Yavuz Dede

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.

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

Intracellular Modulation of Excited-State Dynamics in a Chromophore Dyad: Differential Enhancement of Photocytotoxicity Targeting Cancer Cells†

The photosensitized generation of reactive oxygen species, and particularly of singlet oxygen [O2 (a(1) Δg )], is the essence of photodynamic action exploited in photodynamic therapy. The ability to switch singlet oxygen generation on/off would be highly valuable, especially when it is linked to a cancer-related cellular parameter. Building on recent findings related to intersystem crossing efficiency, we designed a dimeric BODIPY dye with reduced symmetry, which is ineffective as a photosensitizer unless it is activated by a reaction with intracellular glutathione (GSH). The reaction alters the properties of both the ground and excited states, consequently enabling the efficient generation of singlet oxygen. Remarkably, the designed photosensitizer can discriminate between different concentrations of GSH in normal and cancer cells and thus remains inefficient as a photosensitizer inside a normal cell while being transformed into a lethal singlet oxygen source in cancer cells. This is the first demonstration of such a difference in the intracellular activity of a photosensitizer.

A Redox Non-Innocent Ligand Controls the Life Time of a Reactive Quartet Excited State - An MCSCF Study of [Ni(H)(OH)]

The electronic structures of the low and high-spin states of the cationic complex [Ni(H)(OH)](+) that was previously found to be highly reactive toward CH(4) and O(2) were examined. Earlier computational work suggested that the low-spin doublet state D(0) of the Ni(III)-d(7) system is significantly lower in energy than its high-spin quartet analogue Q(1). Recent DFT-studies indicated, however, that Q(1) is the reactive species requiring Q(1) to have a sufficiently long lifetime for undergoing thermal reactions with the small molecule reactants under single collision conditions in the gas phase. These observations raise the question as to why Q(1) does not spontaneously undergo intersystem crossing. Our work based on DFT, coupled-cluster and MCSCF calculations suggests that the hydroxyl ligand behaves as a redox noninnocent ligand and becomes oxidized to formally afford an electronic structure that is consistent with a Ni(II)-(OH)* species. As a result, the doublet and quartet ground states are not related by a single electron spin flip and the intersystem crossing becomes inhibited, as indicated by unexpectedly small spin-orbit coupling constants. After extensive sampling of the potential energy surfaces, we concluded that there is no direct way of converting Q(1) to the ground state doublet D(0). Alternative multistep pathways for the Q(1) --> D(0) decay involving doublet excited states were also evaluated and found to be energetically not accessible under the experimental conditions.

Synthesis and dye sensitized solar cell applications of Bodipy derivatives with bis-dimethylfluorenyl amine donor groups

Three Bodipy dyes with strong absorptivities in the visible and near infrared regions were designed, synthesized and their potential as photosensitizers for liquid electrolyte-based dye sensitized solar cells have been evaluated. For the first time Bodipy derivatives with bis-dimethylfluorenyl amine donor groups which were known for their bulky structures as donor groups have been used together. We altered our mostly used triphenylamine group with these and investigated the dye-sensitized solar cell efficiencies of this new class of Bodipy dyes.

Nanostructured organic semiconductor films for molecular detection with surface-enhanced Raman spectroscopy.

π-Conjugated organic semiconductors have been explored in several optoelectronic devices, yet their use in molecular detection as surface-enhanced Raman spectroscopy (SERS)-active platforms is unknown. Herein, we demonstrate that SERS-active, superhydrophobic and ivy-like nanostructured films of a molecular semiconductor, α,ω-diperfluorohexylquaterthiophene (DFH-4T), can be easily fabricated by vapour deposition. DFH-4T films without any additional plasmonic layer exhibit unprecedented Raman signal enhancements up to 3.4 × 103 for the probe molecule methylene blue. The combination of quantum mechanical computations, comparative experiments with a fluorocarbon-free α,ω-dihexylquaterthiophene (DH-4T), and thin-film microstructural analysis demonstrates the fundamental roles of the π-conjugated core fluorocarbon substitution and the unique DFH-4T film morphology governing the SERS response. Furthermore, Raman signal enhancements up to ∼1010 and sub-zeptomole (<10-21 mole) analyte detection were accomplished by coating the DFH-4T films with a thin gold layer. Our results offer important guidance for the molecular design of SERS-active organic semiconductors and easily fabricable SERS platforms for ultrasensitive trace analysis.

