Luminescence Color Tuning from Blue to Near Infrared of Stable Luminescent Solid Materials Based on Bis‐o‐Carborane‐Substituted Oligoacenes

Aryl‐substituted o ‐carboranes have shown highly efficient solid‐state emission in previous studies. To demonstrate color tuning of the solid‐state emission in an aryl‐o ‐carborane‐based system, bis‐o ‐carborane‐substituted oligoacenes were synthesized and their properties were systematically investigated. Optical and electrochemical measurements revealed efficient decreases in energy band gaps and lowest unoccupied molecular orbital (LUMO) levels by adding a number of fused benzene rings for the extension of π‐conjugation. As a consequence, bright solid‐state emission was observed in the region from blue to near infrared (NIR). Furthermore, various useful features were obtained from the modified o ‐carboranes as an optical material. The naphthalene derivatives exhibited aggregation‐induced emission (AIE) and almost 100 % quantum efficiency in the crystalline state. Furthermore, it was shown that the tetracene derivative with NIR‐emissive properties had high durability toward photo‐bleaching under UV irradiation.     

Long‐Lived Room‐Temperature Near‐IR Phosphorescence of BODIPY in a Visible‐Light‐Harvesting N^C^N PtII–Acetylide Complex with a Directly Metalated BODIPY Chromophore

Room‐temperature long‐lived near‐IR phosphorescence of boron‐dipyrromethene (BODIPY) was observed (λ_em=770 nm, Φ_P=3.5 %, τ_P=128.4 μs). Our molecular‐design strategy is to attach Pt^II coordination centers directly onto the BODIPY π‐core using acetylide bonds, rather than on the periphery of the BODIPY core, thus maximizing the heavy‐atom effect of Pt^II. In this case, the intersystem crossing (ISC) is facilitated and the radiative decay of the T₁ excited state of BODIPY is observed, that is, the phosphorescence of BODIPY. The complex shows strong absorption in the visible range (ε=53800 M ^?1 cm^?1 at 574 nm), which is rare for Pt^II–acetylide complexes. The complex is dual emissive with ³M LCT emission at 660 nm and the ³IL emission at 770 nm. The T₁ excited state of the complex is mainly localized on the BODIPY moiety (i.e. ³IL state, as determined by steady‐state and time‐resolved spectroscopy, 77 K emission spectra, and spin‐density analysis). The strong visible‐light‐harvesting ability and long‐lived T₁ excite state of the complex were used for triplet‐triplet annihilation based upconversion and an upconversion quantum yield of 5.2 % was observed. The overall upconversion capability (η=ε×Φ_UC) of this complex is remarkable considering its strong absorption. The model complex, without the BODIPY moiety, gives no upconversion under the same experimental conditions. Our work paves the way for access to transition‐metal complexes that show strong absorption of visible light and long‐lived ³IL excited states, which are important for applications in photovoltaics, photocatalysis, and upconversions, etc.     

On the Phosphorescence of 1H‐Phenalen‐1‐one

Combining the effects of heavy atom and low temperature, the phosphorescence spectrum from 1H‐phenalen‐1‐one has been unveiled. The 0‐0 band is located at 649 nm in methylcyclohexane and shifted to 646 nm in ethanol, which sets the triplet‐energy level to 185 and 186 kJ⋅mol⁻¹, respectively. The emission is unambiguously identified as phosphorescence originating from 1H‐phenalen‐1‐one through complementary transient absorption and emission studies. The quantum yield for triplet formation is confirmed to be unity.     

