Novel phosphorescent bis- and tris-cyclometalated iridium(III) complexes bearing carbazole-appended dendrons were successfully synthesized, and fabrication of non-doped multilayer OLEDs by solution processing was demonstrated by using these complexes as emitting materials. The complexes exhibited sky-blue photoluminescence (PL) at 460–464 nm with excellent PL quantum yields of ca. 0.8, and their neat films also emitted sky-blue photoluminescence: the dendrons effectively suppressed aggregation between the luminescent cores, although referential core complexes, bearing no dendrons, exhibited aggregate-based green PL even when the core complexes were doped into a model host material of the dendron in the same core–dendron molar ratios as the present dendritic bis- and tris-cyclometalated iridium(III) complexes. As hydrophobic tert-butyl groups are placed on the periphery, the dendritic complexes exhibit good solubility in cyclohexane and poor solubility in 2-propanol. Such special properties allow for fabrication of solution-processed non-doped multilayer OLEDs consisting of a stack of the hole-transporting layer (HTL)/emitting layer (the dendritic complex)/electron-transporting layer (ETL). The devices showed sky-blue electroluminescence with excellent electron–hole charge balance factors (γ) of ca. 0.9, which were much higher than those of simple non-doped devices without HTL and ETL (γ; ca. 0.1).

•The 2-(dibenzo[b,d]furan-4-yl)pyridine-based platinum(II) complexes were prepared.•The introduction of a trifluoromethyl group gave rise to a red-shifted emission.•The 4-substituted complexes showed improved PL quantum yields in the polymer films.•PLEDs with the 4- and 5-methyl-substituted complexes showed good device performance.•Efficient excimer EL was obtained from the 4-methyl-substituted complex.A series of heteroleptic cyclometalated platinum(II) complexes based on a 4- (Pt-1) and 5-substituted (Pt-2) cyclometalated ligand were prepared, and their photoluminescence (PL) and electroluminescence (EL) properties were investigated. In dichloromethane, the complexes bearing a fluoro (Pt-1a and Pt-2a) or a methyl (Pt-1b and Pt-2b) substituent exhibited green PL, whereas a trifluoromethyl substituent induced a red shift, affording yellow PL (Pt-1c and Pt-2c). For poly(methyl methacrylate) thin films doped with Pt-1a–c, higher PL quantum yields were obtained in comparison with the solution samples. Polymer light-emitting diodes (PLEDs) containing the platinum(II) complexes were fabricated, and the devices doped with Pt-1b and Pt-2b showed relatively high external quantum efficiencies. PLEDs doped with varying concentrations of Pt-1b were also fabricated, where generation of excimer-based EL led to significant shifts of the Commision Internationale de L'Eclairage chromaticity coordinate from (0.40, 0.56) (green) to (0.51, 0.48) (orange) with the device efficiency almost constant.

Novel donor–acceptor–donor (D–A–D) π-conjugated molecules based on a dipyrido[3,2-a:2′,3′-c]phenazine (dppz) skeleton were synthesized, and their luminescent properties were investigated. Introduction of various aryl substituents to the 10- and 13-positions of dppz allowed us to tune the emission properties through modulation of the intramolecular charge transfer (ICT) character on the D–A–D chromophores. Coordination of platinum(II) to the diimine site of dppz also gave rise to facilitation of the ICT to induce a significant red shift of the emission.

To clarify the ancillary ligand effect on excimer formation of heteroleptic cyclometalated platinum(II) complexes, we investigated photo- and electroluminescence behavior for green-phosphorescent [(dibenzo[b,d]furan-4-yl)pyridinato-N,C3′]platinum(II) 1,3-diketonates Pt-1 and Pt-2 (1,3-diketonate ancillary ligands; 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (bdbp) and dipivaloylmethanate (dpm) for Pt-1 and Pt-2, respectively). The X-ray crystallographic study reveals that both Pt-1 and Pt-2 form dimeric pairs in the solid states, indicating that they are likely to form the excimers. In PMMA films doped with Pt-1, red-shifted photoluminescence at >600 nm, assignable to the excimer emission, increasingly emerges along with the original monomer emission at 516 and 552 nm as the doping level of Pt-1 is increased to 44 wt %. Pt-2-doped films give quite modest excimer-based photoluminescence even when heavily doped to the same extent as Pt-1-doped films. This clearly indicates that the aromatic ancillary ligand (bdbp) effectively facilitates the excimer formation rather than the aliphatic (dpm). Similar efficient excimer formation is also observed for electroluminescent devices such as poly(9-vinylcarbazole)-based polymer light-emitting diodes (PLEDs) doped with Pt-1. The Pt-1-doped PLEDs afford electroluminescence from green to orange when the doping level of Pt-1 is varied from 7.1 to 24 wt %. Thus, referring to the Commission Internationale de L’Eclairage (CIE) chromaticity coordinate, the color tuning is available from CIE = (0.40, 0.57) to (0.56, 0.43) (@maximum luminance). We also found that the excimer-based luminescence of Pt-1 is enhanced in the PLEDs more than in polymer thin films upon photoexcitation. This result shows the difference in the mechanism of triplet excimer formation between photo- and electroluminescence and indicates that the efficient triplet excimer formation is caused by direct charge recombination of anion and cation radicals of Pt-1.

