A squaraine-based far-red/near-infrared fluorescent probe (SQ-DNBS) was exploited for thiophenol detection. SQ-DNBS is a colorimetric and “off–on” fluorometric dual-channel “naked-eye” chemosensor showing high selectivity, high sensitivity (detection limit: 9.9 nM), and rapid response to thiophenol in aqueous solution. SQ-DNBS also can be used in practical applications for the detection of thiophenol in water samples. Photophysical and spectral characterization results revealed that the probing mechanism of SQ-DNBS toward thiophenol lies in the thiolate-mediated cleavage reaction. Our discovery demonstrates the potential of the arylmethylene-squaraine skeleton as a promising fluorophore unit to construct high-performance far-red/near-infrared chemosensors.Keywords: colorimetric; fluorometric; high sensitivity; squaraine; thiophenol; water-soluble;

A cyclometalated iridium(III) complex (Ir-DNBS) was designed and synthesized as a high-performance phosphorescent thiophenol probe. Ir-DNBS displays a distinct phosphorescence “off–on” response toward thiophenol with high selectivity, high sensitivity (detection limit: 2.5 nM) and fast response (10 min). It is noteworthy that the signaling phosphore of Ir-DNBS possesses relatively high photoluminescence quantum efficiency (ΦPL = 0.21) together with relatively long lifetime (τ = 2.07 μs), indicative of its potential in achieving high temporal resolution. Ir-DNBS is also applicable to the detection of thiophenol in actual water samples with high recovery rate. Photophysical and spectral characterization results revealed that the probing mechanism of Ir-DNBS toward thiophenol lies in the thiolate-mediated cleavage reaction, resulting in suppressed photo-induced excited state electron transfer process in the reaction product.

Pure white organic light emitting devices (OLEDs) with high color stability have been fabricated, using a 4-aryloxy-1,8-naphthalimide derivative FluONI as a non-doped blue emitting layer (EML) combined with orange emitting exciplex at the interface of electron donor/FluONI. Three kinds of hole transport materials consisting of amino groups with stepped highest occupied molecular orbital (HOMO) levels are introduced as electron donors for the modulations of exciplex emission band and hole injection barrier. As a result, pure white emission with a standard Commission Internationale de 1'Eclairage (CIE) coordinate of (0.33, 0.33) is achieved by adopting N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine as an electron donor with a relatively deep HOMO level and a 30 nm-thick EML. Meanwhile, excellent color stability is also observed with a slight CIE coordinates shift of (0.01, 0.01) at a luminance range from 100 to 4000 cd/m2. According to the systematic analyses on electroluminescence spectra, the pure white emission benefits from the broad emission bands of both FluONI and exciplex. The pure white emission with high color stability at different drive voltages are attributed to the sustainable equilibrium between FluONI intrinsic and exciplex emission. Our devices based on a single non-doped EML provide a simple way to realize color pure and stable white OLEDs.

By simply grafting a –CN group and/or a –OCH3 group onto the meta- and/or para-site of the C-ring, a series of Ir(III) complexes bearing a similar molecular platform of bis(1,2-diphenyl-1H-benzimidazolato-N,C2′)iridium(III)(acetylacetonate), but showing fine-tuned phosphorescence covering nearly the whole window of the visible spectrum with a wide color-tuning range of 109 nm was acquired. With the help of DFT calculations, it was revealed that if the C-related arene moiety of the C^N ligand (C-ring) contributes substantially to both the HOMO and LUMO of an Ir(III) complex, the concurrent introduction of an electron-donating –OCH3 and an electron-withdrawing –CN groups on the C-ring at the meta- and para-sites relative to the Ir atom may lead to a favorable synergetic substituent effect on the color-tuning direction. This may represent a facile yet effective molecular design strategy for Ir(III) complexes with a desirous emission color. A bluish green organic light-emitting diode (OLED) based on one of the objective complexes displayed a maximum current efficiency of 62.1 cd A−1, an external quantum efficiency of 19.8%, and a brightness of 48040 cd m−2, implying that high-performance red and blue OLED phosphors as well as libraries of Ir(III) complexes bearing similar molecular platforms may be developed through this –OCH3 and –CN synergetic substitution strategy.

