nm for the pentamer. The red shift seems to saturate for GNR models longer than a tetramer, indicating that the exciton coherence length does not exceed beyond the length of ~ 3.8 nm. The lowest energy bright transitions are associated with the formation of charge-transfer (CT) excited state where the electron density shifts to the bipyrimidine edges, as illustrated in the difference density plots shown in Figure . For trimer and tetramer GNR models, the inner bipyrimidine units accumulate the electron density, while the bipyrimidine units located on the periphery exhibit a slight depletion of electronic density while the majority of electron density depletion occurs within the aromatic core. The match between calculated (360 nm, 3.44 eV) and experimental (355 nm, 3.49 eV) absorption wavelengths for H-GQD is excellent, while the comparison of the H-GNR absorption band at 610 nm with the calculated data indicate that the H-GNR solution consists of GNRs with lengths that are longer than 5 monomer units. The exciton size in GNRs was further investigated computationally using a one-particle transition density matrix approach developed previously. In this method, the exciton size dexc is defined as the root-mean-square separation between instantaneous hole and electron positions (𝑥 ⃑ ℎ and 𝑥 ⃑ 𝑒 , respectively):