Synthesis, structures, photoluminescent behaviors, and DFT studies of novel aluminum complexes containing phenoxybenzotriazole derivatives

Article abstract of DOI:10.1021/om9007423

Treatment of AlMe₃ with 2 equiv of 2-(2H-benzo[d][l,2,3]triazol- 2-yl)-4,6-di-tert-pentylphenol (Lig¹H) or 2-ter/-butyl-6-(5-chloro- 2H-benzo[d][l,2,3] triazol-2-yl)-4-methylphenol (Lig₂H) and subsequent addition of ROH afford monomeric and heteroleptic (OR)AlL₂ complexes [R = C₆H₅, L =Lig¹,1; R =p _-C₆H₄Ph, L = Lig¹2; R = C ₆H₅, L = Lig¹,3; R =p_-C ₆H₄Ph, L=Lig²,4]. The simple reaction between a quantitative amount of water and compound 1 gave the novel dimeric aluminum complex 5 bridged by an oxygen atom. The crystal structures of 1,3, and 5 determined from X-ray diffraction studies reveal pentacoordination geometry around the Al center with the preference for the slightly distorted trigonal-bipyramidal geometry. Monomeric complexes 1-4 showed emissions of the blue region (475 nm) with broad emission areas (half-width of peak = 150 nm) in the emission spectrum with high quantum yield. Dimeric aluminum complex 5 with enhanced thermal stability has a similar absorption and emission pattern to complexes 1 and 2, as predicted by DFT calculations. The DFT calculations suggested that the HOMO and LUMO orbitais are localized on the phenoxy group and benzotriazole group of the ligand and the effect of the phenoxide ligand is negligible.

Full text of DOI:10.1021/om9007423

Organometallics 2010, 29, 347–353 347
DOI: 10.1021/om9007423
Synthesis, Structures, Photoluminescent Behaviors, and DFT Studies of
Novel Aluminum Complexes Containing Phenoxybenzotriazole
Derivatives
Junseong Lee,† So Han Kim,‡ Kang Mun Lee,† Kyu Young Hwang,† Hyoseok Kim,†
Jung Oh Huh,† Da Jung Kim,‡ Yoon Sup Lee,† Youngkyu Do,*,† and Youngjo Kim*,‡
†Department of Chemistry, KAIST, Daejeon 305-701, Korea and
‡Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Korea
Received August 26, 2009
Treatment of AlMe3 with 2 equiv of 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di-tert-pentylphenol
(Lig1H) or 2-tert-butyl-6-(5-chloro-2H-benzo[d][1,2,3] triazol-2-yl)-4-methylphenol (Lig2H) and
subsequent addition of ROH afford monomeric and heteroleptic (OR)AlL2 complexes [R =
C6H5, L = Lig1, 1; R = p-C6H4Ph, L = Lig1, 2; R = C6H5, L = Lig2, 3; R = p-C6H4Ph, L = Lig2, 4].
The simple reaction between a quantitative amount of water and compound 1 gave the novel dimeric
aluminum complex 5 bridged by an oxygen atom. The crystal structures of 1, 3, and 5 determined from
X-ray diffraction studies reveal pentacoordination geometry around the Al center with the preference for
the slightly distorted trigonal-bipyramidal geometry. Monomeric complexes 1-4 showed emissions of
the blue region (475 nm) with broad emission areas (half-width of peak = 150 nm) in the emission
spectrum with high quantum yield. Dimeric aluminum complex 5 with enhanced thermal stability has a
similar absorption and emission pattern to complexes 1 and 2, as predicted by DFT calculations. The
DFT calculations suggested that the HOMO and LUMO orbitals are localized on the phenoxy group
and benzotriazole group of the ligand and the effect of the phenoxide ligand is negligible.
