Conventional Exciplex Research Articles

Efficient Solution‐Processed Red Fluorescent OLEDs Using Diluted Exciplex Host to Reduce Non‐Radiative Loss of Triplet Excitons

AbstractHigh‐performance red fluorescent organic light‐emitting diodes (OLEDs) are fabricated via a diluted exciplex host strategy to suppress non‐radiative triplet exciton loss in the exciplex sensitized fluorescent emitter system. The solution‐processed red fluorescent OLEDs based on the diluted TCTA:PO‐T2T exciplex host demonstrate higher external quantum efficiency (EQE) than the theoretical 5% limit for traditional fluorescent OLEDs. In the TCTA:PO‐T2T exciplex sensitized fluorescent emitter system, the excess content of TCTA is employed relative to PO‐T2T to manipulate the decay process of triplet excitons. A part of TCTA adjacent to PO‐T2T involves forming the exciplex pair that efficiently converts triplet excitons to singlet excitons via reverse intersystem crossing (RISC), while the other TCTA donors act as the host for the exciplex sensitizer and fluorescent emitter, which inhibits both concentration quenching of the exciplex sensitizer and short‐range dexter energy transfer from the exciplex sensitizer to the fluorescent emitter DCJTB without affecting ISC, RISC, and long‐range Föster energy transfer rate significantly. With this strategy, the formed triplet excitons can be converted to singlet excitons with reduced loss contributing to efficient fluorescent emission under electric excitation. The solution‐processed red fluorescent OLEDs achieve an EQE of 8.1%, much higher than 3.5% of the conventional exciplex sensitized OLEDs.

Donor/Acceptor Pairs Created by Electrostatic Interaction: Design, Synthesis, and Investigation on the Exciplex Formed Within the Pair.

Exciplexes formed between donors and acceptors have been widely explored but isolating them from each other and tuning the interaction between the donor and acceptor have remained challenges. Here, we report donor/acceptor (D/A) pairs created by electrostatic interaction between a carbazole-based anionic donor and a 1,3,5-triazine-based cationic acceptor and the exciplex formed within the pair. In a diluted film, the D/A pair affords an isolated exciplex which shows thermally activated delayed fluorescence (TADF). By changing the anchoring position of the imidazolium cation in the cationic acceptor, interactions between the donor and acceptor can be changed. Compared to the conventional exciplex formed in a neat film, the isolated exciplex exhibits a substantially higher luminescence efficiency. The D/A pairs show intriguing mechanochromic luminescence and mechanical grinding-induced/reinforced TADF in the solid state and promising performances as emitters in organic light-emitting diodes.

Donor/Acceptor Pairs Created by Electrostatic Interaction: Design, Synthesis, and Investigation on the Exciplex Formed Within the Pair

AbstractExciplexes formed between donors and acceptors have been widely explored but isolating them from each other and tuning the interaction between the donor and acceptor have remained challenges. Here, we report donor/acceptor (D/A) pairs created by electrostatic interaction between a carbazole‐based anionic donor and a 1,3,5‐triazine‐based cationic acceptor and the exciplex formed within the pair. In a diluted film, the D/A pair affords an isolated exciplex which shows thermally activated delayed fluorescence (TADF). By changing the anchoring position of the imidazolium cation in the cationic acceptor, interactions between the donor and acceptor can be changed. Compared to the conventional exciplex formed in a neat film, the isolated exciplex exhibits a substantially higher luminescence efficiency. The D/A pairs show intriguing mechanochromic luminescence and mechanical grinding‐induced/reinforced TADF in the solid state and promising performances as emitters in organic light‐emitting diodes.

Extremely superb efficiency and lifetime of deep blue phosphorescent OLEDs by introducing a hypsochromic emissive intermolecular complex (HEIC) with a negligibly small ΔEST and fast reverse intersystem crossing rate

Concept of a hypsochromic emissive intermolecular complex (HEIC) compared with the conventional exciplex and electroplex. A mixed host system with HEIC exhibits blue-shifted electroluminescence as well as photoluminescence.

Solvent Dependence of Cationic-Exciplex Emission: Limitation of Solvent Polarity Functions and the Role of Hydrogen Bonding.

