Synthesis of Doubly β‐to‐β 1,3‐Butadiyne‐Bridged Diporphyrins: Enforced Planar Structures and Large Two‐Photon Absorption Cross Sections

Abstract

Covalently linked p-conjugated porphyrin oligomers are materials of great importance that display efficient energy and electron transfer, single-molecule conductivity, and nonlinear optical (NLO) properties.[1] Among them, peripherally ethyne-and 1,3-butadiyne-fabricated porphyrins hold a central position owing to their attractive electronic properties. [2][3][4][5] As representative examples, Therien and co-workers reported large hyperpolarizabilities for push-pull meso,-meso'-bis-arylethynylated porphyrins [3] and Anderson et al. reported large two-photon-absorption (TPA) properties of meso-ethyne-and meso-butadiyne-bridged porphyrin oligomers.[4] Most of the electronic properties of these conjugated porphyrins are considered to depend on the extent of overall p conjugation, which is a function of molecular coplanarity. However, there is a continuous distribution of rotational isomers because of small rotational barriers of triple-bond linkages [6] which duplicates their photophysics. [7] As an interesting approach, conformational control has been achieved with the aid of noncovalent interactions between a zinc porphyrin and a pyridine coordination site. [4b, 8] Herein, we report the synthesis of doubly b-to-b butadiyne-bridged diporphyrins as the first example of multiply 1,3-butadiyne-bridged diporphyrins. An apparent advantage of this double bridging strategy is a robust, enforced overall planar conformation, which causes significant enhancements of TPA properties. Large TPA compounds attract considerable interest for several innovative applications such as threedimensional (3D) optical memory, deeper-penetrating photodynamic therapy (PDT) of cancer, 3D microfabrication, and others. [9] Diporphyrins 1 and 2 were prepared from b-borylated porphyrins 3 a and 3 b[10] (Scheme 1). Treatment of mono-and diborylporphyrins 3 a and 3 b with Oxone in a THF/acetone/ water suspension [11] successfully gave the corresponding mono-and dihydroxyporphyrins 4 a and 4 b in 80 % and 88 % yields, respectively. The trifluoromethanesulfonyl derivatives were prepared in DMF by reaction with PhNTf 2 and Cs 2 CO 3 , [12] and following metallation with Zn(OAc) 2 6 aZn and 6 bZn were obtained in excellent yields. Installation of ethynyl groups was conducted through the standard Sonogashira coupling with trimethylsilylacetylene followed by desilylation to provide 8 aZn and 8 bZn in good yields. Then, 8 aZn and 8 bZn were subjected to a Cu II -mediated oxidative coupling reaction to furnish the corresponding butadiynebridged porphyrin dimers 1 Zn and 2 Zn in 82 % and 89 % yields, respectively. Demetallation with trifluoroacetic acid provided 1 H and 2 H in good yields. Synthesis of 1 Ni and 2 Ni was achieved in a similar fashion to 1 Zn and 2 Zn from Ni II triarylporphyrin as the starting material.b-to-b-Linked diporphyrins have been explored to examine the influences of connectivity difference on the electronic interactions between the porphyrins, [1e, 2a, 6a, 13, 14] but 2 H and its metal complexes (2 Zn and 2 Ni) are the fi...

Subporphyrins: A Legitimate Ring-Contracted Porphyrin with Versatile Electronic and Optical Properties

“…Representative examples synthesized through this method include ¢,¢-bis(acrylyl)-substituted porphyrins that can serve as an effective sensitizer in solar cells, 118,119 multi-porphyrin oligomers, 120,121 planar doubly 1,3-butadiyne-bridged diporphyrins, 122 heterocycle-bridged porphyrin rings, 123125 2-borylated corroles, 126 biscorrolebased stable singlet biradicals, 127,128 face-to-face dioxoisobacteriochlorin dimers, 129 porphyrin pincer complexes that exhibited catalytic activity in Heck reactions, 130 Ptpincer complexes that undergo tweezers-like molecular motions responding to the oxidation state of the incorporated Pt metal, 131 and © 2 coordinating Ru(II) complexes. 132 Recent remarkable examples are ¢,¢-doubly linked porphyrin belts with significant molecular curvatures 133 and ring forming porphyrin barrels, 134 both of which can capture C 60 efficiently.…”

Structural Aspects of Porphyrins for Functional Materials Applications

“…Figure 17. A doubly β-to-β 1,3-butadiyne-bridged TPP-based dimer, from data in [7,53]. Each zinc has one axially bound isopropanol ligand.…”

A Cyclic Porphyrin Tetramer Linked by Azo and Butadiyne Bridges

“…The butadiyne and other linkers have been studied extensively, with numerous papers reporting interesting results with potential in various fields, [7,10, including recently two-photon absorption [31,34,[37][38][39][40][41][42][43][44] and two-photon photodynamic therapy. [14,15,41,45] Anderson suggested that an azo linkage would provide the optimum degree of interporphyrin conjugation and be a good bridge for facilitating electronic communication between the porphyrins in the ground state. [29] The azo bridge provides a short distance between the porphyrin rings and a conjugated linker with little steric hindrance.…”