Non-covalent polymer–fullerene interactions and their influence on the optical properties of C 70 in polystyrene matrices

This work presents a systematic solution-state investigation of non-covalent interactions between pristine C70 fullerene and polystyrene (PS) in benzene, addressing a notable gap in polymer–fullerene research where C70 systems have been far less explored than their C60 counterparts. UV–Vis absorption spectroscopy, supported by symmetry-based (D5h) transition analysis, was employed to examine C70/benzene, PS/benzene, and PS/C70/benzene solutions over a controlled range of fullerene concentrations. The characteristic absorption bands of C70 were found to retain their spectral positions upon incorporation into the PS matrix, confirming the absence of covalent bonding and the preservation of the intrinsic π-electronic structure of the fullerene. In contrast, absorption intensities and the optical absorption edge were sensitively modulated by the polymer environment. Optical bandgap energies (Eopt), extracted via Tauc analysis, exhibit a non-monotonic dependence on C70 concentration, reflecting the competition between PS–C70 and C70–C70 interactions mediated by π–π stacking and van der Waals forces. The novelty of this study lies in demonstrating that weak, non-covalent polymer–fullerene interactions alone can induce measurable, reversible tuning of the optical bandgap without altering molecular electronic structure. These findings establish concentration control in π-conjugated polymer solutions as an effective, non-chemical strategy for bandgap engineering in polymer–fullerene systems.