Spin-Dependent Exciton Funneling to a Dendritic Fluorophore

Article pubs.acs.org/cm

Spin-Dependent Exciton Funneling to a Dendritic Fluorophore Mediated by a Thermally Activated Delayed Fluorescence Material as an Exciton-Harvesting Host Naoya Aizawa,*,† So Shikita,†,‡ and Takuma Yasuda*,†,‡ †

INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

‡

S Supporting Information *

ABSTRACT: Thermally activated delayed fluorescence (TADF) materials generate energetically equivalent spin-singlet and spin-triplet excited states. In the presence of an energy acceptor, each excited state undergoes energy transfer on different length scales. However, the lack of quantitative understanding of the length dependence of the excited energy-transfer processes hampers the rational design of molecular systems that control exciton transport in organic light-emitting diodes (OLEDs) using TADF. We herein utilize a dendritic fluorophore G1, which consists of an anthracene-based fluorescent core encapsulated by four insulating tris(4-tert-butylphenyl)methyl groups as an energy acceptor. By combining transient photoluminescence measurements and kinetic modeling, we demonstrate the spin-dependent energy transfer in a binary host−guest system composed of a TADF material as the exciton-harvesting host and G1 as the guest fluorophore. The encapsulated structure with the dendritic shell effectively inhibits triplet excitons on the TADF host from funneling to the fluorescent core, thus allowing efficient reverse intersystem crossing and singlet energy transfer. The utilization of G1 in solution-processed OLEDs leads to a maximum external electroluminescence quantum efficiency as high as 5.2%, which is equivalent to an enhancement by a factor of 1.6 over the corresponding nondendritic fluorophore.

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dopants to fluorescent emitters results in significant losses of triplet excitons via nonradiative energy dissipation. Triplet energy transfer is generally understood through the Dexter model and is associated with a typical energy-transfer distance of less than 1 nm, which is 1 order of magnitude shorter than that required for singlet energy transfer described by the Förster model.36,37 Previous studies have shown that increasing the interchromophore distances between the TADF and conventional fluorescent molecules to within several nanometers in vacuum-deposited stacked films38 or blended films39−41 greatly enhances the ηint of the OLEDs devices, consistent with the different length scales predicted for the two different energy-transfer processes. However, a quantitative understanding of the effect of interchromophore distance on these excited energy-transfer processes is still lacking, which consequently limits insight into how to engineer TADF systems that enable efficient spin-dependent exciton funneling to a fluorescent emitter. Dendrimers42−51 encapsulating fluorescent cores may be ideal platforms for controlling interchromophore distances

INTRODUCTION Organic light-emitting diodes (OLEDs) that utilize thermally activated delayed fluorescence (TADF) have generated widespread research interest, driven by the promise of ∼100% internal quantum efficiencies (ηint) for electroluminescence (EL) without the use of any precious metals.1−25 In TADFbased OLEDs, purely organic fluorophores with small singlet− triplet energy splittings (ΔEST) exploit inherently nonradiative triplet excitons to exhibit delayed fluorescence via reverse intersystem crossing (RISC). To date, considerable research effort has been directed to exploring efficient TADF emitters and understanding their unique excitonic properties, in order to further improve the ηint of OLEDs by harvesting both electrogenerated singlet and triplet excitons.26−32 In principle, spin-dependent energy transfer can dominate the performance of TADF-based OLEDs. A typical example involves an energy-funneling system, referred to as TADFassisted fluorescence (TAF),33−35 consisting of ternary blends of a wide-bandgap host, a TADF assistant dopant, and a conventional fluorescent emitter. In TAF systems, RISC in the TADF assistant dopant, followed by singlet energy transfer to the fluorescent emitter, offer ηint higher than the theoretical maximum of 25% in conventional fluorescent OLEDs. However, inevitable triplet energy transfer from TADF assistant © 2017 American Chemical Society

