Organic thermally activated delayed fluorescent (TADF) materials can harvest 100% of the electrically generated excitons as a result of their small singlet–triplet energy difference. However, maximizing the external quantum efficiency (EQE) of a device also requires enhancing the light out‐coupling efficiency. This work presents a new acceptor–donor–acceptor (ADA) emitter employing an indolocarbazole donor and diphenyltriazine acceptors that show nearly‐completely horizontal orientation regardless of the host matrix, leading to a sky‐blue organic light‐emitting diode (λEL = 483 nm, CIE coordinates of 0.17, 0.32) with EQEMAX of 22.1%, a maximum luminance of 7800 cd m−2, and blue emission.
Six luminophores bearing an OBO-fused benzo[fg]tetracene core as an electron acceptor were designed and synthesized. The molecular structures of three molecules (PXZ-OBO, 5PXZ-OBO, 5DMAC-OBO) were determined by single crystal X-ray diffraction studies and revealed significant torsion between the donor moieties and the OBO acceptor with dihedral angles between 75.5 and 86.2◦. Photophysical studies demonstrate that blue and deep blue emission can be realized with photoluminescence maxima (lPL) ranging from 415 to 480 nm in mCP films. The emission energy is modulated by simply varying the strength of the donor heterocycle, the number of donors, and their position relative to the acceptor. Although the DMAC derivatives show negligible delayed emission because of their large singlet-triplet excited state energy difference, 1EST, PXZ-based molecules, especially PXZ-OBO with an experimental 1EST of 0.25 eV, demonstrate delayed emission in blend mCP films at room temperature, which suggests
triplet exciton harvesting occurs in these samples, potentially by thermally activated delayed fluorescence.
It is well known that in organic solids the collision of two excitons can give rise to delayed fluorescence
(DF). Revived interest in this topic is stimulated by the current endeavor towards the development of efficient organic optoelectronic devices such as organic light-emitting diodes (OLEDs) and solar cells, or sensitizers used in photodynamic therapy. In such devices, triplet excitations are ubiquitously present but their annihilations can be either detrimental, e.g., giving rise to a roll-off of intensity in an OLED, or mandatory, e.g., if the sensitizer relies on up-conversion of long-lived low-energy triplet excitations. Since the employed materials are usually noncrystalline, optical excitations migrate via incoherent hopping. Here, we employ kinetic Monte Carlo simulations (KMC) to study the complex interplay of triplet-triplet annihilation (TTA) and quenching of the triplet excitations by impurities in a single-component system featuring a Gaussian energy landscape and variable system parameters such as the length of the hopping sites, i.e., a conjugated oligomer, the morphology of the system, the degree of disorder (σ), the concentration of triplet excitations, and temperature. We also explore the effect of polaronic contributions to the hopping rates. A key conclusion is that the DF features a maximum at a temperature that scales with σ/kBT. This is related to disorder-induced filamentary currents and thus locally enhanced triplet densities. We predict that a maximum for the TTA process near room temperature or above requires typically a disorder parameter of at least 70 meV.
ABSTRACT: The thermally activated delayed fluorescence (TADF) donor−acceptor (D−A) molecule, DMAC−TRZ, is used as a TADF emitter “probe” to distinguish the environmental effects of a range of solid-state host materials in guest−host systems. Using the guest’s photophysical behavior in solution as a benchmark, a comprehensive study using a variety of typical TADF organic light-emitting diode hosts with different characteristics provides a clearer understanding of guest−host interactions and what affects emitter performance in solid state. We investigate which are the key host characteristics that directly affect chargetransfer (CT) state energy and singlet triplet energy gaps. Using time-resolved photoluminescence measurements, we use the CT state energy distribution obtained from the full width at halfmaximum (fwhm) of the emission band and correlate this with other photophysical properties such as the apparent dynamic red shift of CT emission on-set to estimate the disorder-induced heterogeneity of D−A dihedral angles and singlet triplet gaps. Further, the delayed emission stabilization energy value and time-dependent CT band fwhm are shown to be related to a combination of host’s rigidity, emitter molecule packing, and the energy difference between guest and host lowest energy triplet states. Concentration dependence studies show that emitter dimerization/aggregation can improve as well as reduce emission efficiency depending on the characteristics of the host. Two similar host materials, mCPCN and mCBPCN, with optimum host characteristics show completely different behaviors, and their hosting potential is extensively explored. We demonstrate that type I and type III TADF emitters behave differently in the same host and that the materials with intrinsic small ΔEST have the smallest disorderinduced CT energy and reverse intersystem crossing rate dispersion. We also present an optimized method to define the actual triplet energy of a guest−host system, a crucial parameter in understanding the overall mechanism of the TADF efficiency of the system.
Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100%internal quantumefficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF
emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role.
We present the synthesis and spectroscopic characterization of a twisted push–pull biphenyl molecule undergoing photoinduced electron transfer. Steady‐state and transient absorption spectra suggest, in this rigid molecular structure, a subtle interplay between locally‐excited and charge‐transfer states, whose equilibrium and dynamics is only driven by solvation. A theoretical model is presented for the solvation dynamics and, with the support of quantum chemical calculations, we demonstrate the existence of two sets of states, having either local or charge‐transfer character, that only “communicate” thanks to solvation, which is the sole driving force for the charge‐separation process.
The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ. This is a major requirement for the use of the materials as a host for blue TADF emitters.
For thermally activated delayed fluorescence (TADF) host–guest systems used in organic light-emitting diodes, understanding of the transient photoluminescence (PL) measurements is crucial for accurate determination of the photophysical rates of the emitter. Here, we study how the PL is affected by triplet-exciton deconfinement from the guest to the host molecules. This deconfinement can complicate the analysis of the PL decay and potentially lead to a loss of efficiency. From an analytical model, we find that the transient PL intensity remains bi-exponential in the presence of exciton deconfinement for the case of fast triplet diffusion, albeit with a longer decay time of the delayed component. Deconfinement might, therefore, not always be recognizable from a single transient PL measurement. The role of deconfinement depends on the energetic disorder, the guest concentration, and the energy difference ΔET between triplet-exciton energies on the host and guest molecules and is effectively suppressed for ΔET>0.2eV. We find from analytical modeling and kinetic Monte Carlo simulations that the decay can become non-bi-exponential and even show a distinct third decay step. The shape of the decay curves depends on the characteristic times for guest–host transfer and host diffusion, relative to the prompt and delayed decay times of the TADF emitter. A comparison with available experimental data is included, finding qualitative agreement with dedicated deconfinement studies and indicating the influence of other processes for the often observed power-law decay at long time scales
ABSTRACT: An easy-to-access, near-UV-emitting linearly extended B,Ndoped heptacene with high thermal stability is designed and synthesized in good yields. This compound exhibits thermally activated delayed fluorescence (TADF) at ambient temperature from a multiresonant (MR) state and represents a rare example of a non-triangulene-based MR-TADF emitter. At lower temperatures triplet−triplet annihilation dominates. The compound simultaneously possesses narrow, deep-blue emission with CIE coordinates of (0.17, 0.01). While delayed fluorescence results mainly from triplet−triplet annihilation at lower temperatures in THF solution, where aggregates form upon cooling, the TADF mechanism takes over around room temperature in solution when the aggregates dissolve or when the compound is well dispersed in a solid matrix. The potential of our molecular design to trigger TADF in larger acenes is demonstrated through the accurate prediction of ΔEST using correlated wave-function-based calculations. On the basis of these calculations, we predicted dramatically different optoelectronic behavior in terms of both ΔEST and the optical energy gap of two constitutional isomers where only the boron and nitrogen positions change. A comprehensive structural, optoelectronic, and theoretical investigation is presented. In addition, the ability of the achiral molecule to assemble on a Au(111) surface to a highly ordered layer composed of enantiomorphic domains of
racemic entities is demonstrated by scanning tunneling microscopy.
A novel simulation approach for excitonic organic light‐emitting diodes (OLEDs) is established by combining a continuous one‐dimensional (1D) drift‐diffusion (DD) model for the charge carrier dynamics with a three‐dimensional (3D) master equation (ME) model describing the exciton dynamics in a multilayer OLED stack with an additional coupling to a thin‐film optics solver. This approach effectively combines the computational efficiency of the 1D DD solver with the physical accuracy of a discrete 3D ME model, where excitonic long‐range interactions for energy transfer can be taken into account. The coupling is established through different possible charge recombination types as well as the carrier densities themselves. We show that such a hybrid approach can efficiently and accurately describe steady‐state and transient behavior of optoelectronic devices reported in literature. Such a tool will facilitate the optimization and characterization of multilayer OLEDs and other organic semiconductor devices.
