Anodic oxidation of novel hole-transporting materials derived from tetraarylbenzidines. Electrochemical and spectroscopic characterization

Article abstract of DOI:10.1135/cccc20001403

Fluorenylidene-linked triarylamines, potential hole-transporting materials, have been prepared by the palladium-catalyzed Hartwig-Buchwald animation. Their redox and spectral properties were investigated in solution, applying cyclic voltammetry, UV-VIS and WR spectroscopy, and in situ spectroelectrochemical measurements. N,N,N′,N′-Tetraphenylbenzidine (1), N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (2), and triphenylamine (3) served as model substances in the study of the synthesized complex compounds 4 and S. In structure 4, two triphenylamine centres are linked with a non-conjugated fluorene bridge; in structure 5, two tetraarylbenzidine skeletons with two nitrogens are linked with a conjugated biphenyl-bridge system. In addition, structure 5 contains a non-conjugated fluorene bridge. The presence of the fluorene moiety in the molecular design has a significant influence on the investigated properties of the new materials. In the anodic oxidation of the tetraarylbenzidine-type compounds (1, 2, and 5), two well-defined reversible oxidation peaks were observed. However, the oxidation of the triphenylamine-type structures (3 and 4) is more complex, due to fast consecutive reactions. The dimer-like structures (4 and 5) are characterized by two independent oxidation centres that are simultaneously oxidized at approximately the same potentials. This was confirmed by quantitative cyclovoltammetric as well as UV-VIS investigations.

Full text of DOI:10.1135/cccc20001403

Anodic Oxidation of Tetraarylbenzidines
1403
ANODIC OXIDATION OF NOVEL HOLE-TRANSPORTING MATERIALS
DERIVED FROM TETRAARYLBENZIDINES. ELECTROCHEMICAL AND
SPECTROSCOPIC CHARACTERIZATION
a3,
René FÁBERa1,b, G. Felix MIELKEb1, Peter RAPTAa2, Andrej STAŠKO
*
b2
and Oskar NUYKEN
a
Department of Physical Chemistry, Slovak Technical University, Radlinského 9,
1
SK-812 37 Bratislava, Slovak Republic; e-mail: rene.faber@ch.tum.de,
2
3
rapta@cvt.stuba.sk, stasko@cvt.stuba.sk
Lehrstuhl für Makromolekulare Stoffe, Technische Universität München, Lichtenbergstr. 4,
D-85747 Garching, Germany; e-mail: felix.mielke@ch.tum.de, oskar.nuyken@ch.tum.de
b
1
2
Received March 15, 2000
Accepted Jun e 16, 2000
Dedicated to Professor 4udovít Treindl in honour of his 70th birthday.
Fluoren yliden e-lin ked triarylam in es, poten tial h ole-tran sportin g m aterials, h ave been pre-
pared by th e palladium -catalyzed Hartwig–Buch wald am in ation . Th eir redox an d spectral
properties were in vestigated in solution , applyin g cyclic voltam m etry, UV-VIS an d EPR spec-
troscopy, an d in situ spectroelectroch em ical m easurem en ts. N,N,N′,N′-Tetraph en ylben zidin e
(1), N,N′-di(1-n aph th yl)-N,N′-diph en ylben zidin e (2), an d triph en ylam in e (3) served as m odel
substan ces in th e study of th e syn th esized com plex com poun ds 4 an d 5. In structure 4, two
triph en ylam in e cen tres are lin ked with a n on -con jugated fluoren e bridge; in structure 5, two
tetraarylben zidin e skeleton s with two n itrogen s are lin ked with a con jugated biph en yl-
bridge system . In addition , structure 5 con tain s a n on -con jugated fluoren e bridge. Th e pres-
en ce of th e fluoren e m oiety in th e m olecular design h as a sign ifican t in fluen ce on th e in ves-
tigated properties of th e n ew m aterials. In th e an odic oxidation of th e tetraarylben zidin e-
type com poun ds (1, 2, an d 5), two well-defin ed reversible oxidation peaks were observed.
However, th e oxidation of th e triph en ylam in e-type structures (3 an d 4) is m ore com plex,
due to fast con secutive reaction s. Th e dim er-like structures (4 an d 5) are ch aracterized by
two in depen den t oxidation cen tres th at are sim ultan eously oxidized at approxim ately th e
sam e poten tials. Th is was con firm ed by quan titative cyclovoltam m etric as well as UV-VIS in -
vestigation s.
Key w ord s: EPR/UV-VIS spectroelectroch em istry; Cyclic voltam m etry; Hole-tran sportin g m a-
terials; Tetraarylben zidin es; Hartwig–Buch wald am in ation ; Triarylam in es; Electroch em istry.
Organ ic h ole-tran sportin g m aterials fin d application s in organ ic
ligh t-em ittin g diodes (OLEDs)1. To be can didates for th e use in
h ole-tran sportin g layers, organ ic m aterials sh ould possess a variety of
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1404
Fáber, Mielke, Rapta, Staško, Nuyken:
ch em ical an d ph ysical properties, such as good electroch em ical stability, a
low ion isation poten tial, reversible redox beh aviour, h ole-tran sport an d
h ole-in jection ability from th e an ode in to th e h ole-tran sportin g layer
(HTL). In addition , th erm al stability, ph otostability, an d good film -form in g
properties are required. At presen t, triph en ylam in e derivatives are com -
m on ly used as h ole-tran sportin g m aterials, usually in th e form of
tetraarylben zidin es2–4. However, th ese com poun ds suffer from low th erm al
stability because of th eir low glass-tran sition tem peratures. Several ap-
proach es were con sidered to im prove th e th erm al properties of triarylam in e
HTL m aterials, such as syn th esis of “starburst” m olecules5,6, asym m etric
substitution of ben zidin e derivatives7,8, or th e use of spiro lin kages9–11
.
Low-m olecular-weigh t triarylam in es h ave also been in corporated in to poly-
m ers12.
