Light-Harvesting Hybrid Assemblies

Abstract

Light-harvesting hybrids have gained much importance as they are considered as potential mimics for photosynthetic systems. In this Concept article we introduce the design concepts involved in the building up of light-harvesting hybrids; these resemble the well-studied organic-based assemblies for energy transfer. We have structured this article into three parts based on the strategies adopted in the synthesis of hybrid assemblies, as covalent, semicovalent, and noncovalent procedures. Furthermore, the properties and structural features of the hybrids and analogous organic assemblies are compared. We also emphasize the challenges involved in the processability of these hybrid materials for device applications and present our views and results to address this issue through the design of soft-hybrids by a solution-state, noncovalent, self-assembly process.

Introduction

Supramolecular organization of chlorophyll pigments (chromophores) is crucial for the biological photosynthetic processes that convert solar energy to chemical potentials. These chromophoric arrays act as an antenna to absorb the incident light (light-harvesting) and subsequently funnels the excitation energy to an acceptor reaction center.1 The whole process is composed of a series of photophysical events, such as energy- and electron-transfer processes. In recent years, synthetic chemists have followed nature’s design principles and constructed various artificial light-harvesting assemblies based on π-conjugated molecules to get more insight into the photochemical processes involved in the natural photosynthesis.1 It has been realized that not only the molecular structure but also the spatial organization of these molecules is crucial for an efficient light-harvesting process. More importantly, these chromophoric assemblies are shown to have a great potential for various organic electronic devices, for which the supramolecular organization of the molecules is of great importance for a better performance.2

Herein we focus on the recent developments in hybrid (organic–inorganic) light-harvesting assemblies with a special emphasis on their design principles. This field of research bears resemblance to the well-studied purely organic-based chromophoric assemblies, and interestingly an analogy can be easily drawn to the methods developed for hybrid light-harvesting scaffolds with those established for various organic antennae (vide supra). Accordingly, we organize the hybrid assemblies into covalent, semicovalent and noncovalent systems and discuss about their qualities in the device point of view, in this concept article (Figure 1).

Figure 1. Design principles of artificial light-harvesting assemblies. The analogy between the design of organic and hybrid systems are classified based on a) covalent, b) semicovalent and c) noncovalent approaches (D: donor chromophore, A: acceptor chromophore).

Light-Harvesting Assemblies: From Covalent to Noncovalent Approaches

The process of light-harvesting in natural photosynthetic systems is very well understood. The excitation energy transfer occurs through Förster resonance energy transfer (FRET), which is a through-space mechanism. The efficiency of these FRET process mainly depends on the orientation of transition dipoles and the spectral overlap integral of donor and acceptor chromophores along with their interchromophoric distance. Hence the spatial arrangement of organic chromophores in a specific microenvironment is one of the prerequisites to achieve efficient artificial photon-harvesting antennas, mimicking the natural photosynthetic systems, in which energy- and electron-transfer processes are handled by well-organized donor–acceptor complexes at the microscale. A detailed description of various photophysical aspects of the energy-transfer process is beyond the scope of this concept article and the readers are referred to the excellent reviews for further understanding.1, 3 Over the last two decades, organic chemists have been able to design the required multicomponent assemblies by various covalent and noncovalent synthetic procedures. A selected anthology of various assemblies for light-harvesting is given in Figure 1.

Dendrimers are one of the first classes of organic assemblies that has been extensively studied to construct artificial antenna systems (covalent approach, Figure 1 a).4 The regularly structured branches and the core of the dendrimer were covalently modified with suitable donor and acceptor groups respectively, for efficient light-harvesting. The dendritic design allows the spatial distribution of the chromophores, and hence minimizes the quenching of donor through aggregation, similar to the fencing of chlorophyll molecules by polypeptide cages in natural antennae systems. Entrapment of acceptor chromophores into the dendritic cores (semicovalent approach, Figure 1 b), to some extent reduces the challenging synthetic procedures for the multicomponent covalent donor–acceptor dendrimers.5

