Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.     

Excited state properties of mixed phosphine 2-(2'-pyridyl)quinoline complexes of ruthenium(II)

Mixed ligand complexes of the type Ru(pq)(2)(PP)(2+) (pq = 2,2'-pyridylquinoline and PP = one bidentate or two monodentate phosphine ligands) have been prepared from the appropriate phosphine and Ru(pq)(2)Cl(2). The room temperature absorption spectra and low temperature (77 K) emission spectra, emission lifetimes, and quantum yields have been measured for the series of complexes and compared with those of Ru(pq)(3)(2+) and analogous Ru(bpy)(2)(PP)(2+) complexes (bpy = 2,2'-bipyridine) where possible. Emission spectra have been fit using a single mode Franck-Condon analysis. The visible absorption bands and emission bands are assigned to MLCT transitions that are blue shifted relative to Ru(pq)(3)(2+), while the emission lifetimes and quantum yields are increased. The trends in the nonradiative rate constants, k(nr), are described in terms of the energy gap, E(0), and the Huang-Rhys factor, S(M), which were obtained from the spectral fittings, and are correlated with the phosphine ligand structures.     

Photophysical properties of highly luminescent copper(I) halide complexes chelated with 1,2-bis(diphenylphosphino)benzene

Studies on synthesis, structures, and photophysics have been carried out for a series of luminescent copper(I) halide complexes with the chelating ligand, 1,2-bis[diphenylphosphino]benzene (dppb). The complexes studied are halogen-bridged dinuclear complexes, [Cu(mu-X)dppb]2 (X = I (1), Br (2), Cl (3)), and a mononuclear complex, CuI(dppb)(PPh3) (4). These complexes in the solid state exhibit intense blue-green photoluminescence with microsecond lifetimes (emission peaks, lambdamax = 492-533 nm; quantum yields, Phi = 0.6-0.8; and lifetimes, tau = 4.0-10.4 mus) at 298 K. In 2-methyltetrahydrofuran (2mTHF) solutions at 298 K, only 1 and 4 show weaker emission (Phi = 0.009) with shorter lifetimes (tau = 0.35 and 0.23 mus) and red-shifted spectra (lambdamax = 543 and 546 nm). The emission in the solid state originates from the (M + X)LCT excited state with a distorted-tetrahedral conformation, in which emissive excited states, 1(M + X)LCT and 3(M + X)LCT, are in equilibrium with an energy difference of approximately 2 kcal/mol. On the other hand, the complexes in the 2mTHF solutions emit from the MLCT excited state with an energetically favorable flattened conformation in the temperature range of 298-130 K. The flattened geometry with equilibrated 1MLCT and 3MLCT states has a nonradiative rate at least 2 orders of magnitude larger than that of the distorted-tetrahedral geometry, leading to a much smaller emission quantum yield (Phi = 0.009) at 298 K. Since the flattening motion is markedly suppressed below 130 K, the emission observed in 2mTHF below 130 K is considered to occur principally from the (M + X)LCT state with a distorted-tetrahedral geometry. To interpret the photophysics of 1 and 4 in both the solid and solution states, we have proposed the "2-conformations with 2-spin-states model (2C x 2S model)". The electroluminescence device using (1) as a green emissive dopant showed a moderate EL efficiency; luminous efficiency = 10.4 cd/A, power efficiency = 4.2 lm/W at 93 cd/m(2), and maximum external quantum efficiency = 4.8%.     

Synthesis and spectroscopic characterization of CN-substituted bipyridyl complexes of Ru(II)

