When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

We report on a novel small organic molecule comprising a perylene chromophore fused to a six-membered ring containing a persistent nitroxide radical to give a perylene-nitroxide, or PerNO(•). This molecule is a robust, compact molecule in which the radical is closely bound to the chromophore but separated by saturated carbon atoms, thus limiting the electronic coupling between the chromophore and radical. We present both ultrafast transient absorption experiments and time-resolved EPR (TREPR) studies to probe the spin dynamics of photoexcited PerNO(•) and utilize X-ray crystallography to probe the molecular structure and stacking motifs of PerNO(•) in the solid state. The ability to control both the structure and electronic properties of molecules having multiple spins as well as the possibility of assembling ordered solid state materials from them is important for implementing effective molecule-based spintronics.

Two-dimensional covalent organic frameworks often π stack into crystalline solids that allow precise spatial positioning of molecular building blocks. Inspired by the hydrogen-bonded G-quadruplexes found frequently in guanine-rich DNA, here we show that this structural motif can be exploited to guide the self-assembly of naphthalene diimide and perylene diimide electron acceptors end-capped with two guanine electron donors into crystalline G-quadruplex-based organic frameworks, wherein the electron donors and acceptors form ordered, segregated π-stacked arrays. Time-resolved optical and electron paramagnetic resonance spectroscopies show that photogenerated holes and electrons in the frameworks have long lifetimes and display recombination kinetics typical of dissociated charge carriers. Moreover, the reduced acceptors form polarons in which the electron is shared over several molecules. The G-quadruplex frameworks also demonstrate potential as cathode materials in Li-ion batteries because of the favourable electron- and Li-ion-transporting capacity provided by the ordered rylene diimide arrays and G-quadruplex structures, respectively.

As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress charge recombination. Here we investigate two series of in-chain donor-acceptor copolymers, PTPDnT and PBTInT, based on thieno[3,4-c]pyrrole-4,6-dione (TPD) or bithiopheneimide (BTI) as electron acceptor units, respectively, and oligothiophenes (nTs) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar TPD polymer backbones, however backbone torsion yields greater open-circuit voltages for BTI polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that PTPDnT and PBTInT conjugation lengths scale with nT catenation up to n = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The 3T with "outward" alkylation in PTPD3T and PBTI3T affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm(2)/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for PTPD3T-based PSCs with PCE = 7.7%. The results demonstrate that 3T is the optimal donor unit among nTs (n = 1-4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based PTPD3T blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance.

We use photoluminescence (PL) quenching and photoinduced absorption (PIA) spectroscopy to study charge transfer in bulk heterojunction blends of PbSe quantum dots with the semiconducting polymers poly-3-hexylthiophene (P3HT) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-para-phenylene vinylene] (MDMO-PPV). PIA spectra from the PbSe blends are compared to spectra from similar blends of the polymers with phenyl-C(61)-butyric acid methyl ester (PCBM) and blends with CdSe quantum dots. We find that the MDMO-PPV PL is quenched, and the PL lifetime is shortened upon addition of PbSe quantum dots, while the PL of the P3HT is unaffected upon blending. However, for PbSe blends with both polymers, the PIA spectra show very little polaronic signal, suggesting that few, if any, long-lived charges are being produced by photoinduced charge transfer.

The organization of trisradical rotaxanes within the channels of a Zr 6 -based metal-organic framework (NU-1000) has been achieved postsynthetically by solvent-assisted ligand incorporation. Robust Zr IV -carboxylate bonds are forged between the Zr clusters of NU-1000 and carboxylic acid groups of rotaxane precursors (semirotaxanes) as part of this building block replacement strategy. Ultravioletvisible-near-infrared (UV-Vis-NIR), electron paramagnetic resonance (EPR), and 1 H nuclear magnetic resonance (NMR) spectroscopies all confirm the capture of redox-active rotaxanes within the mesoscale hexagonal channels of NU-1000. Cyclic voltammetry measurements performed on electroactive thin films of the resulting material indicate that redox-active viologen subunits located on the rotaxane components can be accessed electrochemically in the solid state. In contradistinction to previous methods, this strategy for the incorporation of mechanically interlocked molecules within porous materials circumvents the need for de novo synthesis of a metal-organic framework, making it a particularly convenient approach for the design and creation of solid-state molecular switches and machines. The results presented here provide proof-of-concept for the application of postsynthetic transformations in the integration of dynamic molecular machines with robust porous frameworks.mechanically interlocked molecules | metal-organic framework | molecular switches | rotaxanes | radicals T he predictability and reliability with which metal-organic frameworks (MOFs) are assembled (1-11) has accelerated the rate at which porous materials can be developed for applications as diverse as gas storage and separation (7), sensing (8), catalysis (9), and light harvesting (10, 11). Metal oxide joints and organic struts are arranged regularly within MOFs, giving rise to hybrid materials with permanent porosities. It has been proposed (12, 13) that integrating the rigidity and periodicity of MOFs with the addressability and workings of molecular switches and machines, such as bistable mechanically interlocked molecules (MIMs) (14-16), stands a good chance of giving rise to a new class of functional materials that are simultaneously both robust and dynamic. Most switchable MIMs that have been developed operate in solution where they are stochastically oriented and the net movement of a population of switches averages to zero. By integrating such rudimentary molecular switches within highly ordered MOFs, however, they can be organized periodically and precisely in 3D space, allowing their otherwise incoherent motions to be rectified. Although steady progress has been made (17-26) toward this goal in recent years, it still remains a considerable challenge to design such systems that can be addressed by stimuli in the solid state. Electrochemical potential and light would be particularly appealing stimuli on account of their ease of interfacing with current technologies. The avenues of investigation that have been explored up to this point have entailed t...

We use electroabsorption spectroscopy to measure the change in built-in potential (VBI) across the polymer photoactive layer in diodes where indium tin oxide electrodes are systematically modified using dipolar phosphonic acid self-assembled monolayers (SAMs) with various dipole moments. We find that VBI scales linearly with the work function (Φ) of the SAM-modified electrode over a wide range when using a solution-coated poly(p-phenylenevinylene) derivative as the active layer. However, we measure an interfacial parameter of S = eΔVBI/ΔΦ < 1, suggesting that these ITO/SAM/polymer interfaces deviate from the Schottky-Mott limit, in contrast to what has previously been reported for a number of ambient-processed organic-on-electrode systems. Our results suggest that the energetics at these ITO/SAM/polymer interfaces behave more like metal/organic interfaces previously studied in UHV despite being processed from solution.

We present the stereoselective synthesis of a chiral covalent organic cage consisting of three redox-active naphthalene-1,4:5,8-bis(dicarboximide) (NDI) units by dynamic imine chemistry. Single crystal X-ray diffraction analysis shows that host-guest interactions and racemic cocrystallization allow for controlling the solid state structure. Electronic interactions between the NDI units probed by absorption and circular dichroism spectroscopies, electrochemistry and theoretical calculations are shown to be weak. Photoexcitation of NDI leads to intracage charge separation with a longer lifetime than observed in the corresponding monomeric NDI and dimeric NDI cyclophane imines. The EPR spectrum of the singly reduced cage shows that the electron is localized on a single NDI unit at ambient temperatures and transitions to rapid hopping among all three NDI units upon heating to 350 K. Dynamic covalent chemistry thus promises rapid access to covalent organic cages with well-defined architectures to study charge accumulation and electron transport phenomena.