Higher triplet state of fullerene C70 revealed by electron spin relaxation

Spin-lattice relaxation times T-1 of photoexcited triplets C-3(70) in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T-1 was fitted by the Arrhenius law with an activation energy of 172 cm(-1). This indicates that the dominant relaxation process of C-3(70) is described by an Orbach-Aminov mechanism involving the higher triplet state t(2) which lies 172 cm(-1) above the lowest triplet state t(1). Chemical modification of C-70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T-1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in C-3(70) and its absence in triplet C-70 derivatives. The presence of the higher triplet state in C-70 is in good agreement with the previous results from phosphorescence spectroscopy. (C) 2015 AIP Publishing LLC.