Highly Ordered Crystalline Thin Film Bilayers of Para-Hexaphenyl and C 60 Grown by Hot Wall Epitaxy
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N
Neutral
N
Absorption Max. = 389 nm
SbCl 5
N
+
+ Bipolaron Absorption Max. = 689 nm
N
2 SbCl 6-
SYNTHESIS OF DENDRONS AND DENDRIMERS INCORPORATING DIPHENYLAMINOPOLYENYLIC MOIETIES The functionalized polyenes described above can be converted to reactive dendrons as illustrated below in Figure 1 for the n = 1 chromophore. The corresponding dendrimers can be formed by coupling two equivalents of the respective dendrons to a core molecules such as bisphenol A. The resulting G-0 dendrimers contain four reactive chromophore units. We have previously shown2 that oxidative doping of either the chromophore or the dendrons with SbCl5 in solution form bipolarons whose spectra are almost identical. The dendrimers also display the same behavior. For n = 1, the neutral absorbtion is ca. 380 nm, while the bipolaron exhibits strong absorption at ca. 580 nm after oxidative doping. The dendrimers can be isolated and purified by column chromatography (silca gel) and are obtained in good yield (40-60 %). They are soluble in common organic solvents (CH2Cl2, THF, etc.). POTENTIAL FOR OPTICAL POWER LIMITING APPLICATIONS Rapid advances in laser technology have resulted in the need for “smart” passive materials which have a high degree of transparency under ambient conditions, but can undergo rapid structural change to a highly absorbing transient state whose cross-section is much larger than the original π−π* transition8. Materials in this category are usually referred to a reverse saturable absorbers, and several families of materials have been shown to exhibit such behavior, such as porphyrins, phthalocyanines and fullerenes8. The vast majority of these RSA materials derive their capability from triplet states resulting in fast, efficient intersystem crossing from the S1 state initially poulated by the incident laser irradiation. However, much less attention has been paid to other possible nonlinear absorption transient states.
HO 2 equiv,
N
Br + HO
OH
N
K2CO3 / 18-C-6
HO
N
O
O
N
Dendron for Convergent Dendrimer N
Syntheses
N
Figure 1. Formation of model dendron for convergent synthesis In general bipolaronic dications are more highly absorbing than the original π−π* neutral polyene absorption, which suggests their potential to act as RSA optical limiters if these same charge states can be photo-generated rather than by oxidative doping in solution. We have been intrigued by the photophysical potential of dendrimers due to their relatively high chromophore concentration and the ability of all four chromophores in the G-0 dendrimer to be converted to charge states in entirety. At the current time the three G-0 dendrimers discussed above are being evaluated as limiters following a protocol for charge state generation described by Saraciftci and Heeger9 in which C60 is mixed with the donor macromolecule in solution or thin film. Donor pumping (e-transfer) or acceptor pumping (hole transfer) by the laser results in fast, efficient etransfer, generating the highly absorbing transient charge states. We are currently
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