An antiadiabatic approach is proposed to model how the refractive index of the surrounding medium affects optical spectra of molecular systems in condensed phases. The approach solves some of the issues affecting current implementations of continuum solvation models and more generally of effective models where a classical description is adopted for the molecular environment.
Aminoalkyl-substituted heptamethine cyanine dyes are characterized by a large Stokes shift, an uncommon feature for cyanine molecules yet very promising for their application as fluorescent probes in bioimaging and as light harvesting antennas in biohybrid systems for solar energy conversion. The origin of this photophysical feature has not been adequately explored so far, and a combined experimental and theoretical work is herein provided to shed light on the role played by the central aminoalkyl substituent bound to the heptamethine cyanine backbone in defining the unusual properties of the dye. The spectra recorded in solvents of different polarities point to a marginal role of the medium in the definition of the Stokes shift, which conversely can be ascribed to the relaxation of the molecular geometry upon photoexcitation. This hypothesis is supported by an extensive theoretical investigation of the ground and excited states of the dye. TD-DFT results on the aminoalkyl-substituted dye and its unsubstituted precursor demonstrate a very similar cyanine-like structure for both molecules in the relaxed excited state. Conversely, in the ground state the amino substitution disrupts the conjugation in the polymethine chain, leading to a broken-symmetry, non-planar structure.
A new design strategy is introduced to address a persistent weakness with resonance thermally activated delayed fluorescence (R‐TADF) emitters to reduce aggregation‐caused quenching effects, which are identified as one of the key limiting factors. The emitter Mes3DiKTa shows an improved photoluminescence quantum yield of 80% compared to 75% for the reference DiKTa in 3.5 wt% 1,3‐bis(N‐carbazolyl)benzene. Importantly, emission from aggregates, even at high doping concentrations, is eliminated and aggregation‐caused quenching is strongly curtailed. For both molecules, triplets are almost quantitatively upconverted into singlets in electroluminescence, despite a significant (≈0.21 eV) singlet‐triplet energy gap (ΔEST), in line with correlated quantum‐chemical calculations, and a slow reverse intersystem crossing. It is speculated that the lattice stiffness responsible for the narrow fluorescence and phosphorescence emission spectra also protects the triplets against nonradiative decay. An improved maximum external quantum efficiencies (EQEmax) of 21.1% for Mes3DIKTa compared to the parent DiKTa (14.7%) and, importantly, reduced efficiency roll‐off compared to literature resonance TADF organic light‐emitting diodes (OLEDs), shows the promise of this design strategy for future design of R‐TADF emitters for OLED applications.
The desire to boost the reverse intersystem crossing rate and obtain thermally activated delayed fluorescence with sub-microsecond lifetime fosters the search for novel concepts of molecular geometry. Recently, TADF compounds made of acridine, tetramethylcarbazole and triphenylamine donor and triphenyltriazine acceptor units bound by hyperconjugated spacer units were introduced as having very rapid double TADF decay. Here we present an in-depth time-resolved fluorescence analysis of these intriguing donor–σ–acceptor TADF compounds in various surroundings. Extremely weak coupling of electron-donating and electron-accepting units was found for the σ-bridged TADF compounds, resulting in the coexistence of intramolecular and exciplex fluorescence, whose interplay allowed one to tune the emission properties. The initial fluorescence decay in toluene solutions, previously attributed to rapid TADF, was shown to be prompt intramolecular fluorescence with prolonged fluorescence lifetime, susceptible to molecular oxygen. Only the later delayed fluorescence at the microsecond time-scale, originating from the exciplex states, was attributed to TADF. On the contrary, dominant intramolecular TADF was observed in dilute PMMA films with weaker non-radiative decay. The smooth transition from intramolecular to exciplex TADF was observed by increasing the doping concentration of the polymer films. The DF/PF ratio was found to increase with increasing doping concentration due to the emergence of additional exciplex TADF until a 20 wt% doping load, where concentration quenching emerged at larger doping ratios. The presented findings showcase the unusual fluorescence properties of TADF compounds with weakly bound donor and acceptor units and are important for the future design of novel TADF compounds.
