Synthesis and Properties of Bismerocyanines Linked by a 1,8-Naphthylene Skeleton. Novel Solvatochromism Based on Change of Intramolecular Excitonic Coupling Mode

Design and Photonics of Merocyanine Dyes

Merocyanines, thanks to their easily adjustable electronic structure, appear to be the most versatile and promising functional dyes. Their D-π-A framework offers ample opportunities for custom design through variations in both donor/acceptor end-groups and the π-conjugated polymethine chain, and leads to a broad range of practical properties, including noticeable solvatochromism, high polarizability/hyperpolarizabilities, and the ability to sensitize various physicochemical processes. Accordingly, merocyanines are applied and extensively studied in various fields, such as light-converting materials for optoelectronics, nonlinear optics, optical storage, solar cells, fluorescent probes, and antitumor agents in photodynamic therapy. This review encompasses both classical and novel more important publications on the structure-property relationships in merocyanines, with particular emphasis on the results by A.  I. Kiprianov and his followers in Institute of Organic Chemistry in Kyiv, Ukraine.

Dipole-Dipole Interaction Driven Self-Assembly of Merocyanine Dyes: From Dimers to Nanoscale Objects and Supramolecular Materials

π-Conjugation between heterocyclic donor (D) and acceptor (A) groups via a polymethine chain leads to dyes with dipole moments greater than 10 D. These dipole moments direct the self-assembly of the dyes into antiparallel dimer aggregates, even in dilute solution, with binding strengths that are far beyond those observed for other π-scaffolds whose self-assembly is driven primarily by dispersion forces. The combination of directionality and exceptional binding strength of dipolar interactions between D-π-A dyes indeed resembles the situation of the hydrogen bond. Thus, similar to the latter, dipolar interactions between merocyanine dyes, a unique class of D-π-A chromophores, can be utilized to construct sophisticated supramolecular architectures of predictable geometry, particularly in low polarity environments. For bis(merocyanine) dyes it has been demonstrated that the self-assembly pathway is encoded in the tether between the two constituent merocyanine chromophores. If the tether enables the antiparallel stacking of the two appended dyes, folding takes place, which may be followed by further self-assembly into extended H-aggregate π-stacks at higher concentrations in solvents of low polarity. For tethers that do not support folding, the formation of bimolecular complexes of four merocyanine units, cyclic oligomers, and supramolecular polymers has been observed. For the former case, that is, formation of a bimolecular stack of four merocyanine units from tweezer-type molecules, association constants >10(9) M(-1) were measured in chloroform. On the other hand, because only one π-face is utilized in the formation of supramolecular polymers from bis(merocyanine) dyes, higher hierarchical structures typically originate in which the other π-face is surrounded by an antiparallel π-stacked neighbor molecule. Among the observed self-assembled structures, nanorods in particular have attracted considerable attention because their self-assembly into well-defined H-aggregates falls under kinetic control and is slowed tremendously with decreasing solvent polarity. Co-assembly of achiral and chiral merocyanine building blocks or two enantiomers of a chiral merocyanine in different ratios provided insight into "majority rules" and "sergeant-and-soldiers" effects as well as the autocatalytic fiber growth process. With regard to materials applications, it is important to note that the high propensity for dipolar aggregation was disadvantageous for many envisioned applications of these dyes in the area of nonlinear optics. However, this aggregation behavior proved to be advantageous for the recently demonstrated applications of D-π-A dyes, in particular, merocyanines as p-type organic semiconductors in organic electronics and photovoltaics. Thus, organic transistors with hole mobilities >0.5 cm(2)/(V s) and organic solar cells with power conversion efficiencies >6% could be achieved with merocyanine-based organic semiconductor molecules.

Folding-induced exciton coupling in homo- and heterodimers of merocyanine dyes

Folding of a bis(merocyanine) dye with two different chromophores leads to a model system to elucidate electronic interactions in heteroaggregates.

