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The development of stimuli-responsive amphiphilic supramolecular nanostructures is an attractive target for systems based on light-absorbing chromophores that can function as photosensitizers in water. We report here on a water soluble supramolecular carboxylated perylene monoimide system in which charge can be switched significantly by a change in pH. This was accomplished by substituting the perylene core with an ionizable hydroxyl group. In acidic environments, crystalline supramolecular nanoribbons with dimensions on the order of 500 × 50 × 2 nm form readily, while in basic solution the additional electrostatic repulsion of the ionized hydroxyl reduces assemblies to very small dimensions on the order of only several nanometers. The HOMO/LUMO levels were also found to be sensitive to pH; in acidic media the HOMO/LUMO levels are -5.65 and -3.70 eV respectively versus vacuum, whereas is in basic conditions they are -4.90 and -3.33 eV, respectively. Utilizing the assemblies as photosensitizers in photocatalytic production of hydrogen with [Mo3S13]2- as a catalyst at a pH of 4, H2 was generated with a turnover number of 125 after 18 hours. Charge switching the assemblies at a pH of 9-10 and using an iron porphyrin catalyst, protons could again be reduced to hydrogen and CO2 was reduced to CO with a turnover number of 30. The system investigated offers an example of dynamic photosensitizing assemblies that can drive reactions in both acidic and basic media.

Supramolecular light-absorbing nanostructures are useful building blocks for the design of next-generation artificial photosynthetic systems. Development of such systems requires a detailed understanding of how molecular packing influences the material’s optoelectronic properties. We describe a series of crystalline supramolecular nanostructures in which the substituents on their monomeric units strongly affects morphology, ordering kinetics, and exciton behavior. By designing constitutionally-isomeric perylene monoimide (PMI) amphiphiles, the effect of side chain sterics on nanostructure crystallization was studied. Molecules with short amine linked alkyl-tails rapidly crystallize upon dissolution in water, while bulkier tails require the addition of salt to screen electrostatic repulsion and annealing to drive crystallization. A PMI monomer bearing a 3-pentylamine tail was found to possess a unique structure that results in strongly red-shifted absorbance, indicative of charge-transfer exciton formation. This particular supramolecular structure was found to have an enhanced ability to photosensitize a thiomolybdate, [(NH(4))(2)Mo(3)S(13)], catalyst to generate hydrogen gas.

The fill factor (FF) of organic photovoltaic (OPV) devices has proven difficult to optimize by synthetic modification of the active layer materials. In this contribution, a series of small-molecule donors (SMDs) incorporating chalcogen atoms of increasing atomic number (Z), namely oxygen, sulfur, and selenium, into the side chains are synthesized and the relationship between the chalcogen Z and the FF of OPV devices is characterized. Larger Z chalcogen atoms are found to consistently enhance FF in bulk-heterojunction OPVs containing PC61BM as the acceptor material. A significant ∼8% FF increase is obtained on moving from O to S to Se across three series of SMDs. The FF enhancement is found to result from the combination of more ordered morphology and decreased charge recombination in blend films for the high-Z-chalcogen SMDs. Because this FF enhancement is found within three series of SMDs, the overall strategy is promising for new SMD materials design.

Using small-molecule donor (SMD) semiconductors in organic photovoltaics (OPVs) has historically afforded lower power conversion efficiencies (PCEs) than their polymeric counterparts. The PCE difference is attributed to shorter conjugated backbones, resulting in reduced intermolecular interactions. Here, a new pair of SMDs is synthesized based on the diketopyrrolopyrrole–benzodithiophene–diketopyrrolopyrrole (BDT-DPP2) skeleton but having fluorinated and fluorine-free aromatic side-chain substituents. Ternary OPVs having varied ratios of the two SMDs with PC61BM as the acceptor exhibit tunable open-circuit voltages (Vocs) between 0.833 and 0.944 V due to a fluorination-induced shift in energy levels and the electronic “alloy” formed from the miscibility of the two SMDs. A 15% increase in PCE is observed at the optimal ternary SMD ratio, with the short-circuit current density (Jsc) significantly increased to 9.18 mA/cm2. The origin of Jsc enhancement is analyzed via charge generation, transport, and diffus...

