First-principles study of luminescence of fullerene-like clusters

Thermally activated delayed fluorescence (TADF), a unique molecular fluorescence mechanism, plays a keyrole in designing emitters of high efficiency. Carbon fullerenes such as C60 and C70 exhibit strong TADF withintensity even higher than that of the prompt fluorescence, owing to their long lifetimes of triplet state andmodest singlet-triplet energy gaps. Thus, there arises the intriguing question whether other fullerene-likeclusters can also have fluorescence and host the TADF effect. In this work, by time-dependent densityfunctional theory (TD-DFT) calculations, we explore the excited-states of the experimentally reported boronnitride cage clusters B12N12, B24N24 and B36N36, as well as compound clusters B12P12, Al12N12 and Ga12N12 withthe same geometry as B12N12. Using the HSE06 hybrid functional, the predicted energy gaps of these fullerene-like clusters are obtained to range from 2.83 eV to 6.54 eV. They mainly absorb ultraviolet light, and theirfluorescence spectra are all in the visible range from 405.36 nm to 706.93 nm, including red, orange, blue, andviolet emission colors. For the boron nitride cages, the energy gap of excited states increases with the clustersize increasing, accompanied by a blue shift of emission wavelength. For the clusters with B12N12 geometry anddifferent elemental compositions, the excited energy gap decreases as the atomic radius increases, resulting in ared shift of emission wavelength. In addition, the highest occupied molecular orbitals (HOMOs) and lowestunoccupied molecular orbitals (LUMOs) of these compound cage clusters are distributed separately on differentelements, resulting in small overlap between HOMO and LUMO wavefunctions. Consequently, these fullerene-like clusters exhibit small singlet-triplet energy differences below 0.29 eV, which is beneficial for the intersystemcrossing between the excited singlet state and triplet state, and hence promoting the TADF process. Ourtheoretical results unveil the fluorescence characteristics of cage clusters other than carbon fullerenes, andprovide important guidance for precisely modulating their emission colors by controlling the cluster sizes andelemental compositions. These experimentally feasible fullerene-like compound clusters possess many merits asfluorophors such as outstanding stabilities, non-toxicity, large energy gap, visible-light fluorescence, and smallsinglet-triplet energy gap. Therefore, they are promising luminescent materials for applications in display,sensors, biological detection and labelling, therapy, and medicine