3,4,5,6-tetrakis(3,6-di-t-butylcarbazol-9-yl)- 1,2-dicyanobenzene (4CzPN-tBu) has a phthalonitrile core with four carbazole moieties sitting next and opposite to the nitrile (CN) groups around the benzene ring. The highly steric tert-butyl groups at 3,6-positions of each carbazole not only provides greater solubility to the molecule but also prevent the quenching of the excitons by the formation of the excimers between molecules. The tert-butyl substituents at 3,6-position can efficiently prevent the formation of the dimers between the carbazoles.
4CzPN-tBu can be used as a heavy-atom-free triplet photosensitizer by harnessing its thermally active triplet energy to realize visible-to-ultraviolet photon up-conversion via triplet-triplet annihilation (TTA). For 4CzPN-R (R = H, Ph or tBu) derivatives, there is a decreasing luminescence intensity in the order of H > tBu > Ph.