Ashley J. Cadby

The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9, 9-dioctylfluorene)

We have measured the temperature-dependent photoluminescence quantum yields (PLQYs) of poly(9, 9-dioctylfluorene) (PFO) films with four morphologies, namely as-spin-coated (SC) glass, quenched nematic glass, crystalline, and vapour-treated SC glass containing a fraction of 21 helix conformation (β-phase) chains. We find that the room temperature PLQYs of the as-SC, crystalline, and quenched films all increase as the temperature is reduced. However, the PLQY of the film containing β-phase chains decreases at temperatures below 150 K. Via temperature-dependent photoinduced absorption measurements, we show that the polaron population in films containing β-phase PFO chains grows as the temperature is reduced, and is significantly larger than in films with any of the other morphologies. Because of the smaller HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap of the β-phase chains compared to chains in the surrounding glassy PFO matrix, they act as recombination sites for excitons, and as traps for polarons. Hence at low temperatures, the polarons become strongly localized on these chains, where they quench the singlet excitons and reduce the PLQY.

Cell wall elongation mode in Gram-negative bacteria is determined by peptidoglycan architecture

Cellular integrity and morphology of most bacteria is maintained by cell wall peptidoglycan, the target of antibiotics essential in modern healthcare. It consists of glycan strands, cross-linked by peptides, whose arrangement determines cell shape, prevents lysis due to turgor pressure and yet remains dynamic to allow insertion of new material, and hence growth. The cellular architecture and insertion pattern of peptidoglycan have remained elusive. Here we determine the peptidoglycan architecture and dynamics during growth in rod-shaped Gram-negative bacteria. Peptidoglycan is made up of circumferentially oriented bands of material interspersed with a more porous network. Super-resolution fluorescence microscopy reveals an unexpected discontinuous, patchy synthesis pattern. We present a consolidated model of growth via architecture-regulated insertion, where we propose only the more porous regions of the peptidoglycan network that are permissive for synthesis.

Molecular weight dependent vertical composition profiles of PCDTBT:PC71BM blends for organic photovoltaics

We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions. Organic photovoltaic devices were made using a blend of the fullerene acceptor PC71BM with the molecular weight fractions. An average power conversion efficiency of 5.89% (peak efficiency of 6.15%) was measured for PCDTBT blend devices with a number average molecular weight of Mn = 25.5 kDa. There was significant variation between the molecular weight fractions with low (Mn = 15.0 kDa) and high (Mn = 34.9 kDa) fractions producing devices with average efficiencies of 5.02% and 3.70% respectively. Neutron reflectivity measurements on these polymer:PC71BM blend layers showed that larger molecular weights leads to an increase in the polymer enrichment layer thickness at the anode interface, this improves efficiency up to a limiting point where the polymer solubility causes a reduction of the PCDTBT concentration in the active layer.

Excited-state quenching of a highly luminescent conjugated polymer

The optical properties of a luminescent polymer, poly[1,4-phenylene-1,2-di(phenoxyphenyl) vinylene], have been investigated. Its photoluminescence yield increases unusually in the solid-state over solution, 52%–6% respectively. Investigations into the stimulated emission properties of this material were carried out but no amplified spontaneous emission was observed. To investigate the presence of excited-state absorption features, the photoinduced absorption spectrum was measured. An observed polaron absorption band, from 1.5 to 2.25 eV, overlaps the emission spectra and therefore quenches stimulated emission. This highlights the need to consider the effects of excited-state absorption on the emission when synthesizing new materials.

Optical studies of photoexcitations of poly(9,9-dioctyl fluorene)

We have studied the interplay between the morphology and photophysics of poly(9,9-dioctyl fluorene) (PFO) films. Spin-casting from solution produces a glassy film with a broad absorption band and a large Stokes shift due to exciton migration. Upon slowly warming a sample from 80 to 300 K, a fraction of the sample converts into a new phase with extended conjugation, which we refer to as the β-phase. Its absorption band exhibits a clear vibronic structure, and the fluorescence red-shifts by approximately 0.1 eV. The photoinduced absorption (PA) spectrum of the glassy phase is dominated by triplet excitons, whereas, both charged polarons and triplet excitons are seen in the PA spectrum of a sample containing both phases.

