Adamantane‐Based Wide‐Bandgap Host Material: Blue Electrophosphorescence with High Efficiency and Very High Brightness

An adamantane‐based host material, namely, 4‐{3‐[4‐(9H‐carbazol‐9‐yl)phenyl]adamantan‐1‐yl}benzonitrile (CzCN‐Ad), was prepared by linking an electron‐donating carbazole unit and an electron‐accepting benzonitrile moiety through an adamantane bridge. In this approach, two functional groups were attached to tetrahedral points of adamantane to construct an “sp³” topological configuration. This design strategy endows the host material with a high triplet energy of 3.03 eV due to the disruption of intramolecular charge transfer. Although CzCN‐Ad has a low molecular weight, the rigid nonconjugated adamantane bridge results in a glass transition temperature of 89 °C. These features make CzCN‐Ad suitable for fabricating blue phosphorescent organic light‐emitting diodes (PhOLEDs). The devices based on sky‐blue phosphor bis[(4,6‐difluorophenyl)pyridinato‐N,C^2′](picolinato)iridium(III) (FIrpic) achieved a high maximum external quantum efficiency (EQE) of 24.1 %, which is among the best results for blue PhOLEDs ever reported. Furthermore, blue PhOLEDs with bis(2,4‐difluorophenylpyridinato)‐tetrakis(1‐pyrazolyl)borate iridium(III) (FIr6) as dopant exhibited a maximum EQE of 14.2 % and a maximum luminance of 34 262 cd m^?2. To the best of our knowledge, this is the highest luminance ever reported for FIr6‐based PhOLEDs.     

Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene‐Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence

Two new molecules, CzFCBI and CzFNBI , have been tailor‐made to serve as bipolar host materials to realize high‐efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole‐transporting block and N‐phenylbenzimidazole as the electron‐transporting block hybridized through the saturated bridge center (C9) and meta‐conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N‐phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy‐level alignment allows CzFCBI to outperform its N‐connected counterpart CzFNBI while employing other long‐wavelength‐emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue‐to‐red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low‐roll‐off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish‐green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish‐green with a sky‐blue emitter and a red emitter, a CzFNBI ‐hosted single‐emitting‐layer all‐phosphor three‐color‐based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A^?1, 28.3 lm W^?1) and highly stable chromaticity (CIE x=0.43–0.46 and CIE y=0.43) at an applied voltage of 8 to 12 V, and a high color‐rendering index of 91.6.     

Silicon‐Based Material with Spiro‐Annulated Fluorene/Triphenylamine as Host and Exciton‐Blocking Layer for Blue Electrophosphorescent Devices

A novel silicon‐based compound, 10‐phenyl‐2′‐(triphenylsilyl)‐10H‐spiro[acridine‐9,9′‐fluorene] (SSTF), with spiro structure has been designed, synthesized, and characterized. Its thermal, electronic absorption, and photoluminescence properties were studied. Its energy levels make it suitable as a host material or exciton‐blocking material in blue phosphorescent organic light‐emitting diodes (PhOLEDs). Accordingly, blue‐emitting devices with iridium(III) bis[(4,6‐difluorophenyl)‐pyridinato‐N,C²′]picolinate (FIrpic) as phosphorescent dopant have been fabricated and show high efficiency with low roll‐off. In particular, 44.0 cd A^?1 (41.3 lm W^?1) at 100 cd m^?2 and 41.9 cd A^?1 (32.9 lm W^?1) at 1000 cd m^?2 were achieved when SSTF was used as host material; 28.1 lm W^?1 at 100 cd m^?2 and 20.6 lm W^?1 at 1000 cd m^?2 were achieved when SSTF was used as exciton‐blocking layer. All of the results are superior to those of the reference devices and show the potential applicability and versatility of SSTF in blue PhOLEDs.     

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

Using an Organic Molecule with Low Triplet Energy as a Host in a Highly Efficient Blue Electrophosphorescent Device

To achieve high efficiencies in blue phosphorescent organic light‐emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6‐difluorophenylpyridinato‐N,C^2′)iridium(III)] tetrakis(1‐pyrazolyl)borate (FIr6; T₁=2.73 eV). In both the photo‐ and electro‐excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic‐layer device, the maximum current efficiency of 37 cd A^?1 and power efficiency of 40 Lm W^?1 were achieved for the FIr6‐based blue PhOLEDs.     

