Template-Directed Synthesis, Excited-State Photodynamics, and Electronic Communication in a Hexameric Wheel of Porphyrins

Abstract

To investigate new architectures for molecular photonics applications, a shape-persistent cyclic hexameric architecture (cyclo-Zn3Fb3U-p/m) has been prepared that is comprised of three free base (Fb) porphyrins and three zinc porphyrins linked at the meso-positions via diphenylethyne units. The synthesis involves the Pd-mediated coupling of a p/p-substituted diethynyl Zn porphyrin and a m/m-substituted diiodo Fb porphyrin, forming p/m-substituted diphenylethyne linkages. The isolated yield of cyclo-Zn3Fb3U-p/m is 5.3% in the presence of a tripyridyl template. The array has C 3 v symmetry, 108 atoms in the shortest path, and a face-to-face distance of ∼35 Å across the cavity. The excited-state lifetime of the Zn porphyrin in cyclo-Zn3Fb3U-p/m is 17 ps, giving a rate of energy transfer to each adjacent Fb porphyrin of k trans = (34 ps)-1 and a quantum efficiency of Φtrans = 99.2%. This rate is comparable to that in a dimer (ZnFbU-p/m) having an identical linker, but slower than that of a p/p-linked ZnFb dimer, which has k trans = (24 ps)-1. At ambient temperatures, the hole/electron hopping rate in [cyclo-Zn6U-p/m]+ is comparable to or faster than the EPR time scale (∼4 MHz). The hole/electron hopping rate in [cyclo-Zn6U-p/m]+ appears to be more than 2-fold larger than for [Zn2U-p/m]+; [Zn2U-p/m]+ has a rate at least 10-fold slower than for the p/p-linked dimer [Zn2U]+. Both excited-state energy transfer and ground-state hole/electron hopping proceed via through-bond mechanisms mediated by the diphenylethyne linker. The origin of the slightly slower energy-transfer rate, and substantially slower ground-state hole/electron hopping rate, in the p/m-linked arrays versus the p/p-linked analogues, is attributed primarily to the larger electron density of the frontier molecular orbitals at the p- versus m-position of the phenyl ring in the diphenylethyne linker. Collectively, these results indicate that the site of attachment of the porphyrin to the linker could be used to direct energy and/or hole/electron flow in a controlled manner among porphyrins in diverse 3-dimensional (linear, cyclic, tubular) architectures.

Shape-Persistent, Nano-Sized Macrocycles

“…Lindsey examined the Sonogashira coupling of bis(p-ethynylphenyl)-derived porphyrin 63 and bis(m-iodophenyl)-derived porphyrin 64 under templating conditions (Scheme 22). [54,55] In the presence of the rigid tridentate templates 62a or 62b, one of these precursors was used in the metal-free state, while the other was complexed with Zn or Mg and therefore susceptible for the template. The best results were found for treatment of metal-complexed 63b and 63c with metal-free 64a in the presence of terdentate pyridine template 62a.…”

“…Here, the axial coordination sites of porphyrinmetal (mostly zinc) complexes is used to bind guests such as pyridine derivatives. On the basis of Sanders' early fundamental work, [72aϪ72d,117Ϫ119] Lindsey described the use of guest molecules as templates for the directed synthesis of 28 (see above), [54,55,120] and Gossauer their self-association with cycles 29. [56,57] The binding constants in the latter case are as high as K ϭ 1.1 ϫ 10 10 to 1.4 ϫ 10 9  Ϫ1 .…”

“…D 3 HPLC, m.p., MS, NMR, STM 25d [51,52] 2Ϫ12% n.s. D 4 HPLC, m.p., MS, NMR, STM 26a [51,52] R HPLC, m.p., MS, NMR, STM 26b [51,52] R HPLC, m.p., MS, NMR, STM 26c [51,52] R HPLC, m.p., MS, NMR, STM 27 [43] R m.p., MS, NMR, IR, UV, EA 28a [54] 5.3% 20mg B 6 [e] GPC, MS, 1 H NMR, UV 28b [54] R MS, 1H NMR, UV, fl. 28c [55] 13% [56,57] 30% 1.8mg B 1 MS, 1 H NMR, UV 29b [56,57] 15% 0.95mg B 1 MS, 1 H NMR, UV 29c [56,57] 8% 0.39mg B 1 MS, 1 H NMR, UV 29d [56,57] 31% 1.51mg B 1 MS, 1 H NMR, UV 29e [56,57] R M S , 1 H NMR, UV 1.97 g E 3 1 H NMR, EA, X-ray 36d [63] R MS, NMR, EA 37a [29] 14% n.s.…”

Studying the intersystem crossing rate and triplet quantum yield of meso-substituted porphyrins by means of pulse train fluorescence technique

“…The remarkable optical properties of porphyrins continue to attract significant attention, mainly due to their large excited state absorption [1,2] (particularly in the near-IR part of the spectrum [3]), two-photon absorption [4,5], fast intersystem crossing (ISC) [6], excellent optical stability and biocompatibility [7]. As a result of these features porphyrins have extensively been studied with respect to the development of novel optoelectronic and photonic devices [8].…”

Coordination-Driven, Self-Assembly of a Polycyclic, Terpyridine-Based Nanobelt

“…Also, many porphyrin-based nanostructures with organic linkages [20][21][22][23] have been reported to rely on axially coordinating templates; thus, the coordination-driven construction of porphyrin-based supramolecules [24][25][26][27] can proceed via diverse synthetic routes. With such criteria, combinations of the terpyridine moieties with aryl, semi-rigid, functional scaffolds that impart bent framework angles less than 180°s hould provide designer monomers useful for constructing shape-persistent 3D-supramolecular architectures [28,29].…”