Water‐Soluble Luminescent Cyclometalated Gold(III) Complexes with cis‐Chelating Bis(N‐Heterocyclic Carbene) Ligands: Synthesis and Photophysical Properties

A new class of cyclometalated Au^III complexes containing various bidentate C‐deprotonated C^N and cis‐chelating bis(N‐heterocyclic carbene) (bis‐NHC) ligands has been synthesized and characterized. These are the first examples of Au^III complexes supported by cis‐chelating bis‐NHC ligands. [Au(C^N)(bis‐NHC)] complexes display emission in solutions under degassed condition at room temperature with emission maxima (λ_max) at 498–633 nm and emission quantum yields of up to 10.1 %. The emissions are assigned to triplet intraligand (IL) π→π* transitions of C^N ligands. The Au^III complex containing a C^N (C‐deprotonated naphthalene‐substituted quinoline) ligand with extended π‐conjugation exhibits prompt fluorescence and phosphorescence of comparable intensity with λ_max at 454 and 611 nm respectively. With sulfonate‐functionalized bis‐NHC ligand, four water‐soluble luminescent Au^III complexes, including those displaying both fluorescence and phosphorescence, were prepared. They show similar photophysical properties in water when compared with their counterparts in acetonitrile. The long phosphorescence lifetime of the water‐soluble AuIII complex with C‐deprotonated naphthalene‐substituted quinoline ligand renders it to function as ratiometric sensor for oxygen. Inhibitory activity of one of these water‐soluble Au^III complexes towards deubiquitinase (DUB) UCHL3 has been investigated; this complex also displayed a significant inhibitory activity with IC₅₀ value of 0.15 μM .     

Quinoidal Azaacenes: 99 % Diradical Character

Quinoidal azaacenes with almost pure diradical character (y=0.95 to y=0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low‐energy gap ΔE_ST′ of 0.58 to 1.0 kcal mol^?1. The species are persistent in solution (half‐life≈14–21 h) and in the solid state they are stable for weeks.     

Efficient Red‐Emissive Organic Crystals with Amplified Spontaneous Emissions Based on a Single Benzene Framework

Red‐emissive fluorophores generally consist of large π‐extended systems and thus encounter the problem of serious fluorescence quenching in the solid state. A series of structurally simple compounds 2,5‐bis(alkylamino)terephthalates 1 a – c are reported that consist of a very small π‐system (only a single benzene) but display efficient red emission in crystals. Crystal 1 a having a molecular weight of only 252 g mol⁻¹ shows red emission with the maximum of 620 nm and a fluorescence quantum yield of 0.40. The unique emission property of crystal 1 a is mainly because of the planarization of skeleton dominated by the strong intramolecular hydrogen bonds and the packing structure with negligible π–π interactions contributed by the mini π‐system. Moreover, besides efficient red emission, high crystallinity with co‐planar facets endows crystal 1 a with significant amplified spontaneous emission.     

A Stable Trimethylenemethane Triplet Diradical Based on a Trimeric Porphyrin Fused π‐System

Trimethylenemethane (TMM) diradical is the simplest non‐Kekulé non‐disjoint molecule with the triplet ground state (ΔE_ST=+16.1 kcal mol^?1) and is extremely reactive. It is a challenge to design and synthesize a stable TMM diradical with key properties, such as actual aliphatic TMM diradical centers and the triplet ground state with a large positive ΔE_ST value, since such species provide detailed information on the electronic structure of TMM diradical. Herein we report a TMM derivative, in which the TMM segment is fused with three Ni^II meso‐triarylporphyrins, that satisfies the above criteria. The diradical shows delocalized spin density on the propeller‐like porphyrin π‐network and the triplet ground state owing to the strong ferromagnetic interaction. Despite the apparent TMM structure, the diradical can be handled under ambient conditions and can be stored for months in the solid state, thus allowing its X‐ray diffraction structural analysis.     

Molecular design of efficient white‐light‐emitting fluorene‐based copolymers by mixing singlet and triplet emission