Received: June 23, 2017 Revised: July 24, 2017 Published: July 24, 2017 7014

DOI: 10.1021/acs.chemmater.7b02606 Chem. Mater. 2017, 29, 7014−7022

Article

Chemistry of Materials

Figure 1. (a) Molecular structures of materials used. (b) Steady-state UV−vis absorption and PL spectra of G1 and G0 in dichloromethane (1.0 × 10−5 M). The blue dashed lines are the UV−vis absorption spectrum of XAc-XT in dichloromethane and PL spectrum of an XAc-XT film for a comparison. The red-filled area highlights the spectral overlap between the absorption of G0 or G1 and the emission of XAc-XT. (c) Frontier orbitals of G1 at the ground-state geometry optimized by DFT calculations at the B3LYP/6-31G(d) level (isovalue = 0.02).

between the fluorescent core and the surrounding TADF molecules in homogeneous binary blends. To date, a variety of fluorescent and phosphorescent cores have been incorporated into insulating dendritic structures with excellent solution processabilities.52−63 Such topologically encapsulated structures have proven useful for suppressing interchromophore electronic interactions between the functional cores, giving rise to high photoluminescence (PL) quantum efficiencies in solidstate neat films. Here, we demonstrate an unprecedented strategy for efficient spin-dependent energy transfer in a simple binary host−guest system composed of a TADF material (host) and a dendritic fluorophore (guest) in solid thin films. To this end, we designed a new dendritic fluorophore G1 (Figure 1a), incorporating 9,10-bis(diphenylamino)anthracene as a green fluorescent core tethered by four peripheral tris(4-tertbutylphenyl)methyl groups. A comparison between the dendritic G1 and the corresponding nondendritic G0 in binary host−guest systems allows us to quantitatively evaluate the effect of the encapsulated structure on the interchromophore distance and concomitant energy-transfer rates, as well as the ηint in actual OLED devices.

thracene and bis(4-(tris(4-tert-butylphenyl)methyl)phenyl)amine using Pd(OAc)2/P(t-Bu)3HBF4 as the catalyst. The chemical structure of G1 was confirmed by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. Detailed synthetic procedures and characterization data are provided in the Experimental Section. The photophysical properties of G1 and G0 are summarized in Table 1. The optical gaps for both G1 and G0 were determined to be 2.4 eV from their absorption edges in dilute solutions (Figure 1b). Identical optical gaps are also predicted by time-dependent density functional theory (TDDFT) calculations of their lowest excitation energies. Upon photoTable 1. Photophysical Properties of G1, G0, and XAc-XT compound

λPLa (nm)

ΦPLb (%)

Egc (eV)

Ipd (eV)

Eae (eV)

Eabsf (eV)

G1 G0 XAc-XT

531 533 472

95 95 77

2.4 2.4 2.9

5.6 5.6 6.0

3.2 3.2 3.1

2.35 2.34 2.69

a Maximum PL wavelength measured in toluene under N2. bAbsolute PL quantum yields measured in toluene under N2. cOptical energy gaps determined from the absorption edge in dichloromethane. d Ionization potentials measured by photoelectron yield spectroscopy in a solid-state thin film. eElectron affinities determined by subtracting Eg from IP. fThe lowest excitation energies determined from TDDFT calculations at the B3LYP/6-31G(d) level.

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RESULTS AND DISCUSSION Materials Design and Photophysical Properties. A new dendritic fluorophore G1 was synthesized in 90% yield by the double Buchwald−Hartwig reaction between 9,10-dibromoan7015


