Synthesis, Photophysical Properties and Self-Assembly of a Tetraphenylethylene-Naphthalene Diimide Donor-Acceptor Molecule

The development of new π-conjugated molecular structures with controlled self-assembly and distinct photophysical properties is crucial for advancing applications in optoelectronics and biomaterials. This study introduces the synthesis and detailed self-assembly analysis of tetraphenylethylene (TPE) functionalized naphthalene diimide (NDI), a novel donor-acceptor molecular structure referred to as TPE-NDI. The investigation specifically focuses on elucidating the self-assembly behavior of TPE-NDI in mixed solvents of varying polarities, namely chloroform: methylcyclohexane (CHCl3 : MCH) and chloroform: methanol (CHCl3 : MeOH). Employing a several analytical methodologies, including UV-Vis absorption and fluorescence emission spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and dynamic light scattering (DLS), these self-assembled systems have been comprehensively examined. The results reveal that TPE-NDI manifests as distinct particles in CHCl3 : MCH (fMCH =90 %), while transitioning to flower-like assemblies in CHCl3 : MeOH (fMeOH =90 %). This finding underscores the critical role of solvent polarity in dictating the morphological characteristics of TPE-NDI self-assembled aggregates. Furthermore, the study proposes a molecular packing mechanism, based on SEM data, offering significant insights into the design and development of functional supramolecular systems. Such advancements in understanding the molecular self-assembly new π-conjugated molecular structures are anticipated to pave the way for novel applications in material science and nanotechnology.

Luminescence property switching in 1D supramolecular polymerization of organic donor–π-acceptor chromophores

The naphthalene monoimide building block endows with amide functionality undergoes supramolecular polymerization in a J type fashion in a particular co-solvent composition. This leads to luminescent property switching as a result of PET effect.

Fluorene benzothiadiazole co-oligomer based aqueous self-assembled nanoparticles

Self-assembled π-conjugated nanoparticles with tunable optical characteristics are appealing for sensing and imaging applications due to their intrinsic fluorescence, supramolecular organization and dynamics.

Consequences of a cosolvent on the structure and molecular dynamics of supramolecular polymers in water

Polar cosolvents are commonly used to guide the self-assembly of amphiphiles in water. Here we investigate the influence of the cosolvent acetonitrile (ACN) on the structure and dynamics of a supramolecular polymer in water, which is based on the well-known benzene-1,3,5-tricarboxamide motif. Hydrogen/deuterium exchange mass spectroscopy measurements show that a gradual increase in the amount of ACN results in a gradual increase in the exchange dynamics of the monomers. In contrast, the morphology of the supramolecular polymers remains unchanged up to 15% of ACN, but then an abrupt change occurs and spherical aggregates are formed. Remarkably, this abrupt change coincides with the formation of micro-heterogeneity in the water-ACN mixtures. The results illustrate that in order to completely characterize supramolecular polymers it is important to add time-resolved measurements that probe their dynamic behavior, to the conventional techniques that are used to assess the morphology of the polymers. Subsequently we have used time-resolved measurements to investigate the influence of the concentration of ACN on the polymerization and depolymerization rates of the supramolecular polymers. Polymerization occurs within minutes when molecularly dissolved monomers are injected from ACN into water and is independent of the fraction of ACN up to 15%. In the depolymerization experiments-initiated by mixing equilibrated supramolecular polymers with dissolved monomers-the equilibration of the system takes multiple hours and does depend on the fraction of ACN. Interestingly, the longest equilibration time of the polymers is observed at a critical solvent composition of around 15% ACN. The differences in the timescales detected in the polymerization and depolymerization experiments are likely correlated to the non-covalent interactions involved, namely the hydrophobic effect and hydrogen-bonding interactions. We attribute the observed fast kinetics in the polymerization reactions to the hydrophobic effect, whereas the formation of intermolecular hydrogen bonds is the retarding factor in the equilibration of the polymers in the depolymerization experiments. Molecular dynamics simulations show that the latter is a likely explanation because ACN interferes with the hydrogen bonds and loosens the internal structure of the polymers. Our results highlight the importance of the solution conditions during the non-covalent synthesis of supramolecular polymers, as well as after equilibration of the polymers.

