Analysis of the UV/Vis and CD Spectral Line Shapes of Carotenoid Assemblies: Spectral Signatures of Chiral H-Aggregates

Aggregation of One-Dimensional Wires: The Case of Long Oligoynes

We show an unexpected aggregation phenomenon of a long oligoyne (Py[16]) with 16 contiguous triple bonds and endcapped with bulky 3,5-bi(3,5-bis-tert-butylphenyl)pyridine groups. Aggregation of 1D π-conjugated oligoyne chains is rare given the minimal π-π intermolecular interactions as well as its flexibility that works against self-assembly. In dilute solutions, the reversible aggregation of Py[16] initiates at low temperature in the range of 140-180 K, and is not observed for shorter oligoynes in this series. Cryogenic UV/Vis electronic absorption spectra and vibrational Raman spectra with different laser wavelength lines tuning from in-resonance to off-resonance conditions have been used to extract the vibrational features characterizing the monomer and aggregate species. Theoretical calculations complement the spectroscopic findings.

Solubilization of Hydrophobic Astaxanthin in Water by Physical Association with Phytoglycogen Nanoparticles

Astaxanthin (AXT) is a xanthophyll carotenoid with reported health benefits. Realizing its potential as a bioactive is challenging because of its extremely poor solubility in water. We describe a method to improve the effective solubility of AXT in water through its physical association with phytoglycogen (PG), which is produced in sweet corn as compact, highly branched nanoparticles. We combine PG in water with AXT in acetone, evaporate the acetone, and lyophilize. The result is an AXT-PG complex that can be readily redispersed in water, resulting in stable aqueous dispersions. By characterizing the UV-vis absorbance due to different aggregation states of AXT in the AXT-PG complex, we determined the maximum loading of AXT onto PG to be ∼10% by mass. Our results demonstrate the promise of using PG as a solubilizing agent for hydrophobic compounds in water.

Organization of Carotenoid Aggregates in Membranes Studied Selectively using Resonance Raman Optical Activity

In living organisms, carotenoids are incorporated in biomembranes, remarkably modulating their mechanical characteristics, fluidity, and permeability. Significant resonance enhancement of Raman optical activity (ROA) signals of carotenoid chiral aggregates makes resonance ROA (RROA), a highly selective tool to study exclusively carotenoid assemblies in model membranes. Hence, RROA is combined with electronic circular dichroism (ECD), dynamic light scattering (DLS), molecular dynamics, and quantum-chemical calculations to shed new light on the carotenoid aggregation in dipalmitoylphosphatidylcholine (DPPC) liposomes. Using representative members of the carotenoid family: apolar α-carotene and more polar fucoxanthin and zeaxanthin, the authors demonstrate that the stability of carotenoid aggregates is directly linked with their orientation in membranes and the monomer structures inside the assemblies. In particular, polyene chain distortion of α-carotene molecules is an important feature of J-aggregates that show increased orientational freedom and stability inside liposomes compared to H-assemblies of more polar xanthophylls. In light of these results, RROA emerges as a new tool to study active compounds and drugs embedded in membranes.

Induced Chirality in Canthaxanthin Aggregates Reveals Multiple Levels of Supramolecular Organization

Carotenoids tend to form supramolecular aggregates via non-covalent interactions where the chirality of individual molecules is amplified to the macroscopic level. We show that this can also be achieved for non-chiral carotenoid monomers interacting with polysaccharides. The chirality induction in canthaxanthin (CAX), caused by heparin (HP) and hyaluronic acid (HA), was monitored by chiroptical spectroscopy. Electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra indicated the presence of multiple carotenoid formations, such as H- and J-type aggregates. This is consistent with molecular dynamics (MD) and density functional theory (DFT) simulations of the supramolecular structures and their spectroscopic response.

Near-atomic-resolution structure of J-aggregated helical light-harvesting nanotubes

Cryo-electron microscopy has delivered a resolution revolution for biological self-assemblies, yet only a handful of structures have been solved for synthetic supramolecular materials. Particularly for chromophore supramolecular aggregates, high-resolution structures are necessary for understanding and modulating the long-range excitonic coupling. Here, we present a 3.3 Å structure of prototypical biomimetic light-harvesting nanotubes derived from an amphiphilic cyanine dye (C8S3-Cl). Helical 3D reconstruction directly visualizes the chromophore packing that controls the excitonic properties. Our structure clearly shows a brick layer arrangement, revising the previously hypothesized herringbone arrangement. Furthermore, we identify a new non-biological supramolecular motif-interlocking sulfonates-that may be responsible for the slip-stacked packing and J-aggregate nature of the light-harvesting nanotubes. This work shows how independently obtained native-state structures complement photophysical measurements and will enable accurate understanding of (excitonic) structure-function properties, informing materials design for light-harvesting chromophore aggregates.

What is the significance of the chloroform stabilizer C5H10 and its association with MeOH in concentration-dependent polymeric solutions?

