J-Aggregates

• Preface

• Contents

Ultrafast real-time vibrational dynamics in several J-aggregate systems were studied by femtosecond real-time spectroscopy using ultrashort pulses with durations shorter than 5 fs. Despite the weak exciton–phonon coupling in Frenkel exciton systems, coherent molecular vibrations are clearly observed in the time dependence of the transmission change of porphyrin J-aggregates. The experimental results are explained in terms of a transition dipole moment modulated by dynamic intensity borrowing from an intense B transition to a weak Q transition through vibronic interaction. The observed wave packet has been classified as a Herzberg–Teller type in contrast with the frequently studied conventional Franck–Condon type. In pseudoisocyanine J-aggregates, intermolecular vibrations were observed in the lattice phonon coupled to the excitonic transition in J-aggregates and 11 vibrational modes were detected. We applied non-Condon-type exciton–phonon coupling in J-aggregates to explain the oscillatory signals. In addition, we discussed the influence of the excitation pulse duration and the laser spectrum. The vibrational amplitude was found to be proportional to the product of the integral of the absorbed spectrum and the one shifted by the vibrational mode frequency.

Vibronic coupling in molecular aggregates leads to additional signatures for identifying J- and H-aggregates. In most π-conjugated molecules nuclear relaxation subsequent to electronic excitation occurs mainly along a symmetric vinyl-stretching mode with a frequency of 0.17 eV/ħ. Analysis of the impact of intermolecular interactions on the mode progressions in absorption and photoluminescence leads to additional information about the nature of the excited states. In J-aggregates the ratio of the lowest two vibronic peak intensities in the absorption spectrum increases with exciton bandwidth W providing a direct measure of the latter. In addition, the ratio of the 0-0 and 0-1 line strengths in the photoluminescence spectrum provides a direct measure of the exciton coherence number, N_coh. N_coh. increases with W, and decreases with disorder and temperature.

The structural and spectroscopic properties of self-assembled light-harvesting chromophores are discussed. In photosynthetic organisms, light-harvesting aggregates are optimized for absorption of particular wavelengths of the solar spectrum and for energy transfer to the photosynthetic reaction center. Artifical light-harvesting aggregates composed of porphyrins provide model systems for understanding how the hierarchal structure of these assemblies determines their optical properties. Resonance Raman and scanning probe microscopy of porphyrin aggregates reveal the structural features and intermolecular interactions which control this assembly.

Cyanine dyes are known since more than 70 years. While the optical properties have early been explained by molecular excitons, detailed structural data of aggregates in solution were missing for a long time. Here we review the morphological structures of cyanine dye aggregates revealed recently by cryogenic transmission electron microscopy. Soft two-dimensional molecular layers, quasi one-dimensional threadlike and tubular aggregates were detected. Often the structures appear to be hierarchically organized to form more complex superstructures.

Aggregates states and chiralities of cyanine dye are sensitive to the change of solution environment. This property can be utilized in different fields, especially in fields of biology. Due to the distinct change of spectroscopy properties resulting from the interaction between aggregates of cyanine dye and biomolecules, aggregates of cyanine dye become a excellent probe in recognition of different biomolecules. This chapter mainly focuses on the interaction between aggregates of cyanine dyes and biomolecules, as well as the application in recognition of aggregates of cyanine dyes to different biomolecules.

J-aggregates, once widespread in film photography,¹ have fostered renewed interest due to their non-linear optical properties, their quantum mechanical coupling at ambient conditions, and their delocalized molecular excitons. Such Frenkel excitons² contrast with the Wannier-Mott excitons characteristic of inorganic semiconductors, and J-aggregates have been combined with various inorganic semiconductor materials to study the effects of coupling in these hybrid systems.³ This chapter reviews the progress to date on J-aggregates coupled with excitonic inorganic materials. After summarizing the motivation and first proposals in the use of these materials, we discuss the experimental realization of semiconductor quantum wells and quantum dots coupled to J-aggregated dyes. An outlook on future directions is then presented.

We employed biopolymer DNA as a template to form J-aggregates of pseudoisocyanine (PIC), and succeeded to optimize the conditions for their formation in solutions and solid films. The optical characteristics of J-aggregates were investigated by absorption, fluorescence and circular dichroic (CD) spectra. Polyvinylalcohol (PVA) introduced as a matrix for the films was proved to play a role to prevent the precipitation or recrystallization of the dyes in solutions and films. We investigated the dependence on concentrations of dye and DNA, and the ratio of these, and effects from counter ions for PIC. We also prepared the samples for more than 20 types of cyanine dyes, finding few dyes showing J-aggregate peak. These results show possibility of application of J-aggregate into novel optical devices requiring optical nonlinearity or superradiant behavior.

