Jasper  Knoester

Siegfried  Daehne

Prospects of Artificial Light Harvesting Systems: An Introduction

Palladium and silver nanoparticles are formed on the surface of tubular J-aggregates of an amphiphilic 
	tetrachlorobenzimidacarbocyanine dye by reduction of the respective metal cations in aqueous solution. 
	Upon addition of the palladium complex Na2PdCl4 to the aggregate solution, the absorption spectrum shows significant changes which is 
 explained by partial destruction of the aggregates. Cryogenic transmission electron 
 microscopy (cryo-TEM) images show that the tubular J-aggregates are randomly covered by 
 well-separated Pd nanoparticles of approximately 1&#8211;3 nm size. Larger particles and higher particle density along the aggregates are obtained when an auxiliary reducing agent is added to the solution. The presence of 
 the metallic particles leads to efficient fluorescence quenching giving clear evidence for super 
 quenching. In similar experiments using AgNO3,
 silver nanoparticles are grown which are larger in size but less dense distributed along the 
 aggregates. At least in the case of the silver particles, the spontaneous formation of metal 
 nanoparticles is assumed to be initiated by a photo-induced electron transfer process (PET).

Stefan  Kirstein

Hans  von Berlepsch

Christoph  B&#246;ttcher

Photo-induced reduction of Noble metal ions to metal nanoparticles on tubular J-aggregates

The simultaneous chemical linkage of cyanine dye chromophores with both hydrophobic and hydrophilic substituents leads to a new type of  amphiphilic molecules with the ability of spontaneous self-organization into highly ordered aggregates of various structures and morphologies. These aggregates carry the outstanding optical properties of J-aggregates, namely, efficient exciton coupling and fast exciton energy migration, which are essential for the build up of artificial light harvesting systems. The morphology of the aggregates depends sensitively on the molecular structure of the chemical substituents of the dye chromophore. Accordingly, lamellar ribbon-like structures, vesicles , tubes, and bundles of tubes are found depending on the dyes and the structure can further be altered by addition of surfactants, alcohols, or other additives. Altogether the tubular structure is the most noticeable structural motif of these types of J-aggregates. The optical spectra are characterized in general by a complex exciton spectrum which is composed of several electronic transitions. The spectrum is red-shifted as a total with respect to the monomer absorption and exhibits resonance fluorescence from the lowest energy transition. For the tubular structures, the optical spectra can be related to a structural model. Although the molecules itself are strictly achiral, a pronounced circular dichroism (CD) is observed for the tubular aggregates and explained by unequal distribution of left- and right-handed helicity of the tubes. Photo-induced electron transfer (PET) reactions from the dye aggregates to electron acceptor molecules lead to superquenching which proves the delocalization of the excitation. This property is used to synthesize metal nanoparticles on the aggregate surface by photo-induced reduction of metal ions.

Stefan Kirstein

Siegfried Daehne

J-aggregates of amphiphilic cyanine dyes: Self-organization of artificial light harvesting complexes

The theory of the optical properties of linear and tubular molecular J-aggregates is reviewed. The primary optical excitations in these systems are Frenkel excitons, which may be delocalized over many molecules. The collective nature of these
excitations gives rise to special optical properties and dynamics, which are of interest for purely scientific reasons, but also enable the application of J-aggregates as photographic sensitizers and artificial light-harvesting systems. The focus of this paper
is on the effect of aggregate geometry, disorder, and temperature on the absorption and fluorescence spectra. Also transport of excitations between J-aggregates is discussed. Connection is made to experiments on aggregates of cyanine dyes and natural
light-harvesting systems.

Jasper Knoester

Modeling the optical properties of excitons in linear and tubular J-aggregates

The mobility of optically excited excitons on J-aggregates can be demonstrated by the phenomena of  exciton-exciton annihilation. In this intensity-dependent process the collision of two excitons results in their annihilation and hence in a shortening of the mean excitation lifetime. By measuring the intensity-dependent fluorescent lifetime in contrast to the predicted immobilization of the excitons at low temperature we could prove the excellent mobility of the excitons at a temperature (4K), which is far below their expected freezing point.

