Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule

Tailoring raloxifene into single-component molecular crystals possessing multilevel stimuli-responsive room-temperature phosphorescence

Simultaneously achieving room-temperature phosphorescence (RTP) and multiple-stimuli responsiveness in a single-component system is of significance but remains challenging. Crystallization has been recognized to be a workable strategy to fulfill the above task. However, how the molecular packing mode affects the intersystem crossing and RTP lifetime concurrently remains unclear so far. Herein, four economic small-molecular compounds, analogues of the famous drug raloxifene (RALO), are facilely synthesized and further explored as neat single-component and stimuli-responsive RTP emitters via crystallization engineering. Thanks to their simple structures and high ease to crystallize, these raloxifene analogues function as models to clarify the important role of molecular packing in the RTP and stimuli-responsiveness properties. Thorough combination of the single-crystal structure analysis and theoretical calculations clearly manifests that the tight antiparallel molecular packing mode is the key point to their RTP behaviors. Interestingly, harnessing the controllable and reversible phase transitions of the two polymorphs of RALO-OAc driven by mechanical force, solvent vapor, and heat, a single-component multilevel stimuli-responsive platform with tunable emission color is established and further exploited for optical information encryption. This work would shed light on the rational design of multi-stimuli responsive RTP systems based on single-component organics.

Engineering GaN/AuNC core-shell nanowire heterojunctions by gold nanoclusters with excitation-dependent behavior for enhancing the responsivity and stability of self-driven photodetectors

Self-driven broadband photodetectors (PDs) with low-power consumption have great potential applications in the wide range of next-generation optoelectronic devices. In this study, a self-driven broadband PD responding to an ultraviolet-visible range based on gallium nitride/gold nanocluster (GaN/AuNC) core-shell nanowire heterojunctions is fabricated for the first time. By introducing the AuNCs onto the GaN nanowire surfaces, the GaN/AuNC core-shell nanowire heterojunctions can be formed efficiently. It is crucial that AuNCs have the functions of light collectors and hole conductors in heterojunctions due to the suitable energy level alignment. Under the optimized conditions of AuNCs, it is found that GaN/AuNC core-shell nanowires can significantly increase the photocurrent and responsivity of PDs, mainly resulting from the light interreflection within the heterojunctions and the effective improvement of carrier transport. Owing to the excitation-dependent emission behavior of AuNCs, the responsivity of PD with GaN/AuNC core-shell nanowire heterojunctions can be enhanced by around 330% compared with that of PD without AuNCs under visible illumination. Furthermore, GaN/AuNC hybrid nanowires with excitation-dependent fluorescence behavior can modulate the enhanced amplitude performance of broadband PDs. Owing to the high stability of AuNCs, the photocurrent of the PD with AuNCs is still quite stable after continuous operation for more than 20 000 s. Therefore, this study provides an effective method for developing new broadband PDs with high performance and low energy consumption.

Room-Temperature Multiple Phosphorescence from Functionalized Corannulenes: Temperature Sensing and Afterglow Organic Light-Emitting Diode

Corannulene-derived materials have been extensively explored in energy storage and solar cells, however, are rarely documented as emitters in light-emitting sensors and organic light-emitting diodes (OLEDs), due to low exciton utilization. Here, we report a family of multi-donor and acceptor (multi-D-A) motifs, TCzPhCor, TDMACPhCor, and TPXZPhCor, using corannulene as the acceptor and carbazole (Cz), 9,10-dihydro-9,10-dimethylacridine (DMAC), and phenoxazine (PXZ) as the donor, respectively. By decorating corannulene with different donors, multiple phosphorescence is realized. Theoretical and photophysical investigations reveal that TCzPhCor shows room-temperature phosphorescence (RTP) from the lowest-lying T1 ; however, for TDMACPhCor, dual RTP originating from a higher-lying T1 (T1 H ) and a lower-lying T1 (T1 L ) can be observed, while for TPXZPhCor, T1 H -dominated RTP occurs resulting from a stabilized high-energy T1 geometry. Benefiting from the high-temperature sensitivity of TPXZPhCor, high color-resolution temperature sensing is achieved. Besides, due to degenerate S1 and T1 H states of TPXZPhCor, the first corannulene-based solution-processed afterglow OLEDs is investigated. The afterglow OLED with TPXZPhCor shows a maximum external quantum efficiency (EQEmax ) and a luminance (Lmax ) of 3.3 % and 5167 cd m-2 , respectively, which is one of the most efficient afterglow RTP OLEDs reported to date.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

