I was pretty sure that this would've been asked before but I couldn't find anything here. I've noticed that system with multiple benzene rings and pi donating/accepting groups tend to be quite colourful compared to their less conjugated counterparts, like azo dyes or indophenol or methylene blue. My guess is that the extended conjugation somehow pushes down the energy gap between the ground state and excited state, which kicks up the wavelength of light absorption out of the UV spectrum perhaps by molecular orbital magic.
- Is there any rigorous explanation for this observation? (Answered)
- Naphthalene and anthracene are both colourless, so do the nitrogen/oxygen substituents play an important role in imparting colour ie why do heteroatoms seem to create more colourful molecules?