A series of bis-cyclometalated iridium(III) complexes bearing various types of 1,3-diketonate ancillary ligands were prepared, and their photoluminescent (PL) properties were investigated, especially focusing on the emission color tuning. When the dipivaloylmethanate ancillary ligand (O^O-1a) was replaced by conjugated 1,3-diketonates such as 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (O^O-1b) and 1,3-bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-dionate (O^O-1c), the blue-emitting bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C2′]iridium(III) (Ir-1) and bluish green-emitting bis[2-phenylpyridinato-N,C2′]iridium(III) (Ir-2) complexes exhibited significantly red-shifted phosphorescence in solution (λPL; 474–604 nm and 521–661 nm for Ir-1a–c and Ir-2a–c, respectively: the subscripts a, b, and c corresponding to O^O-1a–c). On the other hand, the ancillary ligand replacement was less effective on tuning the emission color of the green-emitting bis[2-(2,4-bis(trifluoromethyl)phenyl)pyridinato-N,C2′]iridium(III) complex (λPL; 541–566 nm for Ir-3a–c), and no PL color change was observed for the red-emitting bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N,C3′]iridium(III) complex (Ir-4). The X-ray crystallographic analysis for Ir-1a–c revealed that these complexes adopt cis-C,C and trans-N,N configuration, indicating that the coordination geometry around the iridium center is not a main factor for the emission spectral differences caused by the O^O ancillary ligands. Furthermore, the ancillary ligand effect on the PL properties of the present iridium(III) complexes is independent on solvent polarity and concentrations. Taking these results into consideration, the ancillary ligand effect should be attributed to the electronic structures of the complexes, and the inter-ligand energy transfer from the triplet metal-to-ligand charge transfer level at the cyclometalated ligand (3MLCTC^N) to the ancillary ligand-related triplet level (3LCO^O or 3MLCTO^O) should play a significant role in the triplet exciton formation. The PL properties of Ir-1–Ir-4 in PMMA films were also investigated, and the remarkable blue shifts of λPLs due to rigidochromism were observed for Ir-1b,c and Ir-2b,c that have the aromatic O^O ancillary ligands. It was found that some of the complexes developed here exhibited larger ΦPLs in PMMA films than in toluene solutions.Graphical abstractHighlights► Ir(III) complexes with various 1,3-diketonate ancillary ligands were prepared. ► The aromatic 1,3-diketonate ancillary ligands give rise to red-shifted PL. ► For the red-emitting complexes, the PL color is independent on the ancillary ligand. ► The ancillary ligand effect on PL is attributed to the inter-ligand energy transfer.

Linear squarylium oligomers bearing extended π-conjugation systems were newly synthesized, and their near-infrared (NIR) light-absorbing properties were investigated. Replacement of the iodo substituent in a 5-iodinated indolino-squarylium dye to the 2-hydroxy-3,4-dioxocyclobut-1-en-1-yl functional group via the Pd-catalyzed cross-coupling with a tributylstannylsquarate followed by condensation with a quaternary indolium salt afforded the semi-squarylium-attached squarylium derivative, i.e., the squarylium dimer. The introduction of the semi-squarylium unit gave rise to a significant bathochromic shift towards the NIR region (λabs = 763 nm in CHCl3 at 298 K). Starting from a 5,5′-diiodinated indolino-squarylium, the iterative extension of semi-squarylium units successfully yielded the linearly π-extended trimer and pentamer, that showed absorption maxima at 862 and 940 nm in CHCl3 at 298 K, respectively. Especially, the pentamer exhibited a considerably low optical band gap of 1.1 eV.Research highlights► We synthesized novel linearly π-extended squarylium oligomers. These dyes exhibited the light absorption in the NIR regions. ► The MO calculations indicated that the NIR-absorption is based on the ICT mechanism. ► The pentameric squarylium showed a considerably low optical band gap of 1.1 eV.

