Poly ground state structure

Poly(phenylene vinylene), PPV, and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Since PPV has a nondegenerate ground state, structural relaxation in the excited state leads to the formation of polarons, bipolarons, and neutral excitons. However, prior to treating the structural relaxation in the excited state, one needs to develop a satisfactory description of the electronic excited states. 

Figure 1.60. Poly(l,6-hcptadiyne) (a) shows the two degenerate ground-state structures with the inequivalent soliton positions indicated in (b), Ri and R2 denote the location of substituents.

A publication, where the results of Raman spectroscopy were combined with theoretical calculations, indicates, however, that the ground-state structure should be quinoid as in the case of polyisothianaphthene. The same authors report that their alkylated poly(thieno[3,4-c]pyrazine) derivative possesses a bandgap of 0.95 eV [632]. 

Monomers that yield radicals in which the unpaired electron is extensively delocalized have ground state structures that are themselves resonance stabilized. The important factor is the relative stability of the product radical, however, because a single electron is more easily delocalized than one in a C=C double bond. Thus resonance stabilization causes as increase in monomer reactivity and a decrease in reactivity of the resulting polymer radical. Styrene is more reactive toward polymerization than vinyl acetate, for example, and the propagation rate in the former polymerization is much slower than in the radical synthesis of poly(vinyl acetate). 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

In the following we will first discuss examples of polytriacetylenes (Fig. 19) and then compare these results with the ones of other molecular structures. Monomers of poly triacetylenes with the three neutral end groups TMS (trimeth-ylsilyl), TES (triethylsilyl), and TIPS (triisopropylsilyl) have been investigated. By deconvolution of the absorption spectra assuming Gaussian fine shapes the wavelengths A00 for the absorption process from the ground state to the zero vibrational mode of the lowest excited state are obtained as described below. 

Figure 5,2. A, Poly p-phenylene) and By 12cas-polyacetylene. These compounds arCy respectively y prototypes for polymers with nondegenerate aromatic structure) and degenerate quinoid structure) ground states. The symbol E denotes...

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