Intramolecular Charge Transfer Compounds Research Articles

Elucidating the non-radiative losses encountered in intramolecular charge transfer compounds with benzodithiophene-4,8-dione acceptors

Elucidating the non-radiative losses encountered in intramolecular charge transfer compounds with benzodithiophene-4,8-dione acceptors

Exploring the Influence of Engineering the Linker between the Donor and Acceptor Fragments on Thermally Activated Delayed Fluorescence Characteristics.

We have expounded the unique molecular design architecture for efficient thermally activated delayed fluorescence (TADF) materials based on a donor-linker-acceptor-linker-donor (D-L-A-L-D) framework, which can be employed as predecessors of organic light-emitting diode (OLED) devices. Different from traditional donor-acceptor-type (D-A-type) TADF scaffolds, the D-L-A-L-D structural design avoids direct coupling amid the D and A fragments allowing the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) to be spatially separated. It results in a reduced overlap between HOMOs and LUMOs, thus realizing fairly a slight singlet-triplet energy gap (ΔE ST) and higher photoluminescence quantum yield (Φ). We revealed that manipulating a linker between D and A fragments in intramolecular charge transfer compounds is an auspicious approach for realizing small ΔE ST. Herein, we report a group of organic electroluminescent D-L-A-L-D-type molecules with different electron-donating and electron-accepting moieties using density functional theory calculations and time-dependent density functional theory calculations. Two types of linkers, the π-conjugated phenylene (-C6H4-) and aliphatic alkyl chains or σ-spacer (-CH2- and -CH2-CH2-), were exploited between D and A fragments. In principle, the conjugation in D-π-A-π-D-type molecules and hyperconjugation in D-σ-A-σ-D type molecules encourage the spatial separation of the HOMO-LUMO causing a reduction in the ΔE ST. All the designed molecules show a blue-shift in the emission wavelengths (λem) over the directly linked parent molecules except DPA-DPS-C6H4 and BTPA-DPS-C6H4 which show a red-shift. Violet-blue to green-yellow (376-566 nm) λem was observed from all of the investigated molecules. Other important properties that affect the efficiency of emission quantum yields like frontier molecular orbital analysis, natural population analysis, electron excitation analysis, exciton binding energies, ionization potentials, electronic affinities, and reorganization energies of the designed molecules were also inspected. We are confident that our work will effectively give a straightforward and distinctive approach to building incredibly effective TADF-OLEDs and a new perspective on their structural design.

Nonlinear properties of benzothiadiazole-based intramolecular charge-transfer compounds

A benzothiadiazole-based chromophore using the molecular design strategy of engineering multipolar charge distributions was synthesized and obtained a large nonlinear response.

Aggregation Induced Emission and Nonlinear Optical Properties of an Intramolecular Charge-Transfer Compound.

Intramolecular charge transfer (ICT) compounds have attracted wide attention for their potential applications in optoelectronic materials and devices such as fluorescent sensors, dye-sensitized solar cells, organic light emitting diodes and nonlinear optics. In this work, we have synthesized a new ICT compound, dimethyl-[4-(7-nitro-benzo[1,2,5]thiadiazol-4-yl)-phenyl]-amine (BTN), and have fabricated it into low dimensional micro/nano structures with well-defined morphologies. These self-assembled nanostructures exhibit high efficiency solid state fluorescence via an aggregation induced emission mechanism, which overcomes the defect of fluorescence quenching caused by aggregation in the solid state of traditional luminescent materials. We also explored and studied the nonlinear optical properties of this material through the Z-scan method, and found that this material exhibits large third-order nonlinear absorption and refraction coefficients, which promises applications of the materials in the fields of nonlinear optics and optoelectronics.

Self‐assembly and tunable optical properties of intramolecular charge transfer molecules

AbstractCrystalline aggregates formed by self‐assembly of organic small molecules have aroused intensive attention in many fields due to their controllable morphologies and functionalization. The advantages of intramolecular charge transfer (ICT) compounds endow them with the ability to self‐assemble into diverse dimensional crystalline microstructures, as potential candidates for advanced materials and smart miniatures in practical applications. In this review, the recent progresses of self‐assembled microstructures of functional ICT molecules in our group for a decade are summarized. The factors influencing the fabrication of the microstructures in solvent‐involved processes are fully discussed. Distinct optical properties induced by self‐assembly behaviors are also depicted. We try to give an outlook of structure‐property relationships in functional microstructure engineering and clarify the crucial issues to be addressed in the future scientific research.

