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The standard uncertainty of detector-based radiance and irradiance responsivity calibrations in the short-wave infrared (SWIR) traditionally has been limited to around 1% or higher by the poor spatial uniformity of detectors used to transfer the scale from radiant power. Pyroelectric detectors offer a solution that avoids the spatial uniformity uncertainty but also introduces additional complications due to alternating current (AC) measurement techniques. Herein, a new, to the best of our knowledge, method for low uncertainty irradiance responsivity calibrations in the SWIR is presented. An absolute spectral irradiance responsivity scale was placed on two pyroelectric detectors (PED) at wavelengths λ from 500 to 3400 nm. The total combined uncertainty ( k = 1 ) was ≈ 0.28 % ( > 1000 nm ), 0.44% (900 nm), and 0.36% ( ≈ 950 nm and < 900 nm ) for PED #1 and 0.34% ( > 1000 nm ), 0.48% (900 nm), and 0.42% ( ≈ 950 nm and < 900 nm ) for PED #2. This was done by utilizing a demodulation technique to digitally analyze the time-dependent AC waveforms, which obviates the use of lock-in amplifiers and avoids associated additional uncertainty components.

Solution-deposited nanoscale films of RuO2 (nanoskins) are effective transparent conductors once calcined to 200 °C. Upon heating to higher temperatures, the nanoskins show increased transmission at 550 nm, indicating changes in the optical behavior of the films. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO2 nanoparticles. The mechanism for the spectral evolution is clearly observed with ultrafast optical measurements. Following excitation at 400 nm, nanoskins calcined at higher temperatures show increased transmission above 650 nm, consistent with the photobleaching of a surface plasmon resonance (SPR) band. Calculations based on the optical constants of RuO2 substantiate the presence of SPR absorption. Sheet resistance and transient terahertz photoconductivity measurements establish that the nanoskins electrically de-wire into separated particles. The plasmonic behavior of the nanoskins has implications for their use in a range of optical and electrochemical applications.

Tin(II) monosulfide (SnS) is a semiconductor material with an intermediate band gap, high absorption coefficient in the visible range, and earth abundant, non-toxic constituent elements. For these reasons, SnS has generated much interest for incorporation into optoelectronic devices, but little is known concerning the charge carrier dynamics, especially as measured by optical techniques. Here, as opposed to prior studies of vapor deposited films, phase-pure colloidal SnS was synthesized by solution chemistry in three size regimes, ranging from nanometer- to micron-scale (SnS small nanoparticles, SnS medium 2D nanosheets, and SnS large 2D μm-sheets), and evaluated by time-resolved terahertz spectroscopy (TRTS); an optical, non-contact probe of the photoconductivity. Dropcast films of the SnS colloids were studied by TRTS and compared to both thermally annealed films and dispersed suspensions of the same colloids. TRTS results revealed that the micron-scale SnS crystals and all of the annealed films undergo decay mechanisms during the first 200 ps following photoexcitation at 800 nm assigned to hot carrier cooling and carrier trapping. The charge carrier mobility of both the dropcast and annealed samples depends strongly on the size of the constituent colloids. The mobility of the SnS colloidal films, following the completion of the initial decays, ranged from 0.14 cm2/V·s for the smallest SnS crystals to 20.3 cm2/V·s for the largest. Annealing the colloidal films resulted in a ~ 20 % improvement in mobility for the large SnS 2D μm-sheets and a ~ 5-fold increase for the small nanoparticles and medium nanosheets.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made towards the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products’ inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multi-point contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.

Ultrafast laser systems are becoming more widespread throughout the research and industrial communities yet eye protection for these high power, bright pulsed sources still require scrupulous characterization and testing before use. Femtosecond lasers, with pulses naturally possessing broad-bandwidth and high average power with variable repetition rate, can exhibit spectral side-bands and subtly changing center wavelengths, which may unknowingly affect eyewear safety protection. Pulse spectral characterization and power diagnostics are presented for a 80 MHz, Ti+3:Sapphire, ≈ 800 nm, ≈40 femtosecond oscillator system. Power and spectral transmission for 22 test samples are measured to determine whether they fall within manufacturer specifications.

