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

Background:
In Hückel’s pivotal model of aromaticity, conjugated cycles with 4n+2 π electrons are energetically stable and aromatic, while those with 4n π electrons are energetically unstable and antiaromatic. The idea that aromaticity and antiaromaticity apply to electronic excited states was first proposed by Baird [1] who argued that ground state aromaticity rules in cyclic conjugated hydrocarbons are reversed in their lowest triplet states. The extension of Baird’s rules to the first singlet excited state was advanced by PBK in 2008 [2]. Excited state aromaticity reversal is significant because it provides insights into photostability, photoreactivity, with applications in photosynthetic mechanisms, photoreactive materials [3] and molecular motors [4]. A great deal of theoretical effort has been applied to demonstrate aromaticity reversal in the excited state, but experimental confirmation has been sparse. Recently [5], high-impact experiments have been applied to this area including time-resolved, transient absorption spectroscopy and time-resolved electron diffraction. Although these experiments provide indications of excited state reversals for aromatic and antiaromatic systems, they do not indicate the route for benchmarking theoretical calculations against experimental results.

Objectives:
We propose to perform a 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 [6]. We will compare the electronic spectra obtained with computational chemistry predictions based on PBK’s recent shielding isosurfaces [7] to verify whether the excited states display aromaticity reversals. 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.

Experimental Approach:
Laser photodissociation measurements will be conducted in a laser-interfaced mass spectrometer (LIMS) with an ESI source that can be used to transfer molecular ions 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 photofragmentation pathways will be monitored. Molecular ion geometries will be optimized using quantum chemistry approaches such as complete-active-space self-consistent field (CASSCF) and equations-of-motion coupled-cluster (EOM-CC). Chemical bonding and aromaticity will be analysed using computed magnetic criteria including NICS, magnetic susceptibilities and shielding isosurfaces.

Novelty:
Excited state dynamics in aromatic/antiaromatic systems is a topic of considerable interest, on which high profile results are being published at an increasing rate. This is a collaboration between theory and experiment in a promising area which is still uncharted. There would be opportunities to explore future collaboration with NTH that would extend the measurements into the time-resolved domain and create additional opportunities for computational modelling.

Training:
PBK group: Computational chemistry techniques including use of non-trivial methods for excited states such as CASSCF requiring careful construction of the wavefunction and assessment of the ordering of the excited states; programming in Fortran and Python. CED group: Mass spectrometry, (soft ionization techniques, CID, mass spectral analysis), laser spectroscopy (class 4 laser systems and diode lasers), handling of gas and vacuum systems. Full training in data analysis, data storage, and data presentation. Computational chemistry and mass spectrometry are valuable transferrable skills. The student will participate in group meetings and in national and international conferences, allowing development of oral and presentation skills.

All Chemistry research students have access to our innovative Doctoral Training in Chemistry (iDTC): cohort-based training to support the development of scientific, transferable and employability skills: https://www.york.ac.uk/chemistry/postgraduate/idtc/

The Department of Chemistry holds an Athena SWAN Gold Award and is committed to supporting equality and diversity for all staff and students. The Department strives to provide a working environment which allows all staff and students to contribute fully, to flourish, and to excel: 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 2020. Induction activities will start on 28 September.