Thomas Lenzer

Synthesis and Photochemical Investigations of Tetrasubstituted Alkenes as Molecular Switches—The Effect of Substituents

Molecular switches based on helical tetrasubstituted alkenes, substituted with either electron-withdrawing (CF(3), F, CN; 2a-c, 3a,c) or -donating substituents (Me, OMe; 2d,e), have been synthesized from acyclic precursors 4 and 5 in a domino carbopalladation/Stille reaction. This palladium-catalyzed process allowed the rapid assembly of two C-C bonds, two six-membered rings, and the tetrasubstituted double bond in a completely diastereoselective fashion. The electronic effects of the substituents on the overall switching process were investigated by alternating irradiation of two different wavelength regions. Although the substituents had only a small influence on the absorption maxima, drastic differences in the switching behavior were observed.

Ultrafast excited state dynamics and spectroscopy of 13,13′-diphenyl-β-carotene

Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newly-synthesized artificial β-carotene derivative 13,13-diphenyl-β-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S(2) state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S(2)→ S(1) and S(1)→ S(0)* with results for β-carotene. Global analysis provides IC time constants and also time-dependent S(1) spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the (1)H NMR spectrum, a small red-shift of the S(0)→ S(2)(0-0) transition energy in the steady-state absorption spectrum relative to β-carotene, and almost the same S(1)→ S(0)* IC time constant as in β-carotene, suggesting a similar S(1)-S(0) energy gap. The oscillator strength of the S(0)→ S(2) transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an S* state. Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S(0)* superimposed on the bleach of room temperature molecules S(0). The S(0)* population is formed by IC from S(1). These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.

On the nature of interactions between ionic liquids and small amino-acid-based biomolecules.

During the last decade, ionic liquids (ILs) have revealed promising properties and applications in many research fields, including biotechnology and biological sciences. The focus of this contribution is to give a critical review of the phenomena observed and current knowledge of the interactions occurring on a molecular basis. As opposed to the huge advances made in understanding the properties of proteins in ILs, complementary investigations dealing with interactions between ILs and peptides or oligopeptides are underrepresented and are mostly only of phenomenological nature. However, the field has received more attention in the last few years. This Review features a meta-analysis of the available data and findings and should, therefore, provide a basis for a scientifically profound understanding of the nature and mechanisms of interactions between ILs and structured or nonstructured peptides. Fundamental aspects of the interactions between different peptides/oligopeptides and ILs are complemented by sections on the experimental (spectroscopy, structural biology) and theoretical (computational chemistry) possibilities to explain the phenomena reported so far in the literature. In effect, this should lead to the development of novel applications and support the understanding of IL-solute interactions in general.

Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent.

We investigated photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the organic solvent acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim](+)[B(CN)4](-) using transient absorption spectroscopy, spectroelectrochemistry, and DFT/TDDFT calculations. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochemistry, where the E6˙(+) radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the positive charge. After photoexcitation of E6 at 520 nm (S0 → S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is observed (time constants ca. 0.8 and 12 ps) which is related to a changing electric field generated by the E6˙(+) radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent molecules. However, pronounced differences are observed on longer timescales. In contact with solvent, one observes an increase in the E6˙(+) absorption band above 600 nm with a time constant of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron-cation recombination occurs on much longer timescales and is multiexponential, with time constants of ca. 100 μs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Additional experiments were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 molecules in S1 quickly decays to S0 (time constants 1 and 35 ps).

Four‐ and Sixfold Tandem‐Domino Reactions Leading to Dimeric Tetrasubstituted Alkenes Suitable as Molecular Switches

A highly efficient palladium-catalyzed fourfold tandem-domino reaction consisting of two carbopalladation and two C-H-activation steps was developed for the synthesis of two types of tetrasubstituted alkenes 3 and 6 with intrinsic helical chirality starting from substrates 1 and 4, respectively. A sixfold tandem-domino reaction was also developed by including a Sonogashira reaction. 20 compounds with different substitution patterns were prepared with yields of up to 97u2009%. Structure elucidation by X-ray crystallography confirmed helical chirality of the two alkene moieties. Photophysical investigations of some of the compounds showed pronounced switching properties through light-controlled changes of their stereochemical configuration.

Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin films

We present an ultrafast UV-Vis-NIR transient absorption study of the donor-acceptor solar-cell dye D49 in diisopropyl ether, THF and acetonitrile, as well as on mesoporous Al2O3 and TiO2 thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramolecular charge transfer character (S1/ICT). On Al2O3 and in the three organic solvents, the dynamics are fully explained in terms of S1/ICT stabilisation (by reorientation of adjacent solvent or D49 molecules and collisional cooling), intramolecular vibrational redistribution and S1/ICT → S0 electronic decay. A substantial decrease of the S1/ICT lifetime is observed with increasing polarity of the surrounding medium suggesting an acceleration of internal conversion. In agreement with these results, the addition of the nonpolar co-adsorbent deoxycholic acid (DCA) to the Al2O3 surface leads to a substantial increase of the S1/ICT lifetime. DCA spacers reduce the local polarity around the dye molecules, thus interrupting D49 self-solvation. These results are in contrast to a recent experimental study for the indoline dye D131 on Al2O3, where charge transfer from electronically excited D131 to adjacent dye molecules was proposed (Cappel et al., Sci. Rep., 2016, 6, 21276). We do not see evidence for charge transfer processes between D49 molecules and also not for electron injection from D49 into Al2O3 trap states. Charge separation is only observed for D49 bound to TiO2 thin films, with efficient injection of electrons into the conduction band of the semiconductor via formation of a [D49˙+e-] complex and a transient Stark effect signalling the formation of mobile electrons upon dissociation of the complex.

