Tailoring Porphyrin-Based Electron Accepting Materials for Organic Photovoltaics

The syntheses, potentiometric responses, optical spectra, electronic structural properties, and integration into photovoltaic devices are described for ethyne-bridged isoindigo-(porphinato)­zinc­(II)-isoindigo chromophores built upon either electron-rich 10,20-diaryl porphyrin (Ar-Iso) or electron-deficient 10,20-bis­(perfluoroalkyl)­porphyrin (Rf-Iso) frameworks. These supermolecules evince electrochemical responses that trace their geneses to their respective porphyrinic and isoindigoid subunits. The ethyne linkage motif effectively mixes the comparatively weak isoindigo-derived visible excitations with porphyrinic π–π* states, endowing Ar-Iso and Rf-Iso with high extinction coefficient (ε ∼ 10⁵ M^–1·cm^–1) long-axis polarized absorptions. Ar-Iso and Rf-Iso exhibit total absorptivities per unit mass that greatly exceed that for poly­(3-hexyl)­thiophene (P3HT) over the 375–900 nm wavelength range where solar flux is maximal. Time-dependent density functional theory calculations highlight the delocalized nature of the low energy singlet excited states of these chromophores, demonstrating how coupled oscillator photophysics can yield organic photovoltaic device (OPV) materials having absorptive properties that supersede those of conventional semiconducting polymers. Prototype OPVs crafted from the poly­(3-hexyl)­thiophene (P3HT) donor polymer and these new materials (i) confirm that solar power conversion depends critically upon the driving force for photoinduced hole transfer (HT) from these low-band-gap acceptors, and (ii) underscore the importance of the excited-state reduction potential (<i>E</i>^–/*) parameter as a general design criterion for low-band-gap OPV acceptors. OPVs constructed from Rf-Iso and P3HT define rare examples whereby the acceptor material extends the device operating spectral range into the NIR, and demonstrate for the first time that high oscillator strength porphyrinic chromophores, conventionally utilized as electron donors in OPVs, can also be exploited as electron acceptors

Valence Band Dependent Charge Transport in Bulk Molecular Electronic Devices Incorporating Highly Conjugated Multi-[(Porphinato)Metal] Oligomers

Molecular electronics offers the potential to control device functions through the fundamental electronic properties of individual molecules, but realization of such possibilities is typically frustrated when such specialized molecules are integrated into a larger area device. Here we utilize highly conjugated (porphinato)­metal-based oligomers (<b>PM</b>_<b>n</b> structures) as molecular wire components of nanotransfer printed (nTP) molecular junctions; electrical characterization of these “bulk” nTP devices highlights device resistances that depend on <b>PM</b>_<b>n</b> wire length. Device resistance measurements, determined as a function of <b>PM</b>_<b>n</b> molecular length, were utilized to evaluate the magnitude of a phenomenological β corresponding to the resistance decay parameter across the barrier; these data show that the magnitude of this β value is modulated via porphyrin macrocycle central metal atom substitution [β­(<b>PZn</b>_<b>n</b>; 0.065 Å^–1) < β­(<b>PCu</b>_<b>n</b>; 0.132 Å^–1) < β­(<b>PNi</b>_<b>n</b>; 0.176 Å^–1)]. Cyclic voltammetric data, and ultraviolet photoelectron spectroscopic studies carried out at gold surfaces, demonstrate that these nTP device resistances track with the valence band energy levels of the <b>PM</b>_<b>n</b> wire, which were modulated via porphyrin macrocycle central metal atom substitution. This study demonstrates the ability to fabricate “bulk” and scalable electronic devices in which function derives from the electronic properties of discrete single molecules, and underscores how a critical device functionwire resistancemay be straightforwardly engineered by <b>PM</b>_<b>n</b> molecular composition

Quasi-Ohmic Single Molecule Charge Transport through Highly Conjugated <i>meso</i>-to-<i>meso</i> Ethyne-Bridged Porphyrin Wires

Understanding and controlling electron transport through functional molecules are of primary importance to the development of molecular scale devices. In this work, the single molecule resistances of <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) structures (<b>PZn</b>_{<b><i>n</i></b>} compounds), connected to gold electrodes via (4′-thiophenyl)­ethynyl termini, are determined using scanning tunneling microscopy-based break junction methods. These experiments show that each α,ω-di­[(4′-thiophenyl)­ethynyl]-terminated <b>PZn</b>_{<b><i>n</i></b>} compound (<b>dithiol-PZn</b>_{<b><i>n</i></b>}) manifests a dual molecular conductance. In both the high and low conductance regimes, the measured resistance across these metal–<b>dithiol-PZn</b>_{<b><i>n</i></b>}–metal junctions increases in a near linear fashion with molecule length. These results signal that <i>meso</i>-to-<i>meso</i> ethyne-bridged porphyrin wires afford the lowest β value (β = 0.034 Å^–1) yet determined for thiol-terminated single molecules that manifest a quasi-ohmic resistance dependence across metal–<b>dithiol-PZn</b>_{<b><i>n</i></b>}–metal junctions

Electron Spin Relaxation of Hole and Electron Polarons in π‑Conjugated Porphyrin Arrays: Spintronic Implications

Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) oligomers (<b>PZn</b>_{<b><i>n</i></b>} compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the <b>PZn</b>_{<b><i>n</i></b>} hole polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b>) and electron polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>}) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b> and <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>} spin–spin (<i>T</i>₂) and spin–lattice (<i>T</i>₁) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin–spin and spin–lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal–dithiol-<b>PZn</b>_{<b><i>n</i></b>}–metal junctions, present <i>meso</i>-to-<i>meso</i> ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications

Tunable Leuko-polymersomes That Adhere Specifically to Inflammatory Markers

The polymersome, a fully synthetic cell mimetic, is a tunable platform for drug delivery vehicles to detect and treat disease (theranostics). Here, we design a leuko-polymersome, a polymersome with the adhesive properties of leukocytes, which can effectively bind to inflammatory sites under flow. We hypothesize that optimal leukocyte adhesion can be recreated with ligands that mimic receptors of the two major leukocyte molecular adhesion pathways, the selectins and the integrins. Polymersomes functionalized with sialyl Lewis X and an antibody against ICAM-1 adhere avidly and selectively to surfaces coated with inflammatory adhesion molecules P-selectin and ICAM-1 under flow. We find that maximal adhesion occurs at intermediate densities of both sialyl Lewis X and anti-ICAM-1, owing to synergistic binding effects between the two ligands. Leuko-polymersomes bearing these two receptor mimetics adhere under physiological shear rates to inflamed endothelium in an in vitro flow chamber at a rate 7.5 times higher than those to uninflamed endothelium. This work clearly demonstrates that polymersomes bearing only a single ligand bind less avidly and with lower selectivity, thus suggesting proper mimicry of leukocyte adhesion requires contributions from both pathways. This work establishes a basis for the design of polymersomes for targeted drug delivery in inflammation

Hapticity-Dependent Charge Transport through Carbodithioate-Terminated [5,15-Bis(phenylethynyl)porphinato]zinc(II) Complexes in Metal–Molecule–Metal Junctions

Single molecule break junction experiments and nonequilibrium Green’s function calculations using density functional theory (NEGF-DFT) of carbodithioate- and thiol-terminated [5,15-bis­(phenylethynyl)-10,20-diarylporphinato]­zinc­(II) complexes reveal the impact of the electrode-linker coordination mode on charge transport at the single-molecule level. Replacement of thiolate (−S^–) by the carbodithioate (−CS₂^–) anchoring motif leads to an order of magnitude increase of single molecule conductance. In contrast to thiolate-terminated structures, metal–molecule–metal junctions that exploit the carbodithioate linker manifest three distinct conductance values. We hypothesize that the magnitudes of these conductances depend upon carbodithoate linker hapticity with measured conductances across Au-[5,15-bis­(4′-(dithiocarboxylate)­phenylethynyl)-10,20-diarylporphinato]­zinc­(II)-Au junctions the greatest when both anchoring groups attach to the metal surface in a bidentate fashion. We support this hypothesis with NEGF-DFT calculations, which consider the electron transport properties for specific binding geometries. These results provide new insights into the origin of molecule-to-molecule conductance heterogeneity in molecular charge transport measurements and the factors that optimize electrode–molecule–electrode electronic coupling and maximize the conductance for charge transport

Caging Metal Ions with Visible Light-Responsive Nanopolymersomes

Polymersomes are bilayer vesicles that self-assemble from amphiphilic diblock copolymers, and provide an attractive system for the delivery of biological and nonbiological molecules due to their environmental compatibility, mechanical stability, synthetic tunability, large aqueous core, and hyperthick hydrophobic membrane. Herein, we report a nanoscale photoresponsive polymersome system featuring a <i>meso</i>-to-<i>meso</i> ethyne-bridged bis­[(porphinato)­zinc] (PZn₂) fluorophore hydrophobic membrane solute and dextran in the aqueous core. Upon 488 nm irradiation in solution or in microinjected zebrafish embryos, the polymersomes underwent deformation, as monitored by a characteristic red-shifted PZn₂ emission spectrum and confirmed by cryo-TEM. The versatility of this system was demonstrated through the encapsulation and photorelease of a fluorophore (FITC), as well as two different metal ions, Zn²⁺ and Ca²⁺

Photoinduced Electron Transfer Elicits a Change in the Static Dielectric Constant of a <i>de Novo</i> Designed Protein

We provide a direct measure of the change in effective dielectric constant (ε_S) within a protein matrix after a photoinduced electron transfer (ET) reaction. A linked donor–bridge–acceptor molecule, PZn–Ph–NDI, consisting of a (porphinato)­Zn donor (PZn), a phenyl bridge (Ph), and a naphthalene diimide acceptor (NDI), is shown to be a “meter” to indicate protein dielectric environment. We calibrated PZn–Ph–NDI ET dynamics as a function of solvent dielectric, and computationally <i>de novo</i> designed a protein <i>SCPZnI3</i> to bind PZn–Ph–NDI in its interior. Mapping the protein ET dynamics onto the calibrated ET catalogue shows that <i>SCPZnI3</i> undergoes a switch in the effective dielectric constant following photoinduced ET, from ε_S ≈ 8 to ε_S ≈ 3