A series of ruthenium complexes having the general form [Ru(bpy)(3-n)(CN-Me-bpy)(n)](PF(6))(2) (where bpy = 2,2'-bipyridine, CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, and n = 1-3 for complexes 1-3, respectively) have been synthesized and characterized using a variety of steady-state and nanosecond time-resolved spectroscopies. Electrochemical measurements indicate that the CN-Me-bpy ligand is significantly easier to reduce than the unsubstituted bipyridine (on the order of ～500 mV), implying that the lowest energy (3)MLCT (metal-to-ligand charge transfer) state will be associated with the CN-Me-bpy ligand(s) in all three compounds. Comparison of the Huang-Rhys factors derived from spectral fitting analyses of the steady state emission spectra of complexes 1-3 suggests all three compounds are characterized by excited-state geometries that are less distorted relative to their ground states as compared to [Ru(bpy)(3)](PF(6))(2); the effect of the more nested ground- and excited-state potentials is reflected in the unusually high radiative quantum yields (13% (1), 27% (2), and 40% (3)) and long (3)MLCT-state room-temperature lifetimes (1.6 μs, 2.6 μs, and 3.5 μs, respectively) for these compounds. Coupling of the π* system into the CN groups is confirmed by nanosecond step-scan IR spectra which reveal a ～40 cm(-1) bathochromic shift of the CN stretching frequency, indicative of a weaker CN bond in the (3)MLCT excited state relative to the ground state. The fact that the shift is the same for complexes 1-3 is evidence that, in all three complexes, the long-lived excited state is localized on a single CN-Me-bpy ligand rather than being delocalized over multiple ligands.     

Probes of the metal-to-ligand charge-transfer excited states in ruthenium-Am(m)ine-bipyridine complexes: the effects of NH/ND and CH/CD isotopic substitution on the 77 K luminescence

The effects of ligand perdeuteration on the metal-to-ligand charge-transfer (MLCT) excited-state emission properties at 77 K are described for several [Ru(L)(4)bpy](2+) complexes in which the emission process is nominally [uIII,bpy-] --> [RuII,bpy]. The perdeuteration of the 2,2'-bipyridine (bpy) ligand is found to increase the zero-point energy differences between the ground states and MLCT excited states by amounts that vary from 0 +/- 10 to 70 +/- 10 cm(-1) depending on the ligands L. This indicates that there are some vibrational modes with smaller force constants in the excited states than in the ground states for most of these complexes. These blue shifts increase approximately as the energy difference between the excited and ground states decreases, but they are otherwise not strongly correlated with the number of bipyridine ligands in the complex. Careful comparisons of the [Ru(L)(4)(d(8)-bpy)](2+) and [Ru(L)(4)(h(8)-bpy](2+) emission spectra are used to resolve the very weak vibronic contributions of the C-H stretching modes as the composite contributions of the corresponding vibrational reorganizational energies. The largest of these, 25 +/- 10 cm(-1), is found for the complexes with L = py or bpy/2 and smaller when L = NH(3). Perdeuteration of the am(m)ine ligands (NH(3), en, or [14]aneN(4)) has no significant effect on the zero-point energy difference, and the contributions of the NH stretching vibrational modes to the emission band shape are too weak to resolve. Ligand perdeuteration does increase the excited-state lifetimes by a factor that is roughly proportional to the excited-state-ground-state energy difference, even though the CH and NH vibrational reorganizational energies are too small for nuclear tunneling involving these modes to dominate the relaxation process. It is proposed that metal-ligand skeletal vibrational modes and configurational mixing between metal-centered, bpy-ligand-centered, and MLCT excited states are important in determining the zero-point energy differences, while a large number of different combinations of relatively low-frequency vibrational modes must contribute to the nonradiative relaxation of the MLCT excited states.     

Excited-state properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py)

The photophysical properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF = tetrahydrofuran, PPh(3) = triphenylphosphine, py = pyridine) were explored upon excitation with visible light. Time-resolved absorption shows that all the complexes possess a long-lived transient (3.5-5.0 micros) assigned as an electronic excited state of the molecules, and they exhibit an optical transition at approximately 760 nm whose position is independent of axial ligand. No emission from the Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py) systems was detected, but energy transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to the (3)pipi excited state of perylene is observed. Electron transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to 4,4'-dimethyl viologen (MV(2+)) and chloro-p-benzoquinone (Cl-BQ) takes place with quenching rate constants (k(q)) of 8.0 x 10(6) and 1.2 x 10(6) M(-1) s(-1) in methanol, respectively. A k(q) value of 2 x 10(8) M(-1) s(-1) was measured for the quenching of the excited state of Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) by O(2) in methanol. The observations are consistent with the production of an excited state with excited-state energy, E(00), between 1.34 and 1.77 eV.     

Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies

The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4'-(X)2-bpy)2)](PF6)3 (where M = Mn(II) or Zn(II), X = CH3 or CF3, mcb is 4'-methyl-4-carboxy-2,2'-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a Ru(II) polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the 3MLCT excited state of the Ru(II) chromophore but with a lifetime (tau(obs) = 5.0 +/- 0.1 ns) and radiative quantum yield (Phi(r) approximately 7 x 10(-4)) that are significantly attenuated relative to the Zn(II) model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (tau(obs) = 730 +/- 30 ns and Phi(r) = 0.024, respectively). Quenching of the 3MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (tau(obs) = 45 +/- 5 ps) is >10(4) shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (tau(obs) = 1.31 +/- 0.05 micros). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form k(nr) = k0 + A x exp{-DeltaE/kB T} with k0 = 1.065 +/- 0.05 x 10(7) s(-1), A = 3.7 +/- 0.5 x 10(10) s(-1), and DeltaE = 1230 +/- 30 cm(-1). Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of lambda approximately 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn(II)2 core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (DeltaG0(ET) = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be approximately 10 cm(-1) and approximately 0.15 cm(-1) in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude. In addition to providing insight into the influence of donor/acceptor proximity on exchange energy transfer, this study underscores the utility of variable-temperature measurements in cases where Dexter and electron-transfer mechanisms can lead to indistinguishable spectroscopic observables.     

(Pyridine)pentaamminechromium(III). Synthesis, Characterization, and Photochemistry

The Cr(NH(3))(5)(py)(3+) ion has been obtained by metathesis of Cr(NH(3))(5)(Me(2)SO)(3+) in pyridine, isolated as the perchlorate salt, and characterized by absorption (lambda(max) at 467, 352, and 260 nm) and emission spectra (lambda(max) at 668 nm, tau = 2.0 &mgr;s at 20 degrees C in water) and by the py aquation rate (k = 5 x 10(-)(4) s(-)(1) at 80 degrees C). Ligand-field (LF) band irradiation in acid aqueous solution (10(-)(2) M HClO(4)) induces photoaquation of py (Phi = 0.26) and NH(3) (Phi = 0.16). HPLC indicates that the latter reaction gives rise to both cis- and trans-Cr(NH(3))(4)(py)(H(2)O)(3+), with the predominance of the cis isomer. This is the first Cr(NH(3))(5)X(z+)() species where Phi(x) > Phi(NH)3: the result is compared with the predictions of various photolysis models and is taken as chemical evidence for pi-acceptance by the py ligand. The photostereochemistry is also discussed. The phosphorescence is totally quenched by Cr(C(2)O(4))(3)(3)(-) (k(q) = 2.7 x 10(9) M(-)(1) s(-)(1)), while the photoreactions are only in part. On 470-nm excitation, the Phi(py)/Phi(NH)()3 ratio is approximately 1 and approximately 2 for the unquenchable and the quenchable contributions, respectively. Such a difference, suggesting at least two reactive precursors, can be interpreted in terms of the photochemistry proceeding from either the lowest doublet and quartet excited states or, alternatively, from the (4)E and (4)B(2) states. Irradiation of the very distinct absorption of coordinated pyridine results in both doublet-state emission and loss of py and NH(3). Comparison of this photobehavior with the LF results gives an efficiency of 0.7 for conversion of the py-localized pipi states into the Cr-localized LF states, confirmed by the wavelength dependence of the relative emission yields. Some py release (Phi = 0.03) is concluded to originate in the pipi states.     

Synthesis and photochemistry of Ru(II) complexes containing phenanthroline-based ligands with fused pyrrole rings

Hydrolysis of 1,10-phenanthrolinopyrrole ethyl ester leads to the acid derivative which is unstable at room-temperature releasing CO(2) and forming 1,10-phenanthrolinopyrrole (php). The ligand reacts with ruthenium(II) to form a series of complexes of the general formula [Ru(php)(n)(bpy)(3-n)](2+), where bpy = 2,2'-bipyridine and n = 1-3. The photochemical properties reveal that the complexes have longer-lived excited states than the standard complex, [Ru(bpy)(3)](2+). Their emission lifetimes range from 9.04 micros (n = 1) to 35.5 micros (n = 3) at 77 K compared to 7.57 micros for the standard. Similarly, at room-temperature, emission lifetimes range from 1.20 micros (n = 1) to 1.70 micros (n = 3) relative to the standard (0.56 micros). The emission quantum yields also have higher values than the standard [Ru(bpy)(3)](2+) under similar conditions. The temperature-dependent studies for the complexes establish the distribution among the radiative, nonradiative, and (3)MLCT to (3)d-d decay channels and are in agreement with the energy gap law.     