Supramolecular Assembly of Phosphole Oxide Based Alkynylplatinum(II) 2,6-Bis(N-alkylbenzimidazol-2'-yl)pyridine Complexes-An Interplay of Hydrophobicity and Aromatic π-Surfaces

A new class of phosphole oxide based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) complexes were synthesized and characterized. Their self-assembly was driven by hydrophobic-hydrophobic and π-π stacking interactions. The self-assembly properties were also investigated by UV/Vis absorption spectroscopy, which revealed that the alkyl-chain length of the bzimpy moiety and the π-surface area of the alkynyl ligand have significant influence on the overall self-assembly process. The alkyl-chain length also affected the morphological structures of the aggregates, which were studied by transmission electron microscopy and scanning electron microscopy.

Effect of Differential Self-Assembly on Mechanochromic Luminescence of Fluorene-Benzothiadiazole-Based Fluorophores

Supramolecular self-assembly is an excellent tool for controlling the optical and electronic properties of chromophore-based molecular systems. Herein, we demonstrate how differential self-assembly affects mechanoresponsive luminescence of fluorene-benzothiadiazole-based fluorophores. We have synthesized two donor-acceptor-donor-type conjugated oligomers consisting of fluorene as the donor and benzothiadiazole as the acceptor. For facile self-assembly, both molecules are end-functionalized with hydrogen-bonding amide groups. Differential self-assembly was induced by attaching alkyl chains of different lengths onto the fluorene moiety: hexyl (FB-C6) and dodecyl (FB-C12). The molecules self-assemble to form well-defined nanostructures in nonpolar solvents and solvent mixtures. Although their optical properties in solution are not affected by the alkyl chain length, significant effects were observed in the self-assembled state, particularly in the excitation energy migration properties. As a result, remarkable differences were observed in the mechanochromic luminescence properties of the molecules. A precise structure-property correlation is made using UV-visible absorption and fluorescence spectroscopy, time-correlated single-photon counting analysis, scanning electron microscopy, and X-ray diffraction spectroscopy.

A self-assembled white-light-emitting system in aqueous medium based on a macrocyclic amphiphile

White light emission was successfully achieved with high quantum yield in a novel FRET platform based on a macrocyclic amphiphile.

Aggregation induced enhanced emission of 2-(2'-hydroxyphenyl)benzimidazole

In this study, the aggregation induced emission enhancement (AIEE) of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) is reported. To investigate the AIEE process of HPBI, absorption/fluorescence spectroscopy, fluorescence imaging and field emission scanning electron microscopy were employed. A comparative study with 2-phenylbenzimidazole (PBI) divulges the significance of the hydroxyl group in the AIEE process. Further, molecular dynamics simulations have been carried out with explicit solvent molecules to follow the aggregation process of HPBI with time. The obtained molecular dynamics simulation results not only predicted the formation of aggregates but also provided detailed insight and information on the molecular interactions. The cellular studies showed aggregates yield higher fluorescence in the visible region inside HeLa cells in comparison to monomeric compounds which failed to exhibit any visible fluorescence inside the cell. The obtained aggregates were further found to be biocompatible and therefore can be used for bio-imaging applications.

Non-coordinating anions assemble cyanine amphiphiles into ultra-small fluorescent nanoparticles

A non-coordinating anion, fluorinated tetraphenylborate, assembles specially designed cationic cyanine amphiphiles into 7-8 nm fluorescent nanoparticles that are >40-fold brighter than a single cyanine dye. This kind of anion, combining hydrophobic and electrostatic forces in aqueous media, constitutes promising building blocks in the self-assembly of functional nanomaterials.

Bottom-Up Hierarchical Self-Assembly of Chiral Porphyrins through Coordination and Hydrogen Bonds