•Two different donor subunits substituted unsymmetrical squaraines exhibited a totally different aggregation.•The USQ-11/PC71BM blend film displayed quite smaller domains size than that of the USQ-12/PC71BM.•The PCE of USQ-11-based solution-processed BHJ-SMOSCs was much higher than that of USQ-12 (4.27% vs. 2.78%).•2,3,3-Trimethylindolenine unit should be an excellent D subunit for construction of squaraines photovoltaic materials.Two unsymmetrical squaraines (USQs) with different donor (D) subunits as photovoltaic materials, namely USQ-11 and USQ-12, were designed and synthesized to investigate the effect of different D subunits on the optoelectronic properties of USQs for the first time. The two USQs compounds were characterized for optical, electrochemical, quantum chemical and optoelectronic properties. By changing the two different D subunits attached to the squaric acid core from 2,3,3-trimethylindolenine to 2-methylbenzothiazole, the HOMO energy levels could be tuned with a stepping of 0.07 eV, and quite different solid state aggregations (H- or J-aggregation) were observed in the thin film by UV-Vis absorption spectra, which were attributed to their distinct steric effects and dipole moments. Solution-processed bulk-heterojunction small molecule organic solar cells fabricated with the USQ-11/PC71BM (1:5, wt%) exhibited extremely higher PCE (4.27%) than that of the USQ-12/PC71BM (2.78%). The much enhanced PCE should be attributed to the simultaneously improved Voc, Jsc and FF.

•A high-performance fluoride ion fluorescence sensor was developed, namely NIMS.•As a rapid response colorimetric/fluorometric F− sensor, NIMS can detect fluoride ion via a desilylation mechanism.•NIMS can be used for practical applications to detect F− in solid films state and in commercial toothpaste.A novel naphthalimide-based fluorescent sensor, namely NIMS, is designed and synthesized for fluoride ion detection. NIMS undergoes a desilylation reaction upon addition of F− ion, thereby shows a colorimetric/fluorometric dual-channel spectral response, i.e., a huge ratiometric absorption value of 229 nm together with distinct colour change from yellow to blue, and drastically quenched fluorescence as well. Additionally, NIMS shows high selectivity, good sensitivity and rapid response toward F− ion, and could be used in qualitative detection of F− ion in the solid state and quantitative detection F− ion in toothpaste samples.

The development of fluorescent materials capable of harvesting triplet excitons efficiently is of great importance in achieving high-performance low-cost organic light-emitting diodes (OLEDs). Among the three mechanisms converting triplet to singlet excitons, triplet fusion delayed fluorescence (TFDF) plays a key role in the demonstration of highly efficient and reliable OLEDs, especially blue devices, for practice applications. This review focuses on the recent development of TFDF materials and their applications in OLEDs. Fundamental TFDF mechanism, molecular design principles, and the structure-property relationship of TFDF materials with a particular emphasis on their different excited state characters, are presented and discussed. Moreover, the future perspectives and ongoing challenges of TFDF materials are also highlighted.This review focuses on the recent development of triplet fusion delayed fluorescence materials and their applications in organic light-emitting diodes.Download high-res image (108KB)Download full-size image

Three red-emissive D–π–A-structured fluorophores with an aromatic amine as the donor, ethene-1,2-diyl as the π-bridge, and 1,8-naphthalimide as the acceptor subunit, namely, (E)-6-(4-(dimethylamino)styryl)-2-hexyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap1), (E)-2-(2,6-di(isopropyl)phenyl)-6-(4-(dimethylamino)styryl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap2) and (E)-2-(2,6-di(isopropyl)phenyl)-6-(2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap3), were designed and synthesized. In-depth investigations on the correlations between their molecular structures and photophysical characteristics revealed that the presence of an electron-rich 4-dimethylaminophenyl donor moiety in compound Nap1 could endow it with a red emission (e.g., λPLmax = 641 nm in the host–guest blend film with a 14 wt% guest composition); moreover, the replacement of the n-hexyl group of Nap1 bonding to the imide nitrogen atom for a more bulky 2,6-di(isopropyl)phenyl one would result in compound Nap2 with more alleviated concentration quenching. Alteration of the 4-(dimethylamino)phenyl donor subunit of Nap2 into a more electron-donating 1,1,7,7-tetramethyljulolidin-9-yl substituent would render compound Nap3 with more improved chromaticity (e.g., λPLmax = 663 nm in a 14 wt% guest-doped film). Consequently, Nap3 could not only emit standard-red fluorescence with satisfactory chromaticity, but it also showed suppressed intermolecular interactions. Using Nap3 as the dopant, a heavily doped standard-red organic light-emitting diode (OLED) with the device configuration of ITO/MoO3 (1 nm)/TcTa (40 nm)/CzPhONI:Nap3 (14 wt%) (20 nm)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm) was fabricated, and the Commission Internationale de L’Eclairage coordinates, maximum external quantum efficiency and maximum current efficiency of this OLED were (0.67,0.32), 1.8% and 0.7 cd A−1, respectively. All these preliminary results indicated that 1,8-naphthalimide derivatives could act as quite promising standard-red light-emitting materials for OLED applications.

