Solid-State Thermochromic Luminescence through Twisted Intramolecular Charge Transfer and Excimer Formation of a Carborane−Pyrene Dyad with an Ethynyl Spacer

Influence of Intermolecular Structural Effects on Radiative Efficiency in Xanthene-Based Carboranyl Luminophores

Two o-carboranes with (i) 9,9-dimethyl-9H-xanthene and (ii) spiro[fluorene-9,9'-xanthene] moieties (XTC and sXTC, respectively) were prepared and characterized. Single X-ray crystallography analysis revealed the presence of intermolecular hydrogen bonds in XTC crystals. Although both compounds did not exhibit emission in tetrahydrofuran solutions at 298 K, intense bluish emission was observed in the solid states and frozen tetrahydrofuran solutions at 77 K. According to the results of theoretical calculations, this emission originated from an intramolecular charge transfer (ICT) transition with the o-carborane moiety. The absolute quantum efficiency (Φem) of the ICT-based emission in the film state equaled 49% for XTC and 20% for sXTC but was as high as 90% for the crystals of both compounds. The crystal structures of XTC and sXTC revealed that the o-carboranyl-appended phenyl plane was orthogonal (85-89°) to the carbon-carbon bonding axis in the o-carborane, indicating the existence of a strong exo-π-interaction, which was identified as the structural basis for the ICT-based transition. These results implied that the intermolecular structural effect of XTC in the randomly aggregated solid state (film) helped maintain the above orthogonality and, hence, the high efficiency from the ICT radiative mechanism. Thus, we concluded that the ICT radiative efficiency of o-carboranyl luminophores in the aggregated solid state can be controlled by specific intermolecular interactions and that the molecular geometric design inducing this feature can be important for developing highly efficient carboranyl luminophores.

Synthesis, Photophysical and Electronic Properties of a D-π-A Julolidine-Like Pyrenyl-o-Carborane

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes)2 (Mes=2,4,6-Me3C6H2) as acceptor groups. Whereas the π-acceptor strength of B(Mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed.

Solid-state excimer emission of o-carborane derivatives and applications for stimuli-responsive luminescent materials

o-Carborane is an icosahedral cluster containing boron and has been paid attention as a versatile platform for constructing stimuli-responsive materials with solid-state luminescence properties by suppressing aggregation-caused quenching (ACQ), which is the emission annihilation process observed in condensed states, such as concentrated solutions, solids and films. Non-specific intermolecular interactions in the condensed states resulting in ACQ are disturbed by the spherical boron cluster. More recently, it has been demonstrated from several reports that o-carborane is capable of assisting solid-state excimer emission in aryl-modified o-carboranes. Moreover, some of these materials provide unique stimuli-responsiveness. In the commodity design for solid-state luminescent materials, o-carboranes are responsible for disturbing intermolecular interactions and subsequently presenting solid-state luminescence. On the other hand, excimer emission, which is inducible from the intermolecular photophysical process, has been discovered in the solid state based on o-carborane materials. In this manuscript, recent progress in stimuli-responsive luminescent materials via excimer formation will be described. Moreover, as the further potential of o-carborane materials, recent findings on the photo-salient effect which can be induced through excimer formation followed by photoreaction will also be explained.

Coordination-Induced Conformational Control Enables Highly Luminescent Metallo-Cages

In recent years, luminescent materials have received a great deal of attention due to their wide range of applications. However, exploring a simple solution to overcome the fluorescence quenching resulting from the aggregation of conventional organic fluorophores remains a valuable area of investigation. In this study, we successfully constructed two metallo-cages, namely, SA and SB, through coordination-driven self-assemblies of the triphenylamine (TPA)-based donor L with different diplatinum(II) acceptors LA and LB, respectively. These metallo-cages take advantage of their steric nature and curved conformation to more effectively limit the free rotation of the benzene ring and hinder π-π stacking in the solid state, which successfully inhibited fluorescence quenching and realizing highly efficient luminescent properties. Therefore, this work offers a new design strategy for preparing materials with excellent luminescent properties.

