Influence of ÏâÏ Hyperconjugation Effect on Thermal, Morphological

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The Influence of # - # Hyperconjugation Effect on Thermal, Morphology and Photoelectronic Properties of Non-Conjugated Pyrene Derivatives Shao-Ya Qiu, Hui Xu, Le Li, Hai-Tao Xu, Ling-Kun Meng, Hu-Sheng Pang, Chao Tang, Zongqiang Pang, Jing Xiao, Xu Wang, Shang-Hui Ye, Quli Fan, and Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02557 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 20, 2017

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The Journal of Physical Chemistry

The Influence of π -π

Hyperconjugation Effect on Thermal,

Morphology and Photoelectronic Properties of Non-Conjugated Pyrene Derivatives

Shao-Ya Qiu,a Hui Xu,a Le Li,a Hai-Tao Xu, a Ling-Kun Meng,a Hu-Sheng Pang,a Chao Tang,a,* Zong-Qiang Pang,a Jing Xiao,c Xu Wang, a Shang-Hui Ye,a* Qu-Li Fan, a*

Wei Huanga,b*

(Shao-Ya Qiu and Hui Xu contributed equally to this work)

a

Key Laboratory for Organic Electronics and Information Displays & Institute of

Advanced

Materials(IAM), Jiangsu National

Synergetic Innovation

Center

for

Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.

b

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China.

C College of Physics and Electronic Engineering, Taishan University, Taian, Shandong 271021, China.

* To whom correspondence should be addressed: Tel: +8625 85866332; Fax: +86 25 85866332; E-mail: [email protected] / [email protected]

1

ACS Paragon Plus Environment


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Abstract: In this paper, four pyrene-fluroene derivatives with conjugated and non-conjugated pyrene substitution were designed and synthesized. In PFP1 and PFP2, there are non-conjugated pyrene substitution on C9 and conjugated pyrene on C2 and/or C7 of the fluorene moiety, and in the control molecules BP1 and BP2 (Scheme 1), there is only the conjugated pyrene in the C2 and/or C7 of the fluorene core. There is special π - π hyperconjugation effect between non-conjugated pyrene and the pyrene-fluorene conjugation in the system (PFP1 and PFP2), which means the electron cloud of such two isolated conjugation systems (non-conjugated pyrene group and pyrene-fluorene group) could be delocalized and transferred to each other. Because of delocalization of electron cloud, the molecule size of PFP1 and PFP2 might have been decreased and led to decreased phase transition temperature compared with that of BP1 and BP2. Also due to the electron transfer between the molecules, the intermolecular force between PFP1 and PFP2 has been improved, which is the reason that they are more amorphous than that of BP1 and BP2. The easily electron transfer also makes the PFP1 and PFP2 show the improved hole injection and device performance compared with that of BP1 and BP2.

1. Introduction In recent years, the organic semiconductors (OSCs) have drawn much attention due to their unexpected potential properties in applications, including organic light emitting diodes (OLED),1 organic field effect transistors (OFET),2 solar cells (OPV)3 2


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and sensors.4 Generally, the photoelectronic properties of OSCs were determined by the molecular structure and aggregation morphology of the materials.5-8 Since the OSCs possessed π

planar conjugated molecular structures, the molecular

aggregation always played the important key to the photoelectronic performances. For example, most of the organic light emitting materials exhibited high emission efficiency in dilute solution while showed decreased or even quenched fluorescence in aggregation state, namely, aggregation caused quenching (ACQ) phenomenon.9 In the solid or aggregation state, the conjugated aromatic hydrocarbon tends to form excimer and/or exciplex due to the intermolecular interaction including π − π stacking, electrostatic attractions and van der Waals force, which, in most time, results in a drastic decrease of their luminescence efficiency.10, 11 Studies on the aggregation state of organic luminescence materials revealed that the aggregation morphology was dominated by molecular structure of the materials,12, 13 temperature,14 solvent15 and other kinetic parameters. In varying solvent or temperature, different aggregation morphology could be prepared, resulting in wide difference in luminescent properties.16,17 As to the molecular structure modifying, the introduction of long-chained alkoxy could increase the solubility and rheological properties of conjugated molecule, and then improve the film-forming capability, leading to optimized spin-coated film morphology.18 Usually, as for one material, a more aggravated aggregation state led to red-shifted and broader absorption spectra.19 In our previous work, we found that in the non-conjugated pyrene substituted conjugated fluorene systems, there was obvious intramolecular through-space 3



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interaction between the pyrene and the main conjugation. Since the intramolecular interaction was between the two isolated large π delocalized systems, and possessed the characteristics of conjugation effect, it was defined to be hyperconjugation

effect.20

Then

the

further

research

of

such

π -π π -π

hyperconjugation effect on the thermal, aggregation morphology and photoelectronic properties should be carried out in this report. Two series of pyrene-fluorene derivatives have been synthesized, with similar conjugated pyrene-fluorene backbone and different substitution on C9 of fluroene moieties (PFP1, PFP2, BP1, BP2) (Scheme

1),

and

the

molecular

structure,

aggregation

morphology

and

photoelectronic properties of the materials were systematically studied to investigate the effect of π - π hyperconjugation.

