Organic radicals are promising building blocks for molecular spintronics. Little is known about the role of unpaired electrons for electron transport at the single-molecule level. Here, we examine ...the impact of magnetic fields on electron transport in single oligo(p-phenyleneethynylene) (OPE)-based radical molecular junctions, which are formed with a mechanically controllable break-junction technique at a low temperature of 4.2 K. Surprisingly huge positive magnetoresistances (MRs) of 16 to 287% are visible for a magnetic field of 4 T, and the values are at least 1 order of magnitude larger than those of the analogous pristine OPE (2–4%). Rigorous analysis of the MR and of current–voltage and inelastic electron-tunneling spectroscopy measurements reveal an effective reduction of the electronic coupling between the current-carrying molecular orbital and the electrodes with increasing magnetic field. We suggest that the large MR for the single-radical molecular junctions might be ascribed to a loss of phase coherence of the charge carriers induced by the magnetic field. Although further investigations are required to reveal the mechanism underlying the strong MR, our findings provide a potential approach for tuning charge transport in metal-molecule junctions with organic radicals.
Abstract
Due to the low corrugation of the Au(111) surface, 1,4-bis(phenylethynyl)-2,5-bis(ethoxy)benzene (PEEB) molecules can form quasi interlocked lateral patterns, which are observed in scanning ...tunneling microscopy experiments at low temperatures. We demonstrate a multi-dimensional clustering approach to quantify the anisotropic pair-wise interaction of molecules and explain these patterns. We perform high-throughput calculations to evaluate an energy function, which incorporates the adsorption energy of single PEEB molecules on the metal surface and the intermolecular interaction energy of a pair of PEEB molecules. The analysis of the energy function reveals, that, depending on coverage density, specific types of pattern are preferred which can potentially be exploited to form one-dimensional molecular wires on Au(111).
We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are ...developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current–voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.
A major goal of molecular electronics is the development and implementation of devices such as single‐molecular switches. Here, measurements are presented that show the controlled in situ switching ...of diarylethene molecules from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation. Both the conductance and the quantum yield for switching of these molecules are within a range making the molecules suitable for actual devices. The conductance of the molecular junctions in the opened and closed states is characterized and the molecular level E
0, which dominates the current transport in the closed state, and its level broadening Γ are identified. The obtained results show a clear light‐induced ring forming isomerization of the single‐molecule junctions. Electron withdrawing side‐groups lead to a reduction of conductance, but do not influence the efficiency of the switching mechanism. Quantum chemical calculations of the light‐induced switching processes correlate these observations with the fundamentally different low‐lying electronic states of the opened and closed forms and their comparably small modification by electron‐withdrawing substituents. This full characterization of a molecular switch operated in a molecular junction is an important step toward the development of real molecular electronics devices.
Optical switching of single diarylethene molecule junctions. The first demonstration of a light‐induced transition to a high‐conductance state in a single‐molecule connected to gold electrodes is given. Full understanding of the current carrying molecular states and their coupling to the electrodes is achieved by analyzing the current–voltage characteristics of the junctions.
Taking the π‐conjugated polymers PBDT2XT (X = H, F) as model systems, the effects of fluorine substitution on main‐chain conformations, packing, and electronic couplings are examined. This ...combination of molecular dynamics simulations and solid‐state NMR shows that a higher propensity for backbone planarity in PBDT2FT leads to more pronounced, yet staggered, chain stacking, which generally leads to higher electronic couplings and binding energy between neighboring chains.
The electrical properties of diarylethene photoswitching molecular devices on flexible substrates are studied. When exposed to UV or visible light, diarylethene molecular devices show two electrical ...states (a high and a low conductance state) with a discrepancy of an order of magnitude in the level of current between the two states. The diarylethene flexible molecular devices exhibit excellent long‐time stability and reliable electrical characteristics in both conductance states when subjected to various mechanical stresses.
We report on the synthesis, material properties, and bulk heterojunction (BHJ) solar cell characteristics of a set of π-conjugated small-molecule (SM) donors composed of ...benzo1,2-b:4,5-b′dithiophene (BDT) and pyrido3,4-bpyrazine (PP) units, examining the perspectives of alkyl-substituted PP acceptor motifs in SM designs. In these systems (SM1–4), both the type of side chains derived from the PP motifs and the presence of ring substituents on BDT critically impact (i) molecular packing, and (ii) thin-film morphologies and charge transport in BHJ solar cells. With the appropriate side-chain pattern, the ring-substituted analogue SM4 stands out, achieving efficiencies of ca. 6.5% with PC71BM, and fine-scale morphologies comparable to those obtained with some of the best-performing polymer donors in BHJ solar cells. 1H–1H DQ-SQ NMR analyses are used to examine the distinct self-assembly pattern of SM4, expected to factor into the development of the BHJ morphology.
A recently reported protolytic gating effect on the ring closing reaction of an oligo(phenylene ethynylene) (OPE) embedded difurylperfluorocyclopentene (
S
) with a dimethylaminophenyl chain link in ...each of the side arms, was quantitatively analyzed in detail. The reaction system (
S
o
,
S
o
H
+
,
S
o
H
2
2+
,
S
c
,
S
c
H
+
,
S
c
H
2
2+
) comprising three protolytic forms in both open and closed configuration, is characterized by four protolytic equilibrium constants and six photochemical quantum yields of ring closing and ring opening. The absorption spectra, conductivity, and effective photochemical quantum yields were measured in acetonitrile as functions of solvent acidity varied by addition of trifluoroacetic acid and triethylamine and characterized by an effective pH
non-aq
. Based on the derivation of a rigorous method for assessing the individual quantum yields of ring closure and ring opening of the six species, it was shown that it is specifically the second protonation step that is responsible for a more than 10-fold increase in the quantum yield of ring closure.
Double protonation (red) increases the ring-closing quantum yield
Q&cmb.a.rharp;
of a molecular switch more than tenfold.
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