Louis E. McNamara

Donor–Acceptor–Donor Thienopyrazines via Pd-Catalyzed C–H Activation as NIR Fluorescent Materials

A series of thienopyrazine-based donor–acceptor–donor (D–A–D) near-infrared (NIR) fluorescent compounds were synthesized through a rapid, palladium-catalyzed C–H activation route. The dyes were studied through computational analysis, electrochemical properties analysis, and characterization of their photophysical properties. Large Stokes shifts of approximately 175 nm were observed, which led to near-infrared emission. Computational evaluation shows that the origin of this large Stokes shift is a significant molecular reorganization particularly about the D–A bond. The series exhibits quantum yields of up to φ = >4%, with emission maxima ranging from 725 to 820 nm. The emission is strong in solution, in thin films, and also in isolation at the single-molecule level. Their stable emission at the single-molecule level makes these compounds good candidates for single-molecule photon sources in the near-infrared.

A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of α-Fe2O3 Photoanode for Solar Water Splitting

Electrochemical reduction method is used for the first time to significantly improve the photo-electrochemical performance of α-Fe2O3 photoanode prepared on fluorine-doped tin oxide substrates by spin-coating aqueous solution of Fe(NO3)3 followed by thermal annealing in air. Photocurrent density of α-Fe2O3 thin film photoanode can be enhanced 25 times by partially reducing the oxide film to form more conductive Fe3O4 (magnetite). Fe3O4 helps facilitate efficient charge transport and collection from the top α-Fe2O3 layer upon light absorption and charge separation to yield enhanced photocurrent density. The optimal enhancement can be obtained for <50 nm films because of the short charge transport distance for the α-Fe2O3 layer. Thick α-Fe2O3 films require more charge and overpotential than thinner films to achieve limited enhancement because of the sluggish charge transport over a longer distance to oxidize water. Electrochemical reduction of α-Fe2O3 in unbuffered pH-neutral solution yields much higher but unstable photocurrent enhancement because of the increase in local pH value accompanied by proton reduction at a hematite surface.

Molecular Design Principles for Near-Infrared Absorbing and Emitting Indolizine Dyes

Desirable components for dye-sensitzed solar cell (DSC) sensitizers and fluorescent imaging dyes include strong donating building blocks coupled with well-balanced acceptor functionalities for absorption beyond the visible range. We have evaluated the effects of increasing acceptor strengths and incorporation of dye morphology controlling groups on molar absorptivity and absorption breadth with indolizine donor-based dyes. Indolizine-based D-A and D-π-A sensitizers incorporating bis-rhodanine, tricyanofuran (TCF), and cyanoacrylic acid functionalities were analyzed for performance in DSC devices. The TCF derivatives were also evaluated as near-infrared (NIR)-emissive materials with the AH25 emissions extending past 1000 nm.

Molecular Engineering of Near Infrared Absorbing Thienopyrazine Double Donor Double Acceptor Organic Dyes for Dye-Sensitized Solar Cells

The thienopyrazine (TPz) building block allows for NIR photon absorption in dye-sensitized solar cells (DSCs) when used as a π-bridge. We synthesized and characterized 7 organic sensitizers employing thienopyrazine (TPz) as a π-bridge in a double donor, double acceptor organic dye design. Donor groups are varied based on electron donating strength and sterics at the donor-π bridge bond with the acceptor groups varied as either carboxylic acids or benzoic acids on the π-bridge. This dye design was found to be remarkably tunable with solution absorption onsets ranging from 750 to near 1000 nm. Interestingly, the solution absorption measurements do not accurately approximate the dye absorption on TiO2 films with up to a 250 nm blue-shift of the dye absorption onset on TiO2. This shift in absorption and the effect on electron transfer properties is investigated via computational analysis, time-correlated single photon counting studies, and transient absorption spectroscopy. Structure-performance relationships were analyzed for the dyes in DSC devices with the highest performance observed at 17.6 mA/cm2 of photocurrent and 7.5% PCE for a cosensitized device with a panchromatic IPCE onset of 800 nm.