Probing the local polarity of alkylammonium formate ionic liquids and their mixtures with water by using a carbonyl carotenoid.

Ultrafast transient absorption experiments have been carried out to determine the local polarity of three alkylammonium formate (AAF) protic ionic liquids (PILs), methlyammonium formate (MAF), ethylammonium formate (EAF), and n-butylammonium formate (BAF), by using 12-apo-β-carotenoic-12-acid (12CA) as a molecular probe. MAF is more polar than methanol; EAF and BAF have polarities similar to ethanol and n-butanol, respectively. In general, the AAF PILs follow rather closely the correlation between the S(1) /intramolecular charge-transfer (ICT) state lifetime and the polarity parameter Δf, which was previously established in organic solvents. This is in contrast to earlier results for the 12CA probe in imidazolium-based ILs, in which the local polarity determined by the probe was much larger than that for dipolar organic solvents with the same dielectric constant. The systematic variation of the composition of EAF/water mixtures shows no indication of significant deviations from the local to the bulk composition. We also characterized the photophysical properties of the deprotonated form of the 12CA probe. It exhibits a structured S(0)→S(2) absorption spectrum, which is blueshifted relative to neutral 12CA. The lifetime of the S(1)/ICT state of the anion is about 170 ps, and therefore, similar to the lifetime of 12CA in less polar solvents. The transient S(1) /ICT spectrum in methanol closely resembles that of nonpolar carotenes. Both observations suggest that the ICT character of the S(1) state of the anion is largely suppressed because the shift of electron density toward the negatively charged carboxylate group is not favorable.

Ultrafast photoinduced dynamics of the 3,6-diaminoacridinium derivative ATTO 465 in solution†

The excited state dynamics of the dye ATTO 465, a well-known fluorescence marker for biological applications, have been characterized in various solvents including THF, ethanol, methanol, water and the highly polar protic ionic liquid 2-hydroxyethylammonium formate (2-OH-EAF) by combining results from time-correlated single-photon counting (TCSPC) and ultrafast pump-supercontinuum probe (PSCP) spectroscopy as well as steady-state absorption and fluorescence. In water, 2-OH-EAF and two fluorinated alcohols, there is a pronounced blue-shift and broadening of the S(0) → S(1) absorption band and also a larger Stokes shift than in the other solvents, indicating a particular influence of hydrogen-bonding interactions. S(1) lifetimes from TCSPC at 25 °C range from 3.3 ns to 5.6 ns. An unusual increase in the S(1) lifetime with temperature is observed for ethanol and methanol, however water behaves in the opposite way. The behavior can be tentatively explained by a solvent- and temperature-dependent proximity effect, where coupling of the close-lying S(1) and S(2) states influences the intramolecular relaxation rate of the dye. In addition, temperature-dependent complex equilibria of ATTO 465 with solvent molecules may influence the measured lifetimes. Several excited-state absorption (ESA) transitions are identified in the PSCP spectra, which are in good agreement with the position of the UV bands in the steady-state absorption spectra. Small shifts of the stimulated emission and ESA bands are consistent with solvation dynamics in the excited electronic state. An additional ~16 ps component in water, visible over the entire spectral range, is tentatively ascribed to a fast IC channel which is accessed by a fraction of ATTO 465 molecules.

Excited-state dynamics of 3,3′-dihydroxyisorenieratene and (3R,3′R)-zeaxanthin: Observation of vibrationally hot S0 species

We report on an ultrafast transient absorption study of all-trans-3,3-dihydroxyisorenieratene (DHIR) and all-trans-(3R,3R)-zeaxanthin in organic solvents covering the wavelength range 350-770u202fnm. The lifetime of the S2 state in both carotenoids is 160-170 fs. Upon internal conversion (IC) non-equilibrated S1 molecules are formed which internally relax on a 300-400 fs time scale. The time constant for IC from S1 depends on the type of terminal substituent: Replacement of the two terminal β-ionone rings of zeaxanthin by two aryl rings in DHIR results in an increase from 9.5 to 10.9 ps in THF. This suggests a mild decrease in the effective conjugation length of DHIR. IC to the ground electronic state prepares vibrationally hot S0* molecules which exhibit characteristic bleach and absorption bands. These are typically denoted as S* features. Collisional cooling of S0* happens with a time constant of 15 ps. Based on our results and the findings from previous studies for other carotenoids, such as macro-β-carotenes, β-carotenes and long-chain apocarotenals, we conclude that S0* spectral features are ubiquitous in carotenoid photophysics: They are particularly easy to observe in systems with a very short S1 lifetime and a high quantum yield for IC to the ground electronic state.