Tailoring Excited-State Properties and Electroluminescence

Jul 13, 2015 - The CT state of PTZ-10-AnP locates at the lowest excited state, while that of PTZ-10P-AnP stays at the higher excited state. For PTZ-3-...
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Tailoring Excited-State Properties and Electroluminescence Performance of Donor−Acceptor Molecules through Tuning the Energy Level of the Charge-Transfer State Liang Yao,† Yuyu Pan,† Xiangyang Tang,† Qing Bai,† Fangzhong Shen,† Feng Li,† Ping Lu,† Bing Yang,*,† and Yuguang Ma*,‡ †

State Key Lab of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, P. R. China Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China

‡

S Supporting Information *

ABSTRACT: Three donor−acceptor (D−A) compounds (PTZ-10-AnP, PTZ-10PAnP, and PTZ-3-AnP) were designed and synthesized through linking the same D and A units with different architectures. The structural change, mainly referring to the torsion angle and distance between D and A, gives rise to the significantly different CT level positions for these compounds. The CT state of PTZ-10-AnP locates at the lowest excited state, while that of PTZ-10P-AnP stays at the higher excited state. For PTZ-3AnP, the CT energy is close to that of the locally π−π* excited state, and a hybridized local and charge-transfer (HLCT) state dominates the S1 state. Photophysical experiment data of these compounds demonstrate that the energy level of the charge-transfer (CT) state plays a decisive role in the emissive state properties of donor−acceptor (D−A) compounds. In addition, the exciton utilization efficiency of the PTZ-3-AnP device exceeds the limit of the spin statistics, which enlightens the molecular design toward harvesting triplet excitons in fluorescent organic light-emitting diodes (OLEDs).

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Particularly, phenothiazine contains two sp3-hybridized heteroatoms (S, N) in the heterocyclic structure, which leads to a nonplanar butterfly-shaped geometry.27 This molecular configuration endows the 3,7-position and 10-position of phenothiazine with significantly different steric hindrances. As a result, the D−A torsion angle should be of difference when introducing the same acceptor to the 3,7-position and 10position of phenothiazine. Through changing the torsion angle and distance between the donor and acceptor, the energy level of the CT state could be adjusted to a certain degree. Hence, a simple relationship between luminescence property and CT state energy level may be established in the D−A system based on phenothiazine with different linkages. In this contribution, we selected anthracene as the acceptor and phenothiazine as the donor for D−A type molecule design. Anthracene, bearing high luminous efficiency and favorable carrier transport property, is one of the most widely used building blocks in the field of organic electroluminescence.28−37 Through the Ullman reaction and Suzuki coupling reaction, anthracene was introduced into the 10-position (PTZ-10-AnP) and 3-position (PTZ-3-AnP) of phenothiazine, respectively. Motivated by adjusting the D−A interaction distance of PTZ-

INTRODUCTION Recently, substantial progress has been made in the field of organic light-emitting diodes (OLEDs), that is, employing triplet excitons through a reverse intersystem crossing (RISC) process in fluorescent OLEDs (FOLEDs), which will certainly bring about exceeding the spin statistical limit of singlet excitons.1−10 This phenomenon is mainly concentrated in donor−acceptor (D−A) molecules with unique excited-state properties. The combination of D and A moieties often results in the formation of a charge-transfer (CT) excited state.11−14 The CT state along with the π−π* state constitute the energy level system in the D−A compound, which determines the optical and electronic properties of the compound. Generally, CT states possess weak binding energy due to their spacially separated transition orbitals.15 This intrinsic merit facilitates the RISC process of triplet excitons.16 On the other hand, the separated transition orbitals of CT states can cause undesirably low radiative transition rates and photoluminescence efficiency.17,18 Consequently, for D−A molecules, their CT energy levels have crucial effects on the final emissive state properties. To deeply understand the relationship between CT energy level and excited-state properties, a systematical modulation for CT energy level in D−A molecules is required. As for molecular design, phenothiazine has been widely used as a donor moiety because of its low ionization potential (around 5.0 eV) and strong electron-donating ability.19−26 © 2015 American Chemical Society