We characterize the long-wavelength transmission of coiled and uncoiled SMF-28 and SM2000 fibers, which is important for extending the range of super-conducting nanowire single-photon detectors (SNSPDs) to infrared wavelengths beyond 1550 nm.

Ultrafast lasers have become increasingly important as research tools in laboratories and commercial enterprises suggesting laser safety, personal protection, and awareness become ever more important. Laser safety eyewear are typically rated by their optical densities (OD) over various spectral ranges, but these measurements are usually made using low power, large beam size, and continuous beam conditions. These measurement scenarios are vastly different than the high power, small beam size, and pulsed laser beam conditions where ultrafast lasers have extremely high peak powers and broad spectra due to the short pulse durations. Many solid-state lasers are also tunable over a broad wavelength range, further complicating the selection of adequate laser safety eyewear. Eighteen laser eyewear filter samples were tested under real-world conditions using a Ti:Sapphire regenerative amplifier with output pulses centered at 800 nm running from 2 Hz to 1 kHz repetition rate. The typical maximum peak laser irradiance employed was ca. 3 TW/cm2 (800 nm wavelength, 450 μJ/pulse with 80 fs full width at half maximum pulse duration) or less when damage occurred, depending on the sample. While many samples maintained their integrity under these test conditions, many plastic samples showed signs of failure which reduced their OD, in some cases transmitting 4–5 orders of magnitude higher than expected. In general, glass filters performed significantly better than plastic filters, exhibiting less physical damage to the substrate and less absorber degradation.Ultrafast lasers have become increasingly important as research tools in laboratories and commercial enterprises suggesting laser safety, personal protection, and awareness become ever more important. Laser safety eyewear are typically rated by their optical densities (OD) over various spectral ranges, but these measurements are usually made using low power, large beam size, and continuous beam conditions. These measurement scenarios are vastly different than the high power, small beam size, and pulsed laser beam conditions where ultrafast lasers have extremely high peak powers and broad spectra due to the short pulse durations. Many solid-state lasers are also tunable over a broad wavelength range, further complicating the selection of adequate laser safety eyewear. Eighteen laser eyewear filter samples were tested under real-world conditions using a Ti:Sapphire regenerative amplifier with output pulses centered at 800 nm running from 2 Hz to 1 kHz repetition rate. The typical maximum peak laser irradian...

Solution-deposited nanoscale films of RuO2 (“nanoskins”) are effective transparent conductors once calcined to 200 °C. Upon heating the nanoskins to higher temperature the nanoskins show increased transmission at 550 nm. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO2 nanoparticles. The mechanism for the spectral evolution is clearly observed with ultrafast optical measurements. Following excitation at 400 nm, nanoskins calcined at higher temperatures show increased transmission above 650 nm, consistent with the photobleaching of a surface-plasmon resonance (SPR) band. Calculations based on the optical constants of RuO2 substantiate the presence of SPR absorption. Sheet resistance and transient terahertz photoconductivity measurements establish that the nanoskins electrically de-wire into separated particles. The plasmonic behavior of the nanoskins has implications their use in a range of optical and electrochemical applications.

Thin-film ruthenium dioxide (RuO2) is a promising alternative material as a conductive electrode in electronic applications because its rutile crystalline form is metallic and highly conductive. Herein, a solution-deposition multi-layer technique is employed to fabricate ca. 70 ± 20 nm thick films (nanoskins) and terahertz spectroscopy is used to determine their photoconductive properties. Upon calcining at temperatures ranging from 373 K to 773 K, nanoskins undergo a transformation from insulating (localized charge transport) behavior to metallic behavior. Terahertz time-domain spectroscopy (THz-TDS) indicates that nanoskins attain maximum static conductivity when calcined at 673 K (σ = 1030 ± 330 S·cm−1). Picosecond time-resolved Terahertz spectroscopy (TRTS) using 400 nm and 800 nm excitation reveals a transition to metallic behavior when calcined at 523 K. For calcine temperatures less than 523 K, the conductivity increases following photoexcitation (ΔE < 0) while higher calcine temperatures yield films composed of crystalline, rutile RuO2 and the conductivity decreases (ΔE > 0) following photoexcitation.

