Effects of Intermolecular Interactions on Luminescence Property in Organic Molecules

The organic solid-state lightemitting materials have attracted more and more attention owing to their promising applications in displays, lasers and optical communications. In contrast to isolated molecule, there are various weak intermolecular interactions in organic solids that sometimes have a large impact on the excited-state properties and energy dissipation pathways, resulting in strong fluorescence/phosphorescence. It is increasingly necessary to reveal the luminescence mechanism of organic solids. Here, we briefly review how intermolecular interactions induce strong normal fluorescence, thermally activate delayed fluorescence and room-temperature phosphorescence in organic solids by examining changes in geometry, electronic structures, electron-vibration coupling and energy dissipation dynamics of the excited states from isolated to aggregated molecules. We hope that the review will contribute to an in-depth understanding of the excited state properties of organic solids and to the design of excellent solid-state light-emitting materials.     

9,9-Dimethylxanthene Derivatives with Room-Temperature Phosphorescence: Substituent Effects and Emissive Properties

Room-temperature phosphorescence (RTP) emitters with ultralong lifetimes are emerging as attractive targets because of their potential applications in bioimaging, security, and other areas. But their development is limited by ambiguous mechanisms and poor understanding of the correlation of the molecular structure and RTP properties. Herein, different substituents on the 9,9-dimethylxanthene core (XCO) result in compounds with RTP lifetimes ranging from 52 to 601 ms, which are tunable by intermolecular interactions and molecular configurations. XCO-PiCl shows the most persistent RTP because of its reduced steric bulk and multiple sites of the 1-chloro-2-methylpropan-2-yl (PiCl) moiety for forming intermolecular interactions in the aggregated state. The substituent effects reported provide an efficient molecular design of organic RTP materials and establishes relationships among molecular structures, intermolecular interactions, and RTP properties.     

9,9‐Dimethylxanthene Derivatives with Room‐Temperature Phosphorescence: Substituent Effects and Emissive Properties

Room‐temperature phosphorescence (RTP) emitters with ultralong lifetimes are emerging as attractive targets because of their potential applications in bioimaging, security, and other areas. But their development is limited by ambiguous mechanisms and poor understanding of the correlation of the molecular structure and RTP properties. Herein, different substituents on the 9,9‐dimethylxanthene core (XCO) result in compounds with RTP lifetimes ranging from 52 to 601 ms, which are tunable by intermolecular interactions and molecular configurations. XCO‐PiCl shows the most persistent RTP because of its reduced steric bulk and multiple sites of the 1‐chloro‐2‐methylpropan‐2‐yl (PiCl) moiety for forming intermolecular interactions in the aggregated state. The substituent effects reported provide an efficient molecular design of organic RTP materials and establishes relationships among molecular structures, intermolecular interactions, and RTP properties.     

荧光/磷光混合型白光有机发光二极管的研究

白光有机发光二极管（white organic light-emitting diodes,WOLEDs）在全色显示、固态照明以及背光源等领域有巨大的应用前景,其研究备受关注.其中,荧光/磷光混合型WOLEDs因兼具荧光材料的长寿命和磷光材料的高效率,被认为是目前最有希望实现照明应用的器件结构.荧光/磷光混合型WOLEDs最重要的问题是要解决荧光材料的单线态激子和磷光材料的三线态激子的协同发光.为了避免单线态激子和三线态激子的相互猝灭问题,必须设计有效的器件结构.本文以两种不同三线态能级的蓝光荧光材料为研究对象,介绍了不同高性能荧光/磷光混合型WOLEDs的结构设计与性能.研究表明,载流子传输平衡的高效结构设计和激子分布宽范围内的有效调控是实现高性能荧光/磷光混合型WOLEDs的关键.     

Stimuli-Responsive Purely Organic Room-Temperature Phosphorescence Materials

This Minireview summarizes the recent progress of stimuli-responsive purely organic phosphorescence materials. Organic phosphorescence is closely related to the intermolecular interactions, because such interactions are beneficial to promote spin orbital coupling (SOC) and boost intersystem cross (ISC) efficiency and finally are conducive to satisfactory phosphorescence. It is found that the intermolecular interactions, which are essential for organic phosphorescence, are easily disturbed by external stimuli such as mechanical force, photon, acid, chemical vapor, leading to the luminescence change. According to this principle, various purely organic phosphorescence materials sensitive to external stimuli have been developed. This Minireview categorizes reported stimuli-responsive purely organic phosphorescence materials on the basis of different stimuli, including mechanochromism, mechanoluminescence, photoactivity, acid-responsiveness and other stimuli. Some prospective strategies for constructing stimuli-responsive purely organic phosphorescence molecules are provided.     

