Similar sample heterogeneity was observed in emission excitation maps of Cl-GNR. Figure shows that the relative intensity of emission peaks at 650 and 775 nm changes with the excitation wavelength and we accordingly assign the 650 nm emission to the monomeric nanoribbons and the 775 nm emission to the aggregated Cl-GNR. The excitation spectra collected at the wavelengths for monomer and aggregate emission (Figure ) indicate that both species contain a main absorption feature at ~500 nm and a shoulder at 600-700 nm. The aggregate absorption features are slightly red-shifted and significantly broader, which is commonly observed in stacked aggregates. The kinetic profiles for the emission at the two bands (collected at 630 and 770 nm, Figure , SI) also differ, confirming the sample heterogeneity. Both emission kinetics could be fit to a model involving one rise component with a lifetime of 0.5 ns and two decay components with lifetimes of 0.6 and 2 ns. However, the relative amplitudes associated with these components were different at 630 and 770 nm with the longer-lived 2 ns component exhibiting much stronger amplitude at 770 nm. Since Cl-GNR samples consist of ribbons with different lengths, the heterogeneity observed in Cl-GNR emission/excitation maps may also be associated with the emission from non-aggregated ribbons of different lengths. However, given the similar heterogeneous emission observed in Cl-GQD and Cl-GNR, it is more likely that the observed behavior originates from aggregation effects. This is also supported by the calculated absorption spectra for different ribbon oligomers (Figure ), which indicate that any ribbon longer than four monomer units is expected to exhibit very similar photophysical properties. Femtosecond transient absorption spectroscopy was employed to study the time-resolved photophysics of our H-and Cl-compounds (Figure ). Transient features of all four samples at early probing times consist of ground-state bleach signals at ~400 nm (for GQD samples) and ~500/600 nm (for GNRs) with broad and featureless excited-state absorption bands in the redder part of the visible range. We assign the positive features to the excited-state absorption of singlet S1 states of GQDs and GNRs. Despite similar spectra features at early times, the temporal evolution of H-compounds is drastically different from Cl-compounds. While most of the transient signal of H-compounds decays back to ground state within a few hundreds of picoseconds, the decays of S1 states of Cl-compounds are longer and associated with a growth of new long-lived transient species with absorption at ~550 nm (for Cl-GQD) and ~700 nm (for Cl-GNR). These results are consistent with the increased fluorescence quantum yields observed for chlorinated compounds in Figure and further confirms that the chlorination of nanographene edges represents an efficient method to prevent excited-state lifetime shortening associated with the -stacking of aromatic units. 54 The formation of a long-lived transient with absorption at ~550 nm in Cl-GQD occurs with a lifetime of 350 ± 10 ps, as determined using the target analysis of the experimental data (Figure ). The kinetic model used in target analysis was a simple 1 → 2 → 3 sequential model and the resulting component spectra match the evolving features at 550 and 700 nm in the experimental data (Figure ). The temporal evolution of the long-lived transient was probed using nanosecond transient absorption spectroscopy (Figures and). The transient spectrum obtained at early times of the nanosecond instrument matches very well with the spectrum obtained at the end of the femtosecond instrument (probed 1.3 ns after the excitation pulse), confirming that the same intermediate is monitored across the two instrumental setups. The nanosecond instrument reveals that the long-lived intermediate decays with a lifetime of 15.7 s in the N2-purged sample, while it is significantly shorter in the presence of oxygen (0.76 s, Figure ). The long-lived nature of this transient, coupled with its sensitivity to molecular oxygen, indicates that this transient is likely the triplet excited T1 state of Cl-GQD. The Jablonski diagram in Figure summarizes the observed photophysical behavior of Cl-GQD. The observation of efficient intersystem-crossing in Cl-GQD is somewhat unusual considering that the molecule consists of light elements that cannot induce significant spin-orbit coupling via the common heavy-atom effect. In general, the rate of intersystem crossing can be expressed within the Fermi golden rule approximation as: 56