Non-contact, optical time-resolved terahertz spectroscopy (TRTS) has been used to study the transient photoconductivity of nanometer-scale metallic films deposited on fused quartz substrates. Samples of 8 nm thick gold or titanium show an instrument-limited (ca. 0.5 ps) decrease in conductivity following photoexcitation due to electron-phonon coupling and subsequent increased lattice temperatures which increases charge carrier scattering. In contrast, for samples of 8 nm gold with a 4 nm adhesion layer of titanium or chromium, a ca. 70 ps rise time for the lattice temperature increase is observed. These results establish the increased transient terahertz transmission sign change of metallic compared to semiconductor materials. The results also suggest nanoscale gold films that utilize an adhesion material do not consist of distinct layers.

The compounds M(2)(O(2)C(t)Bu)(4) and M(2)(O(2)CC(6)H(5))(4), where M = Mo or W, have been examined by femtosecond time-resolved IR (fs-TRIR) spectroscopy in tetrahydrofuran with excitation into the singlet metal-to-ligand charge-transfer ((1)MLCT) band. In the region from 1500 to 1600 cm(-1), a long-lived excited state (2 ns) has been detected for the compounds M(2)(O(2)C(t)Bu)(4) and Mo(2)(O(2)C-C(6)H(5))(4) with an IR absorption at ~1540 cm(-1) assignable to the asymmetric CO(2) stretch, ν(as)(CO(2)), of the triplet metal-metal δ-δ star ((3)MM δδ*) state. The fs-TRIR spectra of W(2)(O(2)C-C(6)H(5))(4) are notably different and are assigned to decay of the MLCT states. In (3)MM δδ*, the removal of an electron from the δ orbital reduces MM δ to CO(2) π* back-bonding and causes a shift of ν(as)(CO(2)) to higher energy by ~30-60 cm(-1), depending on the metal. TRIR spectroscopy also provides evidence for M(2)(O(2)C(t)Bu)(4), where M = Mo or W, having MM δδ* S(1) states with ν(as)(CO(2)) distinct from those of the (3)MM δδ* states.

The singlet S 1 and triplet T 1 photoexcited states of the compounds containing MM quadruple bonds trans -M 2 (T i PB) 2 (O 2 CC 6 H 4 -4-CN) 2 , where T i PB = 2,4,6-triisopropylbenzoate and M = Mo ( I ) or M = W ( I ′ ), and trans -M 2 (O 2 CMe) 2 ((N[ i Pr ]) 2 CC ≡ CC 6 H 5 ) 2 , where M = Mo ( II ) and M = W ( II ′ ), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I ′ but localized for II and II ′ involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMo δδ * for both I and II but delocalized 3 MLCT for I ′ and localized 3 MLCT for II ′ . These differences arise from consideration of the relative orbital energies of the M 2 δ or M 2 δ * and the ligand π ∗ as well as the magnitudes of orbital overlap.

The photophysical properties of the title compound have been studied by fs and ns transient absorption spectroscopy. The electronic absorption spectrum consists of three principle absorptions assigned to terpy 1LLCT at ~300 nm, ruthenium (II) t 2g 6 to terpy 1MLCT at ~470 nm and Mo2 δ to terpycarboxylate at ~670 nm. The compound shows weak room temperature emission in THF solution at ~1,100 nm when excited into each of the aforementioned bands. This emission is assigned to the T1 state, 3MMδδ*. Transient absorption spectroscopy indicates a lifetime for T1 of 9.6 μs.

Metal-Organic Frameworks (MOFs) are three-dimensional coordination polymers that are well known for large pore surface area and their ability to adsorb molecules from both the gaseous and solution phases. In general, MOFs are electrically insulating, but promising opportunities for tuning the electronic structure exist because MOFs possess synthetic versatility; the metal and organic ligand subunits can be exchanged or dopant molecules can be introduced into the pore space. Two such MOFs with demonstrated electrical conductivity are Cu 3 (1,3,5-benzenetricarboxylate) 2 , a.k.a HKUST-1, and Cu[Ni(pyrazine-2,3-dithiolate) 2 ]. Herein, these two MOFs have been infiltrated with the redox active species 7,7,8,8-tetracyanoquinodimethane (TCNQ) and iodine under solution phase conditions and shown to produce redox products within the MOF pore space. Vibrational bands assignable to TCNQ anion and triiodide anion have been observed in the Mid-IR and Terahertz ranges using FTIR Spectroscopy. The MOF samples have been further investigated by Time-Resolved Terehertz Spectroscopy (TRTS). Using this technique, the charge mobility, separation, and recombination dynamics have been followed on the picosecond time scale following photoexcitation with visible radiation. The preliminary results show that the MOF samples have small inherent photoconductivity with charge separation lifetimes on the order of a few picoseconds. In the case of HKUST-1, the MOF can also be supported by a TiO 2 film and initial results show that charge injection into the TiO 2 layer occurs with a comparable efficiency to the dye sensitizer N3, [cis-Bis(isothiocyanato)-bis(2,2’-bipyridyl-4,4’-dicarboxylato ruthenium(II)], and therefore this MOF has potential as a new light absorbing and charge conducting material in photovoltaic devices.

