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SCIENCE CHINA Chemistry, Volume 60 , Issue 10 : 1356-1366(2017) https://doi.org/10.1007/s11426-017-9100-0

Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide unit in alkyl side chain

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  • ReceivedMar 24, 2017
  • AcceptedJun 13, 2017
  • PublishedSep 15, 2017

Abstract

Blue light-emitting polyfluorenes containing dibenzothiophene-S,S-dioxide (SO) unit in alkyl side chain (PF-FSOs and PF-CzSOs) were synthesized. All the polymers show high thermal stability with the decomposition temperatures over 400 °C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the copolymer slightly decrease with the increase of SO content in side chain. PL spectra of the polymers show slightly red shift and broadening with the increase of solvent polarities, indicating unremarkable intramolecular charge transfer (ICT) effect in the polymers containing SO unit in alkyl side chain. EL spectra of the polymers are almost unchanged in the current densities from 100 to 400 mA cm−2, indicating the superb EL stability of the resulted polymers. The EL spectra of the copolymers exhibit obvious blue-shift and narrowing with the CIE of (0.18, 0.11) for PF-FSO10 and (0.17, 0.11) for PF-CzSO10, respectively, compared with PF-SO10 containing SO unit in main chain with the CIE of (0.16, 0.17) and PFO with the CIE of (0.18, 0.18). The superior device performances were obtained with the luminous efficiency (LEmax) of 1.17 and 0.68 cd A−1 for PF-FSO15 and PF-CzSO20, respectively, compared with the LEmax of 0.37 cd A−1 for PFO. The results indicate that linking SO unit to alkyl side chain of the polyfluorene is a promising strategy for efficient blue light-emitting polymers.


Funded by

National Key Basic Research and Development Program of China(2015CB655004)

National Natural Science Foundation of China(51473054,91333206)

and the Fundamental Research Funds for the Central Universities

South China of Technology(2017MS020)


Acknowledgment

This work was supported by the National Key Basic Research and Development Program of China (2015CB655004), the National Natural Science Foundation of China (51473054, 91333206), and the Fundamental Research Funds for the Central Universities, South China of Technology (2017MS020).


Interest statement

The authors declare that they have no conflict of interest.


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  • Figure 1

    DSC curves of the polymers (color online).

  • Scheme 1

    Synthetic routes of the monomers.

  • Figure 2

    CV curves for PF-FSOs (a) and PF-CzSOs (b) (color online).

  • Scheme 2

    Synthetic routes of the polymers.

  • Figure 3

    UV-Vis absorption and PL spectra of the polymers PF-FSOs (a) and PF-CzSOs (b) in toluene solution (color online).

  • Figure 4

    PL spectra of the polymers PF-FSO20 (a) and PF-CzSO20 (b) in different polar solution (color online).

  • Figure 5

    UV-Vis absorption and PL spectra of the polymers PF-FSOs (a) and PF-CzSOs (b) in film (color online).

  • Figure 6

    EL spectra of the polymers PF-FSOs (a) and PF-CzSOs (b) (color online).

  • Figure 7

    EL spectra of the polymers compared with PFO (a) and PF-SO10 (b) (color online).

  • Figure 8

    EL spectra of the polymers PF-FSO15 (a) and PF-CzSO15 (b) with the variation of current densities from 100 to 400 mA cm−2 (color online).

  • Figure 9

    Current densities as a function of electric fields for single carrier devices of PFO (a), PF-FSO15 (b), and PF-CzSO20 (c) (color online).

  • Figure 10

    Current density (J)-voltage (V) curves of the PF-FSOs (a) and PF-CzSOs (b) with a configuration of ITO/PEDOT:PSS/EL/CsF/Al (color online).

  • Figure 11

    Energy levels of the materials used in the devices (color online).

  • Figure 12

    AFM topography (3 μm×3 μm) of PFO (a) and PF-FSO15 (b) (color online).