A red naphthalimide derivative with an intramolecular charge-transfer (ICT) feature, namely (E)-2-(4-(t-butyl)phenyl)-6-(2-(6-(diphenylamino)naphthalen-2-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NA-TNA), was designed and synthesized. Photophysical and magneto-electroluminescence (MEL) characterization results revealed that NA-TNA could harvest triplet excitons via a triplet–triplet annihilation (TTA) process in organic light-emitting diodes (OLEDs) due to the presence of a lower-lying triplet excited state with 3ππ* character. Consequently, using NA-TNA as a guest compound and CzPhONI, another ICT-featured naphthalimide derivative with triplet fusion delayed fluorescence (TFDF) character as host material, a high-performance orange OLED with 6 wt% NA-TNA doped CzPhONI film as the emitting layer was acquired. The maximum current efficiency (LEmax), brightness (Lmax), and external quantum yield (EQEmax) of this OLED is 7.73 cd A−1, 31940 cd m−2 and 5.83%, respectively, while the theoretical EQEmax of this device should not exceed 3.34%. On the contrary, the reference device with a NA-TNA doping level of 1.4 wt% showed much inferior performance, with a LEmax, a Lmax, and an EQEmax of 3.19 cd A−1, 24900 cd m−2 and 2.49%, respectively. The high performance of the 6 wt% NA-TNA doped device was attributed to the efficient harvesting of triplet excitons by both the guest and host materials.

An asymmetrical squaraine dye (Py-3) with its two electron-donating aryl groups directly linked to the electron-withdrawing squaric acid core possesses an ideal bandgap of 1.33 eV, together with an intense and broad absorption band in the range 550–950 nm. Hence, the resulting solution-processed solar cells display an impressive Jsc of 12.03 mA cm−2 and a PCE of 4.35%.

Small molecular neutral Ir(III) complexes have been demonstrated to be promising self-inclusive microcrystalline thin-film oxygen sensors with relatively high sensitivity (Ksv = 6.41), good stability, and linear Stern–Volmer behavior (R2 = 0.9979).

Two novel asymmetrical squaraines based on the indoline unit, ASQ-5-F and ASQ-5-DF, with one and two fluorine substituents, have been developed to investigate the effect of fluorine substituted on small-molecule bulk-heterojunction (BHJ) organic solar cells (OSCs). In comparison with non-fluorine-substituted ASQ-5, both fluorine-substituted ASQ-5-F and ASQ-5-DF possess analogous absorption band gaps but 0.05 and 0.10 eV lowered highest occupied molecular orbital (HOMO) energy levels, respectively. Single-crystal analysis exhibits that ASQ-5-DF shows more desirable intermolecular packing patterns for the hole-carrier collection than ASQ-5 does; hence, higher hole mobility could be acquired. Therefore, solution-processed small-molecule BHJ OSCs fabricated with ASQ-5-F/PC71BM and ASQ-5-DF/PC71BM blends exhibit extremely higher power conversion efficiency (PCE; 5.0% and 6.0%, respectively) than that of ASQ-5/PC71BM (4.5%). The much improved PCE could be attributed to the simultaneously enhanced Voc, Jsc, and FF relative to those of the ASQ-5-based device. To our knowledge, this is the highest PCE (6.0%) among squaraine-based solution-processed BHJ OSCs and the highest PCE in OSCs based on the fluorinated donor segment of small molecules.Keywords: asymmetrical squaraines; fluorine-substituted; packing pattern; small-molecule organic solar cells; solution-processed;

A series of new asymmetrical squaraine derivatives bearing N,N-diarylamino substituents as end-capping groups, namely ASQAr-1–6, were designed and synthesized. In comparison with the reference compound ASQB bearing a N,N-diisobutylamino end-capper, all the six target compounds exhibit improved thermal stability, red-shifted and broadened absorption bands as well as lower HOMO and LUMO energy levels. Despite the hole mobility of most of the compounds still being lower than that of ASQB, solution-processed bulk-heterojunction small molecule organic solar cells (BHJ-SMOSCs) using ASQAr-1–6 as electron donor materials all show drastically higher power conversion efficiency (PCE, 3.08–3.69%) than that of the ASQB-based reference device (PCE = 1.54%). The much enhanced photovoltaic performance of BHJ-SMOSCs based-on ASQAr-1–6 could be attributed to the simultaneously enhanced open-circuit voltage (Voc, 0.81–0.87 V vs. 0.75 V), short-circuit current density (Jsc, 8.07–9.06 mA cm−2 vs. 5.40 mA cm−2), and fill factor (FF, 0.45–0.47 vs. 0.38) relative to those of the reference ASQB-based device.

