Self-assembly of Linear ?-Systems: Electronically Active Supramolecular Architectures
1. Toroidal Nanostructures from Hydrogen-bonded Rosettes of Melamine-Linked Oligo(p-phenyleneethynylene)and Cyanurates.
Self-assembly of rigid p-conjugated oligomers into well defined nanostructures with controlled size, morphology and optical properties is a challenging task in the “bottom-up” construction of supramolecular architectures. Controlling the self–assembly via concerted action of noncovalent interactions is important in such an approach. In this context the complimentary multiple H-bonding modules such as melamines and cyanurates might play a key role in controlling the self assembly of p-conjugated oligomers. Aggregation of these modules leads to the rosette (macrocyclic) and tape-like (linear or crinkled) architectures. Superstructures hierarchically organized from rosette and other related supramolecular macrocyles reported so far are limited to extended columnar architectures. Herein we present an unprecedented self-organization of hydrogen-bonded rosette assemblies of oligo(p-phenyleneethynylene) (OPE) attached melamine and a cyanurate derivative in aliphatic solvent, leading to the formation of toroidal objects of nanometer dimension. These molecular components forms organogels at more concentrated conditions whereas in the solvent-free state liquid crystalline behavior was observed, and hence a unique self-assembly system with multiple functional properties (Angew. Chem. Int. Ed. 2008, 47, (25)).
Self-assembly of melamine-linked OPE 1 and cyanurate 2.
2. Self-Assembly of Oligo(para-phenylenevinylene)s
The arene–perfluoroarene(ArH–ArF) interaction, which has been extensively studied in the field of solid-state chemistry, is exploited in the hierarchical self-assembly of oligo(paraphenylenevinylene)s (OPVs) with controlled longitudinal fiber growth that leads to gelation. The size of the self assembled fibers of a pentafluorophenyl-functionalized OPV 5 could be controlled through C-F···H_C hydrogen bonding and ? stacking. The ability of fluoroaromatic compounds to form excited- state complexes with aromatic amines has been utilized to form a supramolecular exciplex, exclusively in the gel state, that exhibits enhanced emission. Thus, the commonly encountered fluorescence quenching during the self-assembly of OPVs could be considerably prevented by exciplex formation with N,N-dimethylaniline(DMA), which only occurred for the fluorinated OPV and not for the non fluorinated analogue 4. In the former case, a three fold enhancement in the emission intensity could be observed in the gel state, whereas no change in emission occurred in solution. Thus, the major limitations of spontaneous fiber growth and fluorescence self quenching encountered in the self-assembly of OPVs could be controlled to a great extent by using the versatile ArH–ArF interaction.
3. Hybrid gel materials
Single-walled carbon nanotubes (SWNTs) represent a novel class of quasi one-dimensional material that exhibits unique chemical and physical properties, allowing wide range of applications from optoelectronics to biology. The major limitation of SWNTs is their poor solubility in solvents. Covalent functionalization of SWNTs is known to modulate the chemical and physical properties which in many cases may create defect sites thereby severely affecting the electronic properties. An alternative approach is physical interaction of organic molecules with carbon nanotubes (CNTs) leading to their dispersion in organic and aqueous solvents. It enables dispersion and processing while preserving CNTs intrinsic properties. We prepared hybrid materials through self-assembly approach by interacting CNTs with oligo(p-phenylenevinylene)s (OPVs). OPVs are known to form self-assembled architectures with controlled size, shape, and function. Noncovalent interaction of CNT surfaces with OPVs leads to better dispersion of SWNTs in common organic solvents. SWNT is found to accelerate the self-assembly of OPVs in low concentrations leading to the formation of a hybrid p-gel in hydrocarbon solvents. Remarkably, this phenomenon occurs below the normal critical gel concentration of OPVs. The gel stability is significantly enhanced by the addition of SWNTs leading to their uniform dispersion in the gel phase. Optical and morphological studies revealed the presence of exfoliated carbon nanotubes which are encapsulated within the gel matrix. (Angew. Chem. Int. Ed. 2008, 47, 5746)
Chemical structures of OPV derivatives and schematic representation of OPV-CNT gel formation
4. p-Gels and Nanoarchitectures of p-Phenylenevinylenes
Oligo (p-phenylenevinylene)(OPV) derivatives are extensively used in organic electronic devices due to their favorable optoelectronic properties. Structural modifications and intermolecular interactions are two different approaches to modulate their optical properties. When properly functionalized, these molecules self-assemble to form entangled nanostructures via cooperative interactions of H-bonding and p-stacking leading to the gelation of hydrocarbon solvents. Detailed microscopic studies revealed the formation of supramolecular tapes of nano-to micrometer size(Figure 1). Thickness of these tapes are in between 3-20 nm. The self -organization of the chromophores and the resultant gelation has remarkable role in the modulation of the optical properties.(J. Am. Chem. Soc. 2001, 123, 5148; Chem. Eur. J. 2005, 11, 3217; Top. Curr. Chem. 2005, 258, 83; Chem. Commun. 2005, 593). We are currently involved in the self – assembly behavior and properties of a variety of conjugated molecules such as OPVs, OPEs, thiophenes, p-phenylenes and fluorenes.
