Molecular Structure, Optical, and Magnetic Properties of Free‐Base Naphthalocyanine Dianions

Abstract

The reduction of free‐base naphthalocyanine (H2Nc) with sodium fluorenone ketyl and an excess of bis(triphenylphosphoranylidene)ammonium chloride (PPN+)(Cl–) unexpectedly results in the synthesis of the (PPN+)2(H2Nc2–) salt (1). As sodium fluorenone ketyl can generate only H2Nc·– radical anions, the formation of H2Nc2– is most probably realized through the disproportionation of H2Nc·– to H2Nc0 and H2Nc2–. The H2Nc2– dianions in 1 form layers in which π–π stacks of H2Nc2– are formed along the a direction with a short interplanar distance of 3.30 Å and a HOMO–HOMO (HOMO = highest occupied molecular orbiatl) overlap integral of 3.1 × 10–3. The H2Nc2– macrocycle is distorted owing to the formation of alternate short and long C–N(imine) bonds. The distortion of the macrocycle increases with the negative charge on H2Nc owing to the occupation of the lowest unoccupied molecular orbital (LUMO). Calculations indicate that the distortion disappears in the triplet state of H2Nc2– as the occupation of the LUMO and LUMO+1 orbitals averages their effects on the geometry of the macrocycle. The optical spectrum of 1 shows a lowest energy absorption band at λ = 1230 nm, which is attributed to the HOMO → LUMO+1 excitation and a blueshift of the Q‐band of H2Nc. The elecron paramagnetic resonance (EPR) silence of 1 indicates the diamagnetic singlet state of the H2Nc2– dianions. The DFT calculations indicate that the open‐shell broken‐symmetry singlet state is lower in energy than the corresponding closed‐shell singlet and triplet states. The large singlet–triplet energy gap indicates that the corresponding triplet state is thermally inaccessible.