The value is calculated using transition density matrices for each excited state, under the assumption that the one-electron transition density matrix can be used as a representation of the exciton wavefunction. This approach was used to calculate dexc for N-mers with sizes ranging from N = 1 to 4 (Figure ). The results show that the exciton size increases from dexc=5.3 Å in the monomer to 7.1 Å for tetramer, and the value seems to level off, with only a modest increase in size with N-mer length. These results indicate that the exciton size in GNRs likely does not exceed beyond a dimer and are consistent with the previous calculations on a similar all-carbon GNR analog. At a first glance, these results appear contradictory to the continual red-shift observed in calculated UV-vis absorption spectra from monomer to pentamer. Such red-shift has often been interpreted in the literature as a measure of exciton size (the oligomer size at which the red-shift is no longer observed is used as an estimate of dexc). However, our exciton size calculations in Figure clearly show that the exciton size does not exceed beyond a dimer, even though the red-shift in Figure is observed up to the pentamer model. Despite this, the TD-DFT spectra in Figure will still capture shifts due to the electrostatic environment continuing to change as more material is added on either side, slowly converging according to the asymptotic tail of the coulomb force. The calculated natural transition orbitals (NTOs) also indicate that the exciton is not delocalizing past the dimer (Figure ). We obtained similar results in calculations of dexc in one-dimensional -stacks of graphene flakes, indicating that the excitons do not delocalize significantly in these carbon-based chromophores, irrespective of the direction of their growth (along the -stack in graphene flakes or via in-plane bonding in graphene ribbons). It may be noted that the particle-accepting NTO in the trimer appears to be localized over one monomer unit and therefore potentially smaller than the particle NTO of the dimer. However, the hole-leaving NTO is concomitantly larger in the dimer as well, allowing for the total holeparticle displacement to be smaller in the dimer than the trimer. To illustrate further the degree of hole-particle separation, transition density matrices in the tetramer were decomposed into contributions from natural atomic orbitals ; these may be found in the SI (S4).