Phosphorescence Color Tuning by Ligand,and Substituent Effects of Multifunctional Iridium(III) Cyclometalates with 9‐Arylcarbazole Moieties

The synthesis, isomeric studies, and photophysical characterization of a series of multifunctional cyclometalated iridium(III) complexes containing a fluoro‐ or methyl‐substituted 2‐[3‐(N‐phenylcarbazolyl)]pyridine molecular framework are presented. All of the complexes are thermally stable solids and highly efficient electrophosphors. The optical, electrochemical, photo‐, and electrophosphorescence traits of these iridium phosphors have been studied in terms of the electronic nature and coordinating site of the aryl or pyridyl ring substituents. The correlation between the functional properties of these phosphors and the results of density functional theory calculations was made. Arising from the propensity of the electron‐rich carbazolyl group to facilitate hole injection/transport, the presence of such a moiety can increase the highest‐occupied molecular orbital levels and improve the charge balance in the resulting complexes relative to the parent phosphor with 2‐phenylpyridine ligands. Remarkably, the excited‐state properties can be manipulated through ligand and substituent effects that allow the tuning of phosphorescence energies from bluish green to deep red. Electrophosphorescent organic light‐emitting diodes (OLEDs) with outstanding device performance can be fabricated based on these materials, which show a maximum current efficiency of approximately 43.4 cd A^?1, corresponding to an external quantum efficiency of approximately 12.9 % ph/el (photons per electron) and a power efficiency of approximately 33.4 Lm W^?1 for the best device. The present work provides a new avenue for the rational design of multifunctional iridium–carbazolyl electrophosphors, by synthetically tailoring the carbazolyl pyridine ring that can reveal a superior device performance coupled with good color‐tuning versatility, suitable for multicolor‐display technology.     

Core‐and‐Surface‐Functionalized Polyphenylene Dendrimers for Solution‐Processed,Pure‐Blue Light‐Emitting Diodes Through Surface‐to‐Core Energy Transfer

Several pyrene‐based polyphenylene dendrimers (PYPPDs) with different peripheral chromophores (PCs) are synthesized and characterized. Deep blue emissions solely from the core are observed for all of them in photoluminescence spectra due to good steric shielding of the core and highly efficient surface‐to‐core Förster resonant energy transfers (FRETs). Device performances are found in good correlation with the energy gaps between the work function of the electrodes and the frontier molecular orbital (FMO) levels of the PCs. Pure blue emission, luminance as high as 3700 cd m⁻² with Commission Internationale de l'Éclairage 1931 (CIE_xy) = (0.16, 0.21), and a peak current efficiency of 0.52 cd A⁻¹ at CIE_xy = (0.17, 0.20) are achieved. These dendrimers are among the best dendritic systems so far for fluorescent blue light‐emitting materials.

     

Conjugated and partially conjugated 2,5‐diphenylthiophene‐containing light‐emitting copolymers in polymeric light‐emitting diode (PLED) device fabrications

To study the effect of nonconjugation on polymeric and photophysical properties of thiophene‐containing polymers, new light‐emitting copolymers comprising either alternate 2,5‐diphenylthiophene and vinylene or alternate 2,5‐diphenylthiophene and aliphatic ether segments were synthesized. Both copolymers contained 2,5‐diphenylthiophene as the major chromophore and emitted a sky bluish fluorescence in dilute solution (10^?2 mg/mL). With a rigid and planarity structure and the concomitant crystallinity, the former copolymer (fully conjugated) possessed a higher quantum efficiency, a higher glass‐transition temperature, and a better thermal stability. In contrast, the latter copolymer (conjugated–nonconjugated) had better solubility and provided enhanced photophysical properties for the fabricated polymeric light‐emitting diode (PLED) device: at 15 V, the maximum current and brightness were 110 mA/cm² and 4289 cd/m², respectively, and the electroluminescence efficiency remained constant at approximately 4.9 cd/A in a voltage range of 8 to 14 V. The existence of intramolecular/intermolecular aggregates in the latter copolymer was corroborated from the the UV–vis and photoluminescence spectra of its solutions. With an increase in solution concentration, the shape and λ_max of the photoluminescence spectrum were redshifted. In a solution with a concentration as high as 10 mg/mL, the redshift was so drastic that the photoluminescence spectrum was nearly identical to that of a solid‐film. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6061–6070, 2004     

Strong Single‐Band Blue Emission from Colloidal Ce3+/Tm3+‐Doped NaYF4 Nanocrystals for Light‐Emitting Applications

An intense single‐band blue emission at λ=450 nm is observed from Tm³⁺ ions through Ce³⁺ sensitization, for the first time, in colloidal Ce³⁺/Tm³⁺‐doped NaYF₄ nanocrystals. The intense Tm³⁺ emission through broad‐band excitation is advantageous for developing luminescent nanocomposites because they can be easily incorporated into polymers. The composites can easily be coated over UV light‐emitting diodes (LEDs) to develop phosphor‐based blue LEDs.     

