NIR emission of cyclic [4]rotaxanes containing pi-extended porphyrin chromophores

The photophysical properties of a Cu(I) [4]rotaxane 4(4+) and of the demetalated [4] rotaxane 3 have been determined and compared to those of the component Zn porphyrin 2. All samples emit in the NIR region (700-1200 nm). The luminescence from the interlocked structures is bathochromically shifted with respect to 2 and displays a lower emission quantum yield, much lower for 4(4+) than for 3. The occurrence of intra-molecular electron or energy transfer is excluded and the decrease in luminescence yield is discussed in terms of the energy gap law and of electronic interactions between components of the cyclic interlocked structure. In toluene a dual emission behavior, similar to that of 2, is observed for 3 and ascribed to the presence of two non-equilibrated excited states, pi-pi* and CT in nature with lifetimes of 0.80 and 0.14 ns, respectively.