Morphological Versatility in the Self-Assembly of Val-Ala and Ala-Val Dipeptides

Since the discovery of dipeptide self-assembly, diphenylalanine (Phe-Phe)-based dipeptides have been widely investigated in a variety of fields. Although various supramolecular Phe-Phe-based structures including tubes, vesicles, fibrils, sheets, necklaces, flakes, ribbons, and wires have been demonstrated by manipulating the external physical or chemical conditions applied, studies of the morphological diversity of dipeptides other than Phe-Phe are still required to understand both how these small molecules respond to external conditions such as the type of solvent and how the peptide sequence affects self-assembly and the corresponding molecular structures. In this work, we investigated the self-assembly of valine-alanine (Val-Ala) and alanine-valine (Ala-Val) dipeptides by varying the solvent medium. It was observed that Val-Ala dipeptide molecules may generate unique self-assembly-based morphologies in response to the solvent medium used. Interestingly, when Ala-Val dipeptides were utilized as a peptide source instead of Val-Ala, we observed distinct differences in the final dipeptide structures. We believe that such manipulation may not only provide us with a better understanding of the fundamentals of the dipeptide self-assembly process but also may enable us to generate novel peptide-based materials for various applications.

Luminescence of BODIPY and Dipyrrin: An MCSCF Comparison of Excited States

Low lying electronic states of the highly fluorescent BODIPY (boron dipyrromethene, 1) and its nonemissive cousin dipyrrin (2) were investigated by state-of-the-art quantum chemical methods. The opposed luminescence of 1 and 2 is explained by discovering distinct structural and energetic features for the intersection of the ground and first excited singlet state potential energy surfaces, S(0) and S(1). In accessing the intersection region, a B-N σ-bond in 1 has to be broken-an energetically prohibitive change on the nonemissive decay channel. On the contrary, 2 is deactivated via an energetically accessible S(0)/S(1) intersection point. Details of S(0), S(1), S(2), and T(1) wave functions for various regions of the potential energy surfaces were described. Unnoted features for multidimensional vectors that represent S(0) → S(1) and S(0) → T(1) transitions are reported. These correlations regarding S(0) → S(1) and S(0) → T(1) multidimensional vectors were also shown to apply to two highly fluorescent molecules: indole and coumarin.

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

Terminal cobalt(IV)–oxo (CoIV–O) species have been implicated as key intermediates in various cobalt-mediated oxidation reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)CoIV(O)]2+ (2) by irradiating [CoII(13-TMC)(CF3SO3)]+ (1) in the presence of [RuII(bpy)3]2+, Na2S2O8, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (that is, iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diatomic Co–O vibration band at 770 cm−1, which provides the conclusive evidence for the presence of a terminal Co–O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C–H bond activation and olefin epoxidation reactions. The present results lend strong credence to the intermediacy of CoIV–O species in cobalt-catalysed oxidation of organic substrates as well as in the catalytic oxidation of water that evolves molecular oxygen.

Determination of trace element levels in human scalp hair in occupationally exposed subjects by XRF

Trace element levels in hair of individuals living in urban areas were determined by energy dispersive XRF. Two groups of subjects were investigated, the first group was assumed to be from a healthy environment, the other one was exposed to a high level of contamination due to working conditions. The results were compared to data reported in the literature. The concentrations of Ca, Fe, Cu, Zn and Pb in the scalp hair were determined and the correlation between hair trace element levels and environmental effects was discussed. The results given by the second group show that environmental exposure effects hair trace element levels which are related to body trace element concentrations.