Electron delocalization in a ruthenium(II) Bis(2,2':6',2' '-terpyridyl) complex

Photophysical properties have been recorded for a ruthenium(II) bis(2,2':6',2' '-terpyridine) complex bearing a single ethynylene substituent. The target compound is weakly emissive in fluid solution at room temperature, but both the emission yield and lifetime increase dramatically as the temperature is lowered. As found for the unsubstituted parent complex, the full temperature dependence indicates that the lowest-energy triplet state couples to two higher-energy triplets and to the ground state. Luminescence occurs only from the lowest-energy triplet state, but the radiative and nonradiative decay rates indicate that electron delocalization occurs at the triplet level. Comparison of the target compound with the parent complex indicates that the ethynylene group reduces the size of the electron-vibrational coupling element for nonradiative decay of the lowest-energy triplet state. Although other factors are affected by substitution, this is by far the most important feature with regard to stabilization of the triplet state.     

Orientational control of electronic coupling in mixed-valence, binuclear ruthenium(II)-bis(2,2':6',2' '-terpyridine) complexes

A series of binuclear ruthenium(II)-bis(2,2':6',2' '-terpyridine) complexes has been prepared around a central biphenylene unit equipped with a strap of variable length. Partial oxidation forms the mixed-valence complex that displays both ligand-to-metal, charge-transfer, and intervalence charge-transfer (IVCT) transitions in the near-IR region. On the basis of Hush theory, the electronic coupling matrix element for interaction between the metal centers decreases with increasing length of the tethering strap. This effect arises because the strap modulates the torsion angle between the phenyl rings and thereby controls the extent of through-bond electronic coupling. The coupling element favors a maximum for planar geometries and a minimum for orthogonal structures, but the full impact of the torsion angle is not realized due to thermal fluctuations.     

Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2'-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.     

Temperature-induced switching of the mechanism for intramolecular energy transfer in a 2,2':6',2' '-Terpyridine-based Ru(II)-Os(II) trinuclear array

The synthesis and photophysical properties of a linear 2,2':6',2' '-terpyridine-based trinuclear Ru(II)-Os(II) nanometer-sized array are described. This array comprises two bis(2,2':6',2' '-terpyridine) ruthenium(II) terminals connected via alkoxy-strapped 4,4'-diethynylated biphenylene units to a central bis(2,2':6',2' '-terpyridine) osmium(II) core. The mixed-metal linear array was prepared using the "synthesis at metal" approach, and the Ru(II)-Ru(II) separation is ca. 50 A. Energy transfer occurs with high efficiency from the Ru(II) units to the Os(II) center at all temperatures. Forster-type energy transfer prevails in a glassy matrix at very low temperature, but this is augmented by Dexter-type electron exchange at higher temperatures. This latter process, which is weakly activated, involves long-range superexchange interactions between the metal centers. In fluid solution, a strongly activated process provides for fast energy transfer. Here, a charge-transfer (CT) state localized on the bridge is populated as an intermediate species. The CT triplet does not undergo direct charge recombination to form the ground state but transfers energy, possibly via a second CT state, to the Os(II)-based acceptor. The short tethering strap constrains the geometry of the linker, especially in a glassy matrix, such that low-temperature electron exchange occurs across a particular torsion angle of 37 degrees . The probability of triplet energy transfer depends on temperature but always exceeds 75%.     

A theoretical study on the low-lying excited states of 2,2':5',2'-terthiophene and 2,2':5',2':5',2'-quaterthiophene.

The nature and properties of the low-lying singlet and triplet valence excited states of 2,2':5',2'-terthiophene (terthiophene) and 2,2':5',2':5',2'-quaterthiophene (tetrathiophene) are discussed on the basis of high-level ab initio computations. The spectroscopic features determined experimentally for short alpha-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S1 state of terthiophene, the S1 and T2 states become degenerate, which leads to a favourable situation for the occurrence of ISC. The parallel process is expected to be less favoured in tetrathiophene because of the less efficient spin-orbit coupling and the increase of the S1-T2 energy gap.     