The understanding of excitonic transitions associated with polymeric aggregates is fundamental, as such transitions have implications on coherence lengths, coherence numbers and inter- and intra-chain binding parameters. In this context, the investigation of efficient solvents and other ways to control polymer aggregate formation is key for their consolidation as materials for new technologies. In this manuscript, we use Poly(3-hexothiophene) (P3HT) as a probe to investigate the significance of amylene (C5H10) and its association with methanol (MeOH) in both pure and C5H10-stabilized chloroform (CHCl3)-based polymeric solutions. Using the intensity ratio between the first and second vibronic transitions of the P3HT H-aggregates formed, values for their exciton bandwidths and interchain interactions are obtained and correlated with the presence of C5H10 and MeOH as agents determining the CHCl3 quality.

Isomerization of carotenoids in photosynthesis and metabolic adaptation

In nature, carotenoids are present as trans- and cis-isomers. Various physical and chemical factors like light, heat, acids, catalytic agents, and photosensitizers can contribute to the isomerization of carotenoids. Living organisms in the process of evolution have developed different mechanisms of adaptation to light stress, which can also involve isomeric forms of carotenoids. Particularly, light stress conditions can enhance isomerization processes. The purpose of this work is to review the recent studies on cis/trans isomerization of carotenoids as well as the role of carotenoid isomers for the light capture, energy transfer, photoprotection in light-harvesting complexes, and reaction centers of the photosynthetic apparatus of plants and other photosynthetic organisms. The review also presents recent studies of carotenoid isomers for the biomedical aspects, showing cis- and trans-isomers differ in bioavailability, antioxidant activity and biological activity, which can be used for therapeutic and prophylactic purposes.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10^-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Mixing of Excitons in Nanostructures Based on a Perylene Dye with CdTe Quantum Dots

Semiconductor quantum dots of the A₂B₆ group and organic semiconductors have been widely studied and applied in optoelectronics. This study aims to combine CdTe quantum dots and perylene-based dye molecules into advanced nanostructure system targeting to improve their functional properties. In such systems, new electronic states, a mixture of Wannier-Mott excitons with charge-transfer excitons, have appeared at the interface of CdTe quantum dots and the perylene dye. The nature of such new states has been analyzed by absorption and photoluminescence spectroscopy with picosecond time resolution. Furthermore, aggregation of perylene dye on the CdTe has been elucidated, and contribution of Förster resonant energy transfer has been observed between aggregated forms of the dye and CdTe quantum dots in the hybrid CdTe-perylene nanostructures. The studied nanostructures have strongly quenched emission of quantum dots enabling potential application of such systems in dissociative sensing.

Fabrication and characterization of the H/J-type aggregates astaxanthin/bovine serum albumin/chitosan nanoparticles

Astaxanthin is a natural liposoluble ketocarotenoid with various biological activities. Hydrophobic astaxanthin with C_2h symmetry can self-assembly form H-type aggregates and J-type aggregates in hydrated polar solvents. However, astaxanthin and its aggregates are limited by its water insolubility and chemical instability. Here, the biological macromolecules bovine serum albumin (BSA) and chitosan were chosen as protein-polysaccharides based delivery systems for astaxanthin aggregates by molecular self-assembly method. The precise prepared H-ABC-NPs and J-ABC-NPs suspensions were both near spheres with hydrodynamic size around 281 ± 9 nm and 368 ± 5 nm and zeta potentials around +26 mV and +30 mV, respectively. Two types of astaxanthin aggregates were distinguished, water-dispersible, and stable in nanocarriers through UV-vis spectra observation. The encapsulation efficiency of the astaxanthin in ABC-NPs was above 90 %. Fourier transform infrared spectroscopy (FTIR) and circular dichroism (CD) analyses indicated that the dominant driving forces of ABC-NPs formation mainly included electrostatic, hydrophobic interactions and hydrogen bonding. These results offer an elegant opportunity for the protein-polysaccharides delivery systems, and provide an important perspective for applying novel water-dispersed astaxanthin aggregates products in nutrition and medicine industry.

Effect of environmental factors on the aggregation behavior of astaxanthin in water

Molecular aggregation is a common phenomenon widely found in natural organisms, which is crucial for some specific functions of biological systems. To study the aggregating behavior of hydrophobic carotenoids in water, astaxanthin was employed and dispersed under different surroundings to induce aggregation. The results showed that astaxanthin tended to form J- or H-type aggregates when the water content was higher than 60%. Both hydrochloric acid (HCl) and sodium hydroxide (NaOH) were beneficial for the formation of astaxanthin J-aggregates, but they were not good for inducing H-aggregates. Small-molecule electrolytes, like sodium salts, mostly played an enormous hindrance role to the formation of astaxanthin H- and J-aggregates, except for sodium chloride (NaCl) which helped astaxanthin to form J-aggregates. Both sodium periodate (NaIO₄) and sodium acetate (CH₃COONa) could prevent the formation of astaxanthin H- and J-aggregates, but sodium chloride (NaCl) could only hinder the formation of H-aggregates. As for polyelectrolytes chitosan and DNA, the difference of chain structure led to different aggregation effects. The soft single chain of chitosan tended to induce J-aggregates formation, while double-stranded DNA preferred to guide the formation of H-aggregates. By choosing and integrating the advantageous environmental factors that facilitate each type of astaxanthin aggregates, J- and H-type astaxanthin aggregates were stably loaded in DNA/CS nanoparticles with distinct particle sizes. Controlled preparation of either H- or J-type aggregates is of great significance for further studies concerning the structure-activity relationship of carotenoid aggregates.