In th is work, we presen t a n ew con cept for in creasin g th e glass-tran sition
tem perature (Tg) of OLED m aterials. Th e aim is to couple two h ole-
tran sportin g un its in a way th at th erm al an d film -form in g properties are
im proved wh ile th e electron ic ch aracteristics are n ot im paired. Th is can be
ach ieved by a rigid lin ker th at in creases m olecular weigh t an d steric de-
m an ds but does n ot lead to a con jugated system of th e h ole-tran sportin g
subun its. As a lin ker, we ch ose th e sp3-carbon of fluoren e. Th e resultin g
diph en ylfluoren e structure h as been sh own to in crease Tg an d solubility in
polym ers with rigid arom atic structures13,14. Sin ce th e fluoren e un it an d th e
two ph en yl rin gs are n ot con jugated, th is lin kage sh ould preserve th e fa-
vourable electron ic ch aracteristics of th e low-m olecular-weigh t triaryl-
am in es.
Th erefore we in vestigated low-m olecular-weigh t m aterials, N,N,N′,N′-tetra-
ph en ylben zidin e (TPB) (1), N,N′-di(1-n aph th yl)-N,N′-diph en ylben zidin e
(α-NPD) (2), an d triph en ylam in e (TPA) (3) as m odel substan ces to estim ate
redox an d spectral properties of th e n ewly syn th esized com poun ds F-TPA
(4) an d F-1αNPD (5) sh own in Sch em e 1. Th e h ole-tran sportin g an d elec-
troch em ical properties of com poun ds 1–3 h ave already been described, an d
th ey h ave also been em ployed in OLEDs3,15–22
.
Th e syn th esis of 4 by th e Ullm an n couplin g an d its ch aracteristics as a
h ole-tran sportin g layer in OLEDs h ave been reported by Okutsu et al.23. In
our work, we used a tetraarylben zidin e derivative as a h ole-tran sportin g
un it wh ich leads to an electroch em ical poten tial m ore suitable for applica-
tion s of OLEDs. Com poun ds 4 an d 5 were prepared usin g th e palladium -
catalyzed Hartwig–Buch wald am in ation of brom oaren es24–26 (Sch em e 2).
Morph ological in stability un der operatin g con dition s is assum ed to be on e
of th e m ost im portan t failure m odes of OLEDs, th e problem th at can be
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1405
overcom e by h igh er Tg of m aterials. Th e n ew com poun d 5 h as a Tg of 153 °C
wh ich is on e of th e h igh est reported for th e triarylam in es so far used in
OLEDs (refs8,26-29).
Th e ion ization poten tial of h ole-tran sportin g layers is a crucial factor in
m olecular design of h ole-tran sport m aterials suitable for producin g electro-
lum in escen t (EL) devices of h igh durability. In particular, th e form ation of
a sm all en ergy barrier at th e HTL/an ode in terface is essen tial for h igh dura-
bility.
N
N
N
N
1 (TPB)
2 (α-NPD)
N
N
N
4 (F-TPA)
3 (TPA)
N
N
N
N
5 (F-1αNPD)
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1406
Fáber, Mielke, Rapta, Staško, Nuyken:
Th e aim of th is con tribution is to determ in e redox properties of th e
n ewly syn th esized com poun ds 4 an d 5, in cludin g th e ch arge-in jection bar-
rier of th e h ole-tran sport m aterials with respect to in dium -tin -oxide (ITO).
Th e evaluation of th eir HOMO en ergies based on cyclovoltam m etric m ea-
surem en ts, usin g th e ferrocen e/ferrocen ium in tern al redox stan dard30. In
order to fin d a plausible m ech an ism of th e oxidation of 4 an d 5, our
cyclovoltam m etric studies were com pleted with in situ EPR an d UV-VIS
spectroelectroch em ical in vestigation s.
+ Ph2NH;
[Pd2(dba)3]/DPPF
t-BuONa
1. NaNO2/HBr
2. CuBr
4
H2N
NH2
Br
Br
6
7
H
H
N
N
+ 7;
Br
[Pd2(dba)3]/DPPF
[Pd2(dba)3]/DPPF
t-BuONa
t-BuONa
+
5
9
N
N
H
10
8
SCHEME 2
EXPERIMENTAL
Instrumentation
Th e cyclic voltam m ogram s were recorded usin g a platin um wire as workin g an d auxiliary
electrodes. A saturated calom el electrode (SCE) served as referen ce. Th e m easurem en ts were
carried out with a Prin ceton Applied Research (PAR 270) un iversal electroch em ical system .
Ferrocen e/ferricen ium (Fc/Fc+) was used as an in tern al poten tial m arker.
Lam in ated gold-µ-m esh 31, or lam in ated con ductin g ITO coated glass-plate electrodes32
were em ployed in spectroelectroch em ical experim en ts. Redox cyclin g of th e in vestigated
com poun ds with sim ultan eous EPR an d UV-VIS m easurem en ts was carried out in dich loro-
m eth an e solution s con tain in g 0.1 M tetrabutylam m on ium h exafluoroph osph ate (TBAPF6).
Th e referen ce electrode con sisted of a silver wire coated with silver ch loride. EPR spectra
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1407
were recorded on an ESR 200D X-Ban d spectrom eter (Bruker, Germ an y). UV-VIS spectra (λ in
n m ) were recorded on a diode array UV-VIS spectroph otom eter PC 1000 (Ocean Optics,
In c.). A HEKA PG 284 (Lam brech t, Germ an y) poten tiostat/galvan ostat was used as electro-
ch em ical equipm en t for th ese m easurem en ts.
Sin ce n o ch an ges in th e sh ape of th e EPR spectra were observed durin g th e redox cyclin g,
th e in ten sity of th e EPR sign al was also recorded usin g con stan t-field con dition s. Th e m ag-
n etic field was set on th e m axim al in ten sity of th e EPR lin e of th e in vestigated radical in ter-
m ediate an d th e EPR in ten sity was m easured in a tim e-based m ode.