Received: April 27, 2015 Revised: July 6, 2015 Published: July 13, 2015 17800

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

Article

The Journal of Physical Chemistry C

configuration of the ground state (S0), the high excitation energy levels of singlet and triplet states were evaluated using TD-M06-2X/6-31G(d, p). In order to examine the character of excited states, natural transition orbitals (NTOs) were evaluated for S1 states. This approach provides the most compact representation of the electronic transitions in terms of an expansion into single-particle orbitals by diagonalizing the transition density matrix associated with each excitation. Lippert−Mataga Calculation. The impact of solvent polarity on the photophysical properties of PTZ-10P-AnP and PTZ-3-AnP was analyzed by the Lippert−Mataga equation, which is a model that describes the interactions between the solvent and the dipole moment of solute

10-AnP, a phenylene was inserted between the 10-position of phenothiazine and anthracene, obtaining another compound PTZ-10P-AnP. Photophysical measurement (UV−vis, PL, and time-resolved PL) indicated these three molecules exhibited completely different spectral characteristics. On the basis of the photophysical data and DFT calculations, the relative location relationship of the CT state and π−π* state was analyzed in these D−A molecules. Finally, we demonstrated the effect on excited-state properties and electroluminescence properties of D−A molecules by tuning the CT energy levels.

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EXPERIMENTAL SECTION General Information. All the reagents and solvents used for the synthesis were purchased from Aldrich and Acros companies and used without further purification. 1H and 13C NMR spectra were recorded on a Bruker AVANCE 500 spectrometer at 500 and 125 MHz, respectively, using tetramethylsilane (TMS) as the internal standard. The element contents of compounds were characterized by a Flash EA 1112, CHNS-O elemental analysis instrument. The MALDI-TOF-MS mass spectra were measured using an AXIMA-CFRTM plus instrument. UV−vis spectra were tested on a Shimadzu UV3100 spectrophotometer using 1 cm path length quartz cells. Steady state fluorescence spectra, fluorescence lifetime, and photoluminescence quantum yield were carried out with an FLS980 spectrometer. The fluorescence lifetime was measured by using the time-correlated single-photon counting (TCSPC) technique with EPL picosecond pulsed diode lasers (375 nm, 68.9 ps) as the excitation source. The delayed PL spectra were recorded on a LP920 laser flash photolysis spectrometer. Device Fabrication. The device structure utilized for PTZ10P-AnP and PTZ-3-AnP is indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB)/PTZ-10P-AnP or PTZ-3-AnP/1,3,5-tris(1phenyl-1H-benzimidazol-2-yl)benzene (TPBi)/lithium fluoride (LiF)/aluminum (Al). PEDOT:PSS (4083) was purchased from Heraeus. NPB, TPBi, and LiF were purchased from Luminescence Technology Corporation and used without further purification. ITO-coated glass with a sheet resistance of 20 Ω square−1 was used as the substrate. Before device fabrication, the ITO glass substrates were cleaned with isopropyl alcohol and deionized water, dried in an oven at 120 °C, and treated with UV-zone for 20 min. An amount of 40 nm of PEDOT:PSS layer was first spin-coated on the ITO substrate from water solution and then annealed at 120 °C for 30 min. After that, the substrate was transferred to a vacuum deposition system with a base pressure lower than 5 × 10−6 mbar for organic and metal deposition. The hole-transporting material (NPB, 50 nm), emissive materials (PTZ-10P-AnP and PTZ-3-AnP, 20 nm), and electron-transporting material (TPBi, 30 nm) were thermally evaporated at a rate of 1.0 Å s−1. After the organic film deposition, 0.5 nm of LiF and 120 nm of Al were thermally evaporated onto the organic surface. The electroluminescence (EL) characteristics were measured using a Keithley 2400 programmable electrometer and a PR-650 Spectrascan. External quantum efficiencies (EQEs) were calculated from EL results according to the literature, including the current density, luminance, and EL spectrum.38,39 Computational Details. The ground-state geometries were optimized at the level of B3LYP/6-31G(d, p), which is wellknown to provide molecular geometries in good agreement with the experiment. On the basis of the optimized

hc(νa − νf ) = hc(νa0 − νf0) −

2(μe − μg )2 a3

f (ε , n)

where f is the orientational polarizability of the solvent; ν0a −ν0f corresponds to the Stokes shifts when f is zero; μe is the excited-state dipole moment; μg is the ground-state dipole moment; a is the solvent cavity (Onsager) radius, derived from the Avogadro number (N), molecular weight (M), and density (d = 1.0 g/cm3); ε and n are the solvent dielectric and the solvent refractive index, respectively; and f(ε,n) and a can be calculated, respectively, as follows f (ε , n) =