Conventional exciplexes are products of excited-state charge generation reactions between neutral reactants (e.g., A* + D → A•-D•+), whereas cationic exciplexes are products of charge shift reactions of cations with neutral donors (e.g., A+* + D → A•D•+). Compared herein is the solvent-dependent fluorescence of a cationic exciplex with extant data for conventional exciplexes. Although linear correlations of conventional exciplex emission maxima with the Lippert-Mataga solvent polarity function are well documented in low to moderate polarity solvents, the correlations are often poor in more polar solvents. A number of such plots in moderate to high polarity solvents show a strong curvature. Intriguingly, for these same cases, plots of emission maxima versus the solvent dielectric constant (ε) are remarkably linear. Interestingly, emission maxima for the cationic exciplex of 1+ in nitrile and alcohol solvents also correlate linearly with ε. The solvent dependency for cationic exciplex emission maxima on ε is ca 1/3 of that for conventional exciplexes, which is ascribed to solvent stabilization of both the excited state and the ground state for cationic exciplexes. Differences in exciplex emissions between nitrile and alcohol solvents for 1+ are attributed to hydrogen bonding, with larger differences in higher hydrogen-bond acidity solvents.

Highly efficient exciplex organic light-emitting diodes by exciplex dispersion in the thermally activated delayed fluorescence host

Abstract In this work, we describe a new approach of improving the external quantum efficiency (EQE) of exciplex organic light-emitting diodes (OLEDs). A new approach of dispersing the exciplex in the thermally activated delayed fluorescence (TADF) host was developed to increase the EQE of the exciplex OLEDs. The new emitting layer structure was made up of a mixture of TADF material and an n-type material which can generate exciplex with the TADF material. The content of the n-type material was maintained below 10 wt % to have a morphology with the exciplex dispersed in the TADF matrix material. The emitting layer structure with the exciplex dispersed in the TADF host suppressed exciton quenching and improved the photoluminescence quantum yield of the exciplex. As a result, the exciplex OLEDs with the n-type material content of 1 wt% showed high EQE of 15.3% compared with EQE of 10.8% of the conventional exciplex OLEDs with the n-type material content of 50%.

Cationic (Charge Shift) Exciplexes.

Of the many known examples of exciplexes, those formed from bimolecular encounter between a cationic, excited state electron acceptor and a neutral donor in fluid media have not been previously reported. We now show that emissive exciplexes formed from excited N-methyl isoquinolinium cation (NMiQ+) with alkyl benzene donors are readily detected in acetonitrile. These cationic exciplexes result in a charge shift (A+* + D → A•D•+) with no net change in charge, which differs fundamentally from the charge-generation of conventional exciplex formation (A* + D → A•-D•+). We find that cationic and conventional exciplexes show similar trends, e.g., bathochromic shifts and decreases in fluorescence quantum yields with decreasing oxidation potentials of the donors. In the presented examples of NMiQ+ exciplexes, the fluorescence quantum yield decreases by a factor of 30 and the radiative rate constant by 6.6 as the fractional CT character of the exciplex increases from ∼0.79 to ∼0.95. Interestingly, the electronic coupling matrix elements for the NMiQ+ exciplexes, derived from a correlation of the radiative rate constants with the average emission frequencies, are similar to those of related conventional exciplexes, in spite of the absence of Coulombic stabilization in the cationic exciplexes.

電荷移動錯体の選択励起による光不斉合成

The use of photoreactions such as photoredox catalysts in asymmetric synthesis has been expansively recognized as expedient procedure over the past decade. In this paper, a potential application of photoreaction of Charge-Transfer (CT) complex for asymmetric synthesis is described. In chiral donor-acceptor systems, excitation wavelength, along with other environmental factors such as solvent polarity and temperature, plays more substantial role in determining the stereoselectivity of the photochemical outcomes. Accordingly, the excited CT complex produced upon the selective CT-band excitation behaves completely different from the conventional exciplex, which is formed by the direct excitation of donor or acceptor. Such unique wavelength effect, together with other entropic controls, may possibly come to be valuable means in asymmetric photochemical synthesis for the future.

Contrasting Behaviour of Exciplex Ensembles in the Diastereodifferentiating Paternò–Büchi Reaction of Chiral Cyanobenzoate with Naphthyl- and Phenylethenes on Direct or Charge-Transfer Excitation

The diastereodifferentiating Paternò–Büchi reaction of chiral cyanobenzoate with 1-(1-naphthyl)-1-phenylethene was compared with those with 1,1-diphenylethene on direct and charge-transfer excitations. By desymmetrization of the donor, four diastereomeric oxetane products were formed on photolysis in excellent combined yields. Increase in donor strength induced a stronger charge-transfer interaction both in ground and excited states. Thus, the difference in diastereoselectivities with two different excitation modes (i.e. direct versus charge-transfer) became less significant with a naphthyl derivative as donor. A subtle change of donor–acceptor interaction was shown to have profound effect on the nature of the excited-state complexes and thus the product (stereo)selectivities. Despite a small temperature dependence, an Eyring-type study on the diastereoselectivities confirmed that the excited charge-transfer complex is an excited species distinct from the conventional exciplex.