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Chemistry of Materials excitation at 280 nm, G1 and G0 exhibit nearly identical green PL emissions centered around 530 nm in solutions, with high absolute PL quantum yields of 95%. G1 and G0 also show identical ionization potentials of 5.6 eV and electron affinities of 3.2 eV, suggestive of the successful preservation of the electronic structure of the central fluorescent core in G1. In fact, DFT calculations reveal that the frontier molecular orbitals of G1 reside almost entirely on its central core, as is seen in Figure 1c. The essentially identical photophysical and electronic properties of G1 and G0 provide an ideal test bed for determining the effect of the encapsulated structure on excited energy-transfer events in a host−guest system involving a TADF host. Excited Energy-Transfer Dynamics in Host−Guest Systems. To construct a binary host−guest exciton-funneling system, we employed a blue TADF material, XAc-XT (3(spiro[acridan-9,9′-xanthen]-10-yl)xanthone)31 with a wide optical gap (2.9 eV) and minimized concentration quenching, as a suitable exciton-harvesting host. The large spectral overlap between the absorption of G1 (or G0) and the emission of XAc-XT implies that excited energy transfer from the TADF host (XAc-XT) to the guest fluorophores (G1 or G0) effectively takes place. To probe this energy-transfer behavior, we performed steady-state PL quenching experiments in blended films consisting of XAc-XT and either G1 or G0 at different doping concentrations ranging from 0.4 to 1.2 mol % (Figure 2a,b). Upon photoexcitation at 280 nm, the blended films exhibit intense green PL emissions predominately from their guest fluorophores. As expected, spectral contributions from XAc-XT in the higher energy region monotonically decrease with increasing concentrations of the doped guest fluorophores. Given that most of the excitons are initially generated on the XAc-XT host, these observations suggest that efficient excited energy transfers occur from the XAc-XT host to the guest fluorophores even at low doping concentrations. In addition, the PL quenching behavior of the XAc-XT host is more significant in the G0-containing blended films than those containing G1, implying that the encapsulated structure of G1 with the dendritic shell suppresses the excited energy transfer from XAc-XT to the fluorescent core to a certain degree. Transient PL decay measurements using the time-correlated single photon counting method quantify the energy transfers from the XAc-XT host to the doped guest fluorophores in the blended films (Figures 2c−f). In these measurements, we focused solely on the emission band from XAc-XT at 440 nm, which in the absence of G1 and G0, undergoes biexponential decay with intrinsic lifetimes of 16 ns and 1.3 μs for prompt and delayed fluorescence, respectively. The prompt and delayed fluorescence lifetimes both decrease upon addition of the guest fluorophores, suggesting increases in the energy-transfer rates for singlet and triplet excitons in XAc-XT (Figure 3). The decay rates of singlet- and triplet-exciton densities (S1 and T1) of XAc-XT are described by eqs 1 and 2. dS1 = −S1(k r,S + k nr,S + kISC + kET,S) + Tk 1 RISC dt dT1 = −T1(k r,T + k nr,T + kRISC + kET,T) + S1kISC dt

Figure 2. PL characteristics of XAc-XT films blended with G1 (left) and G0 (right) at different concentrations. (a, b) Steady-state PL spectra of the blended films upon excitation at 280 nm. (c−f) Transient PL decay profiles of the blended films on the (c, d) nanosecond and (e, f) microsecond time scales excited at 280 nm and detected at 440 nm (for XAc-XT emission).

Figure 3. Schematic energy diagram depicting excited-state transitions in blended films of XAc-XT and either G1 or G0.

(1)

(phosphorescence), nonradiative decay, RISC, and energy transfer for the triplet excitons, respectively. The solution to eqs 1 and 2 is a biexponential decay given by

(2)

where kr,S, knr,S, kISC, and kET,S are the rates for radiative decay (fluorescence), nonradiative decay, intersystem crossing (ISC), and energy transfer for the singlet excitons, respectively, and kr,T, knr,T, kRISC and kET,T are the rates for radiative decay

S1 , T1 = A1exp( −kPFt ) + A 2 exp( −kDFt )

(3)

where A1 and A2 are the intensities of the prompt and delayed fluorescences, respectively. The measurable decay rates for the 7016


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Figure 4. (a, b) Energy-transfer rates from XAc-XT to G1 and G0 as functions of (a) doping concentration and (b) average interchromophore distance (R). The blue dashed line in (b) is the theoretical singlet energy-transfer rate profile calculated using the Förster model. The red dashed line in (b) is the best fit of the experimental triplet energy-transfer rates to the Dexter model. (c) Efficiencies of the singlet and triplet energy transfers as functions of R. (d) ISC and RISC efficiencies and internal EL quantum efficiencies as functions of R. The dashed lines in (c, d) are the results simulated from the scaling parameters of the spin-dependent energy-transfer rates.

