Direct Conformational Analysis of a 10 nm Long Oligothiophene Wire

Single-step access to a series of D–A π-conjugated oligomers with 3–10 nm chain lengths

A series of five D–A π-conjugated oligo(DPP-co-fluorene)s progressively consisting of 3, 5, 7, 9 and 11 monomers and with chain lengths ranging between 3 and 10 nm have all been synthesized in a single step by direct C–H arylated coupling.

Real-space visualization of conformation-independent oligothiophene electronic structure

We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of the electronic structures of different alkyl-substituted oligothiophenes on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes adopted distinct straight and bent conformations. By combining STS maps with STM images, we visualize, in real space, particle-in-a-box-like oligothiophene molecular orbitals. We demonstrate that different planar conformers with significant geometrical distortions of oligothiophene backbones surprisingly exhibit very similar electronic structures, indicating a low degree of conformation-induced electronic disorder. The agreement of these results with gas-phase density functional theory calculations implies that the oligothiophene interaction with the Au(111) surface is generally insensitive to molecular conformation.

Oligothiophene wires: impact of torsional conformation on the electronic structure

Different torsional conformations of alkyl-substituted oligothiophenes show nearly identical progressions of particle-in-a-box-like electronic orbitals.

Adsorption-induced conformational isomerization of alkyl-substituted thiophene oligomers on Au(111): impact on the interfacial electronic structure

Alkyl-substituted quaterthiophenes on Au(111) form dimers linked by their alkyl substituents and, instead of adopting the trans conformation found in bulk oligothiophene crystals, assume cis conformations. Surprisingly, the impact of the conformation is not decisive in determining the lowest unoccupied molecular orbital energy. Scanning tunneling microscopy and spectroscopy of the adsorption geometries and electronic structures of alkyl-substituted quaterthiophenes show that the orbital energies vary substantially because of local variations in the Au(111) surface reactivity. These results demonstrate that interfacial oligothiophene conformations and electronic structures may differ substantially from those expected based on the band structures of bulk oligothiophene crystals.

Theoretical study on the torsional potential of alkyl, donor, and acceptor substituted bithiophene: the hidden role of noncovalent interaction and backbone conjugation

The torsional potential of substituted bithiophene is influenced not only by steric repulsion, but also by backbone conjugation and noncovalent interactions.

Vibronic coupling density analysis for the chain-length dependence of reorganization energies in oligofluorenes: a comparative study with oligothiophenes

The vibronic coupling constants and reorganization energies of oligofluorenes OF(n) (n = 1-6) are calculated for their cationic states (hole transport). Those of oligothiophenes OT(2n) (n = 1-6) are also calculated for comparison. The vibronic coupling constants of OF(n) are smaller than those of OT(2n), and decrease with increasing n. For the elucidation of the small vibronic couplings of the oligofluorenes, the calculated vibronic coupling constants are analyzed on the basis of the concept of vibronic coupling density. The vibronic coupling density of OF(n) becomes small in the middle of the chain with increasing n because of the reduction in the electron-density difference between the neutral and cationic states. It is found that orbital relaxation plays a crucial role in the distribution of the electron-density difference. From the fragment molecular orbital analyses, the large orbital relaxation in OF(n) is found to originate from the small transfer integral between the fragment molecular orbitals. These findings led to a design principle for a carrier-transporting oligomer/polymer with small vibronic couplings, or small reorganization energy, as follows: the orbital interaction between the monomers should be small from the view of vibronic couplings.

Molecular assembly and electropolymerization of 3,4-ethylenedioxythiophene on Au(111) single crystal electrode as probed by in situ electrochemical STM in 0.10 M HClO4

We have used electrochemical scanning tunneling microscopy (EC-STM) to obtain molecular insights on the adlayer structures and electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on a bare Au(111) single crystal electrode in 0.1 M HClO(4) solution. Cyclic voltammetric (CV) studies showed an increase in anodic current at 0.90 V with the oxidation of EDOT monomer occurring at E = 1.10 V (vs reversible hydrogen electrode). In situ STM revealed, for the first time, that EDOT molecules can spontaneously form organized adlayers on a bare Au(111) surface with 18 muM concentration of EDOT in aqueous solution. Molecularly resolved STM images of the EDOT adlayer showed two domains consisting of disordered and ordered structures with the formation of vacancy islands or "etch pits". Several EDOT structures were observed at +0.60 V, namely, (4 x 7), (5 x square root(37)), and (square root(7) x 3) with calculated coverages of 0.107, 0.114, and 0.111 ML, respectively. Electropolymerization was also carried out using in situ STM in 0.10 M HClO(4) under potential control.

Substrate-induced varied conformation and molecular assemblies: in situ STM observation of beta-substituted oligothiophene adlayers on Au(111)

The adsorption of 3,3'''-dihexyl-2,2':5',2'':5'',2'''-quaterthiophene (4T) molecules on an Au(111) electrode was examined by using in situ scanning tunneling microscopy in 0.10 M HClO(4), revealing internal molecular structures of the tetrathiophene backbones and the hexyl side chains. The 4T admolecules were packed in lamellae with their molecular axis aligned along the main axis of the Au(111) substrate and their hexyl side chains interdigitated to enhance intermolecular interaction. Dynamics of molecular organization incurred by the shifting of potential was also observed in this study. By examining and comparing the adsorption of 4T on HOPG and Au(111), we address the role of the substrate in understanding the arrangement of 4T admolecules.

Direct observation of conformational changes of beta-substituted duodecithiophene on a Au(111)-(square root(3) x 22) substrate using in situ electrochemical STM in 0.1 M HClO4

The adsorption of hexahexylduodecithiophene (12T) on a Au(111) electrode was investigated by using cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM) in 0.10 M HClO(4). Potential control at 0.20 V (vs RHE) revealed adlayer structures of mostly folded and rarely angular (oblique) and extended conformations on a reconstructed Au(111)-(square root(3) x 22) surface. The angular and extended conformations predominate when the electrode potential is increased to 0.35 and 0.60 V. Folded structures are still evident, but dynamic STM studies showed unfolding of this conformation. With molecular STM imaging of 12T adlayers, we address the packing arrangement and conformational changes of 12T admolecules on the reconstructed Au(111) electrode surface.

Importance of side chains and backbone length in defect modeling of poly(3-alkylthiophenes)

Geometric defects in conjugated polymers play a critical role in determining electronic structure and properties such as charge carrier mobility and band gap. Because the relative roles of individual defects are experimentally difficult to discern, computational approaches provide valuable insight if appropriate molecular models are used. Poly(3-alkylthiophenes) are often modeled with very short backbones and without their side chains. We demonstrate the shortcomings of this approach for modeling torsional disorder in poly(3-hexylthiophene) (P3HT). Using a hybrid density functional model, we identify a minimal acceptable model to comprise approximately 10 monomers with explicitly treated alkane side chains. Potential applications of this work extend to polymer electronics and optoelectronics.