SHG-active luminescent thermometers based on chiral cyclometalated dicyanidoiridate(iii) complexes

Abstract

Multifunctional optical materials can be realized by combining stimuli-responsive photoluminescence (PL), e.g., optical thermometry, with non-linear optical (NLO) effects, such as second-harmonic generation (SHG). We report a novel approach towards SHG-active luminescent thermometers achieved by constructing unique iridium(III) complexes, cis-[Ir^III(CN)₂(R,R-pinppy)₂]⁻ (R,R-pinppy = (R,R)-2-phenyl-4,5-pinenopyridine), bearing both a chiral 2-phenylpyridine derivative and cyanido ligands, the latter enabling the formation of a series of molecular materials: (TBA)[Ir^III(CN)₂(R,R-pinppy)₂]·2MeCN (1) (TBA⁺ = tetrabutylammonium) and (nBu-DABCO)₂[Ir^III(CN)₂(R,R-pinppy)₂](I)·MeCN (2) (nBu-DABCO⁺ = 1-(n-butyl)-1,4-diazabicyclo-[2.2.2]octan-1-ium) hybrid salts, (TBA)₂{[La^III(NO₃)₃(H₂O)_0.5]₂[Ir^III(CN)₂(R,R-pinppy)₂]₂} (3) square molecules, and {[La^III(NO₃)₂(dmf)₃][Ir^III(CN)₂(R,R-pinppy)₂]}·MeCN (4) coordination chains. Thanks to the chiral pinene group, 1–4 crystallize in non-centrosymmetric space groups leading to SHG activity, while the N,C-coordination of ppy-type ligands to Ir(III) centers generates visible charge-transfer (CT) photoluminescence. The PL characteristics are distinctly temperature-dependent which was utilized in achieving ratiometric optical thermometry below 220 K. The PL phenomena were rationalized by DFT/TD-DFT calculations indicating an MLCT-type of the emission in obtained Ir(III) complexes with the rich vibronic structure providing a few emission bands that variously depend on temperature due to the role of thermally activated vibrations. As these crucial vibrational modes depend on the crystal lattice, the thermometry performance differs within 1–4 being the most efficient in 4 while the SHG is by far the best also for 4. This proves that pinene-functionalized cyclometalated dicyanidoiridates(III) are great prerequisites for tunable PL-NLO conjunction with the most effective multifunctionality ensured by the insertion of these anions into bimetallic frameworks.