Exciplex Luminescence Research Articles

Strong donor-acceptor interaction of difluoroboron β-diketonates with polyvinylcarbazole: Deeply red-shifted emission of charge-transfer complexes and exciplexes

Donor-acceptor systems exhibiting exciplex luminescence attract much attention due to the wide prospects for their application in the field of OLEDs, nonlinear optics and lasers. Difluoroboron β-diketonates (BF2bdks) are a well-known class of organic dyes, which are able to form effective exciplex fluorescence, but this process has not received due attention for practical application. Strong donor-acceptor interaction in polyvinylcarbazole (donor) and BF2bdks derivatives (acceptor) binary system induces deeply red-shifted emission of charge-transfer complexes and exciplexes with a large bathochromic shift up to ∼170 nm relative to conventional monomeric luminescence. The results of quantum chemistry simulation are in agreement with the experiment data. Luminescence color tuning of the film are capable by the dye concentration, the donor-acceptor properties of the dye substituents and even the excitation wavelength. White light broad spectrum of a dual luminescence of the investigated binary systems has CIE chromaticity coordinates (0.35; 0.33) close to the standard neutral white light CIE coordinates (0.33; 0.33). This study will facilitate photophysical research and industrial applications of donor-acceptor binary systems based on polymer materials.

Exciplex luminescence of difluoroboron meta- and para-Nitrodibenzoylmethanates in solutions and polymer matrix

Molecular systems with intense exciplex luminescence are promising for the creation of OLEDs and light-transforming materials. The luminescent properties of difluoroboron meta- and para-nitrodibenzoylmethanates (1 and 2) were studied by the steady-state and time-resolved luminescence spectroscopy methods and quantum chemistry simulation. The influence of the position of the nitro group on the spectral properties of the complexes was revealed. The nitro group withdraws the electron density upon photoexcitation in the para-isomer in contrast to the meta-isomer. The bond order of the CC bond between the phenyl and chelate rings decreases in the excited relaxed state of 2, which leads to rotation of the substituent and a low quantum yield. Formation of exciplexes of 1 and 2 in benzene, toluene and polystyrene was found. The formation of the ground and excited-state electron donor–acceptor complexes based on difluoroboron nitrodibenzoylmethanates and benzene was revealed. The TDDFT method showed that the complexes are characterized by the presence of short contacts between the fluorine atom of the dye molecule and the hydrogen atoms of the aromatic hydrocarbon molecule. These contacts shorten during the transition from the ground state to the excited state. Polymeric compositions based on polystyrene (PS) and poly(methyl methacrylate) (PMMA) doped with 1 and 2 were obtained. The luminescence intensity of dyes 1 and 2 increases during the transition from PMMA to PS. A significant bathochromic shift of the luminescence maximum of 1 is observed, which is associated with the formation of exciplexes and triplexes. The obtained polymeric materials are promising for the development of optical smart materials.

Efficient deep-blue luminescence based on dual-channel intra/intermolecular exciplexes

Highly efficient and stable blue organic light-emitting diodes (OLEDs) cannot be easily obtained simultaneously. In particular, the efficiency roll-off as a reference index to evaluate the lifetime of deep-blue OLED at high luminescence is still severe. A novel molecule (CzSiTrz) connected with carbazole and triazine fragments by a nonconjugated silicon atom is designed. An intramolecular charge transfer emission and intermolecular exciplex luminescence in the aggregated state are obtained, resulting in a dual-channel intra/intermolecular exciplex (DCIE) emission with fast and efficient reverse intersystem crossing (RISC). Deep-blue OLED with Commission Internationale de l'Eclairage (CIE) coordinates of (0.157, 0.076) and a record-high external quantum efficiency (EQE) of 20.35% at high luminance (5000 cd m-2) is accomplished. Simple molecular synthesis and device fabrication of this strategy give a unique approach to realizing high-performance deep-blue electroluminescence.

