Modulation of strength and position of electron-donating substituent directly linked to a diketopyrrolopyrrole core offers an interesting approach towards fine-tuning their photophysics. Depending on the type of cyclic tertiary aromatic amine either Franck-Condon or charge-transfer excited states have the lowest energy. The N-carbazolyl substituent enables DPPs to have very strong fluorescence while other donors such as phenothiazine red-shift the emission at the expense of quantum yield.

Abstract

Strongly polarized donor-acceptor-donor′ diketopyrrolopyrroles, differing in the type of key donor moiety, were designed and synthesized to examine how this affects the non-radiative decay. Dyes possessing less electron-rich N-carbazolyl substituents are characterized by strong yellow emission from a locally excited (LE) state, whereas replacing this donor with more electron-rich N-phenothiazinyl substituent changes the relative position of charge-transfer (CT) and LE states, leading to weaker, reddish-orange fluorescence. As a result, there is solvent-dependent charge-transfer emission shifted to as far as 700 nm. The opening of the intersystem crossing channel to the triplet state possessing CT character is the most likely cause of the fluorescence quantum yield variation in some cases. These results reveal that the fate of molecules in their excited state can be fine-tuned by very small structural changes.