2. Results and Discussion 2.1 Syntheses and thermal properties The synthesis routes of PFP1 and PFP2 has been published in our previous work.21 The syntheses of BP1 and BP2 are in principle similar to PFP1 and PFP2, which were depicted in Scheme 1. 1-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl) pyrene (PyB), 2-bromo-9-(4’-(2’-ethylhexyloxyphenyl))-fluoren-9-ol

(1a)

and

2,7-dibromo-

9-(4’-(2’-ethylhexyloxyphenyl))-fluoren-9-ol (1b) were synthesized according to our previous work.21 1A and 1b were respectively underwent Friedel-Crafts reaction with toluene under the methanesulfonic acid to get the compound monobromide 2a and dibromide 2b. Then the Suzuki coupling reaction was employed between 2a or 2b and 4


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PyB under the the catalysis of Pd(PPh3)4 to obtain the target compounds BP1 and BP2. They were fully characterized by

1

H and

13

C NMR, matrix-assisted laser

desorption/ionization time-of-flight mass spectrometer (MALDI-TOF-MASS) and elemental analysis. The results were consistent with the proposed structures.

O B O PyB

RO

RO

PFP1

PFP2

RO

RO

RO

ii BP1

OH X

i Br

1

X

iii

Br

RO

2

BP2 a: X=H, b: X=Br, R=2-ethylhexy

Scheme 1. Reagents and conditions: (i) toluene, CH3SO3H (1 equiv), 60 °C, 1h; (ii) 2a/PyB (1:1.25), Pd(PPh3)4, toluene, K2CO3 (2.0 M, aq), 90 °C, 48 h; (iii) 2b/PyB (1:2.5), Pd(PPh3)4, toluene, K2CO3 (2.0 M, aq.), 90 °C, 48 h.

The thermal properties of the four materials were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere at heating rate of 10 °C. min−1. The TGA test results show that the four materials all demonstrate high decomposition temperature (Td). The Td of PFP1, PFP2, 5



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BP1, BP2 are 430℃, 438℃, 412℃,434℃, respectively, which indicate that the decomposition temperature has an rising trend with increasing molecule weight. In the DSC analysis, the glass transition temperatures (Tg) of PFP1, PFP2 and BP1 are 103℃,191℃ and 139℃, respectively. While BP2 shows no obvious phase transition phenomenon up to 250℃. The relevant data are showed in the Table 1. In contrast to the trend of Td, the Tg results indicate that the materials have lower phase transition temperatures (for PFP1 and PFP2) when C9 position of fluorene core is substituted by the larger pyrene group. It is interesting and strange because the Tg generally will increase when the large aromatic group has been incorporated into the system. As to such abnormal phenomenon, there are two possible underlying reasons. Not as the segment motion in the polymer, the phase transition in the small organic molecules needs the rotation motion of some parts of the molecule structure.22 The closer packing of molecular aggregation to solid, the more energy needed to partial rotation motion of molecules, which leads to higher glass transition. As for the BP1 and BP2, the two substitutions on the C9 of fluorene are same phenyl groups. In contrast, the two groups on the C9 of fluorene in PFP1 and PFP2 are phenyl and pyrenyl groups, whose sizes are greatly different, which leads to asymmetrically and less close packing than that of BP1 and BP2. So such is the common explanation why the Tg of BP1 and BP2 are higher than that of PFP1 and PFP2 even if molecular weight and aromatic groups of the latter are larger than the former. However, another reason should be considered when the in-deep investigation is carried out on the essence of the size of the aromatic group. For PFP1 and PFP2, the 6


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pyrene at the C9 of the fluorene core connects with the main conjugation chain by the hyperconjugation,20 in which the pyrene conjugation system and

π -π

pyrene-fluorene conjugation system are isolated by two single bonds rather than direct connection. The hyperconjugation means that the electron cloud from the pyrene will be delocalized and transferred to the main conjugation, which leads to the decreased size of the pyrene group, and also the size of pyrene-fluorene main conjugation could be decreased by the hyperconjugation for the same delocalization and transfer, just as the decreased bond length because of the conjugation. Then the total size of the molecule should be decreased, which at last contributes to the decreased energy for rotation of molecule and thus lower glass transition temperatures. However, the direct evidence for the two kinds of possible reasons are both difficult to obtain, which depends on the development of microscope technology on the microscopic size and electron cloud of organic molecules.

Table 1 Thermal properties of the materials.

Molecules

PFP1

PFP2

BP1

BP2

Tg

103℃

191℃

139℃

/

Td

430℃

438℃

412℃

434℃

2.2. The aggregation morphology

7



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(a): PFP1

(b):PFP2

(c):BP1

(d):BP2

Fig. 1. The SEM images of aggregation morphology

The aggregation morphology was analyzed by scanning electron microscope (SEM) with the silicon wafer as substrate. As the Fig. 1 shows, the aggregation of PFP1 is hemispheroid shape adhering to the substrate. As for BP1, there are also in part hemispheroid shapes, but it is more irregular than that of PFP1. And the aggregation of PFP2 is spherical connected with each other. For BP2, the aggregation become irregularly distorted body with large size. PFP2 and BP2 both pile into distortion aggregation with a three-dimensional structure, and the size of PFP2 aggregation is smaller than BP2. The difference between PFP1 and BP1 is the pyrenyl or phenyl substitution on the C9, which is similar to that in PFP2 and BP2. Even if the former has the larger substitution pyrenyl group than the latter, the former shows higher 8


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uniform in morphology. The film morphology of four materials under AFM (Fig. 2) display the roughness extent agreed with the SEM morphology. BP2 film shows a rough surface and some different size pinholes, which is harm for device properties. While, the films of PFP1 and PFP2 both show more uniform than BP2.