ε−1 n2 − 1 − 2 2ε + 1 2n + 1

a = (3M /4Nπd)1/3

Synthesis. N-(10-Phenylanthracen-9-yl)phenothiazine (PTZ-10-AnP). Phenothiazine (199 mg, 1 mmol) and 9bromo-10-phenylanthracene (366 mg, 1.1 mmol) were dissolved in toluene (10 mL) and degassed with nitrogen. Pd2(dba)3 (18 mg, 0.02 mmol) and HPtBu3BF4 (17 mg, 0.06 mmol) were added, and the mixture was stirred for 10 min. Sodium tert-butoxide (115 mg, 1.2 mmol) was added, and the mixture was stirred at 110 °C for 48 h. After cooling to room temperature, the resulting mixture was extracted with dichloromethane followed by purification by column chromatography on silica gel with petroleum ether/dichloromethane (10:1) as the eluent to offer a yellow solid. The desired compound was obtained in 32% yield (145 mg). 1H NMR (500 MHz, DMSOd6, 25 °C, TMS): δ = 8.21 (d, J = 8.8 Hz, 2H; Ar H), 7.70 (m, 4H; Ar H), 7.64 (m, 3H; Ar H), 7.54 (m, 4H; Ar H), 7.13 (d, J = 7.6 Hz, 2H; Ar H), 6.83 (t, J = 7.4 Hz, 2H; Ar H), 6.73 (t, J = 7.6 Hz, 2H; Ar H), 5.70 (d, J = 8.2 Hz, 2H; Ar H). 13C NMR (125 MHz, THF-d8, 25 °C, TMS): δ = 142.44 (C), 141.54 (C), 137.19 (C), 136.54 (C), 129.35 (C), 129.20 (CH), 128.07 (C), 126.51 (CH), 125.85 (CH), 125.76 (CH), 125.08 (CH), 125.04 (CH), 124.38 (CH), 123.81 (CH), 121.87 (CH), 120.55 (CH), 117.88 (C), 114.01 (CH). MALDI-TOF MS (mass m/z): 452.1 [M+]. Anal. Calcd for C32H21NS: C, 85.11; H, 4.69; N, 3.10; S, 7.10. Found: C, 85.24; H, 4.56; N, 3.08; S, 7.21. N-(4-(10-Phenylanthracen-9-yl)phenyl)phenothiazine (PTZ-10P-AnP). Phenothiazine (67 mg, 0.34 mmol) and 9-(4bromophenyl)-10-phenylanthracene (151 mg, 0.37 mmol) were dissolved in toluene (10 mL) and degassed with nitrogen. Pd2(dba)3 (9 mg, 0.01 mmol) and HPtBu3BF4 (5 mg, 0.017 mmol) were added, and the mixture was stirred for 10 min. Sodium tert-butoxide (37 mg, 0.39 mmol) was added, and the mixture was stirred at 110 °C for 48 h. After cooling to room 17801

