Phosphorus-Based Chromophores: Emitters for OLEDs

Article abstract of DOI:10.1080/10426507.2014.980414

In this paper, we present the development of white OLEDs (WOLEDs) using organophosphorus derivatives as fluorescent emitters. The structural changes that we have performed on and around the phosphole ring (functionalization of the phosphorus atom, nature of the substituents at the 2 and 5 position) allow the ability to tune the emission wavelengths and the color emitted by the OLEDs. Using this concept, we developed organophosphorus dopants emitting orange light, which can be mixed with a blue-emitting matrix for the development of WOLEDs. The OLEDs structure allows the development of flexible OLEDs with good performance and CIE coordinates which are independent from the current density.

Full text of DOI:10.1080/10426507.2014.980414

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Phosphorus-Based Chromophores:
Emitters for OLEDs
Matthew P. Duffya, Pierre-Antoine Bouita, Bernard Geffroyb, Denis
Tondelierc & Muriel Hisslera
a Institut des Sciences Chimiques de Rennes, UMR 622, CNRS-
Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex,
France
b Laboratoire d’Innovation en Chimie des Surfaces et Nanosciences,
CEA Saclay, IRAMIS, NIMBE, IRAMIS, SPCSI, 91191 Gif-sur-Yvette,
France
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c Laboratoire de Physique des Interfaces et Couches Minces, CNRS
UMR 7647, Ecole Polytechnique, 91128 Palaiseau, France
Accepted author version posted online: 23 Dec 2014.
To cite this article: Matthew P. Duffy, Pierre-Antoine Bouit, Bernard Geffroy, Denis Tondelier & Muriel
Hissler (2015) Phosphorus-Based Chromophores: Emitters for OLEDs, Phosphorus, Sulfur, and Silicon
and the Related Elements, 190:5-6, 845-853, DOI: 10.1080/10426507.2014.980414
To link to this article: http://dx.doi.org/10.1080/10426507.2014.980414
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Phosphorus, Sulfur, and Silicon, 190:845–853, 2015
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2014.980414
PHOSPHORUS-BASED CHROMOPHORES: EMITTERS
FOR OLEDS
Matthew P. Duffy,1 Pierre-Antoine Bouit,1 Bernard Geffroy,2
Denis Tondelier,3 and Muriel Hissler1
1Institut des Sciences Chimiques de Rennes, UMR 622, CNRS-Universite´ de Rennes
1, Campus de Beaulieu, 35042 Rennes cedex, France
2Laboratoire d’Innovation en Chimie des Surfaces et Nanosciences, CEA Saclay,
IRAMIS, NIMBE, IRAMIS, SPCSI, 91191 Gif-sur-Yvette, France
3Laboratoire de Physique des Interfaces et Couches Minces, CNRS UMR 7647,
Ecole Polytechnique, 91128 Palaiseau, France
GRAPHICAL ABSTRACT
Abstract In this paper, we present the development of white OLEDs (WOLEDs) using
organophosphorus derivatives as fluorescent emitters. The structural changes that we have
performed on and around the phosphole ring (functionalization of the phosphorus atom, nature
of the substituents at the 2 and 5 position) allow the ability to tune the emission wavelengths
and the color emitted by the OLEDs. Using this concept, we developed organophosphorus
dopants emitting orange light, which can be mixed with a blue-emitting matrix for the develop-
ment of WOLEDs. The OLEDs structure allows the development of flexible OLEDs with good
performance and CIE coordinates which are independent from the current density.
Keywords Phosphole; π-conjugated systems; WOLEDs; optoelectronics
Received 18 September 2014; accepted 20 October 2014.
Address correspondence to Muriel Hissler, Institut des Sciences Chimiques de Rennes, UMR 622, CNRS-
Universite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex. E-mail: muriel.hissler@univ-rennes1.fr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
845

