An Exceedingly Long-Lived Fluorescent State as a Distinct Structural and Dynamic Probe for Supramolecular Association: An Exploratory Study of Host−Guest Complexation by Cyclodextrins

A novel fluorescent probe with n,π* configuration, the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), is responsive to complexation by supramolecular hosts. The n,π* fluorescent probe serves to provide structural and, owing to its exceedingly long fluorescence lifetime (up to 1 μs), also kinetic information on host−guest complexation. The three cyclodextrins (CDs) were selected as prototypal hosts in aqueous solution, and the complexation, in both the ground and excited states, was followed by four techniques:  time-resolved and steady-state fluorescence, UV absorption spectrophotometry, and NMR spectroscopy. The fluorescence quenching rate constants (kq) of DBO by α-, β-, and γ-CD (1.9, 4.0, and 0.78 × 108 M-1 s-1) were determined from the dynamic component of the biexponential time-resolved decay traces, while the static component was assigned to the fluorescence lifetimes of the complexes (τCD) of α-CD (ca. 33 ns) and β-CD (ca. 95 ns). Time-resolved and steady-state fluorescence measurements yielded consistent results. The shorter lifetimes in the complexes are attributed to the propensity of singlet-excited DBO to undergo fluorescence quenching by an “aborted” hydrogen abstraction with the labile glycosidic C−H bonds inside the cavity. Ground-state binding constants (K) could be determined by both UV spectrophotometry (for β-CD, ca. 900 M-1) and, owing to the high water solubility, also by NMR spectroscopy to afford values of 50, 1100, and 6 M-1 for α-, β-, and γ-CD, respectively. The spectroscopic data support the formation of inclusion complexes in both the excited and ground states. The dynamic quenching is attributed to inclusion with subsequent quenching inside the shorter-lived complex. The examination of the complexation dynamics at high guest (DBO) concentration revealed an unprecedented behavior, which may be indicative of singlet energy transfer between the free DBO and the CD·DBO complex. The potential of DBO as a distinct and complementary fluorescent probe is discussed.