The properties of triplet excited states are markedly medium-dependent, which turns
this species into valuable tools for investigating the microenvironments existing in protein binding
pockets. Monitoring of the triplet excited state behavior of drugs within transport proteins
(serum albumins and α1-acid glycoproteins) by laser flash photolysis constitutes a valuable
source of information on the strength of interaction, conformational freedom and protection from
oxygen or other external quenchers. With proteins, formation of spatially confined triplet excited
states is favored over competitive processes affording ionic species. Remarkably, under aerobic atmosphere, the triplet
decay of drug@protein complexes is dramatically longer than in bulk solution. This offers a convenient dynamic range for
assignment of different triplet populations or for stereochemical discrimination. In this review, selected examples of the
application of the laser flash photolysis technique are described, including drug distribution between the bulk solution and
the protein cavities, or between two types of proteins, detection of drug-drug interactions inside proteins, and enzyme-like
activity processes mediated by proteins. Finally, protein encapsulation can also modify the photoreactivity of the guest.
This is illustrated by presenting an example of retarded photooxidation.