Photophysical properties of multichromophoric systems based on a truxene platform

In the frame of exploring dendritic frameworks for the construction of artificial antenna systems the planar heptacyclic polyarene 10,15-dihydro-5H-diindeno[1,2-a;1',2'-c]fluorene (truxene) has been recognized as a new appealing building block for multichromophoric arrays [1]. The interest in this large polycyclic compound derives both from the possibility of its functionalisation in three diverging directions in space, and from its role as a photoactive partner inside the array. Here we report on a series of arrays where Pt(II), Ru(II) and Os(II) centers are arranged around a truxene scaffold. Three Ru-Os dyads, a Pt-Os dyad and a Pt-Ru-Os triad, together with reference mononuclear complexes and ligands, have been synthesized and characterized. In the case of the Pt-Ru-Os triad a novel protocol for the asymmetrical derivatization of the truxene scaffold has been devised. Optical properties and energy transfer features of the arrays have been investigated and details will be presented. The results indicate that upon photoexcitation of the truxene singlet, fast energy transfer processes occur towards the metal based 3CT levels in the dyads and in the triad, that result in the final population of the Os 3CT state via an energy cascade. The nature of the CT triplet in the Pt containing arrays is influenced by the presence of the conjugated truxene scaffold and at low temperature the truxene phosphorescence is observed, also upon excitation of the Pt-based triplet; a 3Pt-3L process is in fact feasible due to the closeness in energy of the two states at low temperature. In the Ru-Os dyads a clear-cut switch from Förster to Dexter energy transfer type of mechanism has been shown to occur in passing from the truxene-metal to the metal-metal energy transfer process, the latter mediated by the conjugated truxene bridge [2].