Materials
9,9-Bis(4-am in oph en yl)fluoren e, triph en ylam in e, diph en ylam in e, 1-brom on aph th alen e,
1,1′-bis(diph en ylph osph in o)ferrocen e (DPPF), an d sodium tert-butoxide were purch ased from
Aldrich ; h ydrobrom ic acid, N,N′-diph en ylben zidin e, tetrabutylam m on ium h exafluoro-
ph osph ate an d dich lorom eth an e from Fluka; N,N,N′,N′-tetraph en ylben zidin e from Syn Tec
(Wolfen , Germ an y); tris(diben zyliden eaceton e)dipalladium (0) (Pd2(dba)3) an d palladium ace-
tate from Strem Ch em icals. All ch em icals were used as received.
Syntheses
All palladium -catalyzed reaction s were carried out in dried solven ts un der n itrogen .
N,N′-Di(1-n aph th yl)-N,N′-diph en ylben zidin e33 (2)
To
a stirred solution of palladium acetate (90 m g, 400 µm ol) an d 1,1′-bis(diph en yl-
ph osph in o)ferrocen e (222 m g, 400 µm ol) in toluen e (100 m l) were added N,N′-diph en yl-
ben zidin e (3.36 g, 10.0 m m ol) an d 1-brom on aph th alen e (6.21 g, 30.0 m m ol). After addition
of sodium tert-butoxide (2.31 g, 24.0 m m ol), th e m ixture was refluxed un der n itrogen for
3 days. After coolin g, 100 m l of brin e were added an d th e m ixture was extracted twice with
ch loroform . Th e com bin ed organ ic ph ases were wash ed with brin e an d dried over an h y-
drous m agn esium sulfate. Th e solven ts were rem oved on a rotary evaporator, an d th e resi-
due was purified by colum n ch rom atograph y on silica gel usin g cycloh exan e–toluen e (4 : 1
v/v) as eluen t. Yield 3.17 g (54%). MS, m/z (%): 588 (M+, 100), 294 (M2+, 22), 217 (8). 1H NMR
(CDCl3): 7.94 (m , 2 H); 7.87 (m , 2 H); 7.76 (m , 2 H); 7.52–7.25 (m , 12 H); 7.19 (m , 4 H);
7.08–6.87 (m , 10 H). 13C NMR (CDCl3): 148.35, 147.27, 143.47, 135.30, 133.84, 131.27,
129.10, 128.38, 127.23, 127.08, 126.48, 126.39, 126.35, 126.13, 124.29, 121.96, 121.92,
121.73. For C44H32N2 (588.8) calculated: 89.76% C, 5.48% H, 4.76% N; foun d: 89.73% C,
5.45% H, 4.69% N.
9,9-Bis(4-brom oph en yl)fluoren e (7)
9,9-Bis(4-am in oph en yl)fluoren e (3.49 g, 10.0 m m ol) was suspen ded in 35% aqueous
hydrobrom ic acid (35 m l) and the suspension was cooled to 0 °C. Ice-cold 2.5 M NaNO2 solu-
tion (8 m l) was added dropwise so th at th e tem perature did n ot rise above 5 °C. Th e yellow
su sp en sion was added to
a so lu tion of Cu Br (2.00 g, 14.0 m m ol) in 48% aq u eou s
h ydrobrom ic acid (10 m l) at 0 °C. After stirrin g for 5 m in , th e m ixture was h eated to 100 °C
un til th e form ation of gas ceased. After coolin g, th e m ixture was extracted with ch loroform
th ree tim es (20 m l each ). Th e com bin ed organ ic ph ases were wash ed with brin e an d dried
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1408
Fáber, Mielke, Rapta, Staško, Nuyken:
over an h ydrous m agn esium sulfate. Th e solven t was rem oved on a rotary evaporator. Th e
residue was purified by colum n ch rom atograph y on silica gel usin g cycloh exan e as eluen t.
Yield 3.24 g (68%). MS, m/z (%): 476 (M+, 100), 397 (27), 395 (28), 321 (26), 319 (27), 315
(14), 239 (55), 198.5 (14), 197.5 (15). 1H NMR (CDCl3): 7.76 (m , 2 H); 7.45–7.22 (m , 10 H); 7.04
(m , 4 H). 13C NMR (CDCl3): 150.16, 144.53, 140.07, 131.44, 129.78, 127.99, 127.93, 125.85,
120.95, 120.42, 64.61. For C25H16Br2 (476.2) calculated: 63.06% C, 3.39% H, 33.56% Br;
foun d: 63.27% C, 3.37% H, 33.43% Br.
9,9-Bis[4-(diph en ylam in o)ph en yl]fluoren e (4)
To a stirred solution of tris(diben zyliden eaceton e)dipalladium (0) (37 m g, 40 µm ol) an d
1,1′-bis(diph en ylph osph in o)ferrocen e (44 m g, 80 µm ol) in toluen e (20 m l) were added
9,9-bis(4-brom oph en yl)fluoren e 7 (952 m g, 2.0 m m ol) an d diph en ylam in e (745 m g, 4.4
m m ol). After addition of sodium tert-butoxide (463 m g, 4.8 m m ol) th e m ixture was refluxed
un der n itrogen for 24 h . After coolin g, 50 m l of brin e were added an d th e m ixture was ex-
tracted twice with dich lorom eth an e. Th e com bin ed organ ic ph ases were wash ed with brin e
an d dried over an h ydrous m agn esium sulfate. Th e solven ts were rem oved on a rotary evapo-
rator, an d th e residue was purified by colum n ch rom atograph y on silica gel usin g cycloh ex-
an e–toluen e (2 : 1 v/v) as eluen t. Yield 675 m g (52%). MS, m/z: 652 (M+, 100), 408 (25), 326
(M2+, 32). 1H NMR (CDCl3): 7.74 (m , 2 H); 7.43 (m , 2 H); 7.34 (m , 2 H); 7.27 (m , 2 H); 7.19
(m , 8 H); 7.08–7.01 (m , 12 H); 6.96 (m , 4 H); 6.89 (m , 4 H). 13C NMR (CDCl3): 151.56,
147.68, 146.15, 140.05, 139.73, 129.14, 128.87, 127.57, 127.35, 126.23, 124.32, 123.11,
122.69, 120.09, 64.50. For C49H36N2 (652.8) calculated: 90.15% C, 5.56% H, 4.29% N;
foun d: 89.59% C, 5.68% H, 4.06% N.
