The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.

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.

The pH-dependent mechanism of the reduction of the nicotinamide adenine dinucleotide (NADH) model complex [Ru(bpy)(2)(5)](2+) (5 = 3-(pyrid-2'-yl)-4-azaacridine) was compared to the mechanism of the previously studied geometric isomer [Ru(bpy)(2)(pbn)](2+) (pbn = 2-(pyrid-2'-yl)-1-azaacridine, previously referred to as 2-(pyrid-2'-yl)-benzo[b]-1,5-naphthyridine) in aqueous media. The exposure of [Ru(bpy)(2)(5)](2+) to CO(2)(*-) leads to the formation of the one-electron reduced species (k = 4.4 x 10(9) M(-1) s(-1)). At pH < 11.2, the one-electron reduced species can be protonated, k = 2.6 x 10(4) s(-1) in D(2)O. Formation of a C-C bonded dimer is observed across the pH range of 5-13 (k = 4.5 x 10(8) M(-1) s(-1)). At pH < 11, two protonated radical species react to form a stable C-C bonded dimer. At pH > 11, dimerization of two one-electron reduced species is followed by disproportionation to one equivalent starting complex [Ru(bpy)(2)(5)](2+) and one equivalent [Ru(bpy)(2)(5HH)](2+). The structural difference between [Ru(bpy)(2)(pbn)](2+) and [Ru(bpy)(2)(5)](2+) dictates the mechanism and product formation in aqueous medium. The exchange of the nitrogen and carbon atoms on the azaacridine ligands alters the accessibility of the dimerization reactive site, thereby changing the mechanism and the product formation for the reduction of the [Ru(bpy)(2)(5)](2+) compound.

The d(1) tungsten-alkylidyne radical [W(CPh)(dppe)2Cl](+) reacts with H2 to give the d(0) hydride [W(CPh)(H)(dppe)2Cl](+), which on deprotonation yields the d(2) photoredox chromophore W(CPh)(dppe)2Cl. This family of reactions results in a cycle by which renewable H2 provides the reducing equivalents for photochemical reductions.

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.