Ultrafast laser systems are becoming more widespread throughout the research and industrial communities, yet eye protection for these high power, bright pulsed sources still requires scrupulous characterization and testing before use. Femtosecond lasers, with pulses naturally possessing a broad-bandwidth and high average power with a variable repetition rate, can exhibit spectral side-bands and subtly changing center wavelengths, which may unknowingly affect eyewear safety protection. Pulse spectral characterization and power diagnostics are presented for a 80 MHz, Ti+3:Sapphire, ≈800 nm, and ≈40 fs oscillator system. Power and spectral transmission for 22 test samples are measured to determine whether they fall within manufacturer specifications.

Charge carrier conductivity and mobility for various semiconductor wafers and crystals were measured by ultrafast above bandgap, optically excited Time-Resolved Terahertz Spectroscopy (TRTS) and Hall Van der Pauw contact methods to directly compare these approaches and validate the use of the non-contact optical approach for future materials and in-situ device analyses. Undoped and doped silicon (Si) wafers with resistances varying over six orders of magnitude were selected as model systems since contact Hall measurements are reliably made on this material. Conductivity and mobility obtained at room temperature by terahertz transmission and TRTS methods yields the sum of electron and hole mobility which agree very well with either directly measured or literature values for corresponding atomic and photo-doping densities. Careful evaluation of the optically-generated TRTS frequency-dependent conductivity also shows it is dominated by induced free-carrier absorption rather than small probe pulse phase shifts, which is commonly ascribed to changes in the complex conductivity from sample morphology and evaluation of carrier mobility by applying Drude scattering models. Thus, in this work, the real-valued, frequency-averaged conductivity was used to extract sample mobility without application of models. Examinations of germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP) and zinc telluride (ZnTe) samples were also made to demonstrate the general applicability of the TRTS method, even for materials that do not reliably make good contacts (e.g., GaAs, GaP, ZnTe). For these cases, values for the sum of the electron and hole mobility also compare very favorably to measured or available published data.

From the reactions between Mo2(T(i)PB)4, where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid) the compounds trans-M2(T(i)PB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4'-nitro-1,1'-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds have been characterized by (1)H NMR, UV-Vis and steady state emission spectroscopy, ns and fs transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions based on electronic structure calculations on model compounds where T(i)PB is substituted for formate. Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I relative to II but this order is reversed in the photo excited S1(1)MLCT state. Attempts to prepare the W2 containing analogs were unsuccessful.

Solutions of acetonitrile (MeCN) in methanol (MeOH) at various concentrations have been investigated by variable temperature Raman spectroscopy. In the ν(CN) region of the spectrum, the variable temperature spectra at each concentration show two overlapping bands from hydrogen bound and free MeCN. These two species undergo dynamic exchange that gives rise to increasing coalescence of the two bands with increasing temperature. By simulation of the band shape, the rate of exchange was determined at each temperature. Arrhenius plots yielded values for the activation energy, Ea, and the natural log of the pre-exponential factor, ln[A/s(-1)], for the hydrogen bond formation/cleavage. Both of these dynamic parameters were found to depend on the relative amounts of MeCN and MeOH in the solutions. In particular, two different concentration regimes of dynamic hydrogen bonding were observed. First, at low MeCN concentration, the dynamics are largely independent of changes in MeCN concentration. Second, at higher MeCN concentration (above ∼0.2 MeCN mole fraction) the dynamics are strongly dependent on further increases of MeCN content. Over the range of MeCN mole fractions that we studied (0.03-0.5), the ln[A/s(-1)] changes from 32.5 ± 0.1 to 30.1 ± 0.2 and Ea changes from 3.73 ± 0.08 to 2.7 ± 0.1 kcal/mol. We suggest the observed changes in dynamics arise from changes in the local solvent microstructure that occur above a critical mole fraction of MeCN.

