Synthesis, crystal structure and characterisation of aquamagnesium phthalocyanine-MgPc(H2O).: The origin of an intense near-IR absorption of magnesium phthalocyanine known as 'X-phase'

The crystals of a triclinic modification of MgPc(H2O) were obtained by dissolving MgPc in benzonitrile with a small amount of water and slow recrystallization at about 80 degreesC. In the crystal there are two independent MgPc(H2O) molecules in the asymmetric unit. The independent molecules are very similar, the 4 + 1 coordinated central magnesium atom is significantly displaced from the N-4-isoindole plane of the distorted Pc rings (similar to 0.45 Angstrom) toward the oxygen atom of coordinated water molecules. The geometry of the MgPc(H2O) molecule in the crystal (C-1-symmetry) is compared with the ab initio full-optimised geometry (C-4v) that corresponds to the conformation of the molecule in solution. In contrast to the monoclinic modification of MgPc(H2O), which is not near-IR active, this triclinic modification shows an intense broad absorption band in this spectral region that is very similar to that observed in the solid state spectrum of beta-MgPc. The near-IR absorption is characteristic only for the solid state samples since the spectra of MgPc(H2O) and beta-MgPc in pyridine solution are similar and show only one intense Q band with characteristic splitting due to the vibronic coupling in the excited state. The intense absorption band observed in the spectrum of solid state of MgPc(H2O) as well as in the spectrum of beta-MgPc originates from the arrangement of the molecules in dimers with strong pi-pi interactions between the distorted Pc macrorings. The molecular distortion (reduction in the symmetry C-4v --> C-1) lifts the double degeneracy of the excited state. (C) 2003 Elsevier Science Ltd. All rights reserved.