Effect of Atom Substitution in Chalcogenodiazole-Containing

Jan 22, 2015 - Department of Automotive Science, Graduate School of Integrated Frontier ... mental studies of ISC in sulfur-containing organic materia...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Effect of Atom Substitution in Chalcogenodiazole-Containing Thermally Activated Delayed Fluorescence Emitters on Radiationless Transition Hiroyuki Tanaka,† Katsuyuki Shizu,† Jiyoung Lee,‡ and Chihaya Adachi*,†,§ †

Center for Organic Photonics and Electronics Research (OPERA) and JST, ERATO, Adachi Molecular Exciton Engineering Project, Department of Automotive Science, Graduate School of Integrated Frontier Sciences, and §International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan

‡

S Supporting Information *

ABSTRACT: Radiationless transition between the lowest singlet (S1) and triplet (T1) excited states in the thermally activated delayed fluorescence (TADF) were investigated with respect to molecular design. The photophysical, transient photoluminescence and electroluminescence (EL) characteristics of two chalcogenodiazole-containing TADF emitters were compared. These contained 1,3,4-oxadiazole or 1,3,4-thiadiazole. The effect of substituting oxygen with sulfur on TADF was caused by an electron-pairaccepting conjugative effect. This effect resulted from the vacant 3d-orbitals of divalent sulfur in the thiadiazole heteroring. Atom substitution narrowed the gap between the highest occupied and lowest unoccupied molecular orbital energy levels, and enhanced S1 → T1 intersystem crossing. These effects resulted from the enhanced acceptor strength and orbital angular momentum by the vacant 3d-orbitals of sulfur. Atom substitution increased the contribution of the delayed fluorescence component to the total EL efficiency (65.1% → 78.0%). This resulted from enhanced reverse intersystem crossing, because of the reduced energy gap between S1 and T1.

■

INTRODUCTION Organic materials with high carrier mobilities or high luminescence quantum efficiencies are important in organic photonics and electronics. Sulfur-containing organic semiconductors have received much recent attention, as building blocks for solutionprocessed thin film transistors and single-crystal light-emitting transistors.1,2 The presence of sulfur can provide short intermolecular contacts giving high carrier mobility, and thiophene and its derivatives can provide electron donation.3−5 Ambipolar materials with high luminescence quantum efficiencies are attractive organic light-emitting diode (OLED) emitters. Thermally activated delayed fluorescence (TADF) materials can up-convert triplet excitons into singlet excitons. Controlling the energy gap between the lowest singlet excited state (S1) and lowest triplet excited state (T1) (ΔES‑T) should be considered when designing highly efficient TADF-based OLED emitters. Understanding upconversion through radiationless S1 ← T1 reverse intersystem crossing (reverse ISC) will also aid molecular design. We recently reported some phenoxazine (PXZ) donor-based TADF emitters.6−8 Their ΔES‑T values were minimized by introducing twisted conformations into their electron donor− acceptor (D−A) structures. This spatially separated the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). TADF is an intramolecular charge transfer (ICT) emission, because the D−A molecules have a small ΔES‑T between the lowest singlet charge transfer (CT) excited state (1CT1) and lowest triplet CT excited state (3CT1). The TADF strongly depends on the ΔES‑T and radiationless transition (ISC and reverse ISC) between 1CT1 and 3CT1. Altering © 2015 American Chemical Society

the electron accepting ability clarified the relationship between HOMO−LUMO spatial overlap, photoluminescence (PL) quantum yield (ϕPL) and ΔES‑T for the PXZ donor-based TADF emitters.6,7 A weak acceptor strength increased the HOMO−LUMO spatial overlap, giving a larger ΔES‑T. However, increasing the HOMO−LUMO spatial overlap also increased the ϕPL. Electroluminescence (EL) efficiencies depend on the emitter ϕPL, so a moderate HOMO−LUMO spatial overlap in the D−A molecule is necessary to achieve a highly efficient TADF emitter. Detailed investigation of ΔES‑T values have been reported for some TADF emitters.6−20 However, the relationship between TADF up-conversion (radiationless transition of ISC and reverse ISC) and molecular design is less well understood, which the present study aims to address. The effect of substituting one oxygen atom for sulfur on the radiationless transition is investigated, by comparing TADF emitters containing the chalcogenodiazole acceptors 1,3,4oxadiazole (OXD) and 1,3,4-thiadiazole (TDZ). Oxygen and sulfur are group 16 elements. The effects of substituting one group 16 atom for another on various characteristics of organic materials have been investigated.21−23 Theoretical and experimental studies of ISC in sulfur-containing organic materials have been reported, such as El-Sayed’s rule for spin−orbit coupling24−26 and internal heavy atom effects.27−32 However, the effect of sulfur in TADF emitters on radiationless transition Received: October 27, 2014 Revised: January 19, 2015 Published: January 22, 2015 2948

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C

while the LUNTO was localized on the 2,5-diphenyl-1,3,4thiadiazole acceptor moieties. This separation of HONTO and LUNTO was attributed to the twisted conformation caused by the PXZ donor, similar to those of reported PXZ donorsubstituted D−A molecules.6−8 No difference in HONTO− LUNTO separation upon single atom substitution was observed from the DFT calculations. Figure 2 shows an

has not been reported. Substituting one oxygen atom for sulfur gives insight into the up-conversion process, and its effect on OLED device performance. Comparing TADF emitters with similar molecular conformations and ΔES‑T values allows the effects of ISC and reverse ISC between 1CT1 and 3CT1 on TADF to be investigated. We previously reported 2,5-bis(4(10-phenoxazyl)phenyl)-1,3,4-oxadiazole (bis-PXZ-OXD) as a green TADF emitter, which had a donor−acceptor−donor (D−A−D) structure and small ΔES‑T.7 Herein, we report the D−A−D type TADF emitter, 2,5-bis(4-(10-phenoxazyl)phenyl)-1,3,4-thiadiazole (bis-PXZ-TDZ). The effect of atom substitution on TADF characteristics is investigated by comparing the photophysical properties, transient PL decay profiles and OLED characteristics of bis-PXZ-OXD and bisPXZ-TDZ (Figure 1a).

