Mechanochromic asymmetric sulfone derivatives for use in efficient blue organic light-emitting diodes

Abstract

Typical π–π stacking is suppressed by the asymmetric molecular design for high fluorescence quantum yield (Φ_f) blue light emission, overcoming the aggregation caused quenching (ACQ) limitation. In this research, two novel blue fluorescent materials with asymmetric structure: 2-(4′-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSC) and 2-(4′-((4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSBC), consisting of a sulfone group as the electron acceptor and two different electron donors, carbazole and phenanthroimidazole, were designed and synthesized. The two compounds have high Φ_f (95.3% for PSC and 81.1% for PSBC) in film because of the restricting π–π stacking, and show apparent mechanochromic properties, i.e., an emission change from deep blue to blue-green resulting from external mechanical stimuli. The emissions display 50 nm/23 nm red shifts after grinding. Organic light emitting diodes (OLEDs) using the two compounds as emitters exhibited good efficiencies: the doped PSC-based device emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.072). The PSBC-based device also emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.068). A maximal external quantum efficiency (EQE) of 5.43% was also achieved.