Light-induced excited spin state trapping (LIESST) is a well-established phenomenon for transition metal complexes, especially those of the Fe(II) d6 ion.1 In the latter, a photoexcitation of the diamagnetic low-spin (LS) electronic configuration can result in effective population (via intersystem crossing) of the S = 2 high-spin (HS) state. This photogenerated state remains trapped for prolonged periods of time (hours to days) if the system is kept sufficiently cold to prevent thermally activated relaxation to the LS state at certain temperature (TLIESST). Many research efforts have been devoted to pushing the TLIESST value toward the room temperature, but the typical values rarely exceed 150 K,2 with the current record set at 180 K.3 Recently, we have reported a fascinating photomagnetic phenomenon in an all-organic material.4 A light-induced splitting of a diamagnetic S···S−S···S bridged σ-dimer of bisdithiazolyl radicals in solid state leads to the formation of a persistent paramagnetic radical state with an unusually high relaxation temperature of 242 K. This light-induced radical trapping (LIRT) is unprecedented in organic chemistry where most efforts historically have been focused on the study of transient photomagnetic states generated by singlet-triplet excitations. We monitored the LIRT effect by magnetic susceptibility measurements, optical absorption spectroscopy, and single crystal X-ray diffraction, conclusively proving the photochemical nature of this process. While conceptually similar to the LIESST effect, the LIRT phenomenon has a different mechanism that relies on the orbital forbiddance of the reverse radical-to-dimer conversion, which thus leads to a substantial energy barrier to the relaxation process. In this contribution, we aim to show the generality of the observed photomagnetic phenomenon and outline the principal requirements for its occurrence by unraveling other systems that demonstrate the LIRT effect. 1. Hauser, A. Top. Curr. Chem. 2004, 234, 155-198. 2. Létard, J. F. J. Mater. Chem. 2006, 16, 2550-2559. 3. Li, D.; Clérac, R.; Roubeau, O.; Harté, E.; Mathonière, C.; Le Bris, R.; Holmes, S. M. J. Am. Chem. Soc. 2008, 130, 252. 4. Phan, H.; Lekin, K.; Winter, S. M.; Oakley, R. T.; Shatruk, M. J. Am. Chem. Soc. 2013, 135, 15674-15677; Lekin, K.; Phan, H.; Winter, S.; Wong, J.; Leitch, A.; Laniel, D.; Yong, W.; Secco, R.; Tse, J.; Desgreniers, S.; Dube, P.; Shatruk, M.; Oakley, R. J. Am. Chem. Soc. 2014, 136, 8050-8062.

Light-induced radical trapping (LIRT): A new approach to molecular photomagnetism from old principles of organic photochemistry

249th National Meeting (2015)

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