A pair of conjoined donor–acceptor butterflies as promising solution-processable aggregation-enhanced emission FR/NIR EL emitters

Abstract

Constructing a novel molecular architecture and creating far-red/near-infrared (FR/NIR) fluorophores with aggregation-enhanced emission (AEE) and solution film-forming ability is of particular interest but has been so far unreported. Here we linked two butterfly-shaped 3,7-bis(alkoxyphenylbenzothiadiazol)phenothiazines by an alkyl spacer to form a pair of conjoined donor–acceptor butterflies (SP6) as FR/NIR electroluminescent emitters. SP6 was found to be a multifunctional dye exhibiting AEE characteristics and reversible mechanofluorochromism, and significantly, a good quality film with strong deep-red emission could be formed by spin-coating the solution. The simplest solution-processed device (ITO/PEDOT:PSS/SP6/CsF/Al) emitted NIR light of 683 nm with a peak external quantum efficiency (EQE) of 0.57% and a CIE coordinate (0.68, 0.31). When the hole-transporting PVK layer was introduced, the still simple solution-processed device (ITO/PEDOT:PSS/PVK/SP6/CsF/Al) emitted FR light of 659 nm with a remarkably increased EQE of 1.82% and a CIE coordinate (0.65, 0.34). The low efficiency roll-offs and high radiative exciton ratios breaking through the theoretical limit signified that the rational modification and integration of the existing EL emitters could develop promising solution-processable EL materials having an inherent light-emitting mechanism.