Chromic phenomena in neutral polythiophene derivatives

Thermochromic phenomena in various regioregular and non-regioregular polythiophene derivatives have been investigated both in the solid state and in solution. These studies have revealed the existence of two different types of thermochromic phenomena which are correlated to the substitution pattern of the polymers. Formation of delocalized conformational defects (twistons) has been observed in regioregular polythiophene derivatives but did not occur in non-regioregular polythiophenes. For the latter polymers, only localized conformational defects can be created along the polymer backbone leading to a monotonic blue shift of the absorption maximum upon heating. It is suggested that all these optical features are driven by order-disorder transitions of the side-chains, side-chain disordering increasing their steric hindrance and/or disrupting short-range ordered assemblies, which then force the thiophene backbone to adopt a nonplanar conformation. It has been also found that non-covalent interactions between alkali metal ions and regioregular ether-substituted polythiophenes can modify the side-chain organization and can lead to interesting ionochromic effects. It is believed that chromic phenomena can be induced via a large range of external stimuli which should lead to the development of novel affinitychromic and photochromic materials.