Ground and Excited States of Bis-4-Methoxybenzyl-Substituted Diketopyrrolopyrroles: Spectroscopic and Electrochemical Studies

A series of symmetrically bis-4-methoxybenzyl (4MB) N-substituted 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) derivatives have been synthesized. The 4MB unit makes the DPP core soluble, and shows subtle modification of up to 0.2 eV in ground and excited states of the core when compared with related alkyl derivatives. Absorption and emission spectroscopy, as well as electrochemical and computational methods have been employed to prove the importance of the peripheral aryl units on the donor/ acceptor properties of the molecules. The 4MB products are highly fluorescent (quantum yields approaching 100 % in solution), with a unique distribution of frontier states shown by spectroelectrochemistry. The solid-state fluorescence correlates with the X-ray crystal structures of the compounds, a Stokes shift of approximately 80 nm is seen for two of the compounds. The frontier energy levels show that this subtle substitutional change could be of future use in molecular energy level tailoring in these, and related, materials for organic (opto)electronics.