Figure 2. ORTEP drawing of bis-PXZ-TDZ. Thermal ellipsoids are shown at the 50% probability level. A solvent molecule (dichloromethane) is omitted for clarity.

ORTEP drawing of the X-ray crystal structure of bis-PXZ-TDZ. Large twisted conformations existed between the PXZ donor and TDZ acceptor moieties. The spatial separation of HONTO and LUNTO suggested that the ΔES‑T of bis-PXZ-TDZ should be small, similar to that of bis-PXZ-OXD. We calculated the 10 lowest singlet and triplet excited states for the optimized ground-state geometry of bis-PXZ-TDZ, using time-dependent DFT37,38 (TD-DFT) calculations with the same functional and basis set (denoted as TD-CAM-B3LYP/cc-pVDZ). The calculated excited states were used to estimate an ΔES‑T of bisPXZ-TDZ of 0.65 eV. This small ΔES‑T was comparable with that of bis-PXZ-OXD (0.57 eV).7 The fluorescence emission is a radiative deactivation based on ICT between the HOMO and LUMO. The fluorescence emission of bis-PXZ-OXD and bis-PXZ-TDZ is influenced by the different electronic structures of the OXD and TDZ acceptor moieties that the LUNTOs are localized on. The chalcogenodiazole acceptors are therefore good candidates for investigating the effect of atom substitution on TADF characteristics. The UV−vis absorption and PL spectra of bisPXZ-TDZ recorded in toluene were compared with those of bis-PXZ-OXD (Figure 3). The absorption spectra of bis-PXZ-OXD

Figure 1. (a) Structures of the chalcogenodiazole-containing D−A−D-type compounds (bis-PXZ-OXD and bis-PXZ-TDZ) and (b) HONTO and LUNTO of bis-PXZ-TDZ calculated at TD-CAMB3LYP/cc-pVDZ level. The results of the calculations on bis-PXZOXD, as reported in ref 7, are very similar to those shown here for bisPXZ-TDZ.

■

RESULTS AND DISCUSSION Quantum chemical calculations were performed on bis-PXZTDZ, to investigate the effect of substituting oxygen for sulfur on the HOMO−LUMO spatial separation. Density functional theory (DFT) calculations were carried out using the Gaussian 09 program package.33 The ground-state geometry of bis-PXZTDZ was optimized using the CAM-B3LYP functional34 and cc-pVDZ basis set.35 The optimized geometry was compared with that of bis-PXZ-OXD.7 The molecular conformations of bis-PXZ-TDZ and bis-PXZ-OXD were predictably similar, except for differing bend-angles resulting from the OXD and TDZ heterorings (see Supporting Information, Figure S1). The geometry of the optimized ground state of bis-PXZ-TDZ was consistent with its X-ray crystal structure. The highest occupied natural transition orbital (HONTO) and lowest unoccupied natural transition orbitals (LUNTO)36 for the optimized groundstate geometry of bis-PXZ-TDZ are shown in Figure 1b. The HONTO was largely distributed on the PXZ donor moieties,

Figure 3. Normalized absorption (dashed) and PL (solid) spectra of bisPXZ-OXD (green) and bis-PXZ-TDZ (orange) in toluene at 1.0 × 10−5 M.

and bis-PXZ-TDZ exhibited CT absorptions at around 396 and 413 nm, respectively. That of bis-PXZ-TDZ was wider than that of bis-PXZ-OXD, which indicated stabilization of its CT excited state.8 The HOMO−LUMO energy gaps of bis-PXZ-OXD 2949

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C

compared benzo[c][1,2,5]chalcogenodiazole acceptors, benzo[c][1,2,5]oxadiazole (BX) and benzo[c][1,2,5]thiadiazole (BT). The acceptor strength of bis-TH-BX (ELUMO = 2.70 eV) was slightly stronger than that of bis-TH-BT (ELUMO = 2.61 eV).39 This trend differed from that of bis-PXZ-OXD and bis-PXZTDZ in the present study. Bis-PXZ-OXD and bis-PXZ-TDZ both exhibited HOMO−LUMO spatial separation. Bis-TH-BX and bis-TH-BT both exhibited a large HOMO−LUMO spatial overlap over their extended π-conjugated structures. The electron density distributions of the LUNTO for bis-PXZ-OXD and bis-PXZ-TDZ were localized on the respective acceptor moieties, as shown in Figure 1. The effect of substituting oxygen for sulfur on acceptor strength was therefore attributed to the nature of sulfur. In 1956, Bordwell and Boutan reported electron-pair accepting conjugative effects in divalent sulfur groupings.40 They suggested that sulfur in divalent sulfurcontaining compounds stabilized unshared electrons of adjacent carbon in a conjugative effect. This expanded the valence shell of sulfur to vacant 3d-orbitals. Conjugative effects in sulfurcontaining compounds were further investigated by Oae and Price et al.41−43 Based on these divalent sulfur grouping reports, we attributed the enhanced acceptor strength of bis-PXZ-TDZ to the electron-pair accepting conjugative effect. This resulted from the vacant 3d-orbitals of divalent sulfur in the TDZ acceptor. The electron-pair acceptor type resonance structures are shown in Scheme 1. These assume single electron transfer,

