Electrocatalytic oxidation of pyrrole on a quasi‐reversible silver nanodumbbell particle surface for supramolecular porphyrin production

This work reports the synthesis of meso-tetrakis(4-hydroxyphenyl)porphyrin through condensation of an electrolyzed pyrrole solution with an acidified 4-hydroxybenzaldehyde at room temperature. It describes the use of silver nanodumbbell particles as sustainable electrode materials for symmetric porphyrin production. Metalation and optical characterizations together with various techniques such as ¹HNMR and ¹³CNMR confirm the synthesis of the targeted product.

Abstract

Photoactive supramolecular porphyrin assemblies are attractive molecules for light-harvesting applications. This is due to their relatively non-toxicity, biological activities and charge and energy exchange characteristics. However, the extreme cost associated with their synthesis and requirements for toxic organic solvents during purification pose a challenge to the sustainability characteristics of their applications. This work presents the first report on the sustainable synthesis, spectroscopic and photophysical characterizations of a near-infrared (NIR) absorbing Ca(II)-meso-tetrakis (4-hydroxyphenyl)porphyrin using an electrolyzed pyrrole solution. The latter was obtained by cycling the pyrrole solution across the silver nanodumbbell particle surface at room temperature. The electrolyzed solution condensed readily with acidified p-hydroxybenzaldehyde, producing the targeted purple porphyrin. The non-electrolyzed pyrrole solution formed a green substance with significantly different optical properties. Remarkable differences were observed in the voltammograms of the silver nanodumbbell particles and those of the conventional gold electrode during the pyrrole cycling, suggesting different routes of porphyrin formation. The rationale behind these formations and the associated mechanisms were extensively discussed. Metalation with aqueous Ca²⁺ ion caused a Stokes shift of 38.75 eV. The current study shows the advantage of the electrochemical method towards obtaining sustainable light-harvesting porphyrin at room temperature without the need for high-energy-dependent conventional processes.

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