Photochemistry is a highly valuable tool for modern organic synthesis.
It has had a very strong first period of prosperity in the early fifties and
sixties of the last century, when numerous light-induced reactions were
discovered, modified and applied to synthetic problems. A milestone of
development is related to the famous Woodward-Hoffmann rules and the
photochemical reactions which served as the experimental basis. Subsequently,
expectations from the ‘‘synthetic community’’ were high and the
actual output in fact remarkable, as chemists became aware of the reactivity
potential of electronic excited states of a molecule. Indeed, photochemistry
means in many cases a multiplication of reactivity options, i.e. in addition
to the ground state, the electronically excited singlets and triplets show
different chemical behavior and often differ so remarkably that they behave
as completely new molecules. If you don’t want to use them, however,
they simply disappear again by radiative or non-radiative pathways (a key
principle also of green chemistry). The techniques of time-resolved spectroscopy
approaching new fascinating time domains (‘‘femtochemistry’’) in the
last decades, enable the photochemistry community to describe a photophysical
process in full detail and also to predict photochemical reactivity.