Near-infrared luminescence from cyclic [4]rotaxanes containing extended zinc porphyrins

The use of light to power molecular machines is very appealing and the study of the interaction of light with complex interlocked structures can open the way to new functionalities and applications. The [4]rotaxanes here presented (2 and 34+) contain two roughly face-to-face porphyrin plates whose mutual arrangement can be controlled by complexation with copper(I) of the four bidentate coordination sites of the rotaxane [1]. Each plate consists of an extended Zn-porphyrin (1), where a central tetraphenyl Zn-porphyrin core is extended by two tetraazaanthracene arms, each one fused on the back of a 2,9-diphenyl-1,10-phenanthroline (Fig. 1 left). All the examined structures interestingly show a near-infrared emission (700-1200 nm). The luminescence of 1, in particular, presents an unusual dual behaviour, that can be interpreted in terms of two non-equilibrated excited states, a high energy one of ?-?* nature and a lower energy one, CT in character, whose energy level is tuned by solvent polarity [2] (Fig. 1 right). The emission from the interlocked structures 2 and 34+ is red-shifted and with a reduced quantum yield with respect to that of 1. In toluene, a dual behavior similar to that of 1 is observed [3]. These emission features, ascribable to the Zn porphyrin units, are discussed in terms of energy gap law and of electronic interactions among the components of the cyclic interlocked structures. References: [1] J. P. Collin, F. Durola, V. Heitz, F. Reviriego, J. P. Sauvage, Y. Trolez, Angew. Chem., Int. Ed., 2010, 49, 10172. [2] B. Ventura, F. Durola, J. Frey, V. Heitz, J.-P. Sauvage, L. Flamigni, Chem. Commun., 2012, 48, 1021. [3] B. Ventura, L. Flamigni, J.-P. Collin, F. Durola, V. Heitz, F. Reviriego, J.-P. Sauvage, Y. Trolez, Phys. Chem. Chem. Phys., in press, DOI: 10.1039/C2CP41381G