1,4‐Bis(alkenyl)‐2,5‐dipiperidinobenzenes: Minimal Fluorophores Exhibiting Highly Efficient Emission in the Solid State

Minimum requirements : Crystals and thin films of 1,4‐bis(alkenyl)‐2,5‐dipiperidinobenzenes, which contain only one benzene ring as the aromatic component, emit visible light with excellent solid‐state quantum yields upon irradiation with UV light. Polystyrene thin films doped with the benzenes also exhibit brilliant fluorescence. By modifying the alkenyl groups, the emission color can be tuned in the range from blue to red.

     

Efficient Emission of Ultraviolet Light by Solid State Organic Fluorophores: Synthesis and Characterization of 1,4-Dialkeny-2,5-dioxybenzenes

The design and development of organic luminophores that exhibit efficient ultraviolet (UV) fluorescence in the solid state remains underexplored. Here, we report that 1,4-dialkenyl-2,5-dialkoxybenzenes and 1,4-dialkenyl-2,5-disiloxybenzenes act as such UV-emissive fluorophores. The dialkenyldioxybenzenes were readily prepared in three steps from 2,5-dimethoxy-1,4-diacetylbenzene or 2,5-dimethoxy-1,4-diformylbenzene via two to four steps from 1,4-bis(diethoxyphosphonylmethyl)-2,5-dimethoxybenzene. The dialkenyldioxybenzenes emit UV light in solution (λ_em=350–387 nm) and in the solid state (λ_em=328–388 nm). In addition, the quantum yields in the solid state were generally higher than those in solution. In particular, the adamantylidene-substituted benzenes fluoresced in the UV region with high quantum yields (Φ=0.37–0.55) in the solid state. Thin films of poly(methyl methacrylate) doped with the adamantylidene-substituted benzenes also exhibited UV emission with good efficiency (Φ=0.27–0.45). Density functional theory calculations revealed that the optical excitation of the dialkenyldimethoxybenzenes involves intramolecular charge-transfer from the ether oxygen atoms to the twisted alkenyl-benzene-alkenyl moiety, whereas the dialkenylbis(triphenylsiloxy)benzenes were optically excited through intramolecular charge-transfer from the oxygen atoms and twisted π-system to the phenyl-Si moieties of each triphenylsilyl group.     

Organic Fluorophores Exhibiting Highly Efficient Photoluminescence in the Solid State

There has been extensive research on the development of organic optoelectronic devices, such as organic light‐emitting diodes, organic field‐effect transistors, and organic solid‐state lasers from various viewpoints, ranging from basic studies to practical applications. As organic materials are used as solids in these devices, the importance of organic chromophores that exhibit intense emissions of visible light in the solid state is greatly increasing in the field of organic electronics. However, highly efficient emission from organic solids is very difficult to attain because most organic emitting materials strongly tend to cause concentration quenching of the luminescence in the condensed phase. Therefore, in order to generate and improve organic optoelectronic devices, it is necessary to design novel chromophores that exhibit superior solid‐state emission performance. This Focus Review covers the recent development of highly emissive organic small molecules whose photoluminescence quantum yields in the solid state have been reported. Following the introduction, the photophysical processes of excited molecules are briefly reviewed. Subsequently, organic solid fluorophores are described with an emphasis on the characteristics of their molecular structures.     

Bridging Small Molecules to Conjugated Polymers: Efficient Thermally Activated Delayed Fluorescence with a Methyl‐Substituted Phenylene Linker

Based on a “TADF + Linker” strategy (TADF=thermally activated delayed fluorescence), demonstrated here is the successful construction of conjugated polymers that allow highly efficient delayed fluorescence. Small molecular TADF blocks are linked together using a methyl‐substituted phenylene linker to form polymers. With the growing number of methyl groups on the phenylene, the energy level of the local excited triplet state (³LE_b) from the delocalized polymer backbone gradually increases, and finally surpasses the charge‐transfer triplet state (³CT). As a result, the diminished delayed fluorescence can be recovered for the tetramethyl phenylene containing polymer, revealing a record‐high external quantum efficiency (EQE) of 23.5 % (68.8 cd A^?1, 60.0 lm W^?1) and Commission Internationale de l′Eclairage (CIE) coordinates of (0.25, 0.52). Combined with an orange‐red TADF emitter, a bright white electroluminescence is also obtained with a peak EQE of 20.9 % (61.1 cd A^?1, 56.4 lm W^?1) and CIE coordinates of (0.36, 0.51).     

