Biological systems, effect wavelengths

It is well known that pyrimidine bases convert to photodimers upon irradiation to UV light near the X max( > 270 nm). This photochemical reaction has a lethal effect in biological systems due to the photochemical transformation of pyrimidine bases of nucleic acids. However the photodimerization is a reversible reaction and the photodimers split to afford the original monomers very efficiently upon irradiation at a shorter wavelengths as shown in Scheme 1(1). 

Nanocrystalline semiconductor quantum dots have attracted attention primarily as sensitisers in solar cells, but have also been used as luminescent probes in biological systems, despite concerns over their toxicity (especially with quantum dots containing heavy metals such as Cd and Pb). The recombination of excitons post-excitation can lead to radiative emission with wavelengths shorter than that of the band gap of the bulk semiconductor due to perturbation of the exciton wave-function - so-called quantum confinement effects - at distances shorter than the exciton Bohr radius. For the popular semiconductor cadmium... 

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