A DFT study of electronic structure, magnetic properties and cyclization reaction of [5]helicene derivatives

A series of 1,14-dimethyl-[5]helicene derivatives comprising radicals ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 1,2,3,5-dithiadiazolyl, 1,5-dimethyl-6-oxo-verdazyl and 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3oxide) was investigated by quantum-chemical modeling of their electronic structure, exchange interactions and cyclization reaction. The performed DFT study of 1,14-dimethyl-[5]helicene, supported by DFT/CASSCF and DFT/NEVPT2 calculations, have shown that cyclic form of this compound represents a singlet biradical. The energy barriers of transition between open and cyclic forms of 1,14-dimethyl-[5]helicene radical derivatives are comparable to the values characteristic of enantiomerization reaction, which indicates a high probability of formation of cyclic isomers under thermal conditions. The TD DFT calculations of electronic transitions predicted the presence of non-overlapping bands in the studied isomers that favors switching between them by irradiation. The variation in a number of paramagnetic centers as a result of interconversion of open and cyclic forms, and the presence of structures bearing strong ferromagnetic exchange interactions, make the considered [5]helicene derivatives a suitable basis for molecular electronics devices.