Engin U. Akkaya

A Panchromatic Boradiazaindacene (BODIPY) Sensitizer for Dye-Sensitized Solar Cells

A novel distyryl-substituted boradiazaindacene (BODIPY) dye displays interesting properties as a sensitizer in DSSC systems, opening the way to further exploration of structure-efficiency correlation within this class of dyes.

Selective manipulation of ICT and PET Processes in styryl-Bodipy derivatives: applications in molecular logic and fluorescence sensing of metal ions.

Remarkably versatile chemistry of Bodipy dyes allows the design and straightforward synthesis of multivalent-multitopic derivatives, which, with judicious selection of metal ion-ligand pairs based on known affinities, affords control and manipulation of photoinduced electron transfer and internal charge transfer processes as desired. We have demonstrated that metal ions acting as modulators (or inputs, in digital design parlance) can generate absorbance changes in accordance with the operation of a half-adder. In addition, an AND logic gate in the emission mode was delivered using a different binucleating arrangement of ligands. A molecular equivalent of a three-input AND logic gate was also obtained exploiting differential binding affinities of metal ions for different ligands. The results suggest that different metal ions can be used as nonannihilating inputs, selectively targeting various ligands incorporated within a single fluorophore, and with careful design, diverse photophysical processes can be selectively modulated, resulting in a range of signals, useful in molecular logic design, and offering an enticing potential for multianalyte chemosensors.

Thinking outside the silicon box: molecular and logic as an additional layer of selectivity in singlet oxygen generation for photodynamic therapy.

A simple derivative of a well-known dye bodipy appears to be a satisfactory sensitizer for singlet oxygen. Moreover, the rate of singlet oxygen generation can be modulated by two cancer-related cellular parameters, sodium ion concentration and acidity. Singlet oxygen generation rate is maximal when sodium ions and an organic acid were added. The operation of this molecular automaton follows AND logic, which introduces an additional layer of selectivity in the photodynamic action of the reagent. It should also be noted that in this system sensing, computing and actuating functions are realized within a single molecule.

A monostyryl-boradiazaindacene (BODIPY) derivative as colorimetric and fluorescent probe for cyanide ions.

We developed a novel boradiazaindacene derivative to detect cyanide ions in solution at micromolar concentrations. This structurally simple chemosensor displays a large decrease in emission intensity and a reversible color change from red to blue on contact with cyanide ions. Highly fluorescent polymeric films can be obtained by doping with the chemosensor. Such polymeric materials can be used for the sensing of the cyanide ions in polymer matrices.

A sensitive and selective ratiometric near IR fluorescent probe for zinc ions based on the distyryl-bodipy fluorophore.

A novel distyryl-substituted boradiazaindacene (bodipy) dye with an emission peak moving hypsochromically from 730 to 680 nm on Zn(II) ion binding seems to be promising as one of the very few water-soluble fluorescent chemosensors emitting in the near IR region.

Design strategies for ratiometric chemosensors: modulation of excitation energy transfer at the energy donor site.

Excitation energy transfer, when coupled to an ion-modulated ICT chromophore, creates novel opportunities in sensing. The direction of energy transfer and the point of ICT modulation can be varied as desired. In our previous work, we have shown that energy transfer efficiency between two energetically coupled fluorophores will be altered by the metal ion binding to the ICT chromophore carrying a ligand. There are two beneficial results: increased pseudo-Stokes shift and expanded dynamic range. Here, we explored the consequences of the modulation of energy transfer efficiency at the energy donor site, in a molecular design which has an ICT type metal ion-sensitive chromophore placed as the energy donor in the dyad. Clear advantages emerge compared to the acceptor site modulation: unaltered emission wavelength in the red end of the visible spectrum, while keeping a large Stokes shift and the ratiometric character.

Designing excited states: theory-guided access to efficient photosensitizers for photodynamic action.

