Triangular Platinum(II) Metallacycles: Syntheses, Photophysics, and Nonlinear Optics

Three triangular platinum­(II) diimine metallacycles incorporating large cyclic oligo­(phenylene-ethynylene) (OPE) bisacetylide ligands are synthesized, and their photophysical properties are studied. Two types of triplet excited states with ligand/metal-to-ligand charge-transfer and acetylide-ligand-centered characteristics respectively, are exhibited by these complexes depending on the size (conjugation length) and electronic features of the cyclic OPE ligands. When the energy levels of the two excited states are close to each other, the lowest triplet state is found to switch between the two in varied solvents, resulting from their relative energy inversion induced by solvent polarity change. Density functional theory and time-dependent density functional theory calculations provide corroborative evidence for such experimental conclusions. More importantly, the designed metallacycles show impressive two-photon absorption (2PA) and two-photon excitation phosphorescing abilities, and the 2PA cross section reaches 1020 GM at 680 nm and 670 GM at 1040 nm by two different metallacycles. Additionally, pronounced reverse saturable absorptions are observed with these metallacycles by virtue of their strong transient triplet-state absorptions

Conjugated Dimeric and Trimeric Perylenediimide Oligomers

Dimeric and trimeric molecules comprising perylenediimide units conjugatively linked by phenylene, ethynylene, or a butadiynylene spacer via the bay positions were prepared. Electrochemical and photophysical characterizations showed that oligomers connected by C−C triple bond(s) exhibited effectively lowered LUMO compared to the monomer. Molecular modeling confirmed that the C−C triple bond realized efficient delocalization of frontier orbitals, while phenylene was less competent in extending the conjugation, partially due to steric interactions

Theoretical Studies on the Thermodynamic Product Size Distribution in Nucleation−Elongation Polymerization under Imbalanced Stoichiometry

Three different models are developed to calculate the thermodynamic product size distribution in a nucleation−elongation polymerization between a pair of A−A and B−B typed comonomers. These monomers are designed to undergo a single step of nucleation prior to an isodesmic chain elongation, namely, a cooperative, step-growth polymerization with dimerization being an energetically less favored process. Particularly, emphasis is laid on analyzing product distribution under conditions of imbalanced functionality stoichiometry. Consistent results are obtained from independent approaches, mechanistic and statistical, demonstrating that when the mole ratio of the comonomers deviates from unity, at polymerization equilibrium such a nucleation−elongation polymerization generates products of substantially higher molecular weights than those from a corresponding isodesmic system having an identical energetics for chain propagation yet without the nucleation process. This higher molecular weight is shown achieved by retaining a large portion of the excess monomer unreacted at equilibrium and selectively compose product chains with comonomers at a roughly stoichiometric ratio. Essentially, such a polymer−monomer coexisting bimodal distribution is a result from destabilization of the oligomeric species due to the nucleation effect

Cyclo-oligomerization of 6,12-Diethynyl Indeno[1,2‑<i>b</i>]fluorenes via Diradical Intermediates

Indeno­[1,2-<i>b</i>]­fluorene derivatives with trimethylsilylethynyl substituents at the 6- and 12-positions were found to undergo cyclo-dimerization, cyclo-trimerization, and higher oligomerizations at room temperature. The cyclic dimer features a novel double-decker motif, composed of two face-to-face stacked bis­(propa­dienylide)­dihydro­indeno­[1,2-<i>b</i>]­fluorenes with a short centroid-to-centroid distance of 3.50 Å. The existence of a cyclic trimer and higher oligomers was confirmed by mass spectroscopy and gel permeation chromatography. The results clearly demonstrate the diradical feature of the indeno­[1,2-<i>b</i>]­fluorene moiety

Sensory Responses in Solution vs Solid State: A Fluorescence Quenching Study of Poly(iptycenebutadiynylene)s

A new series of poly(p-phenylenebutadiynylene)s has been synthesized with unique polymer structural features. In these systems each of the p-phenylene units in the conjugated backbone is the core of a rigid three-dimensional iptycene scaffold. The fluorescence quenching properties of these polymers in response to a series of electron-deficient aromatic compounds have been investigated in both solution and the solid state. It was found that in solution these polymers displayed higher quenching sensitivity toward studied quenchers compared to a more open-structure iptycene-containing poly(p-phenyleneethynylene). The quenching behaviors of the conjugated polymer were shown to be strongly influenced by the configuration of the incorporated iptycences. The thin films investigations revealed differences in both the fluorescence quenching and the recovery processes. Distinct behaviors indicated that the fluorescence quenching in the solid state is dictated by different factors than those in solution. Our results further suggest that poly(p-phenylenebutadiynylene)s containing large iptycene scaffolds that introduce porosity have the ability to efficiently sequester the quencher molecules within thin films as these materials display slow fluorescence recoveries

Folding-Driven Reversible Polymerization of Oligo(<i>m</i>-phenylene ethynylene) Imines: Solvent and Starter Sequence Studies

