Excited state relaxation dynamics and electronic properties of a quinoid carotenoid

A combined study of the quinoid carotenoid DHIRQ by femtosecond transient absorption spectroscopy and quantum chemical calculations revealed its very complex electronic structure and ultrafast relaxation dynamics. The two quinoid end rings are found to cause a strong bathochromic shift of the absorption spectrum and to decrease the main relaxation time of the S1 state to 400 fs. Transient absorption data of DHIRQ show a substantial difference of its spectroscopic features to other carbonyl carotenoids. Various alternative kinetic models including an intramolecular charge transfer (ICT) state are discussed in order to assign the electronic structure and the relaxation dynamics.