and bis-PXZ-TDZ were estimated from the absorption onsets to be 2.76 and 2.62 eV, respectively. The HOMO energy levels of bis-PXZ-OXD and bis-PXZ-TDZ were controlled by the PXZ donor, as shown by the HONTO−LUNTO distributions. Since the HOMO energy levels of those are similar, a deepening of the HOMO−LUMO energy gap implies an increase of the acceptor strength of bis-PXZ-TDZ. This was an obvious effect of the sulfur substitution on the photophysical properties. The wavelengths of the maximum PL intensities of bis- PXZ-OXD and bis-PXZ-TDZ were 501 and 538 nm, respectively. The PL spectrum of bis-PXZ-TDZ was red-shifted compared with that of bis-PXZ-OXD, which indicated narrowing of its HOMO−LUMO energy gap. Sulfur substitution widened the CT absorption and red-shifted CT emission (Figure 3), which reflected the differing electronic structures of bis-PXZ-OXD and bis-PXZ-TDZ. We previously reported the widening of the CT absorption and CT emission, caused by an additional substitution of the PXZ donor unit to an ICT-type D−A molecule. This widening resulted from stabilization of the CT excited states.8 For the current substitution in the chalcogenodiazole-containing D−A−D molecules, widening of the CT absorption of bisPXZ-TDZ was confirmed, but the CT emission of bis-PXZTDZ was red-shifted compared with that of bis-PXZ-OXD. The PL half-widths of bis-PXZ-OXD and bis-PXZ-TDZ were 89 and 94 nm, respectively. The different CT emission behavior reflected the electronic states of the CT excited states. The HOMO and LUMO energy levels (EHOMO and ELUMO, respectively) of bis-PXZ-TDZ were compared with those of bis-PXZ-OXD, by investigating the photophysical properties of neat bis-PXZ-OXD and bis-PXZ-TDZ films fabricated by thermal vacuum deposition. The EHOMO and ELUMO were estimated from UV−vis absorption and work function measurements, which are represented by the schematic energy diagram in Figure 4. The EHOMO values of bis-PXZ-OXD and

Scheme 1. Resonance Structures of bis-PXZ-TDZ

derived from the CT transition from the PXZ donor to TDZ acceptor. The unshared electron at the 2- or 5-position carbon of the TDZ acceptor is stabilized by the electron accepting vacant 3d-orbitals of the divalent central sulfur. This resulted in the enhanced acceptor strength of bis-PXZ-TDZ. This electronpair accepting conjugative ability of the TDZ acceptor in the ICT-type D−A molecular system is the first such example in sulfur-containing electron acceptors. We also attributed the widening of the CT absorption in bis-PXZ-TDZ to the additional resonance structures obtained from the conjugative effect. This stabilized the CT excited state for bis-PXZ-TDZ, as mentioned above. We then investigated the effect of the enhanced acceptor strength of bis-PXZ-TDZ on the CT excited state, using solvatochromic shifts in differing polarity solvents. The solvents used were n-hexane, toluene, 1,4-dioxane, o-dichlorobenzene, and dichloromethane. Their solvent polarities (ET(30) kcal/mol)44 are 31.0, 33.9, 36.0, 38.0 and 40.7, respectively. The emission colors obtained from various solutions, and solvatochromic shifts of PL spectra for bis-PXZ-OXD and bis-PXZ-TDZ are shown in Figure 5. The wavelength of maximum PL intensity for bis-PXZ-OXD red-shifted from 444 (n-hexane) to 558 nm (CH2Cl2), with increasing solvent polarity. A large red-shift from 467 (n-hexane) to 611 nm (CH2Cl2) was observed for bis-PXZ-TDZ. This was consistent

Figure 4. Energy diagram of bis-PXZ-OXD and bis-PXZ-TDZ.

bis-PXZ-TDZ were ∼5.4−5.6 eV, and were governed to the PXZ donor moiety. The ELUMO values of bis-PXZ-OXD and bis-PXZ-TDZ were 2.8 and 3.2 eV, respectively. This confirmed the stronger acceptor strength of bis-PXZ-TDZ, as predicted from the photophysical properties in toluene. The effect of atom substitution in the chalcogenodiazole acceptors on acceptor strength is now discussed. Köse et al. and others have theoretically evaluated the acceptor strength of thiophene (TH) donor-coupled D−A−D chromophores with various acceptor units.39 The effect of single atom substitution among various acceptor units on the electronic and optical properties was reported. In evaluating acceptor strength, we 2950

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C

Figure 5. Emission colors of (a) bis-PXZ-OXD and (b) bis-PXZ-TDZ obtained from solutions in differing polarity solvents. Solvatochromic shifts of PL spectra of (c) bis-PXZ-OXD and (d) bis-PXZ-TDZ.

