Synthesis and properties of salen-aluminum complexes as a novel class of color-tunable luminophores

Article abstract of DOI:10.1002/chem.200900137

A series of salen-aluminum complexes, [{(R⁵)₂- salen(3-tBu)₂}Al(OC₆H₄-ZJ-C₆H ₅)] (salen = N,N'-bis(salicylidene)ethylenediamine; R⁵ = H (1), tBu (2), Br (3), Ph (4), O

Full text of DOI:10.1002/chem.200900137

DOI: 10.1002/chem.200900137
Synthesis and Properties of Salen–Aluminum Complexes as a Novel Class of
Color-Tunable Luminophores
Kyu Young Hwang,[a] Hyoseok Kim,[a] Yoon Sup Lee,*[a] Min Hyung Lee,*[b] and
Youngkyu Do*[a]
Abstract: A series of salen–aluminum
complexes, (3-tBu)2}Al-
[{(R5)2-salen
yses show that they can form amor-
phous glasses with glass transition tem-
peratures of 95–1328C depending on
the C5 substituent. UV/Vis absorption
spectra of the complexes exhibit major
bands at l=338–413 nm assignable to
salen-centered p–p* transitions with a
gradual red shift of the absorption
maximum wavelengths as the substitu-
ent is varied from an electron-with-
drawing (NMe3) to an electron-donat-
ing group (NMe2). The maxima in the
emission spectra of 1–7 occur over the
entire visible region, ranging from l=
438 nm for 7 to l=599 nm for 6, with
high fluorescence quantum efficiencies
of up to F=0.40 for 4 in solution.
DFT calculations suggest that the low-
energy electronic transitions in 1–7 are
characterized by HOMOꢀi–LUMO+1
(i=1 for 1–6 or i=4 for 7) transitions
localized on the salen moiety, with
much involvement of the C5 position
in the HOMOꢀi. Thus, the electronic
alteration at the C5 position of the
phenoxide ring, which mainly affects
the HOMOꢀi energy levels of salen–Al
luminophores, is responsible for the ob-
served emission color-tuning properties
over the entire visible region.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG(OC6H4-p-C6H5)] (salen=N,N’-bis(sali-
cylidene)ethylenediamine; R5 =H (1),
tBu (2), Br (3), Ph (4), OMe (5), NMe2
(6))
and
[{5,5’-(NMe3)2-salen
[OTf]2
ACHTUNGTRENNUNG
tBu)2}Al(OC6H4-p-C6H5)]AHCNUTGTRENNNUG
OTf=CF3SO3) that are electronically
modulated directly at C5 of the phen-
oxide ring in the salen moiety has been
prepared. The crystal structures of 1, 4,
6, and 7 determined by X-ray diffrac-
tion reveal distorted square-pyramidal
geometries around the Al atoms. Com-
plexes 1–7 are all air-stable in both the
solid and solution states and have high
thermal stability (decomp 313–3388C).
Differential scanning calorimetric anal-
Keywords: aluminum · color tun-
ing · luminescence · salen ligands ·
substituent effects
some luminophores based on fluorescent and/or phosphores-
cent metal complexes have been incorporated successfully
into OLEDs to emit over a wide range of visible light, there
has been considerable interest recently in developing color-
tunable emitters for full-color and white-light display appli-
cations.[3] For this purpose, luminophores of which the emit-
ting properties can easily be tuned by varying the electronic
property of the chromophore are highly desirable. It has
been firmly established in the well-known singlet and triplet
emitters such as tris(8-quinolinolato)aluminum [Alq3][4–8] (q:
8-quinolinolato) and fac-tris(2-phenylpyridine)iridium [fac-
Introduction
Luminescent organometallic complexes have been investi-
gated extensively as emitting layer materials in organic
light-emitting diodes (OLEDs), owing to their excellent
emission properties, such as good color purity and high
quantum efficiency, as well as high stability.[1,2] Although
[a] K. Y. Hwang, H. Kim, Prof. Dr. Y. S. Lee, Prof. Dr. Y. Do
Department of Chemistry
KAIST, Daejeon, 305-701 (Republic of Korea)
Fax : (+82)42-350-2810
E-mail: YoonSupLee@kaist.ac.kr
ykdo@kaist.ac.kr
IrACTHNUTRGNEUNG
(ppy)3][9] (ppy: 2-phenylpyridine), respectively, that varia-
tion of the substituent on the ligand leads to controllable
emission color change with respect to their photolumines-
cence (PL) efficiencies, but other systems have not been in-
vestigated systematically in detail, partly due to the lack of
a suitable luminophoric system.
[b] Prof. Dr. M. H. Lee
Department of Chemistry
University of Ulsan
Ulsan, 680-749 (Republic of Korea)
Fax : (+82)52-259-2348
In our search for a novel family of color-tunable lumino-
phores, we have been interested in the pentacoordinated Al
complexes based on the Schiff base salen ligand, [N,N’-bis-
E-mail: lmh74@ulsan.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900137.
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FULL PAPER
ACHTUNGTRENNUNG(salicylidene)ethylenediamine]. Despite the wide use of the
salen–Al complexes as catalysts for various organic transfor-
mations such as asymmetric synthesis[10] and polymeri-
zation,[11,12] however, the photophysical properties of the
salen–Al derivatives were rarely investigated until Cozzi
et al. reported that salen–Al chloride displayed ligand-cen-
tered fluorescence with high quantum efficiency.[13] They
also found that the Al complex was quite stable in solution
compared with the related magnesium and zinc derivatives.
Nevertheless, in contrast to the great attention devoted to
the Al complexes based on the bidentate 8-hydroxyquino-
line ligand, such as the Alq3 and BAlq (bis(2-methyl-8-qui-
nolinolato)-4-phenylphenoxyaluminum)[4,14] derivatives, stud-
ies have not so far been focused on the various salen–Al
complexes. Instead, the PL[15] and electroluminescent[16]
properties of the zinc derivatives have been more actively
investigated. Furthermore, the salen ligands have often been
employed in the phosphorescent emitters based on plati-
num.[17–20] All these reports have prompted us to consider
whether the salen–Al complexes could be utilized as versa-
tile materials for OLEDs; our group has recently demon-
strated that the pentacoordinated salen–Al complexes with
an aryloxy ancillary ligand can serve not only as efficient
hole-blocking materials[21] for phosphorescent OLEDs, but
also as hosts in the dopant assembly OLEDs based on a het-
erodinuclear Al/Ir complex.[22] They found that the salen–Al
complexes possess suitable HOMO and LUMO energy
levels, high thermal stability, and high PL efficiency, all of
which may render them promising as emitting materials in
OLEDs. Accordingly, a detailed investigation of the photo-
physical properties of salen–Al complexes, including color-
tuning properties, may be intriguing and provide an oppor-
tunity to extend their use to light-emitters.
To this end, we decided to introduce the electron-with-
drawing or electron-donating groups particularly at the C5
position (para to the oxygen) of the phenoxide ring in the
salen moiety. This approach has been successful in the 8-hy-
droxyquinoline-based aluminum[2,5] and boron[23] lumino-
phores to tune the emission wavelength; logically it could
also be applicable to the salen–Al luminophores when one
notes that semiempirical molecular orbital (MO) calcula-
tions for the structurally related compounds with the salen,
such as 2-(2’-pyridyl)phenol[24] and salicylaldimine,[25] have
shown that the C5 position of the phenoxide ring is involved
with the large contribution to the HOMO whereas it con-
tributes negligibly to the density of the LUMO. One may
therefore expect the HOMO–LUMO energy gap responsi-
ble for the emission wavelength to be readily manipulated
by introduction of electron-withdrawing or electron-donat-
ing groups at the C5 position of the phenoxide ring, which
may thus allow fine tuning of the emission color.
Results and Discussion
Synthesis and characterization: Synthetic routes to the
salen–Al complexes that are functionalized at the C5 posi-
tion of the salen moiety with various substituents are depict-
ed in Scheme 1. The functionalized salen ligands (S1–S6)
were prepared readily from the reaction of the correspond-
ing 2-hydroxybenzaldehyde with ethylenediamine in a 2:1
molar ratio according to the modified procedure reported
previously.[26] Treatment of the salen ligands with 1.1 equiv
of AlMe3 in toluene afforded the Al precursors of P1–P6 in
1
high yield.[27] Whereas the H NMR signals of the CH2CH2
protons for S1–S6 appear as singlets at around d=4.0 ppm,
those for P1–P6 show two multiplets after complexation.
ꢀ
The characteristic peak of the Al CH3 proton for P1–P6 is
observed around at d=ꢀ1.1 ppm. The precursors were con-
verted into the desired salen–Al complexes 1–6 in good
yield from the reaction of P1–P6 with 4-phenylphenol in re-
fluxing toluene. The ionic complex 7 was synthesized
(Scheme 2) as well as the neutrally modified 1–6, because
the ammonium group at the C5 position of the phenoxide
ring in 7 may exert a highly electron-withdrawing effect.
Methylation of the amino groups in 6 with MeOTf afforded
the di-ionic salt 7 readily as a white solid. In contrast to the
high solubility of 1–6 in common organic solvents such as
toluene, THF, and chloroform, due to its ionic nature 7 is
soluble only in more polar solvents such as acetonitrile. All
the complexes 1–7 are stable in air in both the solid and so-
lution states, similarly to the aryloxide derivatives of 2,[21]
probably because of the steric protection of the Al center
by two tert-butyl groups as well as the strong bonding of the
4-phenylphenoxy ancillary ligand to the Al center. The iden-
tities of 1–7 have been fully characterized by H,13C NMR
spectroscopy, and elemental analysis.
1
The solid-state structures of complexes, 1, 4, 6, and 7 were
determined from X-ray diffraction studies. Single crystals
suitable for X-ray structural determination were obtained by
either cooling a solution of CH2Cl2/n-hexane at ꢀ208C for
1, 4, and 6, or slow diffusion of diethyl ether into a solution
of 7 in MeCN. The ORTEP representations are shown in
Figure 1 and the detailed crystallographic data and selected
bond lengths and angles are summarized in Tables 1 and 2,
respectively.
The complexes 1 and 7 crystallize in space groups C2/c
and P21/n, respectively, while 4 and 6 crystallize in P1. The
bond between the Al atom and the 4-phenylphenoxy group
is nearly perpendicular to the salen–Al coordination plane,
We have prepared a series of salen–Al complexes that are
electronically modulated directly at C5 of the phenoxide
ring in the salen moiety to achieve full emission color tuning
in the visible region. Here we describe the detailed synthe-
sis, characterization, and luminescent properties in conjunc-
tion with theoretical considerations.
¯
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6479

Y. S. Lee, M. H. Lee, Y. Do et al.
Scheme 1. Synthesis of functionalized salen–aluminum complexes (1–6). Reaction conditions: i) ethylenediamine, RT, 2 h/MeOH; ii) AlACHTUNRGTNEG(UN CH3)3, reflux,
1 h/toluene, 67–85%; iii) 4-phenylphenol, reflux, 4 h/toluene, yield 55–80%.
revealing distorted pentacoor-
dination geometry around the
Al center. Geometric analysis
with the index of the degree of
“trigonality” t=(bꢀa)/60 (t=
0 for perfectly square-pyrami-
dal geometry and t=1 for per-
fectly trigonal-bipyramidal ge-
ometry)[28,29] for 1, 2,[21] 4, 6,
and 7 afford t values of 0.38,
0.61, 0.36, 0.44, and 0.19, re-
spectively, indicating that the
complexes 1, 4, 6, and 7 adopt
the square-pyramidal geometry
typically observed for other
salen–Al complexes[11,26–28,30,31]
whereas 2, containing a 5-tert-
butyl group, is more in favor
of trigonal-bipyramidal geome-
try. The Al-O3-Cbiph angles of
126.3–136.58 and the Al–O3
bond lengths of 1.749–1.766 ꢁ
are in a similar range to those
found in salen–Al phenoxide
derivatives.[11,31] The very simi-
lar Al–O1(O2) and Al–
N1(N2) bond lengths in 1–7 in-
dicate little electronic substitu-
ent effect from the C5 position
of the phenoxide ring on the
salen–Al bond lengths.
Scheme 2. Synthesis of functionalized salen–aluminum complex (7). Reaction conditions: i) 3 equiv MeOTf,
RT, 0.5 h/CH2Cl2, yield 63%.