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

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The Journal of Physical Chemistry C temperature, the resulting mixture was extracted with dichloromethane followed by purification by column chromatography on silica gel with petroleum ether/dichloromethane (7:1) as the eluent to offer a white solid. The desired compound was obtained in 67% yield (130 mg). 1H NMR (500 MHz, DMSOd6, 25 °C, TMS): δ = 7.74 (m, 4H; Ar H), 7.69 (m, 4H; Ar H), 7.63 (m, 3H; Ar H), 7.50 (m, 6H; Ar H), 7.20 (d, J = 7.6 Hz, 2H; Ar H), 7.15 (t, J = 7.1 Hz, 2H; Ar H), 6.99 (t, J = 7.5 Hz, 2H; Ar H), 6.62 (d, J = 8.2 Hz, 2H; Ar H). 13C NMR (125 MHz, THF-d8, 25 °C, TMS): δ = 142.42 (C), 139.32 (C), 137.24 (C), 136.57 (C), 135.56 (C), 134.20 (C), 131.67 (CH), 129.25 (CH), 128.07 (C), 128.01 (C), 127.66 (CH), 126.51 (CH), 125.57 (CH), 125.00 (CH), 124.95 (CH), 124.86 (CH), 124.61 (CH), 123.30 (CH), 123.07 (CH), 122.21 (C), 120.89 (CH), 115.43 (CH). MALDI-TOF MS (mass m/z): 527.8 [M+]. Anal. Calcd for C38H25NS: C, 86.49; H, 4.78; N, 2.65; S, 6.08. Found: C, 86.61; H, 4.66; N, 2.54; S, 5.98. 10-Phenyl-3-(10-phenylanthracen-9-yl)phenothiazine (PTZ-3-AnP). N-Phenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenothiazine (602 mg, 1.5 mmol), 9-bromo-10phenylanthracene (600 mg, 1.8 mmol), 12 mL of dry toluene, and 8 mL of aqueous Na2CO3 solution (2.0 M) were placed in a 50 mL round-bottom flask. Pd(PPh3)4 (52 mg, 0.045 mmol) was added, and the mixture was vigorously stirred at 85−90 °C for 2 days. After cooling to room temperature, the resulting mixture was extracted with dichloromethane followed by purification by column chromatography on silica gel with petroleum ether/dichloromethane (8:1) as the eluent to offer a green solid. The desired compound was obtained in 59% yield (467 mg). 1H NMR (500 MHz, DMSO-d6, 25 °C, TMS): δ = 7.75 (m, 2H; Ar H), 7.69 (d, J = 7.9 Hz, 2H; Ar H), 7.65 (t, J = 6.4 Hz, 2H; Ar H), 7.61 (m, 4H; Ar H), 7.55 (m, 2H; Ar H), 7.43 (m, 6H; Ar H), 7.20 (s, 1H; Ar H), 7.14 (d, J = 7.5 Hz, 1H; Ar H), 7.04 (d, J = 8.3 Hz, 1H; Ar H), 7.00 (m, 1H; Ar H), 6.92 (t, J = 8.0 Hz, 1H; Ar H), 6.41 (d, J = 8.3 Hz, 1H; Ar H), 6.24 (t, J = 8.3 Hz, 1H; Ar H). 13C NMR (125 MHz, THF-d8, 25 °C, TMS): δ = 142.60 (C), 142.04 (C), 139.22 (C), 137.93 (C), 137.29 (C), 135.17 (C), 133.98 (C), 133.41 (C), 131.34 (C), 129.27 (CH), 129.23 (CH), 129.20 (CH), 128.97 (CH), 128.12 (C), 128.02 (C), 127.82 (CH), 127.25 (CH), 126.52 (CH), 126.46 (CH), 125.48 (CH), 124.94 (CH), 124.79 (CH), 124.70 (CH), 123.00 (CH), 122.98 (CH), 120.58 (CH), 114.08 (CH), 113.82 (CH). MALDI-TOF MS (mass m/z): 527.8 [M+]. Anal. Calcd for C38H25NS: C, 86.49; H, 4.78; N, 2.65; S, 6.08. Found: C, 86.69; H, 4.75; N, 2.48; S, 5.89.

Scheme 1. Synthetic Routes of PTZ-10-AnP, PTZ-10P-AnP, and PTZ-3-AnPa

(i) Pd2(dba)3, HPtBu3BF4, NaOtBu, toluene, 110 °C, 48 h; (ii) Pd(PPh3)4, Na2CO3, toluene, H2O, 90 °C, 48 h.

a

DFT Simulated Geometries. To gain a further insight into the molecular and electronic structures of these compounds, their ground-state geometries in the gas phase were optimized by DFT-B3LYP/6-31G** calculation. As shown in Figure 1a,

Figure 1. (a) DFT-optimized geometries and D−A torsion angles of PTZ-10-AnP, PTZ-10P-AnP, and PTZ-3-AnP. (b) The ground-state energies of these molecules with different D−A torsion angles.

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all the compounds take highly twisted configurations with the torsion angles (θ1, θ2, and θ3) between phenothiazine and its aryl substituents of 85°, 82°, and 77°, respectively. The reason for these large torsion angles is due to the steric hindrance effect of the hydrogens in the 1,9-position of phenothiazine and 1,4,5,8-position of anthracene. Since the torsion angle may change in actual conditions (such as aggregated state, solvent phase), the ground-state energies at different D−A torsion angles were simulated to understand the changing trend of θ1, θ2, and θ3 (Figure 1b). With rotating the D−A tosion angle in the range of 60−120°, PTZ-10-AnP needs to overcome the largest energy barrier (114 meV) among the three compounds, indicating PTZ-10-AnP possesses the most rigid configuration. In the case of PTZ-10P-AnP, the energy barrier decreases to 88 meV due to the less steric hindrance effect of phenylene than anthracene. Different from θ1 and θ2, merely 18 meV is needed when θ3 varies in the range of 60−120°. The simulated results

