Torsten Fiebig

Electronic energy delocalization and dissipation in single- and double-stranded DNA.

The mechanism that nature applies to dissipate excess energy from solar UV light absorption in DNA is fundamental, because its efficiency determines the vulnerability of all genetic material to photodamage and subsequent mutations. Using femtosecond time-resolved broadband spectroscopy, we have traced the electronic excitation in both time and space along the base stack in a series of single-stranded and double-stranded DNA oligonucleotides. The obtained results demonstrate not only the presence of delocalized electronic domains (excitons) as a result of UV light absorption, but also reveal the spatial extent of the excitons.

Base pair motions control the rates and distance dependencies of reductive and oxidative DNA charge transfer.

In 1999, Wan et al. [Proc. Natl. Acad. Sci. USA 96, 6014–6019] published a pioneering paper that established the entanglement between DNA base pair motions and the transfer time of the charge carrier. The DNA assemblies contained an ethidium covalently bound via a flexible alkyl chain to the 5′ hydroxyl group of the DNA backbone. Although covalently attached, the loose way in which the ethidium was linked to DNA allowed for large degrees of conformational freedom and thus raised some concern with respect to conformational inhomogeneity. In this letter, we report studies on a different set of ethidium DNA conjugates. In contrast to the “Caltech systems,” these conjugates contain ethidium tightly incorporated (as a base pair surrogate) into the DNA base stack, opposite to an abasic site analog. Despite the tight binding, we found that charge transfer from the photoexcited ethidium base pair surrogate across two or more base pairs is several orders of magnitude slower than in case of the DNA systems bearing the tethered ethidium. To further broaden the scope of this account, we compared (oxidative) electron hole transfer and (reductive) electron transfer using the same ethidium chromophore as a charge donor in combination with two different charge acceptors. We found that both electron and hole transfer are characterized by similar rates and distance dependencies. The results demonstrate the importance of nuclear motions and conformational flexibility and underline the presence of a base gating mechanism, which appears to be generic to electronic transfer processes through π-stacked nucleic acids.

Getting to guanine: mechanism and dynamics of charge separation and charge recombination in DNA revisited

The mechanism and dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing a stilbenedicarboxamide linker and a single guanine-cytosine base pair have been reinvestigated. The combination of femtosecond broad-band pump probe spectroscopy, nanosecond transient absorption experiments, and picosecond fluorescence decay measurements permits analysis of the formation and decay of the stilbene anion radical. Reversible hole injection resulting in the formation of the stilbene-adenine contact radical ion pair is found to occur on the picosecond time scale. The mechanism for charge separation across two or more base pairs is revised from single step superexchange to a multi-step process: hole injection followed by hole transport and hole trapping. The mechanism of charge recombination remains assigned to a superexchange process.

Broadband optical parametric amplification in the near UV–VIS

Abstract An optical parametric amplification scheme yielding a broad bandwidth in a BBO crystal (Type I) pumped by the third harmonic of a femtosecond Ti:Sapphire laser has been investigated theoretically and experimentally. A broad amplification bandwidth of 205 THz between 346 and 453 nm is obtained in a noncollinear geometry of parametric interaction. The bandwidth of the amplification is reduced significantly when the pump–signal angle is changed by only 0.1°. A UV-extended white-light continuum is used as a signal seed. Several materials for white-light generation – such as CaF2,MgF2 and LiF – have been investigated.

Tunable femtosecond pulses in the near-ultraviolet from ultrabroadband parametric amplification

Using an ultrabroadband amplification technique in a β-barium borate noncollinear optical parametric amplifier, pumped by the third harmonic of a 1 kHz Ti:sapphire laser, we generate tunable femtosecond pulses in the range of 335–480 nm with energies of a few hundred nJ. The developed setup is an amplification source with a bandwidth of more than 200 THz and provides femtosecond pulses in the near-ultraviolet spectral range using coherent amplification. Parts of the amplified white-light continuum spectrum were compressed to 24–35 fs using a prism pair. Further improvements could make it possible to generate tunable ultraviolet pulses as short as 4–5 fs.

Femtosecond charge transfer dynamics in artificial donor/acceptor systems: switching from adiabatic to nonadiabatic regimes by small structural changes

Abstract The dynamics of intramolecular charge transfer (CT) in a family of conjugated aromatic electron donor/acceptor (D/A) systems has been studied by femtosecond pump-probe spectroscopy. Although the compounds have very similar structures we observed vastly different CT dynamics, ranging from a few 100 fs to the nanosecond time range. These results show that specific modifications in the substitution scheme can lead to alterations of the relevant D/A orbitals and thus to drastic changes in the excited state electronic coupling. As a result the CT process switches from the ultrafast adiabatic to the slow nonadiabatic regime.

Broadband femtosecond circular dichroism spectrometer with white-light polarization control.

A broadband, femtosecond transient circular dichroism (TRCD) spectrometer has been developed and tested in the wavelength range from 350 to 700 nm. The spectrometer uses a femtosecond probe white light with well-defined circular polarization. The latter is modulated by the polarization of a narrowband seed pulse. We have implemented a dual-beam probe geometry with phase-locked detection technique to increase the signal-to-noise ratio and to reduce optical artifacts. The spectrometer allows the acquisition of TRCD spectra with subpicosecond time resolution and typical noise levels of 10(-4) absorbance units. The performance of this instrument has been demonstrated on bis(merocyanine) nanorod aggregates in tetrahydrofurane/methylcyclohexane solution. The case study confirmed that this spectrometer is effective for the investigation of chiral properties in various molecular and nanostructural systems that have transient spectra in the UV-visible spectral range.

Toward an understanding of white-light generation in cubic media—polarization properties across the entire spectral range

An investigation of the polarization properties of a femtosecond laser pulse induced coherent WL continuum generated in a cubic crystal (CaF2) is discussed . For linear input polarization the WL spectrum shows strong depolarization around the input wavelength while the preservation of the input polarization is pronounced towards the blue spectral region. The origin of the observed depolarization effect has been elucidated.

Pyrene as a fluorescent probe for DNA base radicals

The steady-state emission spectra of 5-(1-pyrenyl)-modified pyrimidine and 8-(1-pyrenyl)-modified purine nucleosides in water at different pH values provide important information about the acidity or basicity of photochemically generated DNA base radicals which are key intermediates in DNA-mediated charge transport processes.

Femtosecond probing of the excited state absorption and structural relaxation in biphenyl derivatives

Abstract The excited state absorption spectra of biphenyl and 2,5-dimethyl biphenyl have been investigated by femtosecond broad band pump–probe spectroscopy in organic solvents. The results show vastly different intensities that were analyzed by calculating transition density surfaces for characteristic S 1 →S n absorptions. The analysis revealed strong symmetry-lowering substituent effects in 2,5-dimethyl biphenyl which directly affect the transition dipole moments for excited state absorption. By measuring the temporal evolution of the transient spectra of the biphenyl derivatives we observed a biphasic relaxation mechanism in the excited state characterized by two distinct time scales of 400 fs and 12 ps.