New synthetic routes to biscarbonylbipyridinerhenium(I) complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)n+ (X2bpy = 4,4'-X2-2,2'-bipyridine) via photochemical ligand substitution reactions, and their photophysical and electrochemical properties

Photochemical ligand substitution of fac-[Re(X2bpy)(CO)3(PR3)]+ (X2bpy = 4,4'-X2-2,2'-bipyridine; X = Me, H, CF3; R = OEt, Ph) with acetonitrile quantitatively gave a new class of biscarbonyl complexes, cis,trans[Re(X2bpy)(CO)2(PR3)(MeCN)]+, coordinated with four different kinds of ligands. Similarly, other biscarbonylrhenium complexes, cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (n = 0, Y = Cl-; n = 1, Y = pyridine, PR'3), were synthesized in good yields via photochemical ligand substitution reactions. The structure of cis,trans-[Re(Me2bpy)(CO)2[P(OEt)3](PPh3)](PF6) was determined by X-ray analysis. Crystal data: C38H42N2O5F6P3Re, monoclinic, P2(1/a), a = 11.592(1) A, b = 30.953(4) A, c = 11.799(2) A, V = 4221.6(1) A3, Z = 4, 7813 reflections, R = 0.066. The biscarbonyl complexes with two phosphorus ligands were strongly emissive from their 3MLCT state with lifetimes of 20-640 ns in fluid solutions at room temperature. Only weak or no emission was observed in the cases Y = Cl-, MeCN, and pyridine. Electrochemical reduction of the biscarbonyl complexes with Y = Cl- and pyridine in MeCN resulted in efficient ligand substitution to give the solvento complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(MeCN)]+.     

Synthetic control of excited-state properties in cyclometalated Ir(III) complexes using ancillary ligands

The synthesis and photophysical characterization of a series of (N,C(2')-(2-para-tolylpyridyl))2 Ir(LL') [(tpy)2 Ir(LL')] (LL' = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy)2 Ir(LL')] (dfppy = N,C(2')-2-(4',6'-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)2 Ir(CN)2- and (ppy)2 Ir(NCS)2- (ppy = N,C(2')-2-phenylpyridyl). The (tpy)2 Ir(PPh2CH2)2 BPh2 and [(tpy)2 Ir(CN-t-Bu)2](CF3SO3) complexes have been structurally characterized by X-ray crystallography. The Ir-C(aryl) bond lengths in (tpy)2 Ir(CN-t-Bu)2+ (2.047(5) and 2.072(5) A) and (tpy)2 Ir(PPh2CH2)2 BPh2 (2.047(9) and 2.057(9) A) are longer than their counterparts in (tpy)2 Ir(acac) (1.982(6) and 1.985(7) A). Density functional theory calculations carried out on (ppy)2 Ir(CN-Me)2+ show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-pi and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-pi orbitals. Electrochemical analysis of the (tpy)2 Ir(LL') complexes shows that the reduction potentials are largely unaffected by variation in the ancillary ligand, whereas the oxidation potentials vary over a much wider range (as much as 400 mV between two different LL' ligands). Spectroscopic analysis of the cyclometalated Ir complexes reveals that the lowest energy excited state (T1) is a triplet ligand-centered state (3LC) on the cyclometalating ligand admixed with 1MLCT (MLCT = metal-to-ligand charge-transfer) character. The different ancillary ligands alter the 1MLCT state energy mainly by changing the HOMO energy. Destabilization of the 1MLCT state results in less 1MLCT character mixed into the T1 state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in ln(k(nr)) (k(nr) = nonradiative decay rate). Decreased 1MLCT character in the T1 state also increases the Huang-Rhys factors in the emission spectra, decreases the extinction coefficient of the T1 transition, and consequently decreases the radiative decay rates (k(r)). Overall, the luminescence quantum yields decline with increasing emission energies. A linear dependence of the radiative decay rate (k(r)) or extinction coefficient (epsilon) on (1/deltaE)2 has been demonstrated, where deltaE is the energy difference between the 1MLCT and 3LC transitions. A value of 200 cm(-1) for the spin-orbital coupling matrix element 3LC absolute value(H(SO)) 1MLCT of the (tpy)2 Ir(LL') complexes can be deduced from this linear relationship. The (fppy)2 Ir(LL') complexes with corresponding ancillary ligands display similar trends in excited-state properties.     