N-(1-Naph th yl)-N,N′-diph en ylben zidin e (10)
To
a stirred solution of palladium acetate (45 m g, 200 µm ol) an d 1,1′-bis(diph en yl-
ph osph in o)ferrocen e (111 m g, 200 µm ol) in toluen e (150 m l) were added N,N′-diph en yl-
ben zidin e (6.06 g, 18.0 m m ol) an d 1-brom on aph th alen e (1.255 g, 6.0 m m ol). After addition
of sodium tert-butoxide (692 m g, 7.2 m m ol), th e m ixture was refluxed un der n itrogen for 46 h .
After coolin g, 300 m l of toluen e an d 100 m l of brin e were added, an d excess N,N′-diph en yl-
ben zidin e was filtered off. Th e aqueous ph ase was extracted with toluen e (100 m l). Th e or-
gan ic ph ases were com bin ed, wash ed with brin e an d dried over an h ydrous m agn esium
sulfate. Th e solven ts were rem oved on a rotary evaporator, an d th e residue was purified by
colum n ch rom atograph y on silica gel usin g cycloh exan e–toluen e (3 : 2 v/v) as eluen t. Yield
1.124 g (40%). MS, m/z (%): 462 (M+, 100), 231 (M2+, 22). 1H NMR (THF-d8): 7.97 (m , 1 H);
7.90 (m , 1 H); 7.79 (m , 1 H); 7.52–7.30 (m , 9 H); 7.23–6.98 (m , 12 H); 6.89 (m , 1 H); 6.80
(m , 1 H). 13C NMR (THF-d8): 149.77, 148.21, 144.93, 144.80, 144.00, 136.70, 135.73, 133.40,
132.48, 129.98, 129.40, 128.14, 127.98, 127.74, 127.39, 127.28, 127.21, 127.04, 125.26,
123.27, 122.81, 122.56, 120.97, 118.34 (2 C). For C34H26N2 (462.6) calculated: 88.28% C,
5.66% H, 6.06% N; foun d: 88.29% C, 5.62% H, 5.69% N.
9,9-Bis[4-(N-{4′-[N-(1-n aph th yl)an ilin o]biph en yl-4-yl}an ilin o)ph en yl]fluoren e (5)
To a stirred solution of tris(diben zyliden eaceton e)dipalladium (0) (69 m g, 75 µm ol) an d
1,1′-bis(diph en ylph osph in o)ferrocen e (83 m g, 150 µm ol) in toluen e (40 m l) were added
9,9-bis(4-brom oph en yl)fluoren e (7; 1.19 g, 2.5 m m ol) an d N-(1-n aph th yl)-N,N′-diph en yl-
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1409
ben zidin e (10; 2.31 g, 5.0 m m ol). After addition of sodium tert-butoxide (577 m g, 6.0 m m ol),
th e m ixture was refluxed un der n itrogen for 6 days. After coolin g, 50 m l of brin e was added,
an d th e m ixture was extracted th ree tim es with ch loroform . Th e organ ic ph ases were com -
bin ed, wash ed with brin e an d dried over an h ydrous m agn esium sulfate. Th e solven ts were
rem oved on a rotary evaporator, an d th e residu e was pu rified by colu m n ch rom atogra-
p h y on silica gel u sin g cycloh exan e–ch loroform (1 : 1 v/v) as eluen t. Yield 2.00 g (65%).
MS (MALDI-TOF), m/z (%): 1 239 (M+, 100). 1H NMR (CDCl3): 7.94 (m , 2 H); 7.87 (m , 2 H);
7.74 (m , 4 H); 7.49–7.13 (m , 30 H); 7.09–6.87 (m , 28 H). 13C NMR (CDCl3): 151.53, 148.28,
147.56, 147.36, 146.40, 146.03, 143.43, 140.04, 139.81, 135.28, 134.83, 133.69, 131.24,
129.17, 129.10, 128.90, 128.38, 127.59, 127.35, 127.22, 127.14 (2 C); 126.48, 126.39,
126.34, 126.23, 126.13, 124.43, 124.26 (2 C); 123.21, 122.80, 122.00, 121.82, 121.78,
120.09, 64.51. For C93H66N4 (1 239.6) calculated: 90.11% C, 5.37% H, 4.52% N; foun d:
89.56% C, 6.07% H, 4.07% N.
RESULTS AND DISCUSSION
A ch aracteristic feature of structures 1, 2, an d 5 is th e tetraarylben zidin e
skeleton , wh ere two n itrogen cen tres are lin ked by a con jugated biph en yl
bridge. In addition , structure 5 con tain s a n on -con jugated fluoren e bridge.
Th e ch aracteristic structure of com poun ds 3 an d 4 is a skeleton cen tred
aroun d on ly on e n itrogen atom (triph en ylam in e in 3 an d two triph en yl-
am in es lin ked by a n on -con jugated fluoren e bridge in 4). Th ese attributes
are stron gly reflected in th e results obtain ed.
Th e cyclic voltam m ogram s of th e “tetraarylben zidin e” structures 2 an d 5
with biph en yl-con jugated bridges are sh own in Figs 1a-1 an d 1b-1, respec-
tively. In both cases two ch em ically reversible oxidation s were observed.
Th e extracted data (peak an d h alf-wave poten tials, an d peak-to-peak separa-
tion s) are sum m arized in Table I.
