Polyether-Bridged Sexithiophene as a Complexation-Gated Molecular Wire for Intramolecular Photoinduced Electron Transfer

Mixed Ionic and Electronic Conduction in Small-Molecule Semiconductors

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.

Excitation-Wavelength-Dependent Photoinduced Electron Transfer in a π-Conjugated Diblock Oligomer

Control of photoinduced electron transfer through selective excitation of a π-conjugated diblock oligomeric system featuring tetrathiophene (T₄) and tetra(phenylene ethynylene) (PE₄) donor blocks capped with a naphthalene diimide (NDI) acceptor (T₄PE₄NDI) is demonstrated. Each π-conjugated oligomeric segment has its own discrete ionization potential, electron affinity, and optical band gap which provides an absorption profile that has specific wavelengths that offer selective excitation of the PE₄ and T₄ blocks. Therefore, T₄PE₄NDI can be selectively excited to form a charge-separated state via ultrafast photoinduced electron transfer from the PE₄ segment to NDI when excited at 370 nm, but it does not produce a charge-separated state when excited at 420 nm (T₄). Picosecond transient absorption techniques were performed to probe the excited-state dynamics, revealing ultrafast charge separation (∼4 ps) occurring from the PE₄ segment to NDI when excited at 370 nm, followed by delocalization of the hole over the T₄ segment. On the contrary, electron transfer is suppressed with excitation at longer wavelengths (≥420 nm), where the spectrum is dominated by the T₄ unit. The rate of electron transfer and charge recombination was investigated versus the length of the PE bridge unit in oligomers featuring zero and two PE units (T₄NDI and T₄PE₂NDI). The rate of charge recombination decreases from 1.2 × 10¹¹ to 1.0 × 10⁹ s^-1 with increasing bridge length between the T₄ and NDI components (T₄NDI to T₄PE₄NDI). Furthermore, wavelength-dependent photoinduced electron transfer was not observed in either T₄NDI or T₄PE₂NDI due to an insufficient PE_n bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced electron transfer in a fully π-conjugated oligomer.

Molecular Wires with Controllable π-Delocalization Incorporating Redox-Triggered π-Conjugated Switching Units

A perfluorobiphenyl-2,2'-diyl dication and its corresponding dihydrophenanthrene-type electron donor are interconvertible upon two-electron transfer. Redox-triggered C-C bond-formation/cleavage caused a drastic change in the torsion angle of the biphenyl unit. Thus, π-delocalization ON/OFF switching was observed as a change in the UV absorption upon electrolysis of the linearly extended analogue with two phenylethynyl groups. A further extended π-system with a molecular length of ca. 3.5 nm, which has two switching units, was synthesized. Spectroelectrograms as well as voltammetric analyses showed that the two units act nearly simultaneously because of the very small inter-unit electrostatic repulsion in the tetracationic state. Thus, the present pair is a promising candidate as a switching unit for "molecular wires" with controllable π-delocalization, in which a higher ON/OFF ratio of delocalization could be realized by incorporating multiple switching units.

A crown-ether loop-derivatized oligothiophene doubly attached on gold surface as cation-binding switchable molecular junction

A crown-ether dithiol quaterthiophene is synthesized and immobilized on gold surface by double covalent fixation. UV-vis spectroscopy and cyclic voltammetry show that the corresponding dithioester precursor can complex Pb(2+) in solution and that this property is maintained for monolayers of the dithiol on gold. Current-voltage measurements by eutectic GaIn drop contact on the monolayer show a significant increase (up to 1.6 × 10(3) times) of the current at low bias after Pb(2+) complexation.

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.

Fullerene for organic electronics

This tutorial review surveys and highlights the integration of different molecular wires-in combination with chromophores that exhibit (i) significant absorption cross section throughout the visible part of the solar spectrum and (ii) good electron donating power-into novel electron donor-acceptor conjugates. The focus is predominantly on charge transfer and charge transport features of the most promising systems.