Thermal properties: The solid-
state thermal properties of all
the complexes were investigat-
Figure 1. Molecular structures of 1, 4, 6, and [7]2+ in [7]
ACTHNUGTRNE[UNG OTf]2 (40% thermal ellipsoid). Hydrogen atoms and
OTf groups are omitted for clarity.
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Salen–Aluminum Complexes as Luminophores
FULL PAPER
Table 1. Crystallographic data and parameters for 1, 4, 6, and 7.
1
4
6
7
formula
Mr
C36H39AlN2O ·
659.60
(CH2Cl2)
C48H47AlN2O3
726.86
C40H49AlN4O3
660.81
C42H55AlN4O ·
989.02
3 ACHTUGNTERN(NUNG CF3SO3)2
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
monoclinic
C2/c
29.893(5)
10.3863(18)
24.474(4)
90.00
114.795(11)
90.00
6898(2)
8
triclinic
P1
triclinic
P1
monoclinic
P21/n
12.9371(7)
10.2166(6)
37.065(2)
90.00
95.447(3)
90.00
4876.9(5)
4
¯
¯
10.0705(8)
14.6386(11)
17.1219(14)
113.094(5)
98.009(5)
92.987(5)
2283.3(3)
2
1.057
0.083
772
130(2)
11.9239(12)
13.0986(12)
14.1417(14)
66.950(5)
76.486(6)
85.188(6)
1976.1(3)
2
1.111
0.091
708
130(2)
a [8]
b [8]
g [8]
V [ꢁ3]
Z
1calcd [gcmꢀ3
]
1.270
0.252
2784
130(2)
1.347
0.206
2072
100(2)
m [mmꢀ1
(000)
T [K]
]
F
A
q range [8]
hkl range
measd reflns
1.50–23.19
1.56–28.02
1.60–28.56
1.10–29.15
ꢀ32!21, ꢀ11!11, ꢀ26!27 ꢀ13!13, ꢀ18!18, ꢀ21!22 ꢀ15!15, ꢀ17!17, ꢀ18!18 ꢀ17!17, ꢀ13!10, ꢀ50!50
14704
35052
38575
60194
unique reflns [Rint
reflns. used for refinement 4661
]
4661 [0.0940]
10000 [0.1566]
10000
9807 [0.0735]
9807
12979 [0.0400]
12979
refined parameters
R1[a] (I> 2s(I))
wR2[b] all data
413
493
0.0970
0.2041
1.003
444
0.0728
0.2111
1.006
680
0.0678
0.1901
1.003
0.0586
0.1341
1.001
0.538/ꢀ0.653
GOF on F2
1fin (max/min) [eꢁꢀ3
[a] R1=ꢀjjFoꢀFc jj/ꢀjFo j. [b] wR2={[ꢀw(F2oꢀFc2)2]/[ꢀw(Fo2)2]}1/2
]
0.508/ꢀ0.655
0.605/ꢀ0.371
0.539/ꢀ0.409
.
Table 2. Selected bond lengths [ꢁ] and angles [8] for 1, 4, 6, and 7.
heated, two distinct melting transitions take place at 2188C
and 2358C (Figure 2). The first is followed by instant crystal-
lization at around 2218C to form the second crystal, which
melts at 2358C. The appearance of two glass transitions at
98 and 1328C that differ from that of the final glass of 2
(see below) further implies the formation of polycrystals
from the initial solution-crystallization. Remarkably, when
the melt is cooled (108Cminꢀ1), without rapid cooling, it
forms an amorphous glass with no crystallization process.
Glass formation is further confirmed by observation of a
single glass transition at 1128C after the sample is reheated.
1
4
6
7
ꢀ
Al O1
ꢀ
Al O2
1.806(4)
1.790(4)
1.755(4)
1.994(5)
2.000(4)
1.784(3)
1.815(3)
1.761(4)
2.007(4)
1.975(4)
1.7823(17)
1.800(2)
1.7662(19)
2.017(2)
1.7953(18)
1.8130(17)
1.7486(19)
2.004(2)
ꢀ
Al O3
ꢀ
Al N1
ꢀ
Al N2
1.986(2)
2.002(2)
O1-Al-N2
O2-Al-N1
Al-O3-Cbiph
161.75(19)
138.79(18)
130.3(3)
139.11(18)
160.46(18)
130.4(3)
136.74(9)
163.31(10)
132.83(15)
144.84(9)
156.18(9)
126.31(16)
ed by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). TGA measurements exhibit
high Td5 values (5 wt% decomposition temperatures) of
313–3388C, indicating high thermal stability of the salen–Al
complexes. It is also known that a large difference in Td5
values (ꢁ1008C) is observed for the derivatives of 2, upon
changing an aryloxy ancillary ligand,[21] and the tetracoordi-
nate salen–Pt complexes, which lack the ancillary ligand
show higher Td5 values (ꢁ4008C);[19] therefore the similar
high Td5 values for 1–7 may indicate not only that the salen–
Al bond is thermally robust, but also that the thermal stabil-
ity originates mainly from the pentacoordinated bonding
around the Al center. Most interestingly, DSC analyses
reveal that the complexes are able to form amorphous
glasses with reasonably high glass transition temperatures
(Tg =95–1328C) upon cooling of melt samples. For example,
when a crystalline sample of the complex 2 obtained by
crystallization from a CH2Cl2/n-hexane solvent mixture is
Figure 2. a) The first and b) the second heating DSC curves of crystalline
2 at 108Cminꢀ1
.
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Y. S. Lee, M. H. Lee, Y. Do et al.
Further heating does not induce any crystallization beyond
the Tg and therefore no melting transition; this indicates the
formation of an amorphous solid.
