Functionalized [2.2]Paracyclophanedienes as Monomers for Poly(p-phenylenevinylene)s.

Poly(p-phenylenevinylene)s (PPVs) featuring complex side-chains, to date, have only been synthesized by nonliving polymerization methods which have no control over PPV molecular weights, dispersities, or end groups. [2.2]Paracyclophane-1,9-diene (pCpd) has gained attention as a monomer for its ability to be ring-opened to PPV in a living fashion. pCpd, an organic cyclic scaffold with planar chirality, has seen minimal structural diversity due to the harsh reaction conditions required to afford the highly strained compound. Herein, we introduce a general method to overcome this by targeting the synthesis of a monohydroxy-pCpd via mono-demethylation of a dialkoxy-pCpd. The monohydroxy-pCpd can then be functionalized easily, which we demonstrate using three distinct side-chains with four moieties commonly incorporated in conjugated polymers: an alkyl bromide, an oligo(ethylene glycol) chain, an enantiomerically pure side-chain, and a Boc-protected amine. These monofunctionalized-pCpds were investigated as monomers in the ring-opening metathesis polymerization (ROMP) to afford functionalized PPVs in a living manner. The functional-group-containing PPVs are synthesized with full control over their end groups, repeat units, and dispersities. The feasibility of post-polymerization modifications to incorporate any desired moiety to PPV fabricated by this method was demonstrated using an azide-alkyne click reaction. All synthesized PPVs were soluble in organic solvents and display the same fluorescent emission, indicating their conjugated backbones are unaltered.

Bent and Twisted: Synthesis of an Alkoxy-Substituted (1,5)Naphthalene-paracyclophanediene.

This contribution describes the synthesis of [2.2](1,5)naphthalenoparacyclophane-1,13-diene in four steps from 1,5-bis(bromomethyl)naphthalene and 1,4-benzenedimethanethiol. Consisting of 2,6-dioctyloxynaphthalene and benzene moieties, the effects of differing arene size on the structure, strain energy, and chemical reactivity of the cyclophanediene are examined. Despite a strain energy of 24.3 kcal/mol, the naphthalenoparacyclophanediene was unreactive toward a library of olefin metathesis catalysts. This diminished reactivity can be explained by the steric hindrance of the twisted olefin. Incorporation of an electron donor (naphthalene) into the rigid paracyclophanediene structure can allow for applications in optoelectronics, chiral ligands, and planar chiral materials.

Synthesis and Polymerization of an ortho-para-Substituted Tetraalkoxy [2.2]Paracyclophane-1,9-diene.

This contribution describes the synthesis of an unsymmetrical substituted tetraalkoxy[2.2]paracyclophane-1,9-diene comprised of an ortho-substituted and a para-substituted dioctyloxybenzene. (Sp)-4,5,12,15-tetraoctyloxy-[2.2]paracyclophane-1,9-diene ((Sp)-pCpd) and (Rp)-4,5,13,16-tetraoctyloxy-[2.2]paracyclophane-1,9-diene ((Rp)-pCpd) are formed as planar chiral enantiomers. Unlike other tetraalkoxy-substituted pCpds that form as diastereomers, both the (Sp)-pCpd and the (Rp)-pCpd can be polymerized via ring-opening metathesis polymerization (ROMP) using Grubbs' third generation initiator (G3) as it is achiral. Living ROMP afford copolymers featuring alternating cis,trans-poly(p-phenylenevinylene)s (PPV)s. The polymers' unique, blue-shifted optical properties are due to the alkoxy-substitution in the polymer's backbone and the resulting materials could be photoisomerized to the all-trans polymer. This strategy affords tetraalkoxy-pCpd monomers in high yields for the polymerization of soluble PPVs with low or narrow dispersities.

Secondary Structure in Nonpeptidic Supramolecular Block Copolymers.

