Crystal Structure, Hirschfeld Surface Analysis, and Absorption Spectra of a New Polymorph of Highly Symmetric Tetraphenylporphyrinato Zn(II), ZnTPP

Abstract

The crystal and molecular structure of a new polymorph of four-coordinated Zn(II) tetraphenylporphyrin, with formula ZnTPP (TPP = 5,10,15,20-tetraphenyl porphyrin) has been determined from 3D X-ray diffraction data. The new polymorph crystallizes in the tetragonal space group, I4/m with cell dimensions a = 13.4206(9), c = 9.7210(16) A, V = 1750.9(3) A3, and Z = 4. The previous polymorph was crystallized in triclinic space group, P $$\bar {1}$$ . The single crystal of toluene solvate, a pseudopolymorph of tetraphenylporphinatozinc(II), ZnTPP ⋅ 2C7H8 was also crystallized in the triclinic space group, P $$\bar {1}$$ . The porphyrin cores of all the three centrosymmetric crystalline forms adopt a planar conformation which is attributed to the equivalent size of Zn2+ cations and that of porphyrin cores. In contrast to the two previous triclinic structures, which have two crystallography unique pyrrole rings, the tetragonal structure in the present form has all the pyrrole rings symmetric in nature. Further the symmetric orientations of phenyl rings about the mean N4 plane in the present form, unlike the two previous triclinic forms in which phenyl rings makes two types of dihedral angles with mean N4 plane reflects that the tetragonal structure of ZnTPP is the highly ordered geometry. All the four Zn–N bond distances in the present form are equal i.e., 2.048(2) A and is slightly longer than the average Zn–N bond distances found in the previous triclinic forms. The 3D packing of present form features sandwich type or face to face stacked layers that are bridged by a body centred molecule. The crystal packing is mainly stabilised by C–H⋅⋅⋅C and H⋅⋅⋅H intermolecular forces. Further the role and nature of various intermolecular interactions were quantitatively analysed by 3D Hirschfeld surface analysis and associated 2D fingerprint plots. The optical absorption spectra of ZnTPP and its free base analog, H2TPP have been further analysed in pure organic solvents of CH2Cl2 and CH3OH at different ionic strengths of the media and their results have been discussed in detail.