Highly phosphorescence iridium complexes and their application in organic light-emitting devices

A new series of iridium(III) mixed ligand complexes TBA[Ir(ppy)(2)(CN)(2)] (1), TBA[Ir(ppy)(2)(NCS)(2)] (2), TBA[Ir(ppy)(2)(NCO)(2)] (3), and [Ir(ppy)(2)(acac)] (4) (ppy = 2-phenylpyridine; acac = acetoylacetonate, TBA = tetrabutylammonium cation) have been developed and fully characterized by UV-vis, emission, IR, NMR, and cyclic voltammetric studies. The lowest energy MLCT transitions are tuned from 463 to 494 nm by tuning the energy of the HOMO levels. These complexes show emission maxima in the blue, green, and yellow region of the visible spectrum and exhibit unprecedented phosphorescence quantum yields, 97 +/- 3% with an excited-state lifetimes of 1-3 micros in dichloromethane solution at 298 K. The near-unity quantum yields of these complexes are related to an increased energy gap between the triplet emitting state and the deactivating e(g) level that have been achieved by meticulous selection of ligands having strong ligand field strength. Organic light-emitting devices were fabricated using the complex 4 doped into a purified 4,4'-bis(carbazol-9-yl)biphenyl host exhibiting a maximum of the external quantum efficiencies of 13.2% and a power efficiency of 37 lm/W for the 9 mol % doped system.     

Photophysical and ab Initio Studies of Mononuclear Copper(I) Complexes

Described are the photophysical properties of the mononuclear copper(I) complexes CuL(4)(+) (L = pyridine (py), 4-methylpyridine, 4-phenylpyridine, or acetonitrile), Cu(lut)(3)(+) (lut = 2,6-lutidine), and Cu(lut)(2)(+). Each of these compounds as their solid PF(6)(-) salts display a relatively long-lived (>1 &mgr;s), visible range emission at both ambient temperature and at 77 K but not in fluid solutions. Also reported are the results for ab initio calculations at the restricted Hartree-Fock self-consistent field level to probe the natures of lower energy excited states of the hypothetical species CuL(n)()(+) (L = NH(3), CH(3)CN, or py; n = 1-4). These results point to an assignment of the lowest energy, luminactive excited states as being largely metal centered, d --> s in character for each of the above complexes with the possible exception of the CuL(4)(+) species where L is py or a substituted analogue. In the case of Cu(py)(4)(+) the ab initiocalculations indicate a metal-to-ligand charge transfer to be the lowest energy Franck-Condon state, although the similarities of emission band shapes, energies, and lifetimes among the various complexes suggest a common d --> s assignment.     

Structures, spectroscopic properties and redox potentials of quaterpyridyl Ru(II) photosensitizer and its derivatives for solar energy cell: a density functional study

Scalar relativistic density functional theory (DFT) has been used to explore the spectroscopic and redox properties of Ruthenium-type photovoltaic sensitizers, trans-[Ru((R)L)(NCS)(2)] ((R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-2,2' : 6',2' : 6',2'-quaterpyridine, R = H (1), Me (2), (t)Bu (3) and COOH (4); (R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-cycloquaterpyridine, R = COOH (5)). The geometries of the molecular ground, univalent cationic and triplet excited states of 1-5 were optimized. In complexes 1-4, the quaterpyridine ligand retains its planarity in the molecular, cationic and excited states, although the C≡N-Ru angle representing the SCN → Ru coordination approaches 180° in the univalent cationic and triplet excited states. The theoretically designed complex 5 displays a curved cycloquaterpyridine ligand with significantly distorted SCN → Ru coordination. The electron spin density distributions reveal that one electron is removed from the Ru/NCS moieties upon oxidation and the triplet excited state is due to the Ru/NCS → polypyridine charge transfer (MLCT/L'LCT). The experimental absorption spectra were well reproduced by the time-dependent DFT calculations. In the visible region, two MLCT/L'LCT absorption bands were calculated to be at 652 and 506 nm for 3, agreeing with experimental values of 637 and 515 nm, respectively. The replacement of the R- group with -COOH stabilizes the lower-energy unoccupied orbitals of π* character in the quaterpyridine ligand in 4. This results in a large red shift for these two MLCT/L'LCT bands. In contrast, the lower-energy MLCT/L'LCT peak of 5 nearly disappears due to the introduction of cycloquaterpyridine ligand. The higher energy bands in 5 however become broader and more intense. As far as absorption in the visible region is concerned, the theoretically designed 5 may be a very promising sensitizer for DSSC. In addition, the redox potentials of 1-5 were calculated and discussed, in conjunction with photosensitizers such as cis-[Ru(L(1))(2)(X)(2)] (L(1) = 4,4'-bis(carboxylic acid)-2,2'-bipyridine; X = NCS(-) (6), Cl(-) (7) and CN(-) (8)), cis-[Ru(L(1)')(2)(NCS)(2)] (L(1)' = 4,7-bis(carboxylic acid)-1,10-phenanthroline, 9), [NH(4)][Ru(L(2))(NCS)(3)] (L(2) = 4,4',4'-tris(carboxylic acid)-2,2' : 6',2'-terpyridine, 10) and [Ru(L(2))(NCS)(3)](-) (11).     

