Christoph Thalacker

Fluorescent J‐type Aggregates and Thermotropic Columnar Mesophases of Perylene Bisimide Dyes

A series of perylene tetracarboxylic acid bisimides 3a-e bearing 3,4,5-tridodecyloxyphenyl substituents on the imide N atoms and zero, two, or four phenoxy-type substituents in the bay positions of the perylene core were synthesized. From investigations of their spectroscopic properties and aggregation behavior in low-polarity solvents by absorption and fluorescence optical spectroscopy, not only were these compounds found to form fluorescent J-type aggregates, but also binding constants for aggregation could be derived which reflect the number and steric demand of the phenoxy substituents for bisimides 3a-d. In the pristine state, 3a-d form thermotropic hexagonal columnar mesophases which exist over a broad temperature range from below -30 degrees C to over 300 degrees C. For the tetraphenoxy-substituted compound 3e, however, a layered crystalline structure was found. This difference in behavior can be explained by the concept of microphase segregation of the aromatic cores of the molecules and the alkyl chains at the periphery. The high stability and bright fluorescence of the mesophase of several of the compounds make them promising for applications as polarizers or components in (opto)electronic devices.

Core-Substituted Naphthalene Bisimides: New Fluorophors with Tunable Emission Wavelength for FRET Studies†

Highly colored and photoluminescent naphthalene bisimide dyes have been synthesized from 2,6-dichloronaphthalene bisanhydride 1 by means of a stepwise nucleophilic displacement of the two chlorine atoms by alkoxides and/or alkyl amines. The alkoxy-substituted derivatives are yellow dyes with green emission and low photoluminescence quantum yields, whereas the amine-substituted derivatives exhibit a color range from red to blue with strong photoluminescence up to 76%. Structure-property relationships for this class of two-dimensional chromophores were evaluated based on a single-crystal X-ray analysis for dye 5a, the observed solvatochromism, and quantum-chemical calculations. Owing to the simple tuning of the absorption properties over the whole visible range by the respective substituents, the pronounced brilliancy, and the intense photoluminescence, this class of dyes is considered to be highly suited for numerous applications such as fluorescent labeling of biomacromolecules and light-harvesting in supramolecular assemblies. As an important step towards such applications efficient FRET (fluorescence resonance energy transfer) has been demonstrated for a covalently tethered bichromophoric compound that contains a red and a blue naphthalene bisimide dye.

Chiral Perylene Bisimide–Melamine Assemblies: Hydrogen Bond‐Directed Growth of Helically Stacked Dyes with Chiroptical Properties

Self-aggregation and hydrogen bond-directed superstructure formation with a perylene bisimide dye were investigated for melamines bearing two or four alkyl chains. For a melamine with two n-octadecyl substituents, large micellar aggregates of several hundred nanometers were observed in methylcyclohexane solution by dynamic light scattering. By means of hydrogen bonding these micelles were loaded with various contents of perylene bisimide dyes up to an equimolar amount. Even at low dye contents, islands of strongly aggregated perylene bisimides were formed according to UV-vis absorption spectroscopy. In a series of melamines functionalized with n-octadecyl esters of chiral amino acids, no self-aggregation of the melamines was observed, and the stability of the assemblies with the perylene bisimide dyes showed strong dependence on the melamine substituents. For four melamine–perylene bisimide combinations stable assemblies formed, which exhibited exciton-coupled circular dichroism (CD) effects for the perylene bisimide absorption bands whose sign is controlled by the stereoisomerism of the respective melamine. While for (S,S)-melamines a negative Cotton effect was observed, indicating an M-helical arrangement of the chromophores within the assembly, the (R,R)-enantiomer induced P-helicity, and no CD effect was found for the achiral (R,S)-diastereomer.

Hydrogen bond directed self-assembly of core-substituted naphthalene bisimides with melamines in solution and at the graphite interface

A series of red and blue highly fluorescent core-substituted naphthalene bisimide dyes has been synthesized and they have been investigated as supramolecular building blocks. NMR and UV-Vis titration experiments of these dyes with complementary melamines revealed the formation of triple hydrogen bonds (DAD-ADA arrays) in solution. At stoichiometric ratios, ditopic melamine receptors could dissolve otherwise insoluble bisimides by means of hydrogen bonding, even in aliphatic solvents. At the solution/graphite interface, one-dimensional chains of hydrogen bonded naphthalene bisimides and two-dimensional adlayers of ditopic melamines are formed for the pure compounds but little evidence for heterocomplexes between the two complementary building blocks could be obtained.

Chiral exciton coupling of merocyanine dyes within a well defined hydrogen-bonded assembly.

Multichromophoric hydrogen-bonded assemblies 13⋅(BAR)6 are studied that bear a remarkably close resemblance to commelinin, a naturally occurring assembly responsible for an intense blue color of flowers. The incorporated chromophores exhibit a hypsochromic shift in the UV/visible (Vis) absorption maximum (Δ λmax = 14 nm) compared with the free chromophores. In addition, the chiroptical properties of incorporated chromophores can be rationally controlled by changing the supramolecular chirality of the assembly. These properties have been used to study the stability of this type of assembly with UV and CD spectroscopy at concentrations far below the NMR sensitivity threshold (10−4 M). The determined C50% values of 2–3 μM in benzene show the extremely high stability of these hydrogen-bonded assemblies.

Fabrication and fluorescence properties of perylene bisimide dye aggregates bound to gold surfaces and nanopatterns

Perylene bisimide dyes with two different imide substituents have been synthesized by sequential imidization reactions to give the disulfide 8 bearing two perylene bisimide dyes. Aggregation properties of this bis-perylene dye were studied by UV/Vis absorption and steady-state polarization-dependent as well as time-resolved fluorescence spectroscopy and the results were compared to those of the symmetrical perylene bisimide dye 4. In both cases, aggregation is expressed in a strong bathochromic shift of the excitation and the emission spectra and increasing concentration results in an increase in lifetime from 5 to 8 ns. By means of the thiol linker, self-assembly of 8 on gold surfaces was accomplished leading to surface-bound dye aggregates. Intense fluorescence from these dye aggregates was observed on surfaces decorated with hexagonal gold patterns, whereas the fluorescence is only weak on plain gold substrates.

Optimization of neutrocyanine chromophores based on five-membered heterocycles for photorefractive applications

Exceptionally large photorefractive figures-of-merit are observed for neutrocyanine dyes based on dialkylamino-thiophenes and –furans linked to quinoid five-membered heterocyclic acceptor groups by a methine bridge.

Hydrogen bond-directed aggregation of diazadibenzoperylene dyes in low-polarity solvents and the solid state

The formation of complex superstructures via hydrogen bonding of two ditopic building blocks, diazadibenzoperylenes 1a,b and isophthalic acid 2, has been investigated. It was found that only the phenoxy-substituted diazadibenzoperylene 1a forms extended assemblies with 2 in complexes of a 1:1 stoichiometry, whereas for the 4-tert-butylphenoxy-substituted analogue 1b, no indications for superstructure formation with 2 were found. The different behavior is explained by the presence of additional π–π interactions, which are only observed for [1a⋅2], as revealed by concentration-dependent optical absorption and fluorescence spectroscopy. Based on variable temperature x-ray diffraction studies, a lamellar structure for [1a⋅2] is proposed that takes into account the concept of microphase-segregation.