Parallel triplet formation pathways in a singlet fission material

Harvesting long-lived free triplets in high yields by utilizing organic singlet fission materials can be the cornerstone for increasing photovoltaic efficiencies potentially. However, except for polyacenes, which are the most studied systems in the singlet fission field, spin-entangled correlated triplet pairs and free triplets born through singlet fission are relatively poorly characterized. By utilizing transient absorption and photoluminescence spectroscopy in supramolecular aggregate thin films consisting of Hamilton-receptor-substituted diketopyrrolopyrrole derivatives, we show that photoexcitation gives rise to the formation of spin-0 correlated triplet pair ¹(TT) from the lower Frenkel exciton state. The existence of ¹(TT) is proved through faint Herzberg-Teller emission that is enabled by vibronic coupling and correlated with an artifact-free triplet-state photoinduced absorption in the near-infrared. Surprisingly, transient electron paramagnetic resonance reveals that long-lived triplets are produced through classical intersystem crossing instead of ¹(TT) dissociation, with the two pathways in competition. Moreover, comparison of the triplet-formation dynamics in J-like and H-like thin films with the same energetics reveals that spin-orbit coupling mediated intersystem crossing persists in both. However, ¹(TT) only forms in the J-like film, pinpointing the huge impact of intermolecular coupling geometry on singlet fission dynamics.

Formation of carotenoid supramolecular aggregates in nanocarriers monitored via aggregation-sensitive chiroptical output of enantiopure (3S,3'S)-astaxanthin

The aggregation-sensitive chiroptical (ECD and RROA) output, provided by enantiopure (3S,3'S)-astaxanthin, was used to investigate and control the assembling processes of the carotenoid in Pluronic F-127 nanoparticles. The process of carotenoid J-aggregation inside nanocarriers is interfered with by the formation of kinetically stabilized H1 self-assemblies outside the micelles. Nanocarriers with encapsulated stable J-aggregates provide controlled release of carotenoid molecules to primary murine adipocytes.

Slip-Stacked J-Aggregate Materials for Organic Solar Cells and Photodetectors

Dye-dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near-infrared region and the latter an ultra-narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip-stacked dyes, distinguishing classical J-aggregates with predominant long-range Coulomb coupling from charge transfer (CT)-mediated or -coupled J-aggregates, whose red-shifts are primarily governed by short-range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength-selective OPDs, providing important insights into the rational design of thin-film optoelectronic materials.

Theory of Apparent Circular Dichroism Reveals the Origin of Inverted and Noninverted Chiroptical Response under Sample Flipping

Circular dichroism (CD) finds widespread application as an optical probe for the structure of molecules and supramolecular assemblies. Its underlying chiral light-matter interactions effectively couple between photonic spin states and select quantum-mechanical degrees of freedom in a sample, implying an intricate connection with photon-to-matter quantum transduction. However, effective transduction implementations likely require interactions that are antisymmetric with respect to the direction of light propagation through the sample, yielding an inversion of the chiroptical response upon sample flipping, which is uncommon for CD. Recent experiments on organic thin films have demonstrated such chiroptical behavior, which was attributed to "apparent CD" resulting from an interference between the sample's linear birefringence and linear dichroism. However, a theory connecting the underlying optical selection rules to the microscopic electronic structure of the constituent molecules remains to be formulated. Here, we present such a theory based on a combination of Mueller calculus and a Lorentz oscillator model. The theory reaches good agreement with experimental CD spectra and allows for establishing the (supra)molecular design rules for maximizing or minimizing this chiroptical effect. It furthermore highlights that, in addition to antisymmetrically, it can manifest symmetrically such that no chiroptical response inversion occurs, which is a consequence of a helical stacking of molecules in the light propagation direction.

Simulation of absorption and scattering spectra of crystalline organic nanoparticles with the discrete dipole approximation: Effects of crystal shape, crystal size, and refractive index of the medium

The effect of the shape (habit) of crystalline organic nanoparticles on their absorption spectra is studied by simulations using the discrete dipole approximation, focusing, in particular, on the vibronic structure of the absorption bands in the spectra. Simulations predict a significant effect that, for sufficiently small particles, can be simply rationalized by the depolarization factor. The crystal size and the refractive index of the medium in which the nanoparticles are embedded are also found to have an effect on the absorption spectra. All factors mentioned are found to influence also the spectra of scattered light. These effects, already broadly documented for metallic nanoparticles, are here demonstrated theoretically for the first time for crystalline organic nanoparticles, providing novel insight into the optical response of such particles. The effects are expected to be displayed by all organic nanoparticles, as long as they have a well-defined crystal structure and are large enough for the optical properties to be understandable using a macroscopic dielectric tensor. The effects demonstrated here should be taken into account when rationalizing differences in absorption spectra of a substance in solution and in nanoparticle form, e.g., in deducing the type of intermolecular packing. The effects are much less pronounced for optically isotropic nanoparticles.