Charge-transfer excitation: unconventional yet practical means for controlling stereoselectivity in asymmetric photoreactions

In chiral donor-acceptor (D-A) systems, irradiation wavelength plays vital roles in determining the photochemical consequences. Selective excitation of a D-A complex at the charge-transfer (C-T) band affords an excited C-T complex (ECT), while the local-band excitation of D or A may lead to the formation of a conventional exciplex (EX) upon subsequent interaction with the D-A partner. These two excited species, generated from the same D-A pair, may be categorized formally as excited complexes or exciplexes, but should be distinguished, provided that they significantly differ in structure and reactivity. Indeed, ECT and EX exhibit distinctly different temperature-dependent photophysical and photochemical behaviours, which are assignable to the differences in relative stability, conformational flexibility and/or solvation properties. Fine-tuning excitation wavelength further enabled us to discriminate stereoisomeric intramolecular C-T complexes through preferential excitation, as C-T complexes are generally composed of an ensemble of various geometries. Besides temperature and solvent polarity, the excitation wavelength was shown to be employed as an unconventional yet practical tool for critically controlling the chemo-, regio- and stereoselectivities in molecular and supramolecular photochemistry.

Solvent and Temperature Effects on Diastereodifferentiating Paternó−Büchi Reaction of Chiral Alkyl Cyanobenzoates with Diphenylethene upon Direct versus Charge-Transfer Excitation

In the Paternó-Büchi reaction of chiral p-cyanobenzoates (1) with 1,1-diphenylethene (2), we revealed that the excited charge-transfer (CT) complex formed upon selective excitation at the CT band is distinctly different in structure and reactivity from the conventional exciplex generated through the direct excitation of acceptor 1 which subsequently associates with donor 2. Thus, the favored diastereoface upon photocycloaddition, as well as the temperature- and solvent-dependent behavior of the product's diastereoselectivity, were highly contrasting, often opposite, to each other upon direct versus CT excitation. From the activation parameters obtained by the Eyring analyses of the diastereoselectivity, we are able to infer that the conventional exciplex is relatively flexible and susceptible to the environmental variants, whereas the CT complex is better pi-pi stacked and more rigid in the ground state and also in the excited state, leading to the significantly smaller differential activation enthalpies and entropies. More interestingly, the signs of the differential activation parameters determined for direct and CT excitation are consistently opposite to each other and the isokinetic temperatures calculated therefrom differ significantly, unambiguously revealing the distinctly different nature in structure and reactivity of these two excited-state complex species. Thus, the combined use of irradiation wavelength, temperature, and solvent provides us with a convenient, powerful tool not only for elucidating the mechanistic details of photoreaction but also for critically controlling the stereochemical outcomes of photochirogenic reaction.

Wavelength Control of Diastereodifferentiating Paternó−Büchi Reaction of Chiral Cyanobenzoates with Diphenylethene through Direct versus Charge-Transfer Excitation

In the diastereodifferentiating Paterno-Buchi reaction, the excited CT complex was distinctly different in structure and reactivity from the conventional exciplex, and the inherent diastereofacial selectivity and its temperature dependence were opposite to each other in these two excitation modes. Thus, the combined use of wavelength and temperature not only reveals the mechanistic details but also provides a new convenient, powerful tool for critically controlling the stereochemical outcomes of asymmetric photoreactions.

Bonded Exciplex Formation: Electronic and Stereoelectronic Effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.

Diastereoselective [2 + 2] photocycloaddition of stilbene to chiral fumarate. Direct versus charge-transfer excitation.

The selective excitation of the charge-transfer (CT) complex and the direct excitation of the substrate gave distinctly different product ratios and diastereomeric excesses (de's), as well as their temperature dependencies, in [2 + 2] photocycloaddition of (E)-stilbene to bis((R)-1-methylpropyl) fumarate, clearly demonstrating that the excited CT complex and the conventional exciplex differ in structure and reactivity. This conclusion is supported by the contrasting fluorescence behavior exhibited by the relevant excited species, particularly at low temperatures.