Towards a better understanding of photo-excited spin alignment processes using silole diradicals

The synthesis of two nitroxide-based diradicals connected to a 2,3,4,5-tetraphenylsilole (TPS) unit, especially designed to present high spin photo-excited states, is reported. While the bisnitronylnitroxide (NN) silole-based diradical, TPSNN, is unstable and experiences a spontaneous fragmentation of its imidazolinic ring into a iso-butylammomium salt, the corresponding bisiminonitroxide (IN), TPSIN, is stable both in solution and in the solid state. This diradical crystallizes in the triclinic P-1 space group with a = 10.984(1), b = 11.474(1), c = 17.492(1) angstrom, alpha = 81.10(1), beta = 89.01(1), and gamma = 65.71(2)degrees. Ground state magnetic properties of TPSIN have been investigated by means of SQUID and ESR measurements: the diradical displays weak intramolecular antiferromagnetic interactions (J/k(B) approximate to -1 K), in agreement with its topology and with the molecular packing observed in its crystal structure. In order to investigate the magnetic photo-excited states of TPSIN, time-resolved ESR experiments (TRESR) have been performed on this radical species. Despite the presence of both an appropriate topology for the diradical and a triplet photo-excited state for the TPS coupler, no TRESR signal was observed for this molecule within the timescale of the measurement. In addition to the work already published in this field, this result clearly indicates that besides the radical nature, the pi-topological requirements and the need of photo-tunable spin-states for the coupler, the flexibility of the molecule also plays a crucial role in the achievement of photo-induced spin alignment processes.