846
M. P. DUFFY ET AL.
INTRODUCTION
The development of technologies based on molecular electronics (field-effect transis-
tors (OFET), light-emitting diodes (OLED), solar cells . . . ) was accompanied by a strong
development of the synthesis and photophysical studies of organic and organometallic
compounds having an extended π-conjugated system.1 Indeed, the development of ef-
ficient “plastic electronic” devices is only possible with control of electronic properties
and macroscopic organization of these active compounds. A great deal of attention has,
therefore, been focused on tuning the structure of the organic π-conjugated materials in
order to control their electronic nature (band gap, HOMO and LUMO levels, effective
conjugation length, etc.) in such a way that it suits the desired function (electrolumines-
cence, high-charge mobility, etc.). In this perspective, oligomers and π-linear conjugated
polymers incorporating heteroatoms have received special attention.1,2 The incorporation
of heterocyclopentadienes (thiophene, pyrrole, silole . . . ) allows the emergence of new ge-
ometric and electronic properties directly related to the nature of the heteroelement.3 In this
context, the phosphole ring is an attractive synthon because it displays electronic properties
that are markedly different from those of the widely used highly aromatic thiophene and
pyrrole rings.4 The key property of the phosphole ring for the molecular engineering of π-
conjugated systems is the presence of a reactive phosphorus center due to the low aromatic
character of this P-heterole. In fact, delocalization within the phosphole ring arises from
a hyperconjugation involving the exocyclic P-R σ-bond and the π-system of the dienic
moiety leading to σ-π hyperconjugation phenomenon.4 The insertion of this P-heterocycle
within classical π-conjugated systems results in a lowering of their HOMO-LUMO gaps5c-e
because conjugation is enhanced for macromolecules incorporating units exhibiting low
resonance energies. Moreover, the phosphole building block is a potential source of further
structural variations by (1) changing the nature of the P-substituent, (2) chemical modifica-
tions of the nucleophilic P-atom, (3) using this P-ring as a precursor to other P-heterocycles
featuring a π-system such as phosphametallocenes, or (4) inserting the phosphole into a
2D graphenic scaffold (Figure 1).6 These properties indicate that the phosphole is a good
candidate for the construction of tuneable emissive π-conjugated systems.
RESULT AND DISCUSSION
The introduction of phosphole building blocks in π-conjugated systems allows the
ability to perform molecular engineering of these π-systems using the power of phosphorus
chemistry. Simple chemical modifications performed on the phosphorus atom (oxidation,
coordination chemistry . . . ) allows straightforward access to a wide range of novel π-
conjugated systems with diverse optical and electrochemical properties, without the need
for additional multistep syntheses.5c,d,6 This strategy allowed us to optimize the emission
properties of the organophosphorus chromophores for the development of electrolumines-
cent diodes emitting different colors (yellow, orange . . . ).6a-c,h The performances of these
diodes are notable for fluorescent compounds. Following this work, we decided to use
our expertise to develop a novel series of orange-emitting dopants incorporating a phosp-
hole ring for the development of white OLEDs (WOLEDs). The use of the Fagan-Nugent
conditions on the dialkynes 1a,b to generate the phosphole, followed by oxidation with
S8 afforded thioxophospholes 2a,b (Scheme 1).5c These air stable compounds display 31
P
NMR chemical shifts in the usual range of thioxophopholes (2a: +51.6, 2b: +52.5 ppm)
and all their 1H and 13C NMR spectroscopic data support the proposed structures.

PHOSPHORUS-BASED CHROMOPHORES: EMITTERS FOR OLEDS
847
Figure 1 Example of phosphole-based π-conjugated systems.
Optical properties of 2a,b were studied in CH2Cl2 and in thin films by means of
UV-vis absorption and fluorescence (Table 1). The two compounds show broad absorption
in the visible range (Table 1), attributed to π−π∗ transitions of the extended conjugated
system, and a broad emission in the orange region of the visible spectrum. The replacement
of the methylthiophene substituent by a fluorenyl group does not dramatically modify the
physical properties. The maximum of absorption is blue shifted, the emission is weakly
affected (2b/2a; ꢀλmax = 27 nm, ꢀ λem = 3 nm, Table 1) and the photoluminescence
(PL) quantum yields (QYs) in solution are low. It was of interest to investigate their PL
property in a polymethylmethacrylate (PMMA) matrix because these compounds are used
as solid state emitters in OLEDs. The maxima of emission in PMMA (doping rate, 2 wt.%)
are only slightly blue-shifted compared to those recorded in solution (Table 1) and the PL
QYs of 2a,b in PMMA matrix reach 55%–70%, values that are considerably higher than
those observed in solution (Table 1). These observations, which are very appealing toward
the development of phosphole derivatives 2a,b as emitters for OLEDs, can be attributed to
Table 1 Optical (solution and in thin films) and electrochemical properties for the organophosphorus compounds
2a,b
a
a
b
c
d
λmax
ε (mol−1.L.
cm−1
λ
ꢁf
(%)
λem
(nm])
ϕf
em
Compound
(nm)
)
(nm)
(%)
Eo1xe (V)
Eo2xe (V)
Er1ed e (V)
2a
2b
447
420
29320
14400
575
578
0.5
0.5
549
560
70
55
+0.95
+1.13
+1.28
+1.36
−1.61
−1.61
aMeasured in CH2Cl2;
bfluorescence quantum yields determined using fluorescein as standard, 15%;
cdispersed in PMMA (2 wt.%.);
dmeasured in an integrating sphere;
eall potentials were obtained during cyclic voltammetric investigations in 0.2 M Bu4NPF6 in CH2Cl2. Platinum
electrode diameter 1 mm, sweep rate: 200 mV s−1. All reported potentials are referenced to SCE.