A new strategy to realize efficient white‐light emission from a binary fluorene‐based copolymer (PF‐Phq) with the fluorene segment as a blue emitter and the iridium complex, 9‐iridium(III)bis(2‐(2‐phenyl‐quinoline‐N,C³′)(11,13‐tetradecanedionate))‐3,6‐carbazole (Phq), as a red emitter has been proposed and demonstrated. The photo‐ and electroluminescence properties of the PF‐Phq copolymers were investigated. White‐light emission with two bands of blue and red was achieved from the binary copolymers. The efficiency increased with increasing concentration of iridium complex, which resulted from its efficient phosphorescence emission and the weak phosphorescent quenching due to its lower triplet energy level than that of polyfluorene. In comparison with the binary copolymer, the efficiency and color purity of the ternary copolymers (PF‐Phq‐BT) were improved by introducing fluorescent green benzothiadiazole (BT) unit into polyfluorene backbone. This was ascribed to the exciton confinement of the benzothiadiazole unit, which allowed efficient singlet energy transfer from fluorene segment to BT unit and avoided the triplet quenching resulted from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. The phosphorescence quenching is a key factor in the design of white light‐emitting polyfluorene with triplet emitter. It is shown that using singlet green and triplet red emitters is an efficient approach to reduce and even avoid the phosphorescence quenching in the fluorene‐based copolymers. The strategy to incorporate singlet green emitter to polyfluorene backbone and to attach triplet red species to the side chain is promising for white polymer light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 453–463, 2008     

Enantiopure Cycloiridiated Complexes Bearing a Pentahelicenic N‐Heterocyclic Carbene and Displaying Long‐Lived Circularly Polarized Phosphorescence

A fused π‐helical N‐heterocyclic carbene (NHC) system was prepared and examined through its diastereoisomerically pure cycloiridiated complexes. The latter display light‐green phosphorescence with unusually long lifetimes and circular polarization that depends on both the helical NHC P /M stereochemistry and the iridium Δ/Λ stereochemistry. These unprecedented features are attributed to extended π conjugation within the helical carbenic ligand and efficient helicene‐NHC–Ir interaction.     

Aggregation-Induced Dual Phosphorescence from (o-Bromophenyl)-Bis(2,6-Dimethylphenyl)Borane at Room Temperature

Designing highly efficient purely organic phosphors at room temperature remains a challenge because of fast non-radiative processes and slow intersystem crossing (ISC) rates. The majority of them emit only single component phosphorescence. Herein, we have prepared 3 isomers (o, m, p-bromophenyl)-bis(2,6-dimethylphenyl)boranes. Among the 3 isomers ( o -, m - and p - BrTAB ) synthesized, the ortho-one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. Based on theoretical calculations and crystal structure analysis of o - BrTAB , the short lifetime component is ascribed to the T₁^M state of the monomer which emits the higher energy phosphorescence. The long-lived, lower energy phosphorescence emission is attributed to the T₁^A state of an aggregate, with multiple intermolecular interactions existing in crystalline o - BrTAB inhibiting nonradiative decay and stabilizing the triplet states efficiently.     

From A Fluorescent Chromophore in Solution to An Efficient Emitter in the Solid State

Polycyclic aromatic hydrocarbons (PAHs) normally exhibit efficient fluorescence in dilute solutions, whereas their emission is significantly quenched in the aggregated state due to the formation of π‐aggregates/excimers. The rigid and planar structure of PAHs plays a positive role in terms of fluorescence in solution but a negative one in the aggregated state. To bestow PAHs a luminescent ability in the solid state, we constructed a non‐coplanar PAH‐substituted ethene. By using the planar PAH fluoranthene as a building block, a highly efficient solid‐state emitter with a fluorescence quantum efficiency of unity in the aggregated state was obtained. OLEDs with contain the molecule as an emitter reach a luminance up to 20 520 cd m^?2 and an efficiency of 10 cd A^?1.     

Effect of the Substitution Pattern on the Intramolecular Charge‐Transfer Emissions in Organoboron‐Based Biphenyls,Diphenylacetylenes, and Stilbenes