•OLEDs were fabricated using a charge-transfer-featured host.•Energy transfer and triplet up-conversion were responsible for the high EQE.•Direct exciton formation is involved in phosphorescent OLEDs.•The heavy doping property was elucidated by thermal and morphological analyses.Fluorescent and phosphorescent organic light-emitting diodes (OLEDs) were fabricated using a charge-transfer-featured compound, 6-{3,5-bis-[9-(4-t-butylphenyl)-9H-carbazol-3-yl]-phenoxy}-2-(4-t-butylphenyl)-benzo[de]isoquinoline-1,3-dione, as a host, and the electroluminescent (EL) characteristics of these two kinds of OLEDs were systematically studied. According to the photoluminescent quantum yields (ηpl), it was found that the external quantum efficiencies of both fluorescent and phosphorescent OLEDs were exceeding their theoretical limits. Based on the analysis of EL characteristics, the high device performance of fluorescent and phosphorescent OLEDs was attributed to both efficient energy transfer and triplet energy up-conversion, while direct exciton formation was also involved in phosphorescent OLEDs. In addition, the host film possessed high thermal and morphological stabilities due to the attachment of steric bulks on host molecule, resulting in the high doping concentration for both fluorescent and phosphorescent dyes. These results indicated that charge-transfer-featured material could be the promising host for both fluorescent and phosphorescent OLEDs.

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

A dual-channel naphthalimide-based chemosensor for rapid and sensitive detection of fluoride ion has been developed. Upon addition of F–, it undergoes deprotonation reaction through H-bonding interactions, and its maximum absorption wavelength is red-shifted for 214 nm to the far-red region, together with drastically quenched fluorescence. In addition, it shows high selectivity toward F– anion, thus could be used for practical applications to detecting F– in both solution and solid state. Furthermore, the fluorescence of NIM could be enhanced in protein-containing acidic environments, hence NIM could act as lysosome marker to differentiate cancer cells from normal ones in cell imaging.Keywords: cell imaging; chemosensor; dual-channel; fluoride ions; selectivity;

A charge-transfer-featured naphthalimide derivative with a small exchange energy but a lower lying 3ππ* state than 3CT state is found to contribute to triplet harvesting through a P-type rather than an E-type delayed fluorescence, and could act as a quite promising host to achieve highly efficient OLEDs.

A novel structurally simple and easily prepared pH probe based on a twisted intramolecular charge transfer signalling mechanism, namely Napa-pp, has been designed and synthesized. In aqueous buffer solution, it has a valuable pKa of 5.32 that matches the typical pH range of acidic organelles, hence it is applicable for visualizing lysosomes in living cells.

By introducing a phenyl substituent into the meta-site of the phenyl segment of the 2-phenylbenzothiazole ligand, two novel orange iridium(III) complexes, namely, (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac), have been synthesized. Compared with their parent compound (bt)222Ir(acac), both of them possess much enhanced thermostability and film amorphism, making them suitable candidates as guests for high performance solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). However, (4Phbt)222Ir(acac) bearing para-phenyl possesses worse processability relative to (bt)222Ir(acac) due to spontaneous crystallization stemming from the intense intermolecular interactions. Single-layer solution-processed PhOLEDs with (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac) as guests show peak current efficiency of 17.2 cd A−1 and 15.2 cd A−1, and maximum brightness of 28270 cd m−2 and 27900 cd m−2, respectively. Both are greatly improved compared to the devices employing (bt)222Ir(acac) (10.2 cd A−1 and 14350 cd m−2) and (4Phbt)222Ir(acac) (5.0 cd A−1 and 13790 cd m−2) as phosphors. Moreover, quite low efficiency roll-off is acquired in these devices at high luminance. The much improved electroluminescence performances of these objective complexes could be mainly attributed to the presence of a rigid phenyl on the appropriate substitution site of the cyclometallate ligand, which leads to improved thermostability with compatible alleviated intermolecular interactions, and consequently enhanced film amorphism.