Intramolecular charge transfer-based spirobifluorene-coupled heteroaromatic moieties as efficient hole transport layer and host in phosphorescent organic light-emitting diodes

In this study, we report two efficient intramolecular charge transfer (ICT) compounds with versatile functionality as hole transporting layers (HTL) and hosts in emission layer (EML) of phosphorescent organic light emitting diodes (Ph-OLEDs) with donor-acceptor-donor structures. In both molecules, 9,9′-spirobifluorene (SBF) was employed as a common electron acceptor, while phenoxazine (PXZ) or phenothiazine (PTZ) were used as donors. The molecules 2,7-di(10H-phenoxazin-10-yl)-9,9′-spirobi[fluorene] (SBF-PXZ) and 2,7-di(10H-phenothiazin-10-yl)-9,9′-spirobi[fluorene] (SBF-PTZ) were synthesized via a Buckwald-Hartwig cross-coupling reaction in a single step that exhibited typical ICT phenomena. The extremely twisted geometrical configurations of SBF-PXZ and SBF-PTZ may allow for effective intramolecular charge-transfer. Both molecules exhibited strong absorption and emission characteristics in the visible region in solution as well as in the thin film form; significant positive solvatochromism was also observed for both compounds. The replacement of PTZ donor with the PXZ donor caused a bandgap reduction together with an improved ionization potential (IP). The deeper IP (>5.1 eV), greater glass transition temperature, and higher hole mobility extracted from the hole only devices of both molecules revealed them as superior candidates for HTL applications (with/without exciton blocking layers) in Ph-OLEDs. It is further corroborated by the enhanced half-life time of Ph-OLEDs with SBF-PXZ/SBF-PTZ HTLs. Moreover, the OLED performances with SBF-PTZ and SBF-PXZ as EMLs and hosts for phosphorescent dopant OLEDs have exemplified the versatility of the proposed ICT compounds.

Solvent dependent linear and nonlinear optical properties of triphenylamine unit incorporated difluoroboron β-diketonate complexes

In this work, four intramolecular charge transfer (ICT) compounds, TBT, TVBVT, TSBST and TSVBVST, with difluoroboron β-diketonate as the electron acceptor, triphenylamine unit as the electron donor and different π-conjugated bridges have been synthesized. The linear absorption and emission spectra in different organic solvents indicate a positive solvato-chromism. The multilinear regression analyses of the linear optical spectra in different solvents indicate that the solvato-chromism is mainly caused by the polarity/dipolarity properties of the solvents and possibly the halogen-halide interaction. Time-dependent density functional theory (TD-DFT) calculations were used to investigate the charge density difference between the ground and excited states of the complex molecules of interest and the ICT processes of the excited molecules were verified. The nonlinear optical properties of the molecules were studied using a Z-scan technique. It is found that not only the structure of the molecules influences the nonlinear optical properties, but that also the solvent-solute interaction plays an important role.

1, 3-Indanedione functionalized fluorene luminophores: Negative solvatochromism, nanostructure-morphology determined AIE and mechanoresponsive luminescence turn-on

Luminous efficiency in polar environment and aggregated state, as well as high-contrast luminescence on/off switching under external stimuli are significant issues to be addressed in developing functional fluorescent materials. Combining the advantages of solvatochromic effect in intramolecular charge transfer (ICT) compounds with their nanostructured morphological effects, we herein report a series of A–π–D–π–A fluorene derivatives, which simultaneously exhibit negative solvatochromism, aggregation-induced emission (AIE) and mechanoresponsive luminescence (MRL) turn-on. These molecules contain the same fluorene donor (D) and 1,3-indandione acceptor (A) but differ in alkyl substituents in the 9-position of the fluorene segment, including 9,9-dibutyl (b-DIPF), 9,9-dioctyl (o-DIPF) and 9,9-didodecyl (d-DIPF). The emission colors of these compounds were changed from blue to orange-red, and the fluorescence quantum yields increased upon increasing the solvent polarity from nonpolar hexane to polar dimethyl sulfoxide. Due to the formed random nanostructured aggregates, the compounds exhibited AIE characteristics in THF/H2O mixtures. Oppositely, their emission quenched 0D particles which were afforded by quick evaporation of their dichloromethane solvents under vacuum displayed remarkable mechanoresponsive luminescence turn-on behavior. Our results suggest that rationally design ICT molecules and control their nanostructures would be promising way for realizing multifunctional fluorescent materials.

