Excited states engineering enables efficient near-infrared lasing in nanographenes

The spectral overlap between stimulated emission (SE) and absorption from dark states (i.e. charges and triplets) especially in the near-infrared (NIR), represents one of the most effective gain loss channel in organic semiconductors. Recently, bottom-up synthesis of atomically precise graphene nanostructures, or nanographenes (NGs), has opened a new route for the development of environmentally and chemically stable materials with optical gain properties. However, also in this case, the interplay between gain and absorption losses has hindered the attainment of efficient lasing action in the NIR. Here, we demonstrate that the introduction of two fluoranthene imide groups to the NG core leads to a more red-shifted emission than the precursor NG molecule (685 vs. 615 nm) and also with a larger Stokes (45 nm vs. 2 nm, 1026 cm-1 vs. 53 cm-1, respectively). Photophysical results indicate that, besides the minimisation of ground state absorption losses, such substitution permits to suppress the detrimental excited state absorption in the NIR, which likely arises from a dark state with charge-transfer character. This has enabled NIR lasing (720 nm) from all-solution processed distributed feedback devices with one order of magnitude lower thresholds than those of previously reported NIR-emitting NGs. This study represents an advance in the field of NGs and, in general, organic semiconductors photonics, towards the development of cheap and stable NIR lasers. G.M.P. thanks Fondazione Cariplo for financial support (grant n° 2018-0979). R.M.M., P.G.B., J.M.V., J.A.Q. and M.A.D.G. acknowledge funding from the Spanish Ministry of Economy and Competitiveness (MINECO), the European Regional Development Fund (FEDER) and European Social Funds (ESF; grant n° MAT2015-66586-R and FPI fellowship BES-2016-077681). Q.C., K.M. and A.N. are grateful for the financial support by the Max Planck Society and the ANR-DFG NLE Grant GRANAO by DFG 431450789. K.M. acknowledges a fellowship from Gutenberg Research College, Johannes Gutenberg University Mainz. R.K. and A.N. appreciate the support by the Okinawa Institute of Science and Technology Graduate University. Y.I. and A.N acknowledge financial support from the Japan Society of Promotion Science Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers.