Klaus Schaper

Photoprocesses of Molecules with 2‐Nitrobenzyl Protecting Groups and Caged Organic Acids

The 308 nm photoinduced formation of the nitroso product and the intermediacy of the aci‐nitro form(s) were studied for a series of 2‐nitrobenzyl alkyl and aryl esters (1a–4e) and bis‐(nitrophenyl)methyl acetates (5a–6b) by time‐resolved UV‐vis spectroscopy. A triplet state appears as major transient, when 2‐nitrobenzyl derivatives 1 are substituted by 4,5‐dimethoxy (2) and 4,5‐methylenedioxy (3/4) groups. This triplet of charge transfer character is, however, not part of the route via the aci‐nitro into the 2‐nitroso form. The activation energy and preexponential factor of the longest lifetime component (τaci), i.e. the major part of the aci‐nitro decay, were determined. The carboxylic acids as leaving groups have rather small effects on τaci. An additional nitrated phenyl ring in α‐position (5) leads generally to shorter τaci value. Otherwise, the photogeneration of nitroso products is similar. The quantum yield (Φd) varies only moderately with structure, the yield of the aci‐nitro form and Φd are correlated and little affected by solvent properties.

Synthesis and Photophysical Characterization of a New, Highly Hydrophilic Caging Group

o-Nitrobenzyl-protected bioactive compounds are useful tools in biophysics, allowing controlled photorelease of biologically active compounds with high temporal and spatial precision. Thus, it is possible to study biological processes, such as neurotransmitter-receptor interaction, and many other processes, in much more detail than before. In this respect, these caged compounds have become established as extremely useful tools. In some cases, however, their biological properties (the caged compound should not interact with the biological system), their photochemical properties (quantum yield and kinetics of the photorelease), and their physical properties (high hydrophilicity) are not satisfactory. In order to address the last problem, we examined means to increase the hydrophilicity of caged compounds based on the o-nitrobenzyl moiety. Here, we report the synthesis and the photochemical and biological characterization of a new caged D-aspartate derivative with vastly improved hydrophilicity, compared to derivatives reported previously, and satisfactory photophysical properties. Caged compounds with this improved o-nitrobenzyl group may thus represent a valuable new tool for different kinds of biophysical investigations. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Structural assignment of adenine aggregates in CDCl3

We reinvestigated the self-association of 9-substituted adenine derivatives in CDCl3 solutions and present the infrared spectra of 9-ethyladenine and N-methyl-9-ethyladenine and its aggregates in the spectral regions between 1500 and 1800 cm(-1) and between 2700 and 3600 cm(-1). Wavelength dependent absolute extinction coefficients of the monomer and dimers are presented on the basis of a simple deconvolution method. Comparison of the deconvoluted dimer spectra with quantum chemical calculations allows for a structural assignment of the two dimer structures that coexist in 9-ethyladenine/CDCl3 solutions. In contrast, the dimer spectrum of N-methyl-9-ethyladenine is dominated by a single isomer.

Theoretical Investigation of the Excited States of 2‐Nitrobenzyl and 4,5‐Methylendioxy‐2‐nitrobenzyl Caging Groups†

The photochemistry of caged compounds of the o‐nitrobenzyl type has been investigated thoroughly in the past. However, even recently new side reactions have been discovered. Earlier, we reported [Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol.84 162–171] that we found long‐lived triplet states which do not lead to product formation for the bathochromic absorbing compounds with 4,5‐methylendioxy‐2‐nitrobenzyl caging group. Here, we report on theoretical studies which explain the special behavior of these compounds. These studies reveal that the bathochromic shift of absorption for these compounds compared with o‐nitrobenzyl compounds themselves is not due to a shift in energy of the involved states, but due to a substantial change of oscillator strength of the respective transitions. The lack of reactivity of the triplet state of 4,5‐methylendioxy‐2‐nitrobenzyl compounds can be attributed to state switching. In the triplet manifold the lowest state is a nonreactive charge transfer state, while the lowest state in the singlet manifold is a reactive local excitation in the nitro‐group. From these results we conclude that it will be most likely not possible to create derivatives of caged compounds based on the o‐nitrobenzyl caging group which have absorption which is shifted even more strongly to longer wavelengths.

Diphenylhexatrienes as photoprotective agents for ultrasensitive fluorescence detection.