with the CT excited state of bis-PXZ-TDZ being stabilized by the electron-pair-accepting conjugative effect in the TDZ acceptor. The effect of atom substitution in the chalcogenodiazole acceptors on photophysical properties was evidenced from the widened CT absorption and red-shifted CT emission. Time-resolved fluorescence measurements of bis-PXZ-TDZ were recorded using a streak camera, to elucidate this effect on the transient PL characteristics. To compare the transient PL characteristics of bis-PXZ-TDZ and bis-PXZ-OXD,7 we fabricated a 6 wt % doped film of bis-PXZ-TDZ in bis(2(diphenylphosphino)phenyl)ether oxide (DPEPO). This doped film had a wider HOMO−LUMO energy gap than that of bis-PXZ-TDZ. The ΔES‑T of bis-PXZ-TDZ was estimated to be 0.11 eV, from PL measurements recorded at 10 K. This was calculated from the difference between the onsets of prompt emission derived from 1CT1 → S0 radiative transition after excitation and delayed emission derived from T1 → S0 transition that was observed with an interval of 1 ms (Figure S2). The ΔES‑T of bis-PXZ-TDZ was smaller than that of bis-PXZOXD (0.15 eV). This was expected from the enhanced acceptor strength of bis-PXZ-TDZ. 1CT1 ← 3CT1 reverse ISC occurred from thermally excited vibrational levels of the triplet CT excited state, whose energies were greater than or equal to the ΔES‑T. Thus, the reduced ΔES‑T of bis-PXZ-TDZ promoted upconversion through reverse ISC. The transient PL decay profiles exhibited a thermal activation of the delayed fluorescence component with increasing temperature from 10 to 300 K (Figure 6). This confirmed that bis-PXZ-TDZ is a TADF molecule. The PL quantum yield (ϕPL) of bis-PXZ-TDZ at ambient temperature (300 K) was 68.5%. To reveal the effect of atom substitution on up-conversion of TADF, we analyzed transient PL decay curves obtained from the 6 wt % doped films of bis-PXZ-OXZ and bis-PXZ-TDZ at 300 K. Figure S3 shows transient PL decay curves of bis-PXZ-OXZ and bis-PXZTDZ with fitted curves. The decay curves were fitted using a

Figure 6. Transient PL decay curves for the bis-PXZ-TDZ-doped film in DPEPO, measured at 10−300 K.

triexponential decay function. The decay times of the fitted curve for bis-PXZ-TDZ were 7 μs (τ1), 56 μs (τ2) and 311 μs (τ3). The nanosecond-order prompt decay component was contained within τ1 since the decay spanned a millisecondorder time range. The three decay times for bis-PXZ-OXD were 73 μs (τ1), 249 μs (τ2) and 1420 μs (τ3), and were all larger than those of bis-PXZ-TDZ. The decay times of bis-PXZOXD and bis-PXZ-TDZ indicated that atom substitution influenced the TADF emission process. The transient PL decay curve of bis-PXZ-TDZ was resolved into prompt and delayed decay components. Their contributions to the total ϕPL of 68.5% were 44.1 (ϕprompt) and 24.4% (ϕdelayed), respectively. The ϕprompt and ϕdelayed of bis-PXZ-OXD were 76.6 and 10.4%, respectively.7 The emission processes of TADF generated by light and electrical excitations are shown in Scheme 2. The efficiency of ISC (ϕISC) and reverse ISC (ϕRISC) were estimated from10 ϕISC = 1 − ϕprompt 2951

(1) DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C

(α-NPD) (30 nm)/1,3-bis(9-carbazolyl)benzene (mCP) (10 nm)/ 6 wt % bis-PXZ-TDZ:DPEPO (15 nm)/DPEPO (10 nm)/1,3,5tris(2-N-phenylbenzimidazolyl)benzene (TPBi) (40 nm)/lithium fluoride (LiF) (0.8 nm)/aluminum (Al) (100 nm) (Figure S4). Figure 7 shows the dependence of external quantum efficiency

Scheme 2. Emission Processes for Prompt and Delayed Fluorescence after Light and Electrical Excitations (Upper and Lower, Respectively)

Figure 7. Dependence of EQE on J for OLEDs containing bis-PXZOXD (green) or bis-PXZ-TDZ (orange). Inset: EL spectra obtained from OLEDs containing bis-PXZ-OXD or bis-PXZ-TDZ.

ϕRISC = ϕdelayed /ϕISC

(EQE) on current density (J) and electroluminescence (EL) spectra for OLEDs containing bis-PXZ-OXD (green) and bisPXZ-TDZ (orange). The OLED containing bis-PXZ-TDZ exhibited a maximum EQE of 10.0 ± 0.5%, and a large roll-off in EQE at high J. The wavelengths of maximum EL intensities of bis-PXZ-OXD and bis-PXZ-TDZ were 508 and 537 nm, respectively, which showed a similar red-shift to the PL spectrum. The contributions of prompt and delayed fluorescence to the maximum EL quantum efficiency were estimated from the PL characteristics obtained from the doped films of bis-PXZ-OXD and bis-PXZ-TDZ, according to6−8,10,15 ηext = ηintηout = ηprompt + ηdelayed (4)

(2)

where the decay rate constants of the 1CT1 → S0 radiative transitions (prompt and delayed fluorescence) (krS) were identical.10 The decay rate constant of the 1CT1 → S0 nonradiative transition (knrS) was assumed to be negligible (i.e., knrS = 0).6,10,15 The ϕISC and ϕRISC of bis-PXZ-TDZ were estimated to be 55.9 and 43.7%, respectively. The ϕISC and ϕRISC of bis-PXZ-OXD were 23.4 and 44.4%, respectively.7 The ϕRISC values of bis-PXZ-OXD and bis-PXZ-TDZ with respect to up-conversion were similar. This was attributed to the ϕdelayed values of bis-PXZ-OXD and bis-PXZ-TDZ contributing to ∼50% of their ϕISC values. Substitution of oxygen with sulfur reduced the ΔES‑T of bis-PXZ-TDZ. The rate constant of S1 ← T1 reverse ISC (kRISC) can be represented as45,46 ° exp( −ΔES − T /kBT ) kRISC = kRISC