Lowest electronic excited states of platinum(II) diimine complexes

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.     

Luminescence ranging from red to blue: a series of copper(I)-halide complexes having rhombic {Cu2(mu-X)2} (X = Br and I) units with N-heteroaromatic ligands

A series of Cu(I) complexes formulated as [Cu(2)(mu-X)(2)(PPh(3))(L)(n)] were prepared with various mono- and bidentate N-heteroaromatic ligands (X = Br, I; L = 4,4'-bipyridine, pyrazine, pyrimidine, 1,5-naphthyridine, 1,6-naphthyridine, quinazoline, N,N-dimethyl-4-aminopyridine, 3-benzoylpyridine, 4-benzoylpyridine; n = 1, 2). Single-crystal structure analyses revealed that all the complexes have planar {Cu(2)X(2)} units. Whereas those with monodentate N-heteroaromatic ligands afforded discrete dinuclear complexes, bidentate ligands formed infinite chain complexes with the ligands bridging the dimeric units. The long Cu...Cu distances (2.872-3.303 A) observed in these complexes indicated no substantial interaction between the two Cu(I) ions. The complexes showed strong emission at room temperature as well as at 80 K in the solid state. The emission spectra and lifetimes in the microsecond range were measured at room temperature and at 80 K. The emissions of the complexes varied from red to blue by the systematic selection of the N-heteroaromatic ligands (lambda(em)(max): 450 nm (L = N,N-dimethyl-4-aminopyridine) to 707 nm (L = pyrazine)), and were assigned to metal-to-ligand charge-transfer (MLCT) excited states with some mixing of the halide-to-ligand (XL) CT characters. The emission energies were successfully correlated with the reduction potentials of the coordinated N-heteroaromatic ligands, which were estimated by applying a simple modification based on the calculated stabilization energies of the ligands by protonation.     

Application of time-resolved infrared spectroscopy to electronic structure in metal-to-ligand charge-transfer excited states

Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.     

Photochemical ligand substitution reactions of fac-[Re(bpy)(CO)3Cl] and derivatives

Excitation by high-energy light, such as that of 313 nm wavelength, induces a photochemical ligand substitution (PLS) reaction of fac-[Re(bpy)(CO)3Cl] (1a) to give the solvento complexes (OC-6-34)- and (OC-6-44)-[Re(bpy)(CO)2(MeCN)Cl] (2 and 3) in good yields. The disappearance quantum yield of 1a was 0.01+/-0.001 at 313 nm. The products were isolated, and X-ray crystallographic analysis was successfully performed for 2. Time-resolved IR measurements clearly indicated that the CO ligand dissociates with subpicosecond rates after excitation, leading to vibrationally hot photoproducts, which relax within 50-100 ps. Detailed studies of the reaction mechanism show that the PLS reaction of 1a does not proceed via the lowest vibrational level in the 3MLCT excited state. The PLS reaction gives 2 and (OC-6-24)-[Re(bpy)(CO)2(MeCN)Cl] (5) as primary products, and one of the products, 5, isomerizes to 3. This type of PLS reaction is more general, occurring in various fac-rhenium(I) diimine tricarbonyl complexes such as fac-[Re(X2bpy)(CO)3Cl] (X2bpy=4,4'-X2-bpy; X=MeO, NH2, CF3), fac-[Re(bpy)(CO)3(pyridine)]+, and fac-[Re(bpy)(CO)3(MeCN)]+. The stable photoproducts (OC-6-44)- and (OC-6-43)-[Re(bpy)(CO)2(MeCN)(pyridine)]+ and (OC-6-32)- and (OC-6-33)-[Re(bpy)(CO)2(MeCN)2]+ were isolated. The PLS reaction of rhenium tricarbonyl-diimine complexes is therefore applicable as a general synthetic method for novel dicarbonyls.     

