Presented here are several convergent synthetic routes to conjugated oligo(phenylene ethynylene)s. Some of these oligomers are free of functional groups, while others possess donor groups, acceptor groups, porphyrin interiors, and other heterocyclic interiors for various potential transmission and digital device applications. The syntheses of oligo(phenylene ethynylene)s with a variety of end groups for attachment to numerous metal probes and surfaces are presented. Some of the functionalized molecular systems showed linear, wire-like, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects. Finally, the syntheses of functionalized oligomers are described that can form self-assembled monolayers on metallic electrodes that reduce the Schottky barriers. Information from the Schottky barrier studies can provide useful insight into molecular alligator clip optimizations for molecular electronics.

We report a highly efficient charge separation system, D-Pt-A, where D (triphenylamine) and A (naphthalenediimide) are bonded to the Pt moiety through highly twisted phenylene ethynylene linkages. The quantum yields for the formation of the charge-separated state were determined to be nearly unity. The lifetimes of D(+)-Pt-A(-) were approximately 1 micros at room temperature and much longer at low temperature. The spin-correlated radical ion pair was directly observed by means of time-resolved EPR spectroscopy.

A planar stable triphenylamine radical cation is a fascinating p-electron system that is potentially applicable to electronic and magnetic materials. Hellwinkel and co-workers synthesized an interesting compound 1, [1] from which the radical cation 1C+ was generated in concentrated sulfuric acid [1a] or by oxidation with lead tetraacetate in trifluoroacetic acid.[1c] The triphenylamine framework of 1C + is most likely planar. The dimethylmethylene bridges contribute to its stabilization; however, they disrupt the CT-type intermolecular interaction that is crucial for the construction of electronic and magnetic materials. To overcome this disadvantage and to improve stability, we designed a synthetic route to the new oxygenbridged analogue 2. [2,3] Quite recently, Livant and co-workers described a product fraction showing a molecular ion of m/z = 287 in the thermolysis of tris(2,6-dimethoxyphenyl)amine. They assumed the structure 2 for the MS peak. However, the compound showed no NMR spectroscopic signal.[4] Herein, we report the preparation, structures, and properties of the neutral 2 and the radical cation 2C+ .The synthesis of 2 is outlined in Scheme 1. Sequential aromatic nucleophilic substitution reactions of 2,6-difluoronitrobenzene with 2-bromo-3-methoxyphenolate and then with 2-bromo-3-fluorophenolate gave 4 (71 % yield in two steps). The reduction of 4 proceeded selectively in the presence of p-bromophenol (10 equiv) to avoid a competing reduction of aromatic bromide. Intramolecular cyclization of 5 was performed under Pd 0 -mediated cross-coupling reaction conditions [5] to afford 6 in 35 % yield. Treatment of 6 with BBr 3 gave 7 in good yield. Intramolecular nucleophilic substitution of 7 in DMF with K 2 CO 3 as a base proceeded efficiently under remarkably mild conditions to give the desired compound 2 in good yield. Compound 2 had a reversible oxidation wave at + 0.59 V versus SCE in DMF (SCE = saturated calomel electrode). The chemical oxidation of 2 was performed by using tris(p-bromophenyl)aminium hexafluorophosphate as an oxidant in methylene chloride. The salt 2CÀ can be recrystallized from acetonitrile/ diethyl ether. Figure 1 shows the molecular structures determined by Xray crystallographic analysis of 2 and 2C + .[6] The neutral compound 2 has a shallow bowl structure, whereas the radical cation 2C + has a planar structure. The CÀN bond lengths become shorter and the C-N-C bond angles approach 1208 in the radical cation 2C+ . The bond-length difference is in qualitative accordance with the HOMO shape of 2; that is, the C À N and C À O bonds have an antibonding nature in the HOMO.Triphenylamine radical cations without para substituents are generally unstable because of the large spin densities of Scheme 1. Reaction conditions: a) 2-bromo-3-methoxyphenol, NaH/ DMSO, 130 8C; b) 2-bromo-3-fluorophenol, NaH/DMSO, 130 8C; c) hydrazine hydrate, Pd/C, p-bromophenol/ethanol, reflux; d) NaOtBu, [Pd-(dba) 2 ], P(tBu) 3 /toluene, reflux; e) BBr 3 /CH 2 Cl 2 , À78 8C!room temperature; f) K 2 CO 3 /DMF, ...