Dual and Panchromatic Emission from N-Aryl-phenanthridinones: Extension of the Seesaw Photophysical Model with a Slight Twist

White-light emission from a single organic molecule, known as a single white-light emitter, is a rare phenomenon and desirable property for a class of materials with potential future applications in white lighting. Since N-aryl-naphthalimides (NANs) have been shown to follow excited state behavior and unique dual or panchromatic emission through a substituent pattern prescribed via a seesaw photophysical model, this study investigates the substituent effects on the fluorescence emission of structurally related N-aryl-phenanthridinones (NAPs) dyes. Following a similar placement prescription of an electron-releasing group (ERG) and electron-withdrawing group (EWG) at the phenanthridinone core and N-aryl moiety, we discovered from time-dependent density functional theory (TD-DFT) results that NAPs show a substitution pattern opposite to NANs in order to promote S2 and higher excited states. Interestingly, 2-methoxy-5-[4-nitro-3(trifluoromethyl)phenyl]phenanthridin-6(5H)-one 6e displayed a pronounced dual and panchromatic fluorescence dye depending on the solvent. For the six dyes included in the study, full spectral information in a variety of solvents, as well as fluorescence quantum yield and lifetime are reported. TD-DFT calculations support the predicted optical behavior via mixing of S2 and S6 excited states via anti-Kasha type of emission behavior.

Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Photosensitizers that display "unusual" emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha's rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe₂ group around the core of benzo[g]coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states. For solvents with intermediate polarity, three of the four regioisomers exhibit fluorescence from two different excited states with bands that are well separated in the visible and the near-infrared spectral regions. Computational analysis, employing ab initio methods, reveals that the orientation of an ester on the pyrone ring produces two conformers responsible for the observed dual fluorescence. Studies with solid solvating media, which restricts the conformational degrees of freedom, concur with the computational findings. These results demonstrate how "seemingly inconsequential" auxiliary substituents, such as the esters on the pyrone coumarin rings, can have profound effects leading to "anti-Kasha" photophysical behavior important for molecular photonics, materials engineering, and solar-energy science.

A direct observation of up-converted room-temperature phosphorescence in an anti-Kasha dopant-matrix system

It is common sense that emission maxima of phosphorescence spectra (λ_P) are longer than those of fluorescence spectra (λ_F). Here we report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) with λ_P < λ_F and phosphorescence lifetime > 0.1 s upon doping benzophenone-containing difluoroboron β-diketonate (BPBF₂) into phenyl benzoate matrices. The up-converted RTP is originated from BPBF₂'s T_n (n ≥ 2) states which show typical ³n-π* characters from benzophenone moieties. Detailed studies reveal that, upon intersystem crossing from BPBF₂'s S₁ states of charge transfer characters, the resultant T₁ and T_n states build T₁-to-T_n equilibrium. Because of their ³n-π* characters, the T_n states possess large phosphorescence rates that can strongly compete RTP(T₁) to directly emit RTP(T_n) which violates Kasha's rule. The direct observation of up-converted RTP provides deep understanding of triplet excited state dynamics and opens an intriguing pathway to devise visible-light-excitable deep-blue afterglow emitters, as well as stimuli-responsive afterglow materials.

Conformation-dependent dynamic organic phosphorescence through thermal energy driven molecular rotations

Organic room-temperature phosphorescent (RTP) materials exhibiting reversible changes in optical properties upon exposure to external stimuli have shown great potential in diverse optoelectronic fields. Particularly, dynamic manipulation of response behaviors for such materials is of fundamental significance, but it remains a formidable challenge. Herein, a series of RTP polymers were prepared by incorporating phosphorescent rotors into polymer backbone, and these materials show color-tunable persistent luminescence upon excitation at different wavelengths. Experimental results and theoretical calculations revealed that the various molecular conformations of monomers are responsible for the excitation wavelength-dependent (Ex-De) RTP behavior. Impressively, after gaining insights into the underlying mechanism, dynamic control of Ex-De RTP behavior was achieved through thermal energy driven molecular rotations of monomers. Eventually, we demonstrate the practical applications of these amorphous polymers in anti-counterfeiting areas. These findings open new opportunities for the control of response behaviors of smart-responsive RTP materials through external stimuli rather than conventional covalent modification method.