Evidence, based on femtosecond transient absorption and time resolved infrared spectroscopy, is presented for photoinduced charge transfer from the Mo2δ orbital of the quadruply bonded molecule trans-Mo2(T(i)PB)2(BTh)2, where T(i)PB = 2,4,6-triisopropyl benzoate and BTh = 2,2'-bithienylcarboxylate, to di-n-octyl perylene diimide and di-n-hexylheptyl perylene diimide in thin films and solutions of the mixtures. The films show a long-lived charge separated state while slow back electron transfer, τBET ~ 500 ps, occurs in solution.

Quadruply bonded dinuclear metal complexes of molybdenum and tungsten have the MM configuration σ(2)π(4)δ(2) and a considerable degree of attention has been devoted to studies of the δ → δ(*) transition. For compounds of the type M(2)(O(2)CR)(4), the CO(2) π(*) orbitals introduce a M(2) δ to ligand π(*) transition, a (1)MLCT absorption which may be lower in energy than the δ → δ(*) and is more intense, thus obscuring the observation of the latter. When the R group is a conjugated organic system such as an aryl group, the (1)MLCT shifts to even lower energy and emission is seen from this S(1) state in addition to phosphorescence from the T(1) state which may be either (3)MLCT or (3)MMδδ(*). The latter typically occurs around 1100 nm with a lifetime that ranges from ~1 μs (M = W) to 100 μs (M = Mo). The S(1)(1)MLCT states have lifetimes of ~1-20 ps, allowing for fs and ns studies of the charge distribution/localization with time in both the S(1) and T(1) states, which is quite rare for transition metal coordination complexes. Of particular interest and focus have been complexes of the type trans-M(2)L(2)L'(2) where L and L' are carboxylate or amidinate groups for which only one set of ligands allows for expansive Lπ-M(2)δ-Lπ conjugation and has a low energy (1)MLCT. Compounds of this type have excited states that may be considered as mixed valence (MV) ions [L-M(2)(+)-L(-)] ↔ [L(-)-M(2)(+)-L] where the hole resides on the M(2) unit and the electron is either localized on one ligand, a class I or II MV ion, or is fully delocalized over both ligands, a class III ion in the Robin and Day scheme. Examples of these systems will be described along with the newly prepared complexes trans-M(2)(T(i)PB)(2)(O(2)CC≡C-9-anthracene)(2), M = Mo, W, that have the IR-active reporter groups CO(2) and C≡C.

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and M = Mo or W, in toluene and each of the respective carboxylic acids (2 equiv) the quadruply MM bonded compounds trans-M(2)(T(i)PB)(2)(O(2)C-C≡C-Ar)(2) have been prepared where Ar = p-tolyl and M = Mo, , and M = W, , and Ar = 9-anthracenyl, where M = Mo, 2a, and M = W, 2b. Single crystal X-ray crystallographic studies of 1a and 2b confirmed the trans substitution pattern about the Mo(2)(4+) unit and the centrosymmetric molecules have structural features that indicate extensive Lπ-Mo(2)δ-Lπ conjugation involving the arylethynylcarboxylates. The compounds are intensely colored as a result of the HOMO → LUMO, metal δ-to-ligand π* charge transfer (1)MLCT transition: 1a (orange), 1b (red), 2a (blue) and 2b(green). The compounds 1a, 2a, 1b and 2b have been characterized by UV-Vis-NIR absorption and emission spectroscopy, cyclic voltammetry, femtosecond (fs) and nanosecond (ns) transient absorption spectroscopy and by fs time-resolved infrared spectroscopy in the region of ν(C≡C), ν(CO(2)) and from 1400-1000 cm(-1). Aided by density functional theory, (DFT) and time dependent DFT, the electronic structures of the ground state and the S(1) and T(1) states are described. The molybdenum compounds have short lived (1)MLCT states, 1a ~ 5.0 ps and 2a ~ 10.5 ps, that undergo intersystem crossing to long lived (3)MoMoδδ* states: 1a ~ 101 μs and 2a ~ 83 μs. The tungsten complexes show interesting time-resolved infrared spectra in the ν(C≡C) region when compared with their ground state. Compound 1b shows ν(C≡C) at 1975 cm(-1) for the (1)MLCT state which decays with τ ~ 0.7 ps to ν(C≡C) at 2000 cm(-1) for the (3)MLCT state. For 2b the (1)MLCT is characterized by ν(C≡C) at 2150 cm(-1), τ ~ 19 ps, and a very broad absorption with a maximum ~1970 cm(-1) which is proposed to arise from a low energy electronic transition. The (3)MLCT state for shows no evidence of ν(C≡C) and is suggested to have an electron localized principally on the anthracenyl portion of the ligand, a proposal that finds support from the nature of triplet transient absorption spectrum of 2b.

