100 research outputs found
Analysis of the excited-state absorption spectral bandshape of oligofluorenes
We present ultrafast transient absorption spectra of two oligofluorene
derivatives in dilute solution. These spectra display clear vibronic structure,
which we analyze rigorously using a time-dependent formalism of absorption to
extract the principal excited-state vibrational normal-mode frequencies that
couple to the electronic transition, the configurational displacement of the
higher-lying excited state, and the reorganization energies. We can model the
excited-state absorption spectrum using two totally symmetric vibrational modes
with frequencies 450 (dimer) or 400 cm (trimer), and 1666 cm. The
reorganization energy of the ground-state absorption is rather insensitive to
the oligomer length at 230 meV. However, that of the excited-state absorption
evolves from 58 to 166 meV between the oligofluorene dimer and trimer. Based on
previous theoretical work [Shukla et al., Phys. Rev. B \textbf{67}, 245203
(2003)], we assign the absorption spectra to a transition from the
excited state to a higher-lying state, and find that the energy of the
excited-state transition with respect to the ground-state transition energy is
in excellent agreement with the theoretical predictions for both oligomers
studied here. These results and analysis permit profound understanding of the
nature of excited-state absorption in -conjugated polymers, which are the
subject of general interest as organic semiconductors in the solid state.Comment: Resubmitted to J. Chem. Phys on 28 April 2010; 31 pages, 7 figure
Charge-transfer excitons in strongly coupled organic semiconductors
Time-resolved and temperature-dependent photoluminescence measurements on
one-dimensional sexithiophene lattices reveal intrinsic branching of
photoexcitations to two distinct species: self-trapped excitons and dark
charge-transfer excitons (CTX; > 5% yield), with radii spanning 2-3 sites. The
significant CTX yield results from the strong charge-transfer character of the
Frenkel exciton band due to the large free exciton bandwidth (~400 meV) in
these supramolecular nanostructures.Comment: Physical Review B Rapid Communications (In Press
Salt-induced thermochromism of a conjugated polyelectrolyte
We report here the photophysical properties of a water-soluble conjugated polythiophene with cationic side-chains. When dissolved in aqueous buffer solution (PBS, phosphate buffered saline), there is ordering of the polymer chains due to the presence of the salts, in contrast to pure water, where a random-coil conformation is adopted at room temperature. The ordering leads to a pronounced colour change of the solution (the absorption maximum shifts from 400 nm to 525 nm). Combining resonance Raman spectroscopy with density functional theory computations, we show a significant backbone planarization in the ordered phase. Moreover, the ratio of ordered phase to random-coil phase in PBS solution, as well as the extent of intermolecular interactions in the ordered phase, can be tuned by varying the temperature. Femtosecond transient absorption spectroscopy reveals that the excited- state behaviour of the polyelectrolyte is strongly affected by the degree of ordering. While triplet state formation is favoured in the random-coil chains, the ordered chains show a weak yield of polarons, related to interchain interactions. The investigated polyelectrolyte has been previously used as a biological DNA sensor, based on optical transduction when the conformation of the polyelectrolyte changes during assembly with the biomolecule. Therefore, our results, by correlating the photophysical properties of the polyelectrolyte to backbone and intermolecular conformation in a biologically relevant buffer, provide a significant step forward in understanding the mechanism of the biological sensing
Structural and photophysical templating of conjugated polyelectrolytes with single-stranded DNA
A promising approach to influence and control the photophysical properties of conjugated polymers is directing their molecular conformation by templating. We explore here the templating effect of single-stranded DNA oligomers (ssDNAs) on cationic polythiophenes with the goal to uncover the intermolecular interactions that direct the polymer backbone conformation. We have comprehensively characterized the optical behavior and structure of the polythiophenes in conformationally distinct complexes depending on the sequence of nucleic bases and addressed the effect on the ultrafast excited-state relaxation. This, in combination with molecular dynamics simulations, allowed us a detailed atomistic-level understanding of the structureâproperty correlations. We find that electrostatic and other noncovalent interactions direct the assembly with the polymer, and we identify that optimal templating is achieved with (ideally 10â20) consecutive cytosine bases through numerous Ï-stacking interactions with the thiophene rings and side groups of the polymer, leading to a rigid assembly with ssDNA, with highly ordered chains and unique optical signatures. Our insights are an important step forward in an effective approach to structural templating and optoelectronic control of conjugated polymers and organic materials in general
Managing Local Order in Conjugated Polymer Blends via Polarity Contrast
The optoelectronic landscape of conjugated polymers is intimately related to their molecular arrangement and packing, with minute changes in local order, such as chain conformation and torsional backbone order/disorder, frequently having a substantial effect on macroscopic properties. While many of these local features can be manipulated via chemical design, the synthesis of a series of compounds is often required to elucidate correlations between chemical structure and macromolecular ordering. Here, we show that blending semiconducting polymers with insulating commodity plastics enables controlled manipulation of the semiconductor backbone planarity. The key is to create a polarity difference between the semiconductor backbone and its side chains, while matching the polarity of the side chains and the additive. We demonstrate the applicability of this approach through judicious comparison of regioregular poly(3-hexylthiophene) (P3HT) with two of its more polar derivatives, namely the diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) and the graft polymer poly[3-but(ethylene oxide)thiophene] (P3BEOT), as well as their blends with poly(ethylene oxide) (PEO). Proximity between polar side chains and a similarly polar additive reduces steric hindrance between individual chain segments by essentially "expelling" the side chains away from the semiconducting backbones. This process, shown to be facilitated via exposure to polar environments such as humid air/water vapor, facilitates backbone realignment toward specific chain arrangements and, in particular, planar backbone configurations