Comparison of magnetic field effect and exciplex luminescence in pyrene-N,N,dimethylaniline system in acetone and methanol: Anomalous behavior in alcohols

Steady state magnetic field effect (MFE) on pyrene/di-methyl-aniline exciplex luminescence has been studied in various concentrations of pyrene in acetone and methanol. Photo-induced electron transfer in this system generates radical ion pairs undergoing geminate, non-geminate as well as bulk recombination. It has been observed that MFE in acetone is always greater than that in methanol for identical pyrene concentration. However the exciplex fluorescence yield is always greater in methanol than that in acetone. The data and non-typical behavior of alcoholic solvents with respect to the variation of MFE with permittivity has been explained on the basis of a phenomenological model.

Magnetic field effect on exciplex-forming organic acceptor/donor system: a powerful tool for understanding the preferential solvation

Many acceptor/donor systems can form excited-state charge-transfer complexes (exciplexes) in photo-induced electron transfer reactions. Exciplex can be detected by their luminescence. In addition, the exciplex luminescence is magneto-sensitive. Here, we employ an approach based on the magnetic field effect on the exciplex of 9,10- dimethylanthracene/N,N-dimethylaniline pair in micro-homogeneous and micro-heterogeneous binary solvents to investigate the effects of the preferential solvation processes on solute molecules in solutions. Micro-homogeneous solvent mixtures of propyl acetate (PA)/butyronitrile (BN) allow for a systematic variation of the static dielectric constants, s , in the range from 6.0 to 24.6. The mixtures of toluene (TO)/dimethylsulfoxide (DMSO) with varying the s values in the range from 4.3 to 15.5 are used as micro-heterogeneous binary solvents. In micro-heterogeneous environment, DMSO molecules get preferentially favoured in the solvation shell, forming micro-clusters surrounding the solute molecules. This solvation effect is reflected in the altered magnetic field effects, lifetimes and dissociation rate constants of the exciplexes.

Exciplexes versus Loose Ion Pairs: How Does the Driving Force Impact the Initial Product Ratio of Photoinduced Charge Separation Reactions?

Many donor–acceptor systems can undergo a photoinduced charge separation reaction, yielding loose ion pairs (LIPs). LIPs can be formed either directly via (distant) electron transfer (ET) or indirectly via the dissociation of an initially formed exciplex or tight ion pair. Establishing the prevalence of one of the reaction pathways is challenging because differentiating initially formed exciplexes from LIPs is difficult due to similar spectroscopic footprints. Hence, no comprehensive reaction model has been established for moderately polar solvents. Here, we employ an approach based on the time-resolved magnetic field effect (MFE) of the delayed exciplex luminescence to distinguish the two reaction channels. We focus on the effects of the driving force of ET and the solvent permittivity. We show that, surprisingly, the exciplex channel is significant even for an exergonic ET system with a free energy of ET of −0.58 eV and for the most polar solutions studied (butyronitrile). Our findings demonstrate that exciplexes play a crucial role even in polar solvents and at moderate driving forces, contrary to what is usually assumed.

Role of viscosity in the magnetic field effect on pyrene–DMA exciplex emission at different permittivities

Effect of viscosity variation on the magnetic field effect in pyrene-N,N-dimethylaniline exciplex luminescence has been studied at different permittivity values. The data is compatible to the model of Krissinel et al. (1999) [10] reported earlier to explain the effect probing the escape yield of radical pairs. It is shown that the data can also be explained on the basis of a simple model. It is interesting to note that the present letter also demonstrates the positive slope of MFE with diffusivity at extremely high viscous condition as predicted by Krissinel et al. (1999) [10] which has not been observed in earlier experiments.

Time-Resolved Magnetic Field Effects Distinguish Loose Ion Pairs from Exciplexes

We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor–acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz., the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions.

The Formation of Exciplex and Improved Turn-on Voltage in a Hybrid Organic-Inorganic Light-Emitting Diode

In order to take advantage of organic and inorganic materials, we chose the polymer MEH-PPV as the luminous layer and ZnS as the electron transporting layer to prepare hybrid organic-inorganic light-emitting diodes (HOILEDs): ITO/MEH-PPV(∼70 nm)/ZnS(20 nm)/Al by thermal evaporation and spin coating. Compared with the single-layer device ITO/MEH-PPV(∼70 nm)/Al, spectral broadening and a slightly red shift are observed. Compared with the pure organic device ITO/MEH-PPV(∼70 nm)/BCP (20 nm)/Al and combined with the energy level structure diagram, it is concluded that the spectral broadening and red shift are due to the exciplex luminescence at the interface between MEH-PPV and ZnS or BCP. In addition, the hybrid inorganic-organic device shows a lower turn-on voltage, but the current efficiency is lower than that of the pure organic device with the same structure.

