Dithiafulvene derivatized donor–acceptor norbornadienes with redshifted absorption

Abstract

Photoisomerization of norbornadiene (N) to its metastable isomer quadricyclane (Q) has attracted interest as a strategy for harvesting and storing solar energy. For this strategy to mature the absorption maximum of N has to be moved from the UV to the visible region. Here we show that functionalization of the system with dithiafulvene (DTF) electron donors causes remarkable redshifts of various N derivatives. Thus, some derivatives were found to absorb light with an absorption onset up to 556 nm. The incorporation of DTF units comes, however, with a drawback with regard to achieving reversible N-to-Q and Q-to-N isomerizations. For some derivatives, the photoisomerization was completely quenched. The compounds were subjected to a computational study to shed light on the underlying reason for this reluctance to undergo photoisomerization. The computational study revealed that in these systems, the first excited state (S₁) is positioned close to or lower than the transition state for photoconversion, effectively blocking a possible conversion to Q, thus revealing a practical challenge for the future design of N–Q energy storage systems with an improved solar spectrum match.