Thiophene substituted aza-BODIPY as promising metal-free, pure NIR emitter for OLEDs

Organic semiconductors absorbing or emitting in the near-infrared (NIR) range of the electromagnetic spectrum have emerged in the last twenty years as a novel class of materials with useful optoelectronic properties. Clearly the most appealing features are the solution processing, low-cost fabrication and the possibility of application on flexible, conformable or even stretchable substrates for organic electronics, such as organic and polymer light emitting diodes (OLED and PLED). NIR OLEDs and PLEDs [1] are particularly interesting for night vision-readable displays which are unreadable to the naked eye but could be read with night-vision goggles, or as light source for sensors that operate with NIR light. Furthermore, the semitransparency of biological tissue between 700 and 1000 nm makes these applications appealing for a broad class of biomedical applications, in particular imaging and sensing. However, obtaining organic materials with emission in pure NIR is still a goal to be pursued and in general complexes based on lanthanides are used which however have the problem of scarce abundance and are non-renewable resources. 4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene dyes, more commonly known as BODIPY dyes, since long time have been recognized for their excellent optical properties such as large absorption coefficients, high fluorescence quantum yields, and remarkable photostability [2,3,4] and are particularly promising as IR-emitting dyes. Replacing the C-8 in meso position in the conventional BODIPY core with an aza-N atom lowers the energy of both absorption and emission transitions and the resultant aza-BODIPY dyes absorb and emit in the spectral window 600-750 nm in solution [4]. Here we present a tetra-thiophene substituted aza-BODIPY as pure IR emitter with photoluminescence emission peaked @ 890 nm when dispersed in polystyrene film and electroluminescence tested in PLED devices peaked @ ~ 900 nm. [1] A. Zampetti, A. Minotto, F. Cacialli, Adv. Funct. Mater. 2019, 29, 1807623 [2] J. K. G. Karlsson, A. Harriman, J. Phys. Chem. A 2016, 120. [3] A. Loudet, K. Burgess, Chem. Rev. 2007, 107, 11, 4891. [3] G. Ulrich, R. Ziessel, A. Harriman, Angew. Chem. Int. Ed. 2008, 47, 1184. [4] B.M. Squeo, M. Pasini, Supramol. Chem., 2019, 32, 56.