•Two novel polymers bearing acenaphtho[1,2-b]quinoxaline as electron-deficient unit.•Strong S⋯O interaction(sum of van der Waals radii for S and O is 3.32 Å).•PB and PT possess deep HOMO energy level of −5.4 to −5.5 eV.•PB-based device showed maximum PCE value of 1.20% (Voc = 0.78 V).Dialkoxy substituted acenaphtho [1,2-b]quinoxaline (AQx) is demonstrated to be a promising electronic acceptor subunit for constructing high performance donor-acceptor (D–A) photovoltaic copolymers. Using it as acceptor and benzo[1,2-b:4,5-b′]-dithiophene (BDT) or thiophene (T) as donor units, two D–A conjugated polymers (abbr. as PB and PT respectively) are synthesized via Stille coupling reaction. The incorporation of alkoxy groups with quinoxaline moiety is found to be beneficial to both the enhanced solubility and the maintained coplanarity of the backbone due to strong S⋯O attractive interaction. UV–vis absorption and XRD characterization results indicate that the presence of planar AQx moiety could lead to well-ordered packing in solid state, and PB bearing planar BDT electron-donating subunit exhibits more intensified molecular self-organization. Both PB and PT possess deep HOMO energy level of −5.4∼−5.5 eV, and PB-based photovoltaic devices show maximum PCE value of 1.20% (Voc = 0.78 V, Jsc = 3.58 mA cm−2, FF = 0.43) under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW cm−2.

A neutral phosphorescent coordination compound bearing a benzimidazole ligand, Ir(pbi)2(acac) (Hpbi = 1,2-diphenyl-1H-benzo[d]imidazole; Hacac = acetylacetone), is demonstrated to be the first example of a sulfur-free iridium complex for the detection of Hg2+ cations with high selectivity and sensitivity. Ir(pbi)2(acac) shows a multisignaling response towards mercury(II) ions through UV-vis absorption, phosphorescence and electrochemistry measurements. Upon addition of Hg2+ ions, solutions of this complex change from yellow to colorless, which could be observed easily by the naked eye, while its phosphorescence turns from bright green (λPLmax = 520 nm) into faint skyblue (λPLmax = 476 nm), and the detection limit is calculated to be 2.4 × 10−7 mol L−1. 1H NMR spectroscopic titration as well as ESI-MS results indicate that the decomposition of Ir(pbi)2(acac) in the presence of Hg2+ through rupture of Ir–O bonds is responsible for the significant variations in both optical and electrochemical signals.

Two novel blue light-emitting sextuple hydrogen-bonding self-assembly molecular duplexes bearing 4-phenoxy-1,8-naphthalimide fluorophores, namely PhNIHB and 2TPhNIHB, have been synthesized and characterized. Compared with their small molecular counterparts PhNI and 2TPhNI, the objective compounds exhibit 13–22 nm blue-shifted fluorescent emission, and much higher photoluminescence quantum yields (0.34 vs 0.18 for PhNIHB; 0.42 vs 0.27 for 2TPhNIHB) in solid state; and their thermal and morphological stability have been improved as well. Employing 2TPhNIHB or 2TPhNI as emitter, non-doped solution-processed light-emitting diodes with structure of ITO/PEDOT: PSS (40 nm)/PVK (40 nm)/blue emitter (70–80 nm)/CsF (1.5 nm)/Al (120 nm) have been fabricated. The 2TPhNI-based device gives yellow emission [CIE (0.38, 0.49)] with poor maximum luminous efficiency (LEmax) of 0.13 cd/A and external quantum efficiency (EQEmax) of 0.06%. The 2TPhNIHB-based device, however, gives blue-green emission [CIE (0.25, 0.34)], with much higher efficiency relative to 2TPhNI-based one (LEmax of 0.37 cd/A and EQEmax of 0.35%). The effective isolation of the naphthalimide fluorescent cores as well as the suppressed formation of exciplex at the PVK/emitter interface by these oligoamide motifs are suggested to be responsible for the improved EL performance.Graphical abstractHighlights► Blue multiple H-bonds self-assembly duplexes have been synthesized. ► They exhibit much enhanced PL and EL efficiency. ► They exhibit much improved thermal stability and film morphology.