(3)

ηint = γ[ηr,Sϕprompt + (ηr,SϕISC + ηr,T)ϕRISC]

(5)

ηprompt = γηr,Sϕpromptηout

(6)

ηdelayed = γ(ηr,SϕISC + ηr,T)ϕRISCηout

(7)

where ηext, ηint, ηout, ηprompt, ηdelayed, γ, ηr,S and ηr,T represent the external EL quantum efficiency, internal EL quantum efficiency, light out-coupling efficiency, prompt fluorescence component of the external EL quantum efficiency, delayed fluorescence component of the external EL quantum efficiency, charge recombination factor, singlet exciton production rate (25%), and triplet exciton production rate (75%), respectively. The respective yields and efficiencies obtained from the PL and OLED characteristics of bis-PXZ-OXD and bis-PXZ-TDZ are summarized in Table 1. The maximum EQEs of bis-PXZ-OXD and bis-PXZ-TDZ (14.9 and 10.0%, respectively) at low J (1.0 × 10−2 mA/cm2) were resolved into their prompt and delayed fluorescence components. The large ϕISC of bis-PXZTDZ (55.9%) resulted in the low ηprompt (2.2%). The lower maximum EQE (ηext) of bis-PXZ-TDZ compared with that of bis-PXZ-OXD was attributed to the decreased ϕprompt from the large ϕISC. The ηdelayed values of bis-PXZ-OXD and bis-PXZTDZ were estimated to be 9.7 and 7.8%, respectively. The contribution of ηdelayed to ηext for bis-PXZ-TDZ was 78.0%, which was larger than that for bis-PXZ-OXD (65.1%) (Table 1). The

where k°RISC, kB and T are the limiting rate constant of reverse ISC, Boltzmann constant and absolute temperature, respectively. Equation 3 shows that the reduced ΔES‑T of bis-PXZTDZ increased its kRISC. The comparable ϕRISC values of bisPXZ-OXD and bis-PXZ-TDZ were attributed to the reduced ΔES‑T of bis-PXZ-TDZ. The effect of atom substitution on EL efficiencies resulting from radiationless transition is discussed below. The differing ϕISC values of bis-PXZ-OXD (23.4%) and bis-PXZ-TDZ (55.9%) were attributed to the effect of atom substitution on exciton deactivation. These included radiative and nonradiative transitions, and resulted in the lower decay times of bis-PXZ-TDZ. We attributed the shorter decay times and increased ϕISC of bis-PXZ-TDZ to enhanced 1CT1 → 3CT1 ISC, derived from the vacant 3d-orbitals of sulfur. The effect of atom substitution in the TADF emitter on OLED characteristics was then investigated. To compare the OLED characteristics of bis-PXZ-TDZ with that of bis-PXZOXD, we fabricated the OLED containing bis-PXZ-TDZ as an emitter with the same device structure: indium tin oxide (ITO)/ N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4,4′-diamine 2952

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C

Table 1. PL Characteristics of 6 wt % bis-PXZ-OXD:DPEPO and 6 wt % bis-PXZ-TDZ:DPEPO Doped Films, and Respective EL Efficiencies for OLEDs Containing Their Doped Films As Emitting Layersa ηext/%

a

compounds

ϕprompt/%

ϕdelayed/%

ϕISC/%

ϕRISC/%

γηout/%

ηprompt/%

ηdelayed/%

calc.

exp.

bis-PXZ-OXD bis-PXZ-TDZ

76.6 44.1

10.4 24.4

23.4 55.9

44.4 43.7

27.1 20.1

5.2 (34.9%) 2.2 (22.0%)

9.7 (65.1%) 7.8 (78.0%)

14.9 10.0

14.9 10.0

The contributions of ηprompt and ηdelayed to ηext are shown in parentheses.

larger contribution of ηdelayed for bis-PXZ-TDZ reflected the increased kRISC from the lower ΔES‑T, which was derived from the enhanced acceptor strength of bis-PXZ-TDZ. Thus, the ηdelayed values of the TADF-based OLEDs were sensitive to the ΔES‑T values of their emitters. The EQE versus J plot of bis-PXZ-TDZ exhibited larger rolloff at high J than that of bis-PXZ-OXD, as shown in Figure 7. The transient PL characteristics suggested that the large roll-off for bis-PXZ-TDZ resulted from exciton deactivation through triplet−triplet annihilation (TTA), derived from enhanced 1 CT1 → 3CT1 ISC. To confirm that roll-off resulted from TTA, we analyzed the EQE versus J plots using the fitting equation for TTA exciton deactivation:47 ηT − T = J0 /4J[ 1 + 8J /J0 − 1]η0

its EQE versus J plot. The observed photophysical properties suggested that sulfur in the TDZ acceptor possessed electronpair accepting conjugative ability, derived from its vacant 3dorbitals. This differed from the nature of p-type sulfurcontaining organic compounds. The accepted electron in the 3d-orbitals of sulfur must be directly influenced by spin−orbit coupling (SOC). The rate constant for S1 → T1 ISC (kISC) can be approximated by the golden rule for radiationless transition,26,48 which depends on the SOC matrix element with corresponding SOC Hamiltonian (HSOC). kISC and HSOC can be represented as24,26 kISC = 2π /ℏρ⟨Ψ(S)|HSOC|Ψ(T)2 ⟩ N