Figure 1. Self-assembled gels comprising of nanostructured supramolecular tapes of OPVs
5. Helical Supramolecular Architectures
Incorporation of remote chiral handles on OPVs results in the formation of helical structures. Left-handed-helical nanotapes are formed from higher – order self-assemblies of chiral aggregates during the gelation of a rigid
p-conjugated oligo(p-phenylenevinylene)s with remote chiral handles (C-OPV) (Figure 2). Under higher concentrations, micrometer sized helical ropes are formed. The concentration and temperature dependent CD-signal changes observed for these assemblies are indicative of a helix transition during the hierarchical self-assembly process which is reminiscent of the folding and unfolding of helical biological macromolecules. (Angew. Chem. Int. Ed. 2004, 43, 3422).
Figure 2. Formation of helical structures of chiral OPVs
Coassembly of gel forming chiral and achiral oligo(p-phenylenevinylene)s using the “sergeants and soldiers”approach induces helicity to coassembled p-gels with opposite handedness at very low composition of the sergeants
(Figure 3b). Under higher compositions, the inverted helicity goes back to the original helicity of the sergeants. During the process an unprecedented formation of fused M-and P-helices were observed (Figure 3b). We could provide visual evidence for the formation of inverted and fused helices originated from common chiral centers using AFM techniques which are complemented by circular dichorism spectroscopy. (Angew. Chem. Int. Ed. 2006 45, 1141).
Figure 3. Creation of M- and P- helices of OPVs using the sergeant and soldiers experiments
6. Supramolecular Control of Chromophore Organization and Helicity
Through a rational approach we have exploited cholesterol moieties in directing chromophore assemblies in different pathways leading to helical assemblies with distinct optical, chiroptical and morphological features. The monocholesterol attached oligo(p-phenylenevinylene)s prefer to form pseudo J-aggregates with tilted chromophore packing which showed red-shifted bands in the absorption and emission spectra. However, the corresponding bischolesterol derivatives form pseudo H-aggregates with a twisted chromophore arrangement having blue-shifted absorption and emission bands. The mode of packing of the chromophores and the overall morphologies of the self-assembled structures were established from the detailed optical, chiroptical and AFM studies (Figure 4). (Angew. Chem. Int. Ed. 2006, 45, 456).
Figure 4. Two different supramolecular organizations of OPV chromophore depending upon the substitution pattern of cholesterol
7. Light Harvesting Scaffolds
OPV gels are excellent excitation energy donor scaffolds. Gelation facilitates the transfer of excitation energy via fluorescence resonance energy transfer (FRET) exclusively from the self-assembled structures to entrapped acceptors. Figure5 illustrates energy transfer from OPV gel to entrapped Rhodamine B. Interestingly, FRET is not possible from single OPV molecules thereby establishing the role of the self assembled nanostructures in light harvesting. The emission from the light may be shut off in a thermoreversible fashion since the self assembly breaks above the gel melting temperature. These studies have opened up new avenues to the design of artificial light harvesting assemblies. (Angew. Chem. Int. Ed. 2003, 42, 332; J. Am. Chem. Soc. 2006, 128, 7542). We are currently working to develop efficient light harvesting assemblies using organogels.