Synthesis and Optical Properties of Bifunctional Thiophene Molecules Coordinated to Ruthenium

A series of unsymmetrical bi- and tetrathiophenes have been synthesized with bipyridine and phosphonic acid functional groups. To do this, phosphonic esters were bonded to thiophenes and the thiophenes coupled to bipyridine. After synthesis of the thienylbipyridines, bis(bipyridine) ruthenium was coordinated to them through the bipyridines. The thienylbipyridines absorb visible light and fluoresce; however, on attachment to ruthenium, both their fluorescence and that of ruthenium are quenched. An additional effect of coordinating ruthenium to the thiophenes is a new absorption band around 470 nm. Variation in oligothiophene length and bipyridine substitution position allowed comparison of the effect of these variables on electronic properties. The longer oligothiophenes display lower-energy absorptions and emissions than that of the shorter thiophenes. In contrast, the position of the bipyridine attachment does not have a large effect on the absorbance or emission wavelength, or on the fluorescence quantum yield.

Quinoxalino[2,3-b‘]porphyrins Behave as π-Expanded Porphyrins upon One-Electron Reduction: Broad Control of the Degree of Delocalization through Substitution at the Macrocycle Periphery

The synthesis and redox properties of a series of free-base and metal(II) quinoxalino[2,3-b']porphyrins and their use in an investigation of the substituent effects on the degree of communication between the porphyrin and its beta,beta'-fused quinoxalino component are reported. ESR, thin-layer spectroelectrochemistry, and quantum chemical calculations of the resultant radical anions from one-electron reduction indicate that localization of the unpaired electron across both the porphyrin and the fused quinoxalino group can be controlled, the system as a whole behaving as a highly polarizable pi-expanded porphyrin radical anion. ESR studies on the radical anions of zinc(II) quinoxalino[2,3-b']porphyrin derivatives indicate that nitrogen-atom spin distribution changes as a function of chemical substitution: 27% quinoxaline character when the porphyrin ring bears a 7-nitro substituent, 34% quinoxaline character in the unsubstituted parent, and 51-61% nitroquinoxaline character when the quinoxalino unit has one or more nitro groups. Close analogies are found between the calculated and observed nitrogen-atom spin distributions, indicating that the calculations embody the key chemical effects. The calculations also indicate that the nitrogen-atom spin distributions closely parallel the important total porphyrin, quinoxaline, and nitro spin distributions, indicating that the observed quantities realistically depict the change in the nature of the delocalization of the radical anion as a function of chemical substitution. The profound effects observed indicate long-range communication of the type that is essential in molecular electronics applications.

Twisting of Conjugated Oligomers and Polymers: Case Study of Oligo- and Polythiophene

Interring twisting (change in the dihedral angle between conjugated rings) of polythiophene was studied theoretically using periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level. We find that the band gap of polymers is strongly dependent on the interring twist angle; yet twisting requires very little energy. A twist of 30 degrees increases the band gap by 0.75 eV in polythiophene, while requiring only 0.41 kcal mol(-1) per monomer unit. Such a small energetic value is of the order of crystal packing or van der Waals forces. These results are compared with calculations performed on model oligomers. Sexithiophene, its radical cations, and its dication are optimized at 0-180 degrees end-to-end twist angles (which correspond to 0-36 degrees interring dihedral angles) using the B3LYP/6-31G(d) method. The theoretical results suggest that the HOMO-LUMO gap, ionization potential, and charge distribution of oligomers are strongly dependent on twisting, whereas, similar to the case of polythiophene, twisting of neutral oligothiophenes costs very little energy. In the case of the radical cation, the lowest energy transition is shifted to a longer wavelength region on twisting, while the second-lowest energy transition is shifted to a shorter wavelength region. This implies that twisted, doped conducting polymers (modeled here by an oligomer radical cation), in contrast to planar, doped polymers, should be transparent within a certain optical window (in the far-visible region, at approximately 1.5 eV). This observation is explained on the basis of changes in the shape and overlap of the frontier molecular orbitals.

