Fluorescent Bis-Calix[4]arene-Carbazole Conjugates: Synthesis and Inclusion Complexation Studies with Fullerenes C(60) and C(70)

Supramolecular chemistry has become a central theme in chemical and biological sciences over the last decades. Supramolecular structures are being increasingly used in biomedical applications, particularly in devices requiring specific stimuli-responsiveness. Fullerenes, and supramolecular assemblies thereof, have gained great visibility in biomedical sciences and engineering. Sensitive and selective methods are required for the study of their inclusion in complexes in various application fields. With this in mind, two new fluorescent bis-calix[4]arene-carbazole conjugates (4 and 5) have been designed. Herein, their synthesis and ability to behave as specific hosts for fullerenes C(60) and C(70) is described. The optical properties of the novel compounds and their complexes with C(60) and C(70) were thoroughly studied by UV-Vis and steady-state and time-resolved fluorescence spectroscopies. The association constants (K(a)) for the complexation of C(60) and C(70) by 4 and 5 were determined by fluorescence techniques. A higher stability was found for the C(70)@4 supramolecule (K(a) = 5.6 × 10(4) M(−1); ΔG = −6.48 kcal/mol). Evidence for the formation of true inclusion complexes between the host 4 and C(60)/C(70) was obtained from NMR spectroscopy performed at low temperatures. The experimental findings were fully corroborated by density functional theory (DFT) models performed on the host–guest assemblies (C(60)@4 and C(70)@4).