Lately, noncovalent design principles have taken the center stage, as it helps to mimic light-harvesting assemblies in a supramolecular manner, which is more close to the strategy that nature adopts. In a key paper, Ajayaghosh and co-workers first extended the noncovalent approach to the self-assembled three dimensional fibrous networks made of π-conjugated molecules, for energy transfer.6 In an interesting approach, Meijer and co-workers have designed long one-dimensional π-stacked fibers of donor chromophores, with acceptor chromophores docked in between, which showed fast energy migration through the columns and an enhanced emission from the isolated acceptor molecules (noncovalent approach, Figure 1 c).7 These systems present an efficient antenna for entrapped acceptor molecules, which triggered a series of similar reports on various noncovalent scaffolds.8 The modulation of optical properties in organic assemblies is well studied and for further information the readers are referred to excellent reviews.2, 9

The Concept of Light-Harvesting Hybrid Assemblies

Though a variety of organic assemblies, such as dendrimers and chromophore-functionalized polymers,10 are shown to display efficient light-harvesting and energy-transfer properties, the difficulty in obtaining a high degree of organization at the macroscale limits their use in device applications. Although, the long-range ordering in organic chromophoric assemblies has been achieved by a supramolecular self-assembly approach (1D stacks and 3D organogel network), they suffer from fluorescence quenching due to the aggregation of dye molecules.2, 9 In natural photosynthetic systems, quenching by aggregation of chlorophyll molecules is prevented by fencing them with polypeptide cages. A similar strategy can be achieved in artificial systems by having inorganic scaffolds, like zeolites,3b, 11 mesoporous silica,12 periodic mesoporous organosilica (PMOs),13 metal–organic frameworks (MOF),14 clay,15 and so forth, to organize the donor–acceptor chromophores. The long-range periodicity and nanoscopic space present in these hosts help to spatially organize the dye molecules at the macroscale without giving room for aggregation. The inorganic host would also improve the thermal, mechanical, and photostability of the entrapped chromophore complexes compared to pure organic films, rendering the resultant hybrid materials ideal for the fabrication of high-performance optoelectronic devices. In addition, these scaffolds can be prepared in different morphologies and sizes. Encapsulation of semiconducting polymer or dye molecules in a particular conformation is also possible by using inorganic hosts. Spatial confinement, for example, in the hexagonally arrayed mesoporous silica channels, even helps to separate the interchain versus intrachain energy transfer of conjugated polymer poly[2-methoxy,5-(2-ethyl-hexyloxy)-p phenylenevinylene] (MEH-PPV).16 Moreover, the spatial constraints inside the pores could lock the guest molecules in a particular orientation and therefore arrest the rotational relaxation, leading to the deactivation of excitation energy. The presence of nanochannels in the periodic porous materials can facilitate the anisotropic absorption of light by the encapsulated chromophores.13b In addition to the confinement, the maneuverability of the surface charge inside the pores (or within the layers) and the ability to modify the framework walls with desired chromophores distinctively favor these scaffolds for the spatial organization of guest molecules.

In this article, we highlight the recent accounts of this concept in constructing light-harvesting hybrid assemblies. In this context, we define the light-harvesting hybrid as hybrids in which the organic chromophores (D and A) are spatially distributed in an inorganic matrix. The inorganic matrices include mesoporous silica, PMOs, zeolites, clays, MOFs, and so forth. Three different approaches are usually adopted to link the chromophores to the inorganic matrix, which are summarized in Figure 1 along with the analogous design for corresponding organic systems:

• 1)
  Covalent approach, in which both D and A chromophores are covalently grafted in an inorganic matrix template (Figure 1 a). MOFs with D and A linkers present an interesting example for hybrids based on this design and can be considered similar to dendrite scaffold.
• 2)
  Semicovalent approach, in which the D chromophores are covalently linked to an inorganic support and the A chromophores are encapsulated noncovalently (Figure 1 b). PMOs provide an example for this design, analogous to the dye-doped dendrimer and polymer organic counterparts.
• 3)
  Noncovalent approach in which D and A chromophores are noncovalently distributed in an inorganic template (Figure 1 c). This noncovalent approach which was used for the design of donor–acceptor organogels and 1D nanofibers could be extended for the multicomponent hybrid assemblies with inorganic clay and zeolite templates.