(8)

(9)

n

HSOC = α 2 ∑ ∑ Zμ/ri3μLiSi

where ηT‑T, η0 and J0 represent the external EL quantum efficiency in the presence of TTA, the external EL quantum efficiency in the absence of TTA, and the parameter for fitting the experimental data, respectively. Equation 8 does not include contributions of up-conversion of triplet excitons from 3CT1 to 1 CT1 based on TADF to the external EL quantum efficiencies (ηT‑T and η0).47 The TTA model was fitted to the EQE versus J plot of bis-PXZ-TDZ at the high J (Figure 8). η0 and J0 were

μ

i

(10)

where ρ, α, Z, riμ, Li, and Si are the Franck−Condon weighted density of states representing energy conservation, finestructure constant, effective charge on nucleus μ, distance between the ith electron and μth nucleus, orbital angular momentum of the ith electron and spin angular momentum of the ith electron, respectively. The kISC of bis-PXZ-TDZ was enhanced by the larger Z and L, over than those of bis-PXZOXD in the absence of d-orbitals. The origin of enhanced 1 CT1 → 3CT1 ISC for bis-PXZ-TDZ was attributed to the electron-pair accepting vacant 3d-orbitals of divalent sulfur in the TDZ acceptor. There was no remarkable change in the ϕRISC values of bis-PXZ-OXD and bis-PXZ-TDZ, which led to the large contributions of ηdelayed to ηext. This suggested two effects of sulfur on TADF. First, sulfur in the TDZ acceptor enhanced the ϕISC of bis-PXZ-TDZ by the internal heavy atom effect, derived from its vacant 3d-orbitals. This lowered the ηprompt and resulted in exciton deactivation through TTA, in the OLED device performance. Second, sulfur enhanced the acceptor strength of bis-PXZ-TDZ by the electron-pair accepting conjugative effect. This reduced the ΔES‑T of bisPXZ-TDZ, which increased the kRISC, and complemented the lowering of ηprompt with ηdelayed.

■

Figure 8. EQE versus J of bis-PXZ-TDZ (orange) with the fitting TTA model (black). The fitting parameters η0 and J0 were 6.5% and 2.1 mA/cm2, respectively.

CONCLUSION The effect of substituting oxygen for sulfur in a chalcogenodiazole acceptor-containing TADF emitter on radiationless ISC and reverse ISC was investigated. Substitution in the chalcogenodiazole heteroring endowed bis-PXZ-TDZ with additional electron accepting ability. This resulted from the conjugative effect because of sulfur’s vacant 3d-orbitals, and narrowed the HOMO−LUMO energy gap. This reduced the ΔES‑T, and enhanced 1CT1 → 3CT1 ISC. The enhanced 1 CT1 → 3CT1 ISC of bis-PXZ-TDZ lowered the ηprompt. However, the lower ΔES‑T increased the kRISC and contribution of ηdelayed (78%) to ηext. These are the first insights into radiationless transitions of TADF, with respect to molecular design.

6.5% and 2.1 mA/cm2, respectively. The fitted curve satisfactorily modeled the experimental data at J of 1.0−1.0 × 102 mA/cm2. This confirmed the increase of triplet excitons derived from enhanced 1CT1 → 3CT1 ISC in bis-PXZ-TDZ, and the exciton deactivation pathway through TTA. The difference between the experimental data and fitted curve at J of 1.0 × 10−2 to 1.0 mA/cm2 reflected the contribution of up-conversion of triplet excitons through 1CT1 ← 3CT1 reverse ISC to external EL quantum efficiency. Substitution of oxygen with sulfur enhanced the 1CT1 → 3 CT1 ISC of bis-PXZ-TDZ. This resulted in the large roll-off in 2953