The preparation and characterization of the compounds trans-M(2)(T(i)PB)(2)(O(2)C-CH=CH-2-C(4)H(3)S)(2) where M = Mo or W and T(i)PB = 2,4,6-triisopropylbenzoate are reported. The optical spectra of the new compounds are compared with those of related trans-M(2)(T(i)PB)(2)L(2) compounds where L = O(2)C-C(6)H(4)-4-CN, O(2)C-α,α'-terthienyl (TTh), and O(2)C-4-C(6)H(4)N-B(C(6)F(5))(3), that show strong metal-to-ligand charge transfer bands because of M(2)δ to Lπ conjugation, and are notably temperature dependant due to the various conformations of the two trans-L groups. Upon cooling the spectral features sharpen as the planar geometry that optimizes M(2)δ-Lπ conjugation is favored. As the electronic coupling of the two trans-Lπ systems increases the (0,0) electronic transition gains intensity indicating a greater nesting of the ground state (S(0)) and excited state (S(1)) potential energy surfaces. These features are discussed in terms of the related electronic coupling of [M(2)]-[M(2)] complexes.

The title compounds trans-M(2)(O(2)CMe)(2)[C((i)PrN)(2)C≡C-Ph](2), I (M = Mo) and II (M = W), show electronic absorptions in the visible region of the spectrum assignable to (1)MLCT [M(2)δ to phenylethynylamidinate π*]. These compounds show dual emission from S(1) and T(1) states. For both I and II, S(1) is (1)MLCT, but for I the T(1) state is shown to be MMδδ* while for II T(1) is (3)MLCT. The lifetimes of the S(1) and T(1) states have been determined by femtosecond and nanosecond transient absorption spectroscopy: for I S(1) ∼ 20 ps and T(1) ∼ 100 μs and for II S(1) ∼ 6 ps and T(1) ∼ 5 μs. From solvent dependence of the absorption and emission spectra, we suggest that the S(1) states are localized on one amidinate ligand though the initial absorption is to a delocalized state.

The compounds M(2)(TiPB)(2)(OSC-2-Th)(2) have been prepared from the reactions between M(2)(TiPB)(4) and Th-2-COSH (2 equiv) in toluene solution, where M = Mo (Mo(2)ThCOS) or W (W(2)ThCOS), TiPB = 2,4,6-triisopropylbenzoate and Th = thienyl. The molybdenum and tungsten compounds are pink and blue, air-sensitive, ether soluble solids that show M(+) ions in the mass spectrometer and metal and ligand based reversible oxidation and reduction waves, respectively, by cyclic voltammetry. Electronic structure calculations on the model compounds M(2)(O(2)CH)(2)(OSC-2-Th)(2) indicate that the highest occupied molecular orbital (HOMO) is principally M(2)delta and the lowest unoccupied molecular orbital (LUMO) is thienylthiocarboxylate pi* but with significant metal-sulfur mixing. The intense visible absorptions arise from (1)MLCT, M(2)delta to thienylthiocarboxylate. The photoexcited states of these molecules have been studied by transient absorption spectroscopy and steady state emission. These properties are compared with those of previously reported thienylcarboxylate compounds, M(2)(TiPB)(2)(O(2)C-2-Th)(2), where M = Mo (Mo(2)ThCO(2)) or W (W(2)ThCO(2)).

