Enhancing Fluorescence of Naphthalimide Derivatives by

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Enhancing Fluorescence of Naphthalimide Derivatives by Suppressing the Intersystem Crossing Wenqiang Zhang, Yuwei Xu, Muddasir Hanif, Shitong Zhang, Jiadong Zhou, Dehua Hu, Zengqi Xie, and Yuguang Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07513 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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The Journal of Physical Chemistry

Enhancing Fluorescence of Naphthalimide Derivatives by Suppressing the Intersystem Crossing Wenqiang Zhang, †, ‡, § Yuwei Xu, ‡, § Muddasir Hanif,‡ Shitong Zhang,† Jiadong Zhou,‡ Dehua Hu,*,‡ Zengqi Xie,‡ Yuguang Ma*,‡ † State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 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. ABSTRACT: The control over intersystem crossing (ISC) during an excited state of an organic molecule is critical to realize high-efficiency fluorescent materials. We report newly designed naphthalimide (NMI) based emitters (Ph-NMI and TPA-NMI), in which the photoluminescence (PL) efficiency increased by suppressing the intersystem crossing (ISC). The experimental and theoretical analysis revealed that the enlarged energy gap between the lowest singlet state (S1) and lowest triplet state (T1), and the mismatched electronic configuration of these two states can effectively suppress the ISC process in Ph-NMI and TPA-NMI as compared to the original HNMI. Moreover, the electronic configuration of S1-state changed to the π-π* transition in PhNMI and TPA-NMI (n-π* tansition in H-NMI), resulting in large radiation transition rate constant (Kr). Therefore, Ph-NMI and TPA-NMI exhibit significantly improved PL efficiencies when compared with H-NMI. The optimized OLEDs using TPA-NMI as yellow emitting layers in doped device showed high performance with a maximum external quantum efficiency of

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5.96%, which is the best device performance ever achieved based on the naphthalimide derivatives. 1. Introduction Organic luminescent materials with high photoluminescence (PL) efficiencies play central role for the next generation flat-panel displays and solid-state lighting sources.1-6 Generally, the fluorescent efficiencies of organic materials depend on several types of photophysical process, such as radiative transition, intersystem crossing (ISC) and internal conversion (IC). In some cases, the ISC from radiative lowest singlet excited state (S1) to non-radiative lowest triplet state (T1) and subsequently to the ground state (S0) by vibrational relaxation is a main decay pathway, resulting in lower PL efficiency of the optoelectronic materials.7 It is well known that the ISC process is not always vigorous in the excited state and usually governed by the factors, such as temperature, heavy atom effect, energy gap (∆EST) and electronic configuration (El-Sayed rule) etc. Therefore, controlling these factors allows to suppress the intersystem crossing and enhance the radiative transition, is critical for the organic light emitting diodes (OLED) application. Recently, naphthalene imide based molecules (naphthalene diimide: NDI, naphthalimide: NMI), have been reported for the optoelectronic applications such as organic field effect transistors (OFETs) and organic solar cells (OSCs).8, 9 These molecules have unique advantages such as chemical robustness, photo-stability and redox activity.9 However, the naphthalene imide based materials are seldom reported for the OLEDs.10-14 The main obstacle to apply NMI based materials is their low PL efficiencies caused by the high intersystem crossing process (S1 to dark T1 excited state).15,16 Therefore, to control the ISC process in NMI derivatives is important to


