Photonic Properties of Dendrons and Dendrimers Incorporating Bis-(Diphenylphosphino)Diphenylpolyenes
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HENYLPHOSPHINO-SUBSTITUTED POLYENES, PPV-OLIGOMERS AND DENDRIMERS During the past two years we have reported the syntheses and absorption characteristics of a series of bis-(diphenylphosphino)diphenylpolyenes9,10. In all cases the π−π* transition is blueshifted from the corresponding diphenylaminopolyene by ca. 40-50 nm. We were also able to synthesize the bis-(diphenylphosphino)-PPV dimer and also found a 30 nm blue shift. More recently we synthesized the P equivalent of a dendrimer based on bis-(diphenylamino)-E-stilbene repeat units, finding an amazing 60 nm blue-shift! Regardless of π-conjugated structure, the blue shift in the absorption spectra seems to be general when a diphenylphosphino group replaces a diphenylamino group, the exact opposite that one observes in the S for O replacement in donor groups. The structures for the materials previously studied are illustrated below.
P
P (
)n
P n = 1-5 P
P
P
P
P
SYNTHESIS OF MIXED P/N POLYENES AND DENDRIMERS One observation that has potential ramifications for the use of the diphenylphosphino substituted chromophores in optical limiting or as a hole-transport or emitter material in organic light-emitting diodes is that the fluorescence in the all-N dendrimers seems brighter than the allP dendrimer (detailed quantitative photoluminescence measurements are in progress). Also, we have not been able to generate bipolarons with SbCl5 in solution as we accomplished for the diphenylamino counterparts. Therefore, we have synthesized both a mixed P/N polyene and the two possible mixed P/N G-0 dendrimers via Wittig methodology. Both dendrimers were obtained in good yield and are soluble in common organic solvents. The synthesis of the mixed
P/N-stilbene is shown in Figure 1. The mixed dendrimer syntheses, abbreviated NP3-D and PN3D are shown in Figures 2 and 3.
N CHO PO(OEt) 2 P
KOt-Bu
N
+
P
THF
Figure 1. Synthesis of a mixed diphenylphosphino, diphenylamino-stilbene
HOC
CHO
N
PO(OEt) 2 KOt-Bu P
N +
THF (77%)
CHO
P
N
Figure 2. Synthesis of a mixed N3 P-dendrimer (G-0).
N
P HOC
CHO PBu3 ,Br + N
+
1. KOt-Bu/THF
P
(71%)
BH 3
2. Et2 NH (90%)
CHO
N
P
P
Figure 3. Synthesis of a mixed P3N-dendrimer (G-0). OXIDATIVE DOPING OF SUBSTITUTED STILBENES We were initially interested in the bis-(diphenylphosphino)diphenylpolyene series in our continuing quest for stable, highly absorbing charge states for optical limiting and other potential photonic applications. However, we found that bis-(diphenylamino)- and bis(diphenylphosphino) substituted diphenylpolyenes behaved very differently when subjected to the normal oxidative doping protocol with SbCl5 in solution. As we have previously reported1 , the bis-(diphenylamino)stilbene forms an exceptionally stable bipolaron upon doping, the smallest bipolaron yet formed that is stable under ambient room conditions. The P equivalent, however, does not form stable charge states under the same conditions. Although at the current time we are not sure of the rationale for this dramatic difference in reactivity
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