Strategies for self‐assembling molecule‐based devices are considered in terms of current chemical issues whose resolution appears critical to efficient connection and addressing of electronically active molecules between electrodes. We discuss issues related to the type and shape of the molecules, chemical bonding at junctions, molecular lengths and electrode gap matching, molecular alignment at electrodes, chemistry of deposited metal contacts, and the doping of molecular conductors. Examples of each of these aspects is given using fully conjugated molecules, constituted from rigid rod phenylene‐ethynylene units, self‐assembled onto metal and semiconductor surfaces.

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Photoinduced charge separation of a phenothiazine–anthraquinone dyad 1 with a rigid bicyclo[2.2.2]octane spacer was investigated by means of picosecond and nanosecond transient absorption and time-resolved ESR spectroscopies. Irradiation of the anthraquinone chromophore in 1 produced the charge-separated (CS) state in THF. Time constants of the formation and the decay of the CS state were determined to be 1.3 ns and 1.0 µs, respectively. Time-resolved ESR showed spin-polarized all-emission signals due to the formation of the CS state via the triplet mechanism.

This contribution presents an electrochemical, Raman spectroscopic, and theoretical study probing the differences in molecular and electronic structure of two quinoidal oligothiophenes (3′,4′‐dibutyl‐5,5″‐bis(dicyanomethylene)‐5,5″‐dihydro‐2,2′:5′,2″‐terthiophene and 5,5′‐bis(dicyanomethylene)‐3‐hexyl‐2,5‐dihydro‐4,4′‐dihexyl‐2,2′,5,5′‐tetrahydro‐tetrathiophene) with terminal tetracyanomethylene functionalization and aromatic oligothiophenes where acceptor moieties are positioned at lateral positions along the conjugated chain (6,6′‐dibutylsulfanyl‐[2,2′‐bi‐[4‐dicyanovinylene‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene]). In this way, the consequences of linear and cross conjugation are compared and contrasted. From this analysis, it is apparent that organic field‐effect transistors fabricated with cross‐conjugated tetrathiophene semiconductors should combine the benefits of an electron‐donor aromatic chain with strongly electron‐accepting tetracyanomethylene substituents. The corresponding organic field‐effect transistors exhibit ambipolar transport with rather similar hole and electron mobilities. Moreover, n‐channel conduction is enhanced to yield one of the highest electron mobilities found to date for this type of material.

The recently reported efficient charge-separated system based on bipyridine-diacetylide platinum(ii) complexes was applied to photoelectric conversion systems herein, based on the design and synthesis of two triads: MTA-Pt-NDISAc (3, MTA: dimethoxytriphenylamine, Pt: platinum(ii) complex, NDISAc: thioacetate derivative linked to naphthalenediimide) and MTA-Pt-MNICOOH (4, MNICOOH: naphthaleneimide-4-carboxylic acid). The charge-separated (CS) states of triads 3 and 5 (MOM-protected 4) were effectively generated by photo-induced electron transfer in both THF and toluene, although the rate of formation of the CS state from 5 was relatively slow in toluene. The lifetimes of these CS states were determined to be 730 ns in toluene and 61 ns (70%) and 170 ns (30%) as a double exponential decay in THF for 3, and 600 ns in toluene and 170 ns in THF for 5. The acetylthio group of triad 3 was exploited in the preparation of a self-assembled monolayer (SAM) on a gold surface. Photocurrent was detected upon irradiation of an electrochemical cell comprising Au/3/Na ascorbate/Pt, which was ascribed to the platinum(ii) complex based on the action spectrum. The carboxylic acid group of triad 4 facilitated adsorption on the TiO2 surface, and a dye-sensitized solar cell constructed based on FTO/TiO2/4/electrolyte (LiI-I2)/Pt exhibited a poor energy conversion efficiency (η = 0.20%) based on the incident photon-to-current conversion efficiency spectrum and the I-V curve. This poor efficiency may be derived from the bent molecular shape of 4, or may be due to a possible high energy barrier in the electron injection process through the adsorption site.