TEMPO (2,2,6,6-tetramethylpiperidinyloxy) radical 1 and its derivative
s 2-5 are found to form charge-transfer complexes with some accepters
such as TCNQF(4) ,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) or
DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) being isolated as sol
id substances.