Zeolite presents an interesting inorganic 1D channel for the sequential insertion of D and A chromophores as shown by Calzaferri and co-workers. Dye-filled zeolites are shown to exhibit very efficient energy transfer, the details of which has been reviewed recently.11 Hence we restrict our discussion here to the other light-harvesting hybrids only.

Covalent Systems

Hybrid scaffolds with covalently attached donor (D) and acceptor (A) chromophores on an inorganic material provide an interesting structural motif analogous to the multichromophoric D–A dendrimers (Figure 1 a). This design provides a simple strategy to fabricate stable, multicomponent assemblies with tunable composition and spatial distribution of D and A chromophores. The covalent linking of chromophores in a hybrid scaffold protects them from leaching and phase separation. However, the lack of precision in positioning the chromophores is a drawback here in comparison to organic dendrimers. This would restrict the usage of these structures as a model for the basic understanding of photophysical processes. On the other hand, the mechanical stability and the relatively easy one-step synthetic procedure, compared to the multistep and tedious processes required for the dendrimers, make these hybrid assemblies suitable for large-scale device fabrication.

Several groups observed a very efficient FRET and light-harvesting phenomena in hybrid assemblies in which both types of chromophores are covalently linked to the inorganic matrix, like silicas, MOFs, and so forth. For example, Luh et al. have showed a very efficient energy transfer in hybrid silica thin films consisting of three covalently bound chromophores: stilbene, terphenylenedivinylene and hexathiophene (HT).17 MOFs constructed from polytopic organic linkers (ligands or struts) and metal ions, are another class of attractive hybrid scaffolds for the long-range assembling of chromophores in a covalent manner.18 The MOF can act as an artificial light-harvesting antennae either by the linker-to-linker energy transfer19a, 20 or by harvesting energy from the chromophoric linker scaffold to the (vide infra) noncovalently encapsulated acceptor molecules.19b

Hupp and co-workers have designed a pillared, paddlewheel type MOF (a boron dipyrromethene (BODIPY)–porphyrin-based BOP-MOF) and demonstrated its function as a well-organized donor-acceptor scaffold to promote energy transfer between the ligands (strut-to-strut energy transfer).20 Pyridine bis-functionalized BODIPY with green emission and zincated porphyrin tetraacid derivative with red emission constitutes the pillars and paddlewheels, respectively, for the Zn^II coordinated MOF (Figure 2 a). Excitation of the pillar at 543 nm resulted in the emission of paddlewheel at 667 nm, suggesting the energy transfer from pillar to paddlewheel (Figure 2 b). One highlight of this hybrid system is that the two ligands together show good efficiency across the entire visible spectrum.

Figure 2. a) Molecular structures of pillar and paddlewheel used in the synthesis of BOP-MOF. b) Confocal laser scanning microscopy (CLSM) images (solid white arrows) and the molecular structure (dashed white arrow) of BOP-MOF crystals. For illustrative purposes, when the crystals are shown as black, there is no emission. To denote emission in the “green” and “red” ranges, the remaining crystals are artificially colored green and red, respectively. Adapted from reference 20.

