Bis(phosphine Imide)s: Easily Tunable Organic Electron Donors

The electrochemical, structural, and spectroscopic properties of bis(phosphine imide)s have been
investigated. p-Phenylenebis(phosphine imide)s Ar3PNC6H4NPAr3 (1a−d) have two reversible
single-electron oxidations. The first oxidation potentials can be varied from −0.05 to 0.15 V (versus
SCE) by modification of the substituents on phosphorus (Ar). Electron-donating substituents lower
the oxidation potential, while electron-withdrawing substituents increase the oxidation potential.
The difference between the first and second oxidation potential (ΔE, 0.41−0.50) and the electronic
coupling (Hab, 1.1 eV) are similar for 1a−d. Computational (DFT) and UV−visible−NIR spectroscopic
investigations of 1a−d suggest that the first oxidation leads to a delocalized radical cation 1a•+
while the second oxidation leads to a quinonoidal dicationic state 1a2+. The aromatic linker between
phosphine imides has also been modified. Upon oxidation, N,N‘-4,4‘-biphenylene(bis(triphenyl)phosphine imide) (3) forms radical cationic and a dicationic species similar to 1a−d. While ΔE
(0.18 V) and Hab (0.63 eV) are smaller, suggesting weaker electronic communication between the
two PN units in the radical cationic state, the presence of NIR absorptions with vibrational fine
structure (768, 861, and 983 nm) supports the formation of delocalized radical cation for 3•+.