To verify whether external donors with a limited electron-donating ability (e.g., benzene groups) can induce a charge transfer process to the PDI scaffold, we first dissolved PDI-tetraBr in a series of electron-rich aromatic solvents and examined their photophysical properties (Figures and). Interestingly, noticeable redshifts in both absorption and emission spectra were observed with increasing electron-donating ability of the solvents (from benzene to mesitylene and anisole), accompanied by a gradual decrease of the peak intensity. This strongly indicates the formation of charge transfer complexes between PDI-tetraBr and solvent molecules. No significant variation of the vibrational structure was observed from the spectra, indicating negligible influence from molecular packing/aggregation in dilute solutions. The effect of solvent polarity was not attributed here as hypsochromic shifts were found in solvents with low-to-mid dielectric constants (Figure , F).