A series of chiral synthetic compounds is reported that shows intricate but specific hierarchical assembly because of varying positions of coordination and hydrogen bonds. The evolution of the aggregates (followed by absorption spectroscopy and temperature-dependent circular dichroism studies in solution) reveal the influence of the proportion of stereogenic centers in the side groups connected to the chromophore ring in their optical activity and the important role of pyridyl groups in the self-assembly of these chiral macrocycles. The optical activity spans 2 orders of magnitude depending on composition and constitution. Two of the aggregates show very high optical activity even though the isolated chromophores barely give a circular dichroism signal. Molecular modeling of the aggregates, starting from the pyridine-zinc(II) porphyrin interaction and working up, and calculation of the circular dichroism signal confirm the origin of this optical activity as the chiral supramolecular organization of the molecules. The aggregates show a broad absorption range, between approximately 390 and 475 nm for the transitions associated with the Soret region alone, that spans wavelengths far more than the isolated chromophore. The supramolecular assemblies of the metalloporphyrins in solution were deposited onto highly oriented pyrolitic graphite in order to study their hierarchy in assembly by atomic force microscopy. Zero and one-dimensional aggregates were observed, and a clear dependence on deposition temperature was shown, indicating that the hierarchical assembly took place largely in solution. Moreover, scanning electron microscopy images of porphyrins and metalloporphyrins precipitated under out-of-equilibrium conditions showed the dependence of the number and position of chiral amide groups in the formation of a fibrillar nanomaterial. The combination of coordination and hydrogen bonding in the complicated assembly of these molecules-where there is a clear hierarchy for zinc(II)-pyridyl interaction followed by hydrogen-bonding between amide groups, and then van der Waals interactions-paves the way for the preparation of molecular materials with multiple chromophore environments.

Functional supramolecular polymers for biomedical applications

As a novel class of dynamic and non-covalent polymers, supramolecular polymers not only display specific structural and physicochemical properties, but also have the ability to undergo reversible changes of structure, shape, and function in response to diverse external stimuli, making them promising candidates for widespread applications ranging from academic research to industrial fields. By an elegant combination of dynamic/reversible structures with exceptional functions, functional supramolecular polymers are attracting increasing attention in various fields. In particular, functional supramolecular polymers offer several unique advantages, including inherent degradable polymer backbones, smart responsiveness to various biological stimuli, and the ease for the incorporation of multiple biofunctionalities (e.g., targeting and bioactivity), thereby showing great potential for a wide range of applications in the biomedical field. In this Review, the trends and representative achievements in the design and synthesis of supramolecular polymers with specific functions are summarized, as well as their wide-ranging biomedical applications such as drug delivery, gene transfection, protein delivery, bio-imaging and diagnosis, tissue engineering, and biomimetic chemistry. These achievements further inspire persistent efforts in an emerging interdisciplin-ary research area of supramolecular chemistry, polymer science, material science, biomedical engineering, and nanotechnology.

Self-assembly of benzothiadiazole-functionalized dinuclear platinum acetylide bolaamphiphiles for bio-imaging application

Benzothiadiazole-functionalized dinuclear platinum(ii) acetylide bolaamphiphiles have been demonstrated to form nanoparticles in protic solvents, serving as novel fluorescent labels for bio-imaging applications with good biocompatibility and sufficient stability.

Thermo-responsive white-light emission based on tetraphenylethylene- and rhodamine B-containing boronate nanoparticles

White-light emissive boronate nanoparticles have been prepared, which exhibit reversible and thermo-responsive emission in the investigated temperature range (5–65 °C) with a temperature sensitivity of 1.1% K⁻¹ in water.

Single-chain semiconducting polymer dots

This work describes the preparation and validation of single-chain semiconducting polymer dots (sPdots), which were generated using a method based on surface immobilization, washing, and cleavage. The sPdots have an ultrasmall size of ∼3.0 nm as determined by atomic force microscopy, a size that is consistent with the anticipated diameter calculated from the molecular weight of the single-chain semiconducting polymer. sPdots should find use in biology and medicine as a new class of fluorescent probes. The FRET assay this work presents is a simple and rapid test to ensure methods developed for preparing sPdot indeed produced single-chain Pdots as designed.

A carbazole–fluorene molecular hybrid for quantitative detection of TNT using a combined fluorescence and quartz crystal microbalance method

Self-assembled fluorescent rods and nanoparticles prepared from a carbazole–fluorene molecular hybrid have been used for the sensing of TNT.

Organic–inorganic nanohybrids and their applications in silver extraction, chromogenic Cu2+ detection in biological systems, and hemolytic assay

An organic–inorganic nanohybrid material is prepared for chemosensor development in aqueous media.

