H-type Aggregates Research Articles

Propagation of chirality from gemini surfactants to porphyrin/surfactant heteroaggregates: transcription of the stereochemical information into an organizational feature

The interaction of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin with the chiral gemini surfactants (2R,3R)- and (2S,3S)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane bromide gives, below the critical micellar concentration, porphyrin/surfactant heteroaggregates of defined stoichiometry, where the dye molecules have a column-packed, H-type arrangement. The results of a CD investigation suggest that the gemini structure controls both the chirality of the porphyrin-surfactant unit and the chiral morphology of H-type aggregates.

Synthesis, liquid crystalline and luminescent properties of trimeric phenylenevinylene with end-groups of (R)-(+)-2-methylbutyric acid ester

The trimeric phenylenevinylene with the symmetrical chiral end-groups (ChTPV) was synthesized. The liquid crystalline and luminescent properties of the ChTPV have been studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), absorption and photoluminescence spectra. The results indicated that the ChTPV exhibits mesophase over a wide temperature range and a typical optical texture of smectic phase. In contrast with the spectra of the solution, that of the film showed blue-shift in maximal absorption and red-shift in maximal emission due to H-type aggregation with parallel alignment of the TPV transition dipole moment.

On the Arrangements of R6G Molecules in Organophilic C12TMA/Lap Clay Films for Low Dye Loadings

Absorption and fluorescence spectroscopies with linearly polarized light are applied to characterize the adsorbed species of rhodamine 6G (R6G) laser dye in ordered organophilic laponite (Lap) clay films for low dye loadings. The organophilic character of the clay was controlled by the number of organic surfactant dodecyl-trimethylammonium (C12TMA) cations intercalated into the interlayer space of the clay. Experimental results suggest that for moderate to high surfactant contents (>70% of the total cation exchange capacity, CEC, of the clay) the accessibility of the interlayer space for R6G molecules is reduced. In these cases, the first stage in the adsorption of R6G molecules is at the external surface, where dye molecules can self-associate even for very low dye loading (around 0.1%CEC), probably for the limitation of the external surface area. The presence of a nonfluorescent H-type aggregate with a short-displaced coplanar structure and a fluorescent oblique head-to-tail J aggregate is reported. The inclined structure of this J aggregate is stabilized by the surfactant molecules at the external surface.

Tuning Microstructures in Organogels: Gelation and Spectroscopic Properties of Mono- and Bis-cholesterol-Linked Diphenylbutadiene Derivatives

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (> 10(-4) M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres.

Circular dichroism and theoretical calculations of pinacyanol dimer inclusion in γ-cyclodextrin

Aggregation of pinacyanol chloride is strongly enhanced through inclusion into γ-cyclodextrin, which was studied using both UV/Visible, derivative and Circular Dichroism (CD) spectroscopy. The intensities at 546 and at 507 nm were increased with the growing ratio of γ-cyclodextrin to dye. Both aggregates were shown to be optically active. CD spectra with two and three oppositely signed excitonic bands were shown. The structures of the aggregates have been analysed and discussed in terms of qualitative H-type aggregation and quantitative based on OSCI program. Applying eight different parameter sets aggregates of pinacyanol chloride ranging in size from 2 to 20 dye molecules were considered. The calculated energy splitting was adjusted, via the transition dipole length, to the experimentally observed band shift. Rotational strengths associated with the CD absorptions were calculated and used to obtain an estimate of the inherent twist of the dimer aggregate inside the γ-cyclodextrin cavity.

Synthesis and Characterization of Coil−Rod−Coil Triblock Copolymers Comprising Fluorene-Based Mesogenic Monodisperse Conjugated Rod and Poly(ethylene oxide) Coil

A series of coil−rod−coil triblock copolymers (i.e., F3T8EO8, F3T8EO17, F3T8EO45, and F3T8EO125) with a mesogenic monodisperse conjugated oligomer comprising 3 fluorene, 8 thiophene, and 2 phenyl units as the rod and poly(ethylene oxide) (PEO) as the coil were synthesized. A reference compound, that is F3T8ME2, with the identical rod but without PEO was also prepared for comparison. The volume fraction of PEO (fPEO) was 0, 0.16, 0.28, 0.50, and 0.73 for F3T8ME2, F3T8EO8, F3T8EO17, F3T8EO45, and F3T8EO125, respectively. It was found that the introduction of PEO into the triblock copolymers encouraged the formation of H-type aggregation and fPEO-dependent highly ordered mesophases while fPEO < 0.73. For F3T8ME2, only nematic mesophase was observed. In contrast, F3T8EO8 and F3T8EO17, with fPEO of 0.16 and 0.28, respectively, are smectic A (SA) mesomorphism. For F3T8EO45 with fPEO of 0.50, several highly ordered phases, such as hexagonal columnar crystalline phase and rectangular columnar, smectic B (SB) and ...