Experimental observation of preferential solvation on a radical ion pair using MARY spectroscopy

The effect of preferential solvation on the exciplex luminescence detected magnetic field effect has been studied using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By designing solvent mixtures which can provide a micro-environment around the magneto-sensitive radical ion pair (RIP) from highly heterogeneous to quasi-homogenous, the effect of the polarity scan on an absolute magnetic field effect (χ(E)) and B(1/2) (the field value marking half saturation) has been studied on the system 9,10-dimethylanthracene (fluorophore)/N,N'-dimethylaniline (quencher). While the trend in χ(E) (although with subtle differences) follows the usual norm of passing through maxima with increasing polarity, the B(1/2) values show either a large monotonic decrease (for heterogeneous solvents) or remain constant (for quasi-homogenous systems) with increasing polarity. The observations have been interpreted invoking the concept of amplification of the "cage-effect" as a result of preferential solvation in binary solvents and its influence on the decaying exciplex. The use of ternary solvents further confirms the proposed mechanism. Additionally electron hopping from the radical ion pair to the surrounding neutral donor molecules could also possibly contribute to the observed trend.

Ideal diode equation for organic heterojunctions. II. The role of polaron pair recombination

In paper I [N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010)], we proposed that current transport in a donor-acceptor heterojunction (HJ) depends on the balance of polaron pair (PP) dissociation and recombination. Here, we directly investigate these processes in archetype planar copper phthalocyanine $(\text{CuPc})/{\text{C}}_{60}$ and boron subpthalocyanine chloride $(\text{SubPc})/{\text{C}}_{60}$ HJs. Using intensity-modulated photocurrent spectroscopy (IMPS) along with emission from interfacial $\text{Pc}/{\text{C}}_{60}$ exciplex states, we monitor the geminate PP density at the HJ as a function of bias and illumination intensity. We find that the $\text{SubPc}/{\text{C}}_{60}$ PP density is limited by the dynamics of dissociation, where it increases from short circuit, and peaks at open circuit. In contrast, that of $\text{CuPc}/{\text{C}}_{60}$ is dominated by faster recombination kinetics and declines monotonically over the same voltage domain. We conclude that the PP recombination rate depends on electric field, and propose a simple expression that qualitatively explains the observed exciplex luminescence and IMPS behavior for these HJs. Our results provide insight into polaron pair recombination, which governs the current-voltage characteristics of organic heterojunctions in the dark and under illumination.

Exciplex Formation Accompanied with Excitation Quenching

The competence of the reversible exciplex formation and parallel quenching of excitation (by electron or energy transfer) was considered using a non-Markovian pi-forms approach, identical to integral encounter theory (IET). General equations accounting for the reversible quenching and exciplex formation are derived in the contact approximation. Their general solution was obtained and adopted to the most common case when the ground state particles are in great excess. Particular cases of only photoionization or just exciplex formation separately studied earlier by means of IET are reproduced. In the case of the irreversible excitation quenching, the theory allows specifying the yields of the fluorescence and exciplex luminescence, as well as the long time kinetics of excitation and exciplex decays, in the absence of quenching. The theory distinguishes between the alternative regimes of (a) fast equilibration between excitations and exciplexes followed by their decay with a common average rate and (b) the fastest and deep excitation decay followed by the weaker and slower delayed fluorescence, backed by exciplex dissociation.

