The electronic structure and deexcitation pathways of an isolated metalloporphyrin ion resolved by metal L-edge spectroscopy

Abstract

The local electronic structure of the metal-active site and the deexcitation pathways of metalloporphyrins are crucial for numerous applications but difficult to access by commonly employed techniques. Here, we applied near-edge X-ray absorption mass spectrometry and quantum-mechanical restricted active space calculations to investigate the electronic structure of the metal-active site of the isolated cobalt(iii) protoporphyrin IX cation (CoPPIX⁺) and its deexcitation pathways upon resonant absorption at the cobalt L-edge. The experiments were carried out in the gas phase, thus allowing for control over the chemical state and molecular environment of the metalloporphyrin. The obtained mass spectra reveal that resonant excitations of CoPPIX⁺ at the cobalt L₃-edge lead predominantly to the formation of the intact radical dication and doubly charged fragments through losses of charged and neutral side chains from the macrocycle. The comparison between experiment and theory shows that CoPPIX⁺ is in a ³A_2g triplet ground state and that competing excitations to metal-centred non-bonding and antibonding σ* molecular orbitals lead to distinct deexcitation pathways.

Fig. 1. Structure of metalloporphyrins. The peripheral side chains of the cobalt(iii) protoporphyrin IX cation (CoPPIX⁺) are listed on the right.

Fig. 2. (a) Mass spectrum of CoPPIX⁺ upon resonant absorption at 780.5 eV at the cobalt L₃-edge after subtraction of the below-edge mass spectrum (see Experimental section). (b) Below-edge mass spectrum of CoPPIX⁺. The mass ranges of doubly and triply charged fragments are indicated in green and yellow, respectively.

Scheme 1. Possible photo products upon single and multiple ionisation after resonant cobalt 2p inner-shell excitation of CoPPIX⁺ at 780.5 eV at the cobalt L₃-edge. Photo products which are not observed in the mass spectrum in Fig. 2 and in ESI-2 are crossed out in red.

Fig. 3. Zoom into the mass spectrum of CoPPIX⁺ upon resonant absorption at 780.5 eV at the cobalt L₃-edge after subtraction of the below-edge mass spectrum (see Experimental section, see Fig. 2a). The assignments of the observed peaks are indicated.

Fig. 4. Structure of CoPPIX⁺. The indicated bond cleavages lead to the observed fragments in Fig. 2 and in Fig. 3.

Fig. 5. Comparison between the experimental TIY spectrum and the calculated XAS spectrum of CoPPIX⁺ at the cobalt L-edge, including the orbital-contribution analysis. The positive and negative values of the orbital contribution curves represent electron gain and loss in the transitions from initial state to core-excited states, the constant one-electron loss in the 2p core orbital is not shown.

Fig. 6. (a) Calculated two main orbital contributions to the cobalt L-edge XAS of CoPPIX⁺ (see Fig. 5). (b–e) Measured PIY spectra of product ions produced after single Auger decay following inner-shell excitation of cobalt 2p electrons of CoPPIX⁺ in the photon energy range 770–804 eV. For better comparison the PIY spectra are normalised to the maximum of peak C.

Fig. 7. (a) Calculated two main orbital contributions to the cobalt L-edge XAS of CoPPIX⁺ (see Fig. 5). (b–d) Measured PIY spectra of product ions produced upon multiple ionisation following inner-shell excitation of cobalt 2p electrons of CoPPIX⁺ in the photon energy range 770–804 eV. For better comparison the PIY spectra are normalised to the maximum of peak C.

Scheme 2. Orbital-specific deexcitation pathways of CoPPIX⁺ after resonant cobalt 2p inner-shell excitation at the L₃-edge to selected valence molecular orbitals from RAS calculation.