Experimental and theoretical study of model ladder fluoranthenopyracylene with two-dimensional π-conjugation upon charging: Structure and optical properties

The spectroelectrochemical and theoretical study of thermally and chemically stable model ladder-type molecules with two-dimensional π-conjugation consisting of two repeating fluoranthenopyracylene units is reported. The model compound exhibits a reversible first electron transfer for both reduction and oxidation. Similar UV/vis/NIR spectra were observed for both the cation and the anion applying in situ ESR-UV/vis/NIR spectroelectrochemistry. A single ESR line spectrum (line width: ΔH _p-p = 0.25 mT) was detected for the radical cation, while its radical anion showed a distinct hyperfine pattern pointing to differences in the spin distribution of the positively and negatively monocharged fluoranthenopyracylene molecule. Density functional theory and Hartree-Fock time-dependent calculations were performed for the electronic ground (neutral and its positively/negatively charged forms) and lowest excited states to evaluate the optimal geometries. Two twisted conformations of the central ladder part are stabilized due to the presence of the mutual sterical repulsion of the lateral phenylene rings. Significant bond length changes upon the electric charging are indicated in the central part of the molecular skeleton. The anion shows larger changes in geometry in the central part upon optical excitation. Theoretical data for the absorption and fluorescence spectra are compared with the experimental data to demonstrate the value of the applied theoretical approach.