Figure 5. Gelation assisted excitation energy transfer
In order to design an efficient light harvesting system which can funnel excitation energy to a few acceptor molecules, it is necessary to have an acceptor which is compatible with the donor self-assembly. Recently we have addressed the issue with the help of OPVs having different HOMO-LUMO energy levels as donor and acceptor (Figure 6). The rational choice of OPVs with dipolar end function groups allowed the tuning of the emission in the molecular level and further modulation of the optical properties in the supramolecular level. Detailed studies revealed that the optical properties of the gel forming OPV1 (donor) and OPV4(acceptor) are ideal for energy transfer. Addition of 2.62 mol% of OPV4 to a n-decane gel of OPV1 showed a 90% quenching of the emission of donor and the simultaneous emission from the acceptor.
Figure 6. Structure of OPV derivatives and the photographs of the hexane gels under the illumination at 365nm.
Recently we have designed a supramolecular donor-acceptor organogel with tunable emission from green to red through controlled energy transfer by simply varying the acceptor concentration. Upon excitation of the decane
gels of the donor with 0-2 mol% of the acceptor, quenching of the emission of the former with the formation of the monomer emission of the latter at 555 nm is observed. Upon further addition of acceptor (2-20 mol%), the emission was continuously red shifted to 610 nm which corresponds to the aggregate emission of the acceptor. Thus efficient trapping of excitons by “isolated” or “aggregated” acceptors through a subtle control of the self assembly and the photophysical properties of the donor-acceptor building blocks allowed a continuous shifting of the emission color anywhere between green and red (509-610 nm) in a supramolecular light harvesting system (Figure 7).
Figure 7. Two different supramolecular organizations of OPV chromophore depending upon the substitution pattern of cholesterol
8. Self-assembly of p-Phenyleneethynelenes
In contrast to the self-assembly of OPVs to nanotapes, an analogous OPE, (OPE1) in decane resulted in the formation of nanoparticles, microspheres, superstructures and eventually blue-emitting organogels. The morphology of these structures was confirmed by AFM, SEM, TEM, DLS and optical microscopy analyses. At a concentration of 1 x 10-6 M, nanoparticles with average size of 94 nm were formed whereas up to a concentration of 1 x 10-4 M, microspheres of 5-10 mm with strong blue fluorescence were obtained. Moreover, the spherical aggregates are vesicular in nature as evident from the TEM analysis. However, at a concentration of 3.5 x 10-3 M, ( Critical Gelation Concentration (CGC)) elongated giant structures of micrometer size were formed as evident from the SEM analysis (Figure 8). The present report provides a simple method to prepare stable spherical and extended self-assemblies of a linear p-conjugated molecule with different size and shape from a single solvent. This is the first report on the self-assembly of a short linear OPE in a single solvent resulting in nanoparticles, microspheres and super structured blue- light emitting organogels.
Figure 8. Schematic representation of the self-assembly process.
Co-assembly of OPE1 with a chiral analogue COPE1, exhibited an induced CD from the latter to the former resulting in the formation of helical supramolecular tubules. AFM and TEM analyses of the drop casted samples of OPE1, COPE1 and the coassembly revealed vesicular morphology for OPE1, nonspecific morphology for COPE1 and helical tubular morphology for the coassembly. Simple mixing and sonication of COPE1 and OPE1 revealed the fusion of the vesicles on the way to helical extended tubular aggregates with weak CD which upon heating and cooling resulted in the complete transformation of the fused vesicles to helical tubular assemblies with strong exciton CD signal (Figure 9).
Figure 9. Schematic representation of the vesicle to helical tubular assembly formation
Low Band Gap Polymers and NIR Dyes
We are interested in developing conjugated polymers with low band gaps and intense near infra red absoption, which are rather difficult to achieve by conventional synthetic approaches (Org. Lett. 2001, 3, 2595). We have developed a new strategy which involves the extension of conjugation in donor-acceptor-donor type squaraine dyes, by introducing an electron donating conjugated bridge, thereby facilitating strong electronic coupling between the dye moieties. Using this approach, a variety of polymers with broad window of absorption around 600-1400 nm could be synthesized. The intrinsic conductivities of these polymers could be varied between 10-4 – 10-7 S/cm by controlling the molecular packing with the help of hydrocarbon side chains (Chem. Mater. 2002, 14, 410; Chem. Soc. Rev. 2003, 32, 181).