Conformational Relaxation and Thermally Activated Delayed Fluorescence in Anthraquinone-Based Intramolecular Charge-Transfer Compound

A novel donor-π-acceptor-π-donor-type (D-π-A-π-D-type) chromophore, 2,6-bis[4-(diphenylamino)phenyl]-9,10-anthraquinone (AQ(PhDPA)2), has been reported as an efficient red thermally activated delayed fluorescence (TADF) emitter. Molecular structure and conformation, which directly determine the nature of excited states of a TADF emitter, are critical for obtaining efficient reverse intersystem crossing (rISC) and TADF. In this work, a series of excited-state deactivation processes of AQ(PhDPA)2, from the optical excitation to fluorescence and TADF emitting, have been investigated by theoretical calculations and ultrafast transient absorption (TA) spectroscopy. Theoretical calculations and steady-state spectra suggest that the TADF emitter appears to have conformational twisting in the excited state. Both the relaxed S0 and S1 conformations have a small energy difference between the lowest singlet and triplet excited states (ΔEST) in favor of rISC, whereas ΔEST increases at the relaxed T1 conformation. Ult...

Tuning Luminescence and Conductivity through Controlled Growth of Polymorphous Molecular Crystals

AbstractClear structure–property correlation is of crucial importance for molecular materials for optical and electronic application. Here, a new intramolecular charge transfer compound (E)‐2‐{4‐[4‐(9H‐carbazol‐9‐yl)styryl]benzylidene}malononitrile (TCBR) with a dicyanovinyl group as the electron acceptor and a carbazole group as the electron donor is designed and synthesized. Four different single crystals based on the cis and trans isomeric forms of this compound are grown simultaneously, which are both kinetically and thermodynamically stable. The different packing modes give rise to a dynamic crystal structure change, leading to dramatically different optical and electronic properties. The closer packing can effectively promote the π–π interactions to increase the exciton coupling and orbital overlap between neighbor molecules in crystals, which produces a redshift fluorescence emission and good conductivity. This permits one to tune the packing molecules in the solid state to specific applications in a more precise way. The results of this study highlight the power of designing new functional molecules for optical and electronic applications.

A turn-on type stimuli-responsive fluorescent dye with specific solvent effect: Implication for a new prototype of paper using water as the ink

In this study, we reported the photoluminescence (PL) behaviour of a new intramolecular charge transfer (ICT) compound, ((E)-2-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid, (HABA), which shows ICT solvent effect in aprotic solvents as confirmed by absorption and emission spectra. While in protic solvents including water and ethanol, the charge transfer (CT) band significantly reduces. Remarkable fluorescence enhancement in the blue region was also observed for HABA in polar protic solvents. We described such phenomena as “specific solvent effect”. It can be ascribed to the hydrogen bonding formation between HABA and protic solvents, which not only causes significant reduction in the rate of internal conversion but also elevates the energy gap. Density functional theory (DFT) calculations as well as the dynamics analysis were performed to further verify the existence of hydrogen bonding complexes. Stronger emission turn-on effect was observed on HABA solid film when it is treated with water and base solution. The stimuli-responsive fluorescence of HABA enables a new green printing technique that uses water/base as the ink, affording fluorescent handwritings highly distinct from the background. Thermoanalysis of the dye suggests the nice thermostability, which is highly desired for real-world printing in a wide temperature range.

A chemosensor for fingerprinting liquors: Implication for differentiation and identification

In this work we studied the photoluminescence (PL) behaviour of a new intramolecular charge transfer (ICT) compound, DHAIA. It shows ICT solvent effect in common organic solvents and specific solvent effect in aqueous medium as confirmed by its spectra characterization. The specific solvent effect can be ascribed to the transition from charge-transfer (CT) state to locally excited (LE) state, which is activated by intermolecular hydrogen bonding. The proposed mechanism was confirmed by DFT calculations. The compound can serve as a reliable sensor for identifying the brand of liquors and confirming their authenticity, based on such unique solvent effect intrinsic to this fluorogen. The sensor aimed to fingerprint different liquors via plotting their characteristic factors recorded by spectroscopic analysis. It succeeded in identifying an adulterated liquor sample, once commercially available, provided by Municipal Public Security Bureau, showing great potential in liquor differentiation and identification.

Recent progress on intramolecular charge-transfer compounds as photoelectric active materials

This article summarized the recent advance on the structural design and synthetic strategies of intramolecular charge-transfer compounds as well as their potential applications in two-photon absorption chromophores, organic photovoltaics and organic light-emitting diodes.

Third- and high-order nonlinear optical properties of an intramolecular charge-transfer compound

An oligo(phenylenevinylene) bridged intramolecular charge-transfer (ICT) compound, (TCNQ)2OPV3, has been synthesized and its third- and fifth-order nonlinear optical properties have been determined by measurement with the 4f system with a phase-object.