RESULTS AND DISCUSSION Synthetic Procedures. Scheme 1 illustrates the synthetic procedures of PTZ-10-AnP, PTZ-10P-AnP, and PTZ-3-AnP. The synthesis of precursor reactants can be found in the Supporting Information. PTZ-10-AnP and PTZ-10P-AnP were obtained through Ullmann reactions between phenothiazine and 9-bromo-10-phenylanthracene and 9-(4-bromophenyl)-10phenylanthracene, respectively, and the corresponding yield is 32% and 67%. The preparation of PTZ-3-AnP was achieved by Suzuki coupling reaction between 3-bromo-N-phenyl-phenothiazine and 4,4,5,5-tetramethyl-2-(10-phenylanthracen-9-yl)1,3,2-dioxaborolane in the yield of 59%. The molecular structures of PTZ-10-AnP, PTZ-10P-AnP, and PTZ-3-AnP were confirmed by 1H and 13C NMR, MS, and elemental analysis. 17802

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

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The Journal of Physical Chemistry C indicate the θ3 of PTZ-3-AnP has better flexibility. The difference in molecular configurations of these compounds may cause the different excited-state properties. Electron Transition Properties (S0 → S1). Natural transition orbitals (NTOs) for the S1 state of the compounds are depicted in Figure 2. The S1 state of PTZ-10-AnP is a

Figure 3. Absorption spectra of the compounds in THF at room temperature.

UV spectra of PTZ-10-AnP, and corresponding ε is below 1000 M−1 cm−1. The weak absorption belongs to a forbidden electron transition and could be assigned to a CT band with extremely small orbital overlap. For PTZ-10P-AnP, since a wide optical band gap of 3.03 eV is obtained and the UV spectra are nearly identical to that of 9,10-diphenylanthracene (Figure S1), this characteristic absorption is mainly attributed to the LE state transition of the anthracene unit. The CT transition in PTZ-10P-AnP may locate at higher energy level due to the increased distance between phenothiazine and anthracene. In contrast, the UV spectra of PTZ-3-AnP are obviously broadened in the long-wavelength range with a reduced optical band gap of 2.94 eV. The result indicates certain CT components are introduced in the S0 → S1 electron transition, and the absence of an absorption shoulder indicates the LE components and CT components have been mixed or hybridized into one state. Fluorescence Properties. PTZ-10-AnP does not emit fluorescence upon optical excitation (Figure S2). This is because the entirely separated transition orbitals lead to an extremely low radiative transition rate. The fluorescence properties of PTZ-10P-AnP and PTZ-3-AnP were characterized in different solvents through steady and transient PL spectra. The fluorescence photographs of PTZ-10P-AnP and PTZ-3AnP in different solvents are depicted in Figure 4a and Figure 5a. As shown in Figure 4b, the PL spectra of PTZ-10P-AnP are dependent on the solvent. In nonpolar hexane, PTZ-10P-AnP exhibits a LE emission with a single peak at 430 nm. In polar solvents, a second fluorescence band appears at long wavelengths, and the LE emission maintains at the same time. The PL quantum yield declines rapidly as solvent polarity increased: 0.41 in hexane, 0.06 in toluene, 0.04 in ether, and 0.02 in THF. Since the low-energy emission is significantly red-shifted and its lifetime is enlarged with an increase of solvent polarity (Figure 4c), this emerging band could be assigned to a CT emission.41−43 The dipole moment (μe) of the CT band could be calculated by the Lippert−Mataga equation. The plot of Stokes shift versus orientation polarizability (Δf) exhibits a linear relationship (slope value ∼38 237, R = 0.95) as shown in Figure S3, corresponding to the CT state dipole moment of 28.2 D. The LE emission shows the biexponential decay in polar solvents (such as toluene, ether, and THF), including a fast-decay time constant of ∼500 ps and a slow-decay time constant of ∼5 ns (Figure S4). The slow decay is originated from the LE emission, and the fast decay can be attributed to

Figure 2. Natural transition orbitals (S0 → S1) of the molecules.