Spectral-luminescent and photochemical characteristics of homodimers of the styrylcyanine dye Sbt in solutions

The influence of concentration, solvent nature on the intermolecular interaction and its spectroscopic manifestations in solutions of styrylcyanine dye Sbt ((E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]thiazol-3-ium iodide) and its homodimers, i.e. dyes with two interconnected chromophores, was studied. It was found out that, depending on the concentration, the structure of dye molecules, the nature of the solvent various forms of associated molecules are forms; each of these forms is manifested in the spectra of absorption and fluorescence in its own way. It is shown that the intensity of the main absorption band of the studied dyes in aqueous solutions and in a mixture of water+ethanol decreases in the process of light irradiation and a new band is observed in the region of shorter wavelengths; the intensity of the new band increases with increase of radiation exposure of solutions.

Photoswitchable intramolecular H-stacking of perylenebisimide

Dynamic control over the formation of H- or J-type aggregates of chromophores is of fundamental importance for developing responsive organic optoelectronic materials. In this study, the first example of photoswitching between a nonstacked and an intramolecularly H-stacked arrangement of perylenebisimides (PBI) is demonstrated. The system is composed of a central switching unit (overcrowded alkene) tethered to two PBI chromophores. cis-trans isomerization of the switching unit, induced by alternate irradiation at 312 and 365 nm, can drive two PBI chromophores reversibly between an intramolecularly "aggregated" and "nonaggregated" state. Distinct changes in UV-vis absorption and fluorescence spectra are observed following photoisomerization. This approach allows for efficient control of intramolecular H-stack formation with no significant intermolecular interactions spanning over at least 4 orders of magnitude of concentration (from 10(-8) to 10(-4) M) and a range of solvents and temperatures.

Ultralong carbon-carbon bonds in dispirobis(10-methylacridan) derivatives with an acenaphthene, pyracene, or dihydropyracylene skeleton

Acenapthalene, pyracene, and dihydropyracylene attached to two units of spiroacridan are a novel class of hexaphenylethane (HPE) derivatives that have an ultralong Csp3-Csp3 bond (1.77-1.70 A). These sterically challenged molecules were cleanly prepared by C-C bond formation through two-electron reduction from the less-hindered dications. These ultralong bonds were realized based on several molecular-design concepts including enhanced "front strain" through "multiclamping" by means of fusing or bridging aryl groups in the HPE molecule. The lengths of these ultralong bonds and their relation to the conformation (torsional angle) were also validated by means of theoretical calculations. Bond-fission experiments revealed that the bonds are more easily cleaved than standard covalent bonds to produce the corresponding dication upon oxidation with an increase in the length of the C-C bond.

Antiparallel-aligned neutral-ground-state and zwitterionic chromophores as a nonlinear optical material

Efficient noncentrosymmetric arrangement of nonlinear optical (NLO) chromophores with high first-order hyperpolarizability (beta) for increased electro-optical (EO) efficiency has proven challenging as strong dipolar interactions between the chromophores encourage antiparallel alignment, attenuating the macroscopic EO effect. This work explores a novel approach to simultaneously achieve large beta values while providing an adjustable dipole moment by linking a strong neutral-ground-state (NGS) NLO chromophore with positive beta to a zwitterionic (ZWI) chromophore with negative beta in an antiparallel fashion. It is proposed that the overall beta of such a structure will be the sum of the absolute values of the two types of chromophores while the dipole moment will be the difference. Molecules 1-3 were synthesized to test the feasibility of this approach. Molecular dynamics calculations and NMR data supported that the NGS chromophore component and the ZWI chromophore component self-assemble to an antiparallel conformation in chloroform. Calculations showed that the dipole moment of 1 is close to the difference of the two component chromophores. Hyper-Rayleigh scattering (HRS) studies confirmed that the first hyperpolarizability of 1 is close to the sum of the two component chromophores. These results support the idea that an antiparallel-aligned neutral-ground-state chromophore and a zwitterionic chromophore can simultaneously achieve an increase in beta and a decrease of the dipole moment.