Covalent synthesis of perylenediimide-bridged silsesquioxane nanoribbons and their electronic properties

Perylenediimide-functionalized silsesquioxane nanostructures were prepared from base-catalyzed polymerization of their respective monoalkoxysilane precursor. The shapes of nanostructures varied from nanoribbons to nanochains to nanorods upon changing the base concentration. Transmission electron microscopy confirms the twisted nature of nanoribbons with lengths up to 15 μm, whereas the dimensions of nanorods were in the range of 9 μm in length and 200 nm in width. The photovoltaic performance of the nanoribbons and nanorods were evaluated and compared for bulk heterojunction solar cells and it was discovered that morphology plays an important role in the PV performance.

Ullazine Donor-π bridge-Acceptor Organic Dyes for Dye-Sensitized Solar Cells

A series of four ullazine-donor based donor-π bridge-acceptor (D-π-A) dyes have been synthesized and compared to a prior ullazine donor-acceptor (D-A) dye as well as a triphenylamine donor with an identical π-bridge and acceptor. The D-π-A ullazine series demonstrates an unusually uniform-in-intensity panchromatic UV/Vis absorption spectrum throughout the visible region. This is in part due to the introduction of strong high-energy bands through incorporation of the ullazine building block as shown by computational analysis. The dyes were characterized on TiO2 films and in DSC devices. Performances of 5.6 % power conversion efficiency were obtained with IPCE onsets reaching 800 nm.

Probing Dative and Dihydrogen Bonding in Ammonia Borane with Electronic Structure Computations and Raman under Nitrogen Spectroscopy

Although ammonia borane is isoelectronic with ethane and they have similar structures, BH3NH3 exhibits rather atypical bonding compared to that in CH3CH3. The central bond in ammonia borane is actually a coordinate covalent or dative bond rather than the conventional covalent C-C bond in ethane where each atom donates one electron. In addition, strong intermolecular dihydrogen bonds can form between two or more ammonia borane molecules compared to the relatively weak dispersion forces between ethane molecules. As a result, ammonia boranes physical properties are very sensitive to the environment. For example, gas-phase and solid-state ammonia borane have very different BN bond lengths and BN stretching frequencies, which led to much debate in the literature. It has been demonstrated that the use of cluster models based on experimental crystal structures led to better agreement between theory and experiment. Here, we employ a variety of cluster models to track how the interaction energies, bond lengths, and vibrational normal modes evolve with the size and structural characteristics of the clusters. The M06-2X/6-311++G(2df,2pd) level of theory was selected for this analysis on the basis of favorable comparison with CCSD(T)/aug-cc-pVTZ data for the ammonia borane monomer and dimer. Fourteen unique fully optimized molecular cluster geometries, (BH3NH3)n≤12, and nine crystal models, (BH3NH3)n≤19, were used to elucidate how the local environment impacts ammonia boranes physical properties. Computational results for the BN stretching frequencies are also compared directly to the Raman spectrum of solid ammonia borane at 77 K using Raman under liquid nitrogen spectroscopy (RUNS). A strong linear correlation was found to exist between the BN bond length and stretching frequency, from an isolated monomer to the most distorted BH3NH3 unit in a cluster or crystal structure model. Excellent agreement was seen between the frequencies computed for the largest crystal model and the RUNS experimental spectra (typically within a few wavenumbers).

