title: Flexible Tuning of Unsaturated β-Substituents on Zn Porphyrins: A Synthetic, Spectroscopic and Computational Study.
abstract: A series of zinc porphyrins substituted at adjacent β-positions with a CN group and para-substituted ethenyl/ethynyl-phenyl group have been studied using electronic absorption spectroscopy, resonance Raman spectroscopy and DFT calculations. The oxidative nucleophilic substitution of hydrogen was utilized for the introduction of a cyano substituent on the porphyrin ring. This modification has a remarkable electronic effect on the ring. The resulting porphyrin cyanoaldehyde was further modified in Wittig condensations to give series of arylalkene- and arylalkyne-substituted derivatives. This substitution pattern caused significant redshifting and broadening of the B band, tuning from 433-446 nm. Additionally the Q/B band intensity ratios show much higher values than observed for the parent porphyrin ZnTPP (0.20 vs. 0.03). Careful analysis of the electronic transitions using DFT and resonance Raman spectroscopy reveal that the substituent does not significantly perturb the electronic structure of the porphyrin core, which is still well described by Gouterman's four-orbital model. However, the substituents do play a role in elongating the conjugation length and this results in the observed spectral changes.
authors.affiliation: Department of Chemistry, University of Otago, Union Place, 9016 Dunedin (New Zealand).
authors.forename: Holly
authors.initials: H
authors.lastname: van der Salm
authors.affiliation: Intelligent Polymer Research Institute, University of Wollongong, Wollongong 2522 NSW (Australia). pawel@uow.edu.au.
authors.forename: Pawel
authors.initials: P
authors.lastname: Wagner
authors.affiliation: Intelligent Polymer Research Institute, University of Wollongong, Wollongong 2522 NSW (Australia).
authors.forename: Klaudia
authors.initials: K
authors.lastname: Wagner
authors.affiliation: Intelligent Polymer Research Institute, University of Wollongong, Wollongong 2522 NSW (Australia).
authors.forename: David L
authors.initials: DL
authors.lastname: Officer
authors.affiliation: Intelligent Polymer Research Institute, University of Wollongong, Wollongong 2522 NSW (Australia).
authors.forename: Gordon G
authors.initials: GG
authors.lastname: Wallace
authors.affiliation: Department of Chemistry, University of Otago, Union Place, 9016 Dunedin (New Zealand). keith.gordon@otago.ac.nz.
authors.forename: Keith C
authors.initials: KC
authors.lastname: Gordon
keywords: DFT calculations; Raman spectroscopy; electronic absorption spectroscopy; porphyrins; zinc