(a)

(b)

(c)

(d)

Fig. 2. The AFM morphology of spin-coated film on glass substrate: (a):PFP1, (b):PFP2, (c):BP1, (d):BP2.

As to all the solids, of course the single crystal is the most uniform structure. But the problem is that in devices of OSCs fabricated by generally methods such as vacuum thermal sublimation and spin coating, the organic material always cannot 9



form full single crystal. Part crystal and part amorphous states have been proved to do harm to the stability of the device. Therefore, in contrast, the full amorphous state, especially in OLEDs, is in fact more uniform than the part crystal state, even if part state (or part ordered state) might be good for the mobility in OFETs. The above experiments have proved that the larger pyrenyl group has led to higher uniform morphology, which is in fact the more amorphous state coming from the molecular

Intensity(a.u.)

structures. Therefore, the in-deep reason should be investigated.

Intensity(a.u.)

BP1

BP2

PFP1

20

25

30

35

40

45

50

5

10

15

20

2θ deg

25

30

)

15

PFP2

((((

10

)

5

((((

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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35

40

45

50

2θ deg

Fig. 3. Powder X-ray diffraction for the four materials.

In general, symmetry of molecule always leads to ordered arrangement or orientation of molecule, and higher symmetry always results in higher orientation. As to the two series of molecules, PFP1 and BP1, PFP2 and BP2, the only difference in molecular structure is one substitution group on C9. PFP1 and PFP2 are the pyrenyl substitution, and BP1 and BP2 are phenyl substitution. For BP1 and BP2, the two substitutions on C9 are same phenyl groups. Compared with phenyl group, the pyrenyl group is much larger. And for PFP1 and PFP2, the two substitutions on C9 of fluorene moiety are different groups of phenyl and pyrenyl, with larger difference of 10


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size. Therefore, the symmetry of PFP1 and PFP2 are lower than that of BP1 and BP2. Then, the orientation of the former is difficult than the latter (as showed in Fig 3), which at last results in more amorphous state and more uniform morphology. The symmetry factor is always considered in the research of arrangement and morphology of OSC molecules. But another factor is also important, that is the intermolecular force, which is in sometime more important such as that in the ordered arrangement of amino acid into DNA chain. However, irregular intermolecular force always blocks the molecule from entering the crystal lattice. And most of OSC molecules possessed the irregular intermolecular force, which is reason why most of the OSC molecule cannot form perfect single crystal. As to the molecules in this report, just as the above description, there is π - π hyperconjugation between the isolated non-conjugated pyrene group and the main conjugation, which means there are more electron transporting tunnels in PFP1 and PFP2 than that of BP1 and BP2 (Fig 4, detailed discussion can be seen in 2.3. The electrochemical property). The intermolecular force is stronger but still more irregular in PFP1 and PFP2 than that in BP1 and BP2, which also can contributes to the more amorphous state and more uniform morphology of the former than that of the latter.

2.3. The electrochemical property The electrochemical properties of the materials were analyzed by cyclic voltammetry (CV) with a standard three electrode system in dichloromethane solution under nitrogen atmosphere at room temperature with a scanning rate of 200 mV s−1. 11



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The oxidation onset potentials are 0.69 eV, 0.66 eV and 1.13 eV, 1.03 eV for the materials of PFP1, PFP2, BP1 and BP2, respectively. According to EHOMO = – (Eonset + 4.7 eV), the corresponding HOMO energy levels are calculated to be -5.39 eV, -5.36 eV, -5.83 eV and -5.72 eV, respectively. The corresponding data are summarized in Table 2. It is obvious that PFP1 and PFP2 both have higher HOMO energy levels than that of BP1 and BP2, respectively. The only difference between the above two pairs of molecules are the non-conjugated pyrene substitution on C9 of fluorene core. But because of the large aromatic characteristic of pyrene ring, the electron cloud can be easily transferred from one molecule to another molecule just using pyrene group as a transferring bridge, which in fact means the more transporting paths for carrier.

12


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Fig. 4. Possible intermolecular hopping ways for carriers of PFP2 and BP2.