Similar thermal behavior, including glass formation from
a melt sample, is observed consistently for other complexes
except 1 and 7 (see the Supporting Information). In the case
of 1, despite formation of a glass by cooling, heating of the
glass sample induces crystallization above Tg and then a
melting transition. In contrast, 7 exhibits a crystallization
peak during the first cooling of the melt sample, indicative
of formation of a crystalline solid. The different thermal be-
havior of 7 is probably due to its ionic nature. These unusual
features of salen–Al complexes are likely to be attributable
to both the steric bulkiness of the salen and 4-phenylphe-
noxy ligands, and the distorted structure, which may hinder
crystallization. The Tg values of 95–1328C apparently
depend on the size of the C5 substituent; thus Tg increases
with the size of the substituent, in the order H<OMe<
NMe2 <tBu<Ph. Whereas the Tg values are similar to that
found for the recently reported salen–Al/Ir heterodinuclear
complex (Tg =1068C),[22] they are higher than that of the
structurally related pentacoordinate BAlq (Tg =928C).[32]
Since the latter complex possesses the same 4-phenylphe-
noxy ancillary ligand, this finding suggests that tetradentate
salen can serve as a thermally rigid supporting ligand. All
these thermal properties, including the high thermal stability
and the excellent glass-forming ability with high Tg, may
render the salen–Al complexes suitable for formation by
vacuum deposition of the morphologically stable amorphous
thin films required for OLED applications.
Figure 4. UV/Vis absorption spectra of 2 in various solvents; MeCN
(a), THF (c) and toluene (d)
Figure 5. Emission spectra of 1–7 in solution.
Optical and electrochemical properties: The optical proper-
ties of 1–7 were examined in UV/Vis and PL experiments.
The absorption and emission spectra are displayed in Fig-
ures 3 and 4, and Figure 5, respectively, and the optical data
are listed in Table 3.
All the complexes feature major absorption bands at l=
338–413 nm assignable to salen-centered p-p* transi-
tions.[13,21] The absorption maximum wavelength (labs) is
gradually red-shifted as the substituent is varied from H to
NMe2 (from 1 to 6). These red shifts upon electronic altera-
tion at the C5 position of the phenoxide ring in the salen
moiety are quite similar to those observed for the 8-hydroxy-
AHCTUNGTRENNUNG
quinoline-based aluminum luminophores,[4,5] as well as the
recently reported salophen-Zn (H2salophen=N,N’-phenyl-
ene-bis(salicylidene)phenylenediamine)[33] and zinc bis(2-ox-
azolylphenoxide)[34] derivatives. In particular, the complex 7
containing cationic NMe3 groups exhibits the most blue-
shifted band, of which labs is at a much higher-energy region
(by about 80 nm) than that of the neutral 6. These results
are correlated with the electronic effect of the substituents,
being in parallel with their Hammett constants[35] except for
3 and 4 whose labs values are slightly more red-shifted than
that of the tert-butyl-containing 2. In the case of 3 and 4, the
p–p and p–p conjugations of the lone electron pairs of the
bromine and the p-electrons of the phenyl ring with the
phenoxide p orbitals, respectively, appear to participate in
the formation of the molecular orbitals responsible for the
electronic transition, thus raising the energy levels of the
molecular orbitals (vide infra). The similar red shifts by hal-
ogen[6,36] or phenyl[5] substitution are also found in the Alq3
derivatives.
Solvatochromic study of 2 further shows that the absorp-
tion band undergoes weak blue shifts when the solvent po-
larity is increased (e.g., labs =362 nm in toluene; 357 nm in
THF; 354 nm in MeCN) (Figure 4). Such negative solvato-
Figure 3. UV/Vis absorption spectra of 1–7.
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Salen–Aluminum Complexes as Luminophores
FULL PAPER
Table 3. UV/Vis, PL, and electrochemical data of 1–7.
labs [nm][a] (log e)
lem [nm][a] (solution)
F[b] (solution)
lem [nm] (film)
F[c]
Eg [eV][d]
Vox [V][e]
HOMO [eV][g]
LUMO [eV][h]
(film)
7
1
2
3
4
5
6
338 (4.04)
354 (3.97)
364 (4.04)
366 (3.91)
373 (3.86)
384 (3.99)
413 (3.77)
438
458
480
466
487
518
599
0.16
0.21
0.31
0.06
0.40
434
459
479
472
490
516
600
0.03
0.04
0.13
0.10
0.11
0.24
0.05
3.20
2.96
2.90
2.87
2.82
2.70
2.40
0.80
0.42
0.38
0.34
0.34
0.30
0.02[f]
ꢀ5.60
ꢀ5.22
ꢀ5.18
ꢀ5.14
ꢀ5.14
ꢀ5.10
ꢀ4.78
ꢀ2.40
ꢀ2.26
ꢀ2.28
ꢀ2.27
ꢀ2.32
ꢀ2.40
ꢀ2.28
0.15
<0.01
[a] Measured in CHCl3 (1–6) or in CH3CN (7) (1.0ꢂ10ꢀ5 m). [b] Quinine sulfate (F=0.55) used as a standard. [c] Absolute quantum efficiency measured
by an integrating sphere method. [d] Estimated from the absorption edge. [e] The oxidation onset potential vs. an Fc/Fc+ couple. [f] Half-oxidation po-
tential (E1/2). [g] Calculated from the Vox. [h] Estimated from the HOMO and band-gap (Eg) energies.
chromism is consistent with the polar character of the elec-
tron-rich phenoxide ring in the ground state. This finding
may suggest an involvement of partial charge transfer from
a phenoxide ring to an imine in the salen-centered transi-
tion, similar to that found in the salen-Pt derivatives[17,19]
and the 8-hydroxyquinoline-related metal complexes[20,37]
(vide infra).
to blue, but also to a drastic increase in PL efficiency, from
<0.01 for 6 to 0.16 for 7. The solid-state emission spectra
obtained from solution-cast films also exhibits similar color
tuning features with comparable emission maxima (Fig-
ure S9 in the Supporting Information). Despite the overall
decrease in the quantum efficiencies mainly due to aggrega-
tion quenching, the compounds 5 and 6 bearing electron-do-
nating substituents had increased quantum efficiencies in
the solid state (Table 3).
The cyclic voltammetry (CV) measurements indicate that
the given salen–Al luminophores undergo irreversible oxida-
tion, except for the NMe2-substituted 6, oxidation of which
is reversible (Figures S10 and S11 in the Supporting Infor-
mation). Although the oxidation potential and thereby the
HOMO energy levels[38] estimated for 6 and 7 exhibit the
highest and lowest values (ꢀ4.78 eV and ꢀ5.60 eV, respec-
tively), the energy levels for 1–5 are in the narrow range be-
tween ꢀ5.10 eV and ꢀ5.22 eV (Table 3).
