Processable Star-Shaped Molecules with Triphenylamine Core as Hole-Transporting Materials: Experimental and Theoretical Approach

In this study we report on the characterization of five star-shaped π-conjugated molecules by means of UV–vis absorption spectroscopy and electrochemical cyclic voltammetry. These molecules, with triphenylamine (TPA) core bearing one thienothiophene moiety and a different number of thiophene ones, are designed as hole-transporting materials for dye-sensitized solar cell (DSSC) applications. Theoretical calculations employing the B3LYP functional are also carried out in order to understand the structure–property relationships. UV–vis absorption measurements and time-dependent density functional theory (TDDFT) calculations show the presence of intense UV–vis bands for all compounds. These bands are dominated by two degenerate π–π* excitations mostly involving the HOMO → LUMO and HOMO → LUMO+1 transitions. Electrochemical cyclic voltammetry and DFT calculations show the HOMO (LUMO) energy levels increasing (decreasing) with the number of conjugated heterocyclic rings in these molecules. The HOMO energies have been found to vary between −5.38 and −5.13 eV thus showing good positioning with respect to the Fermi level of gold electrode (DSSC applications). The calculated internal reorganization energies (λi) suggest for these materials promising hole-transport properties. The analysis of the space extension of the HOMO orbitals as a function of the number of conjugated rings in these molecules gives useful information on their design.