The π − π hyperconjugation effect between the non-conjugated pyrene and the main conjugation can also be easily applied to explain this phenomenon. There is 13



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obvious π − π

hyperconjugation effect in PFP1 and PFP2, which means the

non-conjugated pyrene could join to electron transfer besides the main conjugation chain. According to hopping model,23 carrier-injection process could be seen as tunneling hopping process. Thus in a simple physical model analysis, there are three carrier tunnels in PFP1 and PFP2 molecule: (1) tunnel directly from pyrene-fluorene main conjugation of one molecule to main conjugation of other molecule; (2) tunnel from main conjugation to non-conjugated pyrene group of the same molecule, then to other main conjugation; (3) tunnel from main conjugation to non-conjugated pyrene of the same molecule, then tunnel to non-conjugated pyrene of other molecule, and through which finally tunnel to main conjugation of other molecule. However, for BP1 and BP2, there is only one carrier tunnel: from main conjugation to main conjugation of other molecule. Therefore, due to the bridge function of the non-conjugated pyrene group, there are two more possible ways for carriers to tunnel in PFP1 and PFP2, which then show better carrier injection ability than that of BP1 and BP2, respectively (Fig. 4). Although the possible tunnel ways at the present are just physical deduction, it could also be used to understand the reason that PFP1 and PFP2 show higher HOMOs.

14


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Table 2 Photophysical properties of the materials

Molecules

a

PLQYb

λabs,edge /nm(solution)

△E/eV HOMO/eV LUMOa/eV Tol

DCM

THF

PFP1

397

3.12

-5.39

-2.27

0.23

0.19

0.21

PFP2

415

2.99

-5.36

-2.37

0.30

0.22

0.25

BP1

394

3.15

-5.83

-2.68

0.19

0.17

0.16

BP2

412

3.00

-5.72

-2.72

0.26

0.20

0.23

LUMO = HOMO + ∆E , the ∆E is calculated with the solution absorption edges.

b

Measured in CH2Cl2 ,tetrahydrofuran and toluene, using the 9,10-Diphenylanthracene as

standard.

2.4. The photophysical property

Intensity(a.u.)

Intensity(a.u.)

0.8

PFP1

0.6

solution film nano-solution

1.0


1.0

0.4 0.2

0.8

PFP2 0.6 0.4 0.2 0.0

0.0 300

400

500

300

600

1.0

solution film nano-solution BP1

0.6 0.4 0.2 0.0 300

500

600


1.0

Intensity(a.u.)

0.8

400

Wavelength(nm)

Wavelength(nm)

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


0.8

BP2

0.6 0.4 0.2 0.0

400

500

600

300

Waveleth(nm)

400

500

600

wavelength(nm)

Fig. 5. UV–visible absorption spectra of the materials in different state. 15



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The UV-Vis absorption and photoluminescence spectra were measured in dichloromethane solution, thin film by spin-coated on ITO glass and nano-solution state. The results are showed in Fig. 5. For PFP1 and PFP2, both of the absorption peaks at 334nm are rooted in the non-conjugation pyrene group at C9 of the fluorene core.21 The absorption peaks at 352nm resulting from the π - π hyperconjugation is the couple interaction between the non-conjugated pyrene at the C9 position of fluorene core and the pyrene-fluorene main conjugation.20 Due to increasing conjugation length, the absorption peaks of BP1 and BP2 are from 350nm to 364nm. For the four materials, because of the strong π - π stacking of pyrene and fluorene rings, the absorption curves in nano-solution are all broader than in solution. The absorption peak of BP1 and BP2 display a red-shifted (25~30nm) phenomenon in nano-solution. This confirms that the aggregation of four materials belong to J-aggregation. It is noteworthy that the characteristic peak of PFP2 at 352nm in nano-solution is consistent with that in solution. And for PFP1, the absorption peak in nano-solution only takes place a tiny red-shift (5nm) compared with solution. The absorption peaks in nano-solution state are broader than film, which arise from that the materials in nano-solution state have more serious aggregation extent than in film.19

16


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solution nano-solution film

Intensity(a.u.)

0.8

PFP1

0.6


1.0

Intensity（ a.u.）

1.0

0.4 0.2 0.0

0.8

PFP2

0.6 0.4 0.2 0.0

400

450

500

550

600

400

Wavelength (nm)

BP1

0.6

500

550

600


1.0

Intensity (a.u.)

Intensity (a.u.)

0.8

450

Wavelength (nm)


1.0

0.4 0.2 0.0

0.8

BP2

0.6 0.4 0.2 0.0

400

450

500

550

600

400

Wavelenth (nm)

450

500

550

600

Wavelenth (nm)

Fig. 6. PL spectra of the materials in different state.

1000

600

Intensity (a.u.)

800

Intensity (a.u.)

1000

10-5mol/L 10-6mol/L 10-7mol/L 10-8mol/L PFP1

400 200 0

10-5mol/L 10-6mol/L 10-7mol/L 10-8mol/L

800 600

PFP2

400 200 0

400

450

500

550

400

wavelength (nm)

800


600

450

500

550

Wavelength (nm)

1000

Intensity (a.u.)