Although, MOFs provide an appealing architecture for efficient energy transfer, their limited solubility is a serious concern for its processability in the ultimate device applications. Furthermore, the chemically different donor and acceptor struts in Hupp’s system restrict the flexibility to vary the composition of the ligands as it would affect the structure of the MOF.20 These shortcomings can be overcome to a certain extent by designing MOFs in the form of nanostructures that have D and A linkers with similar functional groups and dimensions. With these objectives, Uvdal and co-workers reported nanoscale MOFs (NMOFs) as light-harvesting hybrid assemblies, in which the three dimensional nanoscaffold was achieved by π-conjugated dicarboxylate ligands (with different side-chain length) and lanthanide ions (Ln=Gd, Eu, Yb).19a The D and A linkers of comparable dimensions have been achieved by the clever choice of blue-emitting fluorene (L1, D linker) and the corresponding yellow emitting fluorenone (L2, A linker) based π-conjugated systems (Figure 3 a and b).19a It should be mentioned, however, that the fluorenone derivatives formed by the oxidation of fluorene chromophores are considered as a troublesome impurity for getting a clean blue light. Multicomponent disclike NMOFs fabricated from Gd^III and L1 chromophores (Figure 3 c), with small percentages (ca. 2 %) of L2 ligand showed excitation energy transfer from L1 to L2, when the former is selectively excited at 370 nm. In addition, the incomplete quenching of the L1 in mixed Gd-L1/L2 nanoparticles results in a broad emission covering most of the visible region to give white-light emission. Controlled experiment with the composites of homoligand NMOFs Gd-L1 and Gd-L2 ruled out the possibility of interparticle energy transfer due to physical mixing. This work is of great importance, as the solution-state behavior of NMOFs addresses the limited solubility and processability problems of the corresponding bulk crystalline analogues to certain extent. The fluorene-derived nanodisc donors have been further studied for energy transfer by the noncovalent encapsulation of the acceptor guest molecules (vide infra).

Figure 3. a) Schematic representation of the spatial organization of donor and acceptor chromophores and energy transfer between ligands in a metal-organic framework. b) Chemical structures of donor fluorene (L1) and acceptor fluorenone (L2) based carboxylic acid ligands and c) scanning electron microscopy (SEM) of the nanoscale MOFs formed when they are co-ordinated with Gd^III ions. Adapted from reference 19a.

Since MOFs constitute one of the frontier areas in materials chemistry, many different approaches with a better design for light-harvesting hybrids can be anticipated in near future. The appropriate choice of the linkers and metal ions are very crucial for the design of light-harvesting MOFs. The inorganic metal nodes should not quench or alter the organic chromophoric strut emission, owing to metal–ligand or ligand–metal charge-transfer processes, but it should promote a linker-based emission from the MOF.14 Another point of concern is that, if the metal nodes organize the transition dipoles of the ligands in a geometrically orthogonal fashion, then the energy transfer would not occur. However, this concern did not operate in the pioneering examples, at least, probably due to the slight deviation of the average value of ligand-metal-ligand angle from 90°.20

Semicovalent Systems

One of the most important features of dendrimer chemistry is the possibility to covalently insert required chemical units in predetermined sites in their architecture. Furthermore, dendrimers exhibit internal cavities in which acceptor chromophores can be hosted in a noncovalent manner. If the acceptor molecules can be selectively entrapped into the core of a dendrimer with a donor scaffold, an efficient energy transfer can be envisaged from the periphery to the core of the dendrimer. Selective encapsulation of rhodmaine B dyes was achieved by the elegant design of an amphiphilic poly(propylene imine) dendrimer covalently functionalized with oligo(phenylenevinylene) (OPV) donor branches.5b The selective entrapment of hydrophilic rhodamine at the polar propylene imine core resulted in the energy transfer from OPV shell to the rhodamine dye molecules (Figure 1 b).

A similar design principle has been recently extended to hybrid materials for light-harvesting antenna. This semicovalent approach (one of the chromophores is covalently linked to the matrix and the other is noncovalently entrapped) for the compartmentalization of donor and acceptor molecules was achieved by the use of ordered porous materials, such as periodic mesoporous organosilicas (PMOs) and metal organic frameworks (MOFs). The walls or the skeleton of the frameworks were constructed with fluorescent organic donors. This provides an intensely packed chromophoric scaffold, analogous to π-conjugated dendritic core, for an efficient light absorption and excitation energy transfer. The confined environment provided by the pore channels was used for the noncovalent encapsulation of acceptor molecules. The hybrid design allows chromophores to be located in two spatially separated regions, that is, in the frameworks and in the pore channels, which is an ideal configuration to promote excitation energy transfer. More importantly, the transparent nature of these hybrid systems offers considerable advantage for device fabrication.