Anthracene‐Cored Dendrimer for Solution‐Processible Blue Emitter: Syntheses,Characterizations, Photoluminescence,and Electroluminescence

Summary: A second‐generation blue fluorescent anthracene‐cored dendrimer EH‐G2AN was readily synthesized via a convergent method. Its monodispersity was confirmed by ¹H NMR and MALDI‐TOF mass measurement. The peak emission of EH‐G2AN in a dilute CH₂Cl₂ solution was observed at 416 nm with a shoulder at 434 nm and moved to 418 nm in the solid film with the shoulder at 433 nm. The nearly “perfect” overlap of solution and solid emission spectra revealed the absence of molecular aggregations in the solid film, which was apparently suppressed by the presence of rigid and bulky 1,3,5‐phenylene‐based dendrons and 2‐ethylhexyloxy solubilizing peripheral groups. EH‐G2AN appeared strikingly stable with the onset decomposition temperature above 350 °C and remained at the high temperature of 428 °C where 5% weight loss occurred. The electroluminescent device [ITO/PEDOT:PSS/EH‐G2AN/Ba/Al] showed a peak emission at 442 nm and maximal external device efficiency of 0.82%@170 cd · m⁻². After inserting a PVK layer between the hole injection layer and emitting layer, a maximal external device efficiency of 1.05%@184 cd · m⁻² was obtained with a narrow FWHI of merely ca. 42 nm in the device configuration [ITO/PEDOT:PSS/PVK/EH‐G2AN/Ba/Al].

     

Modulating the Ground‐ and Excited‐State Oxidation Potentials of Diaminonaphthalene by Sequential N‐Methylation

A series of 1,5‐diaminonaphthalene derivatives were synthesized and characterized to provide ground‐ and excited‐state electron donors of similar structure but varying potential. Electrochemical and spectroscopic properties of the series are reported and together illustrate two opposing consequences of alkyl substitution on the aryl amines. Inductive effects of methylation are evident from the decrease in ground‐state oxidation potential for derivatives containing monomethylamino substituents. In contrast, steric effects seem to dominate the increase in the ground‐state oxidation potential of derivatives containing dimethylamino substituents since the conformational constraints created by dimethylation suppress delocalization of the nonbonding electrons. Absorption and emission properties also respond to increasing levels of N‐methylation, and the excited‐state oxidation potentials of the parent 1,5‐diaminonaphthalene and its monomethylamine derivatives (ca. ?3.2 V) are approximately 200 mV lower than the corresponding dimethylamino derivatives (?3.0 V).     

Modulating the Optoelectronic Properties of Large,Conjugated, High‐Energy Gap,Quaternary Phosphine Oxide Hosts: Impact of the Triplet‐Excited‐State Location

The purposeful modulation of the optoelectronic properties was realised on the basis of a series of the large, conjugated, phosphine oxide hosts 9,9‐bis‐{4′‐[2‐(diphenylphosphinoyl)phenoxy]biphenyl‐4‐yl}‐9H‐fluorene (DDPESPOF), 9,9‐bis‐{3′‐(diphenylphosphinoyl)‐4′‐[2‐(diphenylphosphinoyl)phenoxy]biphenyl‐4‐yl}‐9H‐fluorene (DDPEPOF), 9‐[4′‐(9‐{4′‐[2‐(diphenylphosphoryl)phenoxy]biphenyl‐4‐yl}‐9H‐fluoren‐9‐yl)biphenyl‐4‐yl]‐9H‐carbazole (DPESPOFPhCz) and 9‐[4′‐(9‐{3′‐(diphenylphosphoryl)‐4′‐[2‐(diphenylphosphoryl)phenoxy]biphenyl‐4‐yl}‐9H‐fluoren‐9‐yl)biphenyl‐4‐yl]‐9H‐carbazole (DPEPOFPhCz). The last two are quaternary with fluorenyls as linking bridges, diphenylphosphine oxide (DPPO) moieties as electron acceptors and diphenylethers and carbazolyls as two different kinds of electron donors. Owing to the fine‐organised molecular structures and the mixed indirect and multi‐insulating linkages, all of these hosts achieve the same first triplet energy levels (T₁) of 2.86 eV for exothermic energy transfer to phosphorescent dopants. The first singlet energy levels (S₁) and the carrier injection/transportation ability of the hosts were accurately modulated, so that DPESPOFPhCz and DPEPOFPhCz revealed extremely similar optoelectronic properties. However, the T₁ state of the former is localised on fluorenyl, whereas the carbazolyl mainly contributes to the T₁ state of the latter. A lower driving voltages and much higher efficiencies of the devices based on DPESPOFPhCz indicated that the chromophore‐localised T₁ state can suppress the quenching effects through realising independent contributions from the different functional groups to the optoelectronic properties and the embedding and protecting effect on the T₁ states by peripheral carrier transporting groups.     