“pure” CT state. The hole and particle are almost entirely separated and localize at phenothiazine and anthracene, respectively. Owing to the nearly orthogonal configuration between phenothiazine and anthracene, the electron transition from hole to particle is forbidden, and the oscillator strength is close to zero. In contrast, the S1 state of PTZ-10P-AnP exhibits typical locally excited (LE) state property. Both the hole and particle localize at anthracene, and the S1 state primarily derives from the π−π* transition of anthracene. Due to the large orbital overlap between the hole and particle, the oscillator strength (0.2492) is significantly increased compared to that of PTZ-10-AnP. The calculated result demonstrates that the introduction of orthogonal phenylene seriously decreases the orbital overlap between phenothiazine and anthracene, which elevates the CT state energy level in PTZ-10P-AnP and makes the LE state expose to be the lowest excited state. For the S1 state of PTZ-3-AnP, certain spatial separation is observed between hole and particle, and large orbital overlap is obtained simultaneously. The result demonstrates that CT transition from phenothiazine to anthracene is introduced in the S1 state of PTZ-3-AnP, while LE properties are maintained by the large orbital overlap between the hole and particle. Finally, the S1 state of PTZ-3-AnP exhibits an intermediate state between the LE state and CT state. The oscillator strength is further promoted to 0.2995 with respect to PTZ-10P-AnP, which can be explained in terms of the relatively small D−A torsion angle and the enhanced π conjugation between phenothiazine and anthracene. Furthermore, the UV spectra of these compounds are in accordance with simulated NTOs of the S1 state. As shown in Figure 3, all the compounds exhibit well-resolved vibronic fine structured absorption peaks (ε = 9760−15 000 M−1 cm−1) at 395, 375, and 355 nm, respectively. These structured absorptions could be attributed to π−π* transition (LE state) of the rigid anthracene unit.40 Differently, a very weak absorption shoulder appears at the low energy level in the 17803

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

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acetonitrile. The plot of Stokes shift versus Δf shows two sets of linearity (slope value: 9480 and 26 491), indicative of two different excited states in high- and low-polarity solvents (Figure S6). The μe is estimated to be 23.5 and 14.0 D, respectively. In high polarity solvents, a CT state is dominant due to the large dipole moment (23.5 D) and low fluorescence quantum yield (below 0.04). In low-polarity solvents, the S1 state combines certain dipole moment (14.0 D) and relatively high fluorescence quantum yield (between 0.25 in hexane and 0.08 in isopropyl ether), indicating that CT and LE components coexisted simultaneously. Moreover, singleexponential decay is observed for PTZ-3-AnP in different solvents (Figure S7), and the lifetime in the same solvent is independent of the measured wavelength (Figure 5c), which indicates LE and CT states are effectively mixed or hybridized into a new state. We name this unique state as hybridized local and charge-transfer (HLCT) state.44−46 According to the statemixing principle in quantum chemistry, the state mixing or hybridization can be described as the linear combination of two initial states (eq 1), and the degree of two-state mixing is relative to the mixing coefficient (λ, eq 2).47

Figure 4. Fluorescence properties of PTZ-10P-AnP in different solvents at room temperature. (a) The fluorescence photograph taken under UV light. The solvents from a to f are hexane, toluene, ether, ethyl acetate, THF, and dichloromethane. (b) The PL spectra of PTZ10P-AnP in different solvents with the excitation of 375 nm. (c) Lifetime measurement of the CT emission. The fluorescence lifetime values are 5.9 ns (toluene at 540 nm), 7.2 ns (ether at 550 nm), and 10.5 ns (THF at 610 nm).

ψ (S1) = ψ (LE) + λ × ψ (CT)

λ=

(1)

ψLE|H |ψCT ECT − E LE

(2)

Generally, λ is inversely proportional to the energy gap between initial states and directly proportional to the spatial wave function overlap of two states. The effective hybridization of the LE state and CT state demonstrates that the energy level of the CT state is very close to that of the LE state in PTZ-3-AnP. On the basis of the photophysical measurements, we can conclude that the excited -state properties of these three compounds are completely different. A schematic illustration can be found in Figure 6. In PTZ-10-AnP, phenothiazine and

Figure 5. Fluorescence properties of PTZ-3-AnP in different solvents at room temperature. (a) The fluorescence photograph taken under UV light. The solvents from a to f are hexane, toluene, ether, ethyl acetate, THF, and acetonitrile. (b) The PL spectra of PTZ-3-AnP in different solvents with the excitation of 375 nm. (c) Lifetime measurement of PTZ-3-AnP at different wavelength. The fluorescence lifetime values are 1.0 ns (toluene at 500 nm) and 1.0 ns (toluene at 560 nm).