Self-assembled fluorescent organic nanoparticles for live-cell imaging

Fluorescent, cell-permeable, organic nanoparticles based on self-assembled π-conjugated oligomers with high absorption cross-sections and high quantum yields have been developed. The nanoparticles are generated with a tuneable density of amino groups for charge-mediated cellular uptake by a straightforward self-assembly protocol, which allows for control over size and toxicity. The results show that a single amino group per ten oligomers is sufficient to achieve cellular uptake. The non-toxic nanoparticles are suitable for both one- and two-photon cellular imaging and flow cytometry, and undergo very efficient cellular uptake.

Fluorene-based macromolecular nanostructures and nanomaterials for organic (opto)electronics

Nanotechnology not only opens up the realm of nanoelectronics and nanophotonics, but also upgrades organic thin-film electronics and optoelectronics. In this review, we introduce polymer semiconductors and plastic electronics briefly, followed by various top-down and bottom-up nano approaches to organic electronics. Subsequently, we highlight the progress in polyfluorene-based nanoparticles and nanowires (nanofibres), their tunable optoelectronic properties as well as their applications in polymer light-emitting devices, solar cells, field-effect transistors, photodetectors, lasers, optical waveguides and others. Finally, an outlook is given with regard to four-element complex devices via organic nanotechnology and molecular manufacturing that will spread to areas such as organic mechatronics in the framework of robotic-directed science and technology.

Realizing molecular pixel system for full-color fluorescence reproduction: RGB-emitting molecular mixture free from energy transfer crosstalk

A full-color molecular pixel system is realized for the first time using simple mixtures composed of RGB-emitting excited-state intramolecular proton transfer (ESIPT) dyes, each of which has delicately tailored Stokes shift and independent emission capability completely free from energy transfer crosstalk between them. It is demonstrated that the whole range of emission colors enclosed within the RGB color triangle on the CIE 1931 diagram is predictable and conveniently reproducible from the RGB molecular pixels not only in the solution but also in the polymer film. It must be noted that mixing ratios to reproduce the desired color coordinates can be precisely calculated on the basis of additive color theory according to their molecular pixel behavior.

Fluorescent π-conjugated polymer dots versus self-assembled small-molecule nanoparticles: what's the difference?

Fluorescent nanoparticles based on π-conjugated small molecules and polymers are two different classes of π-conjugated systems that have attracted much interest. To date, both emerging classes have only been studied separately and showed no clear differences in their properties. Herein these nanoparticles are compared on the basis of a fluorene co-polymer and its corresponding small molecule. Both systems formed nanoparticles with the same diameter, whereas the fluorescence properties clearly differed. In case of the polymer the fluorescence diminished, whereas for the small molecules the fluorescence increased. In addition, the capability of encapsulation and release of a hydrophobic dye from the fluorescent nanoparticles was studied. For the polymer system, encapsulation was highly efficient and no release was observed, whereas for the small molecule system encapsulation was less efficient and release of the dye was observed. These studies show a clear difference between small molecules and polymers which has important implications for the design of fluorescent nanoparticles.

Conjugated oligomer-based fluorescent nanoparticles as functional nanocarriers for nucleic acids delivery

Oligonucleotides such as siRNA and plasmid DNA (pDNA) have great potential for gene therapies. Multifunctional, environment-resistant carriers with imaging capabilities are required to track the assembly and disassembly of oligonucleotides, monitor the delivery processes, and develop new delivery systems. Conjugated polymers and oligomers can potentially be used as novel materials for functional nanocarriers with both delivery and imaging abilities. In this work, a novel π-conjugated oligomer 4,7-(9,9'-bis(6-adenine hexyl)fluorenyl)-2,1,3-benzothiadiazole (OFBT-A) modified with nucleotide adenine (A) groups in its side chains is synthesized and characterized. Fluorescent nanoparticles based on the π-conjugated oligomers OFBT-A are developed as novel functional nanocarriers for oligonucleotides. Single-stranded DNA (ssDNA) TR-T5 labeled with Texas Red (TR) fluorescent dye is selected as a model payload oligonucleotide. The capture abilities and stability of OFBT-A are investigated by monitoring the fluorescence resonance energy transfer (FRET) efficiency between the OFBT-A nanoparticles and TR labels in solution. The OFBT-A/TR-T5 composites are stable in solution at high ionic strengths (0-500 mM) and have a wide working pH range, from 3.0 to 9.5. The in vitro profile demonstrates that the release of the TR-DNA is induced by the ssDNA A43, which has a high charge density. The release process is monitored by measuring the changes in FRET efficiency and fluorescence color for the OFBT-A/TR-T5 composites. Using this carrier, the uptake of TR-DNA by A549 lung cancer cells is observed. Both the OFBT-A nanoparticles and the OFBT-A/TR-T5 composites show high cytocompatibility. We anticipate that these novel functional nanocarriers will provide a safe strategy for monitoring the gene delivery process.