Inntensity (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


400 BP1

200 0


800 600

BP2

400 200 0

400

450

500

550

400

Wavelength (nm)

450

500

Wavelength (nm)

Fig. 7. PL spectra of the materials in different concentration. 17


550


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

The peaks of photoluminescence spectra are at 417nm, 425nm, and 415nm, 422nm for PFP1, PFP2, BP1 and BP2, respectively. The spectra are red-shifted when the C9 of the fluorene is substituted by pyrene group or the main conjugation chain is stretched, which is consistent with the absorption spectra. In nano-solution state, the photoluminescence spectra of four materials all exhibit red-shifts and broad which originate from the π - π stacking of the pyrene and fluorene rings in aggregation state. Interestingly, the photoluminescence spectra curves of film and nano-solution state are identical, which show materials in film state is also accompanied by the strong

π - π stacking effect, and the intermolecular interactions was similar to

nano-solution. The photoluminescence spectra of four materials were tested in dichloromethane solution of different concentration. The concentration ranged from 10-5, 10-6, 10-7 to 10-8 mol/L. As seen from the spectra (Fig. 7), the intensity of PL increased and the spectra broadened when the concentration of solution increased. For 10-8 mol/L concentration, the emission intensity was very high and the full width at half maximum (FWHM) was narrow. The FWHM of PFP1、PFP2、BP1 and BP2 were 56、57、57 and 59 nm, respectively. For 10-5 mol/L concentration, the corresponding FWHM increased to 60、61、59 and 61 nm. The relevant data of FWHM were described in Table 3.

18


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Table 3 The FWHM of the materials in solution

FWHM (nm) Molecules 10-5mol/L

10-6mol/L

10-7mol/L

10-8mol/L

PFP1

60

59

57

56

PFP2

61

60

60

57

BP1

59

58

58

57

BP2

61

60

59

59

1200

10000

(a)

(b)

2

Current Density (mA/cm )

2.5. The electroluminescence properties

800

2

Luminance (cd/m )

PFP1 PFP2 BP1 BP2

1000

600 400 200

1000 PFP1 PFP2 BP1 BP2

100

10

0 0

2

4

6

8

4

10

6

8

10

Voltage (V)

Voltage (V)

1.8 PFP1 PFP2 BP1 BP2

1.6 1.4 1.2

1.0

PFP1 PFP2 BP1 BP2

0.8

Intensity (a.u.)

Current efficient (cd/A)

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 0.8 0.6 0.4 0.2

0.6 0.4 0.2 0.0

0.0 0

100

200

300

400

500

600

700

400

2

500

600

Wavelength (nm)

Current density (mA/cm )

Fig. 8. (a) The current Density–voltage of the EL Device. (b)The luminance characteristic–voltage of the EL Device. (c)The current efficiency–current density of the EL Device. (d) The EL spectra of the device. 19



0.6

9V 8V 7V 6V 5V

0.4

PFP1

Intensity (a.u.)

0.8

1.0

9V 8V 7V 6V 5V

0.8

Intensity (a.u.)

1.0

0.2 0.0

0.6 0.4

PFP2

0.2 0.0

400

450

500

550

600

650

400

Wavelength (nm)

1.0

550

600

650

9V 8V 7V 6V 5V

0.8

BP1

0.4

500

1.0

Intensity (a.u.)

0.6

450

Wavelength (nm)

9V 8V 7V 6V 5V

0.8

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

Page 20 of 33

0.2 0.0

0.6 0.4

BP2

0.2 0.0

400

450

500

550

600

650

400

Wavelength (nm)

450

500

550

600

650

Wavelength (nm)

Fig. 9. EL spectra of the four light-emitting devices with different voltage.

Table 3 The electroluminescence properties of the devices.

Device

Turn on (V)

Brightness (cd/m2)

LEmax (cd/A)

1(PFP1)

4.9

4846

1.11

2(PFP2)

4.1

8468

1.42

3(BP1)

5.2

2582

0.87

4(BP2)

4.5

5610

1.35

With the long oxyalkyl chain, the four compounds all exhibit good dissolution properties in the common solution of dichloromethane, chloroform, toluene and chlorobenzene. And the corresponding luminescence properties are measured in the 20


Page 21 of 33

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study.

The

devices

with

the

configuration

of

[ITO/PEDOT:PSS(30nm)/PVK:EL:oXD-7(40nm)/TPBI(35nm)/Ca(15nm):Ag(100nm )] were fabricated. And device 1, device 2, device 3 and device 4 were respectively fabricated using PFP1, PFP2, BP1 and BP2 as luminescent layer, respectively. The curves of the current density–voltage, the luminance characteristic–voltage, the luminance efficiency–current density and the electroluminescence characteristic of the four devices were showed in Fig. 8. And the corresponding data were summarized in Table. 3. All the performance of the device 2 (PFP2) is superior to device 4 (BP2). On the one hand, due to the π - π hyperconjugation can improve the electron transfer between molecules, PFP2 has a better film forming ability than BP2, which contribute to uniform device structure. On the other hand, the pyrene at the C9 of the fluorene core is a non-conjugated group in PFP2 molecule, which offers more possible ways for carriers to tunnel, improving the carrier injection efficiency.20 As well, with the high carrier injection efficiency, the PFP1 is superior to BP1 on device performance. From the results, the PFP2 and BP2, whose C2 and C7 of fluorene core are both substituted by pyrene group, demonstrate an obvious advantage than PFP1 and BP1 on

luminescence

properties,

respectively,

which

results

from

the

pyrene-fluorene-pyrene conjugation system has stronger carrier ability than pyrene-fluorene conjugation system. And in the Fig. 8 (d), the electroluminescence curves of the four devices show analogical shape. But the characteristic peaks of PFP2 and BP2 exhibit red-shifts compare with PFP1 and BP1, which is caused by the longer main conjugation chain. The emission spectra (Fig.9) are measured in different 21



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Page 22 of 33

voltage and the consistent curves indicate that the materials all have enough stability with luminescence property.