Photophysical and Structural Characterization of Intrinsically Fluorescent Sterol Aggregates

Self-association of cholesterol into aggregates and crystals is a hallmark of developing atherosclerosis. Intrinsically fluorescent sterols, such as dehydroergosterol (DHE), can be used to study sterol aggregation by fluorescence spectroscopy and microscopy, but a thorough understanding of DHE's photophysical and structural properties in the aggregated state is missing. Here, we show that DHE forms submicron fluorescent aggregates when evaporated from an ethanol solution. Using atomic force microscopy, we find that DHE, like cholesterol, forms compact oblate-shape aggregates of <100 nm in diameter. DHE's fluorescence is lowered in the aggregate compared to the monomeric form, and characteristic spectral changes accompany the aggregation process. Electronic structure calculations of DHE dimers in water indicate that Frenkel-type exciton coupling contributes to the lowered DHE fluorescence in the aggregates. Using molecular dynamics (MD) simulations, we show that DHE forms compact aggregates on the nanosecond scale and with strong intermolecular attraction, in which a broad range of orientations, and therefore electronic couplings, will take place. Tight packing of DHE in aggregates also lowers the apparent absorption cross section, further reducing the molecular brightness of the aggregates. Our results pave the way for systematic solubility studies of intrinsically fluorescent analogues of biologically relevant sterols.

Bright Frenkel Excitons in Molecular Crystals: A Survey

We computed the optical properties of a large set of molecular crystals (∼2200 structures) composed of molecules whose lowest excited states are strongly coupled and generate wide excitonic bands. Such bands are classified in terms of their dimensionality (1-, 2-, and 3-dimensional), the position of the optically allowed state in relation with the excitonic density of states, and the presence of Davydov splitting. The survey confirms that one-dimensional aggregates are rare in molecular crystals highlighting the need to go beyond the simple low-dimensional models. Furthermore, this large set of data is used to search for technologically interesting and less common properties. For instance, we considered the largest excitonic bandwidth that is achievable within known molecular crystals and identified materials with strong super-radiant states. Finally, we explored the possibility that strong excitonic coupling can be used to generate emissive states in the near-infrared region in materials formed by molecules with bright visible absorption and we could identify the maximum allowable red shift in this material class. These insights with the associated searchable database provide practical guidelines for designing materials with interesting optical properties.

Fluorescence Quenching in J-Aggregates through the Formation of Unusual Metastable Dimers

Molecular aggregation alters the optical properties of a system as fluorescence may be activated or quenched. This is usually described within the well-established framework of H- and J-aggregates. While H-aggregates show nonfluorescent blueshifted absorption bands with respect to the isolated monomer, J-aggregates are fluorescent displaying a redshifted peak. In this publication, we employ a combined approach of experiment and theory to study the complex aggregation features and photophysical properties of diaminodicyanoquinone derivatives, which show unusual and puzzling nonfluorescent redshifted absorption bands upon aggregation. Our theoretical analysis demonstrates that stable aggregates do not account for the experimental observations. Instead, we propose an unprecedented mechanism involving metastable dimeric species formed from stable dimers to generate nonfluorescent J-aggregates. These results represent a novel kind of aggregation-induced optical effect and may have broad implications for the photophysics of dye aggregates.

Singlet fission in naturally-organized carotenoid molecules

We have investigated the photophysics of aggregated lutein/violaxanthin in daffodil chromoplasts. We reveal the presence of three carotenoid aggregate species, the main one composed of a mixture of lutein/violaxanthin absorbing at 481 nm, and two secondary populations of aggregated carotenoids absorbing circa 500 and 402 nm. The major population exhibits an efficient singlet fission process, generating μs-lived triplet states on an ultrafast timescale. The structural organization of aggregated lutein/violaxanthin in daffodil chromoplasts produces well-defined electronic levels that permit the energetic pathways to be disentangled unequivocally, allowing us to propose a consistent mechanism for singlet fission in carotenoid aggregates. Transient absorption measurements on this system reveal for the first time an entangled triplet signature for carotenoid aggregates, and its evolution into dissociated triplet states. A clear picture of the carotenoid singlet fission pathway is obtained, which is usually blurred due to the intrinsic disorder of carotenoid aggregates.

