Photophysical characterisation of Tröger's base molecular scaffolds: a combined theoretical and experimental study†
Three Tröger's bases (TBs) 2a–c were synthesised in order to study the photophysical behaviour of the TB scaffold in the ground and excited states. The photophysics was studied using UV-Vis, steady-state and time resolved fluorescence emission spectroscopies in solution. TBs present absorption in the ultraviolet region, ascribed to ππ* electronic transitions, where any evidence of intramolecular charge transfer character (ICT) was observed in the ground state. TBs present emission in the violet-blue regions with a decay time of around 1.5 ns. However, a linear relation of the fluorescence maximum (νmax) versus the solvent polarity function (Δf) from the Lippert–Mataga correlation was observed for 2c, which indicates an intramolecular charge transfer character in the excited state. Theoretical calculations were also performed in order to study the geometry and charge distribution of TBs in their ground and excited electronic states. TD-DFT calculations were performed using the PBE0 functional with cc-pVDZ basis set for geometrical optimisations and the aug-cc-pVDZ basis set to obtain vertical transition energies and electronic properties. Solvent effects were included by IEF-PCM formalism using five solvents with different dielectric constants, and the results have shown a small solvatochromic effect. The computationally predicted transition energies are in good agreement with the experimental results, and have shown that the presence of substituents in the 2,8-positions of the TB scaffold causes a bathochromic shift in the absorption and emission wavelengths.