The emission spectra of 1–7 exhibit maxima (lem) over the
entire visible region ranging from l=438 to 599 nm
(Figure 5). As observed similarly in the absorption spectra,
the gradual red shift of lem is seen distinctly in order from 1
to 6 except for 3; 7 displays the most blue emission, with lem
located near an indigo region (lem =438 nm). Such a shift of
lem when the substituent on the phenoxide ring in the salen
moiety is changed implies that the emission from 1–7 is
salen-centered, with little contribution by the 4-phenylphe-
noxy ancillary ligand. The essentially identical lem value
(480 nm) of 2 with that of the precursor P2 (480 nm) mea-
sured under the same conditions confirms this implication.
This finding is very consistent with the previous report that
the absorption and emission maximum wavelengths of
salen–Al complexes are not affected by the type of the an-
cillary aryloxy ligand.[21] These emission features clearly in-
dicate that the electronic alteration at the C5 position of the
phenoxide ring in the salen–Al luminophores can lead to
facile tuning of the emission color in the whole visible
region. Furthermore, the overall fluorescence quantum effi-
ciencies of up to F=0.40 for 4 are high enough to be com-
parable with those of Alq3 luminophores.[5,7] The complexes
bearing electron-withdrawing and moderately electron-do-
nating substituents show high fluorescence quantum effi-
ciencies whereas the bromine- and strongly electron-donat-
Furthermore, the experimentally determined LUMO
energy levels for 1–7 show only slight fluctuations between
ꢀ2.26 eV and ꢀ2.40 eV. These results may indicate that
HOMOs and LUMOs of the salen–Al luminophores are not
directly related to the optical properties described above,
such as the systematic changes in the absorption and emis-
sion wavelengths by C5 substitution of the phenoxide ring in
the salen moiety.
Calculations: DFT calculations have given an insight into
the electronic transitions and the electronic structures of the
given salen–Al complexes. The geometries of 1, 2, 4, 6, and
[7]2+ for the calculations were optimized from their X-ray
structures, whereas optimization of 3 and 5 was performed
on the X-ray structure of 1 after the C5 hydrogen atom had
been replaced with a bromine and a methoxy group, respec-
tively.
For example, the optimized geometry and MOs for the
ground state of 1 are displayed in Figure 6. The calculation
results show that the transition from HOMOꢀ1 to LUMO+1
has the largest contribution to the low-energy electronic
transitions (Table 4). The delocalized electron density of the
HOMOꢀ1 is located mainly on the phenoxide ring in the
salen moiety with an orbital contribution of 86% whereas
the LUMO+1 is distributed over the imine fragment (58%)
with a reduced contribution from the phenoxide ring (40%).
ing NMe2-substituted 3 and 6 exhibit low efficiencies, as has
[5,7]
been observed similarly in the Alq3
and zinc bis(2-oxazo-
lylphenoxide)[34] luminophores. The greatest reason for the
low fluorescence quantum efficiency of 3 could be related to
the strong spin–orbit coupling in the bromine atoms that fa-
cilitates intersystem crossing (ISC) to the nonradiative trip-
let state. In the case of the NMe2-substituted 6, it is most
likely that the very low quantum yield results from a dimin-
ished HOMO–LUMO band gap, which facilitates the nonra-
diative processes according to the energy-gap law.[5] Interest-
ingly, simple alkylation of the NMe2 group in 6 gives rise
not only to the very large emission shift of 161 nm from red
Chem. Eur. J. 2009, 15, 6478 – 6487
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6483

Y. S. Lee, M. H. Lee, Y. Do et al.
which electrons are excited (that is, particle orbital LUMO+j)
remains intact, allowing tuning of the absorption wave-
length. Furthermore, in 4, which shows a slight deviation of
the electronic transition from that expected on the basis of
the Hammett constant (inductive effect), the delocalized p
electron of the phenyl group takes part in the conjugation
with the phenoxide p orbitals in the HOMOꢀ1 (Figure S15
in the Supporting Information). Consequently, the electron-
donating effect of the C5 substituent may result in destabili-
zation of the HOMOꢀ1 energy level to the red-shifted elec-
tronic transition observed experimentally. Similar destabili-
zation may also be expected in the bromine-substituted 3, as
judged by the participation of the lone electron pairs on the
bromine atom in the conjugation with the phenoxide ring in
the HOMOꢀ1.
Consistently with these MO features, the calculated
HOMOꢀ1 energy levels for 1–6 increase systematically from
1 to 6 whereas the calculated LUMO+1 energy levels are af-
fected less significantly by the electronic substitution, al-
though in the case of the bromine-substituted 3 both levels
are shifted toward lower energy (Figure 7). This result sug-
Figure 6. Optimized geometries and molecular orbitals for the ground-
state 1 (isovalue=0.05 a.u.).
Table 4. Experimental and calculated absorption maximum wavelengths
and oscillator strengths (fcalcd) of 1–7.
lexptl
lcalcd
fcalcd
Major contribution
7
1
2
3
4
5
6
338
354
364
366
373
384
413
327.3
344.7
354.5
359.8
368.1
392.8
406.8
0.1064
0.0719
0.0604
0.0765
0.0548
0.0800
0.0644
HOMOꢀ4!LUMO+1 (82%)
HOMOꢀ1!LUMO+1 (89%)
HOMOꢀ1!LUMO+1 (90%)
HOMOꢀ1!LUMO+1 (89%)
HOMOꢀ1!LUMO+1 (92%)
HOMOꢀ1!LUMO+1 (88%)
HOMOꢀ1!LUMO+1 (86%)
There is little orbital contribution from the Al center, as ex-
pected. It is noteworthy that the HOMO localized on the 4-
phenylphenoxy ancillary ligand hardly contributes to the
electronic transitions. These findings suggest that the low-
energy electronic transitions occur predominantly through
salen-centered p–p* transitions with partial charge-transfer
character from a phenoxide ring to an imine. Although a
similar feature is found in the Alq3 luminophores, the extent
of charge transfer might be less than in those systems, in
which the electronic p–p* transitions localized on the quino-
late ligands are mostly characterized by charge transfer
from a phenoxide donor to a pyridyl acceptor.[39] Moreover,
the HOMOꢀ1 shows a substantial contribution at the C5 po-
sition of the phenoxide ring, whereas the C5 position con-
~
~
*
Figure 7. Calculated HOMO
( ), HOMOꢀ1 ( ), LUMO ( ), and
*
LUMO+1
(
) energy levels for 1–6.
gests that the electronic alteration at the C5 position of the
phenoxide ring mainly affects the HOMOꢀ1 energy levels of
salen–Al luminophores in such a way that an electron-with-
drawing group lowers the level while an electron-donating
group destabilizes it. It is also notable that the calculated
HOMO and LUMO energy levels are almost unchanged, as
observed in the experimental energy levels similarly. In par-
ticular, the HOMO and HOMOꢀ1 energy levels of the
NMe2-substituted 6 are very close to each other, reflecting
that HOMOꢀ1 could participate in the oxidation process.
