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

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

doi:10.1039/d1sc00038a

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.

Highlights

Molecules decorated with donor and acceptor moieties electronically conjugated through an appropriate intervening linking unit are o en referred to as D–p–A molecules or ‘push–pull’ chromophores
We have considered a unique system containing two chromophores in which an electron rich phenol-substituted metalloporphyrin[16] is fused with an electron de cient imidazo aInternational Centre for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
The push–pull charge transfer molecule ZnP-SQ, a novel spiro-locked N-heterocycle-fused zinc porphyrin, undergoes excited state charge separation, which is enhanced by axial FÀ binding to the Zn centre

Summary

Introduction

Molecules decorated with donor and acceptor moieties electronically conjugated through an appropriate intervening linking unit are o en referred to as D–p–A molecules or ‘push–pull’ chromophores. In order to monitor excited state charge transfer events in the present push–pull system, it was necessary to replace Ni(II) with Zn(II) in the porphyrin complex since the Ni(II) complex is essentially non- uorescent.

Results

Conclusion