Non-coherent exciton migration in J-aggregates of the dye THIATS: exciton–exciton annihilation and fluorescence depolarization

Comparison of Frenkel and Excimer Exciton Diffusion in Perylene Bisimide Nanoparticles

Exciton migration is an important process for light harvesting with organic systems and often the bottleneck. Especially the formation of trap states hinders the mobility considerably. Although excimer excitons are often referred to as traps, their mobility has been demonstrated while their nature is still unclear. Here, we compare the mobility of singlet and excimer excitons in nanoparticles consisting of the same type of perylene bisimide molecules. By changing the preparation conditions, nanoparticles with different intermolecular coupling strengths are prepared. Femtosecond transient absorption spectroscopy reveals the formation of excimer excitons from Frenkel excitons. The mobility of both exciton types is determined by evaluating exciton-exciton annihilation processes. In the lower coupling regime, singlet mobility is observed, whereas for stronger coupling the dynamics is dominated by a 10-fold increased excimer mobility. The excimer mobility can thus even be higher than the singlet mobility and is affected by the intermolecular electronic coupling.

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.

Exciton Migration in Multistranded Perylene Bisimide J-Aggregates

Exciton migration in self-assembled supramolecular ensembles of dye molecules is controlled by the electronic coupling between adjacent sites, the delocalization of the excitation and thereby by the packing arrangement. Here, we put emphasis on the packing structure and analyze the exciton migration in two perylene bisimide-based J-aggregates composed of almost identical molecular building blocks but forming double-strand versus quadruple-strand slip-stacked supramolecular architectures. Analyzing ultrafast transient absorption spectra in dependence on the exciton density by a kinetic model for exciton-exciton annihilation based on incoherent transfer demonstrates that the migration is quasi one-dimensional. The migration distance is enhanced by a beneficial geometrical structure. We find a factor of more than two between the diffusion lengths of 188 and 77 nm for the double- and quadruple-stranded system. The supramolecular design efficiently influences the exciton mobility and minor structural changes have a pronounced influence on functional properties of dye aggregates.

Microfluidic out-of-equilibrium control of molecular nanotubes

The bottom-up fabrication of functional nanosystems for light-harvesting applications and excitonic devices often relies on molecular self-assembly. Gaining access to the intermediate species involved in self-assembly would provide valuable insights into the pathways via which the final architecture has evolved, yet difficult to achieve due to their intrinsically short-lived nature. Here, we employ a lab-on-a-chip approach as a means to obtain in situ control of the structural complexity of an artificial light-harvesting complex: molecular double-walled nanotubes. Rapid and stable dissolution of the outer wall was realized via microfluidic mixing thereby rendering the thermodynamically unstable inner tubes accessible to spectroscopy. By measurement of the linear dichroism and time-resolved photoluminescence of both double-walled nanotubes and isolated inner tubes we show that the optical (excitonic) properties of the inner tube are remarkably robust to such drastic perturbation of the system's supramolecular structure as removal of the outer wall. The developed platform is readily extendable to a broad range of practical applications such as e.g. self-assembling systems and molecular photonics devices.

Interplay between structural hierarchy and exciton diffusion in artificial light harvesting

Unraveling the nature of energy transport in multi-chromophoric photosynthetic complexes is essential to extract valuable design blueprints for light-harvesting applications. Long-range exciton transport in such systems is facilitated by a combination of delocalized excitation wavefunctions (excitons) and exciton diffusion. The unambiguous identification of the exciton transport is intrinsically challenging due to the system's sheer complexity. Here we address this challenge by employing a spectroscopic lab-on-a-chip approach: ultrafast coherent two-dimensional spectroscopy and microfluidics working in tandem with theoretical modeling. We show that at low excitation fluences, the outer layer acts as an exciton antenna supplying excitons to the inner tube, while under high excitation fluences the former converts its functionality into an exciton annihilator which depletes the exciton population prior to any exciton transfer. Our findings shed light on the excitonic trajectories across different sub-units of a multi-layered artificial light-harvesting complex and underpin their great potential for directional excitation energy transport.

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.

Fluorescence polarization measures energy funneling in single light-harvesting antennas--LH2 vs conjugated polymers

Numerous approaches have been proposed to mimic natural photosynthesis using artificial antenna systems, such as conjugated polymers (CPs), dendrimers, and J-aggregates. As a result, there is a need to characterize and compare the excitation energy transfer (EET) properties of various natural and artificial antennas. Here we experimentally show that EET in single antennas can be characterized by 2D polarization imaging using the single funnel approximation. This methodology addresses the ability of an individual antenna to transfer its absorbed energy towards a single pool of emissive states, using a single parameter called energy funneling efficiency (ε). We studied individual peripheral antennas of purple bacteria (LH2) and single CP chains of 20 nm length. As expected from a perfect antenna, LH2s showed funneling efficiencies close to unity. In contrast, CPs showed lower average funneling efficiencies, greatly varying from molecule to molecule. Cyclodextrin insulation of the conjugated backbone improves EET, increasing the fraction of CPs possessing ε = 1. Comparison between LH2s and CPs shows the importance of the protection systems and the protein scaffold of LH2, which keep the chromophores in functional form and at such geometrical arrangement that ensures excellent EET.

Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites

Photoluminescence (PL) of organo-metal halide perovskite semiconductors can be enhanced by several orders of magnitude by exposure to visible light. We applied PL microscopy and super-resolution optical imaging to investigate this phenomenon with spatial resolution better than 10 nm using films of CH3NH3PbI3 prepared by the equimolar solution-deposition method, resulting in crystals of different sizes. We found that PL of ∼100 nm crystals enhances much faster than that of larger, micrometer-sized ones. This crystal-size dependence of the photochemical light passivation of charge traps responsible for PL quenching allowed us to conclude that traps are present in the entire crystal volume rather than at the surface only. Because of this effect, "dark" micrometer-sized perovskite crystals can be converted into highly luminescent smaller ones just by mechanical grinding. Super-resolution optical imaging shows spatial inhomogeneity of the PL intensity within perovskite crystals and the existence of <100 nm-sized localized emitting sites. The possible origin of these sites is discussed.

Nanotubular J-aggregates and quantum dots coupled for efficient resonance excitation energy transfer

Resonant coupling between distinct excitons in organic supramolecular assemblies and inorganic semiconductors is supposed to offer an approach to optoelectronic devices. Here, we report on colloidal nanohybrids consisting of self-assembled tubular J-aggregates decorated with semiconductor quantum dots (QDs) via electrostatic self-assembly. The role of QDs in the energy transfer process can be switched from a donor to an acceptor by tuning its size and thereby the excitonic transition energy while keeping the chemistry unaltered. QDs are located within a close distance (<4 nm) to the J-aggregate surface, without harming the tubular structures and optical properties of J-aggregates. The close proximity of J-aggregates and QDs allows the strong excitation energy transfer coupling, which is around 92% in the case of energy transfer from the QD donor to the J-aggregate acceptor and approximately 20% in the reverse case. This system provides a model of an organic-inorganic light-harvesting complex using methods of self-assembly in aqueous solution, and it highlights a route toward hierarchical synthesis of structurally well-defined supramolecular objects with advanced functionality.

Organization of bacteriochlorophylls in individual chlorosomes from Chlorobaculum tepidum studied by 2-dimensional polarization fluorescence microscopy

Chlorosomes are the largest and most efficient natural light-harvesting systems and contain supramolecular assemblies of bacteriochlorophylls that are organized without proteins. Despite a recent structure determination for chlorosomes from Chlorobaculum tepidum (Ganapathy Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8525), the issue of a possible large structural disorder is still discussed controversially. We have studied individual chlorosomes prepared under very carefully controlled growth condition by a novel 2-dimensional polarization single molecule imaging technique giving polarization information for both fluorescence excitation and emission simultaneously. Contrary to the existing literature data, the polarization degree or modulation depth (M) for both excitation (absorption) and emission (fluorescence) showed extremely narrow distributions. The fluorescence was always highly polarized with M ≈ 0.77, independent of the excitation wavelength. Moreover, the fluorescence spectra of individual chlorosomes were identical within the error limits. These results lead us to conclude that all chlorosomes possess the same type of internal organization in terms of the arrangement of the bacteriochlorophyll c transition dipole moments and their total excitonic transition dipole possess a cylindrical symmetry in agreement with the previously suggested concentric multitubular chlorophyll aggregate organization (Ganapathy Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8525).

Kinetics of salt-induced J-aggregation of cyanine dyes

The addition of monovalent, divalent, and trivalent metal ions to three anionic ethyl meso-thiacarbocyanine dyes, an ethyl meso-oxacarbocyanine, and an imidacarbocyanine in aqueous solution at room temperature results in the production of J-aggregates within the range of tens to hundreds of seconds. The rate of formation of J-aggregates correlates with the rate of decay of dimers or monomers and is dependent on the type of metal ion, dye structure, and temperature. The rate of formation of J-aggregates increases as the temperature decreases and the dye and salt concentrations increase, and the rate is highest for trivalent ions and smallest for monovalent ions, independent of the type of anion. The time course of formation of J-aggregates is described in most cases by a sigmoidal curve, and the kinetics and mechanism are discussed within the framework of autocatalysis. Computer simulations reveal that the sigmoidal time dependence is transferred to an exponential-like curve by substantially increasing the rate constant for the noncatalytic step. The reaction pathway into J-aggregates can be switched from dimeric ion pairs as the reactant to monomeric ion pairs, when the rate constant for the catalytic step via the monomer becomes larger with respect to that via the dimer.