This feature might account for the reversible oxidation be-
havior observed for 6. The calculated maximum absorption
wavelengths and the trend in the wavelength shifts for 1–7
are in good agreement with the experimental data (Table 4).
tributes negligibly to the LUMO+1
.
All these features are observed consistently for 2–7
except for the involvement of the HOMOꢀ4 in the case of 7
(Table 4; see the Supporting Information for other com-
plexes), and are similar to that observed in the salophen–
Zn[33] complex. It is thus expected that the introduction of a
substituent at the C5 position of the phenoxide ring would
greatly affect the MO from which electrons are excited (that
is, hole orbital HOMOꢀi) while the molecular orbital into
6484
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 6478 – 6487

Salen–Aluminum Complexes as Luminophores
FULL PAPER
potentials were recorded at a scan rate of 50 mV/s and reported with ref-
erence to the ferrocene/ferrocenium (Fc/Fc+) redox couple.
General synthesis of [{(R5)2-salen
ACHTUNRGTNEG(UN 3-tBu)2}Al(OC6H4-p-C6H5)] (1–6): A
Conclusions
We have demonstrated that the salen–aluminum complexes
with a 4-phenylphenoxy ancillary ligand (1–7) that are elec-
tronically modulated directly at the C5 position of the phen-
oxide ring in the salen moiety can constitute a novel class of
color-tunable luminophores achieving full emission color
tuning in the visible region. The complexes show high air
and thermal stability and adopted pentacoordinated square-
pyramidal geometry in the solid state. They are able to form
amorphous glasses with reasonably high glass transition tem-
peratures (Tg =95–1328C). UV/Vis absorption and emission
spectra of the complexes exhibit salen-centered p–p* transi-
tions with a gradual red shift of both the absorption and
emission maximum wavelengths as the substituent is varied
from an electron-withdrawing to an electron-donating
group. In particular, the emission spectra display maxima
over the entire visible region ranging from l=438 to
599 nm, with high fluorescence quantum efficiencies. DFT
calculations further supported the experimental features, all
of which thus allow predictable emission color tuning in the
given salen–Al luminophores and hold promise for the
salen–Al luminophores as emitting materials in OLEDs.
solution of R5-3-tert-butyl-salen (S1–S6, 2.0 mmol) in toluene (30 mL)
was treated with trimethylaluminum (1.1 mL, 1.1 equiv) at ꢀ788C. The
reaction mixture was allowed to warm slowly to room temperature,
heated at reflux for 1 h, and then cooled to ambient temperature, after
which the solvent was removed under reduced pressure. Washing with n-
hexane followed by drying in vacuo afforded (R5)2-salen
ACTHUNTRGNE(NUG 3-tBu)2AlMe
precursors, P1–P6.
Data for P1: Pale yellow solid; yield=82%; 1H NMR (CDCl3): d=8.23
(s, 2H), 7.42 (dd, J=5.6/1.4 Hz, 2H), 7.03 (dd, J=5.8/1.4 Hz, 2H), 6.66
(t, J=5.8 Hz, 2H), 3.92 (m, 2H), 3.66 (m, 2H), 1.51 (s, 18H), ꢀ1.13 ppm
(s, 3H).
Data for P3: Yellow solid; yield=85%; 1H NMR (CDCl3): d=8.18 (s,
2H), 7.44 (d, J=2.0 Hz, 2H), 7.15 (d, J=2.0 Hz, 2H), 3.94 (m, 2H), 3.71
(m, 2H), 1.46 (s, 18H), ꢀ1.16 ppm (s, 3H).
Data for P4: Bright-yellow solid; yield=71%; 1H NMR (CDCl3): d=
8.30 (s, 2H), 7.70 (d, J=1.9 Hz, 2H), 7.53 (d, J=5.2 Hz, 4H), 7.39 (t, J=
5.6 Hz, 4H), 7.27 (t, J=5.6 Hz, 2H), 7.23 (d, J=1.9 Hz, 2H), 3.96 (m,
2H), 3.72 (m, 2H), 1.58 (s, 18H), ꢀ1.08 ppm (s, 3H).
Data for P5: Greenish-yellow solid; yield=77%; 1H NMR (CDCl3): d=
8.20 (s, 2H), 7.09 (d, J=2.4 Hz, 2H), 6.46 (d, J=2.4 Hz, 2H), 3.93 (m,
2H), 3.74 (s, 3H), 3.62 (m, 2H), 1.48 (s, 18H), ꢀ1.16 ppm (s, 3H).
1
Data for P6: Red solid; yield=67%; H NMR (CDCl3): d=8.22 (s, 2H),
7.17 (d, J=2.3 Hz, 2H), 6.42 (d, J=2.3 Hz, 2H), 3.90 (m, 2H), 3.66 (m,
2H), 2.80 (s, 12H), 1.51 (s, 18H), ꢀ1.16 ppm (s, 3H).
Next, the solution of 4-phenylphenol (0.25 g, 1.5 mmol) in toluene
(10 mL) was added to a solution of each of P1–P6 (1.5 mmol) in toluene
(20 mL) through a cannula. The reaction mixture was refluxed at 1108C
for 4 h and cooled to ambient temperature. Removal of the solvent
under reduced pressure followed by washing with n-hexane gave a yellow
powder. Recrystallization from a CH2Cl2/n-hexane solvent mixture at
ꢀ208C afforded crystalline 1–6, respectively. Single crystals of 1, 4, and 6
suitable for X-ray structural determination were obtained by slow cooling
of a CH2Cl2/n-hexane solvent mixture.
