Curved Nanographenes: Multiple Emission, Thermally Activated Delayed Fluorescence, and Non-Radiative Decay

Abstract

The intriguing and rich photophysical properties of three curved nanographenes (CNG), each containing a corannulene (Cor) and a tert-butylhexa-peri-hexabenzocoronene (HBC-tBu) fragment, have been investigated as a function of their structural design (loose-helicene, CNG 6; cycloheptatrienically constrained helicene, CNG 8; cycloheptatrienically plus sterically constrained helicene, CNG 7). All CNG exhibit narrow fluorescence in the visible range (50<FWHM<75 nm) and singlet oxygen production quantum yields up to 47% that confirm significant participation of triplet excited states in their photophysics even at room temperature. Time-resolved and temperature-dependent photoluminescence (PL) spectra suggest that different singlet and triplet excited states are involved in their photoluminescence properties. CNG 7 and 8 exhibit dual fluorescence, as well as dual phosphorescence at low temperature in the main PL bands, evidenced by wavelength-dependent PL decays. In addition, hot bands are detected in fluorescence as well as phosphorescence and, in the temperature range of 100-140 K, thermally activated delayed fluorescence (TADF) with TADF lifetimes on the millisecond time scale is observed. These findings are rationalized by quantum-chemical simulations which allow to establish the topography of the ground and excited state potential energy surfaces of the three CNG. Whereas the S1 potential of CNG 6 is predicted to exhibit a single minimum, two S1 minima are found for CNG 7 and CNG 8, with considerable geometric reorganization between them, in agreement with the experimentally observed dual fluorescence of these chromophores. Additionally, a higher-lying S2 minimum close to S1 has been optimized for the three CNG, from where emission is also possible due to thermal activation and, hence, non-Kasha behavior. The presence of higher-lying dark triplet states close to the S1 minima provides mechanistic evidence for the TADF phenomena observed in the CNG. Non-radiative decay of the T1 state appears to be thermally activated with activation energies of roughly 100 meV and leads to disappearance of phosphorescence at T > 140 K.