Three series of organoboron‐based molecules, including biphenyls 1a , 1b , 1c , diphenylacetylenes 2a , 2b , 2c , and stilbenes 3a , 3b , 3c , in which the electron‐accepting boryl and the electron‐donating amino groups are introduced at different positions, have been comprehensively investigated to explore the effect of the substitution pattern on the intramolecular charge‐transfer emissions. In cyclohexane solution, the change of substitution pattern from p,p′ to o,p′ by introduction of boryl at the lateral o‐position rather than the terminal p‐position leads to bathochromism in the absorption and emission spectra. With further variation of the amino position from the terminal p′‐position in o,p′‐substitution to the lateral o′‐position in an o,o′‐substitution pattern, a blueshift was observed in the absorption owing to the less‐efficient conjugation extension of the amino group as the result of sp³ hybridization. It is notable that the emission of the three series of molecules changes with completely different trends. Only the emission of the biphenyl is redshifted further from o,p′‐substituted 1b to o,o′‐substituted 1a , whereas o,o′‐substituted diphenylacetylene 2a maintains almost the same spectrum as that of o,p′‐substituted diphenylacetylene 2b and the fluorescence of o,o′‐substituted stilbene 3a is even blueshifted compared with o,p′‐substituted stilbene 3b . As a result, the o,o′‐substituted biphenyl 1a shows the longest emission wavelength despite the limited conjugation of the parent biphenyl skeleton. The long emission wavelength of 1a may arise from its extremely twisted structure, which would cause a significant structural relaxation in the exited state. In the solid state, 1a still keeps almost the longest emission wavelength. In addition, its quantum yield is also among the highest. The unusual properties, intense solid‐state emission together with long emission wavelength, and particularly large Stokes shift, which are difficult to attain by structural modification of other parent π‐conjugated frameworks, have been achieved by the introduction of boryl and amino groups at the o,o′‐positions of the biphenyl skeleton.     

Triggering Thermally Activated Delayed Fluorescence by Managing the Heteroatom in Donor Scaffolds: Intriguing Photophysical and Electroluminescence Properties

Establishment of the structure–property relationships of thermally activated delayed fluorescence (TADF) materials has become a significant quest for the scientific community. Herein, two new donors, 10H‐benzofuro[3,2‐b]indole (BFI) and 10H‐benzo[4,5]thieno[3,2‐b]indole (BTI), have been developed and integrated with a aryltriazine acceptor to design the green TADF emitters benzofuro[3,2‐b]indol‐10‐yl)‐5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)benzonitrile ( BFICNTrz ) and 2‐(10H‐benzo[4,5]thieno[3,2‐b]indol‐10‐yl)‐5‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)benzonitrile ( BTICNTrz ), respectively. The physicochemical and electroluminescence properties of the compounds were tuned by exchanging the heteroatom in the donor scaffold. Intriguingly, the electronegativity of the heteroatom and the ionization potential of the donor unit played vital roles in control of the singlet–triplet energy splitting and TADF mechanism of the compounds. Both compounds showed similar singlet excited states that originated from the charge transfer (CT) states (¹CT), whereas the triplet excited states were tuned by the heteroatom in the donor unit. The origin of phosphorescence in the BTICNTrz emitter was CT emission from the triplet state (³CT), whereas that in the BFICNTrz emitter stemmed from the local triplet excited state (³LE). Consequently, BTICNTrz showed a small singlet–triplet energy splitting of 0.08 eV, compared with 0.26 eV for BFICNTrz . Thus, BTICNTrz showed efficient delayed fluorescence with a high quantum yield and a short delayed exciton lifetime, whereas BFICNTrz displayed weak delayed fluorescence with a relatively long lifetime. Furthermore, a BTICNTrz ‐based device exhibited a maximum external quantum efficiency (EQE) of 15.2 % and reduced efficiency roll‐off (12 %) compared with its BFICNTrz ‐based counterpart, which showed a maximum EQE of 6.4 % and severe efficiency roll‐off (55 %) at a practical brightness range of 1000 cd m^?2. These results demonstrate that the choice of subunit plays a vital role in the design of efficient TADF emitters.     

New Wine in Old Bottles: Prolonging Room‐Temperature Phosphorescence of Crown Ethers by Supramolecular Interactions

Supramolecular macrocyclic hosts have long been used in smart materials. However, their triplet emission and regulation at crystal level is rarely studied. Herein, ultralong and universal room‐temperature phosphorescence (RTP) is reported for traditional crown ethers. A supramolecular strategy involving chain length adjustment and morphological locking through complexation with K⁺ was explored as a general method to tune the phosphorescence lifetime in the solid state. A maximum 10‐fold increase of lifetime after complex formation accompanied with by invisible to visible phosphorescence was achieved. A deep encryption based on this activated RTP strategy was also facilely fabricated. This work thus opens a new world for supramolecular macrocycles and their intrinsic guest responsiveness offers a new avenue for versatile smart luminescent materials.     