2. Conclusion Two series of pyrene-fluorene derivatives have been synthesized to systematically investigate the π - π hyperconjugation effect on the thermal, aggregation morphology and photoelectronic properties. π - π Hyperconjugation effect is between the non-conjugated pyrene group on C9 of fluorene core and the main fluorene-pyrene conjugation, which means the electron cloud of the above two isolated conjugation systems could be transferred to each other and delocalized, which then leads to the size decreasing of the total molecule, and the decreased phase transition temperature of PFP1 and PFP2 compared with the controlled group BP1 and BP2. The electron cloud transfer and delocalization in further increase the intermolecular force in PFP1 and PFP2, which leads to more amorphous and uniform morphology than that of the control group. In addition, the π - π hyperconjugation can improve the injection and transporting of the carrier, which are the reasons that the HOMO of PFP1 and PFP2 are higher than that of the control group. The carrier injection and transporting, beside the more uniform morphology, also contribute to the improved device performance of the PFP1 and PFP2. Based on above systematic research,

in

order to

realize

and explore

π -π

hyperconjugation,

the

non-conjugated pyrene substitution tactics should be seriously considered when 22


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designing the molecular structure of OSCs.

4. Experimental section 4.1. Synthesis experiments 2-Bromo-9-(4’-(2’-ethylhexyloxyphenyl))-9-(4’-(methylphenyl))-fluoren (2a). 1a (1.5g, 3.24mmol), CH3SO3H (0.31g, 3,24mmol) and toluene (10ml) were mixed in a flask, then the reaction mixture was stirred in 100℃ for 1h. After the reaction system was cooled to room temperature, the saturated sodium bicarbonate was added and then and the mixture was extracted twice with water. The organic phase was dried with anhydrous MgSO4. The crude product was purified by column chromatography using petroleum ether/dichloromethane (10:1) as eluent to provide a white solid (1.22 g, 70%). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.71 (d, J = 8.0 Hz, 1H), 7.60 (d, J = 8.0 Hz, 1H), 7.51 – 7.42 (m, 2H), 7.37 – 7.27 (m, 3H), 7.09 – 7.01 (m, 6H), 6.80 – 6.71 (m, 2H), 3.78 (d, J = 4.0 Hz, 2H), 2.29 (s, 3H), 1.71 – 1.67 (m, 1H), 1.33 – 1.26 (m, 8H), 0.93 – 0.86 (m, 6H). 2,7-Dibromo-9-(4’-(2’-ethylhexyloxyphenyl))-9-(4’-(methylphenyl))-fluoren (2b). 2b was synthesized according to the procedure described for 2a using 1b (1.5g, 3.32mmol), CH3SO3H (0.32g, 3.32mmol) and toluene (10ml). The crude product was purified by column chromatography using petroleum ether/dichloromethane (12:1) as eluent to provide a faintly yellow solid (1.42 g, 69%). 1H NMR (400 MHz, CDCl3) δ 23



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Page 24 of 33

(ppm): 7.59 (dd, J = 1.2 Hz, 7.6Hz, 2H), 7.52 – 7.46 (m, 4H), 7.11 – 7.03 (m, 6H), 6.82 – 6.77 (m, 2H), 3.82 (d, J = 5.6 Hz, 2H), 2.33 (s, 3H), 1.71 – 1.67 (m, 1H), 1.44 – 1.25 (m, 8H), 0.95 – 0.89 (m, 6H). 2-Pyrenyl-9-(4’-(2’-ethylhexyloxyphenyl))-9-(4’-(methylphenyl))-fluoren

(BP1).

2a (1.22g, 2.27mmol), PyB (0.94g, 2.84mmol) and Pd(PPh3)4 (0.13g, 0.11mmol) were added into a flask with nitrogen protection. Then toluene (15ml) and 2M K2CO3 (10ml) were added with stirring. The reaction mixture was heated to 90℃ for 48h. After the reaction system was cooled to room temperature, it was quenched with saturated sodium bicarbonate and the mixture was extracted twice with water. The organic phase was dried with anhydrous MgSO4. The crude product was purified by column chromatography using petroleum ether/dichloromethane (15:1) as eluent to provide a yellow solid (1.21 g, 80%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.20 – 8.16 (m, 4H), 8.08 (s, 2H), 8.04 – 7.91 (m, 4H), 7.86 (d, J = 7.2 Hz, 1H), 7.71 (d, J = 0.8 Hz, 1H), 7.65 (dd, J = 8.0 Hz, 7.6Hz, 1H), 7.44 – 7.40 (m, 2H), 7.32 (dd, J = 7.6 Hz, 14.8Hz, 1H), 7.21 (d, J = 6.4 Hz, 4H), 7.06 (d, J = 8.0 Hz, 2H), 6.78 (d, J = 8.8 Hz 2H), 3.78 (d, J = 5.6 Hz, 2H), 2.29 (s, 3H), 1.71 –1.68 (m, 1H), 1.46 – 1.25 (m, 8H), 0.92 – 0.85 (m, 6H). 13C NMR (101 MHz, CDCl3) δ (ppm): 158.18, 152.10, 151.98, 143.18, 140.40, 139.78, 139.20, 137.86, 137.65, 136.21, 131.51, 130.97, 130.55, 129.92, 129.20, 128.96, 128.54, 128.43, 128.10, 127.79, 126.25, 126.00, 125.37, 125.08, 124.94, 124.75, 120.30, 120.04, 114.12, 70.33, 64.71, 39.39, 30.53, 29.68, 29.09, 23.86, 23.05, 20.98, 14.09, 11.12. MALDI–TOF-MS(m/z): Anal. calcd. for C50H44O 660.34, found 659.788. 24


