Aggregation of a Crown Ether Decorated Zinc-Phthalocyanine by Collision-Induced Desolvation of Electrospray Droplets

The aggregation of phthalocyanines is well-known in solution but has never before been studied in the gas phase. We investigated the tetra-[18]crown-6 ether functionalized zincphthalocyanine (ZnPcTetCr, M) with electrospray ionization mass spectrometry (ESI-MS) in the absence of coordinating metal cations. Apart from the molecular ion M+center dot, singly and multiply charged aggregates M-n(z)((+center dot)) were observed, bound together by electrostatic interactions, without alkali metal cations inside the crown ethers. Collision-induced dissociation (CID) experiments indicate that these clusters consist of stacked neutral M and radical cations M+center dot. After the oxidation of individual molecules at the electrospray needle, the aggregation occurs during desolvation of the charged droplets created in the source. Complete evaporation of the solvent and detection of the aggregates was found to require an additional acceleration of the droplets in the transfer region of the instrument, the resulting collisions with neutral gas assisting the desolvation process.