3 are emissive at room temperature in fluid solution. The emission originates from (3)MLCT(Ru) states in all cases. Accordingly, the first oxidation of L1-L3 and 1-3 to [L1](+)-[L3](+) and [1](+)-[3](+) is assigned to the Ru(II/III) couple, while the first reduction to [L1](-)-[L3](-) and [1](-)-[3](-) occurs at the tpy-CO ligand as shown by UV/vis, IR, and EPR spectroscopy of the chemically generated radicals. Under rapid freezing conditions, radicals [2](-) and [3](-) are stabilized as different valence isomers with the odd electron localized at the [bpy-CO](•) bridging unit instead of the [tpy-CO](•). Furthermore, in radical [3](-) this valence equilibrium is shifted from [bpy-CO](•) to [tpy-CO](•) by coordination of [PF(6)](-) counterions to the bridging amide unit and back by replacing the [PF(6)](-) counterion with [BPh(4)](-). Photoinduced electron transfer (λ(exc) = 500 nm) to L1-L3 and to 1-3 is successful using triethanolamine (TEOA) as a reducing agent. Photocatalytic reduction of CO(2) by TEOA and 1-3 is hampered by the wrong site of electron localization in the one-electron reduced species [1](-)-[3](-).

Photoinduced Electron Transfer in Os(terpyridine)-biphenylene-(bi)pyridinium Assemblies

A series of linearly arranged donor-spacer-acceptor (D-S-A) systems 1-3, has been prepared and characterized. These dyads combine an Os(II)bis(terpyridine) unit as the photoactivable electron donor (D), a biphenylene (2) or phenylene-xylylene (3) fragment as the spacer (S), and a N-aryl-2,6-diphenylpyridinium electrophore (with aryl = 4-pyridyl or 4-pyridylium in 1 or 2/3, respectively) as the acceptor (A). Their absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution) have been studied. The electronic structure and spectroscopic properties of a representative compound of the series (i.e., 2) have also been investigated at the theoretical level, performing Density Functional Theory (DFT)-based calculations. Time-dependent transient absorption spectra of 1-3 have also been recorded at room temperature. The results indicate that efficient photoinduced oxidative electron transfer takes place in the D-S-A systems at room temperature in fluid solution, for which rate constants (in the range 4 × 10(8)-2 × 10(10) s(-1)) depend on the driving force of the process and the spacer nature. In all the D-S-A systems, charge recombination is faster than photoinduced charge separation, in spite of the relatively large energy of the D(+)-S-A(-) charge-separated states (between 1.47 and 1.78 eV for the various species), which would suggest that the charge recombination occurs in the Marcus inverted region. Considerations based on superexchange mechanism suggest that the reason for the fast charge recombination is the presence of a virtual D-S(+)-A(-) state at low energy--because of the involvement of the easily oxidizable biphenylene spacer--which is beneficial for charge recombination via superexchange but unsuitable for photoinduced charge separation. To further support the above statement, we prepared a fourth D-S-A species, 4, analogous to 2 but with a (hardly oxidizable) single phenylene fragment serving as the spacer. For such a species, charge recombination (about 3 × 10(10) s(-1)) is slower than photoinduced charge separation (about 1 × 10(11) s(-1)), thereby confirming our suggestions.

Luminescence Rigidochromism and Redox Chemistry of Pyrazolate-Bridged Binuclear Platinum(II) Diimine Complex Intercalated into Zirconium Phosphate Layers

The direct intercalation of a pyrazolate-bridged platinum(II) bipyridyl dimer ([{Pt(dmbpy)(μ-pz)}(2)](2+); dmbpy = 4,4'-dimethyl-2,2'-bipyridine, pz(-) = pyrazolate) within a zirconium phosphate (ZrP) framework has been accomplished. The physical and spectroscopic properties of [{Pt(dmbpy)(μ-pz)}(2)](2+) intercalated in ZrP were investigated by X-ray powder diffraction and X-ray photoelectron, infrared, absorption, and luminescence spectroscopies. Zirconium phosphate layers have a special microenvironment that is capable of supporting a variety of platinum oxidation states. Diffuse reflectance spectra from powders of the blue-gray intercalated materials show the formation of a low-energy band at 600 nm that is not present in the platinum dimer salt. The nonintercalated complex is nonemissive in room-temperature fluid solution, but gives rise to intense blue-green emission in a 4:1 ethanol/methanol 77 K frozen glassy solution. Powders and colloidal suspensions of [{Pt(dmbpy)(μ-pz)}(2)](2+)-exchanged ZrP materials exhibit intense emissions at room-temperature.