Given the particular importance of dye photostability for single-molecule investigations, fluorescence fluctuation spectroscopy, and laser-scanning microscopy, refined strategies were explored for enhancing the fluorescence signal by selectively quenching the first excited triplet state of the laser dye Rhodamine 123 (Rh123). The strategy is to quench the T(1) state by Dexter triplet energy transfer, while undesired quenching of the singlet state via Forster or Dexter singlet energy transfer and the generation of free radicals through electron transfer should be avoided. Diphenylhexatrienes (DPHs) were tested in ethanol for their beneficial effects as a novel class of photoprotective agents using fluorescence correlation spectroscopy. A library of DPHs with electron-donating (dimethlyamino) and withdrawing substituents (e.g., trifluormethyl) was synthesized to optimize the electronic properties. Quantum chemical calculations, optical spectroscopy, and cyclic voltammetry were used to determine the electronic properties. The computed T(1) emission energy of Rh123 and the T(1) excitation energies of all DPHs allow for exergonic triplet energy transfer to the quencher. The parent compound quenches the T(1) state of Rh123 nearly diffusion controlled (4.9 x 10(9) M(-1) s(-1)). All electron-deficient DPHs significantly increase (3x) the fluorescence rate of Rh123 by reducing the triplet state population and by avoiding the formation of other long-lived dark radical states. The quenching constants are reduced by more than a factor of 2, if substituents with increasing size or electronegativity are introduced. The beneficial effect of triplet quenching of substituted DPHs is governed by a delicate interplay of steric, electronic, and intermolecular Coulombic effects.

On the photophysics of 1,6-diphenyl-1,3,5-hexatriene isomers and rotamers.

Herein, the low-lying electronic states of various isomers and rotamers of 1,6-diphenyl-1,3,5-hexatriene (DPH) are studied by a combination of density functional theory and multireference configuration interaction. Starting from the all-trans nuclear arrangement, trans-cis isomerization pathways in the electronic ground state and in the first excited triplet state were determined. Further, spin-orbit coupling calculations were carried out at selected points where singlet-triplet energy gaps are small. The calculations reveal that the primarily excited, optically bright 1(1)B(u) state undergoes a curve crossing with the optically dark multiconfigurational 2(1)A(g) state upon geometry relaxation in the excited state. The strong vibronic coupling of the two singlets in the neighborhood of the conical intersection provides a conclusive explanation for the experimentally observed fast equilibration of the states and the appearance of delayed fluorescence. With regard to the trans-cis isomerization of the central CC bond, the perpendicular conformation is found to represent a maximum on the energy profile not only of the electronic ground state, but also of the low-lying excited states. The lack of a strong driving force along the torsional coordinate explains the low tendency of DPH for isomerization. Finally, the results of our spin-orbit coupling calculations suggest that the intramolecular formation of DPH molecules in the T(1)(1(3)B(u)) state proceeds from the 1(1)B(u) state and involves intermediately the T(2)(1(3)A(g)) state.

The α,5‐Dicarboxy‐2‐nitrobenzyl Caging Group, a Tool for Biophysical Applications with Improved Hydrophilicity: Synthesis, Photochemical Properties and Biological Characterization

Earlier we reported on the synthesis of α,4‐dicarboxy‐2‐nitrobenyzl caged compounds (Schaper, K. et al. [2002] Eur. J. Org. Chem., 1037–1046). These compounds have the advantage of an increased hydrophilicity compared with the well‐established α‐carboxy‐2‐nitrobenzyl caged compounds; however, the release of the active compound becomes slower due to the introduction of the additional carboxy group. Based upon theoretical calculations we predicted that the release would become faster when the additional carboxy group is moved to the 5‐position. Here we describe the synthesis and the photochemical and biological characterization of an α,5‐dicarboxy‐2‐nitrobenyzl caged compound. The high hydrophilicity of the new caging group is maintained due to the fact that the additional carboxy moiety is preserved, while the release of the active species from the new derivative is even faster than for the reference, an α‐CNB caged compound.

NMR–spectroscopic investigation of o-nitrosobenzoic acid

The synthesis of o‐nitrosobenzoic acid 2 has been known for more than 100 years, and the photochemical preparation from o‐nitrobenzaldehyde 1 became a textbook example for [1,5]‐hydrogen shifts. However, neither the 1H–NMR spectra nor the 13C{1H}–NMR of this compound have been reported so far. This fact can most likely be attributed to the monomer–dimer equilibrium of the nitrosobenzoic acid, which leads to rather complex, concentration‐dependent NMR spectra. In this paper, we report a thorough investigation of these spectra. In the 13C‐{1H}‐NMR spectra, all 21 lines could be assigned to the monomeric form, the E‐dimer, and the Z‐dimer. Copyright

Solid state photoelectron spectroscopy of carotenoids

Abstract The threshold energies of carotenoids were determined by solid state photoelectron spectroscopy and their polarization energies were calculated with the dielectrical constant obtained via Lorentz-Lorenz equation. The influences on the threshold energy exerted by the chain-length of the system, the acceptor-strength of the end group, the number and kind of alkyl groups attached to the system and the conformation, especially the dihedral angle between chain and end group were investigated.[1]

Thiophene Syntheses by Ring Forming Multicomponent Reactions

Thiophenes occur as important building blocks in natural products, pharmaceutical active compounds, and in materials for electronic and opto-electronic devices. Therefore, there is a considerable demand for efficient synthetic strategies for producing these compounds. This review focuses on ring-forming multicomponent reactions for synthesizing thiophenes and their derivatives.