Synthesis of main chain polymeric benzophenone photoinitiator via thiol‐ene click chemistry and Its use in free radical polymerization

Main chain polymeric benzophenone photoinitiator (PBP) was synthesized by using “Thiol‐ene Click Chemistry” and characterized with ¹H NMR, FTIR, UV, and phosphorescence spectroscopies. PBP as a polymeric photoinitiator presented excellent absorption properties (ε₂₉₄ = 28,300 mol^?1L^?1cm^?1) compared to the molecular initiator BP (ε₂₅₂ = 16,600 mol^?1L^?1cm^?1). The triplet energy of PBP was obtained from the phosphorescence measurement in 2‐methyl tetrahydrofurane at 77 K as 298.3 kJ/mol and according to phosphorescence lifetime, the lowest triplet state of PBP has an n‐π* nature. Triplet–triplet absorption spectrum of PBP at 550 nm following laser excitation (355 nm) were recorded and triplet lifetime of PBP was found as 250 ns. The photoinitiation efficiency of PBP was determined for the polymerization of Hexanedioldiacrylate (HDDA) with PBP and BP in the presence of a coinitiator namely, N‐methyldiethanolamine (MDEA) by Photo‐DSC. The initiation efficiency of PBP for polymerization of HDDA is much higher than for the formulation consisting of BP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Solid‐State Phosphorescence of trans‐Bis(salicylaldiminato)platinum(II) Complexes Bearing Long Alkyl Chains: Morphology Control towards Intense Emission

Morphology control for intense solid‐state phosphorescence of non‐emissive, but potentially emissive crystals of platinum complexes and the mechanistic rationale are described. A series of trans‐bis(salicylaldiminato)platinum(II) complexes bearing linear alkyl chains ( 1 a : n=5; 1 b : n=8; 1 c : n=12; 1 d : n=14; 1 e : n=16; 1 f : n=18) was synthesized and the solid‐state emission properties were examined by using crystals/aggregates prepared under various precipitation conditions. Crystals of 1 e , prepared using “kinetic” conditions including rapid cooling, high concentrations, and poor solvents, emit intensive yellow phosphorescence (λ_max=545 nm) under UV irradiation at 298 K with an absolute quantum efficiency of 0.36, whereas all the crystals of 1 a – 1 f prepared using “thermodynamic” conditions including slow cooling, low concentrations, and good solvents were either non‐ or less emissive with Φ_298K values of 0.12 ( 1 a ), 0.11 ( 1 b ), 0.10 ( 1 c ), 0.07 ( 1 d ), 0.02 ( 1 e ), and 0.02 ( 1 f ) under the same measurement conditions. The amorphous solid 1 e , prepared by rapid cooling and freeze‐drying, was also non‐emissive (Φ_298K=0.02, 0.02). Temperature‐dependent emission spectra showed that the kinetic crystals of 1 e exhibit high heat‐resistance towards emission decay with increasing temperature, whereas the amorphous solid 1 e is entirely heat‐quenchable. This is a rare example of the change from a non‐emissive crystal into a highly emissive crystal by morphology control through crystal engineering. Emission spectra and powder X‐ray diffraction (XRD) patterns of the emissive, kinetic crystals of 1 e are clearly distinct from those of the less emissive, thermodynamic crystals of 1 a – 1 f . Single‐crystal XRD unequivocally establishes that the thermodynamic crystals of 1 d have a multilayered lamellar structure supported by highly regulated, consecutive π‐stacking interactions between imine moieties, whereas the kinetic crystals of 1 e have a face‐to‐edge lamellar structure with less stacking. These results lead to the conclusion that 1) morphology control of long‐chained complexes exclusively generates a metastable herringbone‐based lamellar packing motif that exhibits intense emission and high heat‐resistance, while 2) a thermodynamically stable, highly regulated, consecutive stacking motif is unfavorable for solid‐state emission.     

