EDOT-type materials: planar but not rigid.

The geometrical, electronic and optical properties of alpha,alpha'-quater(3,4-ethylenedioxythiophene) (4EDOT) and alpha,alpha'-quater(thiophene) (4T) are studied by quantum-chemical calculations; the computed vibronic spectra at low and room temperature (RT) are compared to experiment. Our results suggest that the better resolved RT absorption spectrum of 4EDOT in comparison with 4T is not due to the molecule's rigid nature as commonly assumed but due to very similar out-of-plane modes in the electronic ground and first excited states of 4EDOT, with low frequencies indicative of its rather soft nature.