Towards a Better Understanding of Photo-excited Spin Alignment Processes Using Silole Diradicals.

International audienceStable bis-nitroxide diradicals I (TPSNN, X = O-; TPSIN, X and charge absent), conjugated with silole moiety via 1,4-phenylene groups, were prepd. by reductive cyclization of Me2Si(C.tplbond.CPh)2 followed by cross-coupling of bis-zincated silole with 1,3-dihydroxy-4,4,5,5-tetramethyl-2-phenylimidazolidine with subsequent oxidn. The two nitroxide-based diradicals connected to a 2,3,4,5-tetraphenylsilole (TPS) unit were designed to feature high-spin photo-excited states. While the bisnitronylnitroxide (NN) silole-based diradical, TPSNN, is unstable and experiences a spontaneous fragmentation of its imidazoline ring into a isobutylammonium salt, the corresponding bisiminonitroxide (IN), TPSIN, is stable both in soln. 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) .ANG., a = 81.10(1), b = 89.01(1), and g = 65.71(2)°. Ground state magnetic properties of TPSIN were investigated by means of SQUID and ESR measurements; the diradical displays weak intramol. antiferromagnetic interactions (J/kB » -1 K), in agreement with its topol. and with the mol. packing obsd. in its crystal structure. In order to investigate the magnetic photo-excited states of TPSIN, time-resolved ESR expts. (TRESR) have been performed on this radical species. Despite the presence of both an appropriate topol. for the diradical and a triplet photo-excited state for the TPS coupler, no TRESR signal was obsd. for this mol. within the timescale of the measurement. In addn. to the work already published in this field, this result clearly indicates that besides the radical nature, the p-topol. requirements and the need of photo-tunable spin-states for the coupler, the flexibility of the mol. also plays a crucial role in the achievement of photo-induced spin alignment processes