A peptide based two component white light emitting system

A peptide based two component white light emitting system has been designed and developed on the basis of a co-assembly of a PDI containing peptide system as an acceptor and a stilbene containing peptide system as a donor in organic solvents.

Highly pure solid-state white-light emission from solution-processable soft-hybrids

Highly pure and solution processable white-light-emitting hybrids are presented. These soft-hybrids are designed by an organic-inorganic supramolecular co-assembly in water. White-light emission is achieved by partial energy transfer (ET) between donor and acceptor molecules anchored on the inorganic component. The unique and remarkable processability feature of these hybrids is demonstrated by painting/writing onto large glass and flexible plastic substrates.

Highly fluorescent semiconducting polymer dots for biology and medicine

In recent years, semiconducting polymer nanoparticles have attracted considerable attention because of their outstanding characteristics as fluorescent probes. These nanoparticles, which primarily consist of π-conjugated polymers and are called polymer dots (Pdots) when they exhibit small particle size and high brightness, have demonstrated utility in a wide range of applications such as fluorescence imaging and biosensing. In this review, we summarize recent findings of the photophysical properties of Pdots which speak to the merits of these entities as fluorescent labels. This review also highlights the surface functionalization and biomolecular conjugation of Pdots, and their applications in cellular labeling, in vivo imaging, single-particle tracking, biosensing, and drug delivery. We discuss the relationship between the physical properties and performance, and evaluate the merits and limitations of the Pdot probes for certain imaging tasks and fluorescence assays. We also tackle the current challenges of Pdots and share our perspective on the future directions of the field.

Multivalent glycoconjugate syntheses and applications using aromatic scaffolds

Glycan-protein interactions are of utmost importance in several biological phenomena. Although the variety of carbohydrate residues in mammalian cells is limited to less than a dozen different sugars, their spatial topographical presentation in what is now associated as the "glycocodes" provides the fundamental keys for specific and high affinity "lock-in" recognition events associated with a wide range of pathologies. Toward deciphering our understanding of these glycocodes, chemists have developed new creative tools that included dendrimer chemistry in order to provide monodisperse multivalent glycoconjugates. This review provides a survey of the numerous aromatic architectures generated for the multivalent presentation of relevant carbohydrates using covalent attachment or supramolecular self-assemblies. The basic concepts toward their controlled syntheses will be described using modern synthetic procedures with a particular emphasis on powerful organometallic methodologies. The large variety of dendritic aromatic scaffolds, together with a brief survey of their unique biophysical and biological properties will be critically reviewed. The distinctiveness of the resulting multivalent glycoarchitectures, encompassing glycoclusters, glycodendrimers and molecularly defined self-assemblies, in forming well organized cross-linked lattices with multivalent carbohydrate binding proteins (lectins) together with their photophysical, medical, and imaging properties will also be briefly highlighted. The topic will be presented in increasing order of aromatic backbone complexities and will end with fullerenes together with self-assembled nanostructures, thus complementing the various scaffolds described in this special thematic issue dedicated to multivalent glycoscience.