Magnetic field effect in homogeneous medium for triplet born radical ions : a way for assessment of inter-radical distance in intermolecular photoinduced electron transfer

Photoinduced electron transfer (PET) reaction involves excitation of either donor or acceptor prior to electron transfer. The PET reaction might occur in the singlet or in the triplet electronic state of the chromophore. These reactions could be influenced by the presence of internal or external magnetic field since radical ions with free electrons are generated as intermediates. Magnetic field effect (MFE) in homogeneous medium could be observed on exciplex luminescence, however the change is very small as the rate of recombination is very high owing to the singlet spin correlation of the geminate radical ion pair (RIP). On the other hand for the triplet born RIPs the detection of MFE needs confinement of the triplet species because at ambient temperature the lifetime of the solvent cages containing spin correlated RIP in homogeneous solvent is 10 -10 s whereas the rate of intersystem crossing is 10 -8 s. It is possible to confine the triplet born radicals by using organized assemblies like micelles, reverse micelles, etc., highly viscous solvents at low temperature or long chain biradicals which help to reduce fast escape and thus retain the spin-correlation between the partners of the geminate RIP. MFE is a composite of diffusion dynamics, spin dynamics leading to intersystem crossing and recombination of free ion formation. The effect is optimized only when the inter-radical distance through diffusion becomes sufficient to make exchange interaction between free electrons negligible retaining the original spin-correlation in the solvent cage. In this review we would like to highlight a few systems where MFE for the triplet born radicals could be observed even in homogeneous media due to some specific interactions other than covalent linking, which also helps to assess the inter-radical distance in such intermolecular PET, which is very rare.

Study of magnetic field effect on Py-DMA exciplex luminescence in THF–DMF binary solvents: Evidence of multiple exciplex formation at higher bulk dielectric constant

Magnetic field effects (MFE) on pyrene-DMA (N,N-dimethylaniline) exciplex system in THF–DMF binary solvents of varying bulk dielectric constant ( ε) has been studied. The MFE has been wavelength resolved over the emission band of the exciplex. The data indicates formation of two types of exciplexes at higher bulk value of ε. At lower value of ε ( ε < 9) essentially there is a single type of exciplex which is supported by the wavelength resolved MFE and life-time ( τ) measurements. However, at higher value of ε ( ε > 15) when the solvent reorganization energy is more prominent; the wavelength resolved MFE and life-time studies show evidence of two types of exciplexes.

Exciplex luminescence in D2O solubilized AOT reverse micelle: A magnetic field effect study

Magnetic field modulated pyrene–N,N-dimethylanilne exciplex fluorescence has been studied in reverse micelle of D2O/AOT/n-heptane. Results have been compared with those in H2O/AOT/n-heptane to understand the effect of deuterium on exciplex luminescence. Strikingly the extent of magnetic field effect (MFE) has been found to be much larger in D2O/AOT/n-heptane. The fluorescence lifetime of the exciplexes have been found to be larger in D2O/AOT/n-heptane than that in H2O/AOT/n-heptane. The stronger hydrogen bond strength in D2O compared to that of H2O has been identified to be the prime factor for the effect. Variation of the MFE with temperature in D2O/AOT/n-heptane and H2O/AOT/n-heptane has also been studied.

Temperature-Dependent Magnetic Field Effect Study on Exciplex Luminescence: Probing the Triton X-100 Reverse Micelle in Cyclohexane

The microenvironment within the reverse micelle of the nonionic surfactant Triton X-100 (TX-100) in cyclohexane has been investigated by studying the magnetic field effect (MFE) on pyrene-dimethylaniline exciplex luminescence. The nature of exciplex fluorescence and its behavior in the presence of a magnetic field have been found to vary significantly with the water content of the medium. Results are discussed in light of multiple exciplex formation within the micelle which is further supported by the fluorescence lifetime measurements. Those exciplexes emitting at longer wavelength are found to be magnetic field sensitive while those emitting toward the blue region of the spectrum are insensitive toward magnetic field. Since the exciplex's emission characteristics and magnetic field sensitivity depend on its immediate surrounding, it has been concluded that the environment within the micelle is nonuniform. With an increase in hydration level, different zones of varying polarity are created within the reverse micelle. It has been pointed out that the magnetic field sensitive components reside inside the polar core of the micelle while those located near the hydrocarbon tail are field insensitive. However it has been presumed that an interconversion between the different types of exciplexes is possible. The environment within the reverse micelle is found to be largely affected by the change in temperature, and this is reflected in the exciplex emission property and the extent of magnetic field effect. Interestingly, the variation of MFE with temperature follows different trends in the dry and the wet reverse micelle. A comparison has been drawn with the reverse micelle of the ionic surfactant to get an insight into the difference between the various types of micellar environment.