Improvement in Radiative Efficiency Via Intramolecular Charge Transfer in ortho-Carboranyl Luminophores Modified with Functionalized Biphenyls

In this study, we found that the electronic effects of the functional groups on aromatic units attached to o-carboranyl species can enhance the efficiency of intramolecular charge transfer (ICT)-based radiative decay processes. Six o-carboranyl-based luminophores having attached functionalized biphenyl groups with CF₃, F, H, CH₃, C(CH₃)₃, and OCH₃ substituents were prepared and fully characterized by multinuclear magnetic resonance spectroscopy. In addition, their molecular structures were determined by single-crystal X-ray diffractometry, which revealed that the distortion of the biphenyl rings and the geometries around the o-carborane cages were similar. All compounds exhibited ICT-based emissions in the rigid state (solution at 77 K and film). Intriguingly, the quantum efficiencies (Φ_em) of five compounds (that of the group with CF₃ could not be measured because of its extremely weak emissions) in the film state increased gradually as the electron-donating power of the terminal functional group modifying the biphenyl moiety increased. Furthermore, the nonradiative decay constants (k_nr) for the group with OCH₃ were estimated to be one-tenth of those for the group with F, whereas the radiative decay constants (k_r) for the five compounds were similar. The dipole moments (μ) calculated for the optimized first excited state (S₁) structures gradually increased, from that of the group with CF₃ to that of the group with OCH₃, implying that the inhomogeneity of the molecular charge distribution was enhanced by electron donation. The electron-rich environment formed as a result of electron donation led to efficient charge transfer to the excited state. Both experimental and theoretical findings revealed that the electronic environment of the aromatic moiety in o-carboranyl luminophores can be controlled to accelerate or interrupt the ICT process in the radiative decay of excited states.

Alternately π-Stacked Systems Assisted by o-Carborane: Dual Excimer Emission and Color Modulation by Bcage -Methylation

Herein, we report the unique solid-state excimer emission of three types of acridine-tethered o-carboranes with variable degrees of methylation at the o-carborane unit. They all showed columnar packing structures based on dimer formation, and two types of π-overlapping motifs were alternately stacked. From the photoluminescence (PL) measurements on the crystalline samples, it was found that three types of luminescence bands can simultaneously appear: monomer emission, excimer emission from the moderately π-stacked intra-dimer unit, and excimer emission from the widely π-stacked inter-dimer unit. Consequently, the PL colors were drastically changed by the steric effect of the methyl groups, with a strong correlation found between the π-overlapping and excimer character. In addition, variable-temperature PL measurements revealed that these PL species should be in thermal equilibrium at room temperature, with the intensity ratios sensitive toward temperature changes.

Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-o-carboranyl compounds, in which hydrogen (15CH and 26CH) or trimethysilyl groups (15CS and 26CS) were appended at the o-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while 15CS and 26CS showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the o-carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH. These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo-π-interaction, resulting in a higher quantum efficiency (Φ_em) for ICT-based radiative decay than those of 15CH and 26CH. Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φ_em and lowest k_nr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o-carboranyl luminophores could be achieved by the formation of a specific geometry between the o-carborane cage and the aromatic plane.

Revisiting the Role of Charge Transfer in the Emission Properties of Carborane-Fluorophore Systems: A TDDFT Investigation

In this study, we performed a detailed investigation of the S1 potential energy surface (PES) of o-carborane-anthracene (o-CB-Ant) with respect to the C-C bond length on o-CB and the dihedral angle between o-CB and Ant moieties. The effects of different substituents (F, Cl, CN, and OH) on carbon- or boron-substituted o-CB, along with a π-extended acene-based fluorophore, pentacene, on the nature and energetics of S1 → S0 transitions are evaluated. Our results show the presence of a non-emissive S1 state with an almost pure charge transfer (CT) character for all systems as a result of significant C-C bond elongation (C-C = 2.50-2.56 Å) on o-CB. In the case of unsubstituted o-CB-Ant, the adiabatic energy of this CT state corresponds to the global minimum on the S1 PES, which suggests that the CT state could be involved in emission quenching. Despite large deformations on the o-CB geometry, predicted energy barriers are quite reasonable (0.3-0.4 eV), and the C-C bond elongation can even occur without a noticeable energy penalty for certain conformations. With substitution, it is shown that the dark CT state becomes even more energetically favorable when the substituent shows -M effects (e.g., -CN), whereas substituents showing +M effects (e.g., -OH) can result in an energy increase for the CT state, especially for partially stretched C-C bond lengths. It is also shown that the relative energy of the CT state on the PES depends strongly on the LUMO level of the fluorophore as this state is found to be energetically less favorable compared to other conformations when anthracene is replaced with π-extended pentacene. To our knowledge, this study shows a unique example of a detailed theoretical analysis on the PES of the S1 state in o-CB-fluorophore systems with respect to substituents or fluorophore energy levels. Our findings could guide future experimental work in emissive o-CB-fluorophore systems and their sensing/optoelectronic applications.

