Electronic structures and ionicity in [60]fullerene/polycyclic aromatic hydrocarbon charge transfer complexes studied by UV–Vis and NMR spectroscopic techniques

A detailed UV–Vis spectrophotometric, thermodynamic and NMR studies have been carried out to look insight into the nature of molecular interactions of the electron donor–acceptor complexes of [60]fullerene with three different polycyclic aromatic hydrocarbon (PAH) molecules in CCl4 medium. Well defined charge transfer (CT) absorption bands have been located in the visible region. We extract degrees of charge transfer, oscillator and transition dipole strengths by analyzing the transition energy of the CT band as a function of vertical ionization potentials of the donors studied. The influences of the PAH on the absorption spectrum of [60]fullerene are studied. The experimental results have been explained using a theoretical model that takes into account the interaction between electronic subsystems of PAH with [60]fullerene. Formation constants (K) of the [60]fullerene/PAH complexes are determined spectrophotometrically (utilizing Benesi–Hildebrand equation) at four different temperatures and by 1H NMR spectroscopic technique. Trend in the formation constant values for the [60]fullerene/PAH complexes are discussed in terms of enthalpies and entropies of formation. Free energies of formation values for the [60]fullerene/PAH systems as well as ab initio calculation suggest that nature of substitution and steric effect in the PAH systems play vital role in formation of EDA complexes with [60]fullerene.