A strategy for the synthesis of metal bis(2,2':6',2' '-terpyridine)-terminated molecular dyads having controlled torsion angles at the central biphenyl linker

The synthesis of a series of binuclear complexes comprising bis(2,2':6',2' '-terpyridine)ruthenium(II) and -osmium(II) centers connected via a geometrically constrained 4,4'-biphenyl bridge is described. These compounds have been prepared by a "synthesis-at-metal" approach as well as by the conventional method of synthesizing the ligand and subsequently attaching the metal center. A computational investigation into the behavior of the biphenyl-based bridges has been used to provide lowest-energy conformations and to estimate the degree of internal fluctuation about the mean torsion angle. It is shown that the length of the constraining strap determines both the torsion angle and the internal flexibility, with longer straps twisting the biphenyl group so as to relax stereochemical interactions between the linking oxygen atoms. Longer straps can be formed from poly(ethylene glycol) residues that provide an additional binding site for small cations. Electrospray mass spectrometry carried out on solutions of these crown ether-like bridges confirmed that Li+, Na+, and K+ ions bind in the form of 1:1 complexes. This range of compounds should permit rational examination of how the torsion angle affects the rate of through-bond electron transfer, electron exchange, and charge shift.     

Comparison of the photophysical properties of osmium(II) bis(2,2':6',2' '-terpyridine) and the corresponding ethynylated derivative

The photophysical properties of osmium(II) bis(2,2':6',2' '-terpyridine) have been recorded over a wide temperature range. An emission band is observed and attributed to radiative decay of the lowest-energy metal-to-ligand, charge-transfer (MLCT) triplet state. This triplet is coupled to two other triplet states that lie at higher energy. The second triplet, believed to be of MLCT character, is reached by crossing a barrier of only 640 cm(-1), but the highest-energy triplet, considered to be of metal-centered (MC) character, is separated from the lowest-energy MLCT triplet by a barrier of 3500 cm(-1). Analysis of the emission spectrum shows that both low- and high-frequency modes are involved in the decay process, while weak emission is seen from the second excited triplet state. The magnitude of the low- and high-frequency modes depends on temperature in fluid solution but not in a KBr disk. Apart from a substantial lowering of the triplet energy, the photophysical properties are relatively insensitive to the presence of an ethynylene substituent at the 4' position of each terpyridine ligand. However, the barrier to reaching the MC triplet is markedly reduced, and the vibrational modes become less sensitive to changes in temperature.     

Photophysical properties of ruthenium(II) tris(2,2'-bipyridine) complexes bearing conjugated thiophene appendages

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.     

Electrochemical and ferromagnetic couplings in 4,4',4' '-(1,3,5-benzenetriyl)tris(phenoxyl) radical formation

4,4',4' '-(1,3,5-Benzenetriyl)tris(2,6-di-tert-butylphenol) was prepared by the cross-coupling of 1,3,5-tribromobenzene and [4-(trimethylsiloxy)phenyl]magnesium bromide. X-ray analysis of the single crystal showed a propeller-like structure with a mean dihedral angle of 39 degrees between the hydroxyphenyl and the core benzene. The phenoxyl mono-, di-, and triradicals were generated by the electrochemical oxidation of the trianion. A stepwise radical formation was revealed by a differential pulse voltammogram, electrolytic ESR spectroscopy, and a comproportionation reaction between the radicals, which was discussed as an effect of the pi-conjugated but non-Kekulé-type coupler. The quartet and triplet ground state for the tri- and diradical, respectively, were confirmed by a SQUID measurement.     

