Optical spectroscopy of graphene quantum dots: the case of C132.

We have reinvestigated the optical spectroscopy of C132 graphene quantum dots by absorption, selective fluorescence, excitation and time-resolved spectroscopy, the external heavy atom effect, and DFT based quantum chemical calculations. In particular, wavelength-selective photobleaching provides strong evidence for the assignment of the intrinsic absorption and emission features of the quantum dots and indicates that emissions observed at ∼670 and ∼630 nm and associated relatively narrow features that display vibrational progressions in the selective excitation spectra are due to different species. The emitting state that leads to a broad emission (1700 cm(-1)) centered around 750 nm appears to be a "near-dark" singlet state with a relatively long lifetime of ∼30 ns. Simulated spectra, based on the nuclear ensemble approach, are in qualitative agreement with this finding and indicate very low oscillator strengths with some significant electron-vibrational intensity borrowing.