Photophysical features and semiconducting properties of propeller-shaped oligo(styryl)benzenes

Electronic, optical, and semiconducting properties of a series of propeller-shaped oligo(styryl)benzenes have been systematically investigated to monitor the effect of the number of styryl branches (three, four, and six) around a central benzene core. In order to clarify the relationships between their structures and properties, Density Functional Theory calculations were carried out at several levels of theory considering solvents with different polarity. Absorption and vibrational Raman spectroscopies showed that cruciform, four-branched derivatives present the most effective π-conjugation in agreement with the lowest calculated bond length alternation and bandgap. Deviations from the mirror image symmetry between absorption and fluorescence spectra were related to changes in the molecular conformation upon electronic excitation. Furthermore, in order to investigate the semiconducting behavior of oligo(styryl)benzenes, molecular structure changes and different electronic properties related to ionization processes were calculated and analyzed. Hole and electron reorganization energies were also computed to provide a first approximation on the n- or p-type character of these compounds. In some cases, electron reorganization energies comparable to common n-type semiconductors were found.