Electronic, redox, and photophysical consequences of metal-for-carbon substitution in oligo-phenylene-ethynylenes.

The electronic structures, redox chemistry, and excited-state properties of tungsten-containing oligo-phenylene-ethynylenes (OPEs) of the form W[C(p-C6H4CC)n-1Ph](dppe)2Cl (n = 1-5; dppe =1,2-bis(diphenylphosphino)ethane) are reported and compared with those of organic analogues in order to elucidate the effects of metal-for-carbon substitution on OPE bonding and electronic properties. Key similarities between the metallo- and organic OPEs that bear on materials-related functions include their nearly identical effective conjugation lengths, reduction potentials, and π* orbital energies and delocalization. In addition to these conserved properties, the tungsten centers endow OPEs with reversible one-electron oxidation chemistry and long-lived emissive triplet excited states that are not accessible to organic OPEs. The electronic similarities and differences between metallo- and organic OPEs can be understood largely on the basis of π/π* orbital energy matching between tungsten and organic PE fragments and the introduction of an orthogonal mid-π/π*-gap d orbital in metallo-OPEs. These orbital energies can be tuned by varying the supporting ligands; this provides a means to rationally implement and control the emergent properties of metallo-OPE materials.

1000-fold enhancement of luminescence lifetimes via energy-transfer equilibration with the T1 state of Zn(TPP).

The new zinc porphyrin/tungsten alkylidyne dyad Zn(TPP)-C[triple bond]CC(6)H(4)C[triple bond]W(dppe)(2)Cl (1) possesses novel photophysical properties that arise from a tunable excited-state triplet-triplet equilibrium between the porphyrin and tungsten alkylidyne units. Dyad 1 exhibits (3)(d(xy) <-- pi*(WCR)) phosphorescence with a lifetime that is 20 times longer than that of the parent chromophore W(CC(6)H(4)CCPh)(dppe)(2)Cl (2). The triplet-triplet equilibrium can be tuned by the addition of ligands to the Zn center, resulting in phosphorescence lifetimes for 1(L) that are up to 1300 times longer than that of 2. The "lifetime reservoir" effect exhibited by 1(L) is approximately 1 order of magnitude larger than previously reported examples of the phenomenon.

Experimental and computational studies of the metal-metal stretching vibration in X(3)MMX(3) compounds (X = alkoxide, alkyl, amide).

Raman spectra of a number of triply bonded M(2)X(6) (M = Mo, W; X = alkoxide, alkyl) compounds have been obtained. Several exhibit a band assignable to the metal-metal stretching vibration nu(M)(M). This band was not identified in earlier studies of the M(2)(NMe(2))(6) compounds. We have attempted to correlate the Raman vibrational data with structural data from single-crystal X-ray diffraction studies. Diffraction studies of the M(2)(O-1-4-pentyl[2.2.2]bicyclooctyl)(6) species show a crowded environment around the dimetal core, but the M-M-O angles differ substantially from 90 degrees. Thus, this angle does not solely determine the extent to which the metal-metal and ligand-based vibrational modes couple and, in turn, our ability to observe nu(M)(M). Computational studies of model systems confirm the assignment of the band as being nu(M)(M), although the predicted vibrational energies are consistently too high by ca. 7%. The computational results suggest that a nu(M)(M) band may be present in the published spectra of the M(2)(NMe(2))(6) pair.

Time-Resolved Resonance Raman Spectroscopy at Low Temperature. The Excited-State Metal-Metal Stretching Frequency of Rh(2)(TMB)(4)(2+) (TMB = 2,5-Dimethyl-2,5-diisocyanohexane).