Photodynamic therapy (PDT) is a treatment modality for certain malignant (skin, head and neck, gastrointestinal, gynecological cancers), premalignant (actinic keratosis), and nonmalignant (psoriasis) indications. Broader acceptance by the medical community and applicability is hampered, at least in part, by the less than optimal photophysical characteristics of the porphyrin derivatives. This situation sparked a worldwide search for novel sensitizers leading to new compounds, some holding more promise than others. The primary cytotoxic agent involved in the photodynamic action is singlet oxygen (Dg), the efficient generation of which is linked invariably to the intersystem crossing (ISC) efficiency of the excited sensitizer. Most organic dyes have low triplet quantum yields, and in many recent candidates for photodynamic sensitizers, heavy atoms are incorporated into the structure as a strategy to improve spin–orbit coupling leading to facilitated intersystem crossing. While this approach seems fail-safe, incorporation of heavy atoms such as bromine, iodine, selenium, and certain lanthanides very often leads to increased “dark toxicity”. Unlike traditional chemotherapy agents, in principle, photodynamic therapy sensitizers themselves can be nontoxic, either at cellular or organ levels, even at relatively high concentrations. We have been interested in trying to find alternative ways of achieving increased intersystem crossing without the use of heavy atoms to minimize dark toxicity, turning our attention to the excitedstate properties of the sensitizers. Designing efficient photoinduced O2 generators requires that any existing operative fluorescence cycle of the fluorophore, which is through the S0!S1!S0 states, has to be perturbed so as to minimize or shut down the S1!S0 deactivation, and switch to the triplet surface once S1 is accessed. A general design principle for a favorable S1!T1 transition from an electronic structure viewpoint would in principle require the structural and electronic compatibility of the S1 and T1 states to surpass that of the S1–S0 pair. Once multiple electronic states come into play, quantum mechanical calculations providing a detailed understanding of the electronic structure are extremely helpful. Multi-configurational self-consistent field (MCSCF) techniques are the stateof-the-art computational chemistry approaches, when near degeneracies and excited states are considered. These methods may not reach chemical accuracy ( 2–3 kcalmol ) for computing total energies, but they are crucial for a qualitatively correct description of the excited states and are capable of providing a conceptually complete picture of the photophysics taking place. Therefore, we mainly employed a popular variant of MCSCF techniques; the complete active space SCF (CASSCF) method in combination with relatively large basis sets and different active spaces. Details of CASSCF calculations are provided in the Supporting Information. Our calculations on the parent Bodipy (4,4-difluoro-4-bora-3a,4adiaza-s-indacene, Scheme 1) showed that natural orbital occupancies of the S1 state describe an open-shell singlet with essentially double (> 1.9) or zero (< 0.1) electrons for all orbitals except the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) that are singly occupied (see the Supporting Information, Figure S1). It is no surprise to observe a fluorophore with low triplet quantum yield to have an excited state that possesses only two orbitals with single occupancy. Hence, to achieve our goal of efficient switching to the triplet manifold, we have to access excited states that differ from the ones that arise from simple HOMO!LUMO transitions. Among multiply excited configurations, doubly substituted ones are particularly important in enhancing S1–T1 coupling as shown by the seminal work of Salem and Rowland and the following work by Michl. Thus, the substitutions should invoke a simultaneous two-electron excitation from the Scheme 1. Structure and numbering of the parent Bodipy compound. [*] Y. Cakmak, S. Kolemen, B. Kilic, Prof. Dr. E. U. Akkaya UNAM-Institute of Materials Science and Nanotechnology Bilkent University, Ankara, 06800 (Turkey) E-mail: eua@fen.bilkent.edu.tr

Tetrastyryl-Bodipy Dyes: Convenient Synthesis and Characterization of Elusive Near IR Fluorophores

1,3,5,7-Tetramethyl-Bodipy derivatives undergo Knoevenagel-type condensations with aromatic aldehydes to ultimately yield tetrastyryl-Bodipy derivatives. The resulting dyes absorb and emit strongly in the near IR. As the versatility of the Bodipy dyes are fully appreciated, these new tetrastyryl dyes are likely to be featured in a variety of functional supramolecular systems.

