Photophysical properties of an extended Zinc porphyrin

Porphyrins are key components in the natural processes of light collection and conversion and they have been widely used in artificial photosynthetic systems. Bis-macrocyclic Zn porphyrin 1 (Figure 1) is a component of a [4]rotaxanes used as a switchable molecular receptor.[1, 2] It is also interesting for its optical features, arising from the presence of an extended tetraazaanthracenic conjugation of the ring and two terminal macrocycles. Tetraazaanthracene substitution of the Zn-porphyrin ring is known, in fact, to introduce charge transfer character to the electronic transitions, causing a remarkable solvent effect.[3] The presence of the two macrocycles, in addition, introduces an interesting option for coordination. A detailed photophysical investigation of porphyrin 1 has been performed and the results are presented. The absorption of 1 extends up to 900 nm and it exhibits IR emission. Remarkably, the optical features point to the presence of states with different emissive properties. This aspect has been analyzed considering the effect of solvents and of temperature. Hypothesis on the nature of the excited states and on the mechanism of their deactivation are presented. 1.J.-P. Collin, F. Durola, J. Frey, V. Heitz, F. Reviriego, J.-P. Sauvage, Y. Trolez and K. Rissanen, J. Am. Chem. Soc. 132 (2010) 6840-6850. 2.J.-P. Collin, F. Durola, V. Heitz, F. Reviriego, J.-P. Sauvage, and Y. Trolez, Angew. Chem. 122 (2010) 10370-10373. 3.L. Flamigni, G. Marconi and M.R. Johnston Phys. Chem. Chem. Phys. 3 (2001) 4488-4494.