Self-assembly of functional molecular systems based on intramolecular charge-transfer compounds

Highly polarized compounds exhibiting intramolecular charge transfer (ICT) are used widely as nonlinear optical (NLO) materials and red emitters and in organic light emitting diodes. Low-molecular-weight donor/acceptor (D/A)-substituted ICT compounds are ideal candidates for use as the building blocks of hierarchically structured, multifunctional self-assembled supramolecular systems. This paper describes our recent studies into the development of functional molecular systems with well-defined self-assembled structures based on charge-transfer (CT) interactions. We have developed new methods to synthesize ICT compounds through the introduction of heterocycles or heteroatoms to the π-conjugated systems or through extending the conjugation of diverse aromatic systems via another aromatic ring. For example, we obtained a noncentrosymmetric microfiber structure that possessed a permanent dipole along its fibers’ long axis and a transition dipole perpendicular to it; the independent NLO responses of this material can be separated and tuned spectroscopically and spatially. The encouraging results that we have obtained suggest that such self-assembled ICT molecular materials can guide the design of new nanostructures and materials from organic systems, and that these materials, across a range of compositions, sizes, shapes, and functionalities, can potentially be applied in the fields of electronics, optics, and optoelectronics.

Theoretical Study of Energy Gaps for Naphthalimide-based Charge Transfer Compounds

Thermally activated delayed fluorescence (TADF) materials are promising for the next generation of organic light-emitting diodes. To realize reverse intersystem crossing (RISC) effectively for TADF molecules requires basically small energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) excited states. In this work, 1,8-naphthalimide was selected as a fixed acceptor (A) because of its large rigid conjugate structure with a high radiative decay rate. Twelve naph- thalimide-based intramolecular charge transfer (ICT) compounds were constructed by the conjunction of naphthalimide moi- ety with each of donors (D) including 9,9-dimethyl-9,10-dihydroacridine, phenoxazine, etc. Their energy gaps were calculat- ed with the Gaussian 09 package at TD-DFT level using the optimal Hartree-Fock exchange method reported by Huang Shuping recently. The results show that D-A structure of naphthalimide-based ICT compounds has smaller ΔEST and longer starting fluorescence emission wavelengths (λ) than those for corresponding D-phenyl-A type. However, strong electron do- nor like 9-(9H-carbazol-3-yl)-9H-carbazole (DCZ) seems to have no significant effect on ΔEST and λ at high CT amount. To achieve small ΔEST of these naphthalimide-based ICT compounds, increasing the twisting angle between D and A, together with enhancing electron donating ability of D is found to be a practical strategy to stabilize the energy of the lowest local- ly-excited triplet state ( 3 LE) and substantially lower the energy of its lowest CT triplet excited state ( 3 CT). As a result, 4-(9,9-dimethyl-9,10-dihydroacridine)-N-phenyl-1,8-naphthalimide (4b) and 4-(phenoxazine)-N-phenyl-1,8-naphthalimide (5b) show small ΔEST of 0.01 eV and 0.02 eV, respectively. Their λ and oscillator strengths of vertical absorption from ground state (S0) to S1 are 575 nm and 0.0002 for 4b, while 621 nm and 0.0025 for 5b, respectively. In addition, the λ of compound 5b obtained from its photoluminescence curve is 600 nm, which consists with the computed one with an error of 0.07 eV. Therefore, compounds 4b and 5b can be expected to be potential reddish-orange and red TADF emitters. Keywords naphthalimide; thermally activated delayed fluorescence; energy gaps; quantum chemical calculation

Anthraquinone-based intramolecular charge-transfer compounds: computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence.

Red fluorescent molecules suffer from large, non-radiative internal conversion rates (k(IC)) governed by the energy gap law. To design efficient red thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs), a large fluorescence rate (k(F)) as well as a small energy difference between the lowest singlet and triplet excited states (ΔE(ST)) is necessary. Herein, we demonstrated that increasing the distance between donor (D) and acceptor (A) in intramolecular-charge-transfer molecules is a promising strategy for simultaneously achieving small ΔE(ST) and large k(F). Four D-Ph-A-Ph-D-type molecules with an anthraquinone acceptor, phenyl (Ph) bridge, and various donors were designed, synthesized, and compared with corresponding D-A-D-type molecules. Yellow to red TADF was observed from all of them. The k(F) and ΔE(ST) values determined from the measurements of quantum yield and lifetime of the fluorescence and TADF components are in good agreement with those predicted by corrected time-dependent density functional theory and are approximatively proportional to the square of the cosine of the theoretical twisting angles between each subunit. However, the introduction of a Ph-bridge was found to enhance k(F) without increasing ΔE(ST). Molecular simulation revealed a twisting and stretching motion of the N-C bond in the D-A-type molecules, which is thought to lower ΔE(ST) and k(F) but raise k(IC), that was experimentally confirmed in both solution and doped film. OLEDs containing D-Ph-A-Ph-D-type molecules with diphenylamine and bis(4-biphenyl)amine donors demonstrated maximum external quantum efficiencies of 12.5% and 9.0% with emission peaks at 624 and 637 nm, respectively.