TABLE I
Peak an d h alf-wave poten tials, an d peak separation s extracted from cyclic voltam m ogram s
of com poun ds 1–5 (in V vs SCE, scan rate 100 m V s–1) m easured in 0.001 M solution s of sub-
strate an d 0.1 M solution s of TBAPF6 in CH2Cl2
Com poun d
Ep1a
Ep2a
Ep2c
Ep1c
E11/ 2
E12/ 2
∆Ep1,2
1
2
3
4
5
0.719
0.733
–
0.957
1.002
1.012a
0.982a
1.011a
0.858
0.896
0.859
0.840
0.898a
0.619
0.626
0.642
0.640
0.646a
0.669
0.680
–
0.908
0.949
–
0.238
0.269
–
–
–
–
–
0.762a
0.704
0.955
0.249
a
Tran sfer of two electron s.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1410
Fáber, Mielke, Rapta, Staško, Nuyken:
Th e h alf-wave poten tials for th e first oxidation of th e tetraarylben zidin e
structures (1, 2, an d 5) are fairly sim ilar, an d cen tred at approxim ately 0.69 V
vs SCE. Th e secon d oxidation occurs at about 0.93 V. Th ese processes corre-
spon d to th e form ation of correspon din g m on o- an d dication s, respec-
tively, as was reported3,15 for 1 an d 2. However, as com poun d 5 con tain s
two tetraarylben zidin e redox cen tres lin ked by a n on -con jugated fluoren e
m oiety, we also carried out spectroelectroch em ical in vestigation s in order
to clarify th e m ech an ism of th e oxidation in th is case. Th erefore, our
cyclovoltam m etric studies were perform ed alon g with th e EPR an d UV-VIS
in vestigation s. Th e correspon din g EPR spectra obtain ed at th e poten tials of
th e in itial oxidation of 2 an d 5 are presen ted in Figs 1a-2 an d 1b-2, respec-
tively. From th e sim ulation of th e EPR spectra of 1+• an d 2+• , th e followin g
a1
b1
0.4
0.6
0.8
1.0
1.2
0.4
0.6
0.8
1.0
1.2
E vs SCE, V
E vs SCE, V
a2
b2
10 G
10 G
FIG. 1
Cyclic voltam m ogram s of 2 (a-1) an d 5 (b-1) sh owin g oxidation in CH2Cl2/0.1 M TBAPF6 (Pt
workin g electrode, scan rate 100 m V s–1), alon g with th e EPR spectra of th e correspon din g
cation -radicals of 2 (a-2) an d 5 (b-2) gen erated in th e poten tial region of th e first an odic
peaks
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1411
h yperfin e splittin g con stan ts were obtain ed: 2aN(1) = 0.433 m T an d 2aN(2) =
0.415 m T. Th is is in good agreem en t with th e data reported for N,N′-bis-
(3-m eth yl)-N,N′-diph en ylben zidin e34 (TPD). It is n oteworth y th at th e EPR
spectrum of com poun d 5 does n ot sh ow an y h yperfin e splittin g (see be-
low).
Com poun ds 3 an d 4 sh owed a com pletely differen t cyclovoltam m etric
beh aviour. On ly on e partly reversible oxidation in th e poten tial region of
th e secon d an odic peak (referrin g to th e previous group of com poun ds) was
observed, togeth er with a sm all cath odic peak due to a secon dary oxidation
product on th e reverse scan , as presen ted in Figs 2a-1 (3) an d 2b-1 (4). Th is
in dicates th at redox processes are associated with con secutive ch em ical re-
action s. In accordan ce with th at assum ption , th e n ew redox couples are ev-
iden t after th e first an odic sweep with both com poun ds 3 an d 4 (dash ed
lin es in Figs 2a-1 an d 2b-1) in th e poten tial region from +0.6 to +0.8 V.
Sim ilar voltam m ogram s were previously observed for m odel com poun d 3
(ref.16). Correspon din g in situ EPR spectroelectroch em ical experim en t at th e
an odic poten tial aroun d 1 V led to th e EPR spectra sh own in Figs 2a-2 an d
2b-2 for 3 an d 4, respectively. Th e EPR spectrum in Fig. 2a-2 due to oxi-
dized 3 correspon ds to a radical cen tred with two equivalen t n itrogen s h av-
in g th e splittin g con stan t 2aN = 0.479 m T; addition ally, well-resolved
h yperfin e splittin g is eviden t. An alogously to th e situation with 5, oxida-
tion of 4 on ly results in an un resolved broad EPR sign al. Th e redox poten -
tials of th e n ew couples alon g with th e splittin g con stan ts of two
equivalen t n itrogen s stron gly suggest th e form ation of a tetraph en yl-
ben zidin e skeleton , an alogous to structures 1, 2, an d 5.
Th e cyclovoltam m etric an d EPR results obtain ed m ay be in terpreted in
th e followin g way. Th e isolated triph en ylam in e skeleton in 3 is oxidized at
h igh poten tials (in th e region of th e secon d peak of tetraarylben zidin es)
an d form s cation -radicals, wh ich rapidly dim erize un der th e form ation of
tetraph en ylben zidin e. Con sequen tly, a m ixture of th e EPR spectra ch arac-
teristic of tetraarylben zidin e an d triph en ylam in e cation -radicals is foun d.
Th e observation of a well-resolved h yperfin e splittin g from h ydrogen an d
n itrogen atom s h ere can be explain ed by th e sim ultan eous presen ce of both
cation -radicals. Th e EPR spectrum presen ted in Fig. 2a-2 was successfully
simulated using the splitting parameters of 3+• (aN = 1.017 mT, 6aHo = 0.226 mT,
6aHm = 0.122 mT and 3aHp = 0.327 mT) and of 1+• (2aN = 0.479 m T), assum in g
approxim ately equal relative EPR in ten sities an d g-values of both species35.
