The development of high-performance near-infrared organic light-emitting diodes is hindered by strong non-radiative processes as governed by the energy gap law. Here, we show that exciton ...delocalization, which serves to decouple the exciton band from highly vibrational ladders in the S0 ground state, can bring substantial enhancements in the photoluminescence quantum yield of emitters, bypassing the energy gap law. Experimental proof is provided by the design and synthesis of a series of new Pt(ii) complexes with a delocalization length of 5–9 molecules that emit at 866–960 nm with a photoluminescence quantum yield of 5–12% in solid films. The corresponding near-infrared organic light-emitting diodes emit light with a 930 nm peak wavelength and a high external quantum efficiency up to 2.14% and a radiance of 41.6 W sr−1 m−2. Both theoretical and experimental results confirm the exciton–vibration decoupling strategy, which should be broadly applicable to other well-aligned molecular solids.Pt(ii) complexes allow the fabrication of efficient near-infrared organic light-emitting diodes that operate beyond the 900 nm region.
The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier ...transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm
was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs.
The electron positive boron atom usually does not contribute to the frontier orbitals for several lower‐lying electronic transitions, and thus is ideal to serve as a hub for the spiro linker of ...light‐emitting molecules, such that the electron donor (HOMO) and acceptor (LUMO) moieties can be spatially separated with orthogonal orientation. On this basis, we prepared a series of novel boron complexes bearing electron deficient pyridyl pyrrolide and electron donating phenylcarbazolyl fragments or triphenylamine. The new boron complexes show strong solvent‐polarity dependent charge‐transfer emission accompanied by a small, non‐negligible normal emission. The slim orbital overlap between HOMO and LUMO and hence the lack of electron correlation lead to a significant reduction of the energy gap between the lowest lying singlet and triplet excited states (ΔET‐S) and thereby the generation of thermally activated delay fluorescence (TADF).
Reducing the gap: Using a boron atom as the spiro linker between an electron‐deficient pyridyl pyrrolide and an electron‐donating phenylcarbazolyl or triphenylamine fragment, boron complexes with a narrow HOMO–LUMO orbital overlap, small singlet–triplet energy gap (down to 38 meV), and strong thermally activated delayed fluorescence (TADF) were prepared. For the first time boron‐complex‐based OLEDs show a significant TADF contribution.
A new class of neutral bis‐tridentate Ir(III) metal complexes that show nearly unitary red, green, and blue emissions in solution is prepared and employed for the fabrication of both monochrome and ...white‐emitting organic light‐emitting diodes, among which a green device gives external quantum efficiency exceeding 31%.
A new series of molecules, T1–T4, possessing thermally activated delayed fluorescence (TADF) have been strategically designed and synthesized. Molecules T1–T4 contain the dimethyl acridine as the ...electron donor, which is linked to either symmetrical or unsymmetrical diphenyl pyrimidine as an acceptor. In comparison to the ubiquitous triazine acceptor, the selection of pyrimidine as an acceptor has advantages of facile functionalization and less stabilized unoccupied π orbitals, so that the energy gap toward the blue region can be accessed. Together with acridine donors, the resulting donor–acceptor functional materials reveal remarkable TADF properties. In the solid state, molecules T1–T4 all exhibit intriguing mechanochromism. The crystal structures, together with spectroscopy and dynamics acquired upon application of stressing, lead us to propose two types of structural arrangement that give distinct emission properties, one with and the other without TADF. Upon fabricating organic light‐emitting diodes, the T1–T4 films prepared from sublimation all exhibit dominant TADF behavior; this accounts for their high performance: an electroluminescent emission at λ=490 nm, with an external quantum efficiency of 14.2 %, can be attained when T2 is used as an emitter.
Functional pyrimidine acceptors: A new series of molecules, T1–T4, possessing thermally activated delayed fluorescence (TADF) have been strategically designed and synthesized. Together with acridine donors, the resulting donor– acceptor functional materials reveal remarkable TADF properties. In the solid state, molecules T1–T4 also exhibit intriguing mechanochromism (see figure).
Pt(II) metal complexes are known to exhibit strong solid‐state aggregation and are promising for realization of efficient emission in fabrication of organic light emitting diodes (OLED) with nondoped ...emitter layer. Four pyrimidine–pyrazolate based chelates, together with four isomeric Pt(II) metal complexes, namely: Pt(pm2z)2, Pt(tpm2z)2, Pt(pm4z)2, and Pt(tpm4z)2, are isolated and systematically investigated for their structure–property relationships for practical OLED applications. Detailed single molecular and aggregated structures are revealed by photophysical and mechanochromic measurements, grazing‐incidence X‐ray diffraction, and theoretical approaches. These results suggest that these Pt(II) emitters pack like a deck of playing cards under vacuum deposition, and their emission energy is not only affected by the single molecular designs, but notably influenced by their intermolecular packing interaction, i.e., Pt···Pt separations that are arranged in the order: Pt(tpm4z)2 > Pt(pm4z)2 > Pt(tpm2z)2 > Pt(pm2z)2. Nondoped OLED with emission ranging from green to red are prepared, to which the best performances are recorded for Pt(tpm2z)2, giving maximum external quantum efficiency (EQE) of 27.5% at 103 cd m−2, maximum luminance of 2.5 × 105 cd m−2 at 17 V, and with stable CIEx,y of (0.56, 0.44).
Nondoped organic light emitting diodes (OLED) devices with emission ranging from green to red are prepared using four Pt(II) complexes that exhibit strong solid‐state aggregation and horizontal aligned transition dipole moments, while their stacking characteristics also play a crucial role in manipulating the emission properties.