Possible Formation of H-type Aggregates in H2TPyP/PMMA Guest Host Films

AbstractIn this work we have employed UV-Vis absorbance and excitation techniques in addition to quantum chemical calculations to investigate the probable formation of H-type aggregates of zero charged aryl substituted porphyrin in a guest- host solid state structure. The films were factored via casting technique mixing free-base tetrapyridyl porphyrins (H2TPyP) and polymethilmethacrilate polymer (PMMA) chlorophorm solutions. A pathway for the possible formation of H-type aggregate is suggested.

Theoretical Investigation of the Role of Strongly Coupled Chlorophyll Dimers in Photoprotection of LHCII

Nonphotochemical quenching is the photoprotection mechanism by which the excess excitation energy absorbed by the light harvesting complex LHCII is dissipated through the protein scaffold as heat. Using the quenched structure of LHCII obtained from crystallographic experiments, the potential quenching of photoexcited excitons by aggregates of chlorophylls is theoretically investigated. In monomeric LHCII there is a hierarchy of length scales resulting in a hierarchy of energy scales that determine the interpigment direct Coulomb coupling. We propose a model whereby eight chlorophylls are coupled quantum mechanically into four dimers, with exciton transfer between these dimers and the remaining six single chlorophylls proceeding incoherently via Forster transfer. The chlorophyll dimer Chl a604-Chl b606 possesses a quasi-parallel geometry, resulting in a weakly dipole-allowed low-lying excited state. This weakly allowed state is accessible via exciton transfer to a higher, strongly allowed state followed by fast vibrational relaxation. This parallel, H-type aggregate can potentially function as an exciton trap. Calculated Forster transfer rates between single chlorophylls and chlorophyll dimers are used in a simulation of exciton transfer in monomeric LHCII to explore this possibility. It is found that Chl a604-Chl b606 has a short-lived enhanced population (on the time scale of approximately picoseconds), but not a long-time resident population. The fluorescence quantum yield of the model was calculated to be phi F = 0.38. Comparison of this result with phi F approximately 0.26 for unquenched LHCII in dilute solution and phi F approximately 0.06 for the highly quenched LHCII crystal reveals that the proposed model does not account for the quenching observed in the LHCII crystal. We therefore conclude that the formation of chlorophyll dimers is not the main cause of excitonic NPQ in LHCII.

Preparation and characterization of free-standing pure porphyrin nanoparticles

Preparation and characterization of absolutely pure and stable nanoparticles of 5,10,15,20-meso-tetrakis phenyl porphyrin (TPP) and catalytically repute 5,10,15,20-meso-tetrakis pentaflurophenyl porphyrin (H2F20TPP) by improved ‘reprecipitation method’ is described. The innovation of this modified ‘reprecipitation method’ lies on the judicial selection of organic solvent and amount of porphyrin solution to be injected in the aqueous media. Exactly similar process produces relatively small nanoparticles for TPP than that of H2F20TPP while the stability of the H2F20TPP nanoparticles is bit higher than nanoparticles of TPP. Absorption and emission spectra reveal that the formation of nanoparticles for both the cases is induced by J-and H-type aggregation. DFT calculations predict the optimized geometries and frontier molecular orbital, which favours the strength of face-to-face interaction with neighbour molecules to be more facile for TPP than that of H2F20TPP helping the latter to form bigger and relatively more stable and free-standing nanoparticles. The use of no other compounds except dichloromethane, a highly volatile organic solvent and respective porphyrins give absolutely pure nanoparticles. This improved method will lead to produce organic nanoparticles of π-conjugated systems easily and efficiently.