Two iridium complexes bearing benzothiazole cyclometallate ligands, bis[2-(3′,5′-di-tert-butylbiphenyl-4-yl)benzothiazolato-N,C2′]iridium(III)(acetylacetonate) [(tbpbt)2Ir(acac)] and bis[2-(9,9-dimethyl-9H-fluoren-2-yl)benzothiazolato-N,C2′]iridium(III)(acetylacetonate) [(fbt)2Ir(acac)], have been evaluated as orange and red electrophosphorescent materials. Both X-ray crystallographic analysis and photophysical results indicate that they possess alleviated self-quenching characteristics due to the existence of steric bulky ligands. As a result, phosphorescent organic light-emitting diodes (PhOLEDs) based on them show high performance even in heavily-doped level (≥ 15 wt.%). The (tbpbt)2Ir(acac)-based PhOLED gives efficient orange emission with peak current efficiency of 26.9 cd/A (1280 cd/m2) at doping ratio of 15 wt.%, while the 15 wt.% (fbt)2Ir(acac)-doped device emits efficient red light with Commission Internationale de l'Eclairage coordinates of (0.63, 0.36), and peak current and external quantum efficiency of 28.5 cd/A (1210 cd/m2) and 15.6%, respectively. Moreover, all these heavily-doped PhOLEDs exhibit low efficiency roll-off at relatively high current density.Highlights► Two iridium complexes with bulky ligands are developed as orange/red emitter. ► Organic light-emitting diodes using these phosphors show low efficiency roll-off. ► High performance devices could be achieved under high doping ratio of ≥ 15 wt.%. ► The high device efficiencies arise from the reduced self-quenching of the phosphors.

Four novel iridium(III) complexes bearing biphenyl (7a–7c) or fluorenyl (7d) modified benzothiazole cyclometallate ligands are synthesized. In comparison with the yellow parent complex, bis(2-phenylbenzothiozolato-N,C2′) iridium(III) (acetylacetonate) [(pbt)2Ir(acac)] (λPLmax = 557 nm, φPL = 0.26), 7a–7d show 20–43 nm bathochromic shifted orange or red phosphorescence in solution, with maximum photoluminescence (PL) quantum yield of 0.62, and PL lifetime of 1.8–2.0 μs. Meanwhile, the resulting complexes also exhibit intense orange or red phosphorescence of λPLmax = 588–611 nm in solid films. The complex 7c with two tert-butyl substituents possesses the highest phosphorescent efficiency both in dilute solution and thin solid films, therefore may be a prospective candidate for both doping and host emitting electrophosphorescent material. Furthermore, despite the observation of severe oxygen quenching for 7a–7d in solution, 7a and 7c even show efficient emission intensity quenching by oxygen in their solid state due to the existence of void channels in crystals; consequently, they are promising molecular oxygen sensor reagents. Electrochemical measurement and DFT calculation results suggest that all these chelates own declined LUMOs of 0.1 eV relative to that of (pbt)2Ir(acac) owing to the contribution of the phenyl substituents; whereas only 7d shows a more destabilized HOMO (∼0.1 eV) compared with the parent chelate.

Substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy or tert-butyl modified phenoxy groups, novel naphthalimide derivatives were obtained which emitted blue fluorescence with emission peaks of 425–444 nm in chloroform solution under UV irradiation, with highest relative photoluminescence quantum efficiency of 0.82. When in solid film, only compounds that contained ortho-tert-butylphenoxy substituents displayed blue photoluminescence of 438–451 nm, with highest absolute fluorescence quantum yield of 0.29; whereas other compounds showed greenish blue fluorescence at 471–478 nm, with highest absolute fluorescence quantum yield of 0.42. Cyclic voltammetry studies revealed that the molecules have low-lying energy levels of the lowest unoccupied molecular orbital (LUMO) ranging from −3.29 eV to −3.24 eV, and energy levels of the highest occupied molecular orbital (HOMO) ranging from −6.26 eV to −6.16 eV, suggesting they may possess good electron-transporting or hole-blocking properties. The findings indicate that the molecules offer potential as dopants as well as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue or greenish blue organic light-emitting diodes.