The energy landscape of a supramolecular material can include different molecular packing configurations that differ in stability and function. We report here on a thermally driven crystalline order transition in the landscape of supramolecular nanostructures formed by charged chromophore amphiphiles in salt-containing aqueous solutions. An irreversible transition was observed from a metastable to a stable crystal phase within the nanostructures. In the stable crystalline phase, the molecules end up organized in a short scroll morphology at high ionic strengths and as long helical ribbons at lower salt content. This is interpreted as the result of the competition between electrostatic repulsive forces and attractive molecular interactions. Only the stable phase forms charge-transfer excitons upon exposure to visible light as indicated by absorbance and fluorescence features, second-order harmonic generation microscopy, and femtosecond transient absorbance spectroscopy. Interestingly, the supramolecular reconfiguration to the stable crystalline phase nanostructures enhances photosensitization of a proton reduction catalyst for hydrogen production.

Molecular packing in light harvesting 2D assemblies of photocatalytic materials is a critical factor for solar-to-fuel conversion efficiency. However, structure–function correlations have yet to be fully established. This is partly due to the difficulties in extracting the molecular arrangements from the complex 3D powder averaged diffraction patterns of 2D lattices, obtained via in situ wide-angle X-ray scattering. Here, we develop a scattering theory formalism and couple it with a simple geometrical model for the molecular shape of chromophore 9-methoxy-N-(sodium hexanoate)perylene-3,4-dicarboximide (MeO-PMI) used in our study. This generally applicable method fully reproduces the measured diffraction pattern including the asymmetric line shapes for the Bragg reflections and yields the molecular packing arrangement within a 2D crystal structure with a remarkable degree of detail. We find an approximate edge-centered herringbone structure for the PMI fused aromatic rings and ordering of the carboxypentyl...

A new series of metal-free alkylated tetrathienoacene (TTAR)-based organic chromophores, TPA–TTAR–TA (R = branched-C8H17, 1, TTAR-b8; R = C15H31, 2, TTAR-15; R = C9H19, 3, TTAR-9), are synthesized for application in dye-sensitized solar cells (DSSCs). Due to the extensively conjugated TTAR π-bridge, all three dyes exhibit high extinction coefficients (1 × 105 M−1 cm−1). By systematically exploring the effects of the TTAR alkyl chain substituents, a significant influence of the dye coverage (orientation) on the TiO2 surfaces is observed. The branched-alkyl TTAR-b8 (1) promotes significant tilting and packing distortion on TiO2 in comparison to more ordered monolayers of linear long alkyls TTAR-15 (2) and TTAR-9 (3). Photophysical measurements on the dye-grafted TiO2 films reveal that the branched-alkylated TTA unit in 1 enhances the electron injection efficiency, in agreement with the high quantum efficiency. Notably, by utilizing a three-dimensional (3D) photonic crystal (PhC) layer to enhance the coherent scattering an increase the light absorption, TTAR-b8 exhibits higher short-circuit current densities and achieved a high PCE of 11.18%. TTAR-b8 is thus the best performing fused-thiophene-based organic DSSC dye reported to date.

International audience; Three-dimensional organic–inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley–Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden–Popper phases—layered two-dimensional perovskite films—have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>103), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes. As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.

Although reactive additives have been employed in perovskite solar cells to enhance film morphology and significantly increase device performance, little is known about the effect of these additives on perovskite structural and optical properties. Here we report a systematic study of how the properties of methylammonium lead iodide perovskite (CH3NH3PbI3) are influenced by hydrohalic acid additives (HX; X=I, Br, Cl) in the precursor solution. Detailed structural and optical spectroscopic analysis reveals that all three acids affect the optical properties and alter the unit cell lattice parameters. Depending on the identity and concentration of HX, optical bandgaps widen or compress: addition of HBr yields a wider bandgap, whereas HI compresses the gap at high concentrations; HCl, on the other hand, has no significant effect on the bandgap. These changes can be understood by correlating them with the types of defects present in polycrystalline perovskite thin films in combination with the structural strain induced in very small crystallites. The presence of extra halides from HX in the precursor solution enables filling of the lattice vacancies in the perovskite, thereby altering metal–halogen–metal bond connectivity and consequently cell volumes and optical bandgaps. Remarkably, a room temperature tetragonal→cubic phase transition is observed for CH3NH3PbI3 films treated with high HX concentrations. Further insights into this anomalous phase transformation are obtained from in situ variable-temperature X-ray diffraction in the 25–55 °C (298–328 K) range, revealing a monotonic fall in transition temperature with increasing precursor solution HX concentration.