Side chains control dynamics and self-sorting in fluorescent organic nanoparticles

To develop fluorescent organic nanoparticles with tailored properties for imaging and sensing, full control over the size, fluorescence, stability, dynamics, and supramolecular organization of these particles is crucial. We have designed, synthesized, and fully characterized 12 nonionic fluorene co-oligomers that formed self-assembled fluorescent nanoparticles in water. In these series of molecules, the ratio of hydrophilic ethylene glycol and hydrophobic alkyl side chains was systematically altered to investigate its role on the above-mentioned properties. The nanoparticles consisting of π-conjugated oligomers containing polar ethylene glycol side chains were less stable and larger in size, while nanoparticles self-assembled from oligomers containing nonpolar pendant chains were more stable, smaller, and generally had a higher fluorescence quantum yield. Furthermore, the dynamics of the molecules between the nanoparticles was enhanced if the number of hydrophilic side chains increased. Energy transfer studies between naphthalene and benzothiadiazole fluorene co-oligomers with the same side chains showed no exchange of molecules between the particles for the apolar molecules. For the more polar systems, the exchange of molecules between nanoparticles took place at room temperature or after annealing. Self-assembled nanoparticles consisting of π-conjugated oligomers having different side chains caused self-sorting, resulting either in the formation of domains within particles or the formation of separate nanoparticles. Our results show that we can control the stability, fluorescence, dynamics, and self-sorting properties of the nanoparticles by simply changing the nature of the side chains of the π-conjugated oligomers. These findings are not only important for the field of self-assembled nanoparticles but also for the construction of well-defined multicomponent supramolecular materials in general.

Enhanced brightness emission-tuned nanoparticles from heterodifunctional polyfluorene building blocks

Three-coordinate complexes (bromo)[4-(2,2-dimethyl-1,3-dioxolan-4-yl)-phenyl](tri-tert-butyl-phosphine)palladium (1) and (bromo){4-[(tetrahydro-2H-pyran-2-yloxy)methyl]phenyl}(tri-tert-butyl-phosphine)palladium (2) were used to initiate Suzuki-Miyaura chain growth polymerization of 7'-bromo-9',9'-dioctyl-fluoren-2'-yl-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (3). The polymerization was optionally terminated by end-capping with red-emitting N-(2-ethylhexyl)-1,6-bis(4-tert-octylphenoxy)-9-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-perylene-3,4-dicarboximide. Heterodisubstituted polyfluorenes of adjustable molecular weights between 5 × 10(3) and 1.0 × 10(4) g mol(-1) and narrow molecular weight distribution (M(w)/M(n) < 1.2), bearing precisely one or two hydroxyl groups on one chain end and optionally a dye-label on the opposite end, were obtained virtually devoid of any side-products. Covalent attachment of polyethylene glycol (M(n) = 2 × 10(3) g mol(-1)) to the reactive end groups yielded amphiphilic block copolymer, which afforded stable nanoparticles with diameters in the range of 25-50 nm when dispersed in water. These particles exhibited a bright fluorescence emission with quantum yields as high as Φ = 84%, which could optionally be tuned to longer wavelengths by energy transfer to the perylene monoimide dye. The heterodifunctional nature of these polyfluorenes is crucial for a bright and enduring fluorescence brightness as revealed by comparison to nanoparticles containing physically mixed dye. Further addition of terrylene diimide dye to the nanoparticles of perylene-end-capped polyfluorene block copolymers allows for an energy cascade resulting in emission exclusively in the deep red and near-infrared regime.

Morphology-dependent energy transfer dynamics in fluorene-based amphiphile nanoparticles

Nanoparticles are interesting systems to study because of their large range of potential uses in biological imaging and sensing. We investigated molecular nanoparticles formed by fast injection of a small volume of molecularly dissolved fluorene-derivative amphiphilic molecules into a polar solvent, which resulted in solid spherical particles of ∼80 nm diameter with high stability. Energy transfer studies were carried out on two-component nanoparticles that contained mixtures of donor and acceptor amphiphiles of various fractions. We conducted time-resolved photoluminescence measurements on the two-component nanoparticles in order to determine whether the fundamental donor-acceptor interaction parameter (the Förster radius) depends on the acceptor concentration. The Förster radius was found to be large for very low incorporated acceptor fractions (<0.1%), but it declined with increasing concentration. These changes were concomitant with shifts in the acceptor emission and absorption circular dichroism spectra that indicated an increasing clustering of acceptors into domains as their fraction was raised. In addition, for acceptor fractions below 2% the extracted Förster radii were found to be significantly larger than predicted from donor-acceptor spectral overlap calculations, in accordance with efficient excitation diffusion within the donor matrix, aiding the overall transfer to acceptors. We conclude that energy transfer in two-component nanoparticles shows a complex interplay between phase segregation of the constituent donor and acceptor molecules and excitation diffusion within their domains.