Photophysical properties of binuclear ruthenium(ii) bis(2,2':6',2''-terpyridine) complexes built around a central 2,2'-bipyrimidine receptor

A binuclear complex has been synthesized having ruthenium(ii) bis(2,2':6',2'-terpyridine) terminals attached to a central 2,2'-bipyrimidine unit via ethynylene groups. Cyclic voltammetry indicates that the substituted terpyridine is the most easily reduced subunit and the main chromophore involves charge transfer from the metal centre to this ligand. The resultant metal-to-ligand, charge-transfer (MLCT) triplet state is weakly emissive and has a lifetime of 60 ns in deoxygenated solution at room temperature. The luminescence yield and lifetime increase with decreasing temperature in a manner that indicates the lowest-energy MLCT triplet couples to at least two higher-energy triplets. Cations can bind to the central bipyrimidine unit, forming both 1:1 and 1:2 (ligand:metal) complexes as confirmed by electrospray MS analysis. The photophysical properties depend on the number of bound cations and on the nature of the cation. In the specific case of binding zinc(ii) cations, the 1:1 complex has a triplet lifetime of 8.0 ns while that of the 1:2 complex is 1.8 ns. The 1:1 complexes formed with Ba(2+) and Mg(2+) are more luminescent than is the parent compound while the 1:2 complexes are much less luminescent. It is shown that the coordinated cations raise the reduction potential of the central bipyrimidine unit and thereby increase the activation energy for coupling with the metal-centred state. Complexation also introduces a non-emissive intramolecular charge-transfer (ICT) state that couples to the lowest-energy MLCT triplet and provides an additional non-radiative decay route. The triplet state of the 1:2 complex formed with added Zn(2+) cations decays preferentially via this ICT state.     

Reversible luminescence switching in a ruthenium(II) bis(2,2':6',2'-terpyridine)-benzoquinone dyad

An electroactive luminescent switch has been synthesized that comprises a hydroquinone-functionalized 2,2':6',2'-terpyridine ligand coordinated to a ruthenium(II) (4'-phenylethynyl-2,2':6',2'-terpyridine) fragment. The assembly is sufficiently rigid that the hydroquinone-chromophore distance is fixed. Excitation of the complex via the characteristic metal-to-ligand charge-transfer (MLCT) absorption band produces an excited triplet state in which the promoted electron is localized on the terpyridine ligand bearing the acetylenic group. The triplet lifetime in butyronitrile solution at room temperature is 46 +/- 3 ns but increases markedly at lower temperature. Oxidation of the hydroquinone to the corresponding benzoquinone switches on an electron-transfer process whereby the MLCT triplet donates an electron to the quinone. This reaction reduces the triplet lifetime to 190 +/- 12 ps and essentially extinguishes emission. The rate of electron transfer depends on temperature in line with classical Marcus theory, allowing calculation of the electronic coupling matrix element and the reorganization energy as being 22 cm(-1) and 0.84 eV, respectively. The switching behavior can be monitored using luminescence spectroelectrochemistry. The on/off level is set by temperature and increases as the temperature is lowered.     

How the central torsion angle affects the rates of nonradiative decay in some geometrically restricted p-quaterphenyls

A small series of p-quaterphenyl derivatives has been prepared in which the dihedral angle (phi) for the two central rings is constrained by dialkoxy spacers of varying length. The photophysical properties of these compounds remain comparable, but there is a clear correlation between the rate constants for nonradiative decay of both singlet and triplet excited states and phi in fluid solution. The rates tend toward a minimum as phi approaches 90 degrees . These effects are attributed to the general phenomenon of extended delocalization and can be traced to a combination of changes in the Huang-Rhys factor and the electron-vibrational coupling matrix element, both relating to displacement of the relevant potential energy surfaces and to the medium-frequency vibronic mode coupled to decay. The latter effect arises because of different levels of conjugation in the ground-state molecule. Such findings might have important implications for the design of improved light-emitting diodes. A similar angle dependence is noted for the yield of the pi-radical cation formed on photoionization in a polar solvent, but here, the effect is due to variations in the respective energy gaps between the relevant excited states.     