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C

(9) Endo, A.; Ogasawara, M.; Takahashi, A.; Yokoyama, D.; Kato, Y.; Adachi, C. TADF from Sn4+−Porphyrin Complexes and Their Application to Organic Light-Emitting Diodes  A Novel Mechanism for Electroluminescence. Adv. Mater. 2009, 21, 4802−4806. (10) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient Up-Conversion of Triplet Excitons into a Singlet State and Its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98, 083302. (11) Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic LightEmitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nat. Photonics 2012, 6, 253−258. (12) Nakagawa, T.; Ku, S.-Y.; Wong, K.-T.; Adachi, C. Electroluminescence Based on TADF Generated by a Spirobifluorene Donor−Acceptor Structure. Chem. Commun. 2012, 48, 9580−9582. (13) Lee, S. Y.; Yasuda, T.; Nomura, H.; Adachi, C. High-Efficiency Organic Light-Emitting Diodes Utilizing TADF from Triazine-Based Donor−Acceptor Hybrid Molecules. Appl. Phys. Lett. 2012, 101, 093306. (14) Mehes, G.; Nomura, H.; Zhang, Q.; Nakagawa, T.; Adachi, C. Enhanced Electroluminescence Efficiency in a Spiro-Acridine Derivative through TADF. Angew. Chem., Int. Ed. 2012, 51, 11311− 11315. (15) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (16) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient TADF Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706−14709. (17) Masui, K.; Nakanotani, H.; Adachi, C. Analysis of Exciton Annihilation in High-Efficiency Sky-Blue Organic Light-Emitting Diodes with TADF. Org. Electron. 2013, 14, 2721−2726. (18) Li, J.; Nakagawa, T.; MacDonald, J.; Zhang, Q.; Nomura, H.; Miyazaki, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes Based on a Hidden TADF Channel in a Heptazine Derivative. Adv. Mater. 2013, 25, 3319−3323. (19) Ishimatsu, R.; Matsunami, S.; Shizu, K.; Adachi, C.; Nakano, K.; Imato, T. Solvent Effect on TADF by 1,2,3,5-Tetrakis(carbazol-9-yl)4,6-dicyanobenzene. J. Phys. Chem. A 2013, 117, 5607−5612. (20) Sato, K.; Shizu, K.; Yoshimura, K.; Kawada, A.; Miyazaki, H.; Adachi, C. Organic Luminescent Molecule with Energetically Equivalent Singlet and Triplet Excited States for Organic LightEmitting Diodes. Phys. Rev. Lett. 2013, 110, 247401. (21) Casado, J.; Oliva, M. M.; Delgado, M. C. R.; Ortiz, R. P.; Quirante, J. J.; Navarrete, J. T. L.; Takimiya, K.; Otsubo, T. Hybrid Organic Semiconductors Including Chalcogen Atom in p-Conjugated Skeletons. Tuning of Optical, Redox, and Vibrational Properties by Heavy Atom Conjugation. J. Phys. Chem. A 2006, 110, 7422−7430. (22) Gibson, G. L.; McCormick, T. M.; Seferos, D. S. Atomistic Band Gap Engineering in Donor−Acceptor Polymers. J. Am. Chem. Soc. 2012, 134, 539−547. (23) Nakano, M.; Mori, H.; Shinamura, S.; Takimiya, K. Naphtho[2,3-b:6,7-b’]dichalcogenophenes: Syntheses, Characterizations, and Chalcogene Atom Effects on Organic Field-Effect Transistor and Organic Photovoltaic Devices. Chem. Mater. 2012, 24, 190−198. (24) Lower, S. K.; El-Sayed, M. A. The Triplet State and Molecular Electronic Processes in Organic Molecules. Chem. Rev. 1966, 66, 199− 241. (25) Webster, S.; Peceli, D.; Hu, H.; Padilha, L. A.; Przhonska, O. V.; Masunov, A. E.; Gerasov, A. O.; Kachkovski, A. D.; Slominsky, Y. L.; Tolmachev, A. I.; et al. Near-Unity Quantum Yields for Intersystem Crossing and Singlet Oxygen Generation in Polymethine-like Molecules: Design and Experimental Realization. J. Phys. Chem. Lett. 2010, 1, 2354−2360. (26) Peceli, D.; Hu, H.; Fishman, D. A.; Webster, S.; Przhonska, O. V.; Kurdyukov, V. V.; Slominsky, Y. L.; Tolmachev, A. I.; Kachkovski, A. D.; Gerasov, A. O.; et al. Enhanced Intersystem Crossing Rate in Polymethine-Like Molecules: Sulfur-Containing Squaraines versus Oxygen-Containing Analogues. J. Phys. Chem. C 2013, 117, 2333− 2346.

We aim for a TADF OLED emitter exhibiting 100% conversion efficiency from all generated triplet excitons to the delayed fluorescence, through 1CT1 → S0 radiative transition. This requires HOMO−LUMO spatial separation to give a small ΔES‑T, for the up-conversion of triplet excitons through the successive reverse ISC. It also requires exciton deactivation, that converts all generated singlet excitons into luminescence through efficient 1CT1 → S0 radiative transition (krS and kRISC > kISC).

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and supplementary photophysical properties. CCDC 965103 for bis-PXZ-TDZ. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. K. Tokumaru for fruitful discussion. This research was supported by the Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST Program), and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. We thank Ms. H. Nomura and Ms. N. Nakamura for performing thermal analysis (TG-DTA and DSC) and sublimation measurements, Mr. T. Matsumoto for single-crystal X-ray crystal structure analysis, and Dr. M. Taneda for measuring OLED characteristics. Computations were partly carried out using facilities at the Research Institute for Information Technology, Kyushu University.

■

REFERENCES

(1) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364−367. (2) Bisri, S. Z.; Takenobu, T.; Yomogida, Y.; Shimotani, H.; Yamao, T.; Hotta, S.; Iwasa, Y. High Mobility and Luminescent Efficiency in Organic Single-Crystal Light-Emitting Transistors. Adv. Funct. Mater. 2009, 19, 1728−1735. (3) Perlstein, J. H. “Organic Metals”−The Intermolecular Migration of Aromaticity. Angew. Chem., Int. Ed. Engl. 1977, 16, 519−534. (4) Misaki, Y.; Higuchi, N.; Fujiwara, H.; Yamabe, T.; Mori, T.; Mori, H.; Tanaka, S. DTEDT)[Au(CN)2]0.4: An Organic Superconductor Based on the Novel π-Electron Framework of Vinylogous Bis-Fused Tetrathiafulvalene. Angew. Chem., Int. Ed. Engl. 1995, 34, 1222−1225. (5) Takimiya, K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi, Y. 2,7-Diphenyl[1]benzothieno[3,2-b]benzothiophene, A New Organic Semiconductor for Air-Stable Organic Field-Effect Transistors with Mobilities up to 2.0 cm2 V−1 s−1. J. Am. Chem. Soc. 2006, 128, 12604−12605. (6) Tanaka, H.; Shizu, K.; Miyazaki, H.; Adachi, C. Efficient Green TADF (TADF) from a Phenoxazine−Triphenyltriazine (PXZ−TRZ) Derivative. Chem. Commun. 2012, 48, 11392−11394. (7) Lee, J.; Shizu, K.; Tanaka, H.; Nomura, H.; Yasuda, T.; Adachi, C. Oxadiazole- and Triazole-Based Highly-Efficient TADF Emitters for Organic Light-Emitting Diodes. J. Mater. Chem. C 2013, 1, 4599− 4604. (8) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Twisted Intramolecular Charge Transfer State for Long-Wavelength TADF. Chem. Mater. 2013, 25, 3766−3771. 2954

