Chromophores in molecular nanorings : when is a ring a ring?

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C–C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation

Genome-Wide Meta-Analysis in Alopecia Areata Resolves HLA Associations and Reveals Two New Susceptibility Loci

Alopecia areata (AA) is a prevalent autoimmune disease with ten known susceptibility loci. Here we perform the first meta-analysis in AA by combining data from two genome-wide association studies (GWAS), and replication with supplemented ImmunoChip data for a total of 3,253 cases and 7,543 controls. The strongest region of association is the MHC, where we fine-map 4 independent effects, all implicating HLA-DR as a key etiologic driver. Outside the MHC, we identify two novel loci that exceed statistical significance, containing ACOXL/BCL2L11(BIM) (2q13); GARP (LRRC32) (11q13.5), as well as a third nominally significant region SH2B3(LNK)/ ATXN2 (12q24.12). Candidate susceptibility gene expression analysis in these regions demonstrates expression in relevant immune cells and the hair follicle. We integrate our results with data from seven other autoimmune diseases and provide insight into the alignment of AA within these disorders. Our findings uncover new molecular pathways disrupted in AA, including autophagy/apoptosis, TGFß/Tregs and JAK kinase signaling, and support the causal role of aberrant immune processes in AA

Energy transfer processes in organic and inorganic materials for photovoltaic devices

This thesis is concerned with energy transport processes in a series of low-bandgap copolymers, solution processable hybrid organic-inorganic perovskite materials, and donor-acceptor triad dyes which are used in photovoltaic cells and solar concentrator devices. These processes are investigated using time-resolved photoluminescence (PL) spectroscopy techniques which allow the investigation of transport processes with sub-ps time resolution. Two donor-acceptor-donor triad dyes composed of perylene bisimide units are compared, and rapid energy transfer (< 2 ps) from the donor to a central bay-substituted PBI unit is observed in both molecules. For the linear molecule, in which the transition dipole moments corresponding to the lowest singly excited state are all aligned along the molecule long-axis, such rapid energy transfer is shown to be consistent with predictions of a modified Förster model which takes into account both the delocalisation of the excitation and the short donor-acceptor separation. When the dipole moments of the donor units are perpendicular to that of the central acceptor however, this model is found to strongly underestimate the energy transfer. The energy transfer is found to arise due to a combination of both through-space (Förster type) and through-bond energy transfer, the latter of which is mediated by molecular torsions which break orthogonality and enable conjugation between the two units. This rapid energy transfer also coincides with either retention or rotation of polarisation between absorption and emission. These dyes are therefore shown to be promising candidates for use in luminescent solar concentrators (LSC), as rapid intramolecular energy transfer and control of the emission polarisation are two features which can help to reduce self-absorption and escape losses in LSC devices. The effect of chemical structure on the morphology, energy transport properties and overall photovoltaic device efficiency is determined for a series of low-bandgap polymers comprising benzodithiophene donor and benzothiadiazole acceptor units. Photovoltaic devices incorporating polymer:fullerene blends are found to yield devices with power conversion efficiencies of up to 6%, with the highest PCE observed in devices which form films exhibiting a very low degree of crystallinity in X-ray diffaction patterns and a corresponding low surface roughness in thin films. The influence of crystallite formation on energy transport is probed by time resolved PL quenching of polymer films on a TiO2 quenching layer. Exciton diffusion lengths in these films are standard for low-bandgap polymers, ranging from 4 to 7.5 nm. The diffusion length is found to be higher in films with a higher degree of crystallinity, however direct PL quenching measurements on polymer:PCBM films show however that the vast majority of generated excitons are found to reach an interface and dissociate within 1 ps, showing that exciton diffusion does not present a bottle-neck for device efficiencies. From these observations it is concluded that the boundaries between crystalline and amorphous domains may impede charge extraction at the charge densities found during photovoltaic operation. Finally, the distance over which electron-hole pairs can diffuse before decay or trapping is investigated in two organic-inorganic hybrid perovskite structures (CH3NH3PbI3-xClx) by monitoring the rate and degree of PL quenching in the presence of either an electron or hole acceptor material. The diffusion lengths observed in these materials are on the order of 100 nm for the triiodide perovskite (x = 0), and over 1 μm in the mixed halide material (x > 0). These diffusion lengths are extremely long compared with those observed in other solution processable materials and explain the very high power conversion efficiencies that have been reported in photovoltaic cells containing these and similar perovskite materials. The longer diffusion lengths in particular correlate with good power conversion efficiency when a planar device geometry is used. Similar cells using the triiodide material however show poor efficiencies, attributed to the smaller diffusion length in this material. Application of the PL quenching technique to determine diffusion lengths is therefore shown to be a useful and simple method by which the suitability of a given perovskite material for use in a planar PV cell can be determined

Electroabsorption Spectroscopy as a Tool to Probe Charge-Transfer and State Mixing in Thermally-Activated Delayed Fluorescence Emitters

Solid-state electroabsorption is demonstrated as a powerful tool to probe the charge-transfer (CT) character and state mixing in the low energy optical transitions of two structurally similar thermally-activated delayed fluorescent (TADF) materials with divergent photophysical and device performances. The Liptay model is used to fit differentials of the low energy absorption bands to the measured electroabsorption spectra, with both emitters showing CT characteristics and large changes of dipole moments on excitation despite the associated absorption bands appearing structured. High electric fields then reveal transfer of oscillator strength to a state close to the CT in the better performing molecule. With supporting TDDFT-TDA and DFT/MRCI calculations, this state showed ππ* characteristics of a local acceptor triplet that strongly mixes with the σπ* of the CT. The emitter with poor TADF performance showed no evidence of such mixing

Chromophores in Molecular Nanorings: When Is a Ring a Ring?