Four novel iridium(III) complexes bearing biphenyl (7a–7c) or fluorenyl (7d) modified benzothiazole cyclometallate ligands are synthesized. In comparison with the yellow parent complex, bis(2-phenylbenzothiozolato-N,C2′) iridium(III) (acetylacetonate) [(pbt)2Ir(acac)] (λPLmax = 557 nm, φPL = 0.26), 7a–7d show 20–43 nm bathochromic shifted orange or red phosphorescence in solution, with maximum photoluminescence (PL) quantum yield of 0.62, and PL lifetime of 1.8–2.0 μs. Meanwhile, the resulting complexes also exhibit intense orange or red phosphorescence of λPLmax = 588–611 nm in solid films. The complex 7c with two tert-butyl substituents possesses the highest phosphorescent efficiency both in dilute solution and thin solid films, therefore may be a prospective candidate for both doping and host emitting electrophosphorescent material. Furthermore, despite the observation of severe oxygen quenching for 7a–7d in solution, 7a and 7c even show efficient emission intensity quenching by oxygen in their solid state due to the existence of void channels in crystals; consequently, they are promising molecular oxygen sensor reagents. Electrochemical measurement and DFT calculation results suggest that all these chelates own declined LUMOs of 0.1 eV relative to that of (pbt)2Ir(acac) owing to the contribution of the phenyl substituents; whereas only 7d shows a more destabilized HOMO (∼0.1 eV) compared with the parent chelate.

A neutral phosphorescent coordination compound bearing a benzimidazole ligand, Ir(pbi)2(acac) (Hpbi = 1,2-diphenyl-1H-benzo[d]imidazole; Hacac = acetylacetone), is demonstrated to be the first example of a sulfur-free iridium complex for the detection of Hg2+ cations with high selectivity and sensitivity. Ir(pbi)2(acac) shows a multisignaling response towards mercury(II) ions through UV-vis absorption, phosphorescence and electrochemistry measurements. Upon addition of Hg2+ ions, solutions of this complex change from yellow to colorless, which could be observed easily by the naked eye, while its phosphorescence turns from bright green (λPLmax = 520 nm) into faint skyblue (λPLmax = 476 nm), and the detection limit is calculated to be 2.4 × 10−7 mol L−1. 1H NMR spectroscopic titration as well as ESI-MS results indicate that the decomposition of Ir(pbi)2(acac) in the presence of Hg2+ through rupture of Ir–O bonds is responsible for the significant variations in both optical and electrochemical signals.

By introducing a phenyl substituent into the meta-site of the phenyl segment of the 2-phenylbenzothiazole ligand, two novel orange iridium(III) complexes, namely, (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac), have been synthesized. Compared with their parent compound (bt)222Ir(acac), both of them possess much enhanced thermostability and film amorphism, making them suitable candidates as guests for high performance solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). However, (4Phbt)222Ir(acac) bearing para-phenyl possesses worse processability relative to (bt)222Ir(acac) due to spontaneous crystallization stemming from the intense intermolecular interactions. Single-layer solution-processed PhOLEDs with (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac) as guests show peak current efficiency of 17.2 cd A−1 and 15.2 cd A−1, and maximum brightness of 28270 cd m−2 and 27900 cd m−2, respectively. Both are greatly improved compared to the devices employing (bt)222Ir(acac) (10.2 cd A−1 and 14350 cd m−2) and (4Phbt)222Ir(acac) (5.0 cd A−1 and 13790 cd m−2) as phosphors. Moreover, quite low efficiency roll-off is acquired in these devices at high luminance. The much improved electroluminescence performances of these objective complexes could be mainly attributed to the presence of a rigid phenyl on the appropriate substitution site of the cyclometallate ligand, which leads to improved thermostability with compatible alleviated intermolecular interactions, and consequently enhanced film amorphism.

Three red-emissive D–π–A-structured fluorophores with an aromatic amine as the donor, ethene-1,2-diyl as the π-bridge, and 1,8-naphthalimide as the acceptor subunit, namely, (E)-6-(4-(dimethylamino)styryl)-2-hexyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap1), (E)-2-(2,6-di(isopropyl)phenyl)-6-(4-(dimethylamino)styryl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap2) and (E)-2-(2,6-di(isopropyl)phenyl)-6-(2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap3), were designed and synthesized. In-depth investigations on the correlations between their molecular structures and photophysical characteristics revealed that the presence of an electron-rich 4-dimethylaminophenyl donor moiety in compound Nap1 could endow it with a red emission (e.g., λPLmax = 641 nm in the host–guest blend film with a 14 wt% guest composition); moreover, the replacement of the n-hexyl group of Nap1 bonding to the imide nitrogen atom for a more bulky 2,6-di(isopropyl)phenyl one would result in compound Nap2 with more alleviated concentration quenching. Alteration of the 4-(dimethylamino)phenyl donor subunit of Nap2 into a more electron-donating 1,1,7,7-tetramethyljulolidin-9-yl substituent would render compound Nap3 with more improved chromaticity (e.g., λPLmax = 663 nm in a 14 wt% guest-doped film). Consequently, Nap3 could not only emit standard-red fluorescence with satisfactory chromaticity, but it also showed suppressed intermolecular interactions. Using Nap3 as the dopant, a heavily doped standard-red organic light-emitting diode (OLED) with the device configuration of ITO/MoO3 (1 nm)/TcTa (40 nm)/CzPhONI:Nap3 (14 wt%) (20 nm)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm) was fabricated, and the Commission Internationale de L’Eclairage coordinates, maximum external quantum efficiency and maximum current efficiency of this OLED were (0.67,0.32), 1.8% and 0.7 cd A−1, respectively. All these preliminary results indicated that 1,8-naphthalimide derivatives could act as quite promising standard-red light-emitting materials for OLED applications.