Competition between energy transfer and interligand electron transfer in porphyrin-osmium(II) bis(2,2':6',2' '-terpyridine) dyads

Rapid intramolecular energy transfer occurs from a free-base porphyrin to an attached osmium(II) bis(2,2':6',2' '-terpyridine) complex, most likely by way of the F?rster dipole-dipole mechanism. The initially formed metal-to-ligand, charge-transfer (MLCT) excited-singlet state localized on the metal complex undergoes very fast intersystem crossing to form the corresponding triplet excited state ((3)MLCT). This latter species transfers excitation energy to the (3)pi,pi* triplet state associated with the porphyrin moiety, such that the overall effect is to catalyze intersystem crossing for the porphyrin. Interligand electron transfer (ILET) to the distal terpyridine ligand, for which there is no driving force, competes poorly with triplet energy transfer from the proximal (3)MLCT to the porphyrin. Equipping the distal ligand with an ethynylene residue provides the necessary driving force for ILET and this process now competes effectively with triplet energy transfer to the porphyrin. The rate constants for all the relevant processes have been derived from laser flash photolysis studies.     

Intramolecular energy-transfer processes in a bis(porphyrin)-ruthenium(II) bis(2,2':6',2'-terpyridine) molecular array

A molecular triad has been synthesized comprising two free-base porphyrin terminals linked to a central ruthenium(II) bis(2,2':6',2'-terpyridine) subunit via meso-phenylene groups. Illumination into the ruthenium(II) complex is accompanied by rapid intramolecular energy transfer from the metal-to-ligand, charge-transfer (MLCT) triplet to the lowest-energy pi-pi* triplet state localized on one of the porphyrin subunits. Transfer takes place from a vibrationally excited level which lowers the activation energy. The electronic coupling matrix element for this process is 73 cm(-1). Selective illumination into the lowest-energy singlet excited state (S1) localized on the porphyrin leads to fast singlet-triplet energy transfer that populates the MLCT triplet state with high efficiency. This latter process occurs via Dexter-type electron exchange at room temperature, but the activation energy is high and the reaction is prohibited at low temperature. For this latter process, the electronic coupling matrix element is only 8 cm(-1).     

1,1',3,3',6,6',8,8'-Octachloro-9,9'-bifluorenylidene and perchloro-9,9'-bifluorenylidene,two exceedingly twisted ethylenes

The syntheses of 1,1',3,3',6,6',8,8'-octachloro-9,9'-bifluorenylidene (1), its precursors, and the byproduct 3,3',5,5'-tetrachloro-4-(trichloromethyl)biphenyl (5) are described. Accurate structural X-ray data on 1 and on perchloro-9,9'-bifluorenylidene (2) are reported and discussed. Because of chlorine overcrowding, the dihedral angles between their two identical fluorenylidene moieties are abnormally large, the central-ethylene twist angles being 55 and 66 degrees, respectively. Significant out-of-plane carbon-chlorine bond bending is likewise exhibited. Their ESR spectra and magnetic measurements prove that they are singlet species. The exceptionally large bathochromic displacements of their UV-vis absorption spectrum with respect to that of their parent hydrocarbon are mainly attributed to bond bending and molecular warping.     

Building molecular-scale bridges having restricted rotation

The synthesis is reported of a binuclear ruthenium(II) bis(2,2′:6′,2″-terpyridine) complex for which the polytopic ligand incorporates a central, torsionally-constrained biphenylene group. In principle, the dihedral angle between the two phenylene rings can be controlled by the length of the constraining strap.     

Construction of triangular metallomacrocycles: [M3(1,2-bis(2,2':6',2''-terpyridin-4-yl-ethynyl)benzene)3][M = Ru(II), Fe(II), 2Ru(II)Fe(II)

Complexation of a predesigned (1,2-bis(2,2':6',2'-terpyridin-4-yl-ethynyl)benzene) ligand possessing a 60 degrees angle between two terpyridines with transition metals [Fe(II) and Ru(II)] afforded the self-assembled, triangular metallomacrocycles.     

