Investigating the Vibrational and Electronic States of Various pi-Conjugated Systems

Abstract

The vibrational and electronic properties of a range of pi-conjugated systems were explored. Such systems included some donor-acceptor dyes in which their symmetry made computational analysis of their charge-transfer dynamics challenging, some polythiophene-derived conducting polymers and their intense Raman frequency dispersion, and novel photolytic nitroxyl donor prodrugs.

The truxene systems reported were intensely coloured with their main absorption band being highly solvatochromic. This solvatochromic behaviour indicated the strong charge-transfer properties of these dyes, which was observed in both the UV-Visible spectra, emission spectra and reaffirmed using resonance Raman spectroscopy. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations supported the observations made spectroscopically, however, dipole moment changes between the ground and first excited state were inaccurate. This was attributed to localised asymmetry caused by the physical solvent environment which stabilised the charge-transfer process down one 'arm' of the truxene dye which was unable to be replicated using DFT simulations.

Three polythiophene co-polymers (PThBr, P3HTBr, and PEDOTBr) were examined for their Raman frequency dispersion. It was observed that P3HTBr and PEDOTBr exhibited higher energy electronic transitions than their pristine analogues. These transitions were pi-pi* in character. This was supported by resonance Raman spectroscopy. Furthermore, these co-polymers were found to be highly dispersive, with dispersion rates (D) of between 24 and 42 x 10^-3 eV^-1.

An additional system featuring PEDOT was examined to investigate if a macrocycle was connected to the polymer via 'click'-chemistry. Conclusive proof for this would have presented as triazole vibrations, or electronic transitions associated with the triazole. However, no triazole vibrations were observed as triazoles are not largely Raman active. Additionally, as triazoles are known electronic insulators, electronic transitions featuring the triazole units were unlikely. Analysis of the precursor to the click reaction, PEDOT-N_3, indicated azide bands were present at around 2210 cm^-1, which were not present in pristine PEDOT nor the clicked product. Additionally, when a positive potential was applied resulting in the PEDOT polymer to become oxidised, Raman modes associated with the macrocycle dominated the Raman spectrum, supporting the hypothesis that the macrocycle was chemically linked to the polymer.

Some more donor-acceptor dyes were investigated using resonance Raman spectroscopy to affirm calculations made by another research group. These calculations were supplied, and suggested more complex charge-transfer processes than those of the truxene systems due to their A-D-A-D-A-D-A structure. It was further demonstrated that naphthalimide moieties can function as both a donor and as an acceptor, and that the electronic transitions for the naphthalimides were highly delocalised. Their short-lived excited state was elongated by increasing the electron density on the central benzene core although the lifetimes were still, at most, 1.1 ns.

Lastly, photolytic nitroxyl donor prodrugs were investigated. These systems were explored to examine whether the photocleavage could be examined spectroscopically in order to elucidate the mechanism at which the photocleavage event occurred. Although clear differences were observed between the cleaved and uncleaved species, observation of the photocleavage through resonance Raman spectroscopy proved challenging, owing to the low Raman scattering of the principal bonds associated with the target sulfonamide moiety. DFT simulations, however, proved to be a useful tool as a predictive measure for determining the potential result of photocleavage. Using NAP-6,2-CF_3, a known nitroxyl releasing prodrug, and MeONOBnCH_3, which was demonstrated to not release nitroxyl, as calibrative systems, it was determined that the primary target - the coumarine-based BHC-CF_3 - would successfully release nitroxyl.