Dibenzodiaza-30-crown-10-appended bis(zinc porphyrin) tweezers: synthesis and crown-assisted chiroptical behaviour

In our program for developing chirality manipulation systems, we synthesized bis(zinc porphyrin) 1, with a dibenzodiaza-30-crown-10 as a linker unit. Two structural features were examined. The aza-crown segment exhibited an intermolecular interaction with the zinc(ii) of the porphyrin, capable of causing aggregation to form spherical nanostructures, as inferred by concentration-dependency of (1)H NMR as well as scanning electron microscopy (SEM) observation. We also consider the crown-based conformation flexibility, in which accommodated K(+) tunes the porphyrin orientation into the tweezers conformation, assisting chirality induction upon complexation with chiral diamine 2. The circular dichroism (CD) intensity change essentially reached a plateau at a [(1R,2R)-2] : [1] ratio of 2 : 1 for which a 45% enhancement in the amplitude of CD spectra was observed compared to the K(+)-free conditions. Use of the crown linker of is not limited to promoting chirality induction with diamines in the presence of K(+); chiroptical probing of unprotected amino acids (Lys, His, Trp, and Phe) using 1 was attained through liquid (1 in CH(2)Cl(2))-liquid (the amino acids in 1 N KOH) two-phase extraction. The amphiphilic properties of the crown segment, as well as the K(+)-assisted tweezers conformation, make it possible to explore a potent way to develop chirality sensors for amino acids in water.

Squaraine-Derived Rotaxanes: Highly Stable, Fluorescent Near-IR Dyes

Squaraines are fluorescent, near-IR dyes with promising photophysical properties for biomedical applications. A limitation with these dyes is their inherent reactivity with nucleophiles, which leads to loss of the chromophore. Another drawback is their tendency to form nonfluorescent aggregates in water. Both problems can be greatly attenuated by encapsulating the dye inside an amide-containing macrocycle. In other words, the squaraine becomes the thread component in a Leigh-type rotaxane, a permanently interlocked molecule. Two new rotaxanes are described: an analogue with four tri(ethyleneoxy) chains on the squaraine to enhance water solubility, and a rotaxane that has an encapsulating macrocycle with transposed carbonyl groups. An X-ray crystal structure of the latter rotaxane shows that the macrocycle provides only partial protection of the electrophilic cyclobutene core of the squaraine thread. The stabilities of each compound in various solvents, including serum, were compared with a commercially available cyanine dye. The squaraine rotaxane architecture is remarkably resistant to chemical and photochemical degradation, and likely to be very useful as a versatile fluorescent scaffold for constructing various types of highly stable, near-IR imaging probes.

Tunable interchromophore electronic interaction of a merocyanine dye in hydrogen-bonded supramolecular assemblies scaffolded by bismelamine receptors

The absorption and fluorescence properties of a barbiturate-type donor-pi-acceptor (D-pi-A) merocyanine dye are controlled by complexation with dimeric melamine receptors featuring different tether lengths.

Selective Calcium Ion Sensing with a Bichromophoric Squaraine Foldamer

Several squaraine tethered bichromophoric podand systems 1a-d and a monochromophoric analogue 2 were prepared and characterized. Among these, the bichromophore, 1b, containing five oxygen atoms in the flexible podand moiety was found to specifically bind Ca(2+) in the presence of other metal ions such as K(+), Na(+), and Mg(2+). The selective binding of Ca(2+) is clear from the absorption and emission spectral changes as well as by the visual color change of 1b from light-blue to an intense purple-blue. Benesi-Hildebrand and Job plots confirmed a 1:1 binding between 1b and Ca(2+). Signaling of the binding event is achieved by the cation-induced folding of the bichromophore and the resultant exciton coupling between the squaraine chromophores. The monochromophoric squaraine dye 2 failed to give optical signals upon Ca(2+) binding, due to the absence of exciton interaction in the bound complex. Titration of the folded complex 9 with EDTA released the metal ion from the complex, thereby regaining the original absorption and emission properties of the bichromophore. The squaraine foldamer 1b reported here is the first example of a selective chromogenic Ca(2+) sensor, which works on the principle of exciton interaction in the folded Ca(2+) complex of a bichromophore, the optical properties of which are similar to those of the "H"-type aggregates of analogous squaraine dyes.