Photophysics of monodisperse platinum-acetylide oligomers: delocalization in the singlet and triplet excited states

A series of monodisperse Pt-acetylide polymers that contain the [-CC-(p-C6H4)-CC-(t-Pt(PBu3)2)-]n repeat unit has been prepared for n = 1, 2, 3, 4, 5, and 7. The photophysical properties of the series provide information concerning the relationship between the oligomer length and delocalization in the singlet and triplet excited states of the pi-conjugated electron system. The results imply that the singlet excited state is delocalized over approximately 6 repeat units; however, the triplet state is considerably more localized. The triplet energy is almost invariant with oligomer length, but the phosphorescence spectra and triplet nonradiative decay rates indicate that the electron-vibrational coupling in the triplet state decreases with increasing oligomer length.     

4'-(5'-R-pyrimidyl)-2,2':6',2'-terpyridyl (R = H, OEt, Ph, Cl, CN) platinum(II) phenylacetylide complexes: synthesis and photophysics

A series of new square-planar 4'-(5'-R-pyrimidyl)-2,2':6',2'-terpyridyl platinum(II) phenylacetylide complexes ( 1a- 5a) bearing different substituents (R = H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer ( (1)MLCT) transition, possibly mixed with some ligand-to-ligand charge transfer ( (1)LLCT) character. Photoluminescence arising from the (3)MLCT state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The (1)MLCT/ (1)LLCT absorption bands and the (3)MLCT emission bands for 1a- 5a red-shift in comparison to those of the corresponding 4'-toly-2,2':6',2'-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the (1)MLCT/ (1)LLCT absorption and the (3)MLCT emission bands exhibit a linear correlation with the Hammett constant (sigma p) of the 5'-substituent on the pyrimidyl ring. The lifetime of the (3)MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a- 5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH 3CN than 2a, which has the -OEt substituent, does (lambda T1-Tn (max) = 720 nm, tau T = 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a- 5a vary from 36 600 L.mol (-1).cm (-1) to 115 090 L.mol (-1).cm (-1), and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH 3CN solutions, with the singlet oxygen generation quantum yield (Phi Delta) varying from 0.24 to 0.46.     

Electron exchange in conformationally restricted donor-spacer-acceptor dyads: angle dependence and involvement of upper-lying excited States

The rate constant for triplet energy transfer (k(TET)) has been measured in fluid solution for a series of mixed-metal Ru-Os bis(2,2':6',2'-terpyridine) complexes built around a tethered biphenyl-based spacer group. The length of the tether controls the central torsion angle for the spacer, which can be varied systematically from 37 to 130 degrees . At low temperature, but still in fluid solution, the spacer adopts the lowest-energy conformation and k(TET) shows a clear correlation with the torsion angle. A similar relationship holds for the inverse quantum yield for emission from the Ru-terpy donor. Triplet energy transfer is more strongly activated at higher temperature and the kinetic data require analysis in terms of two separate processes. The more weakly activated step involves electron exchange from the first-excited triplet state on the Ru-terpy donor and the size of the activation barrier matches well with that calculated from spectroscopic properties. The pre-exponential factor derived for this process correlates remarkably well with the torsion angle and there is a large disparity in electronic coupling through pi and sigma orbitals on the spacer. The more strongly activated step is attributed to electron exchange from an upper-lying triplet state localized on the Ru-terpy donor. Here, the pre-exponential factor is larger but shows the same dependence on the geometry of the spacer. Strangely, the difference in coupling through pi and sigma orbitals is much less pronounced. Despite internal flexibility around the spacer, k(TET) shows a marked dependence on the torsion angle computed for the lowest-energy conformation.