Figure 6. Potential energy surface scheme for the LE state and CT state of these molecules in nonpolar solvents.

anthracene are connected directly in an orthogonal configuration, which causes the CT energy level to fall below the LE energy level. Due to the low radiative rate from the completely separated transition orbital, no fluorescence behavior is observed for PTZ-10-AnP. In PTZ-10P-AnP, phenothiazine and anthracene are separated by a phenylene, and both of them are orthogonal with the phenylene, which certainly increases the energy of the CT state and results in the formation of an energy barrier for the interconversion between the LE state and CT state. Consequently, PTZ-10P-AnP shows only LE emission in film or low-polarity solvents. As solvent polarity increased, the stabilization of the CT state makes the energy level of the CT state decline gradually. However, since the

the energy transfer from LE emission to CT emission. In addition, no separated CT emission is observed in the PL spectra of doped and undoped PTZ-10P-AnP film (Figure S5). This result implies that the CT state of PTZ-10P-AnP in the aggregated state stays at the high-lying energy level, similar to the situation in low-polarity solvents. In contrast, PTZ-3-AnP displays individual emissions in different solvents (Figure 5b), and the PL spectrum exhibits a remarkable red-shift as the polarity of the solvent increased: from 462 nm in nonpolar hexane to 650 nm in polar 17804

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

Article

The Journal of Physical Chemistry C

while the ηS value of the PTZ-3-AnP device reaches 62%, which greatly exceeds the limit of 25% for conventional FOLEDs. We tried to explore the reason for the high ηS of the PTZ-3AnP device. First, the S1/T1 energy gap was determined by the fluorescence spectra and phosphorescence spectra of the compounds. As shown in Figure 8a, we did not obtain the phosphorescence of PTZ-10P-AnP or PTZ-3-AnP solutions at 77 K. After PtOEP was added as phosphorescent sensitizer, PTZ-10P-AnP and PTZ-3-AnP radiated phosphorescence with the wavelength over 700 nm (Figure 8b). The emission peak at 650 nm in the delayed PL spectra can be assigned to the phosphorescence of PtOEP (Figure S8). The S1/T1 energy gaps of PTZ-10P-AnP and PTZ-3-AnP are determined as 1.07 and 0.89 eV, respectively. These large S1/T1 energy gaps cannot allow the RISC process from the T1 state to the S1 state.1−4 In addition, since a linear relationship is found between the luminance and current density of PTZ-3-AnP (Figure S9), the high ηS could not be attributed to the triplet−triplet annihilation (TTA) process.48 We also calculated and analyzed the energy levels of the excited states of PTZ-10P-AnP and PTZ-3-AnP. As shown in Figure S10, a large S1/T1 energy gap of 1.34 and 1.32 eV is estimated for PTZ-10P-AnP and PTZ-3AnP, respectively, which is basically consistent with the experiment results. More especially, both PTZ-10P-AnP and PTZ-3-AnP possess a large T2/T1 energy gap and a small S1/T2 energy splitting. The large energy gap between the T2 state and T1 state could significantly decrease the triplet internal conversion rate (from the T2 state to T1 state) according to the energy gap law.49,50 At the same time, the small S1/T2 energy gap could offer the potential for the RISC process along the high-lying excited states (from the T2 state to S1 state). In fact, recently we have demonstrated that a series of D−A molecules with similar excited state energy levels could achieve a very high ηS, namely, “hot exciton” mechanism.7−9,44−46 The results in this work, higher ηS in PTZ-3-AnP device, reveal that the excited state of PTZ-3-AnP with certain CT components effectively promotes the harvest of triplet excitons because of the weak binding energy of CT excitons, while for PTZ-10PAnP the LE state character with strong binding energy is not conducive to realize the spin flip conversion in electroluminescence. Nevertheless, although high ηS has been achieved in the PTZ-3-AnP device, the total performance was restricted by the low fluorescent quantum efficiency (7%), which may be caused by the relatively large torsion angle of 77°. Our present work focuses on decreasing the torsion angle and improving the fluorescent quantum efficiency.

extremely small orbital overlap between phenothiazine and anthracene leads to a very small mixing coefficient between the LE state and CT state in PTZ-10P-AnP, the hybridization of the CT state and LE state cannot occur even if the energy levels of the CT state and LE state are nearly equal in toluene. Hence, dual fluorescence is observed for PTZ-10P-AnP in polar solvents. Different from PTZ-10P-AnP, the less orthogonal configuration and flexible θ3 in PTZ-3-AnP certainly provide much more transition orbital overlap between phenothiazine and anthracene, and therefore, the LE state and CT state in PTZ-3-AnP could effectively hybridize into the HLCT state. Electroluminescence Performances. We investigated the performance of the devices using PTZ-10P-AnP and PTZ-3AnP as the emissive material, respectively. The data of device performance are summarized in Figure 7 and Table 1. The

Figure 7. Luminous efficiency−luminance characteristics of the devices based on PTZ-10P-AnP and PTZ-3-AnP.