The reactions between MM(TiPB)(4), where TiPB = 2,4,6-triisopropylbenzoate and MM = MoW and W(2), and the (2,2':5',2''-terthiophene)-5-carboxylic acid, TThH (2 equiv) leads to the formation of new compounds trans-MM(TiPB)(2)(TTh)(2), II and III, respectively, as well as to the previously reported compound I, when MM = Mo(2). The compounds have been characterized by elemental analysis, (1)H NMR spectroscopy, electronic absorption, and emission spectroscopies together with cyclic voltammetry and differential pulse voltammetry. Calculations on the model compounds I', II', and III', where formate ligands substitute for TiPB, have been carried out employing density functional theory (DFT) and time-dependent DFT. These complexes display intense (1)MLCT absorptions (MMdelta to thienyl carboxylate) and have oxidations and reductions that are metal (MMdelta) and thienyl ligand based, respectively. All compounds show emission in the near-IR region. At low temperature the NIR emission from I and II shows clear evidence of vibronic features due to upsilon(MM) approximately 350-390 cm(-1), and all compounds show evidence of a vibronic feature due to upsilon(CO(2)) approximately 1200 cm(-1). Transient absorption spectroscopy reveals relatively short-lived S(1) states, tau approximately 10 ps, and longer lived T(1) states: tau approximately 72 micros for I, approximately 160 ns for II, and approximately 90 ns for III. The chemistry described here reveals the remarkable influence of MMdelta to TTh pi electronic coupling on the optoelectronic properties of the thienyl chains.

Recently novel metallo-organic (MO) complexes have been synthesized by the Chisholm group at Ohio State, leading to the potential for a MO complex:fullerene bulk heterojuction solar cell that overcomes the limitations of current polymer:fullerene BHJ solar cell by expanding the usable solar spectrum into the near infrared. These metallated oligimers provide for high absorption at longer wavelengths of solar energy and with very long lifetimes of created electrons for high collection efficiency, leading to a potential efficiency enhancement over conventional organic solar cells. However, many fundamental issues dealing with processability and optimum device topology need to be addressed for this new chemistry system to move from the Chemistry lab to a viable device platform. In this work, prerequisites and preliminary findings of MO complexes for efficient photovoltaic operation are mainly discussed.

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2)MoWW(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2)MoWMo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.

The reaction between M(2)(TiPB)(4) (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M(2)(TiPB)(2)(6-carboethoxy-2-azulenecarboxylate)(2). Compound (M = Mo) is blue and compound (M = W) is green. Both are air sensitive, hydrocarbon soluble species that gave the corresponding molecular ions in their mass spectra (MALDI-TOF). They show metal based oxidations and ligand based reductions. Electronic structure calculations (DFT and time dependent DFT) indicate that the two azulene carboxylate pi systems are coupled by their interactions with the M(2)delta orbitals. Their intense colors arise from M(2)delta to azulene pi* electronic transitions. While compound exhibits weak emission at approximately 900 nm, no emission has been detected for . Both and have been studied by fs and ns transient absorption spectroscopy. The X-ray analysis of the molecular structure of in the solid state confirmed the paddlewheel nature of its W(2)(O(2)C)(4) core and the trans orientation of the ligands.

The preparation and structure of Re(2)(dppm)(2)(O(2)CC(6)H(4)-p-NO(2))(2)Cl(2), where dppm = Ph(2)PCH(2)PPh(2), is reported together with its photophysical properties (absorption, steady state emission, fs- and ns-transient absorption spectroscopy) and electrochemistry. These data are compared with photophysical studies on the previously reported Re(2)(dppm)(2)(O(2)CCH(3))(2)Cl(2). The preparation of the complex Re(2)(O(2)CC(6)H(4)-p-NO(2))(4)Cl(2) is also reported together with its photophysical properties which allows for a comparison of the electronic structures and photophysical states of Re(2)(4+) and Re(2)(6+) containing complexes having MM configurations σ(2)π(4)δ(2)δ(*2) and σ(2)π(4)δ(2), respectively. An interesting comparison is also made with the related MM quadruply bonded complexes of molybdenum and tungsten.