Excited State and Charge Photogeneration Dynamics in Conjugated Polymers

Conjugated polymers are becoming interesting materials for a range of optoelectronic applications. However, their often complex electronic and structural properties prevent establishment of straightforward property-function relationships. In this paper, we summarize recent results on the photophysics and excited state dynamics of conjugated polymers, in order to paint a picture of exciton formation, quenching, and generation of charge carriers.

Three-pulse photon echo of an excitonic dimer modeled via Redfield theory

In this article the third-order response of an excitonically coupled dimer is studied. The three-pulse photon echo signals were calculated by extracting polarization components from the total polarization in the corresponding phase-matched directions. The total nonlinear response was obtained by numeric propagation of the density matrix, with the exciton-vibrational coupling modeled via Redfield relaxation theory. The full two-dimensional three-pulse photon echo signals and the peak shift were analyzed in terms of the density-matrix dynamics of coherence dephasing and population relaxation. The location of the two-exciton state was found to be essential for proper modeling of the three-pulse photon echo. In particular, an oscillation in the three-pulse photon echo peak shift is found if the two-exciton state is displaced. The oscillations can be related to the dynamics of the one-exciton coherences.

Mesoscopic organization of two-dimensional J-aggregates of thiacyanine in Langmuir-Schaefer films

When dioctadecyl dimethylammonium bromide (DODAB) is compressed on a subphase containing 3,3'-disulfopropyl-5,5'-dichlorothiacyanine (THIAMS), adsorption of the dye to the DODAB monolayer results in the formation of J-aggregates which spontaneously organize into polygonal domains of micron size. The features of the domains depend on the surface pressure. The fluorescence of the individual domains is polarized. The shapes of the domains determined by fluorescence microscopy and atomic force microscopy (AFM) are identical. The domains differ however significantly from those observed after injection of a 3,3'-disulfopropyl-5,5'-dichloro-9-ethylthiacarbocyanine (THIATS) or THIAMS solution below a precompressed DODAB film, as well as from the domains observed upon compression of a DODAB monolayer on a subphase containing 10(-6) M THIATS.

Effects of NaCl on the J-aggregation of two thiacarbocyanine dyes in aqueous solutions

The effects of NaCl on the aggregation of two typical thiacarbocyanine dyes (3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-phenyl-thiacarbocyanine triethyl ammonium salt (Dye 1) and 3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-methyl-thiacarbocyanine triethyl ammonium salt (Dye 2)) in aqueous solution have been studied by using absorption spectroscopy, fluorescence spectroscopy, and 1H- and 23Na-NMR measurements. It is found that the J-aggregation of two dyes can be promoted by the addition of NaCl and that the effective coherence length of the J-aggregate is shorter than that obtained without NaCl. Fluorescence spectra demonstrate that the fluorescence intensities of the J-aggregates of two dyes are quenched by addition of NaCl. This is consistent with the decrease of the effective coherence length of J-aggregates of the two dyes in the presence of NaCl. 1H-NMR spectra of two dyes show that the Na(+) ions penetrate into the J-aggregates and replace the counterion (triethylammonium ions) in two dyes. The measurements of the chemical shifts of 23Na nuclei provide further information about the interaction between the Na(+) ions and dye anions in the J-aggregates of the two dyes. Due to this interaction, the electrostatic repulsion between the dye anions in the J-aggregates can be reduced and thus accelerate the aggregation of the two dyes in the presence of NaCl. The apparent association constants between Na(+) ions and dye molecules in J-aggregates of Dye 1 and Dye 2 estimated from the measured chemical shifts of 23Na nuclei are about 2.38 M(-1) and 1.35 M(-1), respectively.

Comparison between J-aggregates in a self-assembled multilayer and polymer-bound J-aggregates in solution: a steady-state and time-resolved spectroscopic study

The self-assembly of a thiacarbocyanine dye on a charged substrate via the layer-by-layer alternate adsorption technique was studied in detail. The influence of the chemical structure of the oppositely charged polyelectrolyte (PEL) in the multilayer on J-aggregate formation was investigated by means of absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. J-Aggregates were formed yielding a bathochromic narrow absorption band. Fine tuning of the position of the absorption band is possible by incorporating other polyelectrolytes in the multilayer. The results obtained for the dye-polyelectrolyte multilayers were compared with measurements made on the aqueous dye-polyelectrolyte mixtures. The parameters obtained by fitting the fluorescence decays to several different models allowed us to suggest a mechanism for the non-radiative decay channel. The low fluorescence quantum yields and fast non-exponential fluorescence decays obtained for both the dye-polyelectrolyte multilayers and the aqueous dye polyelectrolyte mixtures were attributed to the trapping of a fast migrating exciton by a non-luminescent trap. The results related to J-aggregate formation, decay time and dimensionality of the aggregates were compared in solution and in a self-assembled multilayer.