On the factors influencing the chiroptical response of conjugated polymer thin films

Magnetic Circular Dichroism Elucidates Molecular Interactions in Aggregated Chiral Organic Materials

Chiral materials formed by aggregated organic compounds play a fundamental role in chiral optoelectronics, photonics and spintronics. Nonetheless, a precise understanding of the molecular interactions involved remains an open problem. Here we introduce magnetic circular dichroism (MCD) as a new tool to elucidate molecular interactions and structural parameters of a supramolecular system. A detailed analysis of MCD together with electronic circular dichroism spectra combined to ab initio calculations unveils essential information on the geometry and energy levels of a self-assembled thin film made of a carbazole di-bithiophene chiral molecule. This approach can be extended to a generality of chiral organic materials and can help rationalizing the fundamental interactions leading to supramolecular order. This in turn could enable a better understanding of structure-property relationships, resulting in a more efficient material design.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10^-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Pivotal role of transition density in circularly polarized luminescence

Realizing high luminescence dissymmetry factor (g) in circularly polarized luminescence (CPL) materials remains a big challenge, which necessitates understanding systematically how their molecular structure controls the CPL. Here we investigate representative organic chiral emitters with different transition density distributions and reveal the pivotal role of transition density in CPL. We rationalize that to obtain large g-factors, two conditions should be simultaneously satisfied: (i) the transition density for the S₁ (or T₁)-to-S₀ emission must be delocalized over the entire chromophore; and (ii) the chromophore inter-segment twisting must be restricted and tuned to an optimal value (∼50°). Our findings offer molecular-level insights into the CPL of organic emitters, with potential applications in the design of chiroptical materials and systems with strong CPL effects.

Gas-assisted blade-coating of organic semiconductors: molecular assembly, device fabrication and complex thin-film structuring

The competitive performance of optoelectronic devices based on advanced organic semiconductors increasingly calls for suitably scalable processing schemes to capitalise on their application potential. With performance benchmarks typically established by spin-coating fabrication, doctor-blade deposition represents a widely available roll-to-roll-compatible means for the preparation of large-area samples and establishing the device upscaling potential. However, the inherently slower film formation kinetics often result in unfavourable active layer microstructures, requiring empirical and material-inefficient optimisation of solutions to reach the performance of spin-coated devices. Here we present a versatile approach to achieving performance parity for spin- and blade-coated devices using in situ gas-assisted drying enabled by a modular 3D-printed attachment. This is illustrated for organic photodetectors (OPDs) featuring bulk heterojunction active layers comprising blends of P3HT and PM6 polymer donors with the nonfullerene acceptor ITIC. Compared to conventionally blade-coated devices, mild drying gas pressures of 0.5-2 bar yield up to a 10-fold enhancement of specific detectivity by maximising external quantum efficiency and suppressing dark-current. Furthermore, controlling gas flux distribution enables one-step fabrication of 1D chain conformation and 2D chain orientation patterns in, respectively, PFO and P3HT:N2200 blend films, opening the possibility for high-throughput fabrication of devices with complex structured active layers.

Conformational Heterogeneity and Interchain Percolation Revealed in an Amorphous Conjugated Polymer

Conjugated polymers are employed in a variety of application areas due to their bright fluorescence and strong biocompatibility. However, understanding the structure of amorphous conjugated polymers on the nanoscale is extremely challenging compared to their related crystalline phases. Using a bespoke classical force field, we study amorphous poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) with molecular dynamics simulations to investigate the role that its nanoscale structure plays in controlling its emergent (and all-important) optical properties. Notably, we show that a giant percolating cluster exists within amorphous F8BT, which has ramifications in understanding the nature of interchain species that drive the quantum yield reduction and bathochromic shift observed in conjugated polymer-based devices and nanostructures. We also show that distinct conformations can be unravelled from within the disordered structure of amorphous F8BT using a two-stage machine learning protocol, highlighting a link between molecular conformation and ring stacking propensity. This work provides predictive understanding by which to enhance the optical properties of next-generation conjugated polymer-based devices and materials by rational, simulation-led design principles.

Circularly polarised luminescence in an RNA-based homochiral, self-repairing, coordination polymer hydrogel

The aqueous equimolar reaction of Ag(i) ions with the thionucleoside enantiomer (-)6-thioguanosine, ((-)6tGH), yields a one-dimensional coordination polymer {Ag(-)tG} _n , the self-assembly of which generates left-handed helical chains. The resulting helicity induces an enhanced chiro-optical response compared to the parent ligand. DFT calculations indicate that this enhancement is due to delocalisation of the excited state along the helical chains, with 7 units being required to converge the calculated CD spectra. At concentrations ≥15 mmol l^-1 reactions form a sample-spanning hydrogel which shows self-repair capabilities with instantaneous recovery in which the dynamic reversibility of the coordination chains appears to play a role. The resulting gel exhibits circularly polarised luminescence (CPL) with a large dissymmetry factor of -0.07 ± 0.01 at 735 nm, a phenomenon not previously observed for this class of coordination polymer.