Synthesis,Crystal Structure,and Photophysical Properties of (1E,3E,5E)‐1,3,4,6‐Tetraarylhexa‐1,3,5‐trienes: A New Class of Fluorophores Exhibiting Aggregation‐Induced Emission

Double Horner–Wadsworth–Emmons reaction of (E)‐2,3‐diaryl‐1,4‐bis(diethylphosphonyl)but‐2‐ene with (p‐substituted) benzaldehydes gave (1E,3E,5E)‐1,3,4,6‐tetraarylhexa‐1,3,5‐trienes in moderate to good yields. Substitution of electron‐withdrawing or ‐donating groups at the para position of the 1,6‐diphenyl groups induced a slight bathochromic shift of UV spectra measured in CHCl₃ compared with that of the parent 1,3,4,6‐tetraphenylhexa‐1,3,5‐triene. Although fluorescence was not observed with all the trienes in CHCl₃, they markedly emitted visible light in powder forms with quantum yields of 0.15–0.44. Introduction of amino groups at the para position of the 3,4‐diphenyl groups induced a bathochromic shift of emission maxima with good solid‐state quantum yields. Thus, the tetraarylated triene framework is found to serve as a new class of fluorophores that exhibit aggregation‐induced emission.     

Fluorescence Emission of Self-assembling Amyloid-like Peptides: Solution versus Solid State

Analysis of the intrinsic UV-visible fluorescence exhibited by self-assembling amyloid-like peptides in solution and in solid the state highlights that their physical state has a profound impact on the optical properties. In the solid state, a linear dependence of the fluorescence emission peaks as a function of excitation wavelength is detected. On the contrary, an excitation-independent emission is observed in solution. The present findings constitute a valuable benchmark for current and future explanations of the fluorescence emission by amyloids.     

Synthesis and Photophysical Properties of Dimethoxybis(3,3,3‐trifluoropropen‐1‐yl)benzenes: Compact Chromophores Exhibiting Violet Fluorescence in the Solid State

Dimethoxybis(3,3,3‐trifluo‐ropropen‐1‐yl)benzenes were prepared through palladium‐catalyzed double cross‐coupling reactions of diiododimethoxybenzenes with CF₃C≡CZnCl, followed by reduction of CF₃C≡C groups with LiAlH₄ or H₂ in the presence of the Lindlar catalyst. The edges of the absorption spectra of 1,2‐(MeO)₂‐4,5‐(CF₃CHC=CH)₂benzenes 1 and 1,3‐(MeO)₂‐4,6‐(CF₃CH=CH)₂benzenes 2 in cyclohexane ranged from 348 to 360 nm, whereas the absorption spectra of 1,4‐(MeO)₂‐2,5‐[(E)‐CF₃CH=CH]₂ benzene ((E)‐ 3 ) ended at 406 nm. These findings indicate that the effective conjugation length of (E)‐ 3 was significantly larger than those of 1 and 2 . Consistently, 1 and 2 in cyclohexane exhibited fluorescence with emission maxima in the UV region, whereas (E)‐ 3 in cyclohexane emitted violet light with an emission maximum at 407 nm. All the fluorescence spectra of 1 – 3 in various solvents redshifted as the solvent polarity increased. The photoluminescence of 1 , E‐1 , Z‐1 , 2 , E‐2 , E‐2H , Z‐2 , E‐3 , E‐3H , Z‐3 in the solid states was also observed with emission maxima in the violet region. It is important to note that the quantum yields of (E)‐ 3 in a neat thin film and in a doped polymer film were 0.37 and 0.49, respectively. Density functional theory calculations suggested that the fluorine atoms contribute to a slight extension of both the HOMOs and the LUMOs, as well as narrowing of the HOMO–LUMO gaps when compared with the corresponding fluorine‐free analogues. In the case of (E)‐ 3 , it is suggested that the HOMO–LUMO transition includes charge transfer from the ethereal oxygen atoms to the C(sp²) CF₃ moieties.     