Blue electroluminescence from 3,6‐silafluorene‐based copolymers

Poly(3,6‐silafluorene) is a typical wide band‐gap conjugated polymer with ultraviolet light emission. The blue electroluminescence from the 3,6‐silafluorene‐based copolymers via intrachain energy transfer was reported in this study. The monomer containing vinylene, anthracene, and tri‐arylamine moieties incorporated into the poly(3,6‐silafluorene) backbone can form efficient deep‐blue emitting copolymers with EL efficiency of 1.1–1.9%. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3286–3295, 2009     

Watson–Crick Base Pairing Controls Excited‐State Decay in Natural DNA

Excited‐state dynamics are essential to understanding the formation of DNA lesions induced by UV light. By using femtosecond IR spectroscopy, it was possible to determine the lifetimes of the excited states of all four bases in the double‐stranded environment of natural DNA. After UV excitation of the DNA duplex, we detected a concerted decay of base pairs connected by Watson–Crick hydrogen bonds. A comparison of single‐ and double‐stranded DNA showed that the reactive charge‐transfer states formed in the single strands are suppressed by base pairing in the duplex. The strong influence of the Watson–Crick hydrogen bonds indicates that proton transfer opens an efficient decay path in the duplex that prohibits the formation or reduces the lifetime of reactive charge‐transfer states.     

NMR signal assignment of cis/trans‐conformational segments in MEH‐CN‐PPV

Poly[2‐(2′‐ethylhexyloxy)‐5‐methoxy‐1,4‐phenylene‐(1‐cyanovinylene)] MEH‐CN‐PPV and its all‐trans model compound 1,4‐bis(α‐cyanostyryl)‐2‐(2‐ethylhexyloxy)‐5‐methyloxybenzene were synthesized via Knoevenagel condensation. All‐cis isomer and cis–trans isomer of 1,4‐bis(α‐cyanostyryl)‐2‐(2‐ethylhexyloxy)‐5‐methyloxybenzene were prepared by the photoisomerization reaction. Comparison of the ¹H NMR spectra between MEH‐CN‐PPV and three model compounds proved the occurrence of cis‐vinylene in the backbone of MEH‐CN‐PPV. According to the ratio between the cis‐vinylene signal and trans‐vinylene signal, the content of the cis‐vinylene could be estimated to be 15% in MEH‐CN‐PPV. This large cis‐vinylene content came from the rapid photochemical isomerization of cyanovinylene and was likely relative to the poor electroluminescence property of MEH‐CN‐PPV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1105–1113, 2008     

Folding‐Induced Modulation of Excited‐State Dynamics in an Oligophenylene–Ethynylene‐Tethered Spiral Perylene Bisimide Aggregate

The excited‐state photophysical behavior of a spiral perylene bisimide (PBI) folda‐octamer ( F8 ) tethered to an oligophenylene–ethynylene scaffold is comprehensively investigated. Solvent‐dependent UV/Vis and fluorescence studies reveal that the degree of folding in this foldamer is extremely sensitive to the solvent, thus giving rise to an extended conformation in CHCl₃ and a folded helical aggregate in methylcyclohexane (MCH). The exciton‐deactivation dynamics are largely governed by the supramolecular structure of F8 . Femtosecond transient absorption (TA) in the near‐infrared region indicates a photoinduced electron‐transfer process from the backbone to the PBI core in the extended conformation, whereas excitation power‐ and polarization‐dependent TA measurements combined with computational modeling showed that excitation energy transfer between the unit PBI chromophores is the major deactivation pathway in the folded counterpart.