1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Department of Physics and Astronomy, Science and Technology, Aarhus University
This chapter provides a brief introduction to biochromophores encountered in nature, and how their π-conjugated structures determine their excitation energies, i.e., their colour. Perturbations of electronic structure by a microenvironment such as water or charge sites are discussed. These may lead to a colour change (or modulation), depending on the character of the electronic transition. As detailed results for particular chromophores are presented in subsequent chapters, future challenges and new aspects within the research field are instead considered as the author sees them.