Page 25 of 33

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2,7-DiPyrenyl-9-(4’-(2’-ethylhexyloxyphenyl))-9-(4’-(methylphenyl))-fluoren (BP2). BP2 was synthesized according to the procedure described for BP1 using 2b (1.42g, 2.29mmol), PyB (1.90g, 5.73mmol) and Pd(PPh3)4 (0.13g, 0.11mmol) toluene (15ml) and 2M K2CO3 (10ml). The crude product was purified by column chromatography using petroleum ether/dichloromethane (15:1) as eluent to provide a faintly yellow solid (1.49 g, 75%). 1H NMR (400 MHz, CDCl3) δ(ppm): 8.20 – 8.16 (m, 8H), 8.10 (s, 4H), 8.06 – 7.99 (m, 8H), 7.78 (s, 2H), 7.72 (dd, J = 1.6 Hz, 7.6Hz, 2H), 7.31 (d, J = 8.4 Hz, 4H), 7.09 (d, J = 8.0 Hz, 2H), 6.81 (d, J = 8.8 Hz, 2H), 3.79 (d, J = 5.2 Hz, 2H), 2.32 (s, 3H), 1.72 – 1.69 (m, 1H), 1.35 – 1.20 (m, 8H), 0.91 – 0.84 (m, 6H). 13C NMR (101 MHz, CDCl3) δ (ppm): 158.26, 152.33, 143.16, 140.55, 138.95, 137.85, 137.68, 136.30, 131.53, 130.99, 130.60, 130.12, 129.28, 129.04, 128.71, 128.47, 128.19, 127.78, 127.46, 126.03, 125.38, 125.12, 124.96, 124.79, 124.69, 120.25, 114.23, 70.39, 64.90, 39.39, 30.53, 29.73, 29.09, 23.86, 23.06, 21.01, 14.10, 11.14. MALDI–TOF-MS(m/z): Anal. calcd. for C66H52O 860.43, found 859.890. 2-Pyrenyl-9-(4’-(2’-ethylhexyloxyphenyl))-9-pyrenylfluorene

(PFP1)

and

2,7-DiPyrenyl-9-(4’-(2’-ethylhexyloxyphenyl))-9pyrenylfluorene

(PFP2)

were

synthesized according to our previous work.21 The general information of PFP1 and PFP2 are follows: PFP1: 1H NMR (400 MHz, CDCl3) δ (ppm): 8.23 – 8.09 (m, 4H), 8.09 – 7.80 (m, 16H), 7.80 – 7.69 (m, 3H), 7.46 (td, J = 6.8 Hz, 14.8Hz, 1H), 7.29 (d, J = 8.0 Hz, 1H) 7.21 (d, J = 9.6 Hz, 1H), 6.82 (dd, J = 7.6 Hz, 15.2Hz, 2H), 3.83 (d, J = 5.6 Hz, 2H), 25



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

1.74 – 1.67 (m, 1H), 1.48 – 1.30 (m, 8H), 0.96 – 0.92 (m, 6H).13C NMR (101 MHz, CDCl3) δ (ppm): 158.20, 152.68, 140.54, 139.71, 138.88, 137.66, 131.43, 130.97, 130.86, 130.51, 130.41, 130.03, 129.68, 128.80, 128.37, 127.97, 127.76, 127.57, 127.47, 127.36, 127.30, 126.51, 125.91, 125.12, 125.08, 124.99, 124.91, 124.84, 124.70, 124.60, 120.61, 120.48, 114.80, 70.46, 66.33, 39.44, 30.56, 29.75, 29.13, 23.88, 23.08, 14.13, 11.17. MALDI–TOF-MS(m/z): Anal. calcd. for C59H46O 770.37, found 769.827. PFP2: 1H NMR (400 MHz, CDCl3) δ (ppm): 8.15 – 8.10 (m, 8H), 8.09 – 7.88 (m, 21H), 7.80 – 7.65 (m, 4H), 7.36 (d, J = 6.0 Hz, 2H), 6.83 (d, J = 8.8 Hz, 2H), 3.81 (d, J = 5.6 Hz, 2H), 1.73 – 1.68 (m, 1H), 1.35 – 1.19 (m, 8H), 0.92 – 0.86 (m, 6H). 13C NMR (101 MHz, CDCl3) δ (ppm): 158.25, 153.05, 140.65, 139.55, 138.69, 138.40, 137.62, 131.43, 131.02, 130.86, 130.54, 130.43, 130.24, 129.72, 128.94, 128.53, 128.38, 127.70, 127.61, 127.46, 127.37, 126.36, 125.92, 125.89, 125.10, 124.99, 124.84, 124.72, 124.62, 120.64, 114.80, 70.48, 66.48, 39.42, 30.55, 29.73, 29.12, 23.87, 23.06, 14.11, 11.16. MALDI–TOF-MS(m/z): Anal. calcd. for C75H54O 970.46, found 970.091.

