Ethynylene - bridged Porphyrin Fluorene Composites

Porphyrin-fluorene conjugates bridged by ethynylene were synthesized through Pd-catalyzed cross-coupling reaction. These molecules showed strong nonlinear absorption when measured using a nanosecond Z-scan method. The largest effective two-photon absorption cross-section was observed for trisporphyrin as 2.7 × 105 GM at 760 nm, which was 10 times larger than that obtained for monoporphyrin. INTRODUCTION Recently, much attention has been focused on the organic dye molecules in view of optical applications such as nonlinear optics including twophoton absorption (2PA) materials.1 Novel functions will be expected by connecting two or more chromophores even if a monomeric dye itself does not exhibit a significant function. We have developed two-photon absorbing molecules using self-assembled multi-porphyrin arrays and found that the connection of π-conjugation between porphyrins with ethynylene bridges is the most significant for the 2PA enhancement.1a, 1b, 2 However, the expansion of π-conjugation raises large red-shift of one-photon absorption (1PA) up to 2PA range.2g In order to prevent such the overlapping of 1PA and 2PA, it is necessary to control the position of one-photon absorption band by tuning the interaction between chromophores. Fluorene is a well-known π-spacer for two-photon absorption materials3 and provides an angle of 160o between ethynylporphyrins when these are connected at 2and 7-positions of fluorene. Previously, we reported fluorene-bridged bisimidazolylporphyrins 1 having two allyl groups at meso-positions have been synthesized to construct macro cyclic arrays 2. The self-assembled structure in the macro cycle was fixed by the ring-closing metathesis reaction of the allyl groups to investigate the effect of cyclization on 2PA by a femtosecond fluorescence method in the range between 800 and 875 nm.2f Further, linear polymers self-organized by fluorene-bridged bisimidazolylporphyrins 3, and their 2PA properties in the range from 740 to 930 nm were reported.4 However, the π-conjugation was disconnected between chromophores at the selfcoordination sites. So, we designed new fluorenebridged porphyrin arrays 4 – 6 without the self-