Host-to-guest photoinduced electron transfer from a didansyl-pillar[5]arene to a viologen derivative

As a part of our ongoing studies on pillar[n]arenes1 and related macrocycles, a pillar[5]arene functionalized with two dansyl fluorophore moieties at the same aromatic unity of its cavity was synthesized. In order to better understand the electron transfer abilities of fluorophore-functionalized pillararenes, we minutely investigated this new host system in the presence of an N,N’-di-(6-bromohexyl)-viologen derivative4 (2+·2PF6–) as the guest. 1H NMR, NOESY and DOSY experiments demonstrated that the pillar[5]arene recognizes the viologen derivative, forming a 1:1 host-guest complex by including a bromoalkyl chain in its cavity. Photophysical and electrochemical studies showed that the fluorescence intensity of the host is progressively quenched by addition of increasing amounts of the guest, and that such quenching occurs as a consequence of a photoinduced electron transfer (PET) process from one of the dansyl groups of the host to the viologen moiety of the guest