Absorption polythiophene

The polaron is characterized by the reversal of bond alternation, which, in the case of polythiophene, extends over five monomer units [30-32], and the appearance of two localized stales in the band gap ). These states have been indeed observed by UV-V1S absorption of both oligomers and polymers, in solution [33— 40] and in the solid state [41-45]. 

FIGURE 2.26 PL and absorption spectra of thin films of regioregular polythiophenes HT-396 (a) and HHTT-396 (b), spin coated on a fused silica substrate. (From Barta, P., Cacialli, F., Friend, R.H., and Zagorska, M., J. Appl. Phys., 84, 6279, 1998. With permission.)... 

First, the above-mentioned sensors have major drawbacks, as the detection and recognition event is a function of the nature and characteristics of the side chains, and the side chain functionalization of the CP requires advanced synthesis and extensive purification of numerous monomeric and polymeric derivatives. Second, this generation of sensors primarily employed optical absorption as the source for detection, resulting in lower sensitivity when compared with other sensing systems for biological processes. However, the use of fluorescence detection within these sensing systems could justify continued development. More recent examples include a fluorescent polythiophene derivative with carbohydrate functionalized side chains for the detection of different bacteria [15] and novel synthesis schemes for ligand-functionalization of polythiophenes [16]. 

Ho et al. were able to verify the a-helical shape of the polymer by circular dichroism (CD) spectra. No structural elements were observed until the formation of the double helical DNA at which point they observed a right-handed a-helix in the polythiophene backbone. Their work demonstrates the power of fluorometric detection as they noted a seven order of magnitude increase in detection sensitivity (20 fM in 200 pi) simply through the use of fluorometric detection as opposed to UV-vis absorption. The polymer in solution has a high fluorescence yield with a maximum at 530 nm (Fig. 11a). Upon formation of the duplex the fluorescence is significantly quenched (Fig. lib), while with the addition of the complementary DNA and triplex formation, the fluorescence intensity is enhanced by a factor of 5 (Fig. 11c). The inherent sensitivity of the spectral shift even allowed distinction between DNA with only one and two mismatched bases (Fig. lOBd, e). 

Polythiophenes functionalized with monosaccharides have been evaluated for their ability to detect the influenza virus and E. coli (Baek et al. 2000). Copolymers of thiophene acetic acid 10 and carbohydrate-modified thiophenes 11 have been prepared via iron(III) chloride mediated polymerization. Addition of influenza virus to a sialic acid containing copolymer resulted in a blue shift of the polymer absorption maximum, resulting in an orange to red chromatic transition. Mannose-containing polythiophenes underwent color changes upon the addition of the lectin ConA or E. coli cells that contain cell surface mannose-binding receptors. A similar biotinylated pol5hhiophene afforded a streptavidin responsive material (Paid and Leclerc 1996). 

Ohkita H, Cook S, Astuti Y, Duffy W, Tierney S, Zhang W, Heeney M, McCulloch I, Nelson J, Bradley DDC, Durrant JR (2008) Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy. J Am Chem Soc 130 3030... 

There are few data concerning the photoconductivity of polythiophene [192,193]. The conductivity change under the combined action of N02 and white light is discussed assuming the participation of photo-induced absorption and desorption processes. 

Photoelectrochemical cells with polythiophene film as an active electrode and a lead plate as a counter electrode in Pb (C104) acetonitrile electrolyte has an open circuit voltage of 0.8 V, short circuit current of 2 x 10 4 A cm-2, efficiency coefficient of 0.03%, fill factor of 15% [194]. The absorption and photosensitivity spectra of such a cell are shown in Fig. 24. The small bathochromic shift in the longwave region for photosensitivity may be related to the photogeneration of the charge carriers via surface states. The photosensitivity maximum is close to the maximum solar intensity. The parameters exceed the ones obtained with polyacetylene. 

Fig. 24. Short circuit photocurrent (1) of the electrochemical cell with polythiophene and its absorption spectra (2) [194]...

Early studies showed that polymers possessing some regiospecificity exhibit improved electrical and magnetic properties when compared to random polythiophenes [234]. In this instance, P3ATs 56 were synthesized by polymerization of 3,3 -dialkyl-2,2 -bithiophene 55 as shown in Scheme 55. Polymerization either electrochemically or via Lewis acid yielded predominantly HH-coupled polymers whose absorption maxima blue shifted -90 nm for 3,3,-dimethyl-2,2 -... 

Whereas in solution the photoluminescence efficiency (Of) of poly(3-alkylthiophenes) (PATs) is 3(Mf)%, it drastically drops to 1-4% and lower in the solid state due to the increased contribution of nonradiative decay via interchain interactions and ISC caused by the heavy-atom effect of sulfur (97MM4608). Optoelectronic devices of this type of compounds have been studied (98SCI(280)1741 06SM(156)1241). Fibers of poly(3-hex-ylthiophene) for photovoltaic applications have been described (07MI1377). Poly(3-octylthiophene) showed a TTA band at 800 nm (96JPC15309). The photophysical properties of some alkyl and aryl polythiophenes have been studied (03JCP(118)1550). The absorption maximum of poly(3-octylthiophene) is at 438 nm, while the fluorescence was... 

Blends between thiazole yellow and the polythiophene 534 have been reported (07MI275). The polythiophene showed an absorption band at 520 nm. Electrical and photovoltaic properties have been studied. 

A similar absorption bands have been reported for a regioregular P3HT pristine film by continuous wave photoinduced absorption (PIA) measurements at 10 K. The PIA spectrum exhibits an absorption peak around 990 nm and was assigned to localized polarons in the disordered portions of the film. Similar rapid decays were observed for the other polythiophene pristine films. Thus, these rapid decays are ascribed to the charge recombination of polaron pairs formed in the pristine film. 

The inclusion of 5 wt% PCBM [l-(3-methoxycarbonyl)propyl-l-phe-nyl-[6,6]C6i] in the spin-coating solutions resulted in efficient polymer emission quenching for all the polythiophenes studied. The transient absorption spectra of the amorphous poly(541)/PCBM blend film. At 10 ps exhibited an absorption peak around 700 nm, similar to that observed for the poly(541) pristine film. The shape of the transient spectrum varied with time, with the absorption peak shifting from 700 nm at 10 is to 900 nm for time delays >100 (is, demonstrating the formation of two distinct transient species in the blend film. The monoexponential lifetime was t = 8 (is under Ar atmosphere and significantly shortened under 02 atmosphere. Monoexponential phase is therefore assigned to the decay of poly(541) triplet excitons. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

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