Langmuir films of p-tert-butylcalix[8]arene. Conformations at the water-air interface and complexation of fullerene C60

Langmuir monolayers of p-tert-butylcalix[8]arene (CAL8) at the water-air interface were studied as a function of temperature (between 288 and 308 K), of the spreading solvent (benzene, toluene, dichloromethane, chloroform, and carbon tetrachloride), and in a mixture with fullerene C-60. Two different mixed systems were studied: (i) spreading isotherms (surface pressure versus molecular area) after separated spreading of CAL8 and C-60 solutions (1/1 molar ratio) and (ii) spreading isotherms of the C-60-CAL18 complex obtained from precipitation of freshly prepared and stabilized solutions of the two components in carbon tetrachloride. The results show that, at the water-air interface, CAL8 possesses a perpendicular "pleated-loop" conformation, regardless of the spreading solvent. However, the limiting areas do depend on the nature of the spreading solvent, indicating that the flexibility of the CAL8 backbone is significantly affected by the interactions with the spreading solvent, in analogy with the "memory effect" with the retention of the host conformation, even after the removal of the guest, found for bulk solutions. The complexation of C60 by CAL8 does not modify the calixarene's conformation. Films obtained by spreading equimolar solutions of CAL8 and C-60 on the water surface show the same II/A isotherms of the [C-60-CAL8] complex, suggesting the formation of stable CAL8/C-60 1/1 complexes at the air-water interface. Electronic spectra performed on Langmuir-Blodgett films and on collapsed material, both transferred on quartz plates, support the presence at the interface of the [C-60-CAL8] host-guest system, indicating that the water interface acts as a complexation promoter.