prompt and delayed fluorescences (kPF and kDF) are expressed as

model,37 which sensitively expresses the energy-transfer rate in terms of R as

(k r,S + k nr,S + kISC + kET,S − kPF)(k r,T + k nr,T + kRISC + kET,T − kPF) − kISCkRISC = 0

R = R 0(kET,SτPF)−1/6 = (4)

(5)

By assuming (i) kPF ≫ kDF and (ii) kr,S, kISC, kET,S ≫ knr,S, kr,T, knr,T, kET,T, eqs 4 and 5 give: kET,S ≈ kPF − k r,S − kISC kET,T ≈

kPFkDF − kRISC(k r,S + kET,S) k r,S + kISC + kET,S

(6)

− k nr,T

128π 5n 4

(kET,SτPF)−1/6

(8)

where R0 is the Förster radius (which is R for an energy-transfer efficiency of 50%), τPF is the prompt-fluorescence lifetime of XAc-XT, JF is the spectral overlap integral between the XAc-XT emission and the guest fluorophore absorption, ΦPL is the PL quantum yield of the neat XAc-XT film (84%), n is the refractive index of the medium, and κ2 is the dipole orientation factor. We assumed n to be 1.8 and κ2 to be 0.845(2/3)1/2, which are typical values for an amorphous organic film with randomly oriented rigid dipoles.64,65 At each doping concentration, R is higher for the blended films containing G1 than those with G0. For example, the blended film with 1.0 mol % G1 provides an R of 4.5 nm, compared to 4.2 nm for a 1.0 mol % G0-containing blended film. This result indicates that the dendritic shell of G1 does indeed enlarge the interchromophore distance and, hence, sterically hinders access of XAc-XT to the fluorescent core of G1. As shown in Figure 4b, kET,T determined from eq 7 is exponentially dependent on R, which is in good agreement with the Dexter model.36 Hence, plots of kET,T were fitted using eq 9:

(k r,S + k nr,S + kISC + kET,S − kDF)(k r,T + k nr,T + kRISC + kET,T − kDF) − kISCkRISC = 0

9JF ΦPLκ 2

(7)

Taking the sensitive length dependences of the energytransfer rates and the low doping concentrations (0.4−1.2 mol %) of the guest fluorophores into account, only one nearest neighbor to XAc-XT is expected to substantially contribute to the total rate (nearest-neighbor model). Consequently, as presented in Figure 4a, the average energy-transfer rates for the singlet and triplet excitons (kET,S and kET,T) were experimentally determined using eqs 6 and 7, respectively. Both kET,S and kET,T are higher for the blended films with G1 than those containing G0 at each doping concentration, consistent with the steadystate PL data that show more significant quenching of the XAcXT emission upon doping with G0 over G1 (Figure 2). We then estimated the average center-to-center interchromophore distance (R) between the XAc-XT host and the G1 or G0 guest fluorophores in each blended film using the Förster

kET,T = KJT e−βR

(9)

where K is a parameter related to the specific orbital interactions, JT is the normalized spectral overlap integral, and B is the attenuation factor. The best fit gives a KJT value of 1.7 × 109 s−1 and a B value of 1.8 nm−1. This B value is higher than that obtained for intermolecular triplet energy transfer in 7017


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Figure 5. (a) Schematic of the device architecture, (b) EL spectra measured at 10 mA cm−2, (c) current density−voltage characteristics, (d) luminance−voltage characteristics, and (e) external EL quantum efficiency−current density characteristics for the solution-processed OLEDs composed of blended films of XAc-XT and either 1.0 mol % G1 or G0, compared with the device fabricated with a neat XAc-XT film as an emission layer.

