Recent years have shown an increasing number of studies dedicated to new light emitters for diverse applications such as optoelectronics, bio-imaging, and quantum technologies. In this context, graphene quantum dots (GQD) have important assets since bottom-up chemistry allows complete control of the structure, opening the way to wide customization of their electronic, optical, and spin properties [1-3]. The full benefit from these opportunities requires addressing GQD’s intrinsic photophysical properties.To do so, single molecule photoluminescence experiment is a powerfull tool [4]. Here, we highlight the influence of vibrations on GQDs’ optical properties, by comparing optical studies to extensive DFT/TDDFT calculations combined with molecular dynamics simulations. Specifically, we discussed their role in the transitions' oscillator strengths [5]. In order to get deeper in the photophysics of GQD, we investigate the spectroscopy of single GQDs at cryogenic temperatures. In particular, we show a narrowing of the emission lines at low temperature, that allows us to characterize and identify vibrational replicas that are characteristic to GQDs [6]. [1] M. G. Debije, J. Am. Chem. Soc . 2004, 126, 4641 [2] X. Yan, X. Cui, and L.-s. Li, J. Am. Chem. Soc . 2010 132, 5944 [3] A. Konishi et al , J. Am. Chem. Soc. 2010, 132, 11021 [4] S. Zhao et al , Nature Communications , 2018 , 9, 3470 [5] T. Liu et al , under review [6] T. Liu et al , in preparation