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C–C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation

Rapid Energy Transfer Enabling Control of Emission Polarization in Perylene Bisimide Donor–Acceptor Triads

Materials showing rapid intramolecular energy transfer and polarization switching are of interest for both their fundamental photophysics and potential for use in real-world applications. Here, we report two donor–acceptor–donor triad dyes based on perylene-bisimide subunits, with the long axis of the donors arranged either parallel or perpendicular to that of the central acceptor. We observe rapid energy transfer (<2 ps) and effective polarization control in both dye molecules in solution. A distributed-dipole Förster model predicts the excitation energy transfer rate for the linearly arranged triad but severely underestimates it for the orthogonal case. We show that the rapid energy transfer arises from a combination of through-bond coupling and through-space transfer between donor and acceptor units. As they allow energy cascading to an excited state with controllable polarization, these triad dyes show high potential for use in luminescent solar concentrator devices

Revising of the Purcell effect in periodic metal-dielectric structures: the role of absorption

Periodic metal-dielectric structures attract substantial interest since it was previously proposed that the spontaneous emission amplification rates (the Purcell factor) in such structures can reach enormous values up to 105. However, the role of absorption in real metals has not been thoroughly considered. We provide a theoretical analysis showing that absorption leads to diminishing values of Purcell factor. We also suggest that using emitting organic compounds such as CBP (4,4-Bis(N-carbazolyl)-1,1-biphenyl) can lead to a moderate increase of about an order of magnitude in the Purcell factor. Defining the experimentally measured Purcell factor as a ratio between the excited state lifetimes in bare CBP and in periodic structure, this increase in the fabricated periodic structure is demonstrated through a 4–8 times decrease in excited state radiative lifetime compared to a bare organic material in a wide emission spectrum

Effect of Nanocrystalline Domains in Photovoltaic Devices with Benzodithiophene-Based Donor–Acceptor Copolymers

We have investigated the effects of thin-film morphology on the photovolatic performance for a series of donor–acceptor copolymers based on benzodithiophene donor and benzothiadiazole acceptor units. Photovoltaic devices incorporating polymer:fullerene blends show highest efficiencies (up to 6%) for those polymers exhibiting the least degree of crystallinity in X-ray diffraction patterns and a corresponding lowest surface roughness in thin films. We find that the existence of such crystalline domains in thin polymer films correlates well with spectral signatures of polymer chain aggregates already present in solution prior to casting of the film. Polymer solubility and casting conditions therefore appear to be crucial factors for enhancing efficiencies of photovoltaic devices based on such donor–acceptor copolymers. To examine why the presence of crystallite domains lowers device efficiencies, we measured exciton diffusion lengths by modeling the time-dependent photoluminescence from thin polymer films deposited on an exciton quencher layer of TiO₂. We find that exciton diffusion lengths in these materials are substantial (4–7.5 nm) and show some variation with polymer crystallinity. However, ultrafast (1 ps) quenching of the polymer emission from polymer:PCBM blends indicates that the vast majority of excitons rapidly reach the charge-dissociating interface, and hence exciton diffusion does not represent a limiting factor. We therefore conclude that the subsequent charge extraction and lifetimes must be adversely affected by the presence of crystalline domains. We suggest that the formed crystallites are too small to offer significant enhancements in long-range charge carrier mobility but instead introduce domain boundaries which impede charge extraction. For this class of materials, polymer designs are therefore required that target high solubility and chain entropy, leading to amorphous film formation

Dichroic Perylene Bisimide Triad Displaying Energy Transfer in Switchable Luminescent Solar Concentrators

Dichroic Perylene Bisimide Triad Displaying Energy Transfer in Switchable Luminescent Solar Concentrator

Efficient UV Luminescence from Organic-Based Tamm Plasmon Structures Emitting in the Strong Coupling Regime

Excitons in organic semiconductors possessing a large oscillator strength demonstrate strong coupling with cavity modes at room temperature. A large Stokes shift in some organic semiconductors enriches and complicates the picture of the emission in strongly coupled systems of organic excitons and light. Here we demonstrate strong coupling of excitons in 4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP) and Tamm plasmons in the UV band, accompanied by a bright emission from the structure. Reflection measurements demonstrate the pronounced formation of the lower and upper polariton modes with Rabi splitting of the magnitude of 0.3 eV, and the emission peak experiences a substantial red shift with respect to the lower polariton mode. Both radiative and non-radiative decay rates in the Tamm plasmon CBP structure are increased with respect to a bare CBP. Such peculiar behavior is attributed to the simultaneous manifestation of strong coupling and weak coupling of the CBP molecule emitters to the Tamm plasmons