Sexithiophene a-6T

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 (<5 <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 (a-6T) and polythiophene. Magnetic resonance is found to en-... 

Before the elucidation of the crystal structure of non-substituted o-6T, a few alkylated sexithiophene a-6T derivatives have been studied. In particular, Herrema et al. [96] have synthesized two stereoregular dialkyl sexithiophenes and investigated their X-ray structure. Both 4, 3 ""-dibutyl-sexithiophene (Scheme 16) and bistri-methylsilyldioctyl-sexithiophene (Scheme 17) crystals are grown from solution. They crystallize with the monoclinic system in respectively the C2/c and P2i/n... 

Figure 2. Structural formulae of a-sexithiophene and derivatives (a) unsubstituted sexithiophene [a-6T], (b) end-capped sexithiophene [EC6T], (c) regio-random /3-substituted didodecyl-6T (2D6T], (d) derivative of 6T with bulky triisopropylsilyl end-groups [DPS6T], (e) oligothiophene with two triphenyleneamine end-groups [BMA-nT],...

In sexithiophene (a-6T), the 2Ag singlet state has been located by two-photon spectroscopy [58] as being 0.1 eV higher in energy than the lowest (allowed) singlet excited state (IBJ. Therefore, internal conversion to a 2Ag state does not represent a nonradiative decay channel for sexithiophene. However, the separation between 2Ag and IBu states is less than the vibrational energy of the C=C stretch mode, so some couphng may be possible. 

On the basis of a 1/n extrapolation of the energies of the 2Ag state and IBu states of bithiophene (a-2T) and sexithiophene (a-6T), it had been suggested [64] that the 2Ag state would lie below the 1 B state for oligothiophenes with more than six rings. More recent photophysical measurements on oligomers up to seven rings [61] show that this is not the case and estimate the crossover to be nearer nine rings. 

Figure 26. Optical spectra of field-induced charge in sexithiophene [a-6T] (a) at the onset of depletion (0 V gate voltage), (b) in accumulation (-4V gate voltage).

From the point of applications a-6T is by far the most important oligothiophene. Like a-5T sexithiophene forms highly ordered or crystalline films on oxidic siuTaces,... 

At a concentration of 1.3 Na/a-6T HREELS spectra of Na-doped sexithiophene [318] reveal a new, very broad loss-structure at 1.7 eV (Fig. 46) (for bare sexithiophene compare also Fig. 38). With further Na-deposition the intensity of this loss peak increases very strongly and dominates the spectrum at the highest concentration of 8.3 Na/a-6T. Simultaneously a loss peak at 3.9 eV arises. Both effects lead to a less clear peak structure at energies between 1.0 and 4.5 eV due to the broadness of the peaks. Depending on the Na concentration there may also maxima be detected at 1.3 and 2.1 eV which arise within the 1.7eV peak structure as shoulders. There may also be a new peak at 0.95 eV for 3.2 Na/a-6T accompanied by a slight increase of the peak intensity at 0.75 eV. This intensity increase can only be explained by an additional electronic energy loss at 0.75 eV because the intensity of the overtone of the C—H stretch vibration is constant. The new peaks can either be explained as... 

FIGURE 4.3.23 Scheme of the CP-AIM experiment in which an An-coated AFM tip (A) is used to make electrical contact to crystal layers of sexithiophene (6T) grown by vapor deposition on Si02/Si. (B) A microfabricated An wire serves as a drain electrode. (C) Highly doped Si substrate is a gate. (From Kelley, T.W. and Frisbie, C.D., J. Phys. Chem. B 105, 4538-4540, 2001. With kind permission.)... 

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