Experimental Section
General considerations: All operations were performed under inert nitro-
gen gas by standard Schlenk and glovebox techniques. Anhydrous grade
toluene, CH2Cl2, Et2O, MeCN, and n-hexane (Aldrich) were purified by
passage through an activated alumina column. All solvents were stored
over activated molecular sieves (5 ꢁ; Yakuri Pure Chemicals Co). Chem-
icals were used without further purification after purchase from Aldrich
(3,5-di-tert-butyl-2-hydroxybenzaldehyde, 3-tert-butyl-2-hydroxybenzalde-
hyde, tetrabutylammonium tribromide, phenylboronic acid, [Pd
Na2CO3, dimethoxyethane, MgCl2, paraformaldehyde, NEt3, Pd/C, ethyl-
enediamine, trimethylaluminum (2.0m solution in toluene), 4-phenylphe-
1
Data for 1: Pale-yellow solid; yield=65%; H NMR (CDCl3): d=8.25 (s,
2H), 7.49 (dd, J=7.6/1.7 Hz, 2H), 7.43 (d, J=8.3 Hz, 2H), 7.31 (t, J=
7.6 Hz, 2H), 7.18 (d, J=7.5 Hz, 1H), 7.14 (d, J=8.6 Hz, 2H), 7.06 (dd,
J=7.7/1.7 Hz, 2H), 6.73(t, J=7.7 Hz, 2H), 6.41(d, J=8.5 Hz, 2H), 3.92
(m, 2H), 3.59 (m, 2H), 1.53 ppm (s, 18H); 13C NMR (CDCl3): d=169.87,
165.09, 160.18, 141.72, 141.65, 132.81, 131.72, 129.81, 128.41, 127.40,
126.20, 125.61, 119.92, 119.29, 116.55, 54.86, 35.41, 29.66 ppm; elemental
analysis calcd (%) for C36H39AlN2O3: C 75.24, H 6.84, N 4.87; found: C
75.14, H 6.84, N 5.01; TGA: Td5 =3198C; DSC: Tm =2468C, Tg =958C.
ACHTUNGRTEN(NUNG PPh3)],
ACHTUNGTRENNUNG
nol, methyl triflate (MeOTf), 4-hydroxy-3-tert-butylanisole. R5-3-tert-
butyl-2-hydroxybenzaldehyde (R5 =Br, Ph, OMe, NMe2),[40] the salen li-
gands (S1–S6),[26] and the complex 2[21] were prepared according to modi-
fied literature procedures. CDCl3 and CD3CN from Cambridge Isotope
Laboratories were dried over activated molecular sieves (5 ꢁ) and used
after vacuum transfer to a Schlenk tube equipped with a J. Young valve.
NMR spectra were recorded on a Bruker Spectrospin 400 spectrometer
Data for 3: Yellow solid; yield=78%; 1H NMR (CDCl3): d 8.30 (s, 2H),
7.50 (d, J=2.6 Hz, 2H), 7.42 (d, J=8.4 Hz, 2H), 7.30 (t, J=7.5 Hz, 2H),
7.23 (d, J=2.6 Hz, 2H), 7.15 (t, J=7.4 Hz, 1H), 7.14 (d, J=6.7 Hz, 2H),
6.37 (d, J=6.5 Hz, 2H), 4.07 (m, 2H), 3.77 (m, 2H), 1.45 ppm (s, 18H);
13C NMR (CDCl3):
d 169.00, 164.10, 159.70, 144.53, 141.53, 135.90,
(400.13 MHz for H, 100.62 MHz for 13C) at ambient temperature. Chem-
ical shifts are given in ppm, and are referenced against external Me4Si
(1H, 13C). Elemental analyses were performed on an EA1110 (Fisons In-
struments) by the Environmental Analysis Laboratory at KAIST. TGA
1
133.03, 130.31, 128.46, 127.56, 126.28, 125.77, 120.45, 119.76, 108.31, 54.96,
35.67, 29.39 ppm; elemental analysis calcd (%) for C36H37AlBr2N2O3: C
59.03, H 5.09, N 3.82; found: C 58.92, H 5.11, N 4.03; TGA: Td5 =3168C;
DSC: Tm =248 (w); 260 (vs)8C, Tg =1208C.
was carried out under
a nitrogen atmosphere at a heating rate of
Data for 4: Bright-yellow solid; yield=58%; 1H NMR (CDCl3): d=8.37
(s, 2H), 7.76 (d, J=2.4 Hz, 2H), 7.55 (d, J=7.7 Hz, 4H), 7.43 (t, J=
7.5 Hz, 6H), 7.29 (m, 6H), 7.16 (m, 3H), 6.45 (d, J=8.5 Hz, 2H), 4.02
(m, 2H), 3.69 (m, 2H), 1.58 ppm (s, 18H); 13C NMR (CDCl3): d=170.07,
164.76, 160.09, 142.17, 141.62, 140.83, 132.19, 130.00, 129.40, 128.78,
128.41, 127.48, 126.50, 126.46, 126.25, 125.64, 119.94, 119.35, 54.95, 35.62,
29.71 ppm; elemental analysis calcd (%) for C48H47AlN2O3: C 79.31, H
6.52, N 3.85; found: C 79.42, H 6.59, N,3.94; TGA: Td5 =3388C; DSC:
Tm =243 (vs), 254 (w)8C; Tg =1328C.
Data for 5: Greenish-yellow solid; yield=72%;. 1H NMR (CDCl3): d
8.27 (s, 2H), 7.41 (d, J=7.6 Hz, 2H), 7.29 (t, J=7.5 Hz, 2H), 7.14 (m,
5H), 6.50 (d, J=3.0 Hz, 2H), 6.40 (d, J=6.7 Hz, 2H), 3.99 (m, 2H), 3.75
(s, 3H), 3.68 (m, 2H), 1.48 ppm (s, 18H);. 13C NMR (CDCl3): d 169.40,
108Cminꢀ1 with a Dupont 9900 Analyzer. The melting transition (Tm)
and glass transition (Tg) temperatures were measured by DSC (TA In-
struments Q100) at heating and cooling rates of 108Cminꢀ1. UV/Vis and
PL spectra were recorded on a Jasco V-530 and a Spex Fluorog-3 Lumi-
nescence spectrophotometer, respectively. Quinine sulfate was used as
the standard for determination of the quantum yields (1ꢂ10ꢀ5 m in 0.5m
H2SO4, FF =0.55) in solution.[41] The quantum yields of solid-state films
were measured by using an integrating-sphere method. Cyclic voltamme-
try measurements (AUTOLAB/PGSTAT12 system) were carried out
with a three-electrode cell configuration consisting of platinum working
and counter electrodes and a Ag/AgNO3 (0.1m in acetonitrile) reference
electrode at room temperature (solvent, acetonitrile; supporting electro-
lyte, tetrabutylammonium hexafluorophosphate (0.1m)). The oxidation
Chem. Eur. J. 2009, 15, 6478 – 6487
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6485