Introduction
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shift, as has been well documented in the literature, although
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stability.2a,4 While most previous work has focused on color
tuning via the introduction of an electronic effect to homo-
leptic aluminum complexes with three five-membered che-
lating rings5 or the synthesis of heteroleptic aluminum
complexes with one or two five-membered chelating rings,6
much less attention has been directed toward the syn-
thesis of homoleptic or heteroleptic aluminum complexes
with six-membered chelating rings.7 Moreover, there is no
The development of new, efficient emitting systems has
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subject since the discovery of OLED (organic light-emitting
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Tris(8-hydroxyquinolinato)aluminum (AlQ3), which has
three five-membered rings and shows a green-emitting prop-
erty (520 nm), is well known among emitting material
systems.3 The introduction of electronic effects in the 2,4-
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*To whom correspondence should be addressed. e-mail: ykim@
chungbuk.ac.kr (Y.K.) or ykdo@kaist.ac.kr (Y.D.). Tel: þ82-43-261-
3395. Fax: þ82-43-267-2279.
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r
2009 American Chemical Society
Published on Web 12/29/2009
pubs.acs.org/Organometallics

348 Organometallics, Vol. 29, No. 2, 2010
Lee et al.
example of an investigation of photophysical properties and
molecular orbital distribution in heteroleptic aluminum
complexes with two six-membered chelating rings.
117.87, 117.26, 116.86, 38.62, 37.62, 37.53, 36.91, 31.52, 28.57,
28.39, 27.13, 26.98, 9.06, 8.76. 27Al NMR (CDCl3, 52.105 MHz,
ppm): δ 82.93. EI-MS: calcd 821, found 821. Anal. Calcd for
C50H61N6O3Al: C, 73.14; H, 7.49; N, 10.24. Found: C, 73.99; H,
7.78; N, 10.45.
Synthesis of 2. The desired product 2 was obtained as yellow
solids in an isolated yield of 80% (1.43 g) in a manner analogous
to the procedure for 1 using AlMe3 (2.0 M solution in toluene,
1.0 mL, 2.0 mmol), Lig1H (1.44 g, 4.0 mmol), and 4-phenylphe-
nol (0.34 g, 2.0 mmol).
In this regard, 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di-
tert-pentylphenol (Lig1H) and 2-tert-butyl-6-(5-chloro-2H-
benzo[d][1,2,3]triazol-2-yl)-4-methylphenol (Lig2H) were
employed as logical chelating ligands, because they are
affordable starting materials used industrially as UV stabi-
lizers and are likely to form heteroleptic aluminum com-
plexes with two six-membered rings due to steric congestion
of Lig1 or Lig2 in the vicinity of the aluminum. Herein we
report on the synthesis, characterization, X-ray structures,
photoluminescent properties, and theoretical DFT studies of
novel aluminum complexes 1-5 containing Lig1 or Lig2.
1H NMR (CDCl3, 400.15 MHz, ppm): δ 8.28 (d, 2H), 8.10 (br
s, 2H), 7.96 (d, 2H), 7.43 (m, 4H), 7.37(d, 2H), 7.28(t, 2H), 7.14
(s, 4H), 6.71 (d, 2H), 1.63 (q, 4H), 1.30 (s, 12H), 1.02 (m, 4H),
0.95 (s, 6H), 0.82 (s, 6H), 0.66 (t, 6H), 0.05 (t, 6H). 13C{1H}
NMR (CDCl3, 100.63 MHz, ppm): δ 158.82, 148.89, 142.46,
141.20, 140.38,139.73, 138.46, 130.53, 129.08, 128.46, 127.90,
127.76, 127.58, 126.19, 125.87, 119.37, 117.93, 117.21, 116.87,
38.64, 37.64, 36.93, 31.53, 28.60, 28.40, 27.16, 27.00, 9.08, 8.80.
27Al NMR (CDCl3, 52.105 MHz, ppm): δ 89.21. EI-MS: calcd
897, found 897. Anal. Calcd for C56H65N6O3Al: C, 74.97; H,
7.30; N, 9.37. Found: C, 74.38; H, 7.44; N, 9.43.
Synthesis of 3. The desired product 3 was obtained as yellow
solids in an isolated yield of 82% (1.20 g) in a manner analogous
to the procedure for 1 using AlMe3 (2.0 M solution in toluene,
1.0 mL, 2.0 mmol), Lig2H (1.26 g, 4.0 mmol), and phenol (0.34 g,
2.0 mmol).
