Synthesis and single crystal structure of a new polymorph of 5, 10, 15, 20-tetrakis-(4-chlorophenyl) porphyrin, H₂TTPCl₄: Spectroscopic investigation of aggregation of H₂TTPCl₄

5, 10, 15, 20-tetrakis-(4-chlorophenyl) porphyrin, H₂TTPCl₄, has been isolated as a new polymorph following the modified Adler’s method and its X-ray crystal structure solved. The new polymorph (<b>I</b>) crystallises in the monoclinic space group, P2₁/n with a = 10.1574(5) Å, b = 8.9827(4) Å, c = 20.9350(8) Å, β = 102.532(4)°, V = 1864.62(15) ų, Z = 2. The previously found polymorph, prepared using Lindsay method was crystallised in monoclinic space group, P2₁/a with a = 15.776 (13) Å, b = 8.646 (3) Å, c = 14.087 (5) Å, β = 96.05 (5)°, V = 1910.7 (3) ų, Z = 2. The main difference between the two polymorphs seems to be the different packing arrangement of molecules in their crystal lattices. The dramatic self aggregation property of new polymorph (<b>I</b>) has also been investigated. The investigation reveals that under certain conditions of solute concentration and pH of the media, the compound exhibits a strong tendency to exist in a prominent self aggregated state in head-to-tail type (j-aggregation) molecular alignment. The self association behaviour of (<b>I</b>) was confirmed by UV-Vis absorption spectra, performed in chloroform by varying concentrations and pH changes and ¹H NMR spectra, performed in deuterated chloroform at varying concentrations and their results have been discussed in detail.