On/off fluorescence emission induced by encapsulation, exchange and reversible encapsulation of a BODIPY-guest in self-assembled organometallic cages

A dynamic guest-binding study of BODIPY-derivative 3 (3 = 1,3,5,7,8-pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) with two Cp*Rh-III-based organometallic cages [Cp*Rh-6(6)(-CA)(3)(tpt)(2)](OTf)(6)1 (H(2)CA = chloranilic acid, tpt = 2,4,6-tripyridyl-1,3,5-triazine) and [Cp*Rh-6(6)(-DHNA)(3)(tpt)(2)](OTf)(6)2 (H(2)DHNA = 6,11-dihydroxynaphthacene-5,12-dione) have been demonstrated. While guest 3 displays a strong green fluorescence emission, the host-guest complexes 3< subset of of>1 and 3 subset of 2 exhibit almost no emission. However, fluorescence emission can be again turned on' by adding coronene (4) or pyrene (5) into the solution of 3 subset of 1 and 3 subset of 2. Guests 4 and 5 show stronger binding affinities toward the organometallic cages 1 and 2 due to the strong electrostatic interactions between the electron-rich guest and electron-deficient tpt resulting in the displacement of 3 by 4 or 5, which eventually helps the fluorescence enhancement. Further guest-binding studies using a Job's plot experiment illustrate the formation of discrete 1:1 host-guest complexes which are also supported by H-1 NMR spectroscopy and ESI mass spectrometry. The anion-exchange study with sodium chloride reveals that the chloride ion can effectively disassemble the host frameworks of 1 and 2 releasing the fluorescent guest 3 which can be again encapsulated into the cavity of reassembled cages promoted by the addition of silver triflate.