Manipulating the Stacking of Triplet Chromophores in the Crystal Form for Ultralong Organic Phosphorescence

Provided here is evidence showing that the stacking between triplet chromophores plays a critical role in ultralong organic phosphorescence (UOP) generation within a crystal. By varying the structure of a functional unit, and different on‐off UOP behavior was observed for each structure. Remarkably, 24CPhCz, having the strongest intermolecular interaction between carbazole units exhibited the most impressive UOP with a long lifetime of 1.06 s and a phosphorescence quantum yield of 2.5 %. 34CPhCz showed dual‐emission UOP and thermally activated delayed fluorescence (TADF) with a moderately decreased phosphorescence lifetime of 770 ms, while 35CPhCz only displayed TADF owing to the absence of strong electronic coupling between triplet chromophores. This study provides an explanation for UOP generation in crystal and new guidelines for obtaining UOP materials.     

Crystallization-induced phosphorescence of benzils at room temperature

Efficient room temperature phosphorescence (RTP) is rarely observed in pure organic luminogens. However, we have newly observed that benzil and its derivatives are nonluminescent in solvents and thin layer chromatography (TLC) plates, but become highly phosphorescent in crystal state at room temperature, exhibiting typical crystallization-induced phosphorescence (CIP) characteristics. The CIP phenomenon is ascribed to the restriction of intramolecular rotations in crystals owing to effective intermolecular interactions. Such intermolecular interactions greatly rigidify the molecular conformation and significantly decrease the nonradiative deactivation channels of the triplet excitons, thus giving boosted phosphorescent emission at room temperature.     

Room-temperature phosphorescence from purely organic materials

Room-temperature phosphorescence (RTP) materials have attracted great attention due to their involvement of excited triplet states and comparatively long decay lifetimes. In this short review, recent progress on enhancement of RTP from purely organic materials is summarized. According to the mechanism of phosphorescence emission, two principles are discussed to construct efficient RTP materials: one is promoting intersystem crossing (ISC) efficiency by using aromatic carbonyl, heavyatom, or/and heterocycle/heteroatom containing compounds; the other is suppressing intramolecular motion and intermolecular collision which can quench excited triplet states, including embedding phosphors into polymers and packing them tightly in crystals. With aforementioned strategies, RTP from purely organic materials was achieved both in fluid and rigid media.     

Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room‐Temperature Phosphorescence

Two stable, purely organic luminogens exhibit both mechano‐ (ML) and photoluminescence (PL) with dual fluorescence–phosphorescence emissions at room temperature. Careful analysis of the crystal structures, coupled with theoretical calculations, demonstrate that room‐temperature phosphorescence and ML properties are strongly related to molecular packing. In particular, the formation and fracture of molecular dimers with intermolecular charge‐transfer properties has a significant effect on intersystem crossing, as well as excited triplet state emissions, in both PL and ML processes.     

Quantum chemical calculation of phosphorescence microwave double resonance spectra

Lifetimes of individual spin sublevels, vibronic intensities of phosphorescence, zero field splitting, hyperfine and nuclear quadruple tensors for the lowest triplet state, g factors, and dipole nature of microwave transitions have been calculated on the bases of CNDO and INDO methods taking into account spin–orbit, spin–spin, vibronic, and hyperfine interactions by perturbation theory. The results are in qualitative agreement with phosphorescence microwave double resonance data. Influence of intermolecular interaction on the zero field splitting are also investigated.     

Room-Temperature Phosphorescence Invoked Through Norbornyl-Driven Intermolecular Interaction Intensification with Anomalous Reversible Solid-State Photochromism

Herein, norbornyl (NB), a bulky annular nonconjugated spacer, is melded into π systems to construct two groups of ladder-type room-temperature phosphorescence (RTP) luminogens. The effect of the NB on π-π interactions, packing modes and RTP performance is explored systematically. The experimental and computational results demonstrate the versatility of NB in reducing π-π distances and synergistically intensifying the intermolecular interactions, which not only induces intersystem crossing from S₁ to T_n but also diminishes the nonradiative decay of triplet excitons. Impressively, 1800-fold phosphorescence lifetime enhancement is achieved in comparison with the reference compounds without NB. The molecular packing and RTP performance can be further modulated by the length of the backbones and terminal end-groups. It is quite peculiar that NB-annulated phthalic acid exhibits reversible photochromism in the solid state, likely due to the formation of persistent radical pairs. Our study paves an ingenious avenue towards enhancing intermolecular interactions and provides significant implications for a better comprehensive understanding of the origin of their RTP and the inherent photophysical mechanism.     