In addition , in th e correspon din g in situ UV-VIS spectroelectroch em ical ex-
perim en t, th e form ation of both m on o- an d dication s derived from 1 as
well as th e form ation of triph en ylam in e cation -radical were con firm ed, as
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1412
Fáber, Mielke, Rapta, Staško, Nuyken:
will be discussed below. In agreem en t with th at, th e reverse cath odic peak
at 0.64 V in Fig. 2 (a-1, b-1) can be assign ed to th e reduction of th e
m on ocation of th e con secutively form ed dim eric form (tetraph en yl-
ben zidin e). Sim ilarly, th e form ation of tetraarylben zidin es was also re-
ported16,18–22. We con firm ed th is dim erization process by h avin g perform ed
in situ UV-VIS electroch em ical oxidation . Sim ilar redox beh aviour of com -
poun ds 3 an d 4 suggests th at a m ech an ism an alogous to th e oxidation
route as foun d for 3 can also be assum ed in th e case of 4.
Wh ereas th e n ature of th e cation -radicals of com poun ds 1, 2, an d 3 is ev-
iden t from th e EPR h yperfin e structure, th e issue is m ore com plex in th e
case of th e syn th esized com poun ds 4 an d 5, wh ere on ly a broad lin e was
foun d. Th e fun dam en tal differen ce between com poun ds 1–3 an d 4, 5 is
a1
b1
0.4
0.6
0.8
1.0
1.2
0.4
0.6
0.8
1.0
1.2
E vs SCE, V
E vs SCE, V
a2
b2
10 G
10 G
FIG. 2
Cyclic voltam m ogram s of 3 (a-1) an d 4 (b-1) sh owin g oxidation in CH2Cl2/0.1 M TBAPF6 (Pt
workin g electrode, scan rate 100 m V s–1, full lin e: first scan , dash ed lin e: secon d scan ), alon g
with th e EPR spectra of th e cation -radicals gen erated in th e poten tial region aroun d 1 V dur-
in g oxidation of 3 (a-2) an d 4 (b-2)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1413
th at th e form er con tain on ly on e redox un it (tetraarylben zidin e in 1 an d 2
or triph en ylam in e in 3) wh ile th e latter con tain two such un its lin ked by a
n on -con jugated bridge (quatern ary carbon of th e fluoren e m oiety). We as-
sum e th at th e dim er-like structures beh ave as two in depen den t oxidation
cen tres an d are very probably sim ultan eously oxidized at approxim ately th e
sam e poten tials. Th is is in dicated by th e closely sim ilar cyclic voltam m o-
gram s for 2 an d 5 with virtually th e sam e position s of th e oxidation peaks.
Furth er eviden ce for th is assum ption is th at at equivalen t m olar con cen tra-
tion s, th e h eigh t of th e peaks in th e case of 5 is approxim ately double com -
pared with th at of 2, as illustrated in Fig. 3a. Stron g eviden ce for th e
sim ultan eous oxidation of both un its in structures 4 an d 5 com es from
UV-VIS in vestigation s. Th e UV-VIS spectrum of 5 exh ibits approxim ately
double th e in ten sity of th at of 2 at th e sam e m olar con cen tration s, as illus-
trated in Fig. 3b. Also, th e EPR spectra are com patible with th is assum ption ;
as in th e case of dim eric structures, th e presen ce of two delocalized spin s
leads to collapse of th e h yperfin e structure accom pan ied by n arrowin g of
th e lin e.
All th e com poun ds studied sh owed durin g oxidation sim ilar UV-VIS
ban ds in th e region 300–800 n m . A precise attribution of th e recorded
UV-VIS spectra to th e applied poten tial was possible from a sim ultan eous
UV-VIS/cyclovoltam m etric experim en t.
Figure 4 sh ows a represen tative UV-VIS spectrocyclovoltam m ogram for
com poun d 1. Before oxidation , all startin g com poun ds 1–5 h ave absorption
a
b
1.0
A
0.5
0.0
300
350
400
450
500
0.4
0.6
0.8
1.0
1.2
λ, nm
E vs SCE, V
FIG. 3
Cyclic voltam m ogram s (a) an d UV-VIS spectra (b) of 2 (dash ed lin es) an d 5 (full lin es) m ea-
sured in CH2Cl2/0.1 M TBAPF6 at th e sam e con cen tration s (0.001 m ol l–1
)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1414
Fáber, Mielke, Rapta, Staško, Nuyken:
m axim a in th e region of 300–350 n m , an d com poun d 1, in addition , h as a
sh oulder aroun d 310 n m . Th e UV-VIS spectra were recorded sim ultan eously
with a CV scan at th e poten tials m arked with circles in th e cyclic
voltam m ogram , as illustrated in Fig. 4a. From Fig. 4 it is eviden t th at th e
absorption ban d at 480 n m correspon ds to th e first oxidation peak of th e
m on ocation . At h igh er poten tials, in th e region of th e secon d peak, th e
UV-VIS ban ds at 550–800 n m , wh ich are attributed to th e dication , pre-
dom in ate. Also th e origin al VIS ban d at 480 n m reappears after droppin g
th e poten tial to th e region of th e first oxidation peak.
