Poly(p‐phenylene ethynylene) Incorporating Sterically Enshrouding m‐Terphenyl Oxacyclophane Canopies

Abstract

A sterically encumbered m-terphenyl oxacyclophane substituted with two aryl iodide substituents has been prepared as a versatile monomer for the preparation of π-conjugated polymers. The monomer has been used to prepare a poly(p-phenylene ethynylene) derivative (P1) incorporating oxacyclophane units as canopies that shield one side of the π-system from inter-chain interactions. The photophysical properties of P1 in dilute solution compare well to those of a poly(p-phenylene ethynylene) derivative (P2) that lacks the canopy. The presence of the steric canopy leads to a diminished inter-chain interaction in the solid state and enhances the kinetic response of P1 to vapors of nitro-organics such as TNT, presumably by increasing the permeability of P1 to these analytes over that of P2.

Covalently Scaffolded Inter‐π‐System Orientations in π‐Conjugated Polymers and Small Molecule Models

“…[39] This dual inter-p-system interaction/steric canopy module strategy was recently extended by fashioning a sterically encumbered m-terphenyl unit into an oxacyclophane for incorporation into PPE derivative 9 ( Figure 9). [42] Despite the significantly larger macrocyclic scaffold in 9 versus that in 8a, its inter-p-system distance is only slightly larger (3.41 Å in 9 versus 3.33 Å in 8a). This is likely due to the greater conformational mobility of the larger macrocycle, which also explains its ability to accommodate an interplanar angle of 288 (cf.…”

Poly(p‐phenylenevinylene) Derivatives with Defined Conjugation Segments and Post‐Polymerization Modification with Sterically Enshrouded Chromophores

“…The m-terphenyl unit has been selected to provide the desired steric cleft. Such m-terphenyl units have found increasing utility as steric protecting groups [19][20][21] to shield chromophores from extraneous interp-system interactions in CPs (Figure 1A [22] and B [23] ) and in ligandmodified scaffolds for conducting metallopolymers ( Figure 1C) [24] as a means of providing steric coordination control against metal-induced cross-linking. [25] Results and Discussion…”

Conjugated Polymers Featuring Oxacyclophane‐Scaffolded π‐Stacking Interactions

“…Polymers feature significantly longer π‐conjugated multi‐aryl segments than small molecules, thus the relative contribution of intermolecular π–π interactions to spectroscopic properties will be greater for polymers. Mixed methoxy/hexyloxy P1, prepared as a baseline CP for spectroscopic properties exhibited by a PPE when no steric constraints on interchain interactions are present, displayed λ max = 427 nm and λ em = 462 nm. Replacing the methyl substituents in P1 with the terphenyl canopy in P2 did not affect the absorption wavelength but produced a 10 nm bathochromic shift in fluorescence ( λ em = 472 nm).…”

“…Theoretical studies have suggested that a face‐to‐face π‐stacking interaction with a plane‐to‐plane distance of 3.2–4.0 Å is ideal for facilitating charge mobility through a CP film . Some investigators have sought to alleviate the complication of inter‐π‐system interaction influences on polymer properties by appending the polymers with sterically encumbered substituents to prevent close contacts between polymer chains . Other researchers have devised molecular scaffolding and covalent crosslinking strategies to constrain adjacent polymer chains in near‐ideal orientations to improving charge/energy transport through a material .…”

“…Other researchers have devised molecular scaffolding and covalent crosslinking strategies to constrain adjacent polymer chains in near‐ideal orientations to improving charge/energy transport through a material . Two of the scaffolding most relevant to the current work are the [2.2]paracyclophane (i.e., in P3 , Figure B) and the oxacyclophane (i.e., in M5 , M6 , CyM , and CyP , Figure ) scaffolding units. The [2.2]paracyclophane unit enforces a rigid face‐to‐face π‐stacking interaction between its two phenyl rings at a separation of 2.98 Å, a distance smaller than what can be achieved with polymer chains that are not covalently scaffolded .…”