A novel 2,7-linked carbazole based “double cable” polymer with pendant perylene diimide functional groups: preparation, spectroscopy and photovoltaic properties

The preparation and chemical analysis of a ‘double-cable’ conjugated polymer comprising a backbone of alternating 2,7-linked carbazole repeat units with covalently attached perylene diimide (PDI) substituents and dithienyl repeat units is presented. The backbone of the new “double-cable” polymer P1 acts as an electron donor while the pendant PDI substituents act as electron acceptors. In order to investigate the influence of the PDI moieties on the polymer backbone as well as to elucidate their photophysical interaction, three reference-compounds were also prepared and analysed: a polymer with the same backbone as that of P1 but without PDI substituents (P2) and two PDI derivatives with branched alkyl side chains (PDI 2 with 12-tricosanyl substituents and PDI 1 with 3-pentyl substituents). We find that polymer P1 shows pronounced electron transfer from the polymer backbone to the PDI chromophore units covalently bound to it, resulting in highly efficient quenching of excitons and strong suppression of fluorescence in solutions and in thin films. The existence of long-lived polaronic states resulting from exciton dissociation on P1 is confirmed using steady state photo-induced absorption spectroscopy. Despite the improved exciton quenching yield shown by P1 over a blend of P2/PDI, photovoltaic devices fabricated from P1 have a low external quantum efficiency (EQE) of around 0.43% at 410 nm; a value that is smaller than that from a conventional BHJ device of P2/PDI which is found to have a peak external quantum efficiency of 3.7% at 463 nm. We tentatively ascribe the reduced EQE of the double-cable polymer to geminate recombination of charge-carriers as a result of poor charge transport and a complete lack of donor acceptor phase separation.

Mapping exciton quenching in photovoltaic-applicable polymer blends using time-resolved scanning near-field optical microscopy

We have used time-resolved scanning near-field microscopy to image the fluorescence decay lifetime across a phase-separated blend of the photovoltaic-applicable polymers poly(9,9′-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly(9,9′-dioctylfluorene-alt-bis- N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB). We show that the efficiency of local fluorescence quenching is composition dependent, with excitons on F8BT molecules being more effectively quenched when F8BT is trapped at a low concentration in a PFB-rich phase. Despite such presumed differences in charge-carrier generation efficiency, our results demonstrate that charge extraction from F8BT:PFB devices is the most dominant mechanism limiting their operational efficiency.

Optical changes induced in Zn porphyrin solutions and LB films by exposure to amines

The 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphinato zinc(II) (ZnEHO) is highly stable and exhibits a colorful absorption spectrum in the visible range. Exposure of a chloroform solution of ZnEHO to amines is shown to induce changes in the characteristic optical spectrum owing to charge transfer between the amine and the delocalized π-electron system within the highly conjugated molecule. Solid state Langmuir Blodgett (LB) films containing only ZnEHO are compared to films containing a mixture of ZnEHO and calix[8]arene. The transparent calix[8]arene does not change the optical response but aids the diffusion of the amine gas into the LB films. Atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) images demonstrate the topological and compositional differences between the samples. The response of the LB films of ZnEHO and calix[8]arene to a variety of different amines demonstrates that this is a good material system for use as an amine sensor.

Spatially modulated structural colour in bird feathers

Eurasian Jay (Garrulus glandarius) feathers display periodic variations in the reflected colour from white through light blue, dark blue and black. We find the structures responsible for the colour are continuous in their size and spatially controlled by the degree of spinodal phase separation in the corresponding region of the feather barb. Blue structures have a well-defined broadband ultra-violet (UV) to blue wavelength distribution; the corresponding nanostructure has characteristic spinodal morphology with a lengthscale of order 150 nm. White regions have a larger 200 nm nanostructure, consistent with a spinodal process that has coarsened further, yielding broader wavelength white reflectance. Our analysis shows that nanostructure in single bird feather barbs can be varied continuously by controlling the time the keratin network is allowed to phase separate before mobility in the system is arrested. Dynamic scaling analysis of the single barb scattering data implies that the phase separation arrest mechanism is rapid and also distinct from the spinodal phase separation mechanism i.e. it is not gelation or intermolecular re-association. Any growing lengthscale using this spinodal phase separation approach must first traverse the UV and blue wavelength regions, growing the structure by coarsening, resulting in a broad distribution of domain sizes.

Reversible switching between nonquenched and quenched states in nanoscale linear arrays of plant light-harvesting antenna complexes.

A simple and robust nanolithographic method that allows sub-100 nm chemical patterning on a range of oxide surfaces was developed in order to fabricate nanoarrays of plant light-harvesting LHCII complexes. The site-specific immobilization and the preserved functionality of the LHCII complexes were confirmed by fluorescence emission spectroscopy. Nanopatterned LHCII trimers could be reversibly switched between fluorescent and quenched states by controlling the detergent concentration in the imaging buffer. A 3-fold quenching of the average fluorescence intensity was accompanied by a decrease in the average (amplitude-weighted) fluorescence lifetime from approximately 2.24 ns to approximately 0.4 ns, attributed to the intrinsic ability of LHCII to switch between fluorescent and quenched states upon changes in its conformational state. The nanopatterning methodology was extended by immobilizing a second protein, the enhanced green fluorescent protein (EGFP), onto LHCII-free areas of the chemically patterned surfaces. This very simple surface chemistry, which allows simultaneous selective immobilization and therefore sorting of the two types of protein molecules on the surface, is a key underpinning step toward the integration of LHCII into switchable biohybrid antenna constructs.