Cruciforms as Functional Fluorophores: Response to Protons and Selected Metal Ions

Aldehyde cruciforms: dosimeters for primary and secondary amines

We demonstrate that aldehyde-substituted donor-acceptor cruciforms [1,4-bis(arylethynyl)-2,5-distyrylbenzenes] are useful dosimeters for primary amines, primary diamines, and secondary amines. The 1,n-diamines are particularly reactive towards this dosimeter and can be detected in less than 100 ppm concentration. Using a single aldehyde-functionalized cruciform in seven different solvents allowed us to discern fourteen different amines by digital photography and statistical evaluation of the response patterns extracted as red, green, blue (RGB) values.

Investigation of 1H NMR chemical shifts of organic dye with hydrogen bonds and ring currents

The (1)H NMR chemical shifts were theoretically computed for the organic dyes 2-(2,6-dimethyl-4H-pyran-4-ylidene)-malononitrile (1), cyano-(2,6-dimethyl-4H-pyran-4-ylidene)-acetic acid methyl ester (2), 2-(2,6-bis(4-(dimethylamino)styryl)-4H-pyran-4-ylidene)-malononitrile (3), and methyl 2-(2,6-bis(4-(dimethylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate (4) at the GIAO/B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory. Moreover, the intramolecular rotational barriers of the molecules were calculated to evaluate the internal flexibility with respect to the torsional degrees of freedom, and the nuclear-independent chemical shifts (NICS) were employed to analyze the ring currents. The difference was explained in terms of intramolecular hydrogen bonds and ring currents of the molecules. The (1)H NMR spectra were reproduced by experiments for the comparison with computationally constructed data. Our results suggest a good guideline in interpreting (1)H NMR chemical shifts using computational methods and furthermore a reliable perspective for designing molecular structures.

Anion-dependent fluorescence in bis(anilinoethynyl)pyridine derivatives: switchable ON-OFF and OFF-ON responses

Urea and sulfonamide derivatives of 1 exhibit ON-OFF and OFF-ON switchable fluorescent and colorimetric responses upon protonation. The magnitude of the fluorescence event is dictated by the anion, resulting in a rare, fully organic "turn-on" fluorescent sensor for chloride.

Lewis acid adducts of narrow band gap conjugated polymers

We report on the interaction of Lewis acids with narrow band gap conjugated copolymers containing donor and acceptor units. Examination of the widely used poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-(diyl-alt-benzo[2,1,3]thiadiazole)-4,7-diyl] (1) shows weaker binding with B(C(6)F(5))(3) when compared with a small molecule that contains a cyclopenta-[2,1-b:3,4-b']dithiophene (CDT) unit flanked by two benzo[2,1,3]thiadiazole (BT) fragments. Studies on model compounds representative of 1, together with a comparison between B(C(6)F(5))(3) and BBr(3), indicate that the propensity for Lewis acid coordination is decreased because of steric encumbrance surrounding the BT nitrogen sites. These observations led to the design of chromophores that incorporate an acceptor unit with a more basic nitrogen site, namely pyridal[2,1,3]thiadiazole (PT). That this strategy leads to a stronger B-N interaction was demonstrated through the examination of the reaction of B(C(6)F(5))(3) with two small molecules bis(4,4-bis(hexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-4,7-pyridal[2,1,3]thiadiazole (8) and bis{2-thienyl-(4,4-bis(hexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)}-4,7-pyridal[2,1,3]thiadiazole (9) and two polymer systems (poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-([1,2,5]thiadiazolo[3,4-c]pyridine)-4,7-diyl] (10) and poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-(4',7'-bis(2-thienyl)-[1,2,5]thiadiazolo[3,4-c]pyridine)-5,5-diyl] (11). From a materials perspective, it is worth pointing out that through the binding of B(C(6)F(5))(3), new NIR-absorbing polymers can be generated with band gaps from 1.31 to 0.89 eV. A combination of studies involving ultraviolet photoemission spectroscopy and density functional theory shows that the narrowing of the band gap upon borane coordination to the pyridal nitrogen on PT is a result of lowering the energies of both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the optically relevant fragments; however, the LUMO is decreased to a greater extent, thereby giving rise to the narrowing of the gap.

Highly sensitive detection of saccharides under physiological conditions with click synthesized boronic acid-oligomer fluorophores

Two phenylboronic acid based saccharide sensors bearing conjugated oligomer fluorophores with linear and cruciform π-frameworks were synthesized in a modular approach utilizing a Cu-catalyzed alkyne azide cycloaddition (click) reaction. The cruciform fluorophore showed excellent saccharide sensing function under physiological conditions in the mM range, whereas the linear fluorophore gave very limited sensing functions. The different fluorescent sensing behaviours highlight the important role of oligomer fluorophore in the development of effective saccharide sensors.

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.

Conjugated polymers with large effective Stokes shift: benzobisdioxole-based poly(phenylene ethynylene)s

Phenyleneethynylene-based conjugated copolymers using benzo[1,2-d:4,5-d']bis[1,3]dioxole (BDO) in the repeating unit are reported. The electronic structure of the BDO unit imparts a localized HOMO topology while the LUMO is delocalized over the polymer backbone, so that the lowest optical absorption band of the polymer has considerable intramolecular charge transfer character. This contrasts with published donor-acceptor polymers with localized LUMO and delocalized HOMO. The very large Stokes shifts of the monomers, which are due to the small oscillator strength of the lowest optical transition, are largely retained in the polymers as a result of covalently constrained dihedral angles in the substituents (not the backbone), as predicted/explained by calculations.

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.

Fine structures of 8-G-1-(p-YC6H4C identical with CSe)C10H6 (G = H, Cl, and Br) in crystals and solutions: ethynyl influence and Y- and G-dependences

Fine structures of 8-G-1-(p-YC₆H₄C ≡ CSe)C₁₀H₆ [1 (G = H) and 2 (G = Cl): Y = H (a), OMe (b), Me (c), F (d), Cl (e), CN (f), and NO₂ (g)] are determined by the X-ray analysis. Structures of 1, 2, and 3 (G = Br) are called A if each Se–C_sp bond is perpendicular to the naphthyl plane, whereas they are B when the bond is placed on the plane. Structures are observed as A for 1a–c bearing Y of nonacceptors, whereas they are B for 1e–g with Y of strong acceptors. The change in the structures of 1e–g versus those of 1a–c is called Y-dependence in 1. The Y-dependence is very specific in 1 relative to 1-(p-YC₆H₄Se)C₁₀H₇ (4) due to the ethynyl group: the Y-dependence in 1 is almost inverse to the case of 4 due to the ethynyl group. We call the specific effect “Ethynyl Influence.” Structures of 2 are observed as B: the A-type structure of 1b changes dramatically to B of 2b by G = Cl at the 8-position, which is called G-dependence. The structures of 2 and 3 are examined in solutions based on the NMR parameters.