Efficient Blue Electroluminescence from a Single‐layer Organic Device Composed Solely of Hydrocarbons

An interesting physical phenomenon, electroluminescence, that was originally observed with a hydrocarbon molecule has recently been developed into highly efficient organic light‐emitting devices. These modern devices have evolved through the development of multi‐element molecular materials for specific roles, and hydrocarbon devices have been left unexplored. In this study, we report an efficient organic light‐emitting device composed solely of hydrocarbon materials. The electroluminescence was achieved in the blue region by efficient fluorescence and charge recombination within a simple single‐layer architecture of macrocyclic aromatic hydrocarbons. This study may stimulate further studies on hydrocarbons to uncover their full potential as electronic materials.     

Alkylation‐, Heating‐, and Doping‐Induced Emission Enhancement of a Polyaromatic Tube in the Solid State

A polyaromatic tube with a subnanometer‐sized cavity was efficiently prepared on a gram‐scale through the stereo‐controlled cyclotrimerization of a diphenylanthracene derivative as a key step. The facile exterior alkylation of the polyaromatic framework leads to a moderately fluorescent tube (R=‐OC₁₀H₂₁; Φ_F=20 %) in the solid state. The emission intensity of the solid‐state alkyl‐substituted tube is remarkably enhanced upon heating (up to 1.6 times, Φ_F=31 %) as well as doping with fluorescent dyes (up to 4.2 times, Φ_F=83 %) through efficient energy transfer.     

Three-Dimensional Cuprous Lead Bromide Framework with Highly Efficient and Stable Blue Photoluminescence Emission

Considering the instability and low photoluminescence quantum yield (PLQY) of blue-emitting perovskites, it is still challenging and attractive to construct single crystalline hybrid lead halides with highly stable and efficient blue light emission. Herein, by rationally introducing d¹⁰ transition metal into single lead halide as new structural building unit and optical emitting center, we prepared a bimetallic halide of [(NH₄)₂]CuPbBr₅ with new type of three-dimensional (3D) anionic framework. [(NH₄)₂]CuPbBr₅ exhibits strong band-edge blue emission (441 nm) with a high PLQY of 32 % upon excitation with UV light. Detailed photophysical studies indicate [(NH₄)₂]CuPbBr₅ also displays broadband red light emissions derived from self-trapped states. Furthermore, the 3D framework features high structural and optical stabilities at extreme environments during at least three years. To our best knowledge, this work represents the first 3D non-perovskite bimetallic halide with highly efficient and stable blue light emission.     

Engineering Stacks of V‐Shaped Polyaromatic Compounds with Alkyl Chains for Enhanced Emission in the Solid State

A V‐shaped bisanthracene derivative with three butyl groups formed two types of emissive solids that display bluish green and blue fluorescence (Φ_F=72 and 32 %, respectively), depending on the preparation conditions. The crystal and powder X‐ray analyses reveal that the highly emissive solid adopts a head‐to‐head arrangement with discrete stacks of the anthracene moieties, whereas the moderately emissive solid adopts a head‐to‐tail arrangement without the stacks. The obtained molecular arrangements are transformed by thermal stimuli accompanying the change in fluorescence. Furthermore, large enhancements of dye emissions (12–45‐fold) through highly efficient host–guest energy transfer were achieved in the solid state by adding minute amounts of various fluorescent dyes (e.g. rubrene and Nile red) to the V‐shaped compound.     

Thermally Stabilized Blue Luminescent Poly(p‐phenylene)s Covered with Polyhedral Oligomeric Silsesquioxanes

A novel series of poly(p‐phenylene)s (PPPs) with polyhedral oligomeric silsesquioxanes (POSSs) on their side chains was prepared. The obtained POSS‐modified PPPs are as follows: 25POSS‐PPP ( 2b , containing 25 mol‐% of POSS units in all side chains), 50POSS‐PPP ( 2c , containing 50 mol‐% of POSS units in all side chains), 100POSS‐PPP ( 2d , containing 100 mol‐% of POSS units in all side chains), and 0POSS‐PPP ( 2a , as a blank polymer). Films polymer 2d showed the same absorption and photoluminescence (PL) spectra as those in CHCl₃ solution, indicating that bulky POSS units strongly suppressed intermolecular aggregation of the PPP backbone. Polymer 2d showed the same PL spectra even after thermal annealing at 150 °C for 6 h. This enhancement of PL stability is due to the significant effect of the bulky POSS units.