Photophysics of diimine platinum(II) bis-acetylide complexes

A comprehensive photophysical investigation has been carried out on a series of eight complexes of the type (diimine)Pt(-C=C-Ar)(2), where diimine is a series of 2,2'-bipyridine (bpy) ligands and -C=C-Ar is a series of substituted aryl acetylide ligands. In one series of complexes, the energy of the Pt --> bpy metal-to-ligand charge transfer (MLCT) excited state is varied by changing the substituents on the 4,4'- and/or the 5,5'-positions of the bpy ligand. In a second series of complexes the electronic demand of the aryl acetylide ligand is varied by changing the para substituent (X) on the aryl ring (X = -CF(3), -CH(3), -OCH(3), and -N(CH(3))(2)). The effect of variation of the substituents on the excited states of the complexes has been assessed by examining their UV-visible absorption, variable-temperature photoluminescence, transient absorption, and time-resolved infrared spectroscopy. In addition, the nonradiative decay rates of the series of complexes are subjected to a quantitative energy gap law analysis. The results of this study reveal that in most cases the photophysics of the complexes is dominated by the energetically low lying Pt --> bpy (3)MLCT state. Some of the complexes also feature a low-lying intraligand (IL) (3)pi,pi excited state that is derived from transitions between pi- and pi-type orbitals localized largely on the aryl acetylide ligands. The involvement of the IL (3)pi,pi state in the photophysics of some of the complexes is signaled by unusual features in the transient absorption, time-resolved infrared, and photoluminescence spectra and in the excited-state decay kinetics. The time-resolved infrared difference spectroscopy indicates that Pt --> bpy MLCT excitation induces a +25 to + 35 cm(-)(1) shift in the frequency of the C=C stretching band. This is the first study to report the effect of MLCT excitation on the vibrational frequency of an acetylide ligand.     

Fe(II), Ru(II) and Re(I) complexes of endotopic, sterically non-hindering, U-shaped 8,8'-disubstituted-3,3'-biisoquinoline ligands: syntheses and spectroscopic properties

The redox behaviour, optical-absorption spectra and emission properties of U-shaped and elongated disubstituted biisoquinoline ligands and of derived octahedral Fe(ii), Ru(ii), and Re(i) complexes are reported. The ligands are 8,8'-dichloro-3,3'-biisoquinoline (1), 8,8'-dianisyl-3,3'-biisoquinoline (2), and 8,8'-di(phenylanisyl)-3,3'-biisoquinoline (3), and the complexes are [Fe(3)(3)](2+), [Fe(2)(3)](2+), [Ru(1)(phen)(2)](2+), [Ru(2)(3)](2+), [Ru(3)(3)](2+), [Re(2)(py)(CO)(3)](+), and [Re()(py)(CO)(3)](+). For the ligands, the optical properties as observed in dichloromethane are in line with expectations based on the predominant (1)pipi* nature of the involved excited states, with contributions at lower energies from (1)npi* and (1)ILCT (intraligand charge transfer) transitions. For all of the Fe(ii), Ru(ii), and Re(i) complexes, studied in acetonitrile, the transitions associated with the lowest-energy absorption band are of (1)MLCT (metal-to-ligand charge transfer) nature. The emission properties, as observed at room temperature and at 77 K, can be described as follows: (i) the Fe(ii) complexes do not emit, either at room temperature or at 77 K; (ii) the room-temperature emission of the Ru(ii) complexes (phi(em) > 10(-3), tau in the micros range) is of mixed (3)MLCT/(3)LC character (and similarly at 77 K); and (iii) the room-temperature emission of the Re(i) complexes (phi(em) approximately 3 x 10(-3), tau < 1 ns) is of (3)MLCT character and becomes of (3)LC (ligand-centered) character (tau in the ms time scale) at 77 K. The interplay of the involved excited states in determining the luminescence output is examined.