Luminescence and diffuse reflectance studies of biacetyl included within p-tert-butylcalixarenes

Powdered solid samples of biacetyl within p-tert-butylcalix[4]arene and p-tert-butylcalix[6]arene were studied with the use of diffuse reflectance techniques to observe the formation of inclusion complexes of the diketone (probe) and those solid powdered matrixes. In all substrates, room temperature phosphorescence was obtained in air equilibrated samples. The decay times vary greatly and the largest lifetime was obtained for biacetyl/p–tert–butylcalix[4]arene, showing that this host cavity well accommodates biacetyl, this way promoting a significant increase of the triplet emission lifetimes of the diketone. Transient absorption spectra of inclusion complexes show the triplet–triplet absorption of biacetyl in the calix[4]arene case, while the ketyl radical formation is observed in the calix[6]arene case. A lifetime distribution analysis has shown a single band, peaking at 1.9 ms for biacetyl/calix[4]arene indicating a unique emissive species and in accordance with an endo inclusion of the probe within the non-polar cavity of the calixarene. Furthermore, in the calix[6]arene inclusion, the lifetime distribution exhibits two maxima peaking at 40 and 174 μs, showing that in this case the probe is emitting in two different environments, a more confined one within the non-polar cavity of the calixarene, while some others probe molecules emit from a location within the polar lower rim, in accordance with an exo-calix complex formation.