Multiple Luminescence Responses towards Mechanical Stimulus and Photo-Induction: The Key Role of the Stuck Packing Mode and Tunable Intermolecular Interactions

Organic luminescence with different forms continues to be one of the most active research fields in science and technology. Herein, an ultra-simple organic molecule (TPA-B), which exhibits both mechanoluminescence (ML) and photo-induced room-temperature phosphorescence (RTP) in the crystalline state, provides an opportunity to reveal the internal mechanism of ML and the dynamic process of photo-induced RTP in the same molecule. Through the detailed investigation of photophysical properties together with crystal structures, the key role of molecular packing and intermolecular interactions was highlighted in the luminescence response by mechanical and light stimulus, affording efficient strategies to design potential smart functional materials with multiple luminescence properties.     

Tailoring Intermolecular Interactions for Efficient Room‐Temperature Phosphorescence from Purely Organic Materials in Amorphous Polymer Matrices

Herein we report a rational design strategy for tailoring intermolecular interactions to enhance room‐temperature phosphorescence from purely organic materials in amorphous matrices at ambient conditions. The built‐in strong halogen and hydrogen bonding between the newly developed phosphor G1 and the poly(vinyl alcohol) (PVA) matrix efficiently suppresses vibrational dissipation and thus enables bright room‐temperature phosphorescence (RTP) with quantum yields reaching 24 %. Furthermore, we found that modulation of the strength of halogen and hydrogen bonding in the G1–PVA system by water molecules produced unique reversible phosphorescence‐to‐fluorescence switching behavior. This unique system can be utilized as a ratiometric water sensor.     

Fluorine-induced aggregate-interlocking for color-tunable organic afterglow with a simultaneously improved efficiency and lifetime

Designing organic afterglow materials with a high efficiency and long lifetime is highly attractive but challenging because of the inherent competition between the luminescence efficiency and lifetime. Here, we propose a simple yet efficient strategy, namely fluorine-induced aggregate-interlocking (FIAI), to realize both an enhanced efficiency and elongated lifetime of afterglow materials by stimulating the synergistic effects of the introduced fluorine atoms to efficiently promote intersystem crossing (ISC) and intermolecular non-covalent interactions for facilitating both the generation of triplet excitons and suppression of non-radiative decays. Thus, the fluorine-incorporated afterglow molecules exhibit greatly enhanced ISC with a rate constant up to 5.84 × 10⁷ s⁻¹ and suppressed non-radiative decay down to 0.89 s⁻¹, resulting in efficient organic afterglow with a simultaneously improved efficiency up to 10.5% and a lifetime of 1.09 s. Moreover, accompanied by the efficient phosphorescence emission especially at cryogenic temperature, color-tunable afterglow was also observed at different temperatures. Therefore, tri-mode multiplexing encryption devices by combining lifetime, temperature and color, and visual temperature sensing were successfully established. The FIAI strategy by addressing fundamental issues of afterglow emission paves the way to develop high-performance organic afterglow materials, opening up a broad prospect of aggregated and excited state tuning of organic solids for emission lifetime-resolved applications.

Through the fluorine-induced aggregate-interlocking (FIAI) strategy, the designed afterglow materials showed both improved quantum yields and prolonged lifetimes by breaking through the intrinsic bottlenecks of organic afterglow.     

Circularly Polarized Luminescence in Supramolecular Assemblies of Chiral Bichromophoric Perylene Bisimides

Chiral bichromophoric perylene bisimides are demonstrated as active materials of circularly polarized emission. The bichromophoric system exhibited circularly polarized luminescence with dissymmetry factors typical of that of similar organic chiral chromophoric systems in the monomeric state. Variation in solvent composition led to the formation of stably soluble helical aggregates through intermolecular interactions. A large enhancement in the dissymmetry of circularly polarized luminescence was exhibited by the aggregated structures both in the solution and solid states. The sum of excitonic couplings between the individual chromophoric units in the self‐assembled state results in relatively large dissymmetry in the circularly polarized luminescence, thereby giving rise to enhanced dissymmetry factors for the aggregated structures. The spacer between chiral center and chromophoric units played a crucial role in the effective enhancement of chiroptical properties in the self‐assembled structures. These materials might provide opportunities for the design of a new class of functional bichromophoric organic nanoarchitectures that can find potential applications in the field of chiroptical memory and light‐emitting devices based on supramolecular electronics.     

