Keywords: Schiff bases, iridium complexes, luminescence, solid-state lighting, WLEDs.

The near-UV-to-red downshifting converter Ag[Ir(ppz)₂(BS1)] complex may be used as coating of near-UV-excited warm WLEDs due to the high emission color purity and the high photostability.

Ir-based complexes find several applications such as in catalysis, optoelectronics, biomedicine, solar energy conversion as well as in solid-state lighting (SSL).[i] Focusing our attention on SSL, Ir-based complexes can be used as coatings of near-UV-excited light-emitting diodes (LEDs) towards the fabrication of warm white-emitting LEDs (WLEDs) due to their strong absorption within the near-UV region and tunable light emission.[ii] Although a couple of blue or green-emitting Ir complexes featuring high emission quantum yield have already been reported[iii], there is a lack of near-UV-to-red downshifting converter Ir-complexes[iv]. In this context, herein, we introduce the synthesis of two Schiff base ligands and their red-emitting Ir-complexes in order to evaluate their use as coatings of near-UV-excited LEDs. The Schiff bases 2-(((3-methyl-1-phenyl-1H-pyrazol-5-yl)imino)methyl)phenol (BS1) and 5-(diethylamino)-2-(((3-methyl-1-phenyl-1H-pyrazol-5-yl)imino)methyl)phenol (BS2) were synthesized via condensation reaction of a primary amine and an aldehyde. Those ligands were used to synthesize the Ag[Ir(ppz)₂(BS1)] and Ag[Ir(ppz)₂(BS2)] complexes (ppz = phenylpyrazole) through the reaction with the Ir-precursor, [(ppz)₂Ir(μ‑Cl)Ir(ppz)₂], in dichloromethane at 46°C, assisted by Ag₂O. The structures of BS1, BS2, and their respective Ir-complexes were elucidated by FTIR, UV-Vis, elemental and thermal analysis, ¹H-NMR, and/or MALDI-TOF. Under UV excitation, the ligands exhibit emission in the yellow (BS1) and in the green (BS2). On the other hand, under UV excitation, both Ag[Ir(ppz)₂(BS1)] and Ag[Ir(ppz)₂(BS2)] emit a broadband centered at 627 and 604 nm, and 1931CIE color coordinates of (0.659;0.337) and (0.612;0.380), respectively, both within the red spectral region, with 100 % of color purity. Since Ag[Ir(ppz)₂(BS1)] exhibits red color coordinates ideal for warm WLEDs, it was used to coat a commercial near-UV-emitting LEDs (395 nm), the ligand BS1 was also investigated for comparison. The two LED prototypes emit in the yellow (BS1) and in the red (Ag[Ir(ppz)₂(BS1)]) spectral regions. Moreover, inasmuch as the application of the complexes in SSL depends on their reliability, the radiant photostability of the as-fabricated LEDs were overseen for 24 h of operation at 2.9 V. The BS1-LED prototype featured an intensity decrease of 60 % after 24 h while the intensity of the Ag[Ir(ppz)₂(BS1)]-LED decreased only 10 %, confirming that the complex features near-UV-to-red conversion and photostability desired for application as warm WLED.

Capes (88887.341772/2019-00), CNPq(304003/2018-1) and Fapesp (2015/03400-5).