*This involves synthetic organic chemistry of macrocycles. Molecular sensors, nonlinear optical materials, electronic structure of porphyrin aggregates, dendrimer-porphyrin self-assemblies, liquid crystals, Langmuir-Blodgett films and their photophysical properties. Fluorescence spectroscopy of porphyrins and their fluorescence based sensor applications. Supramolecular chemistry of porphyrin/fullerene cocrystallates. Application of porphyrins in photodynamic therapy.
The summary of our selected research work is presented below:
1. Mixed β-substituted tetraphenylporphyrins: Unlike porphyrins with similar peripheral substituents, the introduction of mixed substituents at the antipodal β-pyrrole positions of the versatile synthetic analogue, meso-tetraphenylporphyrin, H2TPP revealed unique physicochemical properties. The precursor 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrin, H2TPPBr4 was prepared regioselectively by our group in very good yield.1 It was further derivatised to produce other antipodal mixed beta-octasubstituted porphyrins and their metal complexes in good to excellent yields. These porphyrins revealed tunable electronic absorption and structural3 properties.
Scheme 2. Synthesis of unsymmetrically mixed substituted porphyrins.
Tunable physicochemical properties of mixed substituted porphyrins.
2. Supramolecular assemblies of β-tetrasubstituted meso-tetraphenylorphyrins: A new family of β-tetra(phenylethynyl/4′-methoxyphenyl/4′-carboxymethylesterphenyl)-meso-tetra(4′-n-butyl phenyl)-porphyrins and their metal complexes have been synthesized and explored their physicochemical properties. Notably, single crystal XRD analysis of MT(4′-BuPh)P(PE)4 (M = 2H, Co(II), Cu(II) and Zn(II)) derivatives showed supramolecular assemblies in the solid-state. Few of these porphyrins serve as precursors for the synthesis of new type of molecular building blocks for the generation of framework solids.
3. Supramolecular chemistry of porphyrin/fullerene cocrystallates: Because of the flexible nature of the macrocycle in antipodal beta-tetrasubstituted TPPs, they serve as versatile hosts for the spherical fullerenes. The cocrystallization of planar MTPP(Ph)4 with C60 generated non-solvated cocrystallates, MTPP(Ph)4•C60 (M = 2H, Co(II) and Cu(II)) bearing nonplanar macrocycles. This is evidenced from the deviation (DCb) of the β-pyrrole carbons by ±0.42 Å relative to the planar parent 24-atoms macrocycle (DCb, ±0.01 Å). Surprisingly, the crystallisation of nonplanar (DCb, ±0.54–0.71 Å), MTPP(2′-thienyl)4 (M = Co(II) and Cu(II)) porphyrins with C60 produced MTPP(2′-thienyl)4•C60 cocrystallates with planar macrocycle (DCb, ±0.08 Å). The unusual switch in conformation of the macrocycle in these cocrystallates has been ascribed to greater attractive interaction between porphyrin and C60 and/or crystal packing forces.
4. Nonlinear optical materials: The influence of substituents on the optical nonlinearities of a series of selectively mixed β-pyrrole functionalized tetraphenylporphyrins, MTPP(CHO)(R)2 (R = H, Br, 2-thienyl, Ph, phenylethynyl (PE)) and their metal (Cu(II), Zn(II)) complexes have been examined in toluene. An increase in the values of third order nonlinear susceptibility (3) and the second order hyperpolarizabilty ⟨⟩ for electron-withdrawing groups was observed whereas an opposite trend was noticed for the electron-donating groups at the beta-pyrrole positions. Fluorescence measurements on MTPP(CHO)(R)2 derivatives revealed red-shifted emission bands relative to corresponding MTPPs. In some cases, the increased radiationless transition was noticed perhaps due to nature of the metal ion and also the distortion of the porphyrin ring.
5. Sterically hindered liquid crystalline porphyrins and their molecular recognition properties: A series of sterically hindered meso-tetrakis(2′,6′-di-n-alkyloxyphenyl)porphyrin, H2T(2′,6′-OCnP)P and their metal (Cu(II) and Zn(II)) complexes were synthesised. These porphyrins exhibited low melting liquid crystalline behaviour due to decreased pi-pi stacking interactions. The molecular recognition properties of these sterically hindered Zn(II)-centre was probed by axial ligation of nitrogenous bases of varying shapes and sizes in the condensed media. These Zn(II)-porphyrins showed enhanced Keq values relative to less sterically hindered 2,6-dimethoxy Zn(II)-porphyrin, ZnT(2′,6′-DMP)P indicating greater attractive interaction between the amines with the hydrophobic pocket on both faces of the porphyrin. The greater selectivity of ZnT(2′,6′-OC4P)P to butylamine has been ascribed to similar size as evidenced from its crystal structure.