Mapping the low-lying excited states of aromatic and antiaromatic ions via experiment and theory

Aromaticity is one of the most fundamental concepts in chemistry. Huckel’s pivotal model of aromaticity proposed that planar cyclic -conjugated molecules with 4n+2  electrons are energetically stable and aromatic, while molecular systems with 4n  electrons are energetically unstable and antiaromatic. The idea that aromaticity and antiaromaticity can apply to molecules in their electronic excited states was first proposed by Baird,1 who argued that the ground state aromaticity/antiaromaticity rules in cyclic conjugated hydrocarbons are reversed in their lowest triplet states. Subsequently, the extension of Baird’s rules to the first singlet excited state was de facto formulated and proved by Karadakov in 2008.2,3 Excited state reversal is highly significant because completely reversed aromaticity in the excited state provides crucial insight into photostability, photoreactivity, and potential applications in photosynthetic mechanisms and photoreactive materials.4 While a great deal of theoretical effort has been applied to demonstrate aromaticity reversal in the excited state,4 experimental confirmation has been sparse to date. Very recently,5 a number of high-impact experiments have been applied to this area including time-resolved, transient absorption spectroscopy and time-resolved electron diffraction.6,7 Although these experiments have provided strong indications confirming ground-excited state reversal for aromatic and antiaromatic systems, these recent experiments have not provided the clearest route for benchmarking theoretical calculations against directly comparable experimental results.
In this project, we propose to perform a new series of experiments where aromatic and antiaromatic molecular ions are produced in the gas-phase, and their electronic spectroscopy is studied via laser photodissociation spectroscopy within a laser-interfaced mass spectrometer.8 We will directly compare the electronic spectra obtained with high-level computational chemistry predictions (which will also be performed as part of the project) to verify whether the excited states correspond to the expected reversed aromatic/antiaromatic states. The initial aromatic and antiaromatic ions will be produced via direct electrospray ionization (ESI), collision-induced dissociation (CID) of suitable precursors, or laser photodissociation of suitable precursor ions.
Methods
Laser photodissociation measurements will be conducted in a laser-interfaced mass spectrometer (LIMS) which includes an ESI source that can be used to transfer a wide range of molecular ions directly into the gas-phase. Ions will be mass selected and isolated in an ion trap prior to CID and/or laser excitation. Electronic spectra will be recorded via action spectroscopy, and all photofragmentation pathways will be monitored. The geometries of the aromatic and antiaromatic molecular ions will be optimized using high-level ab intio quantum chemistry approaches such as complete-active-space self-consistent field (CASSCF) and equations-of-motion coupled-cluster (EOM-CC). Chemical bonding, aromaticity and antiaromaticity will be analysed using computed magnetic criteria including nucleus-independent chemical shifts (NICS), magnetic susceptibilities and magnetic shielding contour plots and isosurfaces.2,3
Project Plan Outline
We envisage that the project will employ three different approaches to produce suitable molecular ions that we can then study with spectroscopy. The electronic spectra will all be recorded via laser photodissociation in the gas-phase providing highly detailed spectra over the very wide spectral range available from an OPO laser system. The spectra will be compared against theoretical calculations, conducted as described above. Methods to produce the aromatic/antiaromatic ions will be:
• Electrospray of suitable solutions, e.g. tropylium cations.9
• Collision Induced Dissociation techniques, e.g. the anthracenyl anion.10
• Photodissociation of precursor ions. Similar to the CID route, but a photon is used to generate the desired ion.8

All research students follow our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills.

The experimental part of the project will be conducted in the Dessent group and training will include mass spectrometry, (soft ionization techniques, CID, and mass spectral analysis), laser spectroscopy (use of class 4 laser systems and diode lasers), as well as routine handling of gas and vacuum systems. Full training in data analysis, data storage, and data presentation will also be provided. In the Karadakov group, full training in computational chemistry techniques (ab initio and DFT). This training will include the use of non-trivial methods for excited states such as CASSCF which require careful construction of the wavefunction and assessment of the ordering of the low-lying excite states, as well as some programming in Fortran and/or Python. Mass spectrometry and computational chemistry are both highly-valuable, transferable skills.

The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students: https://www.york.ac.uk/chemistry/ed/. This PhD project is available to study full-time or part-time (50%).

This PhD will formally start on 1 October 2019. Induction activities will start on 30 September.