Conjugation Length Dependence of Intramolecular Singlet Fission in a Series of Regioregular Oligo 3-Alkyl(thienylene-vinylene)s.

Activated intramolecular singlet fission is known to occur in the conjugated polymer polythienylene-vinylene (P3TV). Instead, efficient intersystem crossing has been observed in a short 3-alkyl(thienylene-vinylene) dimer. Here, we investigate a series of oligomers covering the conjugation length gap between the dimer and polymer.

Polymer Connectivity Governs Electrophotocatalytic Activity in the Solid State.

The reductive functionalization of inert substrates like chloroarenes is a critical yet challenging transformation relevant to both environmental remediation and organic synthesis. Combining electricity and light is an emerging approach to access the deeply reducing potentials required for single electron transfer to chloroarenes, yet this approach is held back by the poor stability and mechanistic ambiguity of current homogeneous systems. Incorporating redox-active moieties into insoluble organic materials represents a promising strategy to unlock new heterogeneous catalytic activity while improving catalyst stability.

The Acetylene Bridge in Intramolecular Singlet Fission: A Boon or A Nuisance?

Various analogues of the alkylsilylacetylene group are frequently used as auxiliary groups to enhance the solubility and stability of the acene dimer core, widely used as platforms to investigate intramolecular singlet fission (iSF) mechanisms. However, while in the 2,2'-linked dimers they are primarily auxiliary groups, these are essential fragments of the bridging units in 6,6'/5,5'-linked dimers, the two preferred choices for dimerization. The starkly different iSF dynamics observed in the two variants raise the question of what role the acetylene bridges play.

Coherent photoexcitation of entangled triplet pair states.

The functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S→TT). The dark state TT is typically generated from bright S after optical excitation; however, the mechanistic details are hotly debated.

Radiative pumping in a strongly coupled microcavity filled with a neat molecular film showing excimer emission.

Strong light-matter interactions have attracted much attention as a means to control the physical/chemical properties of organic semiconducting materials with light-matter hybrids called polaritons. To unveil the processes under strong coupling, studies on the dynamics of polaritons are of particular importance. While highly condensed molecular materials with large dipole density are ideal to achieve strong coupling, the emission properties of such films often become a mixture of monomeric and excimeric components, making the role of excimers unclear.

Small to Large Polaron Behavior Induced by Controlled Interactions in Perovskite Quantum Dot Solids.

The polaron is an essential photoexcitation that governs the unique optoelectronic properties of organic-inorganic hybrid halide perovskites, and it has been subject to extensive spectroscopic and theoretical investigation over the past decade. A crucial but underexplored question is how the nature of the photogenerated polarons is impacted by the microscopic perovskite structure and what functional properties this affects. To tackle this question, we chemically tuned the interactions between perovskite quantum dots (QDs) to rationally manipulate the polaron properties.

Controlling Intramolecular Singlet Fission Dynamics via Torsional Modulation of Through-Bond versus Through-Space Couplings.

Covalent dimers, particularly pentacenes, are the dominant platform for developing a mechanistic understanding of intramolecular singlet fission (iSF). Numerous studies have demonstrated that a photoexcited singlet state in these structures can rapidly and efficiently undergo exciton multiplication to form a correlated pair of triplets within a single molecule, with potential applications from photovoltaics to quantum information science. One of the most significant barriers limiting such dimers is the fast recombination of the triplet pair, which prevents spatial separation and the formation of long-lived triplet states.

Conjugation-length dependence of regioregular oligo 3-alkyl(thienylene-vinylene)s demonstrates polyene-like behaviour with weak electron-electron correlations.

Poly(3-alkyl(thienylene-vinylene)) (P3TV) and its longer oligomers have negligibly low photoluminescence quantum yields, however, the reason for their low yields is currently debated. Here, we prepare a series of regioregular (3-dodecyl)thienylene-vinylene oligomers with = 2-8 repeat units by iterative Horner-Wadsworth-Emmons reactions, and report their steady-state, transient absorption, and emission spectroscopy. The results presented here demonstrate that 3-alkyl(thienylene-vinylene) oligomers form part of the polyene family.

Soluble Diphenylhexatriene Dimers for Intramolecular Singlet Fission with High Triplet Energy.

Intramolecular singlet fission (iSF) facilitates single-molecule exciton multiplication, converting an excited singlet state to a pair of triplet states within a single molecule. A critical parameter in determining the feasibility of SF-enhanced photovoltaic designs is the triplet energy; many existing iSF materials have triplet energies too low for efficient transfer to silicon via a photon multiplier scheme. In this work, a series of six novel dimers based upon the high-triplet-energy, SF-active chromophore, 1,6-diphenyl-1,3,5-hexatriene (DPH) [(T) ∼ 1.

Enhancing Dynamic Spectral Diffusion in Metal-Organic Frameworks through Defect Engineering.

The crystal packing of organic chromophores has a profound impact on their photophysical properties. Molecular crystal engineering is generally incapable of producing precisely spaced arrays of molecules for use in photovoltaics, light-emitting diodes, and sensors. A promising alternative strategy is the incorporation of chromophores into crystalline metal-organic frameworks (MOFs), leading to matrix coordination-induced emission (MCIE) upon confinement.

