Liquid-crystalline dual fluorescent dyes for white light emission

Abstract

Three isomeric phenanthrolines- 2,9-CzPhen, 3,8-CzPhen and 4,7-CzPhen - are formed when the heterocyclic core is functionalised 2,9-, 3,8- or 4,7- with 3,6-bis-(3,4-dioctyloxybenzene)-9H-carbazole. All are liquid crystalline and show a single columnar hexagonal phase, although that of 2,9-CzPhen has a very much higher clearing point (308 °C) compared to 3,8-CzPhen and 4,7-CzPhen (175 °C and 158.5 °C). This is rationalised through the sterically driven disposition of the carbazole groups on the phenanthroline ring alongside likely columnar packing motifs. Photophysical properties were investigated in toluene solution and in a polystyrene film. All compounds are luminescent (λ_max 440–455 nm) with decent (40%, 3,8-CzPhen) to modest (4,7-CzPhen, 13% and 2,9-CzPhen, 18%) solution fluorescence quantum yields. However, in a polystyrene film, 3,8-CzPhen and 4,7-CzPhen also emit at 520 nm and 570 nm, representing a still relatively rare example of anti-Kasha emission. The 520 nm emission of 4,7-CzPhen shows a low-intensity decay component that involves a triplet state and is assigned to a thermally activated delayed fluorescence (TADF). At high emitter loadings in an OLED device, two emission frequencies result in the appearance of white light. Dual emission is understood through the relevant theory and considers rates of internal conversion following photoexcitation that populates the two emitting states – a carbazole-localised, singlet ¹[π_Czπ*_Cz] state and a charge-transfer ¹[π_Czπ*_phen] state. The rate and efficiency of internal conversion (IC) from the Cz-localised excited state to the charge-transfer state, involving electron transfer from the substituent(s) to 1,10-phenanthroline, appear relatively slow so that fluorescence from the higher-lying state competes. This is consistent with anti-Kasha behaviour as a result of a slow internal conversion between electronically decoupled states.