SUPRAMOLECULAR PHOTOCHEMISTRY AND PHOTOPHYSICS - ABSORPTION-SPECTRA AND LUMINESCENCE PROPERTIES OF A LARGE CAGE SUPERMOLECULE AND OF ITS PROTONATED FORMS

The cage-like species 1 is an endoreceptor supermolecule with a large cavity made of three convergent 2,9-dianisyl-1,10-phenanthroline (dap) units connected by two 1,3,5-trimethylbenzene-type spacers. This supramolecular host species shows absorption and luminescence properties (CH-Cl2 solution, 298 K; absorption: lambda(max) = 284 nm; fluorescence: lambda(max) = 400 nm, tau = 2.2 ns, PHI = 0.26) very similar to those exhibited by isolated dap units. Addition of trifluoroacetic acid to CH2Cl2 solutions of 1 causes strong changes in the absorption and luminescence properties, which indicate the occurrence of two distinct and well-separated protonation processes. The protonated form of the cage obtained after addition of a large excess of acid displays absorption and luminescence properties (CH2Cl2 solution, 700 equivalents of acid, 298 K: absorption: lambda(max) = 302 nm; fluorescence: lambda(max) = 580 nm, tau = 7.6 ns, PHI = 0.030) very similar to those exhibited by isolated dap.H+ units, showing that the three chromophoric components of the cage are independently protonated. The absorption and luminescence properties obtained at moderate acid concentration (CH2Cl2 solution, 15 equivalents of acid, 298 K: absorption: lambda(max) = 279 nm; fluorescence: lambda(max) = 544 nm, tau = 37.1 ns, PHI = 0.092) indicate that under such conditions the three chromophoric units of the cage are collectively engaged in a protonated species that contains only one proton.