Supramolecular Donor–Acceptor Assembly Derived from Tetracarbazole–Zinc Phthalocyanine Coordinated to Fullerene: Design, Synthesis, Photochemical, and Photoelectrochemical Studies

A functional photosynthetic antenna–reaction center mimicking donor–acceptor polyad has been newly designed, synthesized, and characterized. The polyad was comprised of four entities of carbazole covalently linked to the macrocycle periphery of a zinc phthalocyanine (ZnPc). Efficient singlet excitation transfer from the carbazole to zinc phthalocyanine has been witnessed from the emission studies. Axial coordination of phenylimidazole-functionalized fulleropyrrolidine to ZnPc served as an electron acceptor in the polyad. Optical absorption and emission along with computational studies revealed stable complex formation wherein the evaluated binding constant <i>K</i> was 7.7 ± 0.2 × 10⁵ M^–1, an order of magnitude higher than that observed earlier for similar complexes due to the electronic effect induced by the carbazole entities. From the free-energy calculations, photoinduced electron transfer from the ¹ZnPc* to fullerene within the polyad was established to be an exothermic process. Kinetics of charge separation, <i>k</i>_CS, monitored by time-resolved emission was found to be 2.8 × 10⁹ s^–1, indicating a relatively fast charge-separation process. The electron-transfer products were characterized by nanosecond transient absorption spectroscopic technique; the presence of ZnPc^+• radical cation at 890 nm and fulleropyrrolidine anion radical at 1000 nm was clear from this study. The kinetics of charge recombination, <i>k</i>_CR, evaluated from the decay of either of the radical ions, was found to be 6.25 ± 0.2 × 10⁷ s^–1, revealing the persistence of the radical ion-pair species to some extent. Further, photoelectrochemical studies, performed by constructing photocells by electrophoretic deposition of the studied polyad on nanocrystalline SnO₂ modified surface, revealed a higher value of incident photon-to-current conversion efficiency covering the wide visible–near IR spectral region and good on–off switchability. Better charge injection from the excited polyad to the conduction band of the semiconductor was evident from the electrochemical impedance spectral measurements of electron recombination resistance calculations

Phenothiazine-Sensitized Organic Solar Cells: Effect of Dye Anchor Group Positioning on the Cell Performance

Effect of positioning of the cyanoacrylic acid anchoring group on ring periphery of phenothiazine dye on the performance of dye-sensitized solar cells (DSSCs) is reported. Two types of dyes, one having substitution on the C-3 aromatic ring (Type 1) and another through the N-terminal (Type 2), have been synthesized for this purpose. Absorption and fluorescence studies have been performed to visualize the effect of substitution pattern on the spectral coverage and electrochemical studies to monitor the tuning of redox levels. B3LYP/6-31G* studies are performed to visualize the frontier orbital location and their significance in charge injection when surface modified on semiconducting TiO₂. New DSSCs have been built on nanocrystalline TiO₂ according to traditional two-electrode Grätzel solar cell setup with a reference cell based on N719 dye for comparison. The lifetime of the adsorbed phenothiazine dye is found to be quenched significantly upon immobilizing on TiO₂ suggesting charge injection from excited dye to semiconducting TiO₂. The performances of the cells are found to be prominent for solar cells made out of Type 1 dyes compared to Type 2 dyes. This trend has been rationalized on the basis of spectral, electrochemical, computational, and electrochemical impedance spectroscopy results

Control over Photoinduced Energy and Electron Transfer in Supramolecular Polyads of Covalently linked azaBODIPY-Bisporphyrin ‘Molecular Clip’ Hosting Fullerene

A ‘molecular clip’ featuring a near-IR emitting fluorophore, BF₂-chelated tetraarylazadipyrromethane (aza-BODIPY) covalently linked to two porphyrins (MP, M = 2H or Zn) has been newly synthesized to host a three-dimensional electron acceptor fullerene via a ‘two-point’ metal–ligand axial coordination. Efficient singlet–singlet excitation transfer from ¹ZnP* to aza-BODIPY was witnessed in the dyad and triad in nonpolar and less polar solvents, such as toluene and <i>o</i>-dichlorobenzene, however, in polar solvents, additional electron transfer occurred along with energy transfer. A supramolecular tetrad was formed by assembling bis-pyridine functionalized fullerene via a ‘two-point’ metal–ligand axial coordination, and the resulted complex was characterized by optical absorption and emission, computational, and electrochemical methods. Electron transfer from photoexcited zinc porphyrin to C₆₀ is witnessed in the supramolecular tetrad from the femtosecond transient absorption spectral studies. Further, the supramolecular polyads (triad or tetrad) were utilized to build photoelectrochemical cells to check their ability to convert light into electricity by fabricating FTO/SnO₂/polyad electrodes. The presence of azaBODIPY and fullerene entities of the tetrad improved the overall light energy conversion efficiency. An incident photon-to-current conversion efficiency of up to 17% has been achieved for the tetrad modified electrode

