1. Functional dyes and their applications
Perylene dyes are key chromophores in dyestuff chemistry. The solubility, absorption, and emission behavior of this class of materials can be efficiently controlled by functionalization using a variety of different synthetic procedures. Moreover, these chemical modifications have allowed their widespread application as functional dyes. New NIR-absorbers have been synthesized by introducing additional naphthalene units to the perylene scaffold to form the higher homologues of the series of rylenediimides. Such dyes can be used for laser welding and laser marking of polymers. Highly fluorescent nanoparticles containing multiple chromophores have been prepared by decorating polyphenylene dendrimers with rylenes on the surface. Depending on the chromophoric interactions, these types of materials can serve as single photon sources. When in addition to the surface, the interior and the dendritic scaffold are equipped with donor and acceptor dyes, synthetic light-harvesting complexes are obtained, which collect light from the whole visible spectrum. Due to the outstanding chemical and photochemical stabilities as well as their high fluorescence quantum yields, the perylene- and terrylenediimides have been established as key chromophores for single molecule spectroscopy allowing photophysical processes, e.g. energy and electron transfer to be studied on the single molecule level over extended periods of time. With water soluble rylene dyes single molecule measurements regarding biological problems can now be addressed as well. The same set of favorable properties also makes the rylene dyes excellent candidates for applications in organic electronic devices, for example, donors and acceptors functionalized perylene can be used as sensitizers for dye-sensitized solar cells.
a) Higher rylenes and new NIR absorbing dyes
By extending the aromatic π-system also the absorption coefficients increase from ε = 60,000 M-1cm-1 (perylenediimide/PDI), ε = 93,000 M-1cm-1 (terrylenediimide/TDI), ε = 167,000 M-1cm-1 (quaterrylenediimide/QDI) over ε = 235,000 M-1cm-1 (pentarylenediimide/5DI) to ε = 293,000 M-1cm-1 (hexarylenediimide/HDI). For organic NIR-dyes such high absorption coefficients are unprecedented. In addition to these remarkable photophysical properties, all the presented NIR-absorbers still display the excellent chemical, photochemical and thermal stability seen for their smaller counterparts the PDIs.
b) Multichromophoric Systems Based on rylene Dyes
By multistep synthesis a dendritic triad is generated containing a central TDI as energy acceptor, four peryleneimides as energy donor within the dendritic scaffold and eight naphthaleneimides as antennae at the rim. Detailed photophysical measurements reveal that Intramolecular Forster-type excitation energy transfer (FRET) occurs from peryleneimide to terrylenediimide with an efficiency of 99.5%. On excitation of the naphthalenes, they transfer their excitation energy either directly or in a cascade-type fashion to the core, the latter case involving scaffold-substituted perylene monoimides (PMIs) as intermediate acceptors. The spatial positioning and the spectral properties of the chosen rylene chromophores make this multichromophoric system an efficient light collector, able to capture light over the whole visible spectral range and to transfer it finally to the TDI, the latter releasing it as red fluorescence. In addition to generating light-harvesting systems from exclusively synthetic units, a bioorganic hybrid combining a natural antenna system and a terryleneimide dye have been constructed. Energy transfer occurs from the chlorophyll units as donors to the terrylene acceptor, which is covalently attached close to the N-terminus of the protein scaffold.
c) Rylene dyes for single molecule spectroscopy
Severe requirements for functional dyes are nowadays defined by single molecule spectroscopy (SMS) where the fluorescence of single emitter molecules rather than of their ensemble is recorded. The rylenes have been proved to be of outstanding value for SMS. Looking at single molecules, in contrast to ensemble measurements which yield information on average properties, provides information on individuals such as distributions and time trajectories of properties that would otherwise be hidden. In this way, static disorder or dynamic disorder of individual molecules can be detected, which is not possible by averaging the observable of a bulk system. A good fluorophore for such experiments exhibits a high extinction coefficient, a high fluorescence quantum yield and good chemical and photochemical stability. Photobleaching severely limits the observation time, and thus, the amount of collected information during the experiment. Since most PDI and TDI chromophores excellently fulfill these requirements they have been established as key chromophores for SMS.
In recent years, self-organization of liquid crystalline PDIs have increasingly gained attention because of the opportunities presented by the combination of their liquid crystalline, photophysical, semiconducting, and photoconducting properties. PDIs in general have been widely used as dyes and pigments because of their brilliant orange and red colors. In addition, their high extinction coefficients and high thermal, chemical, and photochemical stabilities make them suitable as functional dyes in electronic devices. The higher homologues of PDIs, namely, TDIs and QDIs, exhibit the same attractive properties. Their absorption maximum is bathochromically shifted to 600 and 800 nm so that this series of rylene dyes covers practically the visible light and the near-infrared region.
e) Ultrastable fluorescent labels based on Water Soluble Perylene and Terrylenediimide Dyes
Fluorescence microscopy is the most widely used tool for visualizing subcellular structures and for localizing proteins within cells. Single-molecule spectroscopy has gone beyond that and has revealed information about complex biological molecules and processes, which are difficult to obtain from ensemble measurements. Thereby, one critical issue is the label. It should be water-soluble, highly fluorescent in aqueous environment, and exhibit an exceptional photostability to visualise or track biomolecules for a sufficient period of time. These criteria are very well fulfilled by water soluble perylene and terrylenediimide dyes bearing ionic groups at the phenoxylated bay-positions. However, what is missing are reactive groups to guarantee stable attachment to the biomolecule. This goal has been achieved by generating a toolbox of rylene chromophores allowing labelling of proteins or enzymes. Two strategies have been followed relying on either non-specific or site specific labelling. In the first approach perylene and terrylene chromophores were synthesized possessing N-hydroxysuccinimide ester and maleimide groups to target lysine and cysteine residues, respectively. For site specific attachment of the above mentioned dyes the luminophores were functionalized as thioesters and with nitrilotriacetic acid groups. This allows attachment of the emitters to the N-terminus of proteins by native chemical ligation or complexation with His-tagged polypeptides at the N- or C-termini, respectively. The performance of the new fluorescent probes was assessed by single molecule enzyme tracking, in this case phospholipase acting on phospholipid supported layers (Figure).
2. Dye-sensitized solar cells
Dye-sensitized solar cells (DSSCs) based on nanocrystalline semiconductors have been the subject of intense investigation owing to their potential low cost, easy processing and good performance. In these cells, a dye monolayer is adsorbed on a mesoporous film of titania. Upon light absorption, the dye injects electrons into the TiO2 conduction band, where they are transported to the anode. The neutral dye is regenerated by electron transfer either from an electrolyte containing a redox system or from a solid-state hole conductor. With a closed external circuit and under illumination, the device then constitutes a photovoltaic energy-conversion system, which is regenerative and stable. In this technology, ruthenium complexes maintained a clear lead in performance amongst the thousands of dyes tested, yielding power conversion efficiencies of 10?11%. However, in view of the cost and availability of 4d metals as well as their environmental non-compatibility, many metal-free organic dyes have been developed. Rylene derivatives have been widely applied in various optical devices owing to their outstanding chemical, thermal and photochemical stability and non-toxicity. New rylene dyes have been synthesized as sensitizers in DSSCs.