Preparation of n-type semiconducting polymer nanoarrays by covalent synthesis followed by crystallization

In situ covalent synthesis followed by solution crystallization of the n-type semiconducting polymer derived from perylenediimide-bridged silsesquioxanes (PDIB-SSQ) yielded 1D-nanoarrays of 40–100 nm widths and up to 80 μm lengths. Morphologies and dimensions of nanostructures resulting from different base concentrations were characterized by SEM, fluorescence optical microscopy, SAXS, elemental analysis, MALDI-TOF-MS, and absorption and fluorescence spectroscopies. As revealed by SAXS, the nanostructures are composed of crystalline unit cells with cell parameters of d[001] = 24.8 A and d[100] = 10.3 with multiple π–π stacking distances ranging from 4.71 to 2.57 A. The ordering in polymer nanoarrays is favoured by the π–π interactions between the cofacially arranged PDI cores, resulting closely packed polymer arrays with the d-spacing ranging from 3.67 A to 3.24 A. The spectroscopic traces of nanoarrays in solution resembled that of the monomer except the slightly red shifted features of the emission spectrum associated with π–π stacking of polymer chains in aggregated form. Thin film emission spectra followed the similar spectral pattern with a noticeable shoulder corresponds to cofacial π–π interactions. The excited state lifetimes of aggregated polymer nanoarrays in both solution and solid phase were nearly identical. The electrical characterization of thin films made from polymer nanoarrays shows typical semiconducting behaviour with the electrical conductivity of 0.48 × 10−3 S cm−1. The covalent synthesis followed by solution-based crystallization of PDIB-SSQ reported herein provides a new synthesis path to make ordered-crystalline semiconducting polymer nanoarrays and ultimately a benefit for better understanding of their role in organic electronics.

Indolizine–Squaraines: NIR Fluorescent Materials with Molecularly Engineered Stokes Shifts

The development of deep red and near infrared emissive materials with high quantum yields is an important challenge. Several classes of squaraine dyes have demonstrated high quantum yields, but require significantly red-shifted absorptions to access the NIR window. Additionally, squaraine dyes have typically shown narrow Stokes shifts, which limits their use in living biological imaging applications due to dye emission interference with the light source. Through the incorporation of indolizine heterocycles we have synthesized novel indolizine squaraine dyes with increased Stokes shifts (up to >0.119 eV, >50 nm increase) and absorptions substantially further into the NIR region than an indoline squaraine benchmark (726 nm versus 659 nm absorption maxima). These materials have shown significantly enhanced water solubility, which is unique for squaraine dyes without water-solubilizing substituents. Absorption, electrochemical, computational, and fluorescence studies were undertaken and exceptional fluorescence quantum yields of up 12 % were observed with emission curves extending beyond 850 nm.

Synthesis of MoS2 from [Mo3S7(S2CNEt2)3]I for enhancing photoelectrochemical performance and stability of Cu2O photocathode toward efficient solar water splitting

Cu2O is a typical p-type semiconductor that can efficiently absorb visible light and has a high absorption coefficient due to its narrow forbidden band. Thus, it finds potential applications in solar energy conversion and photocatalysis. However, Cu2O photocathodes suffer from a major issue of chemical stability and sluggish proton reduction for splitting water using sunlight. We present here a facile method of coating a thin MoS2 layer onto Cu2O to significantly improve its stability and proton reduction efficiency. MoS2 coating on top of Cu2O is achieved by spin coating a [Mo3S7(S2CNEt2)3]I precursor combined with a thermal annealing process to obtain the optimal stoichiometry. MoS2 thin films synthesized using this method show good prospects as both a protection layer and an electrocatalyst for hydrogen evolution reactions (HER) due to excellent stability and high electrocatalytic activity. The proton reduction performance of spin-coated MoS2/FTO electrodes is studied to determine the optimal synthesis conditions using various derivatives of MoS2 precursors. Our study suggests that the rate-limiting kinetic step of MoS2 synthesized in this method is the desorption of adsorbed hydrogen atoms to form molecular hydrogen, and that nanocrystalline MoS2 with copiously exposed S edges are more active for HER. Photoelectrochemical measurements demonstrate the highest activity for 3-layered (<40 nm thick) MoS2/Cu2O photocathode fabricated at 450 °C with a photocurrent density of ∼6.5 mA cm−2 at −0.2 V vs. RHE. Additionally, the MoS2 coating helps minimize the dark current of the Cu2O photocathode.