TTF-porphyrin dyads as novel photoinduced electron transfer systems

A TTF-linked porphyrin dyad and its zinc complex have been synthesized as novel photosystems with a redox-active pendant. The two chromophores of these dyads are not interactive in the absorption spectra, but the fluorescence of the porphyrin chromophore is dramatically quenched by intramolecular electron transfer from the TTF pendant.     

Control of photoinduced energy- and electron-transfer steps in zinc porphyrin-oligothiophene-fullerene linked triads with solvent polarity

The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C(60)) linked triads (ZnP-nT-C(60)) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C(60), an energy transfer took place from the (1)ZnP moiety to the C(60) moiety, generating ZnP-nT-(1)C(60). In polar solvents, the CS process also took place directly via the (1)ZnP moiety, generating ZnP(*+)-nT-C(60)(*-), as well as the energy transfer to the C(60) moiety. After this energy transfer, an indirect CS process took place from the (1)C(60) moiety. In the less polar solvent anisole, the radical cation (hole) of ZnP(*+)-nT-C(60)(*-) shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 10(8) s(-1); then, the final CS states ZnP-nT(*+)-C(60)(*-) were lasting for 6-7 mus. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT(*+)-C(60)(*-) and ZnP(*+)-nT-C(60)(*-) were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 mus) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nT(*+)-C(60)(*-) was confirmed with apparent short lifetimes (0.6-0.8 mus), which can be explained by the fast hole shift to more stable ZnP(*+)-nT-C(60)(*-) followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C(60); that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H(2)P-nT-C(60), in which H(2)P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C(60) are approximately 30 times longer than those in o-DCB.     

2,6-Diphenylbenzo[1,2-b:4,5-b']dichalcogenophenes: a new class of high-performance semiconductors for organic field-effect transistors

2,6-Diphenylbenzo[1,2-b:4,5-b']dichalcogenophenes including thiophene, selenophene, and tellurophene analogues as organic semiconductors for field-effect transistors were effectively synthesized in three steps from commercially available 1,4-dibromobenzene. All three benzodichalcogenophenes acted as good p-type semiconductors, and particularly the selenophene analogue, 2,6-diphenylbenzo[1,2-b:4,5-b']diselenophene, showed high FET mobility of 0.17 cm2 V-1 s-1.     

Polyether-bridged sexithiophene as a complexation-gated molecular wire for intramolecular photoinduced electron transfer

The porphyrin-sexithiophene-fullerene triad 2, where the two central thiophene units of the sexithiophene spacer are bridged with a crown-ether-like polyether chain, undergoes efficient intramolecular electron transfer from the photoexcited porphyrin moiety to the fullerene through the sexithiophene. However, complexation with a sodium cation in the crown ether ring causes complete suppression of electron transfer as a result of a drastic conformational change of the sexithiophene backbone. Furthermore, decomplexation resumes the photoinduced electron transfer. This on/off switching phenomenon indicates that the polyether-bridged sexithiophene can function as a complexation-gated molecular wire.     

Quinoid-Aromatic Resonance for Very Small Optical Energy Gaps in Small-Molecule Organic Semiconductors: A Naphthodithiophenedione-oligothiophene Triad System

Organic semiconductors with very small optical energy gaps have attracted a lot of attention for near-infrared-active optoelectronic applications. Herein, we present a series of donor-acceptor-donor (D−A−D) organic semiconductors consisting of a highly electron-deficient naphtho[1,2-b:5,6-b′]dithiophene-2,7-dione quinoidal acceptor and oligothiophene donors that show very small optical energy gaps of down to 0.72 eV in the solid state. Investigation of the physicochemical properties of the D−A−D molecules as well as theoretical calculations of their electronic structures revealed an efficient intramolecular interaction between the quinoidal acceptor and the aromatic oligothiophene donors in the D−A−D molecules; this significantly enhances the backbone resonance and thus reduces the bond length alternation along the π-conjugated backbones. Despite the very small optical energy gaps, the D−A−D molecules have low-lying frontier orbital energy levels that give rise to air-stable ambipolar carrier transport properties with hole and electron mobilities of up to 0.026 and 0.043 cm² V⁻¹ s⁻¹, respectively, in field-effect transistors.     

Oligo(octithienylene-diethynylene)s as unprecedentedly long conjugated nanomolecules

[reaction: see text] A series of oligo(octithienylene-diethynylene)s alternately composed of octithiophene and diacetylene units have been prepared by a random Eglinton coupling reaction among mono- and diethynyloctithiophenes. The largest compound isolated in the oligomeric series is comprised of twelve octithiophene units and eleven diacetylene units, and its molecular length reaches ca. 43 nm, which is the longest among single-component conjugated nanomolecules.     

Synthesis and spectroscopic properties of

The title compound has been synthesized as the first oligothiophenophane that has the typical stacking structure of a layered cyclophane and can behave as an ideal pi-dimer model.     

The first electrochemically active cuppedophanes: bis(tetrathiafulvalene)cuppedophanes

The synthesis of the first tetrathiafulvalene-containing cuppedophanes is reported, together with conformational study of the structures by (1)H NMR spectroscopy. The bis(TTF)cuppedophane 6 represents a novel type of cuppedophane.     

Synthesis and characterization of benzo[1,2-b:3,4-b':5,6-b']trithiophene (BTT) oligomers

Two dimers (2 and 3), dendritic tetramer (4), hexamer (5), and decamer (6) of benzo[1,2-b:3,4-b':5,6-b']trithiophene (BTT), a potential π-core unit with C(3h) symmetry, were synthesized, characterized, and evaluated for possible use as organic semiconductors. Single crystal X-ray analyses of the dimers (2 and 3) revealed that they have planar molecular structures with dihedral angles of almost 180° between two BTT units. In accordance with the rigid and planar molecular structure, the unsubstituted dimer (2) is poorly soluble, whereas the octyl-substituted dimer (3) has improved solubility. Although the solubility of the dendritic tetramer (4) is decreased, further extended systems, i.e., the dendritic hexamer (5) and decamer (6), have solubilities better than that of 4. With increasing numbers of BTT units in the molecule, the experimentally determined energy levels of HOMO shift upward slightly and the HOMO-LUMO energy gaps become smaller, but the extent of HOMO destabilization and reduction of the HOMO-LUMO gap are not significant. Taking into account the energy levels of the frontier orbitals, 3-6 could be useful as p-channel organic semiconductors rather than n-channel. In fact, the spin-coated thin film of 3 with edge-on molecular orientation acted as an active channel of field-effect transistors that showed hole mobilities as high as 0.14 cm(2) V(-1) s(-1), indicating that the BTT core is a useful π-conjugated system for application to organic semiconductors, although 4-6 gave FET characteristics rather inferior to those of 3, owing to their amorphous nature in the thin film state.