The effect of electric field on porphyrin J-aggregates both in polymer film and in aqueous solution was studied by electroabsorption spectroscopy with a multichannel lock-in detection technique. A large enhancement of electrooptic (EO) Kerr response was observed due to the formation of J-aggregates from monomer molecules. The enhancement factor of EO Kerr parameters (polarizability difference between the excited and ground states) is as large as 100 for the polymer film and more than 100 000 for the aqueous solution. This is explained by the molecular rearrangement model, where the dipole–dipole interaction energy is modified by field-induced change in the angle between the molecular transition dipole moment and the aggregation axis. The model was supported by various experimental facts. We also found electric-field induced dissociation and association of J-aggregates in aqueous solution. This may lead to the way to control the coherent aggregation number of J-aggregates with a stationary electric field.

Excitons in J-aggregates possess mobility probably superior to that in any other organic system. Recently exciton migration over millions of dyes molecules was demonstrated at cryogenic temperatures for several types of J-aggregates. It is clear that instead of commonly accepted incoherent exciton hopping one has to think in terms of coherent wave-like exciton motion in order to account for the experimental results. However, there is still no consistent theoretical explanation of such excellent exciton mobility. Moreover, efficient exciton transport at low temperatures seems to contradict with the well-accepted theoretical model of the non-monotonous temperature dependence of the Stokes shift in J-aggregates. The latter effect is explained by the idea that excitons at low temperatures have very small mobility and therefore cannot sample exciton states localized on spatially different segments of the chain.

We examine the unique spectroscopic features which give rise to quantum information processing attributes of J-aggregate systems via several entanglement entities such the von Neumann entropy, Wootters concurrence, and the Wei-Goldbart geometric measure. We also discuss two other measures, the quantum discord and its geometric measure which are intrinsically distinct from other known quantum correlation measures. The role of dispersion and resonance terms in the exciton-phonon interaction are analyzed using Green's function formalism. We show that the scaling properties of the third order optical response χ⁽³⁾ with exciton delocalization size is a notable feature that can be harnessed as an experimentally demonstrable measure of quantifying multipartite entanglement in J-aggregate systems. Results of the quantum discord for a linear monomer chain highlights the potential of J-aggregates as sources of robust non-classical correlations which can be utilised for their superior optical properties.

During the last decade detailed information on the supramolecular organization of perylene bisimide (PBI) dyes in their aggregates in solution has been achieved by spectroscopic methods. Whilst the preferential packing for the parent PBIs, i.e. dyes without core-substituents and specific directing groups, affords sandwich-type aggregates with dominant H-type absorption and strongly bathochromically displaced excimer-type emission bands, construction of dye assemblies with J-type coupling was achieved by supramolecular design. In particular, the combination of contorted PBI cores by bay substituents and directional hydrogen bonding afforded PBI J-aggregates that exhibit significantly bathochromically displaced absorption bands, band narrowing and small Stokes shifts. Owing to the nucleation-induced instantaneous growth of extended and well-defined thermodynamically stable J-aggregates and their high fluorescence quantum yield of almost unity, a variety of spectroscopic investigations could be performed for these J-aggregates. For individual PBI J-aggregates, fluorescence blinking corresponding to collective quenching of up to 100 PBI monomers has been observed that could be related to an exciton diffusion length of up to 70 nm in these assemblies at room temperature

Facile and co-operative 2D syntheses of new functionals (E)-1-(3-chloro-4-(alkoxy)-phenyl)-2-phenyl-diazene and their stoichiometrically ordered assembly of parallel-stacked unit mixed-H type aggregates as building blocks are described upon neat addition of charged fluorescent units of Rh B to the interfacial photochromes. For these integrated assemblies, molecular order was encoded in the shape and chemical functionality and was governed by the strength and directionality of secondary interactions. The solution phase H- type aggregates from the amphiphiles required long range orientational order; transformation into more stable tubular architectures with controlled molecular alignment at 2D was energetically favorable solely on account of surface-induced phenomena. Rh B influenced π–π interaction and anisotropic polarizability in addition to electronic interactions, while the diazene derivatives supported coulombic binding.

