Monotrimethylene-bridged bis- P -Phenylenediamine radical cations and dications: Spin states, conformations, and dynamics

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 (CH₂)₃ to dimethylated CH ₂CMe₂CH₂ 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.