Enhanced π Conjugation around a Porphyrin[6] Nanoring

Abstract

Dedicated to Professor Jeremy Sanders on the occasion of his 60th birthdayBelt-shaped chromophores provide fascinating insights into electronic p delocalization over curved surfaces with radially oriented p orbitals.[1] Examples include the cyclic para-phenylacetylenes [2] and the [4 6 ]paracyclophanedodecayne of Tsuji and coworkers, [3] as well as fullerenes and carbon nanotubes. A variety of belt-shaped porphyrin arrays have been synthesized; [4] however, the vast majority of them lacks a complete pconjugation pathway around the whole macrocycle. Recently we reported the synthesis of a belt-shaped D 8h symmetric porphyrin[8] nanoring on an octadentate template.[5] Herein we present an efficient synthesis of an even more strained p conjugated D 6h porphyrin [6] nanoring 1, by template-directed trimerization of a porphyrin dimer 2 on a hexapyridyl template 3 (Scheme 1). This route is more direct than the synthesis of the cyclic octamer since both starting materials, 2 and 3, are readily accessible. The cyclic hexamer complex 1·3 is phenomenally stable (K f = 7 10 38 m À1 ; EM = 340 m) but the free macrocycle can be liberated from the 1·3 complex with amines such as quinuclidine. The UV/Vis/NIR absorption and emission show that there is efficient p conjugation around the porphyrin[6] nanoring 1, and that its S 0 -S 1 gap is even smaller than that of the corresponding linear porphyrin hexamer 4; this conclusion is supported by time-dependent density functional (TD-DFT) calculations.The key to the synthesis of the hexamer nanoring is the design of a complementary template (Scheme 2). The hexadentate template 3 has a calculated nitrogennitrogen distance of 20.1 , which is a good fit for the cavity of cyclic hexamer 1 with a zinc-zinc ring diameter of 24.2 (assuming a Zn À N bond length of 2.2 ). The template was synthesized by a six-fold Suzuki coupling of 4-pyridineboronic acid with hexakis(4-bromophenyl)benzene [6] in 50 % yield (Supporting Information). Two versions of cyclic hexamer 1 were synthesized-1 a·3 with tert-butyl side chains in 44 % and 1 b·3 with octyloxy side chains in 33 % yield-by oxidative

Supramolecular nesting of cyclic polymers

“…We investigated the palladium-catalysed oxidative coupling of the linear zinc-porphyrin 10-mer l-P10 in the presence of hexa-pyridyl template T6 23,34 and octa-pyridyl template T8 35 ( Fig. 1).…”

Macrocyclization Reactions: The Importance of Conformational, Configurational, and Template-Induced Preorganization

“…Using this approach, Anderson and co-workers have described an efficient templated synthesis of the strained π conjugated D 6h porphyrin [6] nanoring 518 where the template 517 is bonded to the building blocks 516 by the formation of Zn−N(pyridine) bonds. 506 The stepwise effective molarities justify the good macrocyclization yields (33−44%) ( Figure 160). 507 It was found that the effective molarity in the presence of the hexameric template 517 is in the range 100−1000 M and they have demonstrated that this is a cooperative process; i.e., each step is more favored than the previous one.…”

“…Templated porphyrin [6] nanoring synthesis. 506,507 Chemical Reviews Review DOI: 10.1021/acs.chemrev.5b00056…”

All‐Carbon Scaffolds by Rational Design

“…[66,67] Worth mentioning are also the great variety of stable metallamacrocycles, obtained largely by insertion of Pt(II) into the dehydroannulenes under formation of bis(s-alkynyl)Pt(II) linkers. [68,69] While all these efforts have not led to regular 2D all-carbon polymers, which currently remain ''just a dream of synthetic chemists,'' [70] the search for new modifications of carbon has produced several new classes of macrocycles that feature conjugated all-carbon backbones without annellated benzene rings and display highly interesting optoelectronic properties.…”