136 research outputs found
Spin Signatures of Photogenerated Radical Anions in Polymer-[70]Fullerene Bulk Heterojunctions: High Frequency Pulsed EPR Spectroscopy
Charged polarons in thin films of polymer-fullerene composites are
investigated by light-induced electron paramagnetic resonance (EPR) at 9.5 GHz
(X-band) and 130 GHz (D-band). The materials studied were
poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester
(C60-PCBM), and two different soluble C70-derivates: C70-PCBM and
diphenylmethano[70]fullerene oligoether (C70-DPM-OE). The first experimental
identification of the negative polaron localized on the C70-cage in
polymer-fullerene bulk heterojunctions has been obtained. When recorded at
conventional X-band EPR, this signal is overlapping with the signal of the
positive polaron, which does not allow for its direct experimental
identification. Owing to the superior spectral resolution of the high frequency
D-band EPR, we were able to separate light-induced signals from P+ and P- in
PHT-C70 bulk heterojunctions. Comparing signals from C70-derivatives with
different side-chains, we have obtained experimental proof that the polaron is
localized on the cage of the C70 molecule
Anticancer Activity and Biophysical Reactivity of Copper Complexes of 2-(benzo[d][1,3]dioxol-5-ylmethylene)-N-Alkylhydrazinecarbothioamides
A series of copper complexes were synthesized from benzo[d][1,3]dioxole-5-carbaldehyde (piperonal) thiosemicarbazones (RHpTSC where R = H, CH3, C2H5 or C6H5 (Ph)). The complexes show interesting variations in geometry depending on the thiosemicarbazone; a dinuclear complex [Cu(HpTSC)Cl]2, a mononuclear complex [Cu(RHpTSC)2Cl2] (R = CH3 or C2H5) and another mononuclear complex [Cu(PhHpTSC)(PhpTSC)Cl] was generated. The complexes bind in a moderately strong fashion to DNA with binding constants on the order of 104 M− 1. They are also strong binders of human serum albumin with binding constants near 104 M− 1. The complexes show good in vitro cytotoxic profiles against two human colon cancer cell lines (HCT-116 and HT29) and two human breast cancer cell lines (MCF-7 and MDA-MB-231) with IC50 values in the low millimolar concentration range
Charge separation and triplet exciton formation pathways in small molecule solar cells as studied by time-resolved EPR spectroscopy
Funding: EPSRC EP/G03673X/1 (SAJT), Royal Society Wolfson research merit award (IDWS).Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.PostprintPostprintPeer reviewe
The Chemistry of Phospholipid Binding by the Saccharomyces cerevisiae Phosphatidylinositol Transfer Protein Sec14p as Determined by EPR Spectroscopy
The major yeast phosphatidylinositol/phosphatidylcholine transfer protein Sec14p is the founding member of a large eukaryotic protein superfamily. Functional analyses indicate Sec14p integrates phospholipid metabolism with the membrane trafficking activity of yeast Golgi membranes. In this regard, the ability of Sec14p to rapidly exchange bound phospholipid with phospholipid monomers that reside in stable membrane bilayers is considered to be important for Sec14p function in cells. How Sec14p-like proteins bind phospholipids remains unclear. Herein, we describe the application of EPR spectroscopy to probe the local dynamics and the electrostatic microenvironment of phosphatidylcholine (PtdCho) bound by Sec14p in a soluble protein-PtdCho complex. We demonstrate that PtdCho movement within the Sec14p binding pocket is both anisotropic and highly restricted and that the C5 region of the sn-2 acyl chain of bound PtdCho is highly shielded from solvent, whereas the distal region of that same acyl chain is more accessible. Finally, high field EPR reports on a heterogeneous polarity profile experienced by a phospholipid bound to Sec14p. Taken together, the data suggest a headgroup-out orientation of Sec14p-bound PtdCho. The data further suggest that the Sec14p phospholipid binding pocket provides a polarity gradient that we propose is a primary thermodynamic factor that powers the ability of Sec14p to abstract a phospholipid from a membrane bilayer
Optical and microstructural characterization of Er doped epitaxial cerium oxide on silicon
Rare-earth ion dopants in solid-state hosts are ideal candidates for quantum
communication technologies such as quantum memory, due to the intrinsic
spin-photon interface of the rare-earth ion combined with the integration
methods available in the solid-state. Erbium-doped cerium oxide (Er:CeO) is
a particularly promising platform for such a quantum memory, as it combines the
telecom-wavelength (~1.5 m) 4f-4f transition of erbium, a predicted long
electron spin coherence time supported by CeO, and is also near
lattice-matched to silicon for heteroepitaxial growth. In this work, we report
on the epitaxial growth of Er:CeO thin films on silicon using molecular
beam epitaxy (MBE), with controlled erbium concentration down to 2 parts per
million (ppm). We carry out a detailed microstructural study to verify the
CeO host structure, and characterize the spin and optical properties of the
embedded Er ions. In the 2-3 ppm Er regime, we identify EPR linewidths
of 245(1) MHz, optical inhomogeneous linewidths of 9.5(2) GHz, optical excited
state lifetimes of 3.5(1) ms, and spectral diffusion-limited homogenoeus
linewidths as narrow as 4.8(3) MHz in the as-grown material. We test annealing
of the Er:CeO films up to 900 deg C, which yields modest narrowing of the
inhomogeneous linewidth by 20% and extension of the excited state lifetime by
40%. We have also studied the variation of the optical properties as a function
of Er doping and find that the results are consistent with the trends expected
from inter-dopant charge interactions.Comment: 15 pages, 6 figures (including supplemental information
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