Experimental Section
General Considerations. All manipulations were carried out
under a dinitrogen atmosphere using standard Schlenk and
glovebox techniques.8 All other chemicals were purchased from
Aldrich and were used as supplied unless otherwise indicated.
Toluene, hexane, and tetrahydrofuran (THF) were dried with
Na/K alloy with benzophenone and were stored over activated
9
˚
3 A molecular sieves. All deuterium solvents were dried over
˚
activated molecular sieves (3 A) and were used after vacuum
1H NMR (CDCl3, 400.15 MHz, ppm): δ 8.11 (s, 3H), 8.05-
7.83 (m, 3H), 7.42 (m, 2H), 7.06 (s, 2H), 6.92 (m, 1H), 6.67 (q,
2H), 6.56 (t, 1H), 2.29 (s, 6H), 0.80 (s, 18H). 13C{1H} NMR
(CDCl3, 100.63 MHz, ppm): δ 149.13, 144.75, 142.31, 135.31,
133.72, 130.60, 130.12, 129.39, 127.21, 126.30, 119.36, 118.93,
118.36, 116.91, 116.69, 34.75, 31.57, 28.78, 22.63, 20.84, 14.08.
27Al NMR (CDCl3, 52.105 MHz, ppm): δ 80.80. EI-MS: calcd
749, found 748. Anal. Calcd for C40H39N6O3Cl2Al: C, 64.09; H,
5.23; N, 11.21. Found: C, 63.84; H, 5.57; N, 11.85.
Synthesis of 4. The desired product 4 was obtained as yellow
solids in an isolated yield of 87% (1.44 g) in a manner analogous
to the procedure for 1 using AlMe3 (2.0 M solution in toluene,
1.0 mL, 2.0 mmol), Lig2H (1.26 g, 4.0 mmol), and 4-phenylphe-
nol (0.34 g, 2.0 mmol).
transfer to a Schlenk tube equipped with a J. Young valve.9
Measurements. 1H and 13C{1H} spectra were recorded at
ambient temperature on a Bruker AVANCE 400 NMR spectro-
meter using standard parameters. The chemical shifts are refer-
enced to the peaks of residual CDCl3 (δ 7.24, 1H NMR; δ 77.0,
13C{1H} NMR). Elemental analyses and EI-mass data were
performed by EA 1110-FISONS(CE) and JMS 700, respec-
tively. UV-vis and PL spectra were recorded on a Jasco
V-530 and a Spex Fluorog-3 Luminescence spectrophotometer,
respectively. Thermogravimetric analyses (TGA) were carried
out under a nitrogen atmosphere at a heating rate of 10 °C/min
with a Dupont 9900 analyzer. UV-visible absorption and
fluorescence measurements were performed in THF with a
1 cm quartz cuvette at ambient temperature. Quantum yields
were determined using quinine sulfate (Fluka) as the standard
(1 ꢀ 10-6 M in 0.5 M H2SO4, ΦF = 0.55).10
1H NMR (CDCl3, 400.15 MHz, ppm): δ 8.14 (s, 3H), 8.10-
7.75 (m, 3H), 7.50-7.15 (m, 9H), 7.08 (s, 2H), 6.71 (m, 2H), 2.32
(s, 6H), 0.85 (s, 18H). 13C{1H} NMR (CDCl3, 100.63 MHz,
ppm): δ 158.38, 149.16, 149.10, 142.96, 142.00, 140.95, 140.54,
135.39, 133.77, 131.14, 130.72, 130.20, 130.16, 129.46, 128.70,
128.51, 128.10, 128.05, 128.02, 127.29, 127.25, 126.27, 126.08,
119.40, 119.35, 119.17, 118.99, 118.38, 116.98, 116.65, 34.79,
28.80, 20.88. 27Al NMR (CDCl3, 52.105 MHz, ppm): δ 83.00.
EI-MS: calcd 825, found 825. Anal. Calcd for C46H43N6O3-
Cl2Al: C, 66.91; H, 5.25; N, 10.18. Found: C, 66.52; H, 5.44; N,
10.15.
