Ultrafast dynamics of relaxation in well-dispersed quantum-confined nanographenes

Recently developed graphene quantum dots (G-QDs), presenting exceptional dispersion stability, a precisely controlled number of conjugated carbon rings and a photoluminescence quantum yield of almost unity, allow scrutinization of their intrinsic photophysics and potential quantum-confined effects related to their excited-state dynamics. Here we use transient absorption with 30-40 fs resolution to probe electronic relaxation in rectangular G-QDs composed of exactly 96, 114 and 132 carbon atoms. Through the growth of excited-state emission signals over the ground-state bleaching ones, the dynamics of relaxation are unveiled. The relaxation time ranges between 130 and 180 fs, which leads to a maximum global energy-loss rate of 5 eV ps^-1. Energy-selective excitation measurements show that this ultrafast relaxation rate is limited by vibrational relaxation rather than the internal conversion process. This reveals the proximity between the excited-state energy surfaces and the key role of high-frequency vibrational modes in driving these ultrafast relaxation dynamics.