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improve their PL efficiencies, extending their application to OLEDs and other fluorescent sensors. In this contribution, we report two new high-efficiency NMI derivatives (Ph-NMI and TPANMI) by introducing phenyl and triphenylamine substituents at the 4-position of 1, 8napthalimides respectively. The photophysical properties demonstrated that the higher PL efficiencies of Ph-NMI and TPA-NMI (0.24 and 0.57 in chloroform solution) mainly originate from decreased ISC process, as well as more allowed S1 radiative tansitions state. The optimized OLEDs using TPA-NMI as emitting layers showed high performance with a low turn-on voltage of 3.1 V, a maximum current efficiency of 15.5 cd A-1, a maximum power efficiency of 12.0 lm W-1 and a maximum external quantum efficiency of 5.96%, which is the best device performance ever achieved based on the naphthalimide derivatives.10-14 2. EXPERIMENTAL SECTION General information: All the reagents and solvents used for the synthesis and characterization were purchased from Aldrich and Acros and used without further purification if not noted. The 1H NMR and 13C NMR spectra were measured on a Bruke ASCEND 500 spectrometer at 500 MHz, using tetramethylsilane (TMS) as the internal standard, CDCl3 as solvent. The mass spectra were measured using an AXIMA-CFRTM plus instrument. Thermal gravimetric analysis (TGA) was measured on a Perkin-Elmer thermal analysis system from 30 ̊C to 800 ̊C at a heating rate of 10 K/min under nitrogen flow rate of 80 mL/min. Differential scanning calorimetry (DSC) was performed on a NETZSCH (DSC-204) unit from 30 ̊C to 350 ̊C at a heating rate of 10 K/min and cooling rate of 5 K/min under nitrogen atmosphere. The electrochemical properties were measured via cyclic voltammetry (CV) measurements by using a standard one-


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compartment, three-electrodes BAS 100 W bioanalytical electrochemical workstation, with glass carbon disk as the working electrode, platinum wire as the auxiliary electrode, and a porous glass wick Ag/Ag+ as the pseudo-reference electrode with standardized against ferrocene/ferrocenium. The solution was CH2Cl2 with 0.1 M n-Bu4NPF6 as supporting electrolyte at a scan rate of 100 mV/s.

UV-vis

and

fluorescence

spectra

were

recored

on

a

Shimadzu

UV-3100

spectrophotometer using 1 cm path length quartz cells. The fluorescence lifetime and PLQY (φ F)

of solutions and solid films were measured by FLS920 spectrometer. The ground-state (S0)

and the lowest singlet excited state (S1) geometries were optimized at the B3LYP/6-31G (d, p) level, which is a common method to provide molecular geometries and the optimized outcome is in good agreement with experiment result. The HOMO/LUMO distributions are calculated on the basis of optimized S0 state. Device Fabrication and characterization: The sheet resistance of ITO glass used in the experiment was 20 Ω square-1. The ITO glass substrates were cleaned with isopropyl alcohol, acetone, and deionized water, dried in an oven at 120 ̊C, treated with UV-zone for 20 min, and finally transferred to a vacuum deposition system with a base pressure lower than 6×10-6 mbar for organic and metal deposition. The deposition rate of all organic layers was 1.0 Å/s. The cathode LiF (1 nm) was deposited at a rate of 0.1 Å/s and then the capping Al metal layer (100 nm) was deposited at a rate of 4.0 Å/s. The electroluminescent (EL) characteristics were measured using a Keithley 2400 programmable electrometer and a PR-650 Spectroscan spectrometer under ambient condition at room temperature. 3. RESULTS AND DISCUSSION 3.1 Synthesis and Characterization


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All the starting materials used in synthesis (Chart 1 and Scheme S1) of Ph-NMI and TPA-NMI were purchased from commercial sources and used without further purification. The initial step for the synthesis of Br-NMI (2) involves a amine condensation reaction between brominated naphthalic anhydride and 2, 6-isopropylphenylamine to get a well-soluble intermediate product. The Ph-NMI and TPA-NMI were synthesized in high yields by Suzuki cross-coupling reactions by using brominated 1, 8-naphthalimides and corresponding boronic acid or ester. The products were

purified

using

column

chromatography,

preparative

high

performance

liquid

chromatography (HPLC) and vacuum sublimation in sequence and white and orange yellow powders were obtained respectively. These two molecular structures were characterized by the nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS). The molecular structures of Ph-NMI and TPA-NMI were further confirmed by the single crystal Xray diffraction analysis (supporting information).