     

小分子有机硅单体N-(3-三甲氧基硅基乙基)乙二胺的有效蓝色发光

详细研究了N-(3-三甲氧基硅基乙基)乙二胺(TMSEEDA)的电子结构、光物理性质以及热稳定性.通过密度泛函理论计算获得了基态和第一激发态的结构参数.计算结果还表明在Si-O骨架内存在离域电子,这导致了长波长的吸收.在270nm紫外光的激发下,溶液和固态均能观察到一个宽且强的蓝色发射,其峰位于430nm,固态的强度高出纯物质5倍左右.在乙醇溶液中,发光强度随着TMSEEDA浓度的增大而增加,并在纯物质时达到最大值.这些结果说明TMSEEDA不存在浓度淬灭效应.我们提出了在Si-O骨架内电子离域和d-pπ键模型来解释长波长吸收和蓝色发射.     

Three‐Dimensional Cuprous Lead Bromide Framework with Highly Efficient and Stable Blue Photoluminescence Emission

Considering the instability and low photoluminescence quantum yield (PLQY) of blue‐emitting perovskites, it is still challenging and attractive to construct single crystalline hybrid lead halides with highly stable and efficient blue light emission. Herein, by rationally introducing d¹⁰ transition metal into single lead halide as new structural building unit and optical emitting center, we prepared a bimetallic halide of [(NH₄)₂]CuPbBr₅ with new type of three‐dimensional (3D) anionic framework. [(NH₄)₂]CuPbBr₅ exhibits strong band‐edge blue emission (441 nm) with a high PLQY of 32 % upon excitation with UV light. Detailed photophysical studies indicate [(NH₄)₂]CuPbBr₅ also displays broadband red light emissions derived from self‐trapped states. Furthermore, the 3D framework features high structural and optical stabilities at extreme environments during at least three years. To our best knowledge, this work represents the first 3D non‐perovskite bimetallic halide with highly efficient and stable blue light emission.     

Versatile Benzimidazole/Triphenylamine Hybrids: Efficient Nondoped Deep‐Blue Electroluminescence and Good Host Materials for Phosphorescent Emitters

Two new bipolar compounds, N,N,N′,N′‐tetraphenyl‐5′‐(1‐phenyl‐1H‐benzimidazol‐2‐yl)‐1,1′:3′,1′′‐terphenyl‐4,4′′‐diamine ( 1 ) and N,N,N′,N′‐tetraphenyl‐5′‐(1‐phenyl‐1H‐benzimidazol‐2‐yl)‐1,1′:3′,1′′‐terphenyl‐3,3′′‐diamine ( 2 ), were synthesized and characterized, and their thermal, photophysical, and electrochemical properties were investigated. Compounds 1 and 2 possess good thermal stability with high glass‐transition temperatures of 109–129 °C and thermal decomposition temperatures of 501–531 °C. The fluorescence quantum yield of 1 (0.52) is higher than that of 2 (0.16), which could be attributed to greater π conjugation between the donor and acceptor moieties. A nondoped deep‐blue fluorescent organic light‐emitting diode (OLED) using 1 as the blue emitter displays high performance, with a maximum current efficiency of 2.2 cd A⁻¹ and a maximum external efficiency of 2.9 % at the CIE coordinates of (0.17, 0.07) that are very close to the National Television System Committee’s blue standard (0.15, 0.07). Electrophosphorescent devices using the two compounds as host materials for green and red phosphor emitters show high efficiencies. The best performance of a green phosphorescent device was achieved using 2 as the host, with a maximum current efficiency of 64.3 cd A⁻¹ and a maximum power efficiency of 68.3 lm W⁻¹; whereas the best performance of a red phosphorescent device was achieved using 1 as the host, with a maximum current efficiency of 11.5 cd A⁻¹, and a maximum power efficiency of 9.8 lm W⁻¹. The relationship between the molecular structures and optoelectronic properties are discussed.     

Spiroconjugated Tetraaminospirenes as Donors in Color-Tunable Charge-Transfer Emitters with Donor-Acceptor Structure

Charge-transfer emitters are attractive due to their color tunability and potentially high photoluminescence quantum yields (PLQYs). We herein present tetraaminospirenes as donor moieties, which, in combination with a variety of acceptors, furnished 12 charge-transfer emitters with a range of emission colors and PLQYs of up to 99 %. The spatial separation of their frontier molecular orbitals was obtained through careful structural design, and two DA structures were confirmed by X-ray crystallography. A range of photophysical measurements supported by DFT calculations shed light on the optoelectronic properties of this new family of spiro-NN-donor-acceptor dyes.