Unsymmetrically substituted four-armed tolanes: new multichromophoric molecules

Cross-conjugated cruciform fluorophores

In optoelectronic devices, chromophores can be designed at the molecular level to create materials with properties desired for advanced applications. Organic fluorophores in particular can be constructed with macroscopic properties that arise from two distinct contributions: (i) the collective impact of the molecular backbone and substituents and (ii) the connectivity within the molecule (that is, the spatial molecular architecture). Accordingly, the exploration of novel conjugated architectures is a productive area of current research. Different two-dimensional, "X-shaped" conjugated materials have been synthesized for a variety of applications. They include spiro compounds, paracyclophanes, swivel-type dimers, bisoxazole-derived cruciforms, tetraethynylethenes, and tetrasubstituted tolanes. A subset of these compounds are constructed from two "perpendicular" pi-conjugated linear arms connected through a central aromatic core; examples of these include tetrakis(arylethynyl)benzenes, tetrakis(styryl)benzenes, and tetrasubstituted thiophenes. In this Account, we evaluate 1,4-distyryl-2,5-bis(arylethynyl)benzenes or cruciforms (XFs). Electronic substitution of this "X-shaped" cross-conjugated scaffold tunes both the energy levels of the frontier molecular orbitals (FMOs) and their spatial distribution in XFs. The resulting fluorophores exhibit FMO separation, imbuing XFs with unusual yet desirable properties for sensory applications. Using model analytes, we examine how the underlying FMO arrangement and the nature of analyte interaction elicit observable responses. These studies provide a foundation for accessing functional responsive ratiometric cores, demonstrating the importance and unique potential of FMO-separated fluorophores. We also highlight the essential contribution of serendipity in materials development. Moving beyond one-dimensional molecular wire-type fluorophores to two-dimensional "X-shaped" materials provides access to materials with unexpected and exciting properties. XFs represent such novel conjugated architectures, and their successful development has frequently has hinged on inspiration from structural components and principles developed in diverse research areas.

Controlling the conformational changes in donor-acceptor [4]-dendralenes through intramolecular charge-transfer processes

The synthesis of two [4]-dendralene compounds incorporating thiophene-(p-nitrophenyl) donor-acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron-donating components of the dendralenes are represented by bromothienyl (in 13) and ethylenedioxythiophene(EDOT)-thienyl (in 15) end-groups. The most facile transformation involves the isomerisation of donor-acceptor conjugated systems (a conformers) into structures in which only the thiophenes are conjugated (b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single-crystal X-ray diffraction studies and the structural changes in both compounds have been monitored by 1H NMR spectroscopy and absorption studies. The transformations were found to be first-order processes with rate constants of k=0.0027 s(-1) and k=0.00022 s(-1) for 13 and 15, respectively. Density functional theory calculations at the B3LYP/6-31G* level give credence to the proposed mechanism for the a-->b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative (15) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.

Terpyridine-Based Cruciform−Zn2+ Complexes as Anion-Responsive Fluorophores

The synthesis of a terpyridine-appended, zinc-complexed cruciform, 3-Zn2+, and the anion-reactive modulation of its emissive properties in acetone-water mixtures are reported.

Phenothiazine Cruciforms: Synthesis and Metallochromic Properties

We report the synthesis and characterization of five novel phenothiazine-containing cruciforms (5-9). The targets were prepared by a NaH-promoted Horner reaction of tetraethyl(2,5-diiodo-1,4-phenylene)bis(methylene)diphosphonate with 10-hexyl-10H-phenothiazine-3-carbaldehyde. The formed intermediary 3,3'-(1E,1'E)-2,2'-(2,5-diiodo-1,4-phenylene)bis(ethene-2,1-diyl)bis(10-hexyl-10H-phenothiazine) was reacted with several different aromatic alkynes (1-tert-butyl-4-ethynylbenzene, N,N-dibutyl-4-ethynylaniline, 1-ethynyl-3-(trifluoromethyl)benzene, and 1-ethynyl-3,5-bis(trifluoromethyl)benzene) to give the corresponding cruciform fluororphores (XF). The XFs were fully characterized by NMR and IR spectroscopy and then exposed to trifluoroacetic acid as well as to several metal triflates. The XFs show dramatic shifts in emission and to a lesser extent in absorption when exposed to magnesium triflate or zinc triflate. In the case of magnesium triflate, a blue shift in emission was observed; in contrast, addition of zinc triflate results in either quenching or a red-shifted emission. Due to the electronic situation, these XFs display spatially separated frontier molecular orbitals, allowing the HOMO or the LUMO of the XFs to be addressed independently by addition of zinc or magnesium ions. Phenothiazine XFs may have potential in array-type sensory applications for metal cations.

Control of Surface Functionality in Poly(phenylenevinylene) Dendritic Architectures

The efficient synthesis of new asymmetric poly(phenylenevinylene) dendritic macromolecules using a stepwise convergent-growth approach is described. By an iterative methodology that made use of the Horner-Wadsworth-Emmons (HWE) reaction, dendrons and dendrimers up to the third generation, with eight different functional groups located at the periphery, were prepared in good yields. Both the number and placement of functionalities can be accurately controlled to afford a large variety of dendritic architectures.