Chart 1.Chemical structures of H-NMI, Ph-NMI and TPA-NMI. 3.2. Thermal and Electrochemical Properties The thermal properties of Ph-NMI and TPA-NMI were investigated by the thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere (Figure


5


S1). Both Ph-NMI and TPA-NMI compounds show high thermal decomposition temperatures (Td, corresponding to 5% weight loss) of 300 ̊C and 376 ̊C, respectively. The high decomposition temperature could avoid material decomposition during the device fabrication process by vacuum thermal deposition. Both Ph-NMI and TPA-NMI showed a glass transition process (DSC) at 97.3 ̊C and 122.5 ̊C, respectively. The high glass-transition temperatures are beneficial to make stable film morphology required for device operation.17,18 The electrochemical properties of Ph-NMI and TPA-NMI were investigated by the cyclic voltammetry (CV, Figure S4a). This experiment allows to calculate highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels (Table 1).

UV intensity (a.u.)

3.3. Photophysical Properties

(a)

H-NMI

(b)

Ph-NMI

(c)

TPA-NMI

300

400

500

600

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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700

Wavelength (nm)


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Figure 1. Absorption and emission spectra of H-NMI (a), Ph-NMI (b) and TPA-NMI (c) in CHCl3 solutions. Concentration: 4×10-5 mol L-1.Inset: Photographs of solutions under violet light radiation. The Figure 1 shows UV-vis and PL spectra of Ph-NMI, TPA-NMI, and H-NMI recorded in CHCl3. The original H-NMI has fine-structure of absorption peaks with λmax at 334 nm and a shoulder at 347 nm (Figure 1a), attributed to the π-π* and n-π* transitions of the conjugated backbone. This assignment can be verified by the high molar extinction coefficients (ε) ~1.56× 104 M-1 cm-1 and the theoretical calculations (section in Figure 3). However, H-NMI shows almost no emission under UV light excitation, and weak PL signal (λmax at 380 nm) was detected. This is caused by the strong ISC process from S1 to a dark T1 state (see later discussion). Different with H-NMI, after introducing phenyl substituent, the UV spectrum of Ph-NMI showed a red-shift and structureless band with an ε value of 1.71 × 104 M-1 cm-1, indicating π-π* transition of the conjugated molecular framework (Figure 1b). More interestingly, the Ph-NMI solution exhibits a deep blue emission with a PL peak at 416 nm. While for TPA-NMI (Figure 1c), its absorption clearly consists of two bands. The band around 325 nm is associated with the π-π* transitions, largely localized on the triphenylamine moiety. The low-energy absorption bands around 430 nm can be assigned to the charge transfer (CT) transition 1CT←S0.19 The ε of this CT transition is 1.12×104 M-1 cm-1, which is still a high value for the allowed transition. Due to this strong CT transition, the PL spectra of TPA-NMI red-shift to longer wavelength region with a peak at 586 nm, showing a bright orange yellow emission. The PL quantum yield (PLQY) of H-NMI, Ph-NMI and TPA-NMI in CHCl3 solution were evaluated using an integrating sphere to be ＜0.1, and 0.24 and 0.57, respectively. Combined


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with their fluorescence lifetimes, relevant parameters of various photophysical processes can be obtained by the following equations (1) - (4).

KF =

1

τF

ΦF

(1)

τF

= K r + K nr

(2)

K nr = K ISC + K IC

ΦISC =

KF

(3)

K ISC + K ISC + K IC

(4)

Where ΦISC and Φ F represent the quantum efficiency of intersystem crossing and fluorescence;

K ISC and K F are the rate of intersystem crossing and fluorescence radiation. The rate of internal conversion: K IC being very small number could be omitted. The τ F is the transition lifetime of fluorescence. Here the ISC process is the main non-radiative process rather than the vibrational de-activation in NMI derivatives, thus the non-radiative rate K nr = K ISC . The data (Table 1) shows that K ISC of Ph-NMI and TPA-NMI decreased when compared to the H-NMI. Obviously, the HNMI has a much larger ΦISC but a much smaller K F than that of Ph-NMI and TPA-NMI.

