Design, Synthesis and Characterization of New Bimechanistic Optical Power Limiters Based on Reverse Saturable and Two-ph
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Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Materials Research Society
thereby giving access to the highly absorbing triplet state manifold 5". However, much less attention has been paid to other possible nonlinear absorption transient states. For several years our research group has been studying the formation and stabilization of polaron-like radical 3 cations and bipolaron-like dications by the oxidative doping of neutral polyenes in solution'- . In solution, the dication appears to be the dominant species, particulary if mesomerically stabilized by electron-donating substituents. Most recently we have synthesized a series of bis(diphenylamino)diphenylpolyenes that form exceptionally stable, highly absorbing bipolarons, even the stilbene derivative"4 . This is by far the shortest stable bipolaron yet studied, and the bipolaron formation is illustrated below, along with the n-7n* maxima for both neutral and bipolaron forms illustrating the dramatic red-shift in oscillator strength upon doping.
Absorption Max. =389 nm
Y+
SbCIs
2 SbCls-
Absorption Max. = 689 nm
In almost all cases studied, both the polaronic radical-cation and bipolaronic dication are more highly absorbing than the corresponding So to S, initial one-photon process. The question of the ability of these charge states to function as RSAs is whether they can be efficiently photogenerated on an appropriate time scale for optical limiting. Over the past few years, other research groups have addressed the problem of the efficiency of photo-generated charge state formation in conjugated molecules and polymers. Saricitci and Heeger have studied charge transfer between conjugated polymers such as MEHPPV and poly[3-alkylthiophene] and electron acceptors such as C60 15 ,16. Light-induced ESR measurement suggested the formation of Donor radical-cations and Acceptor radical-anions (polarons). These workers have suggested that optical pumping of either the donor or acceptor could in theory give access to the excited state mainifold RSA by either photo-induced electron transfer or photo-induced hole transfer"5 . Janssen and coworkers"7 were able to demonstrate PIA in a series of oligothiophenes in solution due to both metastable triplet and oligothiophene radical cations (polarons). Addition of an electron acceptor such as TCNE quenches the triplet state formation and efficiently produces the radical-cations. The nature of the transient species were also solvent dependent. Thus to a certain extent the identity and absorption characteristics of the photo-induced transient species can be controlled by such things as the solvent and identity of the acceptor species. It should also be noted that composite films of C60 and oligothiophenes display PIA spectra characteristic of 370
the oligomer radical-cations and C60 radical-anion. Thus charge state generation in thin film configuration holds promise for practical OPL devices. THE DESIGN OF POTENTIAL BIMECHANISTIC OPTICAL LIMITERS At the current time the two approaches to organic optical power limiters involve b
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