9,9-Bis[4-(N-aryl)phenyl]methylidene-xanthylidene Derivatives Displaying Mechano-, Crystallo-, and Thermochromism

Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

Excitation-Dependent Multicolour Luminescence of Organic Materials: Internal Mechanism and Potential Applications

Excitation-dependent emission (Ex-de) materials have been of considerable academic interest and have potential applications in real life. Such multicolour luminescence is a characteristic exception to the ubiquitously accepted Kasha's rule. This phenomenon has been increasingly presented in some studies on different luminescence systems; however, a systematic overview of the mechanisms underlying this phenomenon is currently absent. Herein, we resolve this issue by classifying multicolour luminescence from single chromophores and dual/ternary chromophores, as well as multiple emitting species. The underlying processes are described based on electronic and/or geometrical conditions under which the phenomenon occurs. Before we present it in categories, related photophysical and photochemical foundations are introduced. This systematic overview will provide a clear approach to designing multicolour luminescence materials for special applications.

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp³ C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T₁^H and T₁^L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp² C-linked D-A counterparts, RTP from one locally excited T₁ state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

[4]Triangulenes Modified by Three Oxygen-Boron-Oxygen (OBO) Units: Synthesis, Characterizations, and Anti-Kasha Emissions

Modification of π-conjugated systems using a boron atom as the dopant has become a powerful approach to create new structures and new properties. Herein, we report a facile synthesis of replacing the carbon edges of [4]triangulene by three oxygen-boron-oxygen (OBO) units. The OBO-modified [4]triangulenes are structurally similar to [4]triangulene and isoelectronic to the trianion of [4]triangulene. The structure of OBO-modified [4]triangulene is confirmed by single-crystal X-ray diffraction analysis, revealing an off-plane core with three edge-modified OBO units. These OBO-modified [4]triangulenes exhibit excellent thermal stability. These compounds have phosphorescence with lifetime longer than 1 s at 77 K. Both theoretical calculations and photophysical investigation of OBO-modified [4]triangulenes indicate that this kind of molecules display a rare anti-Kasha fluorescence and phosphorescence emissions from multiple higher excited states.

Thermally Activated and Aggregation‐Regulated Excitonic Coupling Enable Emissive High‐Lying Triplet Excitons**

Room‐temperature phosphorescence (RTP) originating from higher‐lying triplet excitons remains a rather rarely documented occurrence for purely organic molecular systems. Here, we report two naphthalene‐based RTP luminophores whose phosphorescence emission is enabled by radiative decay of high‐lying triplet excitons. In contrast, upon cooling the dominant phosphorescence originates from the lowest‐lying triplet excited state, which is manifested by a red‐shifted emission. Photophysical and theoretical studies reveal that the unusual RTP results from thermally activated excitonic coupling between different conformations of the compounds. Aggregation‐regulated excitonic coupling is observed when increasing the doping concentration of the emitters in poly(methylmethacrylate) (PMMA). Further, the RTP quantum efficiency improves more than 80‐fold in 1,3‐bis(N‐carbazolyl)benzene (mCP) compared to that in PMMA. This design principle offers important insight into triplet excited state dynamics and has been exploited in afterglow‐indicating temperature sensing.