Table 1. Summary of optical properties of NMIs derivatives. Compd

H-NMI

HOMO

LUMO

τF

ΦF

KF

ΦISC

KISC

(eV)

(eV)

(ns)

(%)

(s-1)

(%)

(s-1)

n.a.

n.a.

1.5

0.7

1.02×106

99.3

6.62×108


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Ph-NMI

-6.20

-3.20

3.6

23.8

6.61×107

76.2

2.11×108

TPA-NMI

-5.33

-3.20

6.9

57.1

8.28×107

42.9

6.22×107

∆ EST=0.02 eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fluorescence Phosphorescence H-NMI

∆ EST=0.66 eV

Ph-NMI ∆ EST=0.31 eV

TPA-NMI 400 450 500 550 600 650 700 750

Wavelength (nm) Figure 2. Fluorescence and phosphorescence spectra of naphthalene derivatives under 77 K, HNMI; Ph-NMI and TPA-NMI. Solvent: tetrahydrofuran, 4×10-5 mol/L. Delay time: 1 ms.

E =

1240

λem

∆E ST = E S − E T

(5)

(6)

To explain the different photophysical properties, we measured phosphorescence spectra (77 K) in THF solution and compared them with their PL spectra (Figure 2 and Table 2), then the ∆EST could be calculated according to the equation (5) and (6) as shown above. Where λem represents the wavelength of main peaks of fluorescence and phosphorescence; ES and ET represent the energy level of lowest singlet state S1 and triplet state T1, respectively. The phosphorescence of H-NMI, almost overlapped with the PL spectrum, indicating a very small energy gap between the S1 and T1 (∆EST = 0.02 eV) in H-NMI. While for Ph-NMI and TPA-NMI,


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the phosphorescence spectra were red-shifted with respect to their PL spectra. Therefore, the ∆EST of the two molecules (Ph-NMI: 0.66 eV and TPA-NMI: 0.31 eV) is much larger than that of H-NMI. It is well known that the ISC can be suppressed by enlarging the ∆EST, thereby decreased ISC can be expected for the Ph-NMI and TPA-NMI. Table 2. Summary of energy gap of NMIs derivatives. Fluorescence

Phosphorescence

λtriplet (nm) em

λtriplet (nm) em

H-NMI

403

Ph-NMI TPA-NMI

Materials

ES (eV)

ET (eV)

ΔEST (eV)

406

3.07

3.05

0.02

393

495

3.16

2.50

0.66

529

613

2.33

2.02

0.31

3.4. Electronic Configuration

Figure 3. Natural transition orbitals for S0→S1 and S0→T1 excitation for the H-NMI (a), PhNMI (b) and TPA-NMI (c), respectively. f: oscillator strengths; the weight of the hole-particle contribution to the excitation is also included.


10

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Natural transition orbitals (NTOs) were calculated by the TD-DFT (Figure 3) with M06-2X/631G (d,p) basis to analyze the electronic configuration of their S1 and T1 states.20 For H-NMI, the S0→S1 transition (Figure 3a), the “hole” is mainly located at carbonyl six-membered ring, while the “particle” is distributed on the naphthalene skeleton. Therefore, we can conclude that the S0→S1 transition in H-NMI is an n-π* transition, which shows almost zero oscillator strength (f = 0.0001). The allowed transition is S0→S2 (π, π*) with an oscillator strength of 0.29 (Figure S6). In contrast, the S0→T1 transition of H-NMI is a π-π* transition (Figure 3a). After structural modification, both S0→S1 and S0→T1 in Ph-NMI and TPA-NMI exhibit π-π* transition character (Figure 3b and 3c). Thus the oscillator strengths of S0→S1 increased to 0.50 and 0.63 for PhNMI and TPA-NMI respectively, indicating more allowed transition. According to El-Sayed rule, the rate of ISC is also determined by the molecular orbital type of singlet and triplet states, for instance, a (π, π*) singlet state could transition to a (n, π*) triplet state, but not to a (π, π*) triplet state and vice versa.21 Therefore, a schematic diagram of the photophysical process in these materials emerged in sight (Figure 4). For H-NMI, the ISC is allowed with transition type from a (n, π*) state to a (π, π*) state. Meanwhile, the small ∆EST in H-NMI also prompts the ISC process. While for Ph-NMI and TPA-NMI, the mismatch of molecular orbital type together with larger energy difference