A star-shaped 1,3,5-triazine/cyano hybrid molecule CN-T2T was designed and synthesized as a new electron acceptor for efficient exciplex-based OLED emitter by mixing with a suitable electron donor ...(Tris-PCz). The CN-T2T/Tris-PCz exciplex emission shows a high ΦPL of 0.53 and a small ΔE T‑S = −0.59 kcal/mol, affording intrinsically efficient fluorescence and highly efficient exciton up-conversion. The large energy level offsets between Tris-PCz and CN-T2T and the balanced hole and electron mobility of Tris-PCz and CN-T2T, respectively, ensuring sufficient carrier density accumulated in the interface for efficient generation of exciplex excitons. Employing a facile device structure composed as ITO/4% ReO3:Tris-PCz (60 nm)/Tris-PCz (15 nm)/Tris-PCz:CN-T2T(1:1) (25 nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm), in which the electron–hole capture is efficient without additional carrier injection barrier from donor (or acceptor) molecule and carriers mobilities are balanced in the emitting layer, leads to a highly efficient green exciplex OLED with external quantum efficiency (EQE) of 11.9%. The obtained EQE is 18% higher than that of a comparison device using an exciplex exhibiting a comparable ΦPL (0.50), in which TCTA shows similar energy levels but higher hole mobility as compared with Tris-PCz. Our results clearly indicate the significance of mobility balance in governing the efficiency of exciplex-based OLED. Exploiting the Tris-PCz:CN-T2T exciplex as the host, we further demonstrated highly efficient yellow and red fluorescent OLEDs by doping 1 wt % Rubrene and DCJTB as emitter, achieving high EQE of 6.9 and 9.7%, respectively.
The “tablelands” in Taiwan are sedimentary terraces occurring in the foreland basin west of the Neogene mountain ranges. The Miaoli Tableland consists of elevated Late Quaternary sedimentary ...successions, representing a change from tidal to coastal and fluvial to eolian depositional environments. The present-day morphology is a result of combined processes, including differential tectonic uplift, ongoing fluvial aggradation, and incision. Selected deposits in 10 outcrops were sampled and studied by optical dating. The deposition of fluvial sediments started after the last interglacial (<100 ka) in the southeast of the tablelands. Uplift and sea-level lowering caused a base-level fall, resulting in a stepwise redeposition of the fluvial sediments. Additionally, enhanced remobilization of fluvial sediments occurred during the cold/dry climate during Marine Oxygen Isotope Stages (MIS) 4 and 2. The depositional ages of the coastal sediments enabled the estimation of long-term uplift rates of ca. 0.5 to 3.5 mm/yr. The eolian cover sediments yielded MIS 3 (east) to Holocene ages (west). Our results provide new insight into the interplay of climate, sea-level changes, remobilization of sediments, and tectonism leading to tableland formation during the Late Quaternary.
Bis‐tridentate Ir(III) metal complexes are expected to show great potential in organic light‐emitting diode (OLED) applications due to the anticipated, superb chemical and photochemical stability. ...Unfortunately, their exploitation has long been hampered by lack of adequate methodology and with inferior synthetic yields. This hurdle can be overcome by design of the first homoleptic, bis‐tridentate Ir(III) complex Ir(pzpyph)(pzHpyph) (1), for which the abbreviation (pzpyph)H (or pzHpyph) stands for the parent 2‐pyrazolyl‐6‐phenyl pyridine chelate. After that, methylation and double methylation of 1 afford the charge‐neutral Ir(III) complex Ir(pzpyph)(pzMepyph) (2) and cationic complex Ir(pzMepyph)2PF6 (3), while deprotonation of 1 gives formation of anionic Ir(pzpyph)2NBu4 (4), all in high yields. These bis‐tridentate Ir(III) complexes 2–4 are highly emitted in solution and solid states, while the charge‐neutral 2 and corresponding t‐butyl substituted derivative Ir(pzpyBuph)(pzMepyBuph) (5) exhibit superior photostability versus the tris‐bidentate references Ir(ppy)2(acac) and Ir(ppy)3 in toluene under argon, making them ideal OLED emitters. For the track record, phosphor 5 gives very small efficiency roll‐off and excellent overall efficiencies of 20.7%, 66.8 cd A−1, and 52.8 lm W−1 at high brightness of 1000 cd m−2. These results are expected to inspire further studies on the bis‐tridentate Ir(III) complexes, which are judged to be more stable than their tris‐bidentate counterparts from the entropic point of view.
Homoleptic bis‐tridentate Ir(III) complexes are synthesized using a facile and simplified methodology. One monomethylated derivative (i.e., a t‐butyl emitter) exhibits very small efficiency roll‐off and excellent efficiencies of 20.7%, 66.8 cd A−1, and 52.8 lm W−1, at a high brightness of 1000 cd m−2.
In this work, we report a series of bis-tridentate Ir(III) metal complexes, comprising a dianionic pyrazole-pyridine-phenyl tridentate chelate and a monoanionic chelate bearing a peripheral carbene ...and carboline coordination fragment that is linked to the central phenyl group. All these Ir(III) complexes were synthesized with an efficient one-pot and two-step method, and their emission hue was fine-tuned by variation of the substituent at the central coordination entity (i.e., pyridinyl and phenyl group) of each of the tridentate chelates. Their photophysical and electrochemical properties, thermal stabilities and electroluminescence performances are examined and discussed comprehensively. The doped devices based on Ir(cbF)(phyz1) (Cb1) and Ir(cbB)(phyz1) (Cb4) give a maximum external quantum efficiency (current efficiency) of 16.6% (55.2 cd/A) and 13.9% (43.8 cd/A), respectively. The relatively high electroluminescence efficiencies indicate that bis-tridentate Ir(III) complexes are promising candidates for OLED applications.
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