Highly emissive planarized B,N-diarylated benzonaphthoazaborine compounds for narrowband blue fluorescence

Near-infrared phosphorescence in a ruthenium(II) complex equipped with a pyridyl-1,2-azaborine ligand

The 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene molecule Hazab-py has been successfully used, for the first time, as a ligand in a ruthenium(II) polypyridine complex A (with the formula [Ru(dtbbpy)2(azab-py)]+, where dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This compound was characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and its electrochemical and photophysical properties were fully investigated and compared to those of its homoleptic analogue [Ru(dtbbpy)3]2+ (B), an archetypical mono-cationic cyclometalated complex C (with the formula [Ru(dtbbpy)2(ppy)]+, where Hppy = 2-phenylpyridine), and the more structurally similar analogue [Ru(dtbbpy)2(naft-py)]+ (D), where the B-N unit of the azaborine ligand is replaced by a standard CC one, resulting in the 2-(naphthalen-2-yl)pyridine ligand (Hnaft-py). The presence of the novel 1,2-azaborine ligand induces a 0.51 V decrease in the redox gap of complex A, compared to that of B, leading to electrochemical and photophysical properties that resemble those of C and D. Accordingly, the azaborine complex displays an emission band extending up to the near infrared region of the spectrum (with the maximum at 765 nm in room-temperature acetonitrile solution), arising from a triplet metal-to-ligand charge-transfer (3MLCT) state. As in the case of other mono-cationic cyclometalated ruthenium(II) complexes, A shows modest photoluminescence quantum yields (PLQYs), but higher PLQYs when compared to those of its direct CC analogue D (e.g., PLQY = 0.6 vs. 0.1% in a PMMA matrix at 298 K). Density functional theory (DFT) calculations were used to provide complete rationalization of the electronic properties of all the complexes and to identify lower-lying metal-centred triplets (3MC), responsible for the low PLQYs of such an azaborine-based ruthenium(II) complex.

Achievement of efficient thermally activated delayed fluorescence materials based on 1,8-naphthalimide derivatives exhibiting piezochromic and thermochromic luminescence

In this study, we developed a D-A type imide derivative based on 1,8-naphthalimide, NI-mPCz, which exhibited outstanding thermally activated delayed fluorescence (TADF) properties. Additionally, it demonstrates characteristics of piezochromic and thermochromic luminescence. The thermochromic luminescence observed is attributed to crystalline transformations occurring during the heating process, as evidenced by differential scanning calorimetry (DSC) and microscopic examinations. Moreover, the good compatibility of NI-mPCz with HeLa cells and its excellent imaging performance indicate its potential for application in the field of biological imaging. These results provide valuable insights for the design and development of new organic electronic and bioimaging materials with high-efficiency TADF characteristics.

Efficient orange and red thermally activated delayed fluorescence materials based on 1,8-naphthalimide derivatives

Thermally activated delayed fluorescence (TADF) molecules have emerged as a promising class of third-generation organic light-emitting diode (OLED) emitters that can achieve 100% internal quantum efficiency without the use of noble metals. However, the design of high-efficiency red TADF materials has been challenging due to limitations imposed by the energy-gap law. To overcome this challenge, two new TADF emitters, namely, 6-(4-(diphenylamino)phenyl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (NI-TPA) and 6-(10H-phenothiazin-10-yl)-2-phenyl-1H-benzo[de]-isoquinoline-1,3(2H)-dione (NI-Pz), have been synthesized and characterized. These compounds exhibit strong TADF characteristics with a small energy gap (ΔEST) between the lowest excited singlet and triplet states, short delayed fluorescence lifetimes, high thermal stability, and high photoluminescence quantum yields. The OLED devices fabricated using NI-TPA and NI-Pz as emitters show orange and red electroluminescence with emission peaks at 593 nm and 665 nm, respectively, and maximum external quantum efficiencies (EQEs) of 11.3% and 7.6%, respectively. Furthermore, applying NI-TPA to cell imaging yielded excellent imaging results, indicating the potential of red TADF materials in the field of biological imaging.