Design and Synthesis of New Acceptor Molecules for Photo-Induced Electron Transfer Reverse Saturable Absorption
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ps). Over the same time span, other research groups have addressed the problem of the efficiency of photo-generated charge state formation in conjugated molecules and polymers. Of particular interest to the problem of optical limiting is a recent review by Saraciftci and Heeger7 on the photophysics of charge separation between conjugated polymers and and fullerenes. C60 is a recognized optical limiter based on RSA from the excited state manifold8-10. Saraciftci and coworkers have also demonstrated that a photo-induced electron transfer can occur between a conducting polymer, such as MEHPPV or poly-3-alkylthiophene, and C60, which produces different absorption species than either component irradiated individually11,12. Light-induced ESR studies indicate formation of both positive (from donor) and negative (from acceptor) polarons., with concomittant quenching of the photoluminescence. While the above studies were carried out by optically pumping the donor, ideally optical pumping of the acceptor could lead to the same excited state manifold. Thus RSA behavior in a D/A pair should be observed by either photo-induced electron transfer or photo-induced hole transfer7. Janssen and coworkers13 have been able to demonstrate photo-induced absorption (PIA) in a series of oligothiophenes in solution, wherein both triplets and radical-cations could be identified. Addition of TCNE quenched the triplet state formation and efficiently produced radical-cations. Other conjugated polymers formed long-lived charge-separated states in the presence of a series of TCNQ derivatives, and finally, composite films of C60 and the oligothiophenes display PIA spectra characteristic of polymer radical-cations and fullerene radical anions14. Examples of donor polymers and electron-acceptors that display this PIA behavior is illustrated in Figure 1. NC
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Figure 1. Typical acceptors and donor conjugated polymers that display PIA. We have recently been examining dendrimers based on bis-(diphenylamino)stilbene repeat units as efficient donor species for PIA RSA. The synthesis of the two G-0 dendrimers, 1 and 2, have been reported recently, and these materials are of interest for OPL due to their stable, highly absorbing bipolaronic charge states15. The structures of 1 (a three-arm dendrimer) and 2 ( a four arm dendrimer are shown below. We have irradiated these dendrimers in solution in the presence of C60, and observed efficient PIA, and the nature of the charge-states formed (polaronic or bipolaronic) is currently under investigation16. For the design of practical optical limiting devices, however, it will be necessary to fabricate donor/acceptor pairs as polymer films. We recently have been able to demonstrate that dendrimer 1 can be vacuum deposited as a thin film. The dendrimers can also be spin-coated from solution. While C60 is a well-studied and useful
acceptor for OPL studies, its relative lack of solubility in common organic solven
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