The interplay between exciton delocalization and molecular vibrations profoundly affects optical spectra of molecular aggregates and crystals. The exciton motion occurs on a similar timescale as molecular vibrations, leading to a complex and intrinsically non-adiabatic problem that has been handled over the years introducing several approximation schemes. Here we discuss systems where intermolecular distances are large enough so that only electrostatic intermolecular interactions enter into play and can be treated in the dipolar approximation. Moreover, we only account for interactions between transition dipole moments, as relevant to symmetric molecules, with negligible permanent (multi)polar moments in the ground and low-lying excited states. Translational symmetry is fully exploited to obtain numerically exact solutions of the relevant Hamiltonian for systems of comparatively large size. This offers a unique opportunity to assess the reliability of different approximation schemes. The so-called Heitler–London approximation, only accounting for the effects of intermolecular interactions among degenerate electronic states, leads to the celebrated exciton model, widely adopted to describe optical spectra of molecular aggregates and crystals. We demonstrate that, mainly due to a cancellation of errors, the exciton model approximates well the position of exciton bands and reasonably well the bandshapes, but it fails to predict spectral intensities, leading to underestimated intensities in J-aggregates and overestimated intensities in H-aggregates. This general result is validated against an exact sum-rule. Finally, we address the validity of several approximation schemes adopted to reduce the dimension of the vibrational basis.
The photophysics of a structurally unique aza‐analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture was studied in detail. The combined experimental and computational investigation reveals that the lowest excited state has charge‐transfer character, in spite of the absence of any peripheral electron‐withdrawing groups. The exceptionally electron‐rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, which is qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states that are close in energy but have different polarization directions.
In this work we report the optimisation of the solution processable TADF exciplex emitter in OLED devices formed by the small molecules 9-[2,8]-9-carbazole-[dibenzothiophene-S,S-dioxide]-carbazole (DCz-DBTO2) and 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC). This exciplex, previously reported by Jankus et al. , has gave vacuum deposited devices having respectively current efficiency, power efficiency and EQE of 32.3 cd/A, 26.7 lm/W and 10.3 % obtained for with DCz-DBTO2:TAPC wt% ratio of 30:70. In this work we optimised the thickness and ratio of the exciplex layer using two different solvents, chlorobenzene and chloroform. The best results were achieved when the two solvents were mixed, adding 5 vol% of chlorobenzene to chloroform. With this solvent mixture comparable results to evaporated devices were achieved, 27.5 ± 3.5 cd/A, 16.5 ± 2.0 lm/W and EQE of 8.9 ± 0.6 % at the same DCz-DBTO2:TAPC wt% ratio of 30:70, demonstrating the suitability of small molecule TADF exciplexes as solution processable emissive layer for OLEDs.
Two new donor (D)–acceptor (A) type molecules, PXZ-DBTO2 and PXZ-Ph-DBTO2, configured with phenoxazine donor and dibenzothiophene-S,S-dioxide acceptor are reported. PXZ-Ph-DBTO2, with a phenyl group introduced at the ortho position of PXZ, was used to probe the effects of the congested aryl substitution on the molecular conformation and electronic coupling toward the acceptor core, as well as the thermally activated delayed fluorescence behavior. The highly twisted donor–acceptor configurations of these two molecules were confirmed by X-ray analysis. Different D–A conformations stemmed from the steric interactions between the phenyl group and acceptor core, which leads the nitrogen lone pair electrons of the PXZ-Ph-DBTO2 donor to conjugate across the D–A bridge, whereas in PXZ-DBTO2 the lone pairs remain localized on the donor and strongly mix with the donor π electrons. However, both PXZ-Ph-DBTO2 and PXZ-DBTO2 have the same energy splitting between the charge-transfer states and local donor triplet states, ΔEST, close to 70 meV. PXZ-DBTO2 exhibits a far more efficient thermally activated delayed fluorescence due to nearly 2 orders of magnitude faster reverse intersystem crossing rate as compared to that of PXZ-Ph-DBTO2. Detailed photophysical analysis of both molecules indicates that the presence of the phenyl group on the donor disrupts the π–π/n−π orbital mixing across the N–C bridge that plays a fundamental role in the excited state dynamics and vibronic coupling governing the reverse intersystem crossing rate and thus the efficiency of thermally activated delayed fluorescence. Devices employing PXZ-DBTO2 as an emitting dopant gave an external quantum efficiency (EQE) of 16.7% (42 cd m–2) and a limited efficiency roll-off (15.7% at 1000 cd m–2), whereas the device based on PXZ-Ph-DBTO2 produced a maximum EQE up to 20.6%, but with a significant efficiency roll-off (8.8% at 1000 cd m–2) ascribed to the much faster reverse intersystem crossing rate of PXZ-DBTO2.