Proteins contain a level of complexity-secondary and tertiary structures-that polymer chemists aim to imitate. The bottom-up synthesis of protein-mimicking polymers mastering sequence variability and dispersity remains challenging. Incorporating polymers with predefined secondary structures, such as helices and π-π stacking sheets, into block copolymers circumvents the issue of designing and predicting one facet of their 3D architecture. Block copolymers with well-defined secondary-structure elements formed by covalent chain extension or supramolecular self-assembly may be considered for localized tertiary structures.In this Account, we describe a strategy toward block copolymers composed of units bearing well-defined secondary structures mixed in a "plug-and-play" manner that approaches a modicum of the versatility seen in nature. Our early efforts focused on the concept of single-chain collapse to achieve folded secondary structures through either hydrogen bonding or quadrupole attractive forces. These cases, however, required high dilution. Therefore, we turned to the ring-opening metathesis polymerization (ROMP) of [2.2]paracyclophane-1,9-dienes (pCpd), which forms conjugated, fluorescent poly(p-phenylenevinylene)s (PPVs) evocative of ß-sheets. Helical building blocks arise from polymers such as poly(isocyanide)s (PICs) or poly(methacrylamide)s (PMAcs) containing bulky, chiral side groups while the coil motif can be represented by any flexible chain; we frequently chose poly(styrene) (PS) or poly(norbornene) (PNB). We installed moieties for supramolecular assembly at the chain ends of our "sheets" to combine them with complementary helical or coil-shaped polymeric building blocks.Assembling hierarchical materials tantamount to the complexity of proteins requires directional interactions with high specificity, covalent chain extension, or a combination of both chemistries. Our design is based on functionalized reversible addition-fragmentation chain-transfer (RAFT) agents that allowed for the introduction of recognition motifs at the terminus of building blocks and chain-terminating agents (CTAs) that enabled the macroinitiation of helical polymers from the chain end of ROMP-generated sheets and/or coils. To achieve triblock copolymers with a heterotelechelic helix, we relied on supramolecular assembly with helix and coil-shaped building blocks. Our most diverse structures to date comprised a middle block of PPV sheets, parallel or antiparallel, and supramolecularly or covalently linked, respectively, end-functionalized with molecular recognition units (MRUs) for orthogonal supramolecular assembly. We explored PPV sheets with multiple folds achieved by chain extension using alternating pCpd and phenyl-pentafluorophenyl ß-hairpin turns. Using single-molecule polarization spectroscopy, we showed that folding occurs preferentially in multistranded over double-stranded PPV sheets. Our strategy toward protein-mimicking and foldable polymers demonstrates an efficient route toward higher ordered, well-characterized materials by taking advantage of polymers that naturally manifest secondary structures. Our studies demonstrate the retention of distinct architectures after complex assembly, a paradigm that we believe may extend to other polymeric folding systems.

Directed Polymorphism and Mechanofluorochromism of Conjugated Materials through Weak Non-Covalent Control.

Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness "soft" non-covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet fluorescent crystal-to-crystal transitions occur for single crystals of CO2-1 and CO2-3. Finally, the PEs show reversible violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and tunable stimuli-responsive behavior in solids.

Reversible mechanofluorochromism of aniline-terminated phenylene ethynylenes.

Seven three-ring phenylene-ethynylene (PE) structural analogs, differing only in the lengths of alkyl chains on terminal aniline substituents, show 50-62 nm bathochromic shifts in emission maxima in response to mechanical force (mechanofluorochromism, MC). These shifts are fully reversible with heat or solvent fuming. Shearing of these solids yields a transition from green-emitting crystalline phases to orange-emitting amorphous phases as established by differential scanning calorimetry and X-ray diffraction. Molecules with shorter alkyl chain lengths required higher temperatures to recover the hypsochromically shifted crystalline phases after grinding, while the recovery with chain lengths longer than butyl occurred at room temperature. In addition to this structure-dependent thermochromism, these compounds retain their MC properties in polymer hosts to various extents. The crystalline phases of these materials have PE chromophores that are twisted due to non-covalent perfluoroarene-arene (ArF-ArH) interactions involving perfluorophenyl pendants and the terminal rings of the PE chromophore, resulting in interrupted conjugation and an absence of chromophore aggregation. The MC behavior of an analog without the perfluoroarene rings is severely attenuated. This work demonstrates the general utility of twisted PEs as stimuli-responsive moieties and reveals clear structure-property relationships regarding the effects of alkyl chain length on these materials.