848
M. P. DUFFY ET AL.
Figure 2 EL spectra of the device A recorded at 30 mA/cm2.
rotational restriction in the solid state and to steric protection provided by the tetrahedral P-
center, the endocyclic cyclohexane, and the bulkiness of the fluorene and methylthiophene
substituent of compounds 2a,b preventing the formation of organized stacks in the solid
state.6c,h
The redox properties of these novel phosphole-based π-conjugated systems 2a,b
were determined by cyclic voltammetry (CV) recorded in CH2Cl2 using Bu4NPF6 as the
electrolyte. The compounds 2a,b present reversible reduction and oxidation processes.
Furthermore, the nature of the substituents in position 2 and 5 on the phosphole ring affects
the oxidation potentials while the reduction potentials are not modified (Table 1). The
LUMO level is unchanged and the HOMO level is stabilized going from compound 2a to
2b because molecule 2a (Scheme 1) presents a lower oxidation potential than compound
2b (Table 1).
Since compounds 2a,b are redox and thermally stable up to 300◦C and present an or-
ange emission (Table 1), we decided to coevaporate them with a blue emitter, 4,4’-bis(2,2’-
diphenylvinyl)biphenyl (DPVBi) for the construction of the active layer in WOLEDs.
Effectively, white emission could potentially be obtained by combining orange and blue
emissions if the ratio of the two colours is correct.7 First, we developed a multilayer
WOLED on a glass substrate having an (ITO/CuPc (10 nm)/α-NPB (50 nm)/doped-DPVBi
(50 nm)/Alq3 (10 nm)/LiF (1.2 nm)/Al (100 nm) structure. Doping of the blue matrix with
0.2% of compound 2a lead to an OLED exhibiting a small turn-on voltage (5.2 V) with cur-
rent and power efficiencies of 7.0 cd A−1 and 2.3 lm/W, respectively. Its electroluminescent
spectrum presents well-balanced emission characteristics of the DPVBi (452 nm, FWHM
= 70 nm) and of the dopant 2a (548 nm, FWHM = 115 nm) (Figure 2). The resulting
CIE coordinates of device A (0.31;0.39) are close to those of pure white light (0.33,0.33)
(Table 2) and are independent of the driving current. These data show that phosphole-based
small molecules can be used as dopants for DPVBi toward the development of efficient
WOLEDs. However, the required low doping rate value (ca. 0.2%) to reach white emission
is quite difficult to control and to reproduce within the coevaporation process.
In order to circumvent this problem, thioxophosphole 2b (Scheme 1) was then studied
as the dopant for the DPVBi matrix because this compound possesses a weak ability to