We make use of bispyrroles with different aromatic bridging units. These fluorescent molecules are excellent substrates for the design of electro active polymers, cation sensors, low band gap systems, NIR dyes and photo- and electroluminescent polymers.
Functional Dyes and Fluorophores
1. A Near-Infrared Squaraine Dye as a Latent Ratiometric Fluorophore for the Detection of Aminothiol Content in Blood Plasma
Shown here a new NIR squaraine dye for the detection of thiols and aminothiol level in human blood plasma. The detection is based on the latent activation of fluorescence by thiol induced breaking of conjugation in contrast to the usually used color bleaching or fluorescence quenching which has several drawbacks. The probe selectively responds to thiols and aminothiols which allow their ratiometric detection due to the generation of new, noninterferring absorption and emission bands. We demonstrate a rare example for dual wavelength excitation ratiometric probe. Application of the probe is illustrated with the detection of the total aminothiol level in human blood plasma which has tremendous biological relevance. Using the new probe we could find that aminothiol content is high in the blood of heavy smokers which is responsible for many heart relat:
2. Design of Molecular Probes
Another area of our interest is the use of functional dyes and fluorophores to the design of molecular probes. Recently we have developed a Ca2+ ion sensor based on the principle of a metal ion steered folding and the resultant exciton interaction of a bichromophoric squaraine foldamer. This was the first report where the phenomenon of dye aggregation has been exploited as a signal transduction pathway for the detection of a cation using a bichromophoric foldamer. A rigid-flexible-rigid type squaraine bichromophore containing a flexible oxyethylene chain specifically binds with Ca2+ in the presence of other cations such as Mg2+ , K+ and Na+ with a visual color change from a light blue to an intense purple blue. The emission of the bichromophore gets quenched substantially upon Ca2+ binding (Figure 10). The signal transduction of the binding event is rationalized on the basis of the folding of the chromophores to form a face-to-face stacked conformation,akin to the H-aggregates of squa
Figure 10. Ca2+ ion induced folding of the squaraine dye into “H” foldamer.
Recently, this approach is developed as a general supramolecular strategy to design alkaline earth metal ion sensors. (J. Am. Chem. Soc. 2004, 126, 6590; J. Am. Chem. Soc. 2005, 127, 3156; Acc. Chem. Res. 2005, 38, 449).
Figure 11. Visible color change of the squaraine solution on addition of Mg2+, Na+, K+ and Ca2+
Another important result in this area is the development of a simple molecular probe for the selective sensing of Zn2+ which is a biologically important cation. (J. Am. Chem. Soc. 2005, 127, 14962). Application of this probe for the biological imaging of Zn2+ is under investigation.
Figure 12. Visual color changes of the solution on addition of metal ions and a schematic representation of the binding and signaling events.
3. Self-assembly of functional dyes
The rational design of the tripodal geometry facilitates the propensity of the squaraine aggregation from ill-defined structures to specific supramolecular architectures. They form nano-to-microsized spherical assemblies on mica surface when evaporated from acetonitrile solution. Surprisingly, upon binding with Ca2+ or Mg2+ , the spherical assemblies transform into extended nano sized fibrillar structures. Interstingly, a tripodal dye with attached chiral handles, which is CD silent in acetonitrile, exhibited induced bisignate CD signal upon binding with Ca2+ or Mg2+ . AFM images of the cation bound complex revealed the transformation of the spherical aggregates to helical structures of nanometer width and height with micrometer length. Decomplexation with EDTA showed complete reversal of the optical and chiroptical properties with deformation of the helical structures back to spherical assemblies. Noticeably, the aggregates of the chiral tripodal dye generated from acetonitrile-water mixture do not exhibit any CD signal or helical bias as evident from the AFM images. These observations reveal the role of a specific cation binding on the amplification of molecular chirality leading to supramolecular helicity and the consequent morphology change from spherical to helical structures. This is the first example for a reversible cation controlled spheres to helix transition during the self-assembly of a functional tripodal dye.
Figure 13. AFM textures of the tripodal squaraine self-assembly at different conditions