Excitation-Wavelength-Dependent Organic Long-Persistent Luminescence Originating from Excited-State Long-Range Proton Transfer

Stimuli-responsive functional luminescent materials with tunable color and long-persistent emission have emerged as a powerful tool in information encryption, anticounterfeiting, and bioelectronics. Herein, we prove a novel strategy for manipulating the proton transfer pathways in the salicylaldehyde derivative EQCN solutions/powder to produce excitation wavelength-dependent (Ex-De) performances with switchable emissions (blue-sky, green, and orange). The experiments and theoretical results demonstrated that the different luminous colors are originated from enol (E) form (blue-sky), Keto-1 (K1) form (orange) through the excited-state intramolecular proton transfer (ESIPT) process, and Keto-2 (K2) form (green) through the excited-state long-range proton transfer (ESLRPT) process. We leverage synergistic effects between the dopant and matrix (dimethyl terephthalate, DTT) to manipulate the excited-state proton transfer pathway in EQCN@DTT mixture powders to generate Ex-De long-persistent luminescence (Ex-De-LPL), which can be well applied in multilevel information encryption. This strategy not only paves an intriguing way for the construction and preparation of pure organic Ex-De materials but also offers a guideline for developing LPL materials based on ESLRPT processes.

Secondary through-space interactions facilitated single-molecule white-light emission from clusteroluminogens

Clusteroluminogens refer to some non-conjugated molecules that show visible light and unique electronic properties with through-space interactions due to the formation of aggregates. Although mature and systematic theories of molecular photophysics have been developed to study conventional conjugated chromophores, it is still challenging to endow clusteroluminogens with designed photophysical properties by manipulating through-space interactions. Herein, three clusteroluminogens with non-conjugated donor-acceptor structures and different halide substituents are designed and synthesized. These compounds show multiple emissions and even single-molecule white-light emission in the crystalline state. The intensity ratio of these emissions is easily manipulated by changing the halide atom and excitation wavelength. Experimental and theoretical results successfully disclose the electronic nature of these multiple emissions: through-space conjugation for short-wavelength fluorescence, through-space charge transfer based on secondary through-space interactions for long-wavelength fluorescence, and room-temperature phosphorescence. The introduction of secondary through-space interactions to clusteroluminogens not only enriches their varieties of photophysical properties but also inspires the establishment of novel aggregate photophysics for clusteroluminescence.

Although mature and systematic theories of molecular photophysics have been developed, it is still challenging to endow clusteroluminogens (CLgens) with designed photophysical properties by manipulating through-space interactions. Here, the authors design three CLgens that show multiple emissions and white-light emission in the crystalline state, and emphasize the important role of secondary through-space interactions between the acceptor and non-conjugated donor units.

Synergistic Generation and Accumulation of Triplet Excitons for Efficient Ultralong Organic Phosphorescence

The traditional method to achieve ultralong organic phosphorescence (UOP) is to hybrid nπ* and ππ* configurations in appropriate proportion, which are contradictory to each other for improving efficiency and lifetime of phosphorescence. In this work, through replacing the electron-donating aromatic group with a methoxy group and combining intramolecular halogen bond to promote intersystem crossing and suppress non-radiative transition, an efficient UOP molecule (2Br-OSPh) has been synthesized with the longest lifetime and brightest UOP among its isomers. As compared to CzS2Br, which has a similar substituted position of bromine atom and a larger k_isc (the rate of intersystem crossing), the smaller ΔE_TT* (the energy gap between monomeric phosphorescence and aggregated state phosphorescence) in 2Br-OSPh could accelerate the transition from T₁ to T₁ *. This research indicates that both generation and accumulation of triplet excitons play an important role in realizing efficient UOP materials.

Multicolour Fluorescence Based on Excitation-Dependent Electron Transfer Processes in o-Carborane Dyads

Organic materials with excitation wavelength-dependent (Ex-de) emission are highly attractive for anticounterfeiting, optoelectronics and bioassay applications; however, the realization of Ex-de fluorescence, independent of aggregation states, remains a challenge. We herein report a photoinduced electron transfer (PeT) strategy to design Ex-de fluorescence materials by manipulating the relaxation pathways of multiple excited states. As expected, the o-carborane dyad presents a clear Ex-de fluorescence colour in the aggregated states, resulting from the tunable relative intensity of the dual-fluorescence spectra. Taking TP[1]B as an example, the amorphous powders emitted bright blue-violet, white and yellow colours under 390 nm, 365 nm and 254 nm UV illumination, respectively. Importantly, multicolour, flexible and transparent films as well as an anticounterfeiting application using this o-carborane dyad are demonstrated.