A color-tunable single-component luminescent molecule with multiple emission centers

For achieving smart materials with color-tunable emissions, the development of single-component systems exhibiting high durability and stability is desired but remains challenging, in comparison to multicomponent systems. Here, a single-component luminescent molecule (3-SPhF) with colorful emissions is successfully reported through different expressions of triplet excitons in radiative transitions. The time-resolved spectra confirm the existence of delayed fluorescence (τ = 282.5 μs), monomeric phosphorescence (τ = 497.7 ms) and aggregated-state phosphorescence (τ = 230.0 ms) in the crystal powder of 3-SPhF, which affords time-dependent afterglow and excitation-dependent emissions in a steady state. Furthermore, the relationships between ultra-long luminescence and stacking of the dibenzofuran group in single crystals are explored, providing evidence for the regularity of multiple emission centers in single-component compounds with dibenzofuran substituents.

Color tunable luminescent materials with multiple emission centers showing delayed fluorescence, single molecular phosphorescence and aggregated molecular phosphorescence are achieved, along with time-dependent and excitation-dependent properties.     

Utilizing d–pπ Bonds for Ultralong Organic Phosphorescence

Developing pure organic materials with ultralong lifetimes is attractive but challenging. Here we report a concise chemical approach to regulate the electronic configuration for phosphorescence enhancement. After the introduction of d–pπ bonds into a phenothiazine model system, a phosphorescence lifetime enhancement of up to 19 times was observed for DOPPMO, compared to the reference PPMO. A record phosphorescence lifetime of up to 876 ms was obtained in phosphorescent phenothiazine. Theoretical calculations and single‐crystal analysis reveal that the d–pπ bond not only reduces the (n, π*) proportion of the T₁ state, but also endows the rigid molecular environment with multiple intermolecular interactions, thus enabling long‐lived phosphorescence. This finding makes a valuable contribution to the prolongation of phosphorescence lifetimes and the extension of the scope of phosphorescent materials.     

Facile synthesis of the structural hierarchy in chrysanthemum–snowball-like self-organized polyaniline

A facile synthesis of the “chrysanthemum–snowball”-shaped polyaniline (PAni) has been prepared by using self-assembly polymerization of the host–guest monomeric inclusion complex of β-cyclodextrin (β-CD) with aniline. The amount of the monomer complex plays a role as a structural regulator during fabrication of the inclusion polymer as chrysanthemum–snowball structure/nanorods via intermolecular interactions such as: hydrogen bonding between β-CD and PAni, π–π interactions and cooperative interaction between PAni with FeCl₃ in an aqueous medium. The microstructure and morphology of the resulting materials were investigated by using various analytical techniques such as Fourier transform infrared, wide-angle X-ray diffraction, small-angle X-ray scattering, field emission scanning electron microscopy and transmission electron microscopy. After observing the growth process, a tentative mechanism is proposed to elucidate the formation of the PAni hierarchical structures.     

Host/guest deuteration effects on mixed crystals

The phosphorescence characteristics of twelve guest/host mixed crystal systems were investigated. A large thermally-activated intermolecular effect of host deuteration on the phosphorescence of the guest was noted. All observed effects can be rationalized using a kinetic model which implies thermal activation of triplet guest to triplet host, entrapment of energy at defects in the host exciton band, and the effects of deuteration on the thermal band gaps.     

Nature and kinetics of the self-quenching of excited states of chromium(VI) oxyanions

Absorption and emission spectroscopy, utilizing theoretical group analysis, was used to investigate the nature of triplet states of chrornate and bichromate ions in aqueous solutions. The complex kinetics observed for the quenching of phosphorescence is explained by competition of emission and rapid exchange of energy between triplet sublevels. Quenching of phosphorescence is observed in concentrated chromate ion solutions. The dimer nature of the bichromate ion prevents intermolecular exchange of energy between ions in view of the existence of a more effective intraionic energy transfer.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 6, pp. 337–341, November–December, 1994.