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Figure 4. Scheme of photophysical process of non-fluorescent H-NMI (a) and fluorescent PhNMI and TPA-NMI (b) derivatives, respectively. Table 3. Electronic configuration of NMIs derivatives. Electronic configuration Compounds

Intersystem

S0→S1

S0→T1

crossing

H-NMI

(n, π*)

(π, π*)

allowed

Ph-NMI

(π, π*)

(π, π*)

forbidden

TPA-NMI

(π, π*)

(π, π*)

forbidden

(∆EST) greatly inhibits the ISC process (Table 3). In addition, the electronic transition S0→S1 in H-NMI shows n-π* transition which is non-beneficial for the radiative transition. This electronic transition changed to π-π* transitions after introducing new substituents at 4-position of naphthalene skeleton (Ph-NMI and TPA-NMI), implying a larger KF can be expected. Therefore, Ph-NMI and TPA-NMI showed enhanced PLQY through suppressing ISC process. In addition to the increased ∆EST and mismatched electronic configuration, low temperature, avoiding heavy atoms and degassing by inert gas would also be effective to suppress ISC process.

3.5. Electroluminescence Properties To check the potential applications of Ph-NMI and TPA-NMI in OLEDs, the doped OLED devices were fabricated with configuration of [ITO/PEDOT:PSS(40 nm)/NPB(40 nm)/TCTA(5 nm)/emitters(20 nm)/TPBi(35 nm)/LiF(1 nm)/Al(100 nm)]. The device structures were also shown in Figure 5a, where ITO is used as the anode; PEDOT: PSS as the hole-injection layer;


12

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NPB as the hole-transporting layer; TCTA as the electron-blocking layer; TPBi as the electrontransporting and hole-blocking layer, LiF as the electron-injection layer and Al as the cathode. The emitting layers (EML) were co-deposited mixtures of hosts: (6 wt %) Ph-NMI or TPA-NMI. Considering the different HOMO and LUMO levels of Ph-NMI or TPA-NMI, mCP and MADN

(b) Current Density (mA cm-2)

6

10

TPA-NMI Ph-NMI

103

105

101

104

10-1

103

-3

10

102

-5

101

-7

100

10 10

0

1

2

3

4

5

6

7

8

Luminance (cd m-2)

with matched energy levels were selected as the hosts for Ph-NMI and TPA-NMI, respectively.

9

Voltagle (V)

6 5 TPA-NMI Ph-NMI

4 3 2 1

1

10

100

0 1000 10000 100000

Luminance (cd m-2)

EL intensity (a.u.)

(d)

7

16 14 12 10 8 6 4 2 0

EQE (%)

(c) LE (cd A-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

Ph-NMI TPA-NMI

0.8 0.6 0.4 0.2 0.0

400 450 500 550 600 650 700

Wavelength (nm)

Figure 5. (a) Energy-level diagrams of the doped devices;(b) Current density-voltage-brightness (J-V-L) characteristics curves; (c) Current efficiency and external quantum efficiency versus current density curves; (d) EL spectra for Ph-NMI and TPA-NMI, respectively. The OLED device characteristics (Figure 5) and device performance data (Table 4) indicate that Ph-NMI based device exhibited deep blue EL emission with the λem max at 434 nm (Figure 5d) with the CIE coordinates (0.15, 0.07), very close to the standard blue-light CIE coordinate of (0.14, 0.08) defined by the NTSC (National Television Standards Committee). More importantly, the