Solvent-tunable exciton-charge transfer mixed state enhances emission of functionalized benzo[]pentaphene through symmetry breaking.

A benzo[]pentaphene (BPP) substituted by two bis(methoxyphenyl)amino (MeOPA) groups (BPP-MeOPA) was synthesized and clearly characterized by NMR and single-crystal X-ray analysis. Detailed investigations of its photophysical properties, including transient absorption spectroscopy analyses, revealed that the introduction of the MeOPA groups breaks the symmetry of the BPP core, improving its absorption and emission from an S state with both excitonic and charge-transfer character.

Parallel triplet formation pathways in a singlet fission material.

Harvesting long-lived free triplets in high yields by utilizing organic singlet fission materials can be the cornerstone for increasing photovoltaic efficiencies potentially. However, except for polyacenes, which are the most studied systems in the singlet fission field, spin-entangled correlated triplet pairs and free triplets born through singlet fission are relatively poorly characterized. By utilizing transient absorption and photoluminescence spectroscopy in supramolecular aggregate thin films consisting of Hamilton-receptor-substituted diketopyrrolopyrrole derivatives, we show that photoexcitation gives rise to the formation of spin-0 correlated triplet pair (TT) from the lower Frenkel exciton state.

Tuning the Coherent Propagation of Organic Exciton-Polaritons through Dark State Delocalization.

While there have been numerous reports of long-range polariton transport at room-temperature in organic cavities, the spatiotemporal evolution of the propagation is scarcely reported, particularly in the initial coherent sub-ps regime, where photon and exciton wavefunctions are inextricably mixed. Hence the detailed process of coherent organic exciton-polariton transport and, in particular, the role of dark states has remained poorly understood. Here, femtosecond transient absorption microscopy is used to directly image coherent polariton motion in microcavities of varying quality factor.

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors.

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity.

Untargeted effects in organic exciton-polariton transient spectroscopy: A cautionary tale.

Strong light-matter coupling to form exciton- and vibropolaritons is increasingly touted as a powerful tool to alter the fundamental properties of organic materials. It is proposed that these states and their facile tunability can be used to rewrite molecular potential energy landscapes and redirect photophysical pathways, with applications from catalysis to electronic devices. Crucial to their photophysical properties is the exchange of energy between coherent, bright polaritons and incoherent dark states.

Emissive spin-0 triplet-pairs are a direct product of triplet-triplet annihilation in pentacene single crystals and anthradithiophene films.

Singlet fission and triplet-triplet annihilation represent two highly promising ways of increasing the efficiency of photovoltaic devices. Both processes are believed to be mediated by a biexcitonic triplet-pair state, (TT). Recently however, there has been debate over the role of (TT) in triplet-triplet annihilation.

First principles modeling of exciton-polaritons in polydiacetylene chains.

Exciton-polaritons in organic materials are hybrid states that result from the strong interaction of photons and the bound excitons that these materials host. Organic polaritons hold great interest for optoelectronic applications; however, progress toward this end has been impeded by the lack of a first principles approach that quantifies light-matter interactions in these systems, which would allow the formulation of molecular design rules. Here, we present a theoretical framework that combines first principles calculations for excitons with classical electrodynamics in order to quantify light-matter interactions.

A Thermostable Protein Matrix for Spectroscopic Analysis of Organic Semiconductors.

Advances in protein design and engineering have yielded peptide assemblies with enhanced and non-native functionalities. Here, various molecular organic semiconductors (OSCs), with known excitonic up- and down-conversion properties, are attached to a de novo-designed protein, conferring entirely novel functions on the peptide scaffolds. The protein-OSC complexes form similarly sized, stable, water-soluble nanoparticles that are robust to cryogenic freezing and processing into the solid-state.

Manipulating molecules with strong coupling: harvesting triplet excitons in organic exciton microcavities.

Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic materials are of current interest as active materials for their ability to sustain exciton-polaritons even at room temperature. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation.

A hybrid organic-inorganic polariton LED.

Polaritons are quasi-particles composed of a superposition of excitons and photons that can be created within a strongly coupled optical microcavity. Here, we describe a structure in which a strongly coupled microcavity containing an organic semiconductor is coupled to a second microcavity containing a series of weakly coupled inorganic quantum wells. We show that optical hybridisation occurs between the optical modes of the two cavities, creating a delocalised polaritonic state.

Switching between Coherent and Incoherent Singlet Fission via Solvent-Induced Symmetry Breaking.

Singlet fission in organic semiconductors causes a singlet exciton to decay into a pair of triplet excitons and holds potential for increasing the efficiency of photovoltaic devices. In this combined experimental and theoretical study, we reveal that a covalent dimer of the organic semiconductor tetracene undergoes activated singlet fission by qualitatively different mechanisms depending on the solvent environment. We show that intramolecular vibrations are an integral part of this mechanism, giving rise to mixing between charge transfer and triplet pair excitations.