Solvatochromism in Diketopyrrolopyrrole Derivatives: Experimental and Computational Studies

A series of seven thiophen-substituted diketopyrrolopyrrole derivatives were synthesised and their solution absorption spectra characterised in a range of solvents of varying polarity. The absorption spectra of these diketopyrrolopyrrole derivatives exhibited significant negative solvatochromism. The behaviour is consistent with results of time-dependent density-functional theory calculations of excitation energies. Calculated electronic structures suggest that there is significant charge transfer between the electron-donating thiophen substituents and the diketopyrrolopyrrole core but that the magnitude of this charge shift is reduced in the excited state compared with the ground state. The resulting reduction in polarity of the excited state accounts for the negative solvatochromism observed. The implications of the results for the potential application of diketopyrrolopyrrole compounds as photovoltaic materials are considered.