A new series of heteroleptic cyclometalated platinum(II) complexes Pt-1a–f was synthesized, employing 2-arylpyridine (or 1-arylisoquinoline) (HC∧N-1) and 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dione (HO∧O-1) for cyclometalation and as ancillary ligands, respectively, and photoluminescent properties were investigated. Focusing on red-shifted phosphorescence, C∧N ligands containing π-extended aromatics and electron-rich heterocycles were examined. All obtained complexes exhibited photoluminescence at ambient temperature, and the emission maxima ranged from green (λPL=518 nm) to far red (λPL=708 nm). The large Stokes shifts of more than 100 nm and sub-microsecond or microsecond emission lifetimes revealed that these complexes are phosphorescent emissive. The quantum yield of Pt-1 ranged from 0.02 to 0.59 at ambient temperature and decreased as the emission maximum was red-shifted. In comparison with the reference platinum(II) complexes, Pt-2 bearing an aliphatic ancillary ligand, such as 2,2,6,6-tetramethylheptane-3,5-dionate (O∧O-2), the ligand O∧O-1 did not significantly affect the photoluminescence emission maxima, indicating that the energy gap between the singlet ground state and the triplet level was predominantly dependent on the C∧N ligand.

In order to develop a selective and sensitive colorimetric sensor for metal ions, a series of oligoether-linked bis(spiropyran) podands were prepared and their ability to colorimetrically sense alkaline earth metal ions was investigated. UV–vis absorption and 1H NMR spectroscopic studies showed that the podands isomerized from the colorless spiropyran form to the colored merocyanine form upon complexation with alkaline earth metal ions, whereas spectral changes were not visible in the presence of alkali metal ions. Podands of spiropyran subunits linked by a spacer of a 3-oxapentan-1,5-dioxy group, i.e.X-BSP-1 (X = MeO, tBu, iPr, H, Cl, Br), exhibited high selectivity to Ca2+. Introduction of an electron-donating group to the 5-position of each indoline ring of the podand gave rise to an increase in affinity to alkaline earth metal ions, enhancing the sensitivity. The absorption maximum of the colored metal–X-BSP-1 complex varied over a range of ca. 30 nm, from reddish purple to bluish purple. Clear discrimination of the Ca2+ complex from that of Mg2+ was possible using the naked eye. These results indicate the potential application of the X-BSP-1 podands as a colorimetric sensor for Ca2+.

In order to investigate effective dopants to induce chiral nematic liquid crystalline phases, novel freebase (FbBL) and zinc bilinone (ZnBL) derivatives bearing optically active aliphatic groups ((S)-3,7-dimethyloctyls) at the peripheral positions were prepared. From the CD spectra, it was confirmed that M-helicity in the bilinone frameworks was modestly enriched for ZnBLs, whereas helicity was hardly induced for FbBLs except for the o-xylylene-spaced dimer. When N-(4-methoxybenzylidene)-4-butylaniline (MBBA) was doped with the bilinone derivatives, the chiral nematic phase was effectively induced, and the helical twisting powers (βMs) ranged from −95 to −159 μm−1. The control experiment using (S)-3,7-dimethyl-1-phenyloctane (βM = +14 μm−1) clearly showed that the induced chiral helical frameworks of FbBL and ZnBL predominantly contribute to chiral nematic induction of MBBA.

Structurally controlled zinc porphyrin–anthracene dyads, syn-arranged 1 and anti-arranged 2, were newly synthesized employing a diarylurea linkage, and the excitation energy transfer (EET) from the anthracene to the zinc porphyrin chromophore was investigated by steady-state fluorescence emission spectroscopy as well as fluorescence lifetime measurement, especially focusing on the effect of the chromophoric orientation on the EET. In both of the dyads, intramolecular EET was facilitated upon excitation of the anthracene chromophore (λex = 401 nm), and the zinc porphyrin S1–S0 emission (580–720 nm) was enhanced. The EET in the syn-arranged dyad 1 was more efficient than in the anti-arranged 2: the S1–S0 emission in 1 was 1.8 times larger than that in the zinc porphyrin reference compound 3, whereas that in 2 was enhanced by 1.6 times, compared to that in 3. In the fluorescence lifetime measurement, the quiet short-lived component assignable to the EET was observed for the dyads 1 and 2 beyond the analysis limit (<25 ps). The EET rate constants in the dyads 1 and 2 were estimated as not less than 4.0 × 1010 s−1. However, in the case of 2, the residual long-lived component assigned to the anthracene emission was also observed at 425 nm. These results showed that the syn-arrangement of the zinc porphyrin and anthracene chromophores was more preferred for intramolecular EET to the anti-arrangement.