849

850
M. P. DUFFY ET AL.
Scheme 1 Synthetic routes to phosphole-based dopants.
aggregate in the solid state.6c Effectively, a doping rate of 3.8% was necessary to obtain
CIE coordinates that are close to white emission (0.31, 0.37). The performance (Table 2) of
this WOLED is very satisfying with a brightness of 1158 cd.m−2 at 20 mA/cm2. Moreover,
the brightness regularly increases with the current density showing the good stability of this
WOLED in spite of the rather high doping rate. In this case, the doping rate value to reach
white emission is quite easy to control and to reproduce within the coevaporation process
compared to the first case. Furthermore, a decrease of the doping rate (1.4%, Table 2) has an
effect on the CIE coordinates (0.20, 0.30) but the efficiencies are not modified. The device
C emits a bluish white emission (brightness of 3770 cd.m−2 at 20 mA/cm2) because the
contribution of the emission of DPVBi in the electroluminescence spectrum is increased.
This interesting result prompted us to develop flexible OLEDs because these devices
can be used in completely new lighting designs.8 We kept the structure of the device C
but we changed the glass substrate to poly(ethylene terephthalate) (PET) substrate. The
performances of the device D are very similar to device C. The flexible device D exhibits a
small turn-on voltage (4.8 V) with current and power efficiencies of 6.3 cd A−1 and 1.9 lm/W,
respectively. The CIE coordinates, which are similar to device C, are independent of the
driving current. The performance (Table 2) of this flexible device D is very satisfying with
a brightness of 2915 cd.m−2 at 20 mA/cm2. All these results demonstrate that phosphole-
based emitters can be used for the development of efficient fluorescent OLEDs.
CONCLUSION
These results show the significant potential of phospholes for applications in optoelec-
tronics and namely for the construction of WOLEDs. The major advantage of the phosphole
ring relative to other more traditional heterocyclopentadienes (thiophene, pyrrole) is the
presence of a reactive heteroatom. Thus, from a single organophosphorus compound, it is
possible to prepare easily a family of new chromophores presenting very different optical

PHOSPHORUS-BASED CHROMOPHORES: EMITTERS FOR OLEDS
851
and redox properties. In addition, the possibility of achieving coordination chemistry opens
up broad prospects including the development of diodes using phosphorescence or the
development of biosensors.
EXPERIMENTAL
Device Fabrication and Characterization: EL devices, based on a multilayer struc-
ture have been fabricated onto patterned ITO-coated glass substrates from Xin Yan Technol-
ogy (sheet resistance 20 ꢂ/sqr) or ITO coated on PET substrate from Sigma-Aldrich. The
organic materials (from Lumtec) are deposited onto the ITO anode by sublimation under
high vacuum (<10−6 Torr) at a rate of 0.2–0.3 nm/s. The common structure of all the de-
vices is the following: a thin layer (10-nm thick) of copper phtalocyanine (CuPc) is used as
hole injection layer (HIL) and 50 nm of N,N’-diphenyl-N,N’-bis(1-naphthylphenyl)-1,1’-
biphenyl-4,4’-diamine (α-NPB) as hole transporting layer (HTL). The emitting layer con-
sists of phosphole derivatives (15 nm). A thin layer of 4,4’-bis(2,2’-diphenylvinyl)biphenyl
(DPVBi) (35 nm), a commercially available molecule (from Syntec) is used as electron
transporting layer (ETL). A thin layer of bathocuproine (BCP) (10 nm) is used as hole
blocking layer. Alq3 (10 nm) is used as ETL. Finally, a cathode consisting of 1.2 nm of
LiF capped with 100 nm of Al is deposited onto the organic stack. The entire device is
fabricated in the same run without breaking the vacuum. In this study, the thicknesses of
the different organic layers were kept constant for all the devices. The active area of the
devices defined by the overlap of the ITO anode and the metallic cathode was 0.28 cm2.
The current-voltage-luminance (I-V-L) characteristics of the devices were measured
with a regulated power supply (Laboratory Power Supply EA-PS 3032-10B) combined
with a multimeter and a 1 cm2 area silicon calibrated photodiode (Hamamatsu). The
spectral emission was recorded with a SpectraScan PR650 spectrophotometer. All the
measurements were performed at room temperature and at ambient atmosphere with no
further encapsulation of devices.
Synthesis of Compound 2b: To a THF solution (10 mL) of Cp2ZrCl2 (286 mg,
1.00 mmol) and 1-(9,9’-dimethylfluoren-2-yl)-8-(5-methyl-2-thienyl)-1,7-octadiyne 1b
(395 mg, 1.00 mmol) was added dropwise (ca 1 min), at −78◦C, a hexane solution of
n-BuLi (2.5 M, 0.8 mL, 2.05 mmol). The solution was warmed to room temperature, and
stirred overnight. To this solution was added, at −78◦C, freshly distilled PhPBr2 (0.22 mL,
1.05 mmol). The solution was allowed to warm to room temperature and stirred for 30 h.
The solution was filtered on basic alumina (THF) and the volatile materials were removed
1
under vacuum. The yellow precipitate containing phosphole (31P-{ H} NMR (80 MHz,
CDCl3): δ = +13.0) was dissolved in CH2Cl2 (20 mL) and elemental sulfur (32 mg,
0.12 mmol) was added to this solution. The solution was stirred for 1 day at room tempera-
ture and chromatographed on silica gel (eluent: heptane/ethyl acetate, 95/5, v/v, Rf = 0.4).
2b was obtained as a yellow orange solid (yield = 70 %, 0.7 mmol;, 374 mg). TTꢃA10%
:
365◦C. 1H NMR (300 MHz, CD2Cl2): δ = 7.74 (ddd, 4J(H,H) = 1.5 Hz, 3J(H,H) = 6.9 Hz,
3
3J(P,H) = 13.8 Hz, 2H, Hortho), 7.58 (m, 1H, Hfluorenyl), 7.51 (d, J(H,H) = 7.8 Hz, 1H,
Hfluorenyl), 7.41–7.31 (m, 4H, 2 Hmeta, Hpara, Hfluorenyl) 7.28 (s, 1H, Hfluorenyl) 7.23–7.20
3
(m, 2H, Hfluorenyl), 7.16 (d, J(H,H) = 7.8 Hz, 1H, Hfluorenyl) 7.08 (AB system, J(H,H) =
3.6 Hz, ꢀυ = 150.1 Hz, 2H, Hthienyl), 2.87 (m, 2H, C = CCH2), 2.74 (m, 2H, C = CCH2),
2.38 (s, 3H, CH3thienyl), 1.64 (m, 4H, CH2), 1.29 (s, 6H, CH3fluorenyl). 13C NMR (75 MHz,
2
CD2Cl2): δ = 154.3 (s, Cfluorenyl), 153.9 (s, Cfluorenyl), 148.9 (d, J(P,C) = 22.1 Hz, Cβ),
143.9 (d, 2J(P,C) = 22.4 Hz, Cβ), 142.5 (s, Cthienyl), 139.1 (s, Cfluorenyl), 139.0 (s, Cfluorenyl),