In one of the pioneering examples, Stupp et al. fabricated hybrid silicate films by a sol–gel technique, in which the acceptor rhodamine B derivative, containing a triethoxysilane group, was covalently grafted.21 Doping of these hybrid films with charged oligo(p-phenylene vinylene) (Amph-OPV) amphiphile by spin-casting led to energy transfer from Amph-OPV to rhodamine derivative (Figure 4). Using neat, 100 % chromophore-bridged alkoxysilane precursors, Inagaki and co-workers synthesized PMOs consisting of densely packed arrays of chromophores in the walls with a molecular-scale periodicity (Figure 5).22 The as-synthesized PMOs are filled with the template surfactants in a cylindrical micellar manner, which provides the opportunity to incorporate various dye molecules in their mesochannels. The amphiphilic nature of the surfactant in the mesochannel allows the incorporation of both polar and nonpolar acceptor molecules as shown in Figure 5. Dyes like coumarin and DCM (4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran) are known to stay at the polar head of the micelles, whereas hydrophobic dyes such as ruburene (Rub) prefers to stay at the center of the mesopore filled by hydrophobic tails of surfactant (Figure 5).

Figure 4. Chemical structures of a) oligo(p-phenylene vinylene) amphiphiles (Amph-OPV) and b) the rhodamine B derivative used in the formation of the silicate domain. c) Photographs of Amph-OPV (n=2)/poly(2-hydroxyethyl methacrylate) film (left) and Amph-OPV (n=2)/silicate film (right) from precursor solutions containing 2 mol % of rhodamine B derivative under UV irradiation (λ=365 nm). Adapted from reference 21.

Figure 5. Schematic representation of the organization of a) polar (DCM) and b) nonpolar (Rub) acceptor chromophores in the mesochannels of periodic mesoporous silica having donor bridged walls (cyan objects represents the surfactants). Adapted from reference 22.

Light-harvesting PMOs with biphenyl-bridged walls (Bp-PMOs) and coumarin 1 in their mesochannels, are shown in Figure 6 a.23 The energy-transfer process was investigated in both powder and in the film states of Bp-PMO loaded with coumarin 1. The synthesis process for the film and powder form of Bp-PMO hybrids were different, and so is the acceptor loading into their channels. The Bp-PMO film loaded with coumarin 1 was prepared by polymerizing the mixture of D and A chromophores on the substrate (pre-synthetic loading), whereas the acceptor was loaded separately into the Bp-PMO powder after its preparation (post-synthetic loading). Efficient energy transfer was observed in both cases, although it was more efficient in the powder compared to the film. In the powder form 0.8 mol % coumarin 1 was sufficient enough to completely harvest the emission of Bp-PMO (Figure 6 c). It was estimated that the light energy absorbed by approximately 125 biphenyl groups in the framework is funnelled to a single coumarin 1 molecule in the channel with almost 100 % quantum efficiency, resulting in an antenna effect and a significant enhancement of emission from the coumarin 1 dye. Since visible-light harvesting is preferable for many applications, Bp-PMO (both film and powder forms) has the limitation that biphenyl chromophores absorb only in the UV region (>300 nm). To overcome this, use of extended π-conjugated systems as the antennae chromophores on the PMO walls is recommended. However, the design of such PMOs are challenging, as the bulky and large organic bridges can increase the distance between the silyl linkers, which would affect the ordered periodicity of the resulting polymerized silica. Nevertheless, visible-light absorptive (up to 450 nm) PMO powder (ACD-PMO) with a highly ordered hexagonal structure has been achieved by the use of a relatively less bulky acridone-based organosilane precursor.24 An efficient energy transfer was observed from the green luminescent acridone chromophores in the ACD-PMO, when encapsulated with red-emitting DCM dyes. In both the examples discussed above, the overall quantum yield of the hybrid systems has significantly increased, reiterating the use of energy transfer in amplifying the emission of low emissive materials.

Figure 6. Energy-transfer properties of coumarin 1 dye-doped Bp-PMO. a) Schematic representation of coumarin 1/Bp-PMO, Bp=biphenyl (Black dots represents the loaded coumarin 1 molecules). b) Molecular structures of coumarin 1 acceptor and biphenyl silane (BP-Si) donor. c) Fluorescence spectra for coumarin 1/Bp-PMO powders with 0–2.35 mol % coumarin 1 (excitation wavelength, 270 nm). Adapted from reference 23.

