Regulating the photoluminescence of aluminium complexes from non-luminescence to room-temperature phosphorescence by tuning the metal substituents

Abstract

Although luminescent aluminum compounds have been utilized for emitting and electron transporting layers in organic light-emitting diodes, most of them often exhibit not phosphorescence but fluorescence with lower photoluminescent quantum yields in the aggregated state than those in the amorphous state due to concentration quenching. Here we show the synthesis and optical properties of β-diketiminate aluminum complexes, such as crystallization-induced emission (CIE) and room-temperature phosphorescence (RTP), and the substituent effects of the central element. The dihaloaluminum complexes were found to exhibit the CIE property, especially RTP from the diiodo complex, while the dialkyl ones showed almost no emission in both solution and solid states. Theoretical calculations suggested that undesired structural relaxation in the singlet excited state of dialkyl complexes should be suppressed by introducing electronegative halogens instead of alkyl groups. Our findings could provide a molecular design not only for obtaining luminescent complexes but also for achieving triplet-harvesting materials.

Fig. 1. Aluminum complexes investigated in this study.

a Synthetic scheme of the dialkylaluminum and dihaloaluminum complexes. Single-crystal structures of b LAlMe, c LAlCl, d LAlBr, and e LAlI.

Fig. 2. Photophysical properties of aluminum β-diketiminate complexes.

a Photographic images of solutions and crystals of the complexes. UV, 365 nm. b, c UV–vis absorption and PL spectra in solutions at room temperature, respectively. d PL spectra in crystalline states at room temperature. e, f PL spectra in solutions and crystalline states at 77 K, respectively. g, h Phosphorescence decay curve of LAlBr (detected at 511 nm) and LAlI (detected at 515 nm) in crystalline states at room temperature, respectively. Solid lines represent fitting curves.

Fig. 3. Results of (TD-)DFT calculations.

a Calculated energies of Kohn–Sham HOMO and NBO of the corresponding Al–X (X = Me, Cl, Br, and I) bond. b, c Optimized geometries and energies of LAlCl and LAlMe, respectively, at S₀ and S₁ states. d Kohn–Sham HOMO, LUMO, and NBO. Orange circles highlight the contribution from the substituent on aluminum to each HOMO.

Fig. 4. Excited singlet and triplet states of the complexes.

a Excitation energies of singlet and triplet states relative to each S₀ state. Orbital contributions to each state and SOC constants between S_m and T_n states are shown in gray texts. H and L denote HOMO and LUMO. b Kohn–Sham molecular orbital distributions (isovalue = 0.03).