title: Monotrimethylene-bridged bis-p-phenylenediamine radical cations and dications: spin states, conformations, and dynamics.
abstract: The properties of p-phenylenediamine- (PD-) based systems substantially depend on the molecular topology. The singly bridged PD analogues HMPD and OMPD in which the PD rings are connected by a flexible linker reveal particular electronic properties in their radical cations and dications. The EPR and UV-vis spectra of HMPD(2+••) were found to be exceptionally temperature-sensitive, following a change from the extended conformation (doublet-doublet state) predominant at room temperature to the π-stacked conformation (singlet state) prevailing at dry-ice temperature. Changing the single bridge from (CH(2))(3) to dimethylated CH(2)CMe(2)CH(2) in OMPD(2+••) causes considerably less of the π-stacked conformation to be present at low temperature as a result of the steric interactions with the methyl groups of the bridge. In contrast to HMPD(2+••) and OMPD(2+••), in which the positive charges are localized separately in each PD(+•) ring, in the extended conformation, exchange of the electron ("hole hopping") between the two PD units (fast at the time scale of EPR experiments) was observed for HMPD(+•) and OMPD(+•). This process slows in a reversible manner with decreasing temperature, thus forming the radical cation with the unpaired electron spin density predominantly on one PD core, at low temperatures. Accordingly, a subtle balance between conformational changes, electron delocalization, and spin states could be established.
authors: Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4/I, A-8010 Graz, Austria. zalibera.m@gmail.com
authors: Michal
authors: M
authors: Zalibera
authors: Almaz S
authors: AS
authors: Jalilov
authors: Stefan
authors: S
authors: Stoll
authors: Ilia A
authors: IA
authors: Guzei
authors: Georg
authors: G
authors: Gescheidt
authors: Stephen F
authors: SF
authors: Nelsen