Conjugation length

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

Another important class of electron-transporting emitter is the distyryl-arylenes. These have been explored most extensively by workers at Idemitsu Kosan [31, 32], with the bulk of the published data focusing on a compound designated as DPVBi (see Table 13-2). This class of materials may be considered as small molecule analogs of the PPV polymers, with distyrylbenzene and its derivatives ]33] as prototypical examples. Because of the short conjugation length they tend to be blue emitters. 

The absoiption spectra of these three materials are shown in the bottom panel of Figure 9-16. From these spectra it becomes clear that the m-LPPP shows the longest effective conjugation length 23 the best resolution of vibronic progression, and the steepest onset of absorption [231. Therefore, one would assume the m-LPPP to be a material of the highest chemical definition. This is indeed con-... 

Figure 16 1. Schematic representation of copolymers with well-defined conjugation length A is a conjugated chromophorc, B is an interruption unit (a) alternating copolymer (b) polymer with slereo-chcmically defined non-coplanar linkages.

In contrast to other findings [139], a substantial increase in y is observed upon extending the conjugation length (see 99-101 vs 90-92, Fig. 8). However, this increase in 7 goes along with a considerable increase in molecular mass and, therefore, only a moderate enhancement of the macroscopic which expresses the third-order nonfinearity per unit volume, is observed. 

For the determination of conjugation lengths see Grimme J, Kreyenschmidt M, Uckert F,... 

From UV/Vis investigations of this series of monodisperse, oligomeric model compounds (30) the effective conjugation length of the corresponding polymer... 

The resulting phosphonium salts, 79 (n = 0,l), containing up to 2 phenylenevinylene moieties, were then coupled with terephthalic dialdeyde, 80 (m = 0), or 4,4 -stilbene dicarbaldehyde, 80 (m=l), to produce the target structures, terminally f-butylated OPVs (78) up to the heptamer (x=6). With results based on these model oligomers, the effective conjugation length of unsubstituted PPV 60... 

Fig. 2. A poly(phenylacetylene) dendrimer with an energy gradient across the conjugation length of the acetylene units from the periphery to the focal point...

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