Absorption in conjugated polymers

The maxima of the absorption spectra of conjugated polymers are located in the visible wavelength region. 

Chart 1.2 Chemical structures of poly(l, 4-phenylene vinylene), PPV, and three PPV derivatives. 

Energy transfer from electronically excited molecules to ground-state molecules of different chemical composition represents a highly important intermolecular deactivation path. In general terms, energy transfer occurs according to Eq. (1-7) from a donor to an acceptor, the latter frequently being referred to as a quencher. 

This process is energetically favorable in the case of exothermicity, i.e., if the excitation energy of D exceeds that of A E (D ) E (A ). A typical case concerns the stabiUzation of polymeric plastics. If an electronically excited macromolecule P transfers its excitation energy to an additive A according to Eq. (1-8), hydrogen abstraction [Eq. (1-9)] is inhibited and the macromolecule remains intact 

The dependence of the rate constant, ker. of intermolecular energy-transfer processes on the distance, R, is given by Eqs. (TIO) and (Til) [13]. 

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Hoppe H, Arnold N, Meissner D, Sariciftci NS (2004) Modeling of optical absorption in conjugated polymer/fullerene bulk-heterojunction plastic solar cells. Thin Solid Eilms 451-452 589... 

Table XTV. Maximum of Absorption in Conjugated Polymer Films...

NLO AND REVERSE SATURABLE ABSORPTION IN CONJUGATED POLYMER/ FULLERENE COMPOSITES... 

D McBranch, MB Sinclair. Ultrafast photoinduced absorption in nondegenerate ground-state conjugated polymers Signatures of excited states. In NS Sariciftci, ed. Nature of the Photoexcitations in Conjugated Polymers Semiconductor Band vs. Exciton Model, New York World Scientific, 1997. 

Figure 4.8-2 Optical absorption of conjugated polymers with a degenerate ground state (trans-poly(acetylene)) (a), according to Suzuki et ah, 1980 and a non-degenerate ground state (poly(thiophene)) (b), according to Danno et ah, 1993 in various doping states. Doping concentrations are indicated in % in (a) and by the applied potential in (b).

In conjugated polymers, the stretching of the coiled polymer chain usually results in a better conjugation of the tt electrons. It is expected that the absorption Amax of the film should be red-shifted as the polymer chains are stretched further. This phenomenon is indeed observed experimentally. Shi et al.17 have observed that the absorption A,max of a spin-coated polymer film obtained from a more dilute solution is significantly red-shifted in comparison to that spun from a more concentrated solution. Fig. 6.6 shows the absorption spectra of two films spun at the same speed (8000 rpm) but at two different concentrations a thinner film (180 X) spun from a 0.3-wt% MEH-PPV solution and a thicker film (900 X) spun from a l-wt% MEH-PPV solution in cyclohexanone (CHO). For easy comparison, the spectrum of the thinner film has been normalized. It can be easily seen from Fig. 6.6 that the absorption of the polymer film spun with the more dilute solution is significantly red-shifted (Amax = 510 nm) as compared to that spun with the more concentrated solution (Amax = 496 nm). This indicates that the polymer chains in the film spun from the more dilute solution are more extended and the n electrons in the polymer backbone are more conjugated. 

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