Bright NIR-luminescent [formula omitted] complexes with pyrazole-substituted 1,3-diketones demonstrated an unusual spectral lines branching ratios

Abstract

A series of novel highly NIR-emitting Nd3+ complexes with specially designed 1,3-diketones bearing a pyrazole moiety and a linear CxF2x+1 group (x=1,3,6) was obtained and exhaustively investigated. It was demonstrated that for the complexes with an increased number of fluorine atoms, the triplet level energy of the ligands is decreased and phonon relaxation processes are suppressed. This provides a remarkable increase in the photoluminescence quantum yield up to 1.1%, which is close to the highest values reported for Nd3+ complexes with 1,3-diketones. The photoluminescence spectra of the complexes exhibited the Stark splitting indicative of non-cubic symmetry. Low symmetry of coordination environment was confirmed by the X-ray structural data and analysis of calculated Judd–Ofelt intensity parameters. Besides, it was revealed that the most intense band in the PL spectrum is attributed to the 4F3∕2→4I9∕2 transition (at 880 nm) which is commonly rather weak for simple diketonate complexes. The calculations showed that the unexpectedly high branching ratio of the 4F3∕2→4I9∕2 transition raises with elongation of the fluorinated chain and reaches 59% for the complex with x=6. The studies were conducted for solutions of the complexes as well as for solid samples, and the corresponding changes in their optical behavior were discussed. Uncommon properties of the synthesized Nd3+ complexes make them promising materials for various state-of-the-art luminescent applications, e.g., luminescent markers with an unusual spectral intensity distribution. Based on the electroluminescence studies, we also estimated the potential of the complexes for purely NIR-emitting OLEDs fabrication.