Synthesis of 5. To a stirred yellow solution of complex 1
(0.787 g, 1.0 mmol) in 30 mL of toluene was added H2O (18 μL,
1.0 mmol) at room temperature, and then the mixture was
refluxed for 12 h. The residue, obtained by removing the solvent
under vacuum, was recrystallized in n-hexane. The desired pro-
duct 5 was isolated as pale yellow crystals after the solution
remained at -20 °C in a refrigerator for a few days (0.59 g, 80%).
1H NMR (CDCl3, 400.15 MHz, ppm): δ 8.22 (d, 2H), 8.10
(br s, 1H), 7.74 (br s, 1H), 7.53 (m, 2H), 7.40 (d, 1H), 2.05
(q, 2H), 1.74 (q, 2H), 1.48 (s, 6H), 1.41 (s, 6H), 0.72 (m,
6H). 13C{1H} NMR (CDCl3, 100.63 MHz, ppm): δ 153.76,
147.04, 136.49, 131.58, 129.05, 128.23, 127.45, 118.76, 118.04,
39.49, 38.60, 36.95, 33.46, 28.42, 28.03, 9.56, 9.21. 27Al NMR
Synthesis of Complexes 1-5. Synthesis of 1. To a stirred
colorless solution of AlMe3 (2.0 M solution in toluene, 1.0 mL,
2.0 mmol) in 30 mL of toluene was added dropwise at -78 °C a
solution of Lig1H (1.44 g, 4.0 mmol) in 20 mL of toluene. The
reaction mixture was allowed to warm to room temperature and
then refluxed for 1 h. The residue, obtained by removing the
solvent under vacuum, was redissolved in 20 mL of toluene, and
phenol (0.188 g, 2.0 mmol) in 10 mL of toluene was added and
then refluxed for 1 h. All volatiles were removed under vacuum,
and the residue was washed with 20 mL of n-hexane three times.
The desired product 1 was isolated as yellow crystals after the
methylene chloride/n-hexane solution remained at -20 °C in a
refrigerator for a few days (1.40 g, 85%).
1H NMR (CDCl3, 400.15 MHz, ppm): δ 8.27 (s, 2H), 8.09 (d,
2H), 7.96 (d, 2H), 7.43 (m, 2H), 7.14 (s, 2H), 6.87 (t, 2H), 6.65 (d,
2H), 6.50 (t, 1H), 1.63 (q, 4H), 1.30 (s, 12H), 1.02 (s, 4H), 0.92 (s,
6H), 0.81 (s, 6H), 0.62 (q, 6H), 0.02 (q, 6H). 13C{1H} NMR
(CDCl3, 100.63 MHz, ppm): δ 159.00, 148.92, 142.43, 140.36,
139.72, 138.40, 129.10, 128.96, 127.87, 127.52, 126.19, 119.17,
(8) (a) Shriver, D. F. In The Manipulation of Air-Sensitive Com-
pounds; McGraw-Hill: New York, 1969. (b) Erringtion, R. J. In Advanced
Practical Inorganic and Metalorganic Chemistry; Blackie Academic &
Professional: London, 1997.
(9) Armarego, W. L. F.; Chai, C. L. L. In Purification of Laboratory
Chemicals, 5th ed.; Elsevier: New York, 2003.
(CDCl3, 52.105 MHz, ppm): δ 84.86. Anal. Calcd for C88H112
N12O5Al2: C, 71.81; H, 7.67; N, 11.42. Found: C, 72.29; H, 7.86;
-
N, 11.75.
X-ray Structure Determination for 1, 3, and 5. Reflection data
for 1, 3, and 5 were collected on a Bruker APEX II CCD area
diffractometer with graphite-monochromated Mo KR radiation
(10) (a) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229. (b) Demas,
J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.