Photogeneration of Long-Lived Triplet States through Singlet Fission in Lycopene H-Aggregates

Molecular triplet excitons produced through singlet fission (SF) usually have shorter triplet lifetimes due to exciton-exciton recombination and relaxation pathways, thereby resulting in complex device architectures for SF-boosted solar cells. Using broadband transient absorption spectroscopy, we here show that the photoexcitation of nanostructured lycopene H-aggregates at room temperature produces free triplets with an unprecedented 35-fold enhancement in the lifetime compared to those localized on the monomer backbone. The observed rise of a spectrally blue-shifted correlated T-T pair state in ∼19 ps with distinct vibronic features provides the basis for SF-induced triplet generation.

Exciton Delocalization in Indolenine Squaraine Aggregates Templated by DNA Holliday Junction Scaffolds

Exciton delocalization plays a prominent role in the photophysics of molecular aggregates, ultimately governing their particular function or application. Deoxyribonucleic acid (DNA) is a compelling scaffold in which to template molecular aggregates and promote exciton delocalization. As individual dye molecules are the basis of exciton delocalization in molecular aggregates, their judicious selection is important. Motivated by their excellent photostability and spectral properties, here, we examine the ability of squaraine dyes to undergo exciton delocalization when aggregated via a DNA Holliday junction (HJ) template. A commercially available indolenine squaraine dye was chosen for the study given its strong structural resemblance to Cy5, a commercially available cyanine dye previously shown to undergo exciton delocalization in DNA HJs. Three types of DNA-dye aggregate configurations-transverse dimer, adjacent dimer, and tetramer-were investigated. Signatures of exciton delocalization were observed in all squaraine-DNA aggregates. Specifically, strong blue shift and Davydov splitting were observed in steady-state absorption spectroscopy and exciton-induced features were evident in circular dichroism (CD) spectroscopy. Strongly suppressed fluorescence emission provided additional, indirect evidence for exciton delocalization in the DNA-templated squaraine dye aggregates. To quantitatively evaluate and directly compare the excitonic Coulombic coupling responsible for exciton delocalization, the strength of excitonic hopping interactions between the dyes was obtained by simultaneously fitting the experimental steady-state absorption and CD spectra via a Holstein-like Hamiltonian, in which, following the theoretical approach of Kühn, Renger, and May, the dominant vibrational mode is explicitly considered. The excitonic hopping strength within indolenine squaraines was found to be comparable to that of the analogous Cy5 DNA-templated aggregate. The squaraine aggregates adopted primarily an H-type (dyes oriented parallel to each other) spatial arrangement. Extracted geometric details of the dye mutual orientation in the aggregates enabled a close comparison of aggregate configurations and the elucidation of the influence of dye angular relationship on excitonic hopping interactions in squaraine aggregates. These results encourage the application of squaraine-based aggregates in next-generation systems driven by molecular excitons.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

A Thermostable Protein Matrix for Spectroscopic Analysis of Organic Semiconductors

Advances in protein design and engineering have yielded peptide assemblies with enhanced and non-native functionalities. Here, various molecular organic semiconductors (OSCs), with known excitonic up- and down-conversion properties, are attached to a de novo-designed protein, conferring entirely novel functions on the peptide scaffolds. The protein-OSC complexes form similarly sized, stable, water-soluble nanoparticles that are robust to cryogenic freezing and processing into the solid-state. The peptide matrix enables the formation of protein-OSC-trehalose glasses that fix the proteins in their folded states under oxygen-limited conditions. The encapsulation dramatically enhances the stability of protein-OSC complexes to photodamage, increasing the lifetime of the chromophores from several hours to more than 10 weeks under constant illumination. Comparison of the photophysical properties of astaxanthin aggregates in mixed-solvent systems and proteins shows that the peptide environment does not alter the underlying electronic processes of the incorporated materials, exemplified here by singlet exciton fission followed by separation into weakly bound, localized triplets. This adaptable protein-based approach lays the foundation for spectroscopic assessment of a broad range of molecular OSCs in aqueous solutions and the solid-state, circumventing the laborious procedure of identifying the experimental conditions necessary for aggregate generation or film formation. The non-native protein functions also raise the prospect of future biocompatible devices where peptide assemblies could complex with native and non-native systems to generate novel functional materials.

Chirality of a rhodamine heterodimer linked to a DNA scaffold: an experimental and computational study

The chiroptical properties of multi-chromophoric systems are governed by the intermolecular arrangement of the monomeric units. We report on a computational and experimental study of the linear optical properties and supramolecular structure of a rhodamine heterodimer assembled on a DNA scaffold. The experimental absorption and circular dichroism (CD) profiles confirm the dimer formation. Computationally, starting from low-cost DFT/TDDFT simulations of the bare dimer we attribute the measured -/+ CD sign sequence of the S1/S2 bands to a specific chiral conformation of the heterodimer. In the monomers, as typical for rhodamine dyes, the electric transition dipole of the lowest π-π* transition is parallel to the long axis of the xanthene planes. We show that in the heterodimer the sign sequence of the two CD bands is related to the orientation of these long axes. To account explicitly for environment effects, we use molecular dynamics (MD) simulations for characterizing the supramolecular structure of the two optical isomers tethered on DNA. Average absorption and CD-profiles were modeled using ab initio TDDFT calculations at the geometries sampled along a few nanosecond MD run. The absorption profiles computed for both optical isomers are in good agreement with the experimental absorption spectrum and do not allow one to discriminate between them. The computed averaged CD profiles provide the orientation of monomers in the enantiomer that is dominant under the experimental conditions.

Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid-BF2 Dimers

We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid-BF₂ dyes and the detailed investigation of their solvent-dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly-coupled H-aggregates. The extent of folding and, in turn, of ground-state aggregation is strongly dependent on the nature of the flexible linker. Steady-state and time-resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in-depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two-dimensional ¹ H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore.

Structure of supramolecular astaxanthin aggregates revealed by molecular dynamics and electronic circular dichroism spectroscopy

Biomolecular aggregation is omnipresent in nature and important for metabolic processes or in medical treatment; however, the phenomenon is rather difficult to predict or understand on the basis of computational models. Recently, we found that electronic circular dichroism (ECD) spectroscopy and closely related resonance Raman optical activity (RROA) are extremely sensitive to the aggregation mechanism and structure of the astaxanthin dye. In the present study, molecular dynamics (MD) and quantum chemical (QC) computations (ZIndo/S, TDDFT) are used to link the aggregate structure with ECD spectral shapes. Realistic absorption and ECD intensities were obtained and the simulations reproduced many trends observed experimentally, such as the prevalent sign pattern and dependence of the aggregate structure on the solvent type. The computationally cheaper ZIndo/S method provided results very similar to those obtained by TDDFT. In the future, the accuracy of the combined MD/QC methodology of spectra interpretation should be improved to provide more detailed information on astaxanthin aggregates and similar macromolecular systems.

Giant intrinsic circular dichroism of prolinol-derived squaraine thin films

Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

Photon management in supramolecular peptide nanomaterials

Self-assembling peptides with covalent pi-electron functionality offer new ways to create delocalized conduits within protein-based nanomaterials. My group's recent research is summarized in this regard, detailing foundational self-assembly and photophysical characterizations that validate the electronic couplings existing within the resulting peptidic nanomaterials. Using these initial studies as a benchmark, ongoing studies to create even more complex photonic energy delocalization schemes are presented, spanning excitonic and Förster energy transfer to low-bandgap dopant sites (whereby 46% of the observed photoluminescence could be quenched by the addition of 1 mol% of an energy acceptor), the creation of charge separated states following photoinduced electron transfer that persisted for over a nanosecond, and use of kinetic control to dictate self-sorting (at long time scales, ca. several hours) or intimate coassembly (at short time scales, ca. several seconds) of multiple peptide components. Peptide coassemblies are described that exhibit both directed exciton migration to low-energy sites and follow-up charge separation events, very much in mimicry with relevant photosynthetic processes.

On the Control of Chromophore Orientation, Supramolecular Structure, and Thermodynamic Stability of an Amphiphilic Pyridyl-Thiazol upon Lateral Compression and Spacer Length Variation

The supramolecular structure essentially determines the properties of organic thin films. Therefore, it is of utmost importance to understand the influence of molecular structure modifications on supramolecular structure formation. In this article, we demonstrate how to tune molecular orientations of amphiphilic 4-hydroxy thiazole derivatives by means of the Langmuir-Blodgett (LB) technique and how this depends on the length of an alkylic spacer between the thiazole chromophore and the polar anchor group. Therefore, we characterize their corresponding supramolecular structures, thermodynamic, absorption, and fluorescence properties. Particularly, the polarization-dependence of the fluorescence is analyzed to deduce molecular orientations and their possible changes after annealing, i.e., to characterize the thermodynamic stability of the individual solid state phases. Because the investigated thiazoles are amphiphilic, the different solid state phases can be formed and be controlled by means of the Langmuir-Blodgett (LB) technique. This technique also allows to deduce atomistic supramolecular structure motives of the individual solid phases and to characterize their thermodynamic stabilities. Utilizing the LB technique, we demonstrate that subtle molecular changes, like the variation in spacer length, can yield entirely different solid state phases with distinct supramolecular structures and properties.

EXAT: EXcitonic analysis tool

We introduce EXcitonic Analysis Tool (EXAT), a program able to compute optical spectra of large excitonic systems directly from the output of quantum mechanical calculations performed with the popular Gaussian 16 package. The software is able to combine in an excitonic scheme the single-chromophore properties and exciton couplings to simulate energies, coefficients, and excitonic spectra (UV-vis, CD, and LD). The effect of the environment can also be included using a Polarizable Continuum Model. EXAT also presents a simple graphical user interface, which shows on-screen both site and exciton properties. To show the potential of the method, we report two applications on a a chiral perturbed BODIPY system and DNA G-quadruplexes, respectively. The program is available online at http://molecolab.dcci.unipi.it/tools/. © 2017 Wiley Periodicals, Inc.

