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Using the first principles methods, we performed systematic study on the effect of edge-functional groups on the electronic energy levels and the optical properties of sp 2 carbon clusters. It is found that the intrinsic π and π ∗ orbitals are weakly altered by oxygen-bearing functional groups, but it is significantly disrupted by pyrrolic groups. Thereby the oscillator strength of the lowest-energy transition is found to be much stronger for the pyrrolic group functionalized cluster than for the carboxyl group. From our results being consistent with the experimental reports, we suggest that the photoluminescence enhancement is caused by a perturbation of the intrinsic frontier molecular orbitals by edge groups.