More eviden ce for th is assign m en t, also for th e dim er-like structure 5,
was obtain ed from a sim ultan eous EPR/UV-VIS electroch em ical study usin g
a lam in ated ITO workin g electrode. Figure 5 (represen tative for com poun ds
1, 2, an d 5) sh ows th e UV-VIS (Figs 5a, 5c) an d also th e upper h alf part of
th e EPR spectra (Fig. 5d) of com poun d 5, th at are sim ultan eously m easured
durin g th e an odic (galvan ostatic) oxidation on ITO in th e cavity of th e EPR
spectrom eter. Durin g oxidation , th e basic ban d (300–350 n m ) of th e paren t
com poun ds van ish es, an d a n ew ban d arises with an isosbestic poin t at
392 n m . Durin g furth er oxidation th e m on ocation ban d passes th rough a
m axim um an d is con tin uously replaced with a n ew ban d in th e region
600–800 n m , again with an isosbestic poin t (at 530 n m ). Th e kin etic curves
extracted from th e given UV-VIS spectra are sh own in Fig. 5b. Th is kin etic
beh aviour is ch aracteristic of two reversible on e-electron oxidation pro-
cesses. With th e ban d in crease in th e ran ge 550–800 n m , th e in ten sity of
a
b
0
1.7
E, V
0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
400
450
500
550
λ, nm
600
E, V
FIG. 4
Cyclic voltam m ogram (a) an d UV-VIS spectra (b) recorded durin g a cyclovoltam m etric scan
at th e poten tials m arked with circles (lam in ated ITO workin g electrode, scan rate 10 m V s–1
)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1415
th e EPR lin e decreased. Th erefore we assum e th at both redox cen tres of th e
two tetraarylben zidin e skeleton s in 5 (lin ked with th e n on -con jugated
fluoren e bridge) are oxidized in depen den tly to th e dication . An in terestin g
observation is th at after stoppin g th e oxidation in th e region of th e secon d
peak (550–800 n m ) th e in ten sity of EPR sign al in creases, in dicatin g an in -
creased con cen tration of m on ocation . Th is suggests a com proportion ation
reaction of th e dication with th e n eutral tetraarylben zidin e in solution n ear
th e active electrode surface.
As specified above, th e differen t beh aviour in th e cyclovoltam m etric in -
vestigation s of 3 an d 4 was also reflected in in situ UV-VIS spectro-
electroch em ical experim en ts. Durin g th e oxidation , th ree n ew ban ds were
observed sim ultan eously wh ile th e origin al ban d van ish ed as sh own for
b
a
0.9
A
1.0
A
0.6
0.5
0.3
0.0
0.0
300
400
500
600
700
λ, nm
800
Oxidation
c
d
I
EPR
Oxidation
Oxidation
400
600
λ, nm
800
1.5 G
FIG. 5
UV-VIS spectra recorded durin g th e am perom etric oxidation of 5 in CH2Cl2/0.1 M TBAPF6
on th e lam in ated ITO electrode: a 2D plot; b extracted kin etic curves; c 3D plot, alon g with
d th e sim ultan eously recorded EPR spectra (on ly th e m arked region presen ted, dash ed lin es)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1416
Fáber, Mielke, Rapta, Staško, Nuyken:
com poun d 4 in Fig. 6. Th e absorption m axim um at 350 n m is ch aracteristic
of th e triph en ylam in e cation -radical with furth er two m axim a at 550 an d
650 n m (ref.21). Th e m axim a at 485 an d 700 n m are ch aracteristic of
tetraarylben zidin e-type com poun ds. Th is is in accordan ce with th e an odic
route suggested above for 3, assum in g couplin g of th e in itially form ed un -
stable cation -radical un der form ation of its dim eric form an d furth er
oxidation to th e m on o- an d dication . Th is route, suggested for 3, is also valid
for 4. Wh ereas h ere th e prim ary form ation of two in depen den t redox cen -
tres in th e structure of 4 is assum ed, th e dim erization in solution could be
an altern ative path way.
Th e en ergy of th e HOMO levels of th e in vestigated com poun ds can be
determ in ed from th e oxidation h alf-wave poten tials obtain ed by
cyclovoltam m etric m easurem en ts in solution . Th e oxidation h alf-wave po-
ten tial for com poun d 5 h as been determ in ed as +0.29 V vs Fc/Fc+. Assum in g
th at th e HOMO en ergy for th e ferrocen e/ferricen ium stan dard is –4.8 eV
a
b
0.6
0.6
A
A
0.4
0.4
0.2
0.0
0.2
0.0
Oxidation
300
400
500
600
700
λ, nm
800
c
FIG. 6
UV-VIS spectra
recorded
durin g
th e
am perom etric oxidation of 4 in CH2Cl2/0.1
M
TBAPF6 on th e lam in ated ITO electrode: a 2D
plot; b extracted kin etic curves; c 3D plot
400
600
800
λ, nm
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Anodic Oxidation of Tetraarylbenzidines
1417
with respect to th e zero vacuum level, th e HOMO en ergy for com poun d 5
h as been evaluated to –5.09 eV. Th e en ergy barrier for h ole in jection from
ITO is th erefore expected to be 0.29 V (assum in g ϕ(ITO) = 4.8 eV). Th e
LUMO en ergetic position for com poun d 5 h as been estim ated to be –1.45 eV,
based on th e en ergy m axim um of th e solution absorption ban d. Th e
HOMO-LUMO gap is th en 3.64 eV.
CONCLUSIONS
Fluoren yliden e-lin ked triarylam in es h ave been prepared by th e palladium -
catalyzed Hartwig–Buch wald am in ation . Th ese lin ked triarylam in es, wh en
com pared with th e low-m olecular-weigh t an alogues con ven tion ally used in
OLEDs, sh ow greatly in creased glass-tran sition tem peratures wh ile th eir de-
sirable electron ic ch aracteristics is preserved. EPR in vestigation s gen erally
con firm ed th at for all in vestigated com poun ds, in th e region of th e first ox-
idation peak, param agn etic species are form ed wh ich are cation -radicals. In
th e m odel com poun ds (1, 2, an d 3), th e h yperfin e splittin gs (quin tet
1 : 2 : 3 : 2 : 1) in dicate th e form ation of species with two equivalen t n itro-
gen atom s, th at well correspon d to tetraarylben zidin e cation s. In th e n ewly
prepared com poun ds 4 an d 5, on ly un resolved broad sin gle lin es were ob-
served; h owever, from th e UV-VIS spectroelectroch em ical results it can be
con cluded th at th ese spectra are in dividual tetraarylben zidin e cation s
lin ked with n on -con jugated fluoren e bridges. Th e UV-VIS ban ds at about
480 n m an d th e EPR kin etic curves passed th rough a m axim um in th e re-
gion of th e first oxidation peak an d decreased in th e region of th e secon d
peak. Th is con firm s th at in th e secon d oxidation step, EPR-silen t dication ic
species are form ed from all in vestigated com poun ds (of both m on om eric
an d dim eric type). Th e cyclovoltam m etric in vestigation s com bin ed with
EPR an d UV-VIS m easurem en ts presen ted h ere provide eviden ce th at com -
poun d 5 could be used as an efficien t m aterial for h ole tran sport in OLEDs.
