Optical Power Limiters for Nanosecond Pulses: Design of New Dendritic Chromophores with Exceptionally Large Two-Photon C
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Extend the conjugation length Change the identity of the conjugated bridge Increase the it-donor strength Incorporate more polarizable double bonds Increase the planarity of the chromophore
By applying these design concepts, the best compounds to come out of these studies are illustrated below. However, it should be recognized that these workers carried out their measurements at only one wavelength (800 nm), and only with nanosecond pulses. Therefore, it is most probable that these cross-sections are enhanced by excited state absorption, and that they may not represent the maximum TPA achievable for that structure. 339 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
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heterocyclic e-ecceptor
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Figure 1. Typical structural motifs for enhanced two-photon absorption (Reinhardt). Perry, Marder and coworkers57 have identified similar design parameters, but based on their observation of the effect of strong donor substitution on stilbene derivatives. These workers originally noted that bis-(di-n-butylamino)-E-stilbene had a two-photon cross-section almost 20 times that of stilbene itself, which they related to increased electron delocalization in the first excited state, and subsequent increases in the S, to S2 transition dipole moments. This, and similar observations in PPV dimer (oligomer) models, has led to design criteria for enhanced TPA. Two distinct chromophore types emerge from these considerations, where D and A represent donor and acceptor moieties: Type I: D-A-D Type II: A-D-A Type 1 is typified by a PPV dimer with dibutylamino end groups and CN substitution on the central ring, while Type 2 chromophores are typified by reversing the positions of the D and A groups, as illustrated below. These workers also ndte that the peak of two-photon absorption occurs at wavelengths significantly shorter than 2 times the one photon absorption, thus indicating that the two-photon state is energetically above the lowest Bu one-photon-state. They also noted that two-photon excited state absorption generates a larger "effective" two-photon absorption when ns pulses are compared to ps pulses. This underscores the fact that one must pay particular attention to the conditions under which measurements of cross-section were made when comparing one chromophore to another.
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DESIGN OF DENDRITIC MATERIALS FOR TWO-PHOTON ABSORPTION Most materials proposed as enhanced two-photon absorbers are relatively small molecules, as illustrated in the work described above from the Reinhardt and Perry/Marder groups. For optical limiting applications, however, the two-photon materials need to be incorporated into polymeric or thin film form. While it is possible to functionalize a TPA chromophore for attachment as a pendant group to typical polymers such as PMMA, it is tedious and expensive. In addition, it is unlikely that chromop
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