Towards Unimolecular Luminescent Solar Concentrators: Bodipy‐Based Dendritic Energy‐Transfer Cascade with Panchromatic Absorption and Monochromatized Emission

Today, efficient and effective utilization of solar energy is a high-priority target and is expected to be even more so in the near future. For the large-scale exploitation of the stellar energy source, cost is always the major prohibitive item. The use of polycrystalline silicon, amorphous thin films of silicon, or alternative semiconducting materials such as Cu(In,Ga)Se2 (CIGS), [4] together with dye-sensitized solar cells already have or are expected to have big impacts on the production costs, but more effort in all aspects of the solar energy transduction is needed. One approach is to break down this massive problem into relatively easily addressable components, such as absorption of solar photons and conversion of absorbed solar energy into electricity. Installation and transmission of the produced electrical energy are two other components, which are essentially engineering problems. For the efficient absorption of the solar radiation component, it has been known for some time that even without major changes in solar cell design, it should be possible to obtain substantial enhancements by making use of solar concentrators. Optical solar concentrators have been around for the last four or five decades, however, overheating is always a troublesome issue, with an additional need for solar tracking with most optical concentrators. Luminescent solar concentrators on the other hand seem to be more promising. Conversion of the incident solar radiation into monochromatized light is expected to lead to a large enhancement in the efficiency of solar cells. Key features of the luminescent solar concentrators are the dispersed dye or dyes in a transparent waveguide. Through total internal reflection, reemitted light is trapped within a plastic or glass matrix, and photovoltaic units are fixed to the sides through which the light is channelled out. The advantages are striking: no tracking or cooling is needed and much smaller areas have to be covered by expensive solar-cell components. However, such concentrators are not free from problems; self absorption of the emitted light is a major problem. Recently a different luminescent concentrator design that made use of a mixture of dyes in amorphous thin films placed in a tandem design with one terminal absorber was reported. The other two dyes absorb light at different wavelengths and are expected to transfer the excitation energy to the terminal absorber. The intermolecular Fçrster energy transfer (FRET) was invoked as the operational mechanism of the energy transfer. With the assumption of efficient intermolecular energy transfer in the solid (gel) phase, the only emission will be at the longer wavelength region with large pseudo-Stokes shifts, thus minimizing self-absorption. The intermolecular energy-transfer efficiency is an important limiting factor that requires high concentrations of the dyes for optimal results, but higher concentrations will lead to larger losses caused by self-absorption. Herein, we propose that this apparent dilemma can be addressed at least in principle, by replacing a cocktail of dyes with a dendritic lightharvesting energy gradient with a core molecule as the terminal absorber and emitter. In the dendritic system, energy-transfer efficiency will remain high, regardless of its concentration within the matrix. Unimolecular energy gradients have been reported previously with a number of peripheral antenna molecules and a core chromophore absorbing at a longer wavelength. Typically, they are characterized in solution. In this work however, we explicitly targeted an energy cascade system SC composed of bodipy dyes (see below) with varying degrees of substitution with styryl groups. This approach will ensure strong absorption in most parts of the visible spectrum, however, through efficient energy-transfer processes, emission is expected to originate only from the terminal absorber. An optimal solar cell placed on the sides of the matrix is expected to produce efficient and cost-effective conversion. In addition, we wanted to demonstrate the efficiency of every single step of cascading energy transfers; to that end we synthesized energy-transfer modules of ET-1, ET-2, and ET-3. Bodipy dyes are highly versatile chromophores and can be conveniently derivatized to span the entire visible spectrum and beyond, showing exceptional photochemical and photophysical qualities. These properties of Bodipy dyes, including sharp absorption and emission maxima, were previously exploited in energy-transfer modules. In our design, the goals were to optimize the absorption in a large part of the visible spectrum and also the conversion to emission centered at 672 nm, which is ideally suited for [*] Dr. O. Altan Bozdemir, S. Erbas-Cakmak, O. O. Ekiz, Dr. A. Dana, Prof. Dr. E. U. Akkaya UNAM-Institute of Materials Science and Nanotechnology Bilkent University, Ankara 06800 (Turkey) E-mail: eua@fen.bilkent.edu.tr

Solid-State Dye-Sensitized Solar Cells Using Red and Near-IR Absorbing Bodipy Sensitizers

Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups for attachment to nanocrystalline TiO(2), these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state electrolyte regime offers additional opportunities for practical applications.