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955

Article

The Journal of Physical Chemistry C (27) Jayanthi, S. S.; Ramamurthy, P. Excited Singlet State Reactions of Thiopyrylium with Electron Donors: Electron Transfer, Induction of Triplet by Internal and External Heavy Atom Effect, and Comparison of Pyrylium and Thiopyrylium Reactions. J. Phys. Chem. A 1998, 102, 511−518. (28) Ohulchanskyy, T. Y.; Donnelly, D. J.; Detty, M. R.; Prasad, P. N. Heteroatom Substitution Induced Changes in Excited-State Photophysics and Singlet Oxygen Generation in Chalcogenoxanthylium Dyes: Effect of Sulfur and Selenium Substitutions. J. Phys. Chem. B 2004, 108, 8668−8672. (29) Beljonne, D.; Shuai, Z.; Pourtois, G.; Bredas, J. L. Spin-Orbit Coupling and Intersystem Crossing in Conjugated Polymers: A Configuration Interaction Description. J. Phys. Chem. A 2001, 105, 3899−3907. (30) Ambre, R.; Chen, K.-B.; Yao, C.-F.; Luo, L.; Diau, E. W.-G.; Hung, C.-H. Effects of Porphyrinic meso-Substituents on the Photovoltaic Performance of Dye-Sensitized Solar Cells: Number and Position of p-Carboxyphenyl and Thienyl Groups on Zinc Porphyrins. J. Phys. Chem. C 2012, 116, 11907−11916. (31) Fonseca, S. M.; Pina, J.; Arnaut, L. G.; Seixas de Melo, J.; Burrows, H. D.; Chattopadhyay, N.; Alcacer, L.; Charas, A.; Morgado, J.; Monkman, A. P.; et al. Triplet-State and Singlet Oxygen Formation in Fluorene-Based Alternating Copolymers. J. Phys. Chem. B 2006, 110, 8278−8283. (32) King, S. M.; Matheson, R.; Dias, F. B.; Monkman, A. P. Enhanced Triplet Formation by Twisted Intramolecular ChargeTransfer Excited States in Conjugated Oligomers and Polymers. J. Phys. Chem. B 2008, 112, 8010−8016. (33) Frisch, M. J. et al. GAUSSIAN 09, revision C.01; Gaussian Inc.: Wallingford, CT, 2009. (34) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid ExchangeCorrelation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (35) Dunning, J. T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (36) Martin, R. J. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775−4777. (37) Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic Excitations Within the Adiabatic Approximation of Time Dependent Density Functional Theory. Chem. Phys. Lett. 1996, 256, 454−464. (38) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. Molecular Excitation Energies to High-Lying Bound States from TimeDependent Density-Functional Response Theory: Characterization and Correction of the Time-Dependent Local Density Approximation Ionization Threshold. J. Chem. Phys. 1998, 108, 4439−4449. (39) Köse, M. E. Evaluation of Acceptor Strength in Thiophene Coupled Donor−Acceptor Chromophores for Optimal Design of Organic Photovoltaic Materials. J. Phys. Chem. A 2012, 116, 12503− 12509 and references cited therein.. (40) Bordwell, F. G.; Boutan, P. J. Conjugative Effects in Divalent Sulfur Groupings. J. Am. Chem. Soc. 1956, 78, 854−860. (41) Oae, S.; Price, C. C. Effects of Dimethylsulfonio and Trimethylammonio Groups on the Dissociation of Substituted Phenols. J. Am. Chem. Soc. 1958, 80, 3425−3427. (42) Oae, S.; Price, C. C. The Effect of m-Dichloro and m-Dibromo Groups on the Dissociation and Ultraviolet Spectra of pDimethylsulfoniophenols. J. Am. Chem. Soc. 1958, 80, 4938−4941. (43) Oae, S.; Zalut, C. Steric Effects of m-Methyl Groups on the Conjugative Abilities of Thia, Sulfinyl and Sulfonio Functions in pSubstituted Phenols. J. Am. Chem. Soc. 1960, 82, 5359−5363. (44) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319−2358. (45) Parker, C. A.; Hatchard, C. G. Triplet-Singlet Emission in Fluid Solutions. Phosphorescence of Eosin. Trans. Faraday Soc. 1961, 57, 1894−1904. (46) Fister, J. C.; Harris, J. M. Time- and Wavelength-Resolved Delayed-Fluorescence Emission from Acridine Yellow in an Inhomogeneous Saccharide Glass. Anal. Chem. 1996, 68, 639−646.

(47) Adachi, C.; Baldo, M. A.; Forrest, S. R. Electroluminescence Mechanisms in Organic Light Emitting Devices Employing a Europium Chelate Doped in a Wide Energy Gap Bipolar Conducting Host. J. Appl. Phys. 2000, 87, 8049−8055. (48) Lawetz, V.; Orlandi, G.; Siebrand, W. Theory of Intersystem Crossing in Aromatic Hydrocarbons. J. Chem. Phys. 1972, 56, 4058− 4072.

2955

DOI: 10.1021/jp510751n J. Phys. Chem. C 2015, 119, 2948−2955