Prompt and Long-Lived Anti-Kasha Emission from Organic Dyes

Anti-Kasha behavior has been the subject of intense debate in the last few years, as demonstrated by the high number of papers appearing in the literature on this topic, dealing with both mechanistic and applicative aspects of this phenomenon. Examples of anomalous emitters reported in the last 10 years are collected in the present review, which is focused on strictly anti-Kasha organic molecules displaying radiative deactivation from S_n and/or T_n, with n greater than 1.

Reversible On-Off Switching of Excitation-Wavelength-Dependent Emission of a Phosphorescent Soft Salt Based on Platinum(II) Complexes

Excitation-wavelength-dependent (Ex-De) emission materials show excellent potential in diverse advanced photonic areas. Of significant importance is the on-demand regulation of the Ex-De luminescence behavior of these materials, which is previously unprecedented. In this study, we report on a platinum(II) complex-based phosphorescent soft salt S1 ([Pt(tpp)(ed)]⁺[Pt(ftpp)(CN)₂]^- (where ttp = 2-(4-(trifluoromethyl)phenyl)pyridine, ed = ethane-1,2-diamine, and ftpp = 2-(4-fluoro-3-(trifluoromethyl)phenyl)pyridine)) with Ex-De photoluminescence (PL) property. UV-visible absorption and PL spectra of S1 were recorded in DMSO-H₂O mixture (1 × 10^-3 M) with various H₂O fractions to investigate its ground and excited states. Interestingly, the PL spectra of S1 powder show that its maximum emission peak is red-shifted from 595 to 644 nm upon excitation at different wavelengths from 360 to 520 nm, accompanied by an obvious emission color change from yellow-orange to red. Furthermore, confocal laser scanning fluorescence microscopy was employed to determine the PL property of self-assembled uniform S1 nanostructure, and the result shows that the Ex-De emission behavior is absent. On the basis of these results, we conclude the various Pt(II)···Pt(II) distances that exist are the major factor responsible for the properties of the Ex-De PL of S1 powder. Thus, for the first time, reversible on-off switching of Ex-De PL of S1 was achieved by manipulating its Pt(II)···Pt(II) distances through mechanical stress and vapor fuming. Finally, we demonstrate the high-level anticounterfeiting applications via on-demand multicolor displays.

Rational design of pyrrole derivatives with aggregation-induced phosphorescence characteristics for time-resolved and two-photon luminescence imaging

Pure organic room-temperature phosphorescent (RTP) materials have been suggested to be promising bioimaging materials due to their good biocompatibility and long emission lifetime. Herein, we report a class of RTP materials. These materials are developed through the simple introduction of an aromatic carbonyl to a tetraphenylpyrrole molecule and also exhibit aggregation-induced emission (AIE) properties. These molecules show non-emission in solution and purely phosphorescent emission in the aggregated state, which are desirable properties for biological imaging. Highly crystalline nanoparticles can be easily fabricated with a long emission lifetime (20 μs), which eliminate background fluorescence interference from cells and tissues. The prepared nanoparticles demonstrate two-photon absorption characteristics and can be excited by near infrared (NIR) light, making them promising materials for deep-tissue optical imaging. This integrated aggregation-induced phosphorescence (AIP) strategy diversifies the existing pool of bioimaging agents to inspire the development of bioprobes in the future.

Unravelling the intricate photophysical behavior of 3-(pyridin-2-yl)triimidazotriazine AIE and RTP polymorphs

The development of purely organic materials showing multicolor fluorescent and phosphorescent behaviour represents a formidable challenge in view of practical applications. Herein the rich photophysical behaviour of 3-(pyridin-2-yl)triimidazotriazine (TT-Py) organic molecule, comprising excitation-dependent fluorescence and phosphorescence under ambient conditions in both blended films and the crystalline phase, is investigated by means of steady state, time resolved and ultrafast spectroscopies and interpreted on the basis of X-ray diffraction studies and DFT/TDDFT calculations. In particular, by proper excitation wavelength, dual fluorescence and dual phosphorescence of molecular origin can be observed together with low energy phosphorescences resulting from aggregate species. It is demonstrated that the multiple emission properties originate from the copresence, in the investigated system, of an extended polycyclic nitrogen-rich moiety (TT), strongly rigidified by π-π stacking interactions and short C-H···N hydrogen bonds, and a fragment (Py) having partial conformational freedom.