4.2. Morphology Testing In order to further explore the relationship between molecule structure, aggregation morphology and device performance, the materials grew into nanoparticle by reprecipitation method, respectively. Firstly, the compound was added into 1mL Tetrahydrofuran (THF) to obtain 2mM solution. Then the solution was rapidly poured 26


Page 26 of 33

Page 27 of 33

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into 5mL deionized water with stirring. Five minutes later, the mixture solution was put into 25℃ circumstance for 24h. The SEM samples were prepared by dropping the nano-solution onto the silicon substrates, then which were annealed in 60℃ circumstance for 10h. The AFM tests were carried on the ITO substrate. The 40mg/ml chlorobenzene solution of the material was spin-coated onto the substrate, then was dried at 80℃ for 40min in the nitrogen atmosphere. The AFM morphology was tested by DI MultiMode NanoScope IIIa Vecco.

4.3.Testing and Device Fabrication Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were measured in a purged nitrogen atmosphere at a heating rate of 10 °C min−1. Absorption and photoluminescence (PL) emission spectra of the compounds were tested using a SHIMADZU UV-3600 spectrophotometer and a SHIMADZU RF-5301PC spectrophotometer, respectively. The spectra of solution state were measured in the 10-5 M dichloromethane solution. The film spectra were measured using thin film on ITO glass, which was prepared by spin-coated with toluene solution. The nano-solution was prepared by the follow procedure: the compound was added into 1mL Tetrahydrofuran (THF) to obtain 2mM solution; then the solution was rapidly poured into 5mL deionized water with stirring; five minutes later, the mixture solution was put into 25℃ circumstance for 24h. The cyclic voltammetry (CV) was achieved by an Eco Chemie’s Autolab instrument. In the process, glassy carbon electrode was used as the working electrode, platinum filament was used as the 27



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Page 28 of 33

counter electrode, Ag/AgNO3 was used as the reference electrode, and dicyclopentadienyl iron was used as the reference. The HOMO, LUMO and the energy gap between them ( ∆ E ) were calculated by cyclic voltammetry (CV). The HOMO/LUMO energy levels of the compounds were estimated based on the reference

energy

level

of

ferrocene

(4.7

eV

below

the

vacuum):

E HOMO = −(Eonset + 4.7eV ) and Eonset is the onset potential of the oxidation. E g = hc / λonset = 1240 / λonset , E LUMO = E HOMO + E g , the λonset is the terminal of absorption peak. The

OLED

devices

with

configuration

[ITO/PEDOT:PSS(30nm)/

PVK:EL:oXD-7(40nm)/TPBI（35 nm）/Ca(15nm):Ag(100nm)] were fabricated by spin-coated on the preprocessed indium-tin oxide (ITO), which were successively cleaned with acetone, detergent, deionized water, and 2-propanol under ultrasonic condition. Then the Oxygen plasma treatment was executed on substrate for 5 min to increase

the

work

function.

The

thin

layer

(30nm)

of

poly(ethylene-

dioxythiophene)-polystyrene sulfonic acid (PEDOT:PSS) was spin-coated onto the preprocessed ITO substrate and then was put in the vacuum oven for 10h at 80℃. The PEDOT:PSS layer acted as the electron hole injection, which could reduce the potential barrier between the anode and the electroluminescent layer (EL). The solution of the emission materials layer was spin-coated onto PEDOT:PSS layer in a glovebox under nitrogen atmosphere. The mixture of the electroluminescent layer was dried at 60℃ for 30 min. And the thickness of EL was 40nm. Then a thin layer of 35 nm TPBI was processed onto the substrate by the vacuum evaporation in a mask. 28


Page 29 of 33

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Subsequently, calcium layer and aluminum layer were carried out by means of evaporation in the same condition. The electroluminescence properties were measured by different instrumental analysis. The current-voltage (I-V) characteristics were measured by the Keithley 236 source meter. The electroluminescence spectra were analyzed by the PR 705 photometer.

Acknowledgements This work was financially supported by the Project Funded by The National Basic Research Program of China (973 Program, 2014CB648300),

Priority Academic

Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001), Program for

Changjiang

Scholars and Innovative Research Team in University

(IRT1148), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),

Synergetic

Innovation Center for Organic Electronics and Information

Displays, National Natural Science Foundation of China under Grant 11604158 and 61574098, Natural Science Foundation of Nanjing University of Posts and Telecommunications under Grant NY214090 and NY215160.

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Influence of ÏâÏ Hyperconjugation Effect on Thermal, Morphological

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