Conformation-Dependent Electron Donation of Nido-Carborane Substituents and Its Influence on Phosphorescence of Tris(2,2′-bipyridyl)ruthenium(II) Complex

In this work, we report the synthesis of the nido-carborane-substituted ruthenium complexes and the substituent effects of nido-carboranes on the optical properties. Initially, from the optical measurements, it is shown that deep-red phosphorescence was obtained from the synthesized molecule, and the phosphorescent quantum yields were significantly improved by loading onto a polyethylene glycol film. This result represents that nido-carborane can work as a strong electron donor and should be an effective unit for enhancing the solid-state phosphorescence of ruthenium complexes. Further, it is suggested that the electron-donating properties of the nido-carborane units and subsequently the optical properties can be tuned by controlling the conformation of the nido-carborane units with the steric substituents. We demonstrate in this study the potential of nido-carborane as a building block for constructing optical materials as well as fundamental information regarding electronic interactions with π-conjugated systems.

Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2'-yl)-o-carborane

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2'-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive 'mixed' state of 1-(pyren-2'-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

Effects of molecular geometry on the efficiency of intramolecular charge transfer-based luminescence in o-carboranyl-substituted 1H-phenanthro[9,10-d]imidazoles

The distinct difference in solid-state emission efficiency between 1H-phenanthro[9,10-d]imidazole–o-carboranyl luminophores showed that the geometric orientation is a key factor for controlling intramolecular charge transfer-based radiative decay.

Manipulation of electronic and structural effects on the solid-state emission of multiple linked anthracenyl-o-carborane dyads

A direct boost of emission efficiency is presented based on the strategy of linking multiple weak AIE-active anthracenyl-o-carborane dyads together for improving solid-state luminescence.

Alteration of intramolecular electronic transition via deboronation of carbazole-based o-carboranyl compound and intriguing 'turn-on' emissive variation

The conversion of closo-o-carborane-containing compounds to the nido-o-species via deboronation causes photophysical changes that could be used for sensing applications. 9-Methyl-9H-carbazole-based closo- (closo-Cz) and nido-o-carboranyl (nido-Cz) compounds were prepared and fully characterised by multinuclear NMR spectroscopy and elemental analysis, and the solid-state molecular structure of closo-Cz was analysed by X-ray crystallography. Although the closo-compound exhibited an emissive pattern centred at λ em = ca. 530 nm in the rigid state only (in THF at 77 K and as a film), nido-Cz demonstrated intense emission in the near-UV region (λ em = ca. 380 nm) in both solution and film states at 298 K. The positive solvatochromic effect of nido-Cz and the results of theoretical calculations for both the o-carboranyl compounds supported that these emissive features originate from intramolecular charge transfer (ICT) corresponding to the o-carborane. Furthermore, the calculations verified that the electronic role of the o-carboranyl unit changed from acceptor to donor upon deboronation from closo-Cz to nido-Cz. Investigations of the radiative decay mechanisms of closo-Cz and nido-Cz according to their quantum efficiencies (Φ em) and decay lifetimes (τ obs) suggested that the ICT-based radiative decays of closo-Cz and nido-Cz readily occur in the film (solid) and solution state, respectively. These observations implied that the emission of closo-Cz in the solution state could be drastically enhanced by deboronation to nido-Cz upon exposure to an increasing concentration of fluoride anions. Indeed, turn-on emissive features in an aqueous solution were observed upon deboronation, strongly suggesting the potential of closo-Cz as a turn-on and visually detectable chemodosimeter for fluoride ion sensing.

Dimerization-Induced Solid-State Excimer Emission Showing Consecutive Thermochromic Luminescence Based on Acridine-Modified o-Carboranes

Although excimer emission is a useful luminescent phenomenon for fabricating optical sensors and probes, it is difficult to apply excimer emission for film sensors due to critical concentration quenching in the solid state. Therefore, robust molecular designs for solid-state excimer emission are still being explored. One of the key examples is the previously reported acridine-ethynyl-o-carborane AcE1, which showed a bright solid-state excimer emission assisted by characteristic C_cageH···N interactions. In this paper, we report the newly synthesized acridine-diehynyl-o-carborane AcE2 and comprehensively compare it to AcE1. Both compounds had the same crystalline packing mode based on dimer formation, resulting in an efficient π-overlapping area and solid-state excimer emission. Variable-temperature photoluminescence (VT-PL) measurements revealed the consecutive thermochromic luminescence of these compounds. Finally, on the basis of the easily accessible spray-coating method, we constructed the thermochromic luminescent sensors on quartz substrates. According to the mechanistic studies, it is demonstrated that the design strategy based on a dimer-induced solid-state excimer should have great potential for applications as a molecular thermometer.