For the same amphiphiles to form J aggregates, a two component interfacial matrix stabilization technique was necessitated and devised. Langmuir isotherms of (E)-1-(3-chloro-4-(alkyloxy)phenyl)-2-phenyldiazene (C_nCD, n = 8, 10, 12) with stearic acid (STA) as the two dimensional matrix revealed a phase separation of the C_nCD from the matrix at a compression pressure of 10 mN/m. While a one component C_nCD failed to form a 2D monolayer, the STA supported C_nCD binary system crossed a compression induced mixed monolayer phase with formation of highly ordered matrix stabilized and phase separated C_nCD J-aggregate upon proper tuning of STA- CnCD ratio, suggesting a 'know-how' for the preparation of J-aggregates from hydrophobic functionals.

The dimeric association of a Fullerene (C₆₀)-N,N-dimethylaminoazobenzene dyad (DPNME), a donor-bridge-acceptor hybrid has been explored. The study started by presenting a comprehensive DFT-study on the dyad, continued by investigation of its tunable self assembly as a function of solvent polarity in a more exploratory manner and finished with an extensive discussion on its dimerization, where the electronic coupling of the azo chromophore with the substituted 3D Fullerene C₆₀ had a predominant impact on the structure organization. For a binary solvent dielectric constant 7.56 ≤ ε ≤ 66, a solvent polarity dependent dynamic equilibrium between the monomer ⅉ J-aggregate ↔ H-aggregate was established. A synergetic effect of the intermolecular donor-acceptor interaction between the electron deficient fullerene ring and the electron rich N,N-dimethylamino substituted aromatic ring governed the process of aggregation with predominant van der Waals and π-π interaction between a pair of dimers. The linear geometry of the donor and the conformational rigidity of the Fullerene C₆₀ acceptor played crucial roles in envisaging the resulting π-extended network. An important part of the work comprised of our emphasis on the observed finite oscillator strength of the hitherto forbidden anti-parallel H-aggregates. The results established a novel chromophoric system in fullerene (C₆₀)- azobenzene hybrid for the construction of supramolecular assemblies with opto-electronically active constituents. Importantly, substitution of fullerene C₆₀ with the J-aggregate forming AZNME chromophore yielded the anti-parallel H-dimer. The overall process of H and J aggregate formation represented significant advance towards controlled molecular self assembly with predictable shapes by controlling the surface molecular density and bulk phase chemical structure and solvent polarity. The results exposed interesting new model systems for dye aggregates from which laws of cluster formation could be deduced and the ability to organize the building blocks in a specified direction could provide insight for nano-electronics design.

Fabrication of highly organized systems such as J-aggregates from photofunctional molecular building blocks opens up new vistas for the exploration of photonic nanodevice concepts. This review delineates formation and direct observation of J-aggregates with various mesoscopic structures in "solution phases" (in bulk solution and at solution/solid interfaces). Fluorescence microscopy, polarized-light microscopy (PLM) and atomic force microscopy (AFM) are used to characterize their morphologies which include quasi-one-dimensional string, tubular-rod and two-dimensional sheet structures as well as three-dimensional multilayers. The optically anisotropic nature of such J-aggregates is quantitatively evaluated. The J-aggregates carrying the outstanding spectroscopic properties with efficient exciton coupling represent a versatile rationale to realize structurally complex nanosystems with specific shape and functions, so the study on the nanoscopic/mesoscopic structures of such aggregates will be significant in relation to future supramolecular engineering.

Studies on structure and optical properties of J-aggregates of amphiphilic merocyanine dyes formed in thin film systems fabricated by Langmuir-Blodgett (LB) technique have been reviewed. After giving a theoretical framework for treating structure and optical properties of the dye thin-film systems, nucleation and aggregation number characterized by optical spectroscopy are presented. Studies on reorganization of the J-band in a merocyanine (DS)-arachidic acid (C₂₀) binary LB system using hydrothermal treatment, which is an annealing under relative humidity of 100%, have been also introduced. Furthermore, as a method to induce the H-aggregate formation in the binary systems of the DS-C₂₀ LB system, addition of n-alkane as a third component has been touched upon. Finally a study on structure of J-aggregate in a merocyanine dye LB film determined by grazing incidence X-ray diffraction by Kato et al. has been introduced; it has been found that the interaction between static dipole moments upon excitation contributes significantly to the band shift as well as the transition-dipole interaction.

The following sections are included:

• Index