title: Electrolytic electrospray ionization mass spectrometry of quaterthiophene-bridged bisporphyrins: beyond the identification tool.
abstract: Several quaterthiophene-bridged bisporphyrins were analyzed by electrospray ionization mass spectrometry (ESI-MS). The active centers of these molecular assemblies are two porphyrins moieties complexed (Z) or not (H) with a metal ion, typically Zn(2+), and the spacer is a quaterthiophene. The two end-groups were chemically linked to the quaterthiophene spacers by (i) a C--C single bond, (ii) a trans double bond or (iii) a triple bond. The formation of charged species either by protonation ([M + H](+) and [M + 2H](2+)) or electron(s) loss (M(+) and M(2+)), account for the occurrence of electrochemical processes in the basic operation of an electrospray source acting in a non-aqueous solvent. The nature of the observed charged species is correlated with the electro-oxidation properties and proton production by electro-oxidation of residual water. The occurence of these electrochemical reaction is proposed when the electroactivity of the electrosprayed substrates is not sufficient to support the current demand of the ESI source. In this way, the results obtained from the analysed series suggest the occurrence of such a process when the interfacial potential of the metal capillary reaches a value of 0.75 V vs Ag/AgCl. The results of theoretical calculations confirm the importance of the ionization energy with regard to the protonation energy in the course of the ionization reaction. The structural differences at the porphyrin-linker junctions lead to significantly smaller ionization energy in the case of the trans double bond. The MS observation of discharged dimers from molecular assemblies, including two complexed porphyrins ZZ or two free bases HH as end-group and a triple bond as the quaterthiophene-bisporphyrin junction, indicates together with molecular modelling (carried out at the semi-empirical PM3 level), that the planar and symmetric structures favour stacking.
authors.affiliation: Service Commun d'Analyses Spectroscopiques, Université d'Angers, France. david.rondeau@univ-angers.fr
authors.forename: David
authors.initials: D
authors.lastname: Rondeau
authors.forename: Françoise
authors.initials: F
authors.lastname: Rogalewicz
authors.forename: Gilles
authors.initials: G
authors.lastname: Ohanessian
authors.forename: Eric
authors.initials: E
authors.lastname: Levillain
authors.forename: Fabrice
authors.initials: F
authors.lastname: Odobel
authors.forename: Pascal
authors.initials: P
authors.lastname: Richomme