Th ese in vestigation s as well as th e syn th esis of furth er fluoren e-lin ked com -
poun ds are curren tly un der way.
Financial support from the Bayerische Forschungsstiftung (R. F.), the Fonds der Chemischen Industrie
(G. F. M.), the Bayerischer Forschungsverbund Katalyse (FORKAT), the Deutsche Forschungs-
gemeinschaft, the Deutscher Akademischer Austauschdienst (P. R.), the BMBF and the Slovak Grant
Agency (Project VEGA 1/4206/97), is gratefully acknowledged. We thank Dipl.-Chem. T. Braig for
helpful discussions.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1418
Fáber, Mielke, Rapta, Staško, Nuyken:
REFERENCES
1. Tang C. W., Van Slyke S. A.: Appl. Phys. Lett. 1987, 51, 913.
2. Adachi Ch., Nagai K., Tamoto N.: Appl. Phys. Lett. 1995, 66, 2679.
3. Van Slyke S. A., Chen C. H., Tang C. W.: Appl. Phys. Lett. 1996, 69, 2160.
4. Adachi C., Tsutsui T., Saito S.: Appl. Phys. Lett. 1989, 55, 1489.
5. Shirota Y., Kobata T., Noma N.: Chem. Lett. 1989, 1145.
6. Inada H., Ohnishi K., Nomura S., Higuchi A., Nakano H., Shirota Y.: J. Mater. Chem.
1994, 4, 171.
7. Koene B. E., Loy D. E., Thompson M. E.: Chem. Mater. 1998, 10, 2235.
8. O’Brien D. F., Burrows P. E., Forrest S. R., Koene B. E., Loy D. E., Thompson M. E.: Adv.
Mater. (Weinheim, Ger.) 1998, 10, 1108.
9. SalbeckJ., Yu N., Bauer J., Weissörtel F., Bestgen H.: Synth. Met. 1997, 91, 209.
10. Bach U., Lupo D., Comte P., Moser J. E., Weissörtel F., SalbeckJ., Spreitzer H., Grätzel M.:
Nature 1998, 395, 583.
11. SalbeckJ., Weissörtel F., Bauer J.: Macromol. Symp. 1997, 125, 121.
12. Kido J., Komada M., Harada G., Nagai K.: Polym. Adv. Technol. 1995, 17, 703.
13. ParkK. H., Tani T., Kakimoto M.-A., Imai Y.: Macromol. Chem. Phys. 1998, 199, 1029.
14. KorshakV. V., Vinogradova S. V., Vygodskii Y. S.: J. Macromol. Sci., Part C 1974, 11, 45.
15. Thelakkat M., Fink R., Haubner F., Schmidt H.-W.: Macromol. Symp. 1997, 125, 157.
16. Seo E. T., Nelson R. F., Fritsch J. M., Marcoux L. S., Leedy D. W., Adams R. N.: J. Am.
Chem. Soc. 1966, 5, 3498.
17. Anderson J. D., McDonald E. M., Lee P. A., Anderson M. L., Ritchie E. L., Hall H. K.,
Hopkins T., Mash E. A., Wang J., Padias A., Thayumanavan S., Barlow S., Marder S. R.,
Jabbour G. E., Shaheen S., Kippelen B., Peyghambarian N., Wightman R. M., Armstrong
N. R.: J. Am. Chem. Soc. 1998, 120, 9646.
18. Creason S. C., Wheeler J., Nelson R. F.: J. Org. Chem. 1972, 37, 4440.
19. Marcoux L. S., Adams R. N., Feldberg S. W.: J. Phys. Chem. 1969, 73, 2611.
20. Debrodt H., Heusler K. E.: Z. Phys. Chem. (Munich) 1981, 125, 35.
21. Sumiyoshi T.: Chem. Lett. 1995, 645.
22. Oyama M., Nozaki K., Okazaki S.: Anal. Chem. 1991, 63, 1387.
23. Okutsu S., Onikubo T., Tamano M., Enokida T.: IEEE Trans. Electron Devices 1997, 44,
1302.
24. Driver M. S., Hartwig J. F.: J. Am. Chem. Soc. 1996, 118, 7217.
25. Wolfe J. P., Wagaw S., Buchwald S. L.: J. Am. Chem. Soc. 1996, 118, 7215.
26. Louie J., Hartwig J. F.: J. Am. Chem. Soc. 1997, 119, 11695.
27. Tanaka H., Tokito S., Taga Y., Okada A.: Chem. Commun. 1996, 2175.
28. Thelakkat M., Schmidt H.-W.: Adv. Mater. (Weinheim, Ger.) 1998, 10, 219.
29. Katsuma K., Shirota Y.: Adv. Mater. (Weinheim, Ger.) 1998, 10, 223.
30. Pommerehne J., Westweber H., Guss W., Mahrt R. F., Bässler H., Porsch M., Daub J.: Adv.
Mater. (Weinheim, Ger.) 1995, 7, 551.
31. NeudeckA., Kress L.: J. Electroanal. Chem. 1998, 437, 141.
32. Rapta P., NeudeckA., Petr A., Dunsch L.: J. Chem. Soc., Faraday Trans. 1998, 94, 3625.
33. Yamamoto T., Nishiyama M., Koie Y.: Tetrahedron Lett. 1998, 39, 2367.
34. Veregin R. P., Harbour J. R.: J. Phys. Chem. 1990, 94, 6231.
35. Neugebauer F. A., Bamberger S., Groh R.: Chem. Ber. 1975, 108, 2406.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)