Strongly Coupled Excitonic States in H-Aggregated Single Crystalline Nanoparticles of 2,5-Bis(4-methoxybenzylidene) Cyclopentanone

This paper reports that extremely strongly coupled excitonic states were formed in H-aggregated monocrystalline nanosheets and semicrystalline nanowires of coplanar organic molecules of 2,5-bis(4-methoxybenzylidene) cyclopentanone, due to the highly regular face-to-face stacking of molecular excitons. It was demonstrated that the spectral absorption and fluorescence emission behaviors are dependent on the routes of molecular aggregation and the ordered degree of molecular arrangement in aggregated nanoparticles. In particular, the H-type aggregation of molecules through a highly ordered molecular arrangement in the monocrystalline nanosheets led to the formation of a new exciton coupling state with an energy band higher than that in normal semi-/noncrystalline H-aggregation. A strong symmetric absorption at higher energy bands was thus observed in the solution of nanosheets. Furthermore, the strongly coupled excitonic state may hold all the oscillator strength, leading to the extinction of the original intramolecular electronic transitions of individual molecules and the appearance of new strong absorption and fluorescence emissions at high-energy bands. These results show a perspective that the ability to control the molecular structure and its arrangement in aggregates holds promise for creating novel optical properties in molecular materials.

Effect of Temperature and Oxygen on Luminescence Spectra and Polarization of Divinylbenzoxazolylbiphenyl Thin Films

H-type molecular aggregation as assembly with chromophore dipoles arranged parallel to each other has been observed in absorption and luminescence spectra of divinylbenzoxazolylbiphenyl thin films deposited on quartz glass substrate by thermovacuum method. The reversible changes of fluorescence anisotropy have been observed under film heating below the glass transition temperature correlating with lifetime changes. The addition of oxygen was found to cause an essential luminescence quenching. The hexyloxy group in the side chain of dibenzoxazolylbiphenyl molecule decreases H-type molecular aggregation and energy migration, and increases luminescence quenching induced by adsorbed oxygen owing to the formation of more porous film morphology.

Controlling molecular assembling by photons: reversible light-powered monomer–aggregate interconversion of porphyrins

Reversible formation and destruction of H-type aggregates of an anionic porphyrin can be controlled exclusively through light inputs of different energy exploiting the different interactions with the two isomeric forms of a cationic photoresponsive azobenzene-based surfactant.

Study of assembly of arachidic acid/LDHs hybrid films containing photoactive dyes

Hybrid monolayers formed with an floating arachidic acid (AA) anions monolayer binding with a densely packed layered double hydroxides (LDHs) monolayer at an air/LDHs suspension interface has been studied by π– A isotherms and TEM images. An ordered multilayer film of AA/LDHs has been fabricated by Langmuir–Blodgett technique on various substrates. The photoactive dyes (methyl orange, MO, and Congo red, CR) can be incorporated into the galleries of LDHs in the AA/LDHs hybrid LB film by an ion intercalation method. The results of FTIR and UV–vis absorption spectra can approve the formation of AA/LDHs/dyes composite films. In addition, UV–vis absorption spectra and LAXRD analyses also provide evidence for the good vertical uniformity and stable layered periodic structure of AA/LDHs/dyes films. More interestingly, it was found that the dye molecules intercalated can be induced by a positively charged LDHs sheet to align in a special orientation and form different aggregates: MO molecules form sandwich H-type aggregates, while CR molecules form head-to-tail J-type aggregates. On the basis of these data, a possible model of the AA/LDHs/dyes composite films was proposed. Also, the dye molecules incorporated into AA/LDHs films exhibit excellent configuration stability under the irradiation of UV light because the LDHs matrix offers a more rigid and constrained environment for them.

Molecular aggregation in Langmuir–Blodgett films of azo dye/liquid crystal mixtures

Studies of azo dye/liquid crystal mixtures in monolayers formed at an air–water interface (Langmuir films) and on a solid surface (Langmuir–Blodgett films) were performed. Six azo dyes with various molecular structures and two liquid crystal materials: 4-octyl-4′-cyanobiphenyl (8CB) and trans-4-octyl(4′-cyanophenyl)-cyclohexane (8PCH) were used. The dyes were added to the liquid crystal matrices at various concentrations and the surface pressure–mean molecular area isotherms for the resultant Langmuir films were recorded. On the basis of the isotherm runs, conclusions concerning the molecular organization and the miscibility of the components in the ultrathin films were drawn. Absorption spectra for the Langmuir–Blodgett films, using both natural and linearly polarized light were recorded. Information regarding intermolecular interactions in the mixtures of the nonamphiphilic dye and the liquid crystal, with the polar terminal group was obtained. The ability of dye molecules to form self-aggregates at the air–solid substrate interface was observed. The influence of the molecular structure and the concentration of a dye on aggregates' geometry is discussed. It was found that at low dye concentrations in the liquid crystal J-type aggregates are formed. High dye contents are conducive to the creation of H-type aggregation, which improve the molecular packing in the Langmuir–Blodgett film.