The time-resolved resonance Raman spectrum of the short-lived triplet (dsigmapsigma) excited state of Rh(2)(TMB)(4)(2+) (TMB = 2,5-dimethyl-2,5-diisocyanohexane) was obtained by lowering the temperature of a 3:1 ethanol/methanol solution until the excited-state lifetime became much greater than the width of the pulsed laser excitation source. The metal-metal stretching frequency is 151 cm(-)(1) in the excited triplet state, as compared to 50 cm(-)(1) in the ground state. The diatomic harmonic force constants derived from these frequencies are in a 9.12:1 ratio (excited state/ground state), consistent with the simple molecular orbital description that predicts that the Rh-Rh bond order is greater in the excited state than in the ground state. A comparison of Rh(2)(TMB)(4)(2+) and Rh(2)b(4)(2+) (b = 1,3-diisocyanopropane) Raman data indicates that the nature of the bridging ligand considerably affects the ground- and excited-state metal-metal stretching frequencies and that the population of the psigma orbital may have very little effect on the bonding in the excited triplet state.

Molecular Structures, Vibrational Spectroscopy, and Normal-Mode Analysis of M(2)(C&tbd1;CR)(4)(PMe(3))(4) Dimetallatetraynes. Observation of Strongly Mixed Metal-Metal and Metal-Ligand Vibrational Modes.

The nature of the skeletal vibrational modes of complexes of the type M(2)(C&tbd1;CR)(4)(PMe(3))(4) (M = Mo, W; R = H, Me, Bu(t)(), SiMe(3)) has been deduced. Metrical data from X-ray crystallographic studies of Mo(2)(C&tbd1;CR)(4)(PMe(3))(4) (R = Me, Bu(t)(), SiMe(3)) and W(2)(C&tbd1;CMe)(4)(PMe(3))(4) reveal that the core bond distances and angles are within normal ranges and do not differ in a statistically significant way as a function of the alkynyl substituent, indicating that their associated force constants should be similarly invariant among these compounds. The crystal structures of Mo(2)(C&tbd1;CSiMe(3))(4)(PMe(3))(4) and Mo(2)(C&tbd1;CBu(t)())(4)(PMe(3))(4) are complicated by 3-fold disorder of the Mo(2) unit within apparently ordered ligand arrays. Resonance-Raman spectra ((1)(delta-->delta) excitation, THF solution) of Mo(2)(C&tbd1;CSiMe(3))(4)(PMe(3))(4) and its isotopomers (PMe(3)-d(9), C&tbd1;CSiMe(3)-d(9), (13)C&tbd1;(13)CSiMe(3)) exhibit resonance-enhanced bands due to a(1)-symmetry fundamentals (nu(a) = 362, nu(b) = 397, nu(c) = 254 cm(-)(1) for the natural-abundance complex) and their overtones and combinations. The frequencies and relative intensities of the fundamentals are highly sensitive to isotopic substitution of the C&tbd1;CSiMe(3) ligands, but are insensitive to deuteration of the PMe(3) ligands. Nonresonance-Raman spectra (FT-Raman, 1064 nm excitation, crystalline samples) for the Mo(2)(C&tbd1;CSiMe(3))(4)(PMe(3))(4) compounds and for Mo(2)(C&tbd1;CR)(4)(PMe(3))(4) (R = H, D, Me, Bu(t)(), SiMe(3)) and W(2)(C&tbd1;CMe)(4)(PMe(3))(4) exhibit nu(a), nu(b), and nu(c) and numerous bands due to alkynyl- and phosphine-localized modes, the latter of which are assigned by comparisons to FT-Raman spectra of Mo(2)X(4)L(4) (X = Cl, Br, I; L = PMe(3), PMe(3)-d(9))(4) and Mo(2)Cl(4)(AsMe(3))(4). Valence force-field normal-coordinate calculations on the model compound Mo(2)(C&tbd1;CH)(4)P(4), using core force constants transferred from a calculation on Mo(2)Cl(4)P(4), show that nu(a), nu(b), and nu(c) arise from modes of strongly mixed nu(Mo(2)), nu(MoC), and lambda(MoCC) character. The relative intensities of the resonance-Raman bands due to nu(a), nu(b), and nu(c) reflect, at least in part, their nu(M(2)) character. In contrast, the force field shows that mixing of nu(M(2)) and nu(C&tbd1;C) is negligible. The three-mode mixing is expected to be a general feature for quadruply bonded complexes with unsaturated ligands.