Self-Assembly of Intramolecular Charge-Transfer Compounds into Functional Molecular Systems

Highly polarized compounds exhibiting intramolecular charge transfer (ICT) are used widely as nonlinear optical (NLO) materials and red emitters and in organic light emitting diodes. Low-molecular-weight donor/acceptor (D/A)-substituted ICT compounds are ideal candidates for use as the building blocks of hierarchically structured, multifunctional self-assembled supramolecular systems. This Account describes our recent studies into the development of functional molecular systems with well-defined self-assembled structures based on charge-transfer (CT) interactions. From solution (sensors) to the solid state (assembled structures), we have fully utilized intrinsic and stimulus-induced CT interactions to construct these functional molecular systems. We have designed some organic molecules capable of ICT, with diversity and tailorability, that can be used to develop novel self-assembled materials. These ICT organic molecules are based on a variety of simple structures such as perylene bisimide, benzothiadiazole, tetracyanobutadiene, fluorenone, isoxazolone, BODIPY, and their derivatives. The degree of ICT is influenced by the nature of both the bridge and the substituents. We have developed new methods to synthesize ICT compounds through the introduction of heterocycles or heteroatoms to the π-conjugated systems or through extending the conjugation of diverse aromatic systems via another aromatic ring. Combining these ICT compounds featuring different D/A units and different degrees of conjugation with phase transfer methodologies and solvent-vapor techniques, we have self-assembled various organic nanostructures, including hollow nanospheres, wires, tubes, and ribbonlike architectures, with controllable morphologies and sizes. For example, we obtained a noncentrosymmetric microfiber structure that possessed a permanent dipole along its fibers' long axis and a transition dipole perpendicular to it; the independent NLO responses of this material can be separated and tuned spectroscopically and spatially. The ready processability and intrinsically high NLO efficiency of these microfibers offer great opportunities for applications in photonic devices. We have also designed molecular sensors based on changes in the efficiency of the ICT process upon complexation of an analyte with the D or A moieties in the ICT compounds. Such sensors, which display evident Stokes shifts or changes in quantum yields or fluorescence lifetimes, have promise for applications in chemical and biological recognition and sensing. In this Account, we shed light on the structure-function relationships of these functional molecular systems with well-defined self-assembled structures based on ICT interactions. The encouraging results that we have obtained suggest that such self-assembled ICT molecular materials can guide the design of new nanostructures and materials from organic systems, and that these materials, across a range of compositions, sizes, shapes, and functionalities, can potentially be applied in the fields of electronics, optics, and optoelectronics.

Self-Assembly of Cyanostilben-Based Intramolecular Charge-Transfer Compounds

A handful of cyano-stilbene derivatives are successfully synthesized through Williamson and Suzuki reaction. The dipole-dipole interaction of intramolecular charge-transfer molecule is used to self-assembly microstructure of the as-prepared materials. The CNS-1 microstructures are large-scale spherical particles. However, compound CNS-2 forms straight one-dimensional (1-D) microribbons. Additionally, the morphology of micro-structures obtained from compounds CNS-3 and CNS-5 are large-scale spheres with several micrometres.

Aggregation-induced emission enhancement materials with large red shifts and their self-assembled crystal microstructures

Two simple intramolecular charge transfer compounds (BzBMN and BzFAN), derived from 2-benzyloxy-4-diethylaminobenzaldehyde, have been synthesized. Unlike typical AIEE molecules, the enhanced emission of these compounds in the solid state is accompanied by large red shifts in emission wavelength. Absorption and photoluminescence (PL) spectra of the compounds in solution, film and the X-ray single crystal structures were investigated to understand the mechanism of enhanced emission in the solid state. The results indicate that the bathochromic shift excimer-like emissions originated from dimers induced by the dipole–dipole interaction and long-range ordered arrangement (e.g. J-aggregation), and the enhanced emission is attributed to the restriction of intramolecular rotations caused by multiple intermolecular hydrogen bonding interactions. In addition, it was also observed that BzBMN and BzFAN can facilely form single crystal microbelts and microtubes by a simple reprecipitation method.