Versatile Synthetic Route for Β-Functionalized Chlorins and Porphyrins By Varying the Size of Michael Donors: Syntheses, Photophysical and Electrochemical Redox Properties
Authors: Grover Nitika , Chaudhri Nivedita , Sankar Muniappan 
Published: February 2020

The β-functionalization of meso-tetraphenylporphyrins is of great interest since the electronic properties of the macrocycle can be altered by small changes in the substituents.1
β-nitroporphyrins exhibited interesting catalytic, photophysical and nonlinear optical properties2 and are important precursor for further functionalization to attain a wide variety of chlorins.3 The electronic properties of chlorins allow to capture of more light of longer wavelength region for light harvesting, PDT, and singlet oxygen generation.4 To better understand the effects of distinct substituents, Herein we have developed rational methods for the synthesis of stable tailorable chlorins and porhyrins, wherein each chlorin bears vicinal groups at the reduced pyrrole ring whereas each porphyrin bears mono-substituent at β-position. The size dependent approach has been applied for the fine tuning of product formation from porphyrins to chlorins. The regioselectivity displayed in these reactions highlight the importance of method that facilitates the direct and predictable introduction of cyclic Michael donors at β-pyrrolic position.5

A hypsochromic shift (~ 8-10 nm) in the longest wavelength (Qy) absorption band was observed upon substitution of electron withdrawing groups (IND and BENAC) at 2,3-postion of meso-tetraphenylchlorin. Single crystal X-ray diffraction analysis revealed the quasi planar to moderate nonplanar conformation of chlorins due to trans orientation of β-substituents, whereas porphyrin exhibited higher mean plane deviation from 24-atom core (Δ24) as compared to chlorins. Electronic nature of synthesized macrocyclic skeletons was authenticated by electrochemical and protonation/deprotonation studies which revealed that the porphyrin core was more susceptible towards protonation, whereas deprotonation of chlorin macrocycle was more facile.

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