The FRET strategy in PMOs has been further extended to obtain color-tunable visible-light emission by the multiple doping of appropriate dyes in the mesochannels. Among different emission colors, the designing of white-light LEDs has always been a challenge. The general approach for the design of white-light-emitting materials involves either the mixing of blue-, green-, and red-emitting, noninteracting chromophores, or by the partial energy transfer between a carefully selected D–A pair.25 White-light emission has been achieved in PMO hybrids by using an oligo(phenylenevinylene)silane–ruberene donor–acceptor pair (OPV-Si-Rub, Figure 7).26 In this case, meso-structured OPV-Si films (mOPV-F) were prepared by placing the OPV-Si precursor on a glass substrate. The resulting blue-emitting silica framework was highly transparent and showed more than 99 % light transmittance at 600 nm. Rub, a yellow emissive (520–700 nm) chromophore was selected as acceptor dye due to good overlap in the absorption spectra of OPV-Si. As the concentration of Rub increased, the emission of the mOPV-F slowly decreased and 100 % FRET was achieved with 5 mol % Rub (Figure 7 a). Careful tuning of the OPV-Si and Rub composition led to pseudo-white-light emission at 0.6 mol % loading of the acceptor, with (0.31, 0.41) CIE (International Commission on Illumination) co-ordinates (Figure 7 b). Perfect-white-light-emitting PMOs could also be achieved by using tetraphenyl pyrene (TPPy) organosilane precursors and rhodamine 6G (Rhd6 G) acceptor molecules (Figure 7 c and d).27

Figure 7. a) Fluorescence spectra of mOPV-F films doped with Rub (0–10 mol % ratio to the OPV-Si unit, λ_exc=400 nm) and molecular structure of OPV-Si is shown in the inset. b) Emission colors in the CIE 1931 chromaticity diagram calculated from the fluorescence spectra of Rub doped mOPV-F films and the photographs of the films under 365 nm UV irradiation. c) Molecular structures of TPPy organosilane precursors and Rhd6 G. d) Photograph of the LED lamp (emission wavelength: 390 nm) coated with a TPPy-PMO/Rhd6 G (1.0 mol %) film. Adapted from references 26, 27.

The flexibility of loading the acceptor dyes in the mesochannels and the possibility to incorporate a wide range of organic chromophore bridges into the walls helps to produce a variety of luminescent PMOs with tunable energy transfer and emission color. Furthermore, PMOs can also be made as highly transparent films, which would avoid the loss of luminescence efficiency due to scattering and hence more suitable for the fabrication of LEDs, although processability is a concern here as well.

Uvdal and co-workers have also used their nanoscale MOFs (vide supra), formed from fluorene donors (L3) and lanthanides (Ln), for the noncovalent encapsulation of the guest molecules to promote energy transfer (Figure 8).19b Cationic guest, trans-4-styryl-1-methylpyridiniumiodide (SMP), was used as the acceptor and a temperature-dependent encapsulation process was found to be driven by the negatively charged carboxylate environment near the metal connectors. The energy-transfer efficiency was found to be dependent on the temperature and the method of acceptor loading. The disc-shaped MOFs synthesized in the presence SMP have shown better light-harvesting properties than that with post-synthetic loading (with SMP). The resultant MOFs have good visible luminescence, which can be used for various applications. Similarly, SMP has also been encapsulated in the one-dimensional nanostructures of the fluorene (L1)-based co-ordination polymers to achieve efficient light harvesting.19c

Figure 8. Schematic illustration of light-harvesting process in nanoscale MOFs. L3 is a fluorene based donor ligand and a trans-4-styryl-1-methylpyridiniumiodide (SMP) acceptor was encapsulated during the synthesis of MOF with Gd(OAc)₃. Adapted from reference 19b.

