Electronic spectroscopy and structure of gas-phase molecular ions

Abstract

This thesis describes investigations of gas-phase molecular cations --- protonated azobenzenes, ruthenium sulfoxide coordination complexes, and the ferrocenium and tropylium cations --- using electronic action spectroscopy. Protonated azobenzenes and ruthenium sulfoxide complexes are prototypical molecular photoswitches, which reversibly change isomeric form in response to light, and are building blocks of light-activated molecular machines. The ferrocenium cation is, along with its reduced form ferrocene, a common redox standard and an archetypal organometallic complex. Tropylium is a 6 pi-aromatic molecule with rare sevenfold symmetry and is involved in the combustion of aromatic hydrocarbons. The electronic spectra of these molecular cations, recorded by measuring photoisomerisation or photodissociation yields as a function of wavelength, provide insight into their electronic and molecular structures.

Action spectra of the protonated azobenzene and 4-aminoazobenzene cations are obtained by measuring the trans-to-cis photoisomerisation yield over the 350--600 nm range. One band corresponding to the S1 <- S0 pi-pi* transition is observed for each species with maxima at 435 and 525 nm for protonated azobenzene and 4-aminoazobenzene, respectively. The transitions are assigned based on density functional theory and coupled cluster calculations. The experimental and computational results indicate that photoisomerisation occurs following pi-pi* excitation in protonated azobenzenes and that the lowest-energy pi-pi* transitions are significantly red-shifted compared to their neutral azobenzene counterparts.

Ruthenium bis-sulfoxide bipyridyl coordination complexes undergo linkage photoisomerisation, where the binding modes of two sulfoxide groups in a chelating ligand change from S- to O-bound following electronic excitation. Photoisomerisation is observed over the 295--515 nm range following excitation of distinct metal-to-ligand charge transfer and ligand-centred pi-pi* transitions. Concerted isomerisation of both sulfoxide groups from S- to O-bound following single-photon absorption is the predominant gas-phase pathway, whereas sequential isomerisation of each sulfoxide group, requiring two photons of two different wavelengths, is the only process observed in solution. Comparison of the gas-phase results with time-resolved experiments in solution suggest the apparent change in dynamics upon solvation is caused by rapid quenching of vibrational energy in the electronically-excited state associated with isomerisation.

Electronic spectra of the ferrocenium cation, Fe(C5H5)2+, are measured over the 560--640 nm range through photodissociation of its weakly-bound complexes with neon and argon. Band systems with origin transitions at 15830 cm-1 and 15809 cm-1 for the neon and argon complexes, respectively, are assigned to the B 2E1' <- X 2E2' ligand-to-metal charge transfer transition, on the basis of multireference electronic structure calculations. The dominant vibrational progression corresponds to the totally-symmetric ligand-metal-ligand stretching mode v4 with a spacing of ~285 cm-1. Several non-Franck-Condon bands likely arise from a weak Jahn-Teller effect or from spin-orbit coupling. The vibronic bands measured for the neon complexes are sharp, whereas those for the argon complexes are broad and show substructure.

Vibronic coupling in the S1 1E3' state of the tropylium cation, C7H7+, is characterised using density functional theory, coupled cluster, and multiconfigurational self-consistent field electronic structure calculations. The weak, forbidden A 1E3' <- X 1A1' transition, recorded over the 250--285 nm range through photodissociation of tropylium-argon complexes, is found to gain intensity through Herzberg-Teller coupling to the bright B 1E1' state mediated by vibrational modes with e2' and e3' symmetries. A weak Jahn-Teller effect occurs in the A 1E3' state associated with the v7 (e1') mode, although its influence on the electronic spectrum is predicted to be small.

The spectroscopic investigations of these molecular cations lay the foundations for further exploration of their photoisomerisation dynamics and vibronic interactions. Time-resolved experiments should complement the photoisomerisation action spectra by following the excited-state dynamics of these photoswitches in real time. The vibrationally-resolved spectra of ferrocenium and tropylium should provide starting points for high-resolution spectroscopic investigations of these cations. The reported spectra should serve as benchmarks for accurate electronic structure theories, which may help to guide future experiments.