PTZ-10P-AnP device exhibits a turn-on voltage of 4.4 V, maximum luminous efficiency of 1.14 cd A−1, and maximum brightness of 3960 cd m−2. The device based on PTZ-3-AnP shows significantly improved performance with the turn-on voltage of 3.2 V, maximum luminous efficiency of 2.14 cd A−1, and maximum brightness of 14 060 cd m−2. In addition, the EQE of the PTZ-3-AnP device only decreases by 12% at high brightness of 10 000 cd m−2, indicating that the PTZ-3-AnP device possesses high operation stability. The radiative exciton ratio (ηS) in these FOLEDs are evaluated by the following equation ηext = γηSηPL ηout where ηS is the radiative exciton ratio; ηext is the external quantum efficiency; ηout is the light out-coupling efficiency (∼20%); ηPL is the intrinsic photoluminescence efficiency (12% for PTZ-10P-AnP, 7% for PTZ-3-AnP); and γ is the recombination efficiency of injected holes and electrons which is ideally 100% only if holes and electrons are fully balanced and completely recombined to form excitons. Therefore, the ηS value of the PTZ-10P-AnP device is 24%,

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CONCLUSION In conclusion, we constructed three D−A molecules with completely different excited states based on the different connections of phenothiazine and anthracene. The experimental and theoretical results demonstrated the excited-state properties are closely related to the CT energy levels. In

Table 1. Electroluminescence Characteristics of the Devices LE [cd A−1] device

Von [V]

PTZ-10P-AnP PTZ-3-AnP

4.4 3.2

a

max

1000 cd m−2

10000 cd m−2

EQE [%]

brightness [cd m−2]

EL [nm]b

1.14 2.14

0.85 1.87

----1.87

0.58 0.87

3960 14060

492 492

Calculated with a luminance of 1 cd m−2. bMeasured at 5 V. Device structure: ITO/PEDOT:PSS (40 nm)/NPB (50 nm)/EML (20 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al (120 nm).

a

17805

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

Article

The Journal of Physical Chemistry C

Figure 8. PL spectra and delayed PL spectra of PTZ-10P-AnP and PTZ-3-AnP at 77 K. PTZ-10P-AnP and PTZ-3-AnP were dissolved in chloroform with the concentration of 5 × 10−5 mol L−1. In the delayed PL spectra, PtOEP as the phosphorescent sensitizer was added with the same concentration with PTZ-10P and PTZ-3-AnP. The emission peak at 650 nm in the delayed PL spectra was attributed to the phosphorescence of PtOEP.

PTZ-10-AnP, a “pure” CT state located at the S1 state leads to no fluorescence emitting due to the forbidden transition. For PTZ-10P-AnP, a phenylene as a spacer was introduced between phenothiazine and anthracene, which decreased the D−A orbital overlap and lifted the CT energy level. As a result, only LE fluorescence can be observed in the low-polarity solvents or aggregated state, while the CT transition appeared at the highlying energy level. For PTZ-3-AnP, the S1 state is a HLCT state, which originated from the less orthogonal configuration and flexible θ3. The HLCT state combined the certain dipole moment and large orbital overlap, resulting in an individual fluorescence emission with high radiative exciton ratio. It is of great significance to tune the excited-state property by simply tailoring the energy level of the CT state. The foregoing results not only facilitate an understanding of the relationship between excited-state properties and CT state energy in a D−A system but also provide a novel approach for harvesting triplet excitons in fluorescent molecules.

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51473063) and National Basic Research Program of China (973 Program grant number 2013CB834705, 2013CB834801, 2015CB655003).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03996. The synthesis and purification procedures for reactants; lifetime measurement for the LE band of PTZ-10P-AnP in different solvents; and linear correlation of orientation polarization (Δf) of solvent media with the Stokes shift (υa−υf) for the CT emission of PTZ-10P-AnP and PTZ3-AnP (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from National Science Foundation of China (grant number 91233113, 51273078, 17806

DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808

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DOI: 10.1021/acs.jpcc.5b03996 J. Phys. Chem. C 2015, 119, 17800−17808