ance, we fabricated and characterized solution-processed OLEDs based on the XAc-XT blended films with either 1.0 mol % G1 or G0 as the emission layer. A neat XAc-XT film was also used instead of the blended films for a control device. The devices have the following layer sequence (Figure 5a): indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate):tetrafluoroethyleneperfluoro-3,6-dioxa-4methyl-7-octenesulfonic acid copolymer (PEDOT:PSS:PFI, 30 nm)/poly(N-vinylcarbazole) (PVK, 20 nm)/XAc-XT:1.0 mol % G1 or G0 (30 nm)/2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF, 50 nm)/8-hydroxyquinolinato lithium (Liq, 1 nm)/Al (100 nm), in which the emission layers were sandwiched between the wide-bandgap materials, PVK and PPF, to confine the electrogenerated excitons within the emission layers. Figure 5b−e shows the EL spectra, the current density− voltage−luminance, and external EL quantum efficiency (ηext)− current density characteristics of the fabricated devices. The devices incorporating the G1 or G0 guest fluorophores exhibit intense green EL emissions originating from their identical fluorescent cores without additional emissions from the adjacent layers. Although the maximum ηext value for the device using G0 is only 3.2% (at 3.9 mA cm−2 and 442 cd m−2), the device using G1 exhibits a much higher ηext of 5.2% (at 2.4 mA cm−2 and 417 cd m−2), which is equivalent to an enhancement by a factor of 1.6. Remarkably, this efficiency is comparable to that observed for a device using a neat XAc-XT emission layer (ηext = 5.4%), indicating that the encapsulated structure of G1 with the dendritic shell minimizes triplet excitons on the TADF host from funneling to the fluorescent core, enabling the TADF host to undergo RISC and generate singlet excitons. Although the expected enhancement in ηint upon replacement of G0 with G1 was a factor of 1.1, the efficiency enhancement was measured to be a factor of 1.6. This discrepancy between the theoretical and experimental results

the typical TADF emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6dicyanobenzene (4CzIPN) (B = 0.8 nm−1), as reported by Menke and Holmes,28 indicating the steeper length dependence of triplet energy transfer in the present system. This propensity is favorable for inhibiting funneling from the TADF host molecules to the nearby guest fluorophoric molecules. We also note that the KJT value is nearly one-order of magnitude higher than that for 4CzIPN (KJT = 1.8 × 108 s−1), which is most likely due to the larger spectral overlap in the present host−guest system. The efficiencies of the singlet and triplet energy-transfer processes as functions of R are plotted in Figure 4c, together with the numerically simulated results obtained using the scaling parameters; the quantitatively different length scales of the singlet and triplet energy-transfer processes are evident, as is expected from the Förster and Dexter models. On the basis of these results, we simulated ηint by accounting for multiple ISC and RISC cycles (Figure 4d).2 In contrast to the photoexcitation process, recombination of spin-uncorrelated charges leads to the formation of singlet and triplet excitons in a 1:3 ratio during the electroexcitation process in OLEDs. Hence, theoretical ηint values are determined from eq 10: ∞

ηint = 0.25 ∑ (1 − ΦISC)(ΦISCΦRISC)n n=0 ∞

+ 0.75 ∑ ΦRISC(1 − ΦISC)(ΦISCΦRISC)n n=0

(10)

where ΦISC is the ISC efficiency and ΦRISC is the RISC efficiency. We note that excitons are assumed to be initially generated on XAc-XT. For a doping concentration of 1.0 mol %, G1 is expected to enhance ηint by a factor of 1.1 over that for G0 because of the different length scales associated with the singlet and triplet energy-transfer processes. OLED Performance. To evaluate the impact of the encapsulated dendritic structure of G1 on its OLED perform7018


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Chemistry of Materials can be explained if a fraction of holes and electrons are trapped and recombine directly on the fluorophores in the devices, which is not accounted for in eq 10. In fact, such direct carrier trapping on the guest fluorophore is evident in the current density−voltage characteristics (Figure 5c) that show lower current densities for devices incorporating the guest fluorophores, compared to the control device based on the neat XAcXT emission layer. Similar phenomena have also been reported for several host−guest systems in OLEDs.66−69 Furthermore, the decrease in current density is more significant for G0 than G1, indicating that the encapsulated structure of G1 partially inhibits direct carrier trapping on the fluorescent core. Therefore, we conclude that the improved EL efficiency of the device incorporating G1 is attributable to the suppression of both triplet energy transfer and direct carrier trapping, which minimizes the nonradiative decay of the triplet excitons.