Synthesis and radical coupling of pyridine-bridged π-extended tetrathiafulvalene (TTF)-type donors and push–pull analogues

A new family of pi-extended TTF analogues (3a-c) and D-pi-A chromophores (5a-c), in which the electroactive units (1,3-dithiole rings and 2,2-dicyanovinyl groups) are connected through a pyridine bridge with a meta substitution pattern, is reported. The redox behavior of these compounds has been investigated by cyclic voltammetry and theoretical calculations performed at the B3P86/6-31G** level. Unlike many pi-extended TTF derivatives, the 1,3-dithiole rings in compounds 3a-c do not behave independently and two oxidation processes are observed with an anodic separation ranging from 50 to 150 mV. Calculations show that electrons are equally extracted from both dithiole rings. A biradical structure is predicted for the dication state due to the near-degeneracy of the HOMO and HOMO - 1 orbitals. The presence of both donor (D) and acceptor (A) fragments in conjugates results in irreversible oxidation and reduction processes associated with the 1,3-dithiole ring and with the 2,2-dicyanovinyl moiety, respectively. An electrochemical-chemical-electrochemical (ECE) process takes place for all the compounds reported. The chemical process implies the dimerization of the radical cation for compounds 5 and the oligomerization of the biradical dication for compounds 3. The ECE process therefore generates new neutral dimeric (5) or oligomeric (3) species that incorporate the TTF vinylogue core.

Giant Macrocycles Composed of Thiophene, Acetylene, and Ethylene Building Blocks

Fully conjugated giant macrocyclic oligothiophenes with 60pi, 90pi,120pi, 150pi, and 180pi frames (1, 2, 3, 4 and 5) have been designed, and their butyl-substituted derivatives (1a, 2a, 3a, 4a, and 5a) have been synthesized using modified Sonogashira and McMurry coupling reactions as key steps. The 60-180pi systems 1-5 are circular with 1.8-6 nm inner cavities and 3.3-7.5 nm outside molecular diameters. Compound 1a containing ten 3,4-dibutyl-2,5-thienylene, eight ethynylene, and two vinylene units has been converted into macrocyclic oligo(3,4-dibutyl-2,5-thienylene-ethynylene) 6a using bromination/dehydrobromination procedure. Giant macrocycles 1a-6a exhibit a red shift of their absorption spectra and a fairly strong fluorescence with a large Stokes shift as compared to a linear conjugated counterpart having five thiophene rings. Compounds 1a-6a exhibit multistep reversible redox behaviors with fairly low first oxidation potentials, reflecting their cyclic conjugation. Furthermore, chemical oxidation of 1a-6a with FeCl3 shows drastic changes of spectroscopic properties due to intramolecular and intermolecular pi-pi interactions. Doping of 1a-3a with iodine forms semiconductor due to its pi-donor properties and pi-pi stacking ability. X-ray analysis of 1a confirmed a round, planar structure with nanoscale inner cavity, and revealed host ability for alkanes and unique packing structure. Interestingly, 2a and 3a self-aggregate in the solid state to form "molecular wires," which are about 200 nm thick and more than 1 mm long. The internal structures of fibrous aggregates have been investigated by optical microscope, scanning electron microscopy, atomic force microscopy, and X-ray diffraction analyses.

Metal ion dependent fluorescence quenching in a crown ether bridged porphyrin–fullerene dyad

The fluorescence decay kinetics from a benzonitrile solution of a dibenzo-18-crown-6 ether bridged porphyrin-fullerene dyad has been studied in the presence of a range of metal ions. Dual-exponential fluorescence decay behaviour has been attributed to conformational flexibility of the molecule influencing quenching interactions between the photo-excited porphyrin and fullerene. Additions of sodium, potassium and lithium ions significantly modulate the observed fluorescence decay processes while the larger tetrabutylammonium ion has only a minor affect. The results are discussed in terms of ion inclusion within the crown ether affecting both the bridge conformational properties and donor-acceptor electronic interactions.