Electron Transfer Across Modular Oligo-p-phenylene Bridges in Ru(bpy)2(bpy–phn–DQ)4+(n= 1–5) Dyads. Unusual Effects of Bridge Elongation

A series of dyads of general formula Ru(bpy)(2)(bpy-ph(n)-DQ)(4+) (n = 1-5), based on a Ru(II) polypyridine unit as photoexcitable donor, a set of oligo-p-phenylene bridges with 1-5 modular units, and a cyclo-diquaternarized 2,2'-bipyridine (DQ(2+)) as electron acceptor unit, have been synthesized. Their spectroscopic and photophysical properties have been investigated in CH(3)CN and CH(2)Cl(2) by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. The experimental study has also been complemented with a computational investigation carried out on the whole series of dyads. The absorption spectra of the dyads show new spectroscopic transitions, in addition to those characteristic of the donor, bridge, and acceptor fragments. DFT calculations suggest the assignment of such bands as bridge-to-acceptor (π ph(n)) → (π* DQ) charge-transfer transitions. This assignment is consistent with the solvatochromic and spectroelectrochemical behavior of the new bands. For all the dyads at room temperature in fluid solution, the typical (3)MLCT luminescence of the Ru(II) polypyridine unit is strongly (>90%) quenched, supporting the occurrence of an efficient intramolecular photoinduced electron transfer. The study has revealed, however, that the photophysical mechanism is actually more complex than presumed on the basis of a simple photoinduced electron-transfer scheme. For n = 1, very fast (few picoseconds) photoinduced electron transfer from the MLCT state localized on the substituted bpy ligand to the DQ unit has been observed, followed by slower interligand hopping and charge recombination. For n = 2-5, MLCT excited-state quenching takes place without transient detection of charge-separated product, indicating that charge recombination is faster than charge separation. This behavior can be rationalized in terms of the superexchange couplings expected through this type of bridges for the two processes. The kinetics of MLCT quenching in the dyads with n = 1-5 does not follow the usual exponential falloff with bridge length: after a regular decrease for n = 1-3, the rate constants become almost insensitive to bridge length for n = 3-5. The rationale of this uncommon behavior, as suggested by DFT calculations, lies in a switch in the MLCT quenching mechanism with increasing bridge length, from oxidative quenching by the DQ acceptor to reductive quenching by the bridge.

Dinuclear bis(terpyridine)ruthenium(II) complexes by amide coupling of ruthenium amino acids: Synthesis and properties

Two redox-asymmetric amide-bridged bis(terpyridine)ruthenium(II) complexes (3a, 3b) have been prepared by amide coupling of a carboxylic acid functionalized complex with an amine substituted complex and they were fully characterized by spectroscopic analyses. They emit at 692 and 750nm at room temperature in fluid solution with quantum yields larger than 10−3 and 3MLCT lifetimes of 22ns. Ru···Ru distances were estimated from DFT models as 17.7 and 13.4Å for 3a and 3b, respectively. Cyclic voltammetry gives two oxidation potentials for the differently substituted ruthenium sites with splittings of 0.10 and 0.23V for 3a and 3b, respectively. Oxidation of 3b with CeIV ions gives the corresponding mixed-valent RuII–RuIII system which is valence-localized according to NIR spectroscopic and theoretical analyses.

Hybrid complexes: Pt(ii)-terpyridine linked to various acetylide-bodipy subunits

By tailoring the chemistry of the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) core in order to change its spectroscopic properties, we linked some modified bodipy dyes to Pt(ii)-terpyridine centers thereby preparing three novel hybrid complexes. In one of such species, two pyrene subunits are also present. The absorption spectra and luminescence properties (both at room temperature in acetonitrile solution and at 77 K in butyronitrile rigid matrix) of the new hybrid complexes and of their bodipy parent species have been investigated, and compared with those of previously studied compounds. Absorption spectra showed that the different chromophoric subunits roughly keep their own features in the multicomponent systems, indicating the supramolecular nature of the compounds. Efficient photoinduced intercomponent energy transfer takes place in the various hybrid Pt(ii)-terpyridine-bodipy compounds, by taking advantage of different mechanisms: at room temperature in fluid solution very efficient energy transfer from the (3)MLCT state(s) of the Pt residue to the lowest-lying (3)bodipy is mediated by a charge transfer state via a two-step electron transfer process. Direct energy transfer from (3)MLCT state(s) to the (3)bodipy states takes place at 77 K, most likely via the Dexter mechanism. When two pyrene fragments are grafted to the boron, cascade energy transfer from the pyrene to the (1)bodipy also occurs by the Förster mechanism. For one of the new compounds, (3)bodipy emission with a maximum at 738 nm is also observed at cryogenic temperature.

Solution luminescence from chloro(2,2′:6′,2″-terpyridine)platinum(II) chloride in micelles

Abstract Previously characterized as being non-luminescent in room-temperature fluid solution, the coordination compound chloro(2,2′:6′,2″-terpyridine)platinum(II) chloride can display two types of luminescence in certain microenvironments. In aqueous solutions of anionic and neutral surfactants having concentrations near or above their critical micelle concentration, [Pt(terpy)Cl]Cl (10–50 μM) displays broad emission centered at ∼610 nm that is characterized as metal-to-ligand charge transfer phosphorescence (3MLCT). In high concentration (10–100 mM) solutions having no surfactant, [Pt(terpy)Cl]Cl aggregates form. Excitation in the 470–540 nm region results in a long-wavelength emission centered at ∼720 nm that is characterized as metal–metal-to-ligand charge transfer phosphorescence (3MMLCT). This emission can also be detected in lower concentration solutions (10–50 μM) with surfactant concentration below its critical micelle concentration. Enhancement of 3MLCT luminescence is also found for the related phenylacetylide complex cation [Pt(terpy)(C CPh)]+ in micelles of the anionic surfactant sodium dodecyl sulfate.