Sexithiophenes

Figure 14-13. Evolution of the field-effect mobility of OFETs for five organic materials polythio-phenc (PT) and its derivatives, qualerthiophcne (4T), scxithio-phenc (6T), dihcxyl-sexithiophene (DH6T). and pcntaecnc.

Analysis of the spin-1/2 2-PADMR spectrum is more complicated as it contains two features with increased and decreased transmission. As the photoexcitation density is proportional to -AT, we describe these as PA-quenching (Sn<0) and PA-enhancing (Sn >0) bands. We defer detailed discussion of the spectrum to the following section, where we compare the doping, PA, and PADMR studies of a-sexithiophene (u-6T) and polythiophene. Magnetic resonance is found to enhance... 

Two years later, the same research group published an alternative to the solvent-free method, since this method cannot be applied to the synthesis of insoluble thiophene oligomers such as sexithiophene [43]. Obviously, their insolubility prevents separation of the AI2O3 solid support. The effectiveness of PdCl2(dppf)/KF in solution phase was tested in the synthesis of soluble quinquethiophene from diiodoterthiophene and 2-thi-... 

A recent publication by the group of Barbarella has disclosed the rapid preparation of poorly soluble unsubstituted and modified a-quinque- and sexithiophenes by the extensive use of bromination/iodination steps and microwave-assisted Suzuki and Sonogashira cross-couplings (Scheme 6.16) [42]. Suzuki reactions were either carried out under solvent-free conditions on a strongly basic potassium fluoride/ alumina support for the synthesis of soluble oligothiophenes, or in solution phase for the preparation of the rather insoluble a-quinque- and sexithiophenes. In both cases, 5 mol% of [l,l -bis(diphenylphosphino)ferrocene]dichloropalladium(II)... 

Thienyl-bridged ligands are of particular interest. Bridging ligands in which two bpy ligands are connected by one (L ), three (L ) or six (L ) thienyl units have been developed. In [(bpy)2Ru(p-L )-Ru(bpy)2] ", both Ru centers are oxidized at the same potential, and initial reduction processes are terminal bpy-centered. For [(bpy)2Ru(//-L )Ru(bpy)2] concurrent Ru oxidation processes come between two terthiophene oxidations, whereas in [(bpy)2Ru(//-L )Ru(bpy)2] , oxidations of the Ru centers follow two sequential sexithiophene oxidation processes. " ... 

Weiser G, Moller S (2002) Directional dispersion of the optical resonance of k-k transitions of alpha-sexithiophene single crystals. Phys Rev B 65 045203... 

Oike T, Kurata T, Takimiya K et al (2005) Polyether-bridged sexithiophene as a complexa-tion-gated molecular wire for intramolecular photoinduced electron transfer. J Am Chem Soc 127 15372-15373... 

Servet B, Horowitz G, Ries S, Lagorsse O, Alnot P, Yassar A, Deloffre F, Srivastava P, Hajlaoui R, Lang P, Gamier F (1994) Polymorphism and charge-transport in vacuum-evaporated sexithiophene films. Chem Mater 6 1809... 

Loiacono MJ, Granstrom EL, Frisbie CD (1998) Investigation of charge transport in thin, doped sexithiophene crystals by conducting probe atomic force microscopy. J Phys Chem B 102 1679-1688... 

Horowitz G, Haijlaoui R, Delannoy P (1995) Temperature dependence of the field-effect mobility of sexithiophene, determination of the density of traps. J Phys III 5 355-371... 

A femtosecond time-resolved study on dodecyl-sexithiophene appeared (94CPL(228)651). A transient absorption forms immediately after excitation and remains stable between 400 fs and 15 ps. Induced fluorescence raised about 400 fs after excitation and increases further up to 15 ps. 

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

The synthesis of the first n-type sexithiophene conductor, a,co-diperfluorohexylsexithiophene (374) has been described (00AGE4547) the synthesis of other oligomers of the same shape has been reported (04CM4715). 

Tetradecafluorosexithiophene 428 was considered as a potential n-type semiconductor for FETs for the following reasons sexithiophene is an excellent p-type semiconductor with high hole mobility perfluorination is an effective way to convert a p-type organic semiconductor to a n-type one (01JA4643). The absorption and emission maxima of 428 (421 and 471 nm, respectively) shifted to higher energies relative to those of sexithiophene (435 and 508 nm, respectively). 

The theory predicts a strong dependence of photogeneration efficiencies on the field and it approaches unity at high field. The temperature sensitivity decreases with the increase in field. The theory has found satisfactory explanations in the photogeneration process in many organic disordered systems, such as PVK (Scheme la) [25], and triphenylamine doped in polycarbonate [26], Figure 4 shows an example of the field dependence of c() calculated from Eq. (22) (the solid lines) to fit the quantum efficiency data at room temperature for hole and electron generation in an amorphous material. The material consists of a sexithiophene covalently linked with a methine dye molecule (compound 1) (Scheme 2). 

This work was partially supported by NATO (Grant No PST.CLG.979685). Fig. 4 and Figs. 6-7 are reprinted with permission from Elsevier from Synthetic Metals, 109, Jursenas et al., Femtosecond excited-state dynamics in N,N-dimethylaminobenzylidene-l,3-indandione DMABI films, p. 171, 2000, and from Chemical Physics Letters, 320, G. Klein, Transient femtosecond spectroscopy in a-sexithiophene single crystals, p. 67, 2000, respectively. 

A particularly interesting fact is that in the case of a-sexithiophene, or a6T, work on prototype devices such as MISFETs has shown that thin films of a6T molecules actually exhibit better performance characteristics than the corresponding polymer, polythiophene, and rival those of amorphous silicon41. a6T also has been used in light emitting diodes42-44. 

Due to the high degree of conjugation, oh go(thiophene)s advanced as attractive candidates for molecular bridges. For instance, Sato et al. [34] constructed hexyl-sexithiophene and methoxy-terthiophene derivatives bearing two terminal ferrocenyl groups. These served as model compounds for molecular wires. In the hexyl-sexithiophene derivative, the resultant oxidized states spread over both the ferrocene and the sexithiophene moieties. Similarly, in the methoxy-terthiophene derivative, the oxidized species spreads over the entire molecule containing the terthiophene and the other ferrocene moiety. In both cases, CT between the terminal units is inferred as it is mediated via the oh go(thiophene)s. 

Because molecules located in the same layer are much closer to each other than those situated in different layers, charge transport is expected to be much more efficient in the direction along the layers than across them. This has largely been confirmed by X-ray diffraction measurements on sexithiophene-based devices [27, 28], which indicated that the highest performance is attained when molecules are standing upright on the insulator. Similar behavior was found for pentacene [29, 30]. This is illustrated in Fig. 1.7, in which the molecules are depicted as short rods. 

Comparing single crystal and vapor-grown devices for these two compounds is difficult, because reports on evaporated tetracene OTFTs are rather scarce [99-101], and despite several (unpublished) attempts, fabrication of an operating thin-film device from rubrene has not yet been successfully achieved. For both compounds the problem seems to arise from an improper deposition mechanism, which, in contrast with experience with pentacene and sexithiophene, does not favor two-dimensional growth. 

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