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EL spectra remained almost unchanged over a wide range of driving voltages, indicating good color stability. The performance of Ph-NMI based device was inferior with a maximum luminance (Lmax) of 948 cd m-2, a maximum current efficiency (CEmax) of 0.90 cd A-1 and a maximum external quantum efficiency (EQEmax) of 1.58%. Due to its higher PL efficiency and smaller charge carriers energy barriers between the injecting layer and emitting layer, the TPANMI based doped device exhibited excellent performance, which shows a lower turn-on voltage (Von) of 3.1 V, an Lmax of 64344 cd m-2, a CEmax of 15.5 cd A-1, and an EQEmax of 5.96% (Figure 5b and 5c). This is the best device performance ever achieved based on the naphthalene imide derivatives. Moreover, at a high brightness of 30000 cd m-2, the efficiency of device still maintained at a high level (CE: 13.6 cd A-1 and EQE: 5.24%), indicating a highly desired small efficiency roll-off for the OLEDs.22 The excellent device performance indicated the NMI derivatives can be used as promising emitters for OLED applications.

Table 4. EL performance of doped devices for Ph-NMI and TPA-NMI mateirals. Vona

Lmaxb

CEmaxc

PEmaxd

(V)

(cd m2 )

(cd A1 )

(lm W-1)

Materi al

EQE e max

(%)

V

CE

EQE

CIE

V

CE

EQE

CIE

(V)

(cd A1 )

(%)

(x, y)

(V)

(cd A-1)

(%)

(x, y)

at 100 cd m-2 PhNMI

3.8

948

0.90

0.68

1.58 5.0

0.65

1.14

at 1000 cd m-2 (0.153, 0.066)

6.0

at 1000 cd m-2 TPANMI

a

3.1

64344

15.5

12.0

5.96 5.0

15.1

5.81

0.46

0.61

(0.153, 0.068)

at 30000 cd m-2 (0.291, 0.595)

7.4

13.6

5.24

(0.281, 0.585)

Turn-on voltage at 1 cd m-2. b Maximum luminance. c Maximum current efficiency. d Maximum power efficiency. e Maximum

external quantum efficiency.


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4. CONCLUSIONS In summary, we have successfully designed and synthesized two new NMI-based emitting materials Ph-NMI and TPA-NMI, which showed high PL efficiencies as compared with their contrastive original H-NMI prototype. The experimental results show that the enhanced PL efficiencies of Ph-NMI and TPA-NMI are mainly attributed to their suppressed ISC process in the excited state, well explained in terms of their enlarged ∆EST and the mismatched electronic configuration of S1 and T1 states. The doped OLEDs using Ph-NMI and TPA-NMI as the emitters exhibited deep-blue and yellow emissions respectively. The TPA-NMI based device reached an Lmax of 64344 cd m-2, a CEmax of 15.5 cd A-1 and a EQEmax of 5.96%, which is the best device performance ever achieved based on NMI derivatives. This research shows that reasonable molecular design of NMI derivatives can inhibit intersystem crossing therefore provides an efficient strategy to achieve high performance organic electroluminescent materials

ASSOCIATED CONTENT Supporting

Information.

Experiment

conditions,

synthesis

and

thermal

properties,

electrochemical curves, crystal structures and theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].


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Notes The authors declare no competing financial interest. § These authors contributed equally to this work. ACKNOWLEDGMENT The authors express their thanks to the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521002), the Natural Science Foundation of China (51403063), Major Science and Technology Project of Guangdong Province (2015B090913002), Foundation of Guangzhou Science and Technology Project (201504010012), China Postdoctoral Science Fund (Grant No. 2014M562174) for their support.

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Enhancing Fluorescence of Naphthalimide Derivatives by

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