The effect on E-stilbazoles second order NLO response by axial interaction with M(II) 5,10,15,20-tetraphenyl porphyrinates (M = Zn, Ru, Os); a new crystalline packing with very large holes

The axial coordination of push–pull E-stilbazoles carrying electron donor or electron withdrawing groups with tetraphenylporphyrinates of Zn(II), Ru(II) and Os(II) does not produce any significant increase in second order NLO response, the exception being the E-stilbazole carrying the electron withdrawing CF3 group. The lack of an increase in the second order NLO response due to the polarising action on the E-stilbazole push–pull system, by the metal centres acting as Lewis acids, is attributed to the axial π backbonding of the metal atom to the E-stilbazole, such backbonding producing a screening effect that opposes σ donation. This effect becomes dominant when the E-stilbazole carries a strong electron withdrawing group. Thus, it appears that the metals in porphyrin complexes may behave in a dual way when they interact axially with soft π delocalised ligands, such as E-stilbazoles where σ acceptor or π donor properties prevail according to the nature of the E-stilbazole itself. Infrared, electronic spectra and dipole moments suggest both axial and equatorial dissipation of the electronic density located on the metal, via π backbonding processes which influence the Lewis acid behaviour of the metal centres. A particular crystal packing, with boxes formed by four porphyrinic moieties and with large empty channels within the boxes, appears to be characteristic of axially substituted tetraphenylporphyrinates, resulting in materials able to absorb gases and solvents in their void spaces.