Strategic molecular design of closo-ortho-carboranyl luminophores to manifest thermally activated delayed fluorescence

In this paper, we propose a strategic molecular design of closo-o-carborane-based donor-acceptor dyad system that exhibits thermally activated delayed fluorescence (TADF) in the solution state at ambient temperature. Planar 9,9-dimethyl-9H-fluorene-based compounds with closo- and nido-o-carborane cages appended at the C2-, C3-, and C4-positions of each fluorene moiety (closo-type: 2FC, 3FC, 4FC, and 4FCH, and nido-type: nido-4FC = [nido-form of 4FC]·[NBu4]) were prepared and characterized. The solid-state molecular structure of 4FC exhibited a significantly distorted fluorene plane, which suggests the existence of severe intramolecular steric hindrance. In photoluminescence measurements, 4FC exhibits a noticeable intramolecular charge transition (ICT)-based emission in all states (solution at 298 K and 77 K, and solid states); however, emissions by other closo-compounds were observed in only the rigid state (solution at 77 K and film). Furthermore, nido-4FC did not exhibit emissive traces in any state. These observations verify that all radiative decay processes correspond to ICT transitions triggered by closo-o-carborane, which acts as an electron acceptor. Relative energy barriers calculated by TD-DFT as dihedral angles around o-carborane cages change in closo-compounds, which indicates that the structural formation of 4FC is nearly fixed around its S0-optimized structure. This differs from that for other closo-compounds, wherein the free rotation of their o-carborane cages occurs easily at ambient temperature. Such rigidity in the structural geometry of 4FC results in ICT-based emission in solution at 298 K and enhancement of quantum efficiency and radiative decay constants compared to those for other closo-compounds. Furthermore, 4FC displays short-lived (∼0.5 ns) and long-lived (∼30 ns) PL decay components in solution at 298 K and in the film state, respectively, which can be attributed to prompt fluorescence and TADF, respectively. The calculated energy difference (ΔE ST) between the first excited singlet and triplet states of the closo-compounds demonstrate that the TADF characteristic of 4FC originates from a significantly small ΔE ST maintained by the rigid structural fixation around its S0-optimized structure. Furthermore, the strategic molecular design of the o-carborane-appended π-conjugated (D-A) system, which forms a rigid geometry due to severe intramolecular steric hindrance, can enhance the radiative efficiency for ICT-based emission and trigger the TADF nature.

Configuration-controllable synthesis of Z/E isomers based on o-carborane-functionalized tetraphenylethene

Two Z/E isomers, namely, Z-TPE-2Car and E-TPE-2Car, with clear configuration were synthesized using an effective route and have high solid-state fluorescence quantum yields, reaching 99% and 90%, respectively.

Experimental proof for emission annihilation through bond elongation at the carbon-carbon bond in o-carborane with fused biphenyl-substituted compounds

Because of their unique luminescence properties, such as aggregation-induced emission (AIE), intense solid-state luminescence and stimuli-responsive luminochromism, aryl-substituted o-carboranes have attracted attention as a platform for developing functional optoelectronic materials. However, there still remains one fundamental issue with the detailed mechanism of solution quenching in AIE behaviors. Aryl-modified o-carboranes with AIE properties exhibit intense emission not in solution but in the solid state. According to quantum calculations and many experimental results, the elongation at the carbon-carbon bond in o-carborane in the excited state, followed by nonradiative decay, has been proposed as a main path for emission annihilation in solution. However, intramolecular rotation would simultaneously occur, and there is a possibility that emission annihilation could be induced by the combination of both bond elongation and rotation. In this study, we designed two types of biphenyl-substituted o-carboranes having fused structures at the neighbor carbon and boron atoms for fixing molecular conformation. In these molecules, bond elongation is allowed, while rotation would be prohibited. From the series of optical measurements and theoretical investigations, we proved that emission annihilation can occur through bond elongation in the absence of rotation. Moreover, we show that bond elongation could be suppressed by introducing a bulky substituent at the adjacent carbon, and emission color tuning was achieved. This is the first example, to the best of our knowledge, to prove that excitation decay can proceed only through bond elongation without electronic perturbation caused by rotation.