A thioxanthone‐based visible photoinitiator

Thioxanthone‐based 9‐(2‐Morpholine‐4yl‐acetyl)‐5‐thia‐napthasen‐12‐one (TX‐MPM) was synthesized and characterized as a one‐component novel visible photoinitiator. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA) was examined in photoreactor and also daylight. Photophysical properties: fluorescence and phosphorescence emission spectra and fluorescence quantum yield of TX‐MPM (?_f = 0.29) were determined. The phosphorescence lifetime was found 131 ms for TX‐MPM and 110 ms for initiator‐attached polymer (PMMA) at 77 K, indicated a π→π* nature of the lowest triplet state. A model compound, morpholino acetonapthone was used as quencher for the triplet states of TX‐MPM and the quenching rate constant was determined (k_q = 1.26 × 10⁹ M^?1s^?1). According to laser flash photolysis studies, intermolecular hydrogen abstraction process was more dominant path to the formation of the initiating radicals. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Structure–Property Relationships and 1O2 Photosensitisation in Sterically Encumbered Diimine PtII Acetylide Complexes

A series of sterically encumbered [Pt( L )(σ‐acetylide)₂] complexes were prepared in which L , a dendritic polyaromatic diimine ligand, was held constant ( L =1‐(2,2′‐bipyrid‐6‐yl)‐2,3,4,5‐tetrakis(4‐tert‐butylphenyl)benzene) and the cis ethynyl co‐ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron‐withdrawing phenyl‐CF₃ substituents ( 4 ) with electron‐donating pyrenes ( 5 ) resulted in a red shift of both the lowest‐energy absorption (ΔE=3300 cm^?1, 61 nm) and emission bands (ΔE=1930 cm^?1, 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited‐state lifetimes, switched from being ³MMLL′CT‐derived (mixed metal–ligand‐to‐ligand charge transfer) when phenyl/polyphenylene substituents ( 3 , 4 , 6 ) were present, to ligand‐centred ³ππ* when the substituents were more conjugated aromatic platforms [pyrene ( 5 ) or hexa‐peri‐hexabenzocoronene ( 7 )]. The novel Pt^II acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited‐state lifetimes suggested they would be good candidates for use as singlet‐oxygen photosensitisers. Determined by in situ photooxidation of 1,5‐dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl‐ 5 as sensitiser (k_obs=39.3×10^?3 min^?1) was over half that of the known ¹O₂ sensitiser tetraphenylporphyrin (k_obs=78.6×10^?3 min^?1) under the same conditions. Measured ¹O₂ quantum yields of complexes 5 and 7 were half and one‐third, respectively, of that of TPP, and thus reveal an efficient triplet–triplet energy‐transfer process in both cases.     

Tailoring Excited State Properties and Energy Levels Arrangement via Subtle Structural Design on D‐π‐A Materials

The donor‐π‐conjugated‐acceptor (D‐π‐A) structure is an important design for the luminescent materials because of its diversity in the selections of donor, π‐bridge and acceptor groups. Herein, we demonstrate two examples of D‐π‐A structures capable to finely modulate the excited state properties and arrangement of energy levels, TPA‐AN‐BP and CZP‐AN‐BP , which possess the same acceptor and π‐bridge but different donor. The investigation of their photophysical properties and DFT calculation revealed that the D‐π‐A structure with proper donor, π‐bridge and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an obvious overlap on the π‐bridge, resulting in a hybridized local and charge‐transfer (HLCT ) excited state with high photoluminescent (PL ) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moieties with large energy gap between T₂ and T₁ , and a small energy gap between S₁ and T₂ , which favor the reverse intersystem crossing (RISC ) from high‐lying triplet levels to singlet levels. As a result, the sky‐blue emission non‐doped OLED based on the TPA‐AN‐BP reached maximum external quantum efficiency (EQE ) of 4.39% and a high exciton utilization efficiency (EUE ) of 77%. This study demonstrates a new strategy to construct highly efficient OLED materials.     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.