Where 𝛹 𝑆 𝑎 and 𝛹 𝑇 𝑏 𝛼 are the initially populated singlet state and ISC-formed triplet state, respectively. ℋ ̂𝑆𝑂 is the spin-orbit operator and 𝜌(𝐸 𝑎𝑗 ) is the vibrational density of states at the energy of the initial state, 𝛹 𝑆 𝑎 . The subscript 𝑞 0 denotes that the integral is solved for when 𝛹 𝑆 𝑎 and 𝛹 𝑇 𝑏 𝛼 have the same equilibrium geometry, i.e. the crossing point of their potential energy surfaces. Based on this expression, the rate of intersystem crossing can be enhanced by one of the two parameters: (i) by increasing the spin-orbit coupling matrix element or (ii) by increasing the density of states with small S-T splitting (E=ES-ET). The spin-orbit coupling matrix was shown both experimentally and computationally to increase in non-planar polyaromatic hydrocarbons, such as helicenes. Computational analysis indicates that this increase is associated with the enhancements of the spin-orbit coupling matrix element, due to lower symmetry of nonplanar molecules: planar molecules contain molecular plane of symmetry, so only the out-of-plane component of their angular moment operator contributes to antisymmetric ,* states. In contrast, nonplanar molecules lack the plane of symmetry, enabling nonvanishing matrix element contributions from all three components of the angular moment operator. Given that chlorination of GQDs results in a loss of planarity in Cl-GQD (Figure ), we considered the possibility that the observed efficiency of intersystem crossing arises due to the nonplanarity of Cl-GQD. This hypothesis was tested by comparing the photophysical behavior of H-GQD and Cl-GQD in organic solvents. The solubility of H-GQD (unlike H-GNR) in organic solvents (such as o-dichlorobenzene) was sufficient to enable a direct comparison of transient absorption features of the planar H-GQD (Figure ) and nonplanar Cl-GQD (Figure ). Interestingly, H-GQD in organic solution (Figure ) exhibits similar photophysical behavior as Cl-GQD, with an intersystem crossing time constant of 350±13 ps (the value is ~200 ps for H-GQD). The longlived character observed of H-GQD in organic solvent allows us to conclude that the compound had better solubility than in aqueous solution. Based on this finding, we conclude that the nonplanarity of GQDs is not responsible for the observed triplet state formation. Thus, we assign the observed triplet formation yields to the second term in the equation 2: an increased density of states with small S-T splitting (in the weak coupling regime, the "energy gap law" dictates an exponential decay of the transition probability as E increases. The increased delocalization in graphene quantum dots is known to result in improved intersystem-crossing rates, which was attributed to a decrease in the S-T energy gaps. We hypothesize that a similar effect explains the efficient triplet formation in Cl-GQD.