Perylene nanotubes fabricated by the template method

Aligned organic nanotubes of perylene with uniform diameters, lengths and wall thicknesses were fabricated via a simple immersion technique using a porous alumina membrane as the template.

Supramolecular Polymerization and Gel Formation of Bis(Merocyanine) Dyes Driven by Dipolar Aggregation

Two highly dipolar merocyanine dyes were tethered by a rigid tris(n-dodecyloxy)xylylene unit that preorganizes the dyes for a supramolecular polymerization process through intermolecular aggregation of the dyes. UV/vis spectroscopy revealed a solvent dependent equilibrium between monomeric dyes and two different types of dye aggregates that are characterized by hypsochromically shifted D- and H-type absorption bands. Taking into account the ditopic nature of the supramolecular building blocks, the occurrence of the D-band indicates the formation of an oligomeric/polymeric supramolecular chain whereas the observation of the H-band suggests a higher order assembly. For the H-aggregated dyes, intrinsic viscosities exceed 0.65 L g(-1) in methylcyclohexane, values typically found for macromolecular solutions. At higher concentration, further association of these aggregates takes place by entanglement of the alkyl groups leading to a substantial increase in viscosity and gelation. Rheology studies show linear viscoelastic behavior which was attributed to the formation of an entangled dynamic network. AFM and cryo-TEM studies of the gel reveal long and stiff rod-type assemblies. X-ray diffraction studies for a solid film show columnar mesomorphism. Based on these results, a structural model is proposed in which six helically preorganized strands of the supramolecular polymer intertwine to form a rod with a diameter of about 5 nm. Within these rods all dyes are tightly aggregated in a tubular fashion giving rise to delocalized excitonic states, and the pi-conjugated tube is jacketed by the tridodecyloxy groups.

Chromophore design for photorefractive organic materials

During the last years, significant progress has been achieved in understanding the mechanism of the photorefractive effect in amorphous organic materials. New chromophores could be devised which provided a substantial increase in the electrooptical response and lead to photorefractive materials with unprecedented refractive index modulation (delta n = 10(-2) at E = 28 V micron-1) and two-beam coupling gain. These improvements could only be accomplished by optimizing the electronic structure of highly conjugated merocyanine dyes to perfectly balanced dyes in the charge resonance limit (such as aminothienyl oxopyridone (ATOP) and indoline dimethine oxopyridone (IDOP) derivatives), considering effects of supramolecular ordering (dipolar aggregation), and adjusting the compatibility of the dyes to photoconducting polymers (like poly-N-vinylcarbazole). In particular, optimized glass-forming dyes (such as 2BNCM and ATOP-4) combine the dual functionalities of charge transport and electrooptical response and exhibit photorefractivity even in absence of any additional photoconductor.

Conformational switching and exciton interactions in hemicyanine-based bichromophores

Conformational changes in two hemicyanine-based bichromophores were demonstrated by varying the polarity as well as temperature of the medium. Dramatic changes in the ground and excited singlet state properties were observed upon folding of the bichromophores, due to the formation of intramolecular aggregates of H-type. These aspects were studied, in detail, using steady-state absorption and time-resolved fluorescence spectroscopy. Time-resolved fluorescence studies indicate that both the bichromophores exhibit a monoexponential decay, with a short lifetime, in mixed toluene-CH(2)Cl(2) solvents having lower proportions of toluene. Interestingly, biexponential decay with short and long-lived species was observed at higher proportions of toluene, due to the presence of unfolded and folded forms. Folding results in the intramolecular stacking of the chromophores which restrict their torsional dynamics, leading to a longer lifetime. Upon laser excitation, the folded form of the bichromophore undergoes rapid conformational changes, due to photoinduced thermal dissociation.