Anion-enhanced excited state charge separation in a spiro-locked N-heterocycle-fused push-pull zinc porphyrin

Mandeep K Chahal; Anuradha Liyanage; Ajyal Z Alsaleh; Paul A Karr; Jonathan P Hill; Francis D'Souza

doi:10.1039/d1sc00038a

Anion-enhanced excited state charge separation in a spiro-locked N-heterocycle-fused push-pull zinc porphyrin

Chem Sci. 2021 Feb 24;12(13):4925-4930. doi: 10.1039/d1sc00038a.

Authors

Mandeep K Chahal¹, Anuradha Liyanage², Ajyal Z Alsaleh², Paul A Karr³, Jonathan P Hill¹, Francis D'Souza²

Affiliations

¹ International Centre for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS) Namiki 1-1, Tsukuba Ibaraki 305-0044 Japan Jonathan.Hill@nims.go.jp.
² Department of Chemistry, University of North Texas 1155 Union Circle, #305070 Denton TX 76203-5017 USA Francis.dsouza@unt.edu.
³ Department of Physical Sciences and Mathematics, Wayne State College 111 Main Street Wayne Nebraska 68787 USA.

Abstract

A new type of push-pull charge transfer complex, viz., a spiro-locked N-heterocycle-fused zinc porphyrin, ZnP-SQ, is shown to undergo excited state charge separation, which is enhanced by axial F^- binding to the Zn center. In this push-pull design, the spiro-quinone group acts as a 'lock' promoting charge transfer interactions by constraining mutual coplanarity of the meso-phenol-substituted electron-rich Zn(ii) porphyrin and an electron deficient N-heterocycle, as revealed by electrochemical and computational studies. Spectroelectrochemical studies have been used to identify the spectra of charge separated states, and charge separation upon photoexcitation of ZnP has been unequivocally established by using transient absorption spectroscopic techniques covering wide spatial and temporal regions. Further, global target analysis of the transient data using GloTarAn software is used to obtain the lifetimes of different photochemical events and reveal that fluoride anion complexation stabilizes the charge separated state to an appreciable extent.