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.

We review the physical properties, linear and nonlinear optical characteristics, and phase‐matching configurations of BiB₃O₆ (BIBO), the first low‐symmetry (monoclinic) inorganic nonlinear crystal that has found broad applications for frequency conversion of laser sources from the UV, across the visible, to the near‐IR based on three‐wave interactions. We describe in detail the most relevant optical properties that make this material an attractive candidate for nonlinear frequency conversion of laser light in general, and ultrafast femtosecond laser sources in particular. With special focus on ultrafast frequency conversion, characteristics such as group‐velocity mismatch and spectral acceptance, parametric gain bandwidth, group‐velocity dispersion, as well as angular acceptance and spatial walk‐off are evaluated and optimum configurations for the attainment of maximum conversion efficiency, minimum pulse duration, and highest spatial beam quality are identified and compared with the most widely established alternative borate crystal, β‐BaB₂O₄. Experimental results are presented on both parametric up‐ and down‐conversion of femtosecond pulses, from the high‐energy, low‐repetition‐rate (1 kHz) to the low‐energy, high‐repetition‐rate (56–76 MHz) regime, demonstrating the unique versatility of BIBO for efficient frequency conversion of femtosecond pulses with broad tunability from 250 nm in the UV, throughout the visible, up to ∼ 3000 nm in the IR.

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.