Recent Progress in the Development of Solid-State Luminescent o-Carboranes with Stimuli Responsivity

o-Carborane, a cluster compound containing boron and adjacent carbon atoms, displays intriguing luminescent properties. Recently, compounds containing o-carborane units were found to show suppressed aggregation-induced quenching and intense solid-state emission; they also show potential for the development of stimuli-responsive luminochromic materials. In this Minireview, we introduce three kinds of fundamental photochemical properties: aggregation-induced emission, twisted intramolecular charge transfer in crystals, and environment-sensitive excimer formation in solids. Based on these properties, several types of luminochromism, such as thermos-, vapo-, and mechanochromism, have been discovered. Based mainly on results from recent studies, we illustrate these mechanisms as well as unique luminescent behaviors of o-carborane derivatives.

A new type of carborane-based electron-accepting material

Direct attachment of the ethynyl group significantly affects the electrochemical properties of o-Cb systems.

Insights into the effects of substitution position on the photophysics of mono-o-carborane-substituted pyrenes

Mono-o-carborane-substituted pyrenes were prepared and apparently showed the effects of substitution position on their photophysical property.

Multiple Photoluminescent Processes from Pyrene Derivatives with Aggregation- and Mechano-Induced Excimer Emission

Two novel pyrene-based isocyanide gold(I) complexes have been designed and synthesized. The different structures lead to distinct and diverse photophysical properties both in solution and in the aggregate state. Multiple photoluminescence, involving monomer emission, locally excited emission and excimer emission, are observed. Notably, an excimer is formed by aggregation in solution and external mechanical stimulation in the solid state, showing aggregation- and mechano-induced excimer emission.

Carborane-Induced Excimer Emission of Severely Twisted Bis-o-Carboranyl Chrysene

The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o-carboranyl groups is reported. The molecule exhibits a complex, excitation-dependent photoluminescence, including aggregation-induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o-carborane. This is the first time that o-carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene-based small molecule in solution. Bis-o-carboranyl chrysene is thus an initial member of a new family of o-carboranyl phenacenes exhibiting a novel architecture for highly-efficient multi-luminescent fluorophores.

Mechanically triggered reversible stepwise tricolor switching and thermochromism of anthracene-o-carborane dyad

A novel single organic molecule–carborane conjugate, CAN, was synthesized in a high yield via a modified nickel-catalyzed cross-coupling reaction incorporating an anthracene unit and an o-carborane moiety.

A novel single organic molecule–carborane conjugate, CAN, was synthesized in a high yield via a modified nickel-catalyzed cross-coupling reaction incorporating an anthracene unit and an o-carborane moiety. CAN exhibits multiple functions of tricolored mechanochromism and mechanically triggered thermochromism. The fluorescence could be switched from blue to bright yellow then to pink by grinding. The robust and reversible thermochromic process was triggered by the mechanical force. The locally excited (LE) state emission, intermolecular excimer formation and twisted intermolecular charge transfer (TICT) are the primary origins of this tricolor switching property. High temperature sensitivity of the heavily ground CAN powders contribute to the mechanical force induced TICT emission enhancement and color switching.

Enhancement of Aggregation-Induced Emission by Introducing Multiple o-Carborane Substitutions into Triphenylamine

The enhancement of aggregation-induced emission (AIE) is presented on the basis of the strategy for improving solid-state luminescence by employing multiple o-carborane substituents. We synthesized the modified triphenylamines with various numbers of o-carborane units and compared their optical properties. From the optical measurements, the emission bands from the twisted intramolecular charge transfer (TICT) state were obtained from the modified triphenylamines. It was notable that emission efficiencies of the multi-substituted triphenylamines including two or three o-carborane units were enhanced 6- to 8-fold compared to those of the mono-substituted triphenylamine. According to mechanistic studies, it was proposed that the single o-carborane substitution can load the AIE property via the TICT mechanism. It was revealed that the additional o-carborane units contribute to improving solid-state emission by suppressing aggregation-caused quenching (ACQ). Subsequently, intense AIEs were obtained. This paper presents a new role of the o-carborane substituent in the enhancement of AIEs.