Scheme 1. Synthesis of G1

Synthesis of 2. A mixture of 1 (2.80 g, 4.70 mmol) and diphenylamine (0.39 g, 2.3 mmol) in AcOH (100 mL) was refluxed for 12 h. After cooling to room temperature, the solution was concentrated under reduced pressure. Methanol (150 mL) and aqueous HCl (11 M, 10 mL) were added, and the mixture was refluxed for 20 h. After cooling to room temperature, the reaction mixture was poured into aqueous NaHCO3 (1.5 M) and then extracted with dichloromethane. The combined organic layers were concentrated under reduced pressure and poured into methanol. The precipitate was collected by filtration and washed with several portions of methanol to afford 2 as a white solid (0.81 g, 55%). 1H NMR (400 MHz, CDCl3, δ) 7.23 (d, J = 8.8 Hz, 12H), 7.09 (d, J = 8.5 Hz, 12H), 7.04 (d, J = 8.8, 4H), 6.94 (d, J = 8.8 Hz, 4H), 5.63 (s, 1H), 1.30 (s, 54H). 13C NMR (100 MHz, CDCl3, δ) 148.29, 144.17, 140.80, 139.83, 132.07, 130.76, 124.01, 116.32, 63.18, 34.30, 31.40. MS (MALDI-TOF) m/z: 989.85 [M]+; calcd for C74H87N 989.68. Synthesis of G1. A mixture of 2 (0.204 g, 0.176 mmol), 9,10dibromoanthracene (27 mg, 80 μmol), Pd(OAc)2 (0.54 mg, 2.4 μmol), P(t-Bu)3HBF4 (3.5 mg, 12 μmol), and t-BuONa (35 mg, 0.36 mmol) in dry toluene (1.0 mL) was refluxed for 24 h under a nitrogen atmosphere. After cooling to room temperature, the reaction mixture was poured into water and extracted with toluene. The combined organic layers were concentrated under reduced pressure. The crude product was purified by silica-gel column chromatography (eluent: dichloromethane/n-hexane = 1:3, v/v) to afford G1 as a yellow solid (0.18 g, 90%). 1H NMR (400 MHz, CDCl3, δ) 8.16 (dd, J = 6.8, 3.3 Hz, 4H), 7.34 (dd, J = 6.8, 3.3 Hz, 4H), 7.18 (d, J = 8.8 Hz, 24H), 7.02 (d, J = 8.5 Hz, 24H), 6.90−6.97 (m, 16H), 1.27 (s, 108H). 13C NMR (100 MHz, CDCl3, δ) 148.26, 145.49, 144.05, 139.78, 137.25, 131.91, 131.79, 130.80, 126.30, 125.36, 123.91, 118.96, 63.15, 34.26, 31.39. MS (MALDI-TOF) m/z: 2153.41 [M]+; calcd for C162H180N2 2153.41. Anal. Calcd (%) for C162H180N2: C, 90.28; H, 8.42; N, 1.30. Found: C, 90.01; H, 8.48; N, 1.30. Quantum Chemical Calculations. All quantum chemical calculations were performed using the Gaussian 09 suite of programs. Gas-phase geometry optimizations for the ground states were carried out using DFT calculations at the B3LYP/6-31G(d) level of theory.73−76 The lowest singlet excitation energies at the groundstate minimum were calculated by linear response TDDFT at the same level of theory. Photophysical Measurements. Thin films of the materials were deposited by spin coating from THF solutions onto cleaned quartz substrates. UV−vis absorption spectra were measured using a V-670 spectrometer (Jasco). Steady-state PL spectra were recorded on a FP8600 spectrophotometer (Jasco) at the excitation wavelength of 280 nm. Absolute PL quantum yields were determined using an ILF-835 integrating sphere system (Jasco) under a nitrogen atmosphere. Ionization potentials were determined using an AC-2 ultraviolet photoelectron yield spectrometer (Riken Keiki). Electron affinities were deduced by subtracting optical gaps from ionization potentials (ignoring the exciton binding energies included in the optical gaps). Transient PL decay measurements were carried out using a C11367 fluorescence lifetime spectrometer (Hamamatsu Photonics) with a 280