Noncovalent Systems

Though the hybrid assemblies discussed above have several advantages, the processability is one of the major concerns in these systems. Unlike organic assemblies and polymers, these hybrids are in the form of powders (hard hybrids), which would definitely affect the fabrication of devices. The more efficient approach towards large-area applications would be solution-processable hybrids, which can be easily spin-coated on to surfaces. We consider that the nanoscale approach in MOFs (NMOF) is an interesting initiative towards this direction, although the efficiency and versatility of this approach are yet to be proved.19 The next generation of light-harvesting assemblies require careful design in order to endow them with easy processability and in this aspect soft-hybrid materials would be of particularly importance.

Self-assembly through noncovalent interactions, an efficient bottom-up approach to design functional soft-materials is well established in organic systems.2, 9, 28 A similar approach has been extended to the two-component self-assembly of D and A chromophores for the design of soft supramolecular materials.29 The resultant soft, multichromophoric systems are shown to be efficient light-harvesting assemblies. However, the extension of this concept to hybrid organic–inorganic systems is challenging, mainly because of the hard and crystalline nature of the inorganic counterparts. The most sought-after design principle in this direction would be the noncovalent self-organization of organic donor and acceptor chromophores on the transparent inorganic counterpart. Some of the initial approaches were to exploit the interlayer galleries of multilayered inorganic minerals as hosts for the organic guests.30 The high charge density in these layers make them versatile 2D platforms to host many charged guest molecules.31 α-Zirconium phosphate (α-ZrP)32 and sumecton SA (SSA)33 both fall in this category of layered materials and have been used to organize the donor and acceptor chromophores to demonstrate energy transfer. Takagi et al. have used saponite clay, which is anionically charged, for the organization of cationic D and A porphyrin and showed a very efficient energy transfer.34

However, these studies are more analogous to host–guest chemistry rather than supramolecular self-assembly approach. It is evident that a noncovalent hybrid approach is possible only if the inorganic material can also be dispersed in solution so that a true organic–inorganic hybrid co-assembly can be envisaged. This noncovalent approach would also impart unprecedented reversibility and dynamics to the light-harvesting hybrid assemblies. With this objective in mind, we have chosen an amino-group-functionalized organoclay for the design of hybrid clay materials. The repulsion between the layers due to the protonation of amino groups in water makes this clay easily exfoliable, even at higher concentration, leading to a clear solution.30 Furthermore, the regularly ordered functional amino groups can be exploited for the noncovalent attachment of the charged D and A molecules. The aminoclay (AC) that we have used is a layered magnesium organosilicate with a structure analogous to 2:1 trioctahedral smectites with an approximate composition of R₈Si₈Mg₆O₁₆(OH)₄, in which R represents covalently linked aminopropyl substituents.30 We have used tetracarboxylates of coronene (CS, blue emission) and perylene (PS, green emission) as donor–acceptor molecules, because of their high solubility in water (Figure 9 a).35 The negatively charged carboxylate groups of these dyes interact electrostatically with the positively charged aminoclay, and the bifunctional nature of both organic and inorganic building blocks further facilitated a three-dimensional self-assembly in aqueous solution, thus resulting in noncovalent hybrid materials (Figure 9 b). Most importantly, the extent of cross-linking and hence the size of these hybrid assemblies in solution could be tuned by the dye to clay ratio. Very efficient energy transfer from CS to PS was observed in highly cross-linked hybrids in solution when the former was excited at 350 nm, resulting in an intense green emission from perylene chromophores. An antenna effect is involved in this light-harvesting process, because the energy-transfer-sensitized emission intensity of PS is 11 times higher compared to its direct excitation. One of the unique characteristics of this noncovalently assembled light-harvesting hybrid is the fast dynamics of molecules, even in the cross-linked state, which was probed with FRET kinetics after the introduction of acceptor molecules to pre-formed CS-clay hybrids.

Figure 9. a) Chemical structures of a single layer of aminoclay (AC) and anionic dyes (CS and PS). b) Schematic representation of the self-assembly and energy transfer in clay/dye hybrid gels.