Role of Surfactants and Salt in Aqueous Two-Phase Separation of Carbon Nanotubes toward Simple Chirality Isolation

Aqueous two-phase extraction has recently been demonstrated as a new method to separate single-wall carbon nanotubes (SWCNTs). In this work, we determined that the mechanism of separation is driven by the hydrophobicity of the surfactant, or combination of surfactants, at the SWCNT surface. This knowledge allowed us to develop a simple approach for obtaining highly enriched single-chirality suspensions in only 1 or 2 steps. These results were obtained by strategically combining multiple surfactants with different diameter-dependent binding affinities for SWCNTs and salts that readjust the surfactant structure within the mixed micelle surrounding the SWCNTs. The procedure is successfully applied to SWCNTs from different sources (CoMoCAT and HiPco) with various diameter distributions (from 0.53 to 1.2 nm). Each separation step is characterized by optical absorption, resonant Raman, and photoluminescence excitation spectroscopies. By determining the SWCNT sorting mechanism, we were able to develop a new set of parameters that separated another chirality

Sequential Photoinduced Energy and Electron Transfer Directed Improved Performance of the Supramolecular Solar Cell of a Zinc Porphyrin–Zinc Phthalocyanine Conjugate Modified TiO₂ Surface

Improved performance of a photosynthetic antenna–reaction center mimicking supramolecular solar cell is demonstrated. Toward this, porphyrin–phthalocyanine conjugates connected by amide linkers, as wide-band capturing solar energy harvesting materials, have been newly synthesized and characterized. Efficient singlet–singlet energy transfer from the zinc or free-base porphyrin to phthalocyanine is evidenced from steady-state emission and transient absorption studies in nonpolar and polar solvents. Further, the dyad was immobilized via axial coordination of zinc porphyrin of the dyad on semiconducting TiO₂ surface modified with axial coordinating ligand functionality, phenylimidazole. Photoelectrochemical studies revealed improved performance of this cell compared to either zinc porphyrin or zinc phthalocyanine only modified electrodes under similar experimental conditions. Transient absorption studies performed on the dyad immobilized on glass/TiO₂ surface suggested that upon excitation of the axially coordinated zinc porphyrin of the dyad singlet–singlet energy transfer to zinc phthalocyanine occurs within 0.2 ps instead of a competing charge injection reaction from the singlet excited zinc porphyrin to TiO₂. Further, sequential photoinduced electron transfer from the newly formed singlet excited zinc phthalocyanine to zinc porphyrin producing ZnP^•––ZnPc^•+ with a 2 ps time constant and followed by electron injection from the ZnP^•– to TiO₂ within 30 ps has been proposed as a mechanism of photocurrent generation in the biomimetic supramolecular photocell

Developing Monolithic Nanoporous Gold with Hierarchical Bicontinuity Using Colloidal Bijels

We report a universal platform for the synthesis of monolithic porous gold materials with hierarchical bicontinuous morphology and combined macro- and mesoporosity using a synergistic combination of nanocasting and chemical dealloying. This robust and accessible approach offers a new design paradigm for the parallel optimization of active surface area and mass transport in porous metal electrodes

Pyrazinacenes: aza analogues of acenes

A series of edge-sharing condensed oligopyrazine analogues of acenes, the pyrazinacenes, were synthesized and characterized. X-ray crystallographic determinations revealed intermolecular interactions that affect the propensity of the molecules to undergo π-π stacking. Increasing heteroatom substitution of the acene framework induces shorter intermolecular π-π stacking distances (shorter than for graphite) probably due to lower van derWaals radius of nitrogen atoms. Hydrogen bonding is also a determining factor in the case of compounds containing reduced pyrazine rings. Combined electrochemical, electronic absorption, and computational investigations indicate the substantial electron deficiency of the compounds composed of fused pyrazine rings. The pyrazinacenes are expected to be good candidates as materials for organic thin film transistors