Photophysical characterizations of 2-(4-biphenylyl)-5 phenyl-1,3,4-oxadiazole in restricted geometry

Langmuir and Langmuir–Blodgett (LB) films of non-amphiphilic 2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazole (abbreviated as PBD) mixed with stearic acid (SA) as well as with the inert polymer matrix poly(methyl methacrylate) (PMMA) have been studied. Surface pressure versus area per molecule ( π– A) isotherm studies suggest that PBD molecules very likely stand vertically on the air–water interface and this arrangement allows the PBD molecules to form stacks and remain sandwiched between SA/PMMA molecules. At lower surface pressure, phase separation between PBD and matrix molecules occurs due to repulsive interaction. However, at higher surface pressure, PBD molecules form aggregates. The UV–vis absorption and steady-state fluorescence spectroscopic studies of the mixed LB films of PBD reveal the nature of the aggregates. H-type aggregate predominates in the mixed LB films, whereas I-type aggregate predominates in the PBD-PMMA spin-coated films. The degree of deformation produced in the electronic levels are largely affected by the film thickness and the surface pressure of lifting.

Metallo Supramolecular Assemblies of Bis-squaraines by Allosteric Ca2+ Ion Binding

Alkyl chain tethered bis-squaraines bind to Ca2+ ions through the participation of the negatively charged oxygen of the central cyclobutene moiety to form folded H-type aggregates. The initially formed Ca2+ complex is preorganized to facilitate cooperative allosteric binding of Ca2+, resulting in the formation of extended supramolecular arrays. The electronic absorption, IR, and ESI-MS studies support the formation of metallo supramolecular architectures of the folded H-type dimers of the bis-squaraines.

Spectral Properties of Rhodamine 3B Adsorbed on the Surface of Montmorillonites with Variable Layer Charge

Montmorillonite was thermally treated at several temperatures to reduce the charge density of its layer surface. Absorption and fluorescence (steady-state and time-resolved) spectroscopies are now applied to study the adsorption of rhodamine 3B (R3B) laser dye in reduced charge montmorillonites (RCMs) in aqueous suspensions. The decrease in the charge density increases the intermolecular distance between adsorbed R3B molecules, reducing the tendency of the dye to self-associate. H-type and J-type aggregates of R3B in RCMs are spectroscopically characterized, the fluorescent J-aggregates being more extensively formed by decreasing the charge density. Both the reduction in the dye aggregation and the formation of J-type aggregates enhance the fluorescence efficiency of R3B dye adsorbed in montmorillonite particles. Absorption with linearly polarized light reveals that the H-aggregates are more disposed toward the perpendicular of the clay surface than the monomer and J-aggregates species.

Self-assembly of acridine orange dye at a mica/solution interface: Formation of nanostripe supramolecular architectures

Optical waveguide spectroscopy and atomic force microscopy (AFM) have been used to characterize the supramolecular architectures of acridine orange (AO) dye self-assembled at a mica/aqueous solution interface. Under the saturated adsorption conditions, optical waveguide spectroscopy revealed that the dye formed H-type aggregates at the interface. In situ AFM visualized interesting morphology of the dye aggregates showing nanosized meandering stripes with the width of ∼ 1.5 nm (or brightness periodicity of ∼ 3 nm ). Electrostatic adsorption of the dye cations onto a mica surface as well as the intermolecular π – π stacking brought about the ordered nanostructures. We propose an interfacial aggregation model that shows a meandering staircase structure with the intermolecular slip angle of 60°. According to the model, the AO molecule occupies a surface area of about 1.0 nm 2.

Hydrogen‐Bonded Oligo(p‐phenylenevinylene) Functionalized with Perylene Bisimide: Self‐Assembly and Energy Transfer

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor-acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.

Harvesting Infrared Photons with Tricarbocyanine Dye Clusters

The absorption of 4,5-benzoindotricarbocyanine dye (IR125) in the infrared can be tuned by controlling the type of aggregation in different media. Molecular clusters of this dye formed in a mixed solvent show broad absorption in the 550-950 nm region as compared to the absorption bands of J- and H-type aggregates. The molecular clusters of the carbocyanine dye are electrophoretically deposited as thin film on optically transparent electrodes using a dc electric field. These tricarbocyanine dye cluster films are photoactive in the infrared region and produce cathodic current when employed as photocathode in a photoelectrochemical cell. Transient absorption spectroscopy of the molecular clusters show short-lived singlet state in the picosecond time scale (lifetime 6 ps) and a charge separated state in the nanosecond time scale. Implication of such dye cluster films for harvesting infrared photons in a photoelectrochemical cell is discussed.