Bis(imino) end-functionalized oligo(m-phenylene ethynylene)s were equilibrated in a closed system under conditions that promote reversible imine metathesis. The metathesis reaction joins two oligomers and produces a small molecule byproduct. In polar solvents, equilibration gave high molecular weight polymers while equilibration in chloroform produced only low molecular weight oligomers. This polymerization is hypothesized to be driven by the free energy gained from the folding of the long polymer chains directed by the noncovalent, intramolecular aromatic stacking and solvophobic interactions. This polymerization was also conducted in a series of solvents in which m-phenylene ethynylene oligomers have previously shown varied, intermediate folding stabilities. These experiments revealed a good correlation of the product molecular weight with the stability of the m-phenylene ethynylene helix. The equilibrium state of the metathesis reaction was also demonstrated to depend on the chain length of the starter sequences. With a pair of trimeric precursors, macrocyclization instead of polymerization takes place. Consistent with the notion that the polymerization is a consequence of the intramolecular solvophobic chain association, higher degrees of polymerization followed from enhanced solvophobicity of the m-phenylene ethynylene backbone, achieved by appending a methyl substituent to half of the repeat units. The considerably longer equilibration time required by these more stabilized sequences suggests that the elongation process may involve unfolding or partial unfolding of the chain; alternatively, intermolecular association may be responsible for the slow chain growth

Nucleation−Elongation Polymerization under Imbalanced Stoichiometry

As a result of the helical structure of the polymeric product, the folding-driven polymerization of oligo(m-phenyleneethynylene) imines in solution should inherently show nucleation−elongation in chain growth. Here, we present evidence for this behavior based on results of polymerizations conducted under conditions of imbalanced stoichiometry. Because the polymerization proceeds via imine metathesis between a pair of bifunctional monomers of types A−A and B−B, the molar ratio of the polymerizing functional groups can be arbitrarily varied. Alternatively, stoichiometry can be controlled by the addition of a monofunctional oligomer. Similar results were obtained in both cases whereby the molecular weight distribution was significantly different from that expected for classical step-growth polymerizations. At equilibrium, high molecular weight polymers were observed to coexist with the monomer in excess. Thermodynamic equilibrium was established by showing that the same distribution was reached starting either from a monomer mixture or from high polymers to which one monomer was added. These results are in great contrast to the low molecular weight oligomers that were produced when the reaction was conducted by melt condensation of bifunctional aldehyde and amine monomers, a polymerization that apparently proceeds without the nucleation event. An equilibrium model that captures the features of nucleation−elongation under conditions of imbalanced stoichiometry qualitatively supports the monomer−polymer distribution observed experimentally

Synthesis and Self-Association of an Imine-Containing <i>m</i>-Phenylene Ethynylene Macrocycle

The purpose of this study was to test the suitability of the imine bond as a structural unit within the backbone of phenylene ethynylene macrocycles and oligomers by determining the ability of m-phenylene ethynylene macrocycle 1 to form π-stacked aggregates in both solution and the solid state. Macrocycle 1, with two imine bonds, was synthesized in high yield from diamine 4 and dialdehyde 5. The imine-forming macrocyclization step was carried out under a variety of conditions, with the best yield obtained simply by refluxing the reactants in methanol. The self-association behavior of 1 in various solvents was probed by 1H NMR. The association constants (KE) in acetone-d6 and tetrahydrofuran-d8 were determined by fitting the concentration-dependent chemical shifts with indefinite self-association models. The results showed that solvophobically driven intermolecular π−π stacking could be preserved in the imine-containing m-phenylene ethynylene macrocycles. Interestingly, in acetone macrocycle 1 exhibited a stronger tendency to form a dimer rather than higher aggregates. We postulate that this behavior may be due to electrostatic attraction between dipolar imine groups. The solid-state packing of 1 was studied by wide- and small-angle X-ray powder diffraction (WAXD and SAXD). Bragg reflections of 1 were consistent with a hexagonal packing motif similar to our previous studies on m-phenylene ethynylene macrocycles that formed columnar liquid crystal phases

Reversible Polymerization Driven by Folding

Bisfunctionalized m-phenylene ethynylene imine oligomers were polymerized in the polar solvent acetonitrile, resulting in high-molecular weight poly(m-phenylene ethynylene imine)s. It is hypothesized that this polymerization, which proceeds through the reversible metathesis of imine bonds, is driven by the folding of the long m-phenylene ethynylene imine chains. Upon conducting the polymerization in a series of solvents in which the m-phenylene ethynylene oligomers exhibit different folding stabilities, it was possible to correlate the molecular weight of the resulting poly(m-phenylene ethynylene imine)s with the helical stability of the corresponding oligomers. The polymerization was also demonstrated to be reversible and responsive to solvent and temperature changes

Cyclo-oligomerization of 6,12-Diethynyl Indeno[1,2‑<i>b</i>]fluorenes via Diradical Intermediates

Indeno­[1,2-<i>b</i>]­fluorene derivatives with trimethylsilylethynyl substituents at the 6- and 12-positions were found to undergo cyclo-dimerization, cyclo-trimerization, and higher oligomerizations at room temperature. The cyclic dimer features a novel double-decker motif, composed of two face-to-face stacked bis­(propa­dienylide)­dihydro­indeno­[1,2-<i>b</i>]­fluorenes with a short centroid-to-centroid distance of 3.50 Å. The existence of a cyclic trimer and higher oligomers was confirmed by mass spectroscopy and gel permeation chromatography. The results clearly demonstrate the diradical feature of the indeno­[1,2-<i>b</i>]­fluorene moiety