Luminescent Copolymers consisting of Hole and/or Electron
Transporting Groups
Yun Chen*, Shiao-Wen Hwang, Jian-Shin Liou, Shinn-Horng Chen
National Cheng Kung University, Department of Chemical Engineering, Tainan, Taiwan
e-mail: yunchen@mail.ncku.edu.tw
Abstract:
(1) Sixteen copoly(aryl ether)s consisting of alternate isolated hole- and electron-transporting
segments have been synthesized and characterized.. The hole-transporting segments include
N-(2-ethylhexyl)-3,6-carbazole, N-(2-ethylhexyl)-3,6-bis(styryl)carbazole, 2,7-bis(styryl)-9,9dihexylfluorene, 2,6-bis (styryl)-1,5-dihexyloxy-naphthalene and 1,4-bis(styryl)-2,5-dihexyloxy
benzene. The p-quaterphenyl or 1,3,4-oxadiazole derivatives were chosen as the electrontransporting segments. These copoly(aryl ether)s are soluble in common organic solvents, and
exhibit good thermal stability with 5% weight loss temperature above 400oC in nitrogen atmosphere.
The photoluminescence emitted from these copoly(aryl ether)s can be classified to three types: (1)
the emissions of polymers are contributed from each fluorophore; (2) the emissions are dominated
by the energy acceptor (the fluorophores with longer emissive wavelength) via re-absorption or
energy transfer; (3) the emissive wavelengths are longer than each fluorophore due to the interchain
interaction (such as excimer or exciplex). The emissive wavelengths of these copoly(aryl ether)s are
located at 407~498 nm in the blue visible region. The electrolumiescent (EL) spectra of most
copolymers are consistent with the PL spectra except P1, P2 and P10 in which additional EL
peaks have been attributed to the emission of interchain exciton.
According to the results of cyclic voltammetric experiments, the hole and electron affinities
of the isolated copolymers can be promoted simultaneously by the introduction of hole- and
electron- transporting segments. However, the isolated hole- and electron-transporting fluorophores
function as hole and electron trap centers. The hole and electron cannot recombine via intrachain
migration. Thus, we can imagine that the carrier-hopping barrier between the hole- and
electron-transporting fluorophores may be a factor to influence the exciton formation.
(2) A series of copolymers containing alternate fluorine and carbazole segments have been
successfully prepared. The obtained neutral polymers PFO, PF5CN, PFCN, PFC readily dissolve in
common organic solvents. After quaternization, the resulting polymer PFCNE shows solubility
characteristics opposite to that of PFCN, being soluble in more polar solvent, such as DMF, DMSO.
Both absorption and PL emission peaks of the alternating copolymers were blue-shifted with an
increase in the carbazole content due to the interruption of the main-chain conjugation by the
presence of the 3,6-carbazole units. Meanwhile, PL efficiency gradually decreases with the
increasing carbazole content because carbazole group has lower emission efficiency than fluorene
group. Moreover, introducing 3,6-carbazole units leads to greater stability toward aggregation
under annealing at 185 °C, which can be ascribed to its torsion conformation that depress the
tendency of π-stacking.
The HOMO energy level is raised, and thus the hole affinity is improved, by introducing
carbazole group. Although, the copolymers exhibited stable PL spectra independent of annealing,
their EL spectra showed significant red-shift with increasing operating voltage. The EL spectra of
PFCN changes apparently and the emission color changes from blue to yellow-green.
(3) Synthesis, optical, and electrochemical properties of novel copolymers P1’~P3’ consisting of
alternate hole-transporting 1,4-bis(hexyloxy)-2,5-distyrylbenzene (HDB) and electron-transporting
4-(4-(hexyloxy)phenyl)-3,5-diphenyl-4H-1,2,4-triazole (EDT) or 2,5-diphenyl-1,3,4-oxadiazole
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(EDO) segments linked via an ether spacer or a twisted σ-bond. These copolymers are soluble in
common organic solvents such as chloroform, NMP, and 1,1,2,2-tetrachloroethane and exhibit good
thermal stability with decomposition temperature higher than 375 oC. P1’~P3’ show efficient
energy transfer from EDT or EDO to EDO fluorophores when photoexcited. Optical and
electrochemical properties of P1’~P3’ are also investigated in detail by comparing with P4’ and P5’
containing similar chromophores. From the cyclic voltammograms, the onset oxidation and
reduction potentials for isolated P1’ and conjugated P2’ are comparable, indicating that the effect
of twisted σ-bond in P2’ is similar to that of ether spacer in P1’. Optimized geometry of P2’ and
P3’ shows that each HDB and EDT or EDB are twisted 83.6o or 89.6o, respectively, based on
MNDO semi-empirical calculations. The large torsion in P2’ and P3’ significantly limits
delocalization of charges between hole- and electron-transporting segments. Accordingly, in P2’
and P3’, the oxidation and reduction starts at the hole- and the electron-transporting, respectively,
like those in isolated P1’. The HOMO and LUMO energy levels of P1’~P3’, estimated from
electrochemical data, are -5.16, -5.17, -5.19 eV and -3.38, -3.38, -3.32 eV, respectively.
*
Electron Transport Segment (ETS)
X
Hole Transport Segment (HTS)
X
n
*
(1) X = O
HTS: carbazole, distyrylcarbazole, distyrylnaphthalene, distyrylfluorene, distyrylbenzene.
ETS: dicyanoquaterphenyl, bistrifluoromethylquaterphenyl, oxadiazole, dioxadiazole.
(2) X = ---; HTS: carbazole derivatives, ETS: fluorene
(3) X = O or ---; HTS: distyrylbenzene, ETS: triazole or oxadiazole
References:
1. Shiao-Wen Hwang and Yun Chen*, “Synthesis, Electrochemical and Optical Properties of
Novel Poly(aryl ether)s with Isolated Carbazole and p-Quaterphenyl Chromophores”,
Macromolecules, 34(9), 2981-6 (2001).
2. Shiao-Wen Hwang and Yun Chen*, “Photoluminescent and Electrochemical Properties of Novel
Poly(aryl ether)s with Isolated Hole-Transporting Carbazole and Electron-Transporting
1,3,4-Oxadiazole Fluorophores”, Macromolecules, 35(14), 5438-43 (2002).
3. Tzi-Yi Wu and Yun Chen*, “Poly(phenylene vinylene)-Based Copolymers Containing
3,7-phenothiazylene and 2,6-pyridylene chromophores: Fluorescence sensors for acids, metal
ions, and oxidation”, J. Polym. Sci.: Part A: Polym. Chem., 42(5), 1272-84 (2004).
4. Tzi-Yi Wu, Rong-Bin Sheu and Yun Chen*, “Synthesis and Optically Acid-Sensory and
Electrochemical Properties of Novel Polyoxadiazole Derivatives”, Macromolecules, 37(3),
725-33 (2004).
5. Shinn-Horng chen and Yun Chen*, “Poly(p-phenylene vinylene) Derivatives containing
Electron-transporting Aromatic Triazole or Oxadiazole Segments, Macromolecules, in
revision.
6. Jian-Shinn Liou, Shinn-Horng Chen, and Yun Chen*, “Synthesis, Optical and
Electrochemical Properties of Copolymers with Alternate Fluorene and Carbazole
Chromophores”, in preparation.
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