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

By simply grafting a –CN group and/or a –OCH3 group onto the meta- and/or para-site of the C-ring, a series of Ir(III) complexes bearing a similar molecular platform of bis(1,2-diphenyl-1H-benzimidazolato-N,C2′)iridium(III)(acetylacetonate), but showing fine-tuned phosphorescence covering nearly the whole window of the visible spectrum with a wide color-tuning range of 109 nm was acquired. With the help of DFT calculations, it was revealed that if the C-related arene moiety of the C^N ligand (C-ring) contributes substantially to both the HOMO and LUMO of an Ir(III) complex, the concurrent introduction of an electron-donating –OCH3 and an electron-withdrawing –CN groups on the C-ring at the meta- and para-sites relative to the Ir atom may lead to a favorable synergetic substituent effect on the color-tuning direction. This may represent a facile yet effective molecular design strategy for Ir(III) complexes with a desirous emission color. A bluish green organic light-emitting diode (OLED) based on one of the objective complexes displayed a maximum current efficiency of 62.1 cd A−1, an external quantum efficiency of 19.8%, and a brightness of 48040 cd m−2, implying that high-performance red and blue OLED phosphors as well as libraries of Ir(III) complexes bearing similar molecular platforms may be developed through this –OCH3 and –CN synergetic substitution strategy.

A charge-transfer-featured naphthalimide derivative with a small exchange energy but a lower lying 3ππ* state than 3CT state is found to contribute to triplet harvesting through a P-type rather than an E-type delayed fluorescence, and could act as a quite promising host to achieve highly efficient OLEDs.

A red naphthalimide derivative with an intramolecular charge-transfer (ICT) feature, namely (E)-2-(4-(t-butyl)phenyl)-6-(2-(6-(diphenylamino)naphthalen-2-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NA-TNA), was designed and synthesized. Photophysical and magneto-electroluminescence (MEL) characterization results revealed that NA-TNA could harvest triplet excitons via a triplet–triplet annihilation (TTA) process in organic light-emitting diodes (OLEDs) due to the presence of a lower-lying triplet excited state with 3ππ* character. Consequently, using NA-TNA as a guest compound and CzPhONI, another ICT-featured naphthalimide derivative with triplet fusion delayed fluorescence (TFDF) character as host material, a high-performance orange OLED with 6 wt% NA-TNA doped CzPhONI film as the emitting layer was acquired. The maximum current efficiency (LEmax), brightness (Lmax), and external quantum yield (EQEmax) of this OLED is 7.73 cd A−1, 31940 cd m−2 and 5.83%, respectively, while the theoretical EQEmax of this device should not exceed 3.34%. On the contrary, the reference device with a NA-TNA doping level of 1.4 wt% showed much inferior performance, with a LEmax, a Lmax, and an EQEmax of 3.19 cd A−1, 24900 cd m−2 and 2.49%, respectively. The high performance of the 6 wt% NA-TNA doped device was attributed to the efficient harvesting of triplet excitons by both the guest and host materials.

Small molecular neutral Ir(III) complexes have been demonstrated to be promising self-inclusive microcrystalline thin-film oxygen sensors with relatively high sensitivity (Ksv = 6.41), good stability, and linear Stern–Volmer behavior (R2 = 0.9979).

An asymmetrical squaraine dye (Py-3) with its two electron-donating aryl groups directly linked to the electron-withdrawing squaric acid core possesses an ideal bandgap of 1.33 eV, together with an intense and broad absorption band in the range 550–950 nm. Hence, the resulting solution-processed solar cells display an impressive Jsc of 12.03 mA cm−2 and a PCE of 4.35%.