Axel Kahnt

Synthesis and Photoinduced Electron‐Transfer Properties of Phthalocyanine–[60]Fullerene Conjugates

A series of three novel ZnPc-C60 conjugates (Pc=phthalocyanine) 1 a-c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc-C60 conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy- towards the preparation of 1 a-c- involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C60. Detailed photophysical investigations of 1 a-c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C60. In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc(*+) radical cation and the C60 (*-) radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1 a).

Molecular Implementation of Sequential and Reversible Logic Through Photochromic Energy Transfer Switching

Photochromic spiropyrans modified with fluorophores were investigated as molecular platforms for the achievement of fluorescence switching through modulation of energy transfer. The dyads were designed in such a way that energy transfer is only observed for the open forms of the photochrome (merocyanine and protonated merocyanine), whereas the closed spiropyran is inactive as an energy acceptor. This was made possible through a deliberate choice of fluorophores (4-amino-1,8-naphthalimide, dansyl, and perylene) that produce zero spectral overlap with the spiro form and considerable overlap for the merocyanine forms. From the Förster theory, energy transfer is predicted to be highly efficient and in some cases of 100% efficiency. The combined switching by photonic (light of λ>530 nm) and chemical (base) inputs enabled the creation of a sequential logic device, which is the basic element of a keypad lock. Furthermore, in combination with an anthracene-based acidochromic fluorescence switch, a reversible logic device was designed. This enables the unambiguous coding of different input combinations through multicolour fluorescence signalling. All devices can be conveniently reset to their initial states and repeatedly cycled.

[2.2]Paracyclophane: a pseudoconjugated spacer for long-lived electron transfer in phthalocyanine–C60 dyads

Charge transfer features in two different ZnPc–C60 donor–acceptor conjugates were contrasted to highlight the benefits of integrating a [2.2]paracyclophane molecular building block instead of just a phenylene vinylene analogue.

Carbon Nanohorns as Integrative Materials for Efficient Dye‐Sensitized Solar Cells

Different nanocarbons, that is, single-wall carbon nanotubes, graphene, single-wall carbon nanohorns (SWCNHs), and their respective oxidized analogs have been used to fabricate novel doped TiO2 electrodes for DSSCs. Our results indicate that all of the nanocarbons significantly enhance the device characteristics when compared to standard TiO2 electrodes. Overall, our most outstanding finding is that SWCNH derivatives are also a plausible material for developing highly-efficient DSSCs.

Temperature Dependence of Charge Separation and Recombination in Porphyrin Oligomer-Fullerene Donor-Acceptor Systems

Electron-transfer reactions are fundamental to many practical devices, but because of their complexity, it is often very difficult to interpret measurements done on the complete device. Therefore, studies of model systems are crucial. Here the rates of charge separation and recombination in donor–acceptor systems consisting of a series of butadiyne-linked porphyrin oligomers (n = 1–4, 6) appended to C60 were investigated. At room temperature, excitation of the porphyrin oligomer led to fast (5–25 ps) electron transfer to C60 followed by slower (200–650 ps) recombination. The temperature dependence of the charge-separation reaction revealed a complex process for the longer oligomers, in which a combination of (i) direct charge separation and (ii) migration of excitation energy along the oligomer followed by charge separation explained the observed fluorescence decay kinetics. The energy migration is controlled by the temperature-dependent conformational dynamics of the longer oligomers and thereby limits the quantum yield for charge separation. Charge recombination was also studied as a function of temperature through measurements of femtosecond transient absorption. The temperature dependence of the electron-transfer reactions could be successfully modeled using the Marcus equation through optimization of the electronic coupling (V) and the reorganization energy (λ). For the charge-separation rate, all of the donor–acceptor systems could be successfully described by a common electronic coupling, supporting a model in which energy migration is followed by charge separation. In this respect, the C60-appended porphyrin oligomers are suitable model systems for practical charge-separation devices such as bulk-heterojunction solar cells, where conformational disorder strongly influences the electron-transfer reactions and performance of the device.

A corrole–azafullerene dyad: synthesis, characterization, electronic interactions and photoinduced charge separation

The preparation and characterization of the first corrole-azafullerene dyad are described. The photophysical and electrochemical properties of the new corrole-C59N dyad were examined and it was found that photoexcitation of the corrole unit leads to the formation of a charge separated state.

A Bis(C60)–Bis(phthalocyanine) Nanoconjugate: Synthesis and Photoinduced Charge Transfer

A novel dimeric phthalocyanine-C60 nanoconjugate, consisting of a bisphthalocyanine core to which two fullerenes are attached, has been prepared. The synthetic strategy implemented the preparation of a diformyl butadiynyl-bridged bisphthalocynaninato-zinc(II) complex by means of palladium-catalyzed cross-coupling reactions and subsequent dipolar cycloaddition reactions. Photophysical experiments confirm that an intramolecular electron transfer, namely, from the photoexcited ZnPc moiety to the electron-accepting C60 unit, governs the overall photoreactivity of the nanoconjugate. Through-space charge-transfer interactions facilitated by the close proximity of the ZnPc and the C60 moieties play a decisive role in determining the lifetimes of the charge-separated state which range from 10(-10) to 10(-9) seconds.

Excited state properties of anodic TiO2 nanotubes

Charge carriers, that is, holes as well as trapped and “free” electrons were investigated by means of time resolved spectroscopy in anodic TiO2 nanotubes that were heat treated at different temperatures. The lifetimes of the charge carrier were compared with those generated in reference layers of Solaronix Ti-Nanoxide D and Degussa P25 nanoparticles. Remarkably long lived “free” electrons were only noted in the TiO2 nanotubes. These findings have significance in view of any photoelectrochemical applications of TiO2 nanotubes.

Controlling charge transfer in fullerene/phthalocyanine electron donor-acceptor conjugates/hybrids

Fullerenes and phthalocyanines are ideally suited for devising integrated, multi-component model systems to transmit and process solar energy. Implementation of C60 as a three-dimensional electron acceptor bears great promises on account of its small reorganization energy in electron transfer reactions and has exerted a noteworthy impact on the improvement of light-induced charge separation. This mini-review describes how the specific composition of phthalocyanines chromophores associated with C60 – yielding artificial light-harvesting antenna and reaction center mimics – have been elegantly utilized to tune the electronic couplings between donor and acceptor sites. Specifically, the effects that these parameters have on the rate, yield and lifetime of the energetic charge-separated states are considered.

Implementing a tripodal relay station in a phthalocyanine–[60]fullerene conjugate

A tripodal architecture 1—based on a tetraphenylmethane core bearing three phthalocyanine arms and a fullerene arm—has been realized en route towards a novel electron donor–acceptor conjugate that reveals long range electron transfer activity.