Aggregation control in natural brush-printed conjugated polymer films and implications for enhancing charge transport

Shear-printing is a promising processing technique in organic electronics for microstructure/charge transport modification and large-area film fabrication. Nevertheless, the mechanism by which shear-printing can enhance charge transport is not well-understood. In this study, a printing method using natural brushes is adopted as an informative tool to realize direct aggregation control of conjugated polymers and to investigate the interplay between printing parameters, macromolecule backbone alignment and aggregation, and charge transport anisotropy in a conjugated polymer series differing in architecture and electronic structure. This series includes (i) semicrystalline hole-transporting P3HT, (ii) semicrystalline electron-transporting N2200, (iii) low-crystallinity hole-transporting PBDTT-FTTE, and (iv) low-crystallinity conducting PEDOT:PSS. The (semi-)conducting films are characterized by a battery of morphology and microstructure analysis techniques and by charge transport measurements. We report that remarkably enhanced mobilities/conductivities, as high as 5.7×/3.9×, are achieved by controlled growth of nanofibril aggregates and by backbone alignment, with the adjusted R² (R²_adj) correlation between aggregation and charge transport as high as 95%. However, while shear-induced aggregation is important for enhancing charge transport, backbone alignment alone does not guarantee charge transport anisotropy. The correlations between efficient charge transport and aggregation are clearly shown, while mobility and degree of orientation are not always well-correlated. These observations provide insights into macroscopic charge transport mechanisms in conjugated polymers and suggest guidelines for optimization.

Structural characterization of astaxanthin aggregates as revealed by analysis and simulation of optical spectra

Carotenoids can self-assemble in hydrated polar solvents to form J- or H-type aggregates, inducing dramatic changes in photophysical properties. Here, we measured absorption and emission spectra of astaxanthin in ethanol-water solution using ultraviolet-visible and fluorescence spectrometers. Two types of aggregates were distinguished in mixed solution at different water contents by absorption spectra. After addition of water, all probed samples immediately formed H-aggregates with maximum blue shift of 31nm. In addition, J-aggregate was formed in 1:3 ethanol-water solution measured after an hour. Based on Frenkel exciton model, we calculated linear absorption and emission spectra of these aggregates to describe aggregate structures in solution. For astaxanthin, experimental results agreed well with the fitted spectra of H-aggregate models, which consisted of tightly packed stacks of individual molecules, including hexamers, trimers, and dimers. Transition moment of single astaxanthin in ethanol was obtained by Gaussian 09 program package to estimate the distance between molecules in aggregates. Intermolecular distance of astaxanthin aggregates ranges from 0.45nm to 0.9nm. Fluorescence analysis showed that between subbands, strong exciton coupling induced rapid relaxation of H-aggregates. This coupling generated larger Stokes shift than monomers and J-aggregates.

Unveiling the Aggregation of Lycopene in Vitro and in Vivo: UV-Vis, Resonance Raman, and Raman Imaging Studies

The present study investigates the structure of lycopene aggregates both in vitro and in vivo using ultraviolet-visible (UV-vis) and Raman spectroscopies. The electronic absorption bands of the J- and H-aggregates in vitro shift to lower and higher energies, respectively, compared to that of the lycopene monomer. Along with these results, the frequencies of the ν₁ Raman bands were shifted to lower and higher frequencies, respectively. By plotting the frequencies of the ν₁ Raman band against the S₀ → S₂ transition energy, a linear relationship between the data set with different aggregation conformations can be obtained. Therefore, the band positions depending on the different conformations can be explained based on the idea that the effective conjugated C═C chain lengths within lycopene molecules are different due to the environmental effect (site-shift effect) caused by the aggregation conformation. Applying this knowledge to the in vivo measurement of a tomato fruit sample, the relationship between the aggregation conformation of lycopene and the spectral patterns observed in the UV-vis as well as Raman spectra in different parts of tomato fruits was discussed in detail. The results showed that the concentration of lycopene (particularly that of the J-aggregate) specifically increased, whereas that of chlorophyll decreased, with ripening. Furthermore, Raman imaging indicated that lycopene with different aggregate conformations was distributed inhomogeneously, even within one sample. The layer formation in tomato tissues with high concentrations of J- and H-aggregates was successfully visualized. In this manner, the presence of lycopene distributions with different aggregate conformations was unveiled in vivo.

The application of a UHPLC system to study the formation of various chemical species by compounds undergoing efficient self-aggregation and to determine the homodimerization constants (KDM) with values in the high range of 106–1010 M−1

This work demonstrates a new concept for the use of UHPLC method for identification of the species formed by a self-aggregating compound depending on its concentration and solvent used and to determine homodimerization constants, K_DM = 10⁶–10¹⁰ M⁻¹.

Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation

Solution shearing has attracted great interest for the fabrication of robust and reliable, high performance organic electronic devices, owing to applicability of the method to large area and continuous fabrication, as well as its propensity to enhance semiconductor charge transport characteristics. To date, effects of the design of the blade shear features (especially the microfluidic shear design) and the prospect of synergistically combining the shear approach with an alternate process strategy have not been investigated. Here, a generic thin film fabrication concept that enhanced conjugated polymer intermolecular alignment and aggregation, improved orientation (both nanoscale and long-range), and narrowed the π-π stacking distance is demonstrated for the first time. The impact of the design of shearing blade microfluidic channels and synergistic effects of fluid shearing design with concomitant irradiation strategies were demonstrated, enabling fabrication of polymer-based devices with requisite morphologies for a range of applications.

Structure Prediction of Self-Assembled Dye Aggregates from Cryogenic Transmission Electron Microscopy, Molecular Mechanics, and Theory of Optical Spectra

Cryogenic transmission electron microscopy (cryo-TEM) studies suggest that TTBC molecules self-assemble in aqueous solution to form single-walled tubes with a diameter of about 35 Å. In order to reveal the arrangement and mutual orientations of the individual molecules in the tube, we combine information from crystal structure data of this dye with a calculation of linear absorbance and linear dichroism spectra and molecular dynamics simulations. We start with wrapping crystal planes in different directions to obtain tubes of suitable diameter. This set of tube models is evaluated by comparing the resulting optical spectra with experimental data. The tubes that can explain the spectra are investigated further by molecular dynamics simulations, including explicit solvent molecules. From the trajectories of the most stable tube models, the short-range ordering of the dye molecules is extracted and the optimization of the structure is iteratively completed. The final structural model is a tube of rings with 6-fold rotational symmetry, where neighboring rings are rotated by 30° and the transition dipole moments of the chromophores form an angle of 74° with respect to the symmetry axis of the tube. This model is in agreement with cryo-TEM images and can explain the optical spectra, consisting of a sharp red-shifted J-band that is polarized parallel to to the symmetry axis of the tube and a broad blue-shifted H-band polarized perpendicular to this axis. The general structure of the homogeneous spectrum of this hybrid HJ-aggregate is described by an analytical model that explains the difference in redistribution of oscillator strength inside the vibrational manifolds of the J- and H-bands and the relative intensities and excitation energies of those bands. In addition to the particular system investigated here, the present methodology can be expected to aid the structure prediction for a wide range of self-assembled dye aggregates.

Stimulus-responsive light-harvesting complexes based on the pillararene-induced co-assembly of β-carotene and chlorophyll

The locations and arrangements of carotenoids at the subcellular level are responsible for their designated functions, which reinforces the necessity of developing methods for constructing carotenoid-based suprastructures beyond the molecular level. Because carotenoids lack the binding sites necessary for controlled interactions, functional structures based on carotenoids are not easily obtained. Here, we show that carotene-based suprastructures were formed via the induction of pillararene through a phase-transfer-mediated host-guest interaction. More importantly, similar to the main component in natural photosynthesis, complexes could be synthesized after chlorophyll was introduced into the carotene-based suprastructure assembly process. Remarkably, compared with molecular carotene or chlorophyll, this synthesized suprastructure exhibits some photocatalytic activity when exposed to light, which can be exploited for photocatalytic reaction studies of energy capture and solar conversion in living organisms.

Interference between Coulombic and CT-mediated couplings in molecular aggregates: H- to J-aggregate transformation in perylene-based π-stacks

The spectroscopic differences between J and H-aggregates are traditionally attributed to the spatial dependence of the Coulombic coupling, as originally proposed by Kasha. However, in tightly packed molecular aggregates wave functions on neighboring molecules overlap, leading to an additional charge transfer (CT) mediated exciton coupling with a vastly different spatial dependence. The latter is governed by the nodal patterns of the molecular LUMOs and HOMOs from which the electron (te) and hole (th) transfer integrals derive. The sign of the CT-mediated coupling depends on the sign of the product teth and is therefore highly sensitive to small (sub-Angstrom) transverse displacements or slips. Given that Coulombic and CT-mediated couplings exist simultaneously in tightly packed molecular systems, the interference between the two must be considered when defining J and H-aggregates. Generally, such π-stacked aggregates do not abide by the traditional classification scheme of Kasha: for example, even when the Coulomb coupling is strong the presence of a similarly strong but destructively interfering CT-mediated coupling results in "null-aggregates" which spectroscopically resemble uncoupled molecules. Based on a Frenkel/CT Holstein Hamiltonian that takes into account both sources of electronic coupling as well as intramolecular vibrations, vibronic spectral signatures are developed for integrated Frenkel/CT systems in both the perturbative and resonance regimes. In the perturbative regime, the sign of the lowest exciton band curvature, which rigorously defines J and H-aggregation, is directly tracked by the ratio of the first two vibronic peak intensities. Even in the resonance regime, the vibronic ratio remains a useful tool to evaluate the J or H nature of the system. The theory developed is applied to the reversible H to J-aggregate transformations recently observed in several perylene bisimide systems.