CYP2J19 mediates carotenoid colour introgression across a natural avian hybrid zone
It has long been of interest to identify the phenotypic traits that mediate reproductive isolation between related species, and more recently, the genes that underpin them. Much work has focused on identifying genes associated with animal colour, with the candidate gene CYP2J19 identified in laboratory studies as the ketolase converting yellow dietary carotenoids to red ketocarotenoids in birds with red pigments. However, evidence that CYP2J19 explains variation between red and yellow feather coloration in wild populations of birds is lacking. Hybrid zones provide the opportunity to identify genes associated with specific traits. Here we investigate genomic regions associated with colour in redâfronted and yellowâfronted tinkerbirds across a hybrid zone in southern Africa. We sampled 85 individuals, measuring spectral reflectance of forecrown feathers and scoring colours from photographs, while testing for carotenoid presence with Raman spectroscopy. We performed a genomeâwide association study to identify associations with carotenoidâbased coloration, using doubleâdigest RAD sequencing aligned to a shortâread whole genome of a Pogoniulus tinkerbird. Admixture mapping using 104,933 single nucleotide polymorphisms (SNPs) identified a region of chromosome 8 that includes CYP2J19 as the only locus with more than two SNPs significantly associated with both crown hue and crown score, while Raman spectra provided evidence of ketocarotenoids in red feathers. Asymmetric backcrossing in the hybrid zone suggests that yellowâfronted females mate more often with redâfronted males than vice versa. Female redâfronted tinkerbirds mating assortatively with redâcrowned males is consistent with the hypothesis that converted carotenoids are an honest signal of quality.DATA AVAILABILITY STATEMENT : The Pogoniulus pusillus genome assembly has been deposited at NCBI SRA in BioProject PRJNA630018, with DDRAD sequencing reads under BioProject PRJNA666541. The master VCF file and gemma and R Code have been deposited in the Dryad Digital Repository (https://doi-org.uplib.idm.oclc.org/10.5061/dryad.jm63xsj87).FP7 Marie Curie Reintegration Grant, a University of Cyprus Research Grant, an AG Leventis Foundation grant and by the AP Leventis Ornithological Research Institute, Jos, Nigeria.http://wileyonlinelibrary.com/journal/mec2021-10-15hj2021Mammal Research InstituteZoology and Entomolog
The Low-redshift Lyman Continuum Survey: Radio continuum properties of low- Lyman continuum emitters
The sources that leak Lyman-continuum (LyC) photons and lead to the
reionisation of the universe are intensely studied using multiple observing
facilities. Recently, the Low-redshift LyC Survey (LzLCS) has found the first
large sample of LyC emitting galaxies at low redshift () with the
Hubble Space Telescope/Cosmic Origins Spectrograph. The LzLCS sample contains a
robust estimate of the LyC escape fraction () for
66 galaxies spanning a wide range of . Here we,
for the first time, aim to study the radio continuum (RC) properties of LzLCS
sources and their dependence on . We present Karl
G. Jansky Very Large Array RC observations at C (4-8 GHz), S (2-4 GHz) and L
(1-2 GHz) bands for a sub-sample of the LzLCS sources. The radio spectral index
() spans a wide range from being flat (
) to very steep (). We find that the strongest leakers in
our sample show flat , weak leakers have
close to normal star-forming galaxies,
and non-leakers are characterized by steep
. We argue that a combination of young
ages, free-free absorption, and a flat cosmic-ray energy spectrum can
altogether lead to a flat for strong
leakers. Non-leakers are characterized by steep spectra which can arise due to
break/cutoff at high frequencies. Such a cutoff in the spectrum can arise in a
single injection model of CRs characteristic of galaxies which have recently
stopped star formation. Such a relation between
and hints
at the interesting role of supernovae, CRs, and magnetic fields in facilitating
the escape (and/or the lack) of LyC photons.Comment: 25 pages, 14 figures, 3 tables, Submitted to Astronomy & Astrophysic
The Low-Redshift Lyman Continuum Survey. Unveiling the ISM properties of low- Lyman continuum emitters
Combining 66 ultraviolet (UV) spectra and ancillary data from the
Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier
studies, we form a statistical sample of star-forming galaxies at
to study the role of the cold interstellar medium (ISM) gas in the leakage of
ionizing radiation. We first constrain the massive star content (ages and
metallicities) and UV attenuation, by fitting the stellar continuum with a
combination of simple stellar population models. The models, together with
accurate LyC flux measurements, allow to determine the absolute LyC photon
escape fraction for each galaxy (). We measure the
equivalent widths and residual fluxes of multiple HI and low-ionization state
(LIS) lines, and the geometrical covering fraction adopting the picket-fence
model. The spans a wide range, with a median (0.16,
0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in
the LyC. The HI and LIS line equivalent widths scale with the UV luminosity and
attenuation, and inversely with the residual flux of the lines. The HI and LIS
residual fluxes are correlated, indicating that the neutral gas is spatially
traced by the LIS transitions. We find the observed trends of the absorption
lines and the UV attenuation are primarily driven by the covering fraction. The
non-uniform gas coverage demonstrates that LyC photons escape through
low-column density channels in the ISM. The equivalent widths and residual
fluxes of the UV lines strongly correlate with : strong
LyC leakers show weak absorption lines, low UV attenuation, and large
Ly equivalent widths. We finally show that simultaneous UV absorption
line and dust attenuation measurements can predict, on average, the escape
fraction of galaxies and the method can be applied to galaxies across a wide
redshift range.Comment: 30 pages, 16 figures, 3 tables; accepted for publication in Astronomy
and Astrophysics on December 16, 2021. Tables A1 to A4 are part of the LzLCS
science products and will be publicly available in a dedicated websit
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