852
M. P. DUFFY ET AL.
133.4 (d, 1J(P,C) = 82.0 Hz, Cipso), 133.3 (d, 2J(P,C) = 17.1 Hz, Cthienyl), 132.2 (d, 4J(P,C)
2
2
= 2.9 Hz, CHpara), 131.8 (d, J(P,C) = 12.3 Hz, Cfluorenyl), 130.9 (d, J(P,C) = 11.5 Hz,
CHortho), 130.1 (d, 1J(P,C) = 72.0 Hz, Cα), 129.1 (d, 3J(P,C) = 12.3 Hz, CHmeta), 129.0 (d,
1J(P,C) = 81.7 Hz, Cα), 128.6 (d, 4J(P,C) = 5.7 Hz, CHthienyl), 128.4 (d, 3J(P,C) = 5.8 Hz,
CHfluorenyl), 127.8 (s, CHfluorenyl), 127.4 (s, CHfluorenyl), 126.0 (s, CHthienyl), 123.7 (d, 3J(P,C)
= 5.5 Hz, CHfluorenyl), 123.0 (s, CHfluorenyl), 120.4 (s, CHfluorenyl), 120.0 (s, CHfluorenyl),
47.1 (s, CH3fluorenyl), 29.3 (s, C = CCH2), 29.1 (s, C = CCH2), 27.2 (s, CH3fluorenyl), 27.1
(s, CH3fluorenyl), 23.2 (s, CH2), 23.0 (s, CH2), 15.4 (s, CH3thienyl). 31P NMR (121 MHz,
CD2Cl2): δ = 52.5 (s). HRMS (ESI, m/z): [M + H]+ calcd for C34H32PS2, 535.16831;
found, 535.1701. Anal. calcd for C34H31PS2: C 76.37 H 5.84 S 11.99; found: C 76.28 H
6.02 S 12.05.
FUNDING
This work is supported by the Ministe`re de la Recherche et de l’Enseignement
Supe´rieur, the CNRS, the Re´gion Bretagne, China-French associated international labora-
tory in “Functional Organophosphorus Materials” and COST CM10302 (SIPS).
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