Vapor-Induced Conversion of a Centrosymmetric Organic–Inorganic Hybrid Crystal into a Proton-Conducting Second-Harmonic-Generation-Active Material

Chemical responsivity in materials is essential to build systems with switchable functionalities. However, polarity-switchable materials are still rare because inducing a symmetry breaking of the crystal structure by adsorbing chemical species is difficult. In this study, we demonstrate that a molecular organic–inorganic hybrid crystal of (NEt4)2[MnN(CN)4] (1) undergoes polarity switching induced by water vapor and transforms into a rare example of proton-conducting second-harmonic-generation-active material. Centrosymmetric 1 transforms into noncentrosymmetric polar 1·3H2O and 1·MeOH by accommodating water and methanol molecules, respectively. However, only water vapor causes a spontaneous single-crystal-to-single-crystal transition. Moreover, 1·3H2O shows proton conduction with 2.3 × 10–6 S/cm at 298 K and a relative humidity of 80%

Enhancement of the Exciton Coherence Size in Organic Semiconductor by Alkyl Chain Substitution

Photophysical properties of molecular aggregates are largely determined by exciton coherence size: a spatial extension of exciton delocalization. Increase in exciton coherence size can lead to fast energy transport as well as efficient charge separation. Here, we demonstrate that introducing alkyl chains to organic molecules can enhance the exciton coherence size significantly. Focusing on the thin films of excellent hole transport materials, dinaphtho­[2,3-<i>b</i>:2,3-<i>f</i>]­thieno­[3,2-<i>b</i>]­thiophene (DNTT) and its alkyl-substituted derivative, we analyze the steady-state and picosecond time-resolved photoluminescence spectra of the films to estimate exciton coherence sizes. The alkyl substitution enhances the coherence size by a factor of 2–3, indicating that a long-range ordering in the molecular aggregates is achieved with the additional van der Waals interaction between saturated alkyl chains. The coherence sizes of both the films decrease with increasing temperature owing to thermal populations within the vibronic exciton manifolds

Helical Nanoribbons for Ultra-Narrowband Photodetectors

This Communication describes a new molecular design that yields ultranarrowband organic photodetectors. The design is based on a series of helically twisted molecular ribbons as the optoelectronic material. We fabricate charge collection narrowing photodetectors based on four different helical ribbons that differ in the wavelength of their response. The photodetectors made from these materials have narrow spectral response with full-width at half maxima of <20 nm. The devices reported here are superior by approximately a factor of 5 to those from traditional organic materials due to the narrowness of their response. Moreover, the active layers for the helical ribbon-based photodetectors are solution-cast but have performance that is comparable to the state-of-the-art narrowband photodetectors made from methylammonium lead trihalide perovskite single crystals. The ultranarrow bandwidth for detection results from the helical ribbons’ high absorption coefficient, good electron mobility, and sharp absorption edges that are defined by the twisted molecular conformation

Persistent Energetic Electrons in Methylammonium Lead Iodide Perovskite Thin Films

In conventional semiconductor solar cells, carriers are extracted at the band edges and the excess electronic energy (<i>E*</i>) is lost as heat. If <i>E</i>* is harvested, power conversion efficiency can be as high as twice the Shockley–Queisser limit. To date, materials suitable for hot carrier solar cells have not been found due to efficient electron/optical-phonon scattering in most semiconductors, but our recent experiments revealed long-lived hot carriers in single-crystal hybrid lead bromide perovskites. Here we turn to polycrystalline methylammonium lead iodide perovskite, which has emerged as the material for highly efficient solar cells. We observe energetic electrons with excess energy ⟨<i>E*</i>⟩ ≈ 0.25 eV above the conduction band minimum and with lifetime as long as ∼100 ps, which is 2–3 orders of magnitude longer than those in conventional semiconductors. The energetic carriers also give rise to hot fluorescence emission with pseudo-electronic temperatures as high as 1900 K. These findings point to a suppression of hot carrier scattering with optical phonons in methylammonium lead iodide perovskite. We address mechanistic origins of this suppression and, in particular, the correlation of this suppression with dynamic disorder. We discuss potential harvesting of energetic carriers for solar energy conversion

Long, Atomically Precise Donor–Acceptor Cove-Edge Nanoribbons as Electron Acceptors

This Communication describes a new molecular design for the efficient synthesis of donor–acceptor, cove-edge graphene nanoribbons and their properties in solar cells. These nanoribbons are long (∼5 nm), atomically precise, and soluble. The design is based on the fusion of electron deficient perylene diimide oligomers with an electron rich alkoxy pyrene subunit. This strategy of alternating electron rich and electron poor units facilitates a visible light fusion reaction in >95% yield, whereas the cove-edge nature of these nanoribbons results in a high degree of twisting along the long axis. The rigidity of the backbone yields a sharp longest wavelength absorption edge. These nanoribbons are exceptional electron acceptors, and organic photovoltaics fabricated with the ribbons show efficiencies of ∼8% without optimization

A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers

Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley–Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, despite weak direct coupling