What determines the colour of an organic dye?
This week I read most of chapter 4 of the book, Colour and Constitution of Organic Molecules by John Griffiths. I think it is a must-read for anyone working on dye molecules. It is at an elementary level and so accessible to both chemists and physicists, experimentalists and theorists. I fear much of the content has been forgotten or is unknown to many working on these materials, whether quantum chemists or biologists trying to find new molecules for fluorescent marker or people working on dye-sensitised solar cells. Griffiths gives a nice discussion of resonance theory and then discusses its "failures", stating: The apparent failure of resonance theory in these systems arises from the assumption that only low energy resonance forms need to be considered as contributing to the ground and first excited states. Indeed, work by Seth Olsen and I (mostly Seth) shows for methine dyes (including flourescent proteins) how to do this in a systematic manner.