  • Table 1   Molecular weight and thermal properties of the polymers

    polymer

    Mn (×104)

    PDI

    SO (mol%) in

    Tg (oC)

    TLC

    (oC)

    Td (oC)

    feed ratio

    polymer a)

    PFO

    3.05

    2.31

    0

    0

    72

    140

    385

    PF-SO10

    3.55

    1.82

    10

    8.5

    156

    397

    PF-FSO5

    10.25

    1.64

    5

    8.9

    155

    411

    PF-FSO10

    15.62

    1.35

    10

    13.1

    79

    150

    410

    PF-FSO15

    14.93

    1.45

    15

    16.6

    83

    148

    411

    PF-FSO20

    11.18

    1.68

    20

    26.5

    94

    139

    412

    PF-CzSO5

    12.39

    1.55

    5

    7.0

    413

    PF-CzSO10

    8.79

    1.90

    10

    14.3

    411

    PF-CzSO15

    16.66

    1.47

    15

    16.1

    405

    PF-CzSO20

    17.17

    1.48

    20

    25.0

    408

    Calculated from the elemental analysis data.

  • Table 2   Photophysical and electrochemical properties of the polymers

    sample

    Eox (V)

    HOMO (eV)

    Ered (V)

    LUMO (eV)

    Egopt a) (eV)

    λabs,f (nm)

    λPL,f (nm)

    QPL b) (%)

    SO

    1.6

    −6.00

    −1.74

    −2.57

    PFO

    1.28

    −5.68

    −2.42

    −1.89

    3.00

    388

    425450

    32

    PF-SO10

    1.44

    −5.75

    −2.19

    −2.12

    2.90

    390

    446468

    42

    PF-FSO5

    1.38

    −5.69

    −2.23

    −2.08

    2.93

    384

    426447

    48

    PF-FSO10

    1.46

    −5.77

    −2.26

    −2.05

    2.93

    385

    423447

    40

    PF-FSO15

    1.40

    −5.71

    −2.28

    −2.03

    2.93

    385

    424,449

    51

    PF-FSO20

    1.46

    −5.77

    −2.36

    −1.95

    2.93

    385

    423,445

    59

    PF-CzSO5

    1.44

    −5.75

    −2.30

    −2.08

    2.94

    382

    423,439

    34

    PF-CzSO10

    1.42

    −5.73

    −2.25

    −2.06

    2.96

    382

    423,448

    32

    PF-CzSO15

    1.46

    −5.77

    −2.34

    −1.97

    2.96

    379

    422,447

    36

    PF-CzSO20

    1.37

    −5.68

    −2.21

    −2.10

    2.96

    375

    424,447

    66

    Calculated from the absorption spectra threshold; b) measured in film.

  • Table 3   Device performances of the polymers

    polymer

    Von (V)

    LEmax (cd A−1)

    EQEmax (%)

    Lmax (cd m−2)

    CIE (x,y) b)

    PFO

    3.4

    0.37

    0.42

    1934

    (0.18, 0.18)

    PF-SO10

    3.2

    2.04

    1.46

    4296

    (0.16, 0.17)

    PF-FSO5

    3.8

    0.74

    0.77

    3911

    (0.17, 0.13)

    PF-FSO10

    4.6

    0.80

    0.83

    3095

    (0.18, 0.11)

    PF-FSO15

    4.8

    1.17

    1.10

    2002

    (0.18, 0.11)

    PF-FSO20

    4.4

    0.70

    0.99

    1288

    (0.17, 0.10)

    PF-CzSO5

    4.2

    0.54

    0.56

    2837

    (0.17, 0.12)

    PF-CzSO10

    3.8

    0.22

    0.23

    1143

    (0.17, 0.11)

    PF-CzSO15

    3.6

    0.51

    0.52

    1764

    (0.17, 0.10)

    PF-CzSO20

    4.1

    0.68

    0.71

    2075

    (0.17, 0.10)

    The device structure: ITO/PEDOT:PSS/EL/CsF/Al; b) measured at 12 mA cm−2 current density.

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