Evidence for electric field dependent dissociation of exciplexes in electron donor–acceptor organic solid films

Electric field effect on time-resolved exciplex luminescence has been examined in electron donor–acceptor organic solid films of undoped and an organic phosphor PtL 2Cl-doped (m-MTDATA:BPT:PC) blends. The applied field leads to a decrease of the initial amplitude and luminescence lifetimes for both singlet and triplet exciplex underlain emission. The largest reduction (up to 50%) of the lifetime has been observed for the triplet exciplex emission and the effect interpreted in terms of the exciplex dissociation into electron–hole pairs driven by hopping separation of such generated charge carriers.

Effect of exciplex formation on organic light emitting diodes based on rare-earth complex

An exciplex can be formed due to the charge transfer between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. By introducing a mixing layer composed of [N,N′-diphenyl-N,N′bis (3-methylphenyl)-1,1′-diphenyl-4,4′-diamine] (TPD) and europium(dibenzoylmethanato)3(bathophenanthroline) [Eu(DBM)3bath] and a graded interface, elimination of light emission from the exciplex and significant luminescence enhancement of trivalent europium ions (Eu3+) in organic light emitting devices have been achieved. The elimination mechanism of exciplex emission based on the concept that an exciplex can be formed between LUMO of the acceptor (Eu complex) and HOMO of donor (TPD) was investigated. To comprehensively understand the mechanism, devices consisting of a Eu(DBM)3bath as the emitting material and the devices using other rare-earth (RE) complex [RE(DBM)3bath] as the emitting material were fabricated with the same device configuration. As a reference, four spin-coated films with the blend composed of TPD and the gadolinium complex [Gd(DBM)3bath] were also fabricated. The electroluminescence (EL) spectra from the devices and photoluminescence spectra from the spin-coating films were fully investigated. The results show that the exciplex was formed by the charge transfer from the donor TPD to the acceptor RE complex, the exciplex state that acted as a transient excited state can be controlled by altering the molecular ratio in the mixing films. The relation of the exciplex formation based on EL devices with the RE complex versus the variety of the RE ions is also discussed by manipulating the energy level of the excited state.

Magnetic field effect on pyrene–DMA exciplex luminescence in non-aqueous AOT reverse micelle

Magnetic field effect (MFE) on pyrene–DMA exciplex luminescence and exciplex lifetime have been studied in reverse micelle of Aerosol-OT in n-heptane in presence of N, N-dimethylformamide and methanol as the polar solvent at different W s ( W s = [polar solvent]/[surfactant]) values. It is found that two types of exciplexes are residing at different locations of the non-aqueous reverse micelle. Out of these only the exciplex emitting at longer wavelength is found to be field sensitive. The higher MFE and shorter lifetime of the exciplex in non-aqueous reverse micelle compared to that of aqueous micelle have been explained on the basis of presence of hydrogen bonding and dipolar interaction between the solvent and the polar head group of the micelle.

Magnetic field effect on exciplex luminescence: A study of multiple exciplex formation dynamics in biomimicking environment

The interfacial environment of AOT based reverse micelle (RM), which has similarity with bio-membranes, has been probed by measuring: (1) magnetic field effect (MFE) on Pyrene–DMA exciplex luminescence; (2) wavelength-dependence of the exciplex lifetime. At least two different types of exciplexes have been identified at different locations within the RM interface. Among these, only the red edge component of the emission centered at ∼500 nm (Ex 500) is field-sensitive. It makes sense to presume that the Ex 500 is localized at the mobile zone of the interface while the other field-insensitive one, centered at ∼430 nm (Ex 430) resides at the immobile zone of the interface. Time resolutions of luminescence at different wavelengths indicate that the field-sensitive exciplex is formed at the expense of Ex 430.