Y. S. Lee, M. H. Lee, Y. Do et al.
160.74, 160.28, 149.94, 143.57, 141.75, 129.82, 128.42, 127.41, 126.24,
125.60, 123.32, 119.96, 118.02, 111.00, 55.75, 55.01, 35.57, 29.55 ppm; ele-
mental analysis calcd (%) for C38H43AlN2O5: C 71.90, H 6.83, N 4.41;
found: C 71.70, H 6.57, N 4.49; TGA: Td5 =3138C; DSC: Tm =2448C;
Tg =1028C.
Data for 6: Red solid; yield=55%; 1H NMR (CDCl3): d=8.29 (s, 2H),
7.42 (d, J=8.5 Hz, 2H), 7.29 (t, J=7.5 Hz, 2H), 7.21 (d, J=3.1 Hz, 2H),
7.15 (d, J=4.5 Hz, 1H), 7.13 (d, J=6.5 Hz, 2H), 6.45 (d, J=3.1 Hz, 2H),
6.41 (d, J=8.5 Hz, 2H), 3.99 (m, 2H), 3.66 (m, 2H), 2.83 (s, 12H),
1.50 ppm (s, 18H); 13C NMR (CDCl3): d=169.77, 160.45, 159.61, 142.34,
142.04, 141.80, 129.54, 128.38, 127.35, 126.21, 125.51, 123.90, 120.00,
118.46, 114.57, 55.01, 42.63, 35.69, 29.63 ppm; elemental analysis calcd
(%) for C40H49AlN4O3: C 72.70, H 7.47, N 8.48; found: C 72.15, H, 7.5, N
8.09, TGA: Td5 =3328C; DSC: Tm =183 (w), 196 (s), 213 (vs)8C; Tg =
1058C.
Acknowledgements
We gratefully acknowledge the Korea Research Foundation (KRF-2007-
313-C00398) for financial support and the PAL (Grant 2008-1041-16) for
the use of the SR facility. Y. S. Lee gratefully acknowledges financial sup-
port from the Korea Science and Engineering Foundation (R01-2007-
000-11015-0). We also appreciate the Refereeꢃs suggestion of reasons for
the low fluorescence quantum yields of compounds 3 and 6.
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17, 1109–1121; b) H. Yersin, Top. Curr. Chem. 2004, 241, 1–26;
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Synthesis of [{5,5’-(NMe3)2-salenACTHUNTGRENN(UG 3-tBu)2}Al(OC6H4-p-C6H5)]ACHTUNGTREN[NUGN OTf]2 (7):
MeOTf (3 equiv) was added to a solution of 6 (0.50 g, 1.1 mmol) in
CH2Cl2 (30 mL) in a glovebox. The reaction mixture was stirred for
30 min and the solution was decanted off through a cannula. Washing
with CH2Cl2 (10 mL) followed by drying in vacuo afforded 7 as a white
solid (0.47 g, 63%). Single crystals could be obtained by slow diffusion of
Et2O into a solution of 7 in MeCN. 1H NMR (CD3CN): d=8.64 (s, 2H),
7.70 (d, J=3.5 Hz, 2H), 7.65 (d, J=3.5 Hz, 2H), 7.42 (d, J=7.5 Hz, 2H),
7.32 (t, J=7.5 Hz, 2H), 7.20 (d, J=7.4 Hz, 1H), 7.15 (d, J=6.6 Hz, 1H),
6.37 (d, J=6.5 Hz, 2H), 4.09 (m, 2H), 3.92 (m, 2H), 3.53 (s, 18H),
1.54 ppm (s, 18H); 13C NMR (CD3CN): d=170.80, 165.62, 161.06, 144.66,
142.07, 136.35, 130.82, 129.64, 128.33, 126.96, 126.86, 124.09, 124.01,
120.69, 119.66, 118.29, 57.98, 55.63, 36.94, 29.66 ppm; elemental analysis
calcd (%) for C40H49AlN4O3: C 53.43, H 5.61, N 5.66; found: C 53.12, H
5.54, N 5.85; TGA: Td5 =3208C; DSC: Tm =2488C; Tg =1058C.
[4] C. Pꢄrez-Bolꢅvar, V. A. Montes, P. Anzenbacher, Jr., Inorg. Chem.
2006, 45, 9610–9612.
X-ray structural determination: Single crystals of suitable size and quality
were selected and mounted on a glass capillary after being coated with
paraton oil. The diffraction data for 4, 6, and 7 were collected on a
Bruker Apex II-CCD area detector diffractometer with graphite-mono-
chromated MoKa radiation (l=0.71073 ꢁ). The reflection data were col-
lected as multiscan frames with 0.58/frame and an exposure time of 10 s/
frame. Cell parameters were determined and refined by means of the
SMART program.[42] SAINT software was used for data reductions.[43]
The data were corrected for Lorentz and polarization effects and empiri-
cal absorption corrections were applied by using the SADABS pro-
gram.[44] The diffraction data for 1 were measured at 193 K with synchro-
tron radiation (l=0.75000 ꢁ) on a 4AMXW ADSC Quantum-210 detec-
tor with a silicon double crystal monochromator at the Pohang Accelera-
tor Laboratory, Korea. The HKL2000 (Version 0.98.689)[45] was used for
data collection, cell refinement, reduction, and absorption correction.
The structures of the compounds were solved by direct methods and re-
fined by full-matrix least-squares methods by using the SHELXTL pro-
gram package with anisotropic thermal parameters for all non-hydrogen
atoms.[46] Hydrogen atoms were placed at their geometrically calculated
positions and refined riding on the corresponding carbon atoms with iso-
tropic thermal parameters. The detailed crystallographic data for 1, 4, 6,
and 7 are given in Table 1.
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Received: January 18, 2009
Published online: May 26, 2009
Chem. Eur. J. 2009, 15, 6478 – 6487
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6487