Coherently Aligned Porphyrin‐Appended Polynorbornenes

Abstract

Six different kinds of coherently aligned porphyrin-appended polynorbornenes derived from 5,6-endo-fused N-arylpyrrolidenonorbornenes have been synthesized. Pi-pi interactions between the pendant groups are essential for dictating the photophysical properties of the polymers and the mechanism for the stereoselective formation of polymers. Splitting of the Soret band of polymers 2a-c, which have alkyl-substituted porphyrin pendant groups, suggests strong exciton coupling between chromophores. No splitting of the Soret band is observed for polymers 2d-f, which have tetraaryl substituents on the porphyrin moiety. Significant fluorescence quenching is found in polymers 2a-e, whereas only slightly reduced quantum yield is observed for 2f. Time-resolved fluorescence measurements also indicate a similar trend. The AFM image of 2d on graphite shows aggregation to form a two-dimensional, ordered pattern.

Hydrogen‐Bonding Induced Cooperative Effect on the Energy Transfer in Helical Polynorbornenes Appended with Porphyrin‐Containing Amidic Alanine Linkers

“…15 These results indicate that the alkenes in the polymer backbone have trans configuration. Similar to those of homopolymer 2a reported previously,11a certain 1 H NMR signals in 2b , 2c , 2d , 2e , 2f , 2g , 2h , 2i , 2j and 3 , 4 , 5 , 6 appeared at higher field and with significant broadening relative to those of the corresponding monomers 1 . Presumably, the anisotropic shielding arising from the adjacent porphyrin moiety may be responsible for such shifts.…”

“…The 1 H NMR spectra of monomers 1 and polymers 2 – 7 are worthy of comment. The olefinic protons in the polymeric backbones appeared in the 1 H NMR spectra around δ =5.2 ppm as a broad symmetrical peak, which is similar to those of related polymers 11a. 15 These results indicate that the alkenes in the polymer backbone have trans configuration.…”

“…The relatively rigid polymeric backbone of PNBs provides a supporting framework for the pending groups coherently aligned towards the same direction 11–16. We recently established that PNBs having endo ‐fused N ‐arylpyrrolidene pendants, obtained by Grubbs I catalyst mediated ring‐opening metathesis polymerization17 of the corresponding norbornene derivatives, adopt comblike structures with homogeneous tacticity and all double bonds in trans configuration 11a. 15 The corresponding double‐stranded poly(bisnorbornene)s or ladderphanes were prepared accordingly,18 single‐handed helical polymers19 being obtained when the chiral substituents are incorporated into these polymers.…”

“…Each of the monomeric units spans about 5.5 Å in these polymers 15a. Exciton coupling,11a, 18c excimer formation,18c fluorescence quenching,11a or enhancement in second‐order optical nonlinearity15 readily occurs in these polymers when photoactive chromophores are used as pendants or linkers. For example, PNBs with certain porphyrin pendants exhibit Soret band splitting and significant fluorescence quenching owing to interactions between these chromophores 11a.…”

“…Exciton coupling,11a, 18c excimer formation,18c fluorescence quenching,11a or enhancement in second‐order optical nonlinearity15 readily occurs in these polymers when photoactive chromophores are used as pendants or linkers. For example, PNBs with certain porphyrin pendants exhibit Soret band splitting and significant fluorescence quenching owing to interactions between these chromophores 11a. 18c It is envisaged that PNBs containing both donor and acceptor pending chromophores may undergo efficient energy and electron transfer between these photoactive species.…”