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CONCLUSIONS To develop an effective spin-dependent exciton funneling system, we employed the dendritic fluorophore G1, consisting of an anthracene-based fluorescent core tethered by four insulating tris(4-tert-butylphenyl)methyl groups. In a solid-state thin film containing 1.0 mol % G1 blended with a XAc-XT TADF host, the encapsulated structure of G1 increases the average interchromophore distance between the fluorescent core and the surrounding XAc-XT host molecules by 0.3 nm without any detrimental change to its electronic structure. Because of the different length scales for the singlet and triplet energy-transfer processes, this increment in the interchromophore distance was expected to enhance ηint by a factor of 1.1 when incorporated in OLEDs. However, the measured efficiency was found to be enhanced by a factor of 1.6. This discrepancy is anticipated under circumstances where fractions of injected charge carriers are trapped and recombine directly on the guest fluorophore in the OLEDs. We therefore propose that the encapsulated structure of G1 suppresses both triplet energy transfer and direct carrier trapping, which inhibit triplet excitons from funneling to the fluorescent core of G1, hence enhancing ηint of the actual device. These results have important implications for the future design of functional molecular materials that control spin-dependent exciton funneling in TADF-based OLEDs. We believe that this study will provide fundamental insight for the further development of high-efficiency light-emitting molecular systems for optoelectronic devices in which exciton funneling along multiple excited states determines the device performance.

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EXPERIMENTAL SECTION

Materials and Methods. Unless otherwise noted, commercially available reagents and solvents were used without further purification. G1 was synthesized by the procedure described in Scheme 1. XAc-XT was prepared according to our previous report.31 Compound 1, PPF, and PEDOT:PSS:PFI were prepared according to procedures described in the literature.70−72 G0, PVK, and Liq were purchased from Lumitec, Sigma-Aldrich, and e-Ray Optoelectronics Technology, respectively. NMR spectra were recorded on an Avance III 400 spectrometer (Bruker). 1H and 13C NMR chemical shifts were determined relative to the signals of tetramethylsilane (δ = 0.00) and CDCl3 (δ = 77.0), respectively, as internal standards. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were collected on an Autoflex III spectrometer (Bruker Daltonics) using dithranol as a matrix. Elemental analyses were carried out using a MT-5 CHN Corder (Yanaco). 7019


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Chemistry of Materials nm LED excitation source under a nitrogen atmosphere. The prompt and delayed lifetimes of a neat XAc-XT film were extracted from the resulting decay by performing biexponential fitting and deconvolution with the instrument response function. Multiexponential fittings were performed to obtain statistically averaged lifetimes of the blended films because of a distribution of energy-transfer rates for each pair of host and guest molecules in solid-state films. OLED Fabrication and Characterization. ITO-coated glass substrates were cleaned with detergent, deionized water, acetone, and isopropanol. The substrates were then subjected to UV−ozone treatment for 30 min. A dispersion of PEDOT:PSS:PFI (1:5:25, wt/ wt/wt) was spin-coated onto the ITO substrates and annealed at 150 °C for 30 min under ambient conditions. A solution of PVK in THF (4.0 mg mL−1) was spin-coated onto the PEDOT:PSS:PFI layer and annealed at 200 °C for 60 min to yield a film that is insoluble in common organic solvents.77 A solution of XAc-XT and G1 or G0 in THF (6.0 mg mL−1) was spin-coated onto the PVK layer. The devices were then loaded into an E-200 vacuum evaporation system (ALS Technology). The PPF and Liq layers were thermally deposited under vacuum (

Spin-Dependent Exciton Funneling to a Dendritic Fluorophore

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