Interestingly, when the concentration of hybrid assemblies was increased, by keeping the dye/clay ratio similar to the highly cross-linked state in water, very stable transparent, self-standing hydrogels were formed (Figure 10). Energy-transfer studies in these hybrid gels showed complete quenching (100 % FRET efficiency) of donor emission (CS) even with small amount of PS (<10 mol %), owing to very efficient energy transfer. These light-harvesting soft materials could be successfully processed onto solid substrates by spin/drop casting and the resultant transparent films showed significant donor quenching, even with less than 1 % acceptor loading. Hence, it is evident that noncovalent approach can yield solution processable light-harvesting hybrid films, which is very important from a device point of view. It is worth mentioning that in this case, the synthesis of hybrids is done in solution, compared to the in situ synthesis of hybrids in the film state after the pre-loading of monomers.23 Furthermore, the energy transfer by co-assembly increases the fluorescence quantum yield of the hybrid gels and films, as the individually assembled dye/clay hybrids were virtually nonfluorescent. In addition, the emission of the hybrid gels were tuned from green to yellow by increasing the acceptor dye concentration, which resulted in the energy transfer from CS to PS aggregates.

Figure 10. Schematic representation of the energy transfer from CS to isolated (1 mol %) and aggregated (50 mol %) PS in clay-dye hybrid gels and films. Adapted from reference 35.

Summary and Reflections

Our aim in this article has been to introduce to the reader the design concepts of light-harvesting hybrid assemblies, a recent but fascinating approach towards artificial photosynthetic systems. The reasons behind this increasing interest are the impressive material properties of these novel class of organic–inorganic hybrids and the intellectual challenges ahead for the preparative chemists in the synthesis of structurally complex, multicomponent organic systems. We have classified the design concepts in hybrid assemblies into three categories analogous to the design principles involved in the well-established area of organic antennae systems (Figure 11). Soft, tuneable inorganic scaffold with high periodicity has the potential to organize the donor and acceptor chromophores to maximize the energy transfer and thus the efficiency of light-harvesting. These organized arrays of chromophores with structural periodicity further reduces the fluorescence quenching due to aggregation, and is reminiscent of the architecture of natural light-harvesting complexes, in which antenna molecules are coated by polypeptides. In addition, the high mechanical and thermal stability, together with the transparent nature of these hybrids are very attractive from a device point of view. The energy transfer and hence the emission color of these hybrids can be fine tuned by an appropriate choice of dye molecules. Noncovalent design principles have further addressed the processability of these hybrid light-harvesting systems.

Figure 11. Classification of hybrid light-harvesting assemblies. a) covalent approach, b) semicovalent approach and c) noncovalent approach. (green: inorganic counterpart, blue: donor chromophore, red: acceptor chromophore).

The concept of light-harvesting hybrids is only in its early stages, but the initial reports have already shown promises in terms of energy-transfer efficiency and device applications. A number of design principles, with a perfect blend of covalent and noncovalent synthetic approaches have been proposed for light-harvesting hybrids. Chemists should begin to address new properties and functions for this novel class of hybrid assemblies. One of the challenges in hybrid assemblies could be the design of an energy gradient for the directional flow of energy through cascade reactions, which has been achieved elegantly in dendrimers. We believe that detailed studies on the photophysics of these hybrids may contribute to the understanding of photosynthesis and hence efforts should be made to develop design principles with predictable structure and precise positioning of components. Since processability of hybrid materials is an important issue for device applications, efforts should be targeted towards the design of solution processable light-harvesting hybrids. The noncovalent design of soft-hybrids through solution state self-assembly is an efficient approach towards this target. Furthermore many interesting applications of the dynamic nature of the multichromophoric hybrids, designed through noncovalent principles, can be envisaged in sensors, molecular recognition and catalysis, although it is too early to draw conclusions. Irrespective of the future developments and their applications, we believe that this concept of light-harvesting hybrid assemblies is a striking example of the success of interdisciplinary research for the development of new functional materials.

Acknowledgements

We are grateful to DST and JNCSAR for financial support. We thank Mr. Amritroop Achari for the artwork. We also acknowledge all our co-workers involved in the project on light-harvesting hybrids. The authors also thank Prof. C. N. R. Rao (FRS) for his kind support and encouragement.