To demonstrate the intramolecular vibronic effect on the spectra, we calculated the vibrationallyresolved absorption spectra of two monomers. The calculated together with the experimentallymeasured absorption spectra 27 are shown in Figure . The theoretical spectra coincide with the experimental measurement very well. As Tables and in the SI show that the first and second singlet excited electronic states (S 1 and S 2 ) of both PDI-F and PDI-CN molecules are bright and dark states, respectively, and there is an energy difference of about 1.0 eV. We thus calculate the vibrationally-resolved absorption spectra of two monomers by just considering the electronic transition from S 0 → S 1 . Figure : Calculated vibrationally-resolved absorption spectra (solid line) of PDI-F and PDI-CN monomera as compared with the experimental results (dashed line). Gaussian lineshape with full width at half maximum(FWHM) of 806 cm -1 was employed. The experimental absorption spectrum was obtained in chloroform (1.0 × 10 -5 M). During the calculations, we adopt a simple method to estimate the position of the 0-0 peak, which subtracts the reorganization energy (E r = j ω j g 2 j ) of S 1 from the vertical excitation energy (E v ). For the PDI-F monomer, the calculated values of E v and E r are 17381 and 1188 cm -1 , resulting in a 0-0 peak position at 16193 cm -1 while for the PDI-CN monomer, they are E v = 16833 and E r = 1251 cm -1 , yielding a 0-0 peak position at 15582 cm -1 . To facilitate comparison with the experimental results, theoretical 0-0 peak positions were blue-shifted by 3339 and 3216 cm -1 for PDI-F and PDI-CN molecules, respectively.