C. Spitz

S. Daehne

Low temperature exciton-exciton annihilation in amphi-PIPE J-aggregates

Interaction of oxacarbocyanines D-G with three polycations in aqueous solutions results in the formation of two types of likely small, distorted aggregates rather than the classical J aggregates. On the contrary, the latter are extensively and almost exclusively obtained in electrostatically self-assembled multilayers (ESAMs) prepared by alternate polycation/dye adsorption on quartz substrates. The J-aggregate growth on supported polycations is qualitatively shared by the four cyanines, a fact that reveals the crucial role of the double anionic substitutions on the dyes. On the other hand, films with D and E, which are known to have a stronger tendency to give dimers in water, exhibit higher J-band intensities and stability upon drying relative to those with F and G. Based on these observations, we suggest that energetic factors associated with cofacial dye/dye van der Waals interactions, ultimately related with the degree of planarity of the conjugated chromophores, may still play a major role in controlling aggregation equilibria in these complex systems.

Andrea Lodi

Glauco Ponterini

Aggregation modes of anionic oxacarbocyanines with polycations in solution and in ESAMs

Zinc bacteriochlorophyll-<mml:math id="E1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>d</mml:mi></mml:math> derivatives possessing a polymerizable moiety at the 
 17-propionate were prepared as model compounds of natural occurring chlorophylls 
 in the main peripheral antennas of green photosynthetic bacteria (chlorosomes). 
 The synthetic compounds self-aggregated in nonpolar organic solvents as well as 
 in the solid state to give large oligomers similar to chlorosomal J-aggregates. 
 Such introduction of the polymerizable groups in the ester did not suppress the ability of 
 self-aggregation.

Hitoshi Tamiaki

Kazuya Nishihara

Reiko Shibata

Synthesis of self-aggregative zinc chlorophylls possessing polymerizable esters as a atable model compound for main light-harvesting antennas of green photosynthetic bacteria

The static and time-resolved optical response of <mml:math id="E1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>5</mml:mn><mml:mo>,</mml:mo><mml:msup><mml:mn>5</mml:mn><mml:mo>&#x2032;</mml:mo></mml:msup><mml:mo>,</mml:mo><mml:mn>6</mml:mn><mml:mo>,</mml:mo><mml:msup><mml:mn>6</mml:mn><mml:mo>&#x2032;</mml:mo></mml:msup></mml:mrow></mml:math>-tetrachlorobenzimidacarbocyanine chromophore-based tubular double-wall molecular aggregates is studied. The linear dichroism and absorption spectra have the same basic structure for all aggregates investigated essentially showing a set of narrow and highly polarized bands originating from inner and outer wall excitations. The exact positions and strengths of absorption bands are influenced by the choice of side groups and various additives. Time-resolved fluorescence measurements show a substantial speeding of the fluorescence decay upon aggregation, mainly resulting from collective effects. Pump-probe experiments evidence slow inner-to-outer and fast outer-to-inner wall energy transfers between the walls of the tubules.

A. Pugzlys

P. R. Hania

R. Augulis

K. Duppen

P. H. M. van Loosdrecht

Cylindrical aggregates of  5,5',6,6'-tetrachlorobenzimida-carbocyanine amphiphilic derivatives: Structure-related optical properties and exciton dynamics

The dye sensitization in photography is achieved by thin layers of
J-aggregated dyes on silver halide grains. Studies by Yoshihara et al. with picosecond
and femtosecond spectroscopies have indicated that the migration of excitons in the dye
layers plays important roles in their charge separation to inject electrons
into the grains. Studies by Seki et al. with ultraviolet photoelectron spectroscopy have
revealed that the electronic structure of the interface between dye layers and a
substrate is characterized by the fact that the vacuum level of the
former is lowered with respect to that of the latter by the electric double layer formed at the interface, and that this characteristic structure is generally valid for the interfaces between organic layers and substrates. Hanna et al. have confirmed the formation of such an electronic double layer at the interfaces between a
liquid crystalline organic semiconductor and electrodes by analyzing the current voltage characteristics.

Tadaaki  Tani

Kazuhiko  Seki

Keitaro  Yoshihara

Junichi  Hanna

Thin organic layers for photography and electronic devices

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Chief Editor, Giulia Grancini, is based at the University of Pavia, Italy. Her current research work aims at solving the stability and toxicity issues of developing multi-dimensional hybrid interfaces as lego-bricks for a new efficient, stable, and environmentally-friendly solar technology.

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