AbstractThis thesis examines and characterises structural transitions in two different organic charge transfer complexes: peryelene-tetracyanoquinodimethane (Perylene-TCNQ) and dibenzotetrathiafulvalene-tetraflourotetracyanoquinodimethane (DTTF-F4TCNQ. Both complexes were grown via physical vapour transport using a Carbolite EHC 12/600B
three zone furnace.
We experimentally established two completely novel polymorphs of Perylene-TCNQ and demonstrated that these polymorphs undergo a reversible temperature dependent phase transition, converting from one to the other, at around 280 K. We characterised the physical structure of both polymorphs using x-ray crystallography and provided estimates for the degree of charge transfer via bond length based methods and infra-red spectroscopy. Notably, we discovered that the benzene rings of perylene are heavily distorted in both polymorphs. This is unusual as it is normally extremely energetically favourable for aromatic rings to be planar. This deviation appears to be a result of nitrogen short contact interactions. These results have been published in "New Polymorphs of Perylene:Tetracyanoquinodimethane Charge Transfer Cocrystals", Cryst. Growth Des. 2018, 18, 4, 2003-2009. (https://doi.org/10.1021/acs.cgd.7b01391)
For DBTTF-F4TNCQ we demonstrated a hitherto unconfirmed high temperature Spin Peierls induced structural transition close to 400 K. This transition can be seen in both the unit cell parameters and the relative inter-molecular distances between adjacent DBTTF and F4TCNQ molecules, as determined by x-ray crystallography. While this behaviour was previously predicted it has never been experimentally confirmed until now. In the process we also ascertained, via both bond length and spectroscopic methods, that the degree of charge transfer does not appear to change over this structural transition. We also performed low temperature measurements to further probe the behaviour of the DBTTF-F4TCNQ system. Electron Paramagnetic Resonance (EPR) measurements were carried out and appear to demonstrate a low temperature transition at around 150 K. We also carried out low temperature x-ray crystallography measurements in an attempt to ascertain any corresponding structural changes, but the results were inconclusive.
|Date of Award||19 Feb 2020|
|Sponsors||Engineering and Physical Sciences Research Council|
|Supervisor||Simon Crampin (Supervisor) & Enrico Da Como (Supervisor)|
- Supramolecular chemistry
- charge transport and recombination
- organic semiconductor
- Organic electronics
- crystal engineering
- x-ray structure