Concentration-assisted optical response and nanoscale aggregation of C ₇₀ fullerene in N,N-dimethylformamide

The concentration-assisted optical response and nanoscale aggregation of C70 fullerene in N,N-dimethylformamide (DMF) were investigated at concentrations of 0.03 and 0.06 mg/mL using UV–Vis absorption spectroscopy, fluorescence spectroscopy, Raman spectroscopy, and nanoparticle tracking analysis (NTA). Semi-logarithmic UV–Vis spectra showed stronger absorption and a more pronounced long-wavelength tail for the 0.06 mg/mL solution. Apparent molar absorptivity values calculated using the Beer-Lambert relation decreased at higher concentration, indicating deviation from ideal molecular absorption behavior and suggesting aggregate-related optical contributions. Fluorescence measurements showed that neat DMF contributes mainly to the short-wavelength emission near 504–542 nm, whereas the broad 620–820 nm emission band arises from C70-related excited states. Gaussian deconvolution revealed a red-shifted low-energy component from ∼710 nm for 0.03 mg/mL to ∼727 nm for 0.06 mg/mL, suggesting an aggregation- and solvation-sensitive excited-state environment. Raman spectra showed retention of the main DMF vibrational bands after C70 addition; the observed small shifts were interpreted cautiously as tentative local perturbations rather than direct evidence of specific solvation-shell restructuring. NTA revealed a decrease in median detected aggregate diameter from 180.55 ± 2.2 nm at 0.03 mg/mL to 133.1 ± 1.1 nm at 0.06 mg/mL, together with a more heterogeneous size distribution at the higher concentration. These findings are consistent with a possible concentration-related redistribution of C70 aggregate populations in DMF. A broader concentration series, time-dependent measurements, and independent morphological characterization are required to establish a complete aggregation mechanism.