Article
Organometallics, Vol. 29, No. 2, 2010 349
Scheme 1. Synthetic Routes to Complexes 1-5
˚
(λ = 0.7107 A). Specimens of suitable quality and size were
selected, mounted, and centered in the X-ray beam by using a
videocamera. The hemisphere of reflection data was collected as
ω-scan frames with 0.3o/frame and an exposure time of 5 s/
frame. Cell parameters were determined and refined by the
SMART program.11 Data reduction was performed using
SAINT software.12 The data were corrected for Lorentz and
polarization effects. An empirical absorption correction was
applied using the SADABS program.13 The structures of the
compounds were solved by direct methods and refined by full
matrix least-squares methods using the SHELXTL program
package with anisotropic thermal parameters for all non-hydro-
gen atoms.14 X-ray crystal structures of 1, 3, and 5 were drawn
by the Diamond Program ver. 2.1e.
6-31G(d) basis sets and using CIS15 with the 6-31G basis set,
respectively. To obtain the electronic transition energies, which
include some account of electron correlation, TD-DFT16 using
the hybrid B3LYP17 (TD-B3LYP) functional was used with the
6-31G(d) basis set. Emission wavelengths were evaluated on the
CIS-optimized structures of the excited state. For comparison
with emission wavelengths, absorption wavelengths were pre-
dicted on the HF-optimized structures of the ground state (see
Supporting Information). All calculations described here were
carried out with the GAUSSIAN 03 program.18
Result and Discussion
Compounds 1-4 were obtained with high yield (>80%)
by adding dropwise a 2 equiv amount of Lig1H or Lig2H in
toluene to a solution of AlMe3 and subsequent addition of
ROH (R = Ph or p-C6H4Ph), as outlined in Scheme 1. The
compounds were purified by washing with n-hexane and
recrystallized in methylene chloride. Complex 1 was also
produced when 1 equiv of Lig1H and excess AlMe3 were
mixed in toluene solution. Thus, the formation of 1 appears
to be kinetically and thermodynamically favored in this
reaction, whereas the compositions of other heteroleptic
aluminum compounds are sensitive to the ratios of the
starting materials.19
Computational Details. The ground-state (S0) structures and
the first singlet excited-state (S1) structures of complexes were
optimized using the ab initio Hartree-Fock (HF) with the
(11) SMART Version 5.0, Data Collection Software; Bruker AXS Inc.:
Madison, WI, 1998.
(12) SAINT Version 5.0, Data Integration Software; Bruker AXS Inc.:
Madison, WI, 1998.
(13) Sheldrick, G. M. SADABS: Program for Absorption Correction
€
with Bruker SMART System; University Gottingen: Germany, 1996.
(14) Sheldrick, G. M. SHELXS-97: Program for the Solution of
€
Crystal Structures; University Gottingen: Germany, 1990.
(15) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J.
J. Phys. Chem. 1992, 96, 135.
The hydrolysis of alkoxide has proven to be a useful route
to the formation of various metallic and metalloidal oxo
complexes.20 Thus the addition of H2O into the toluene
solution of complexes 1 gave oxo-bridged aluminum dimeric
complex 5 with 80% isolated yield, which was remarkably
stable in a solid state for more than a month. In addition,
according to 1H NMR spectroscopy, 5 is very stable at room
temperature for more than a week in chloroform-d1, acetone-
d6, and methanol-d4 solutions contained in capped NMR
tubes. Compound 5 is soluble in a variety of solvents
(16) Runge, E.; Gross, E. K. Phys. Rev. Lett. 1984, 52, 997.
(17) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
J. Phys. Chem. 1994, 98, 11623–11627.
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Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; , Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakr-
zewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; , Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; , Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02;
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350 Organometallics, Vol. 29, No. 2, 2010
Table 1. Crystallographic Data and Parameters for 1, 3, and 5
Lee et al.
1
3
5
empirical formula
fw
cryst syst
C
821.03
triclinic
P1
11.2879(4)
12.6897(5)
17.3676(7)
102.212(2)
104.375(2)
93.132(2)
2340.18(16)
2
50H61AlN6O3
C40H38AlCl2N6O3
748.64
monoclinic
P21/c
15.6237(17)
20.907(2)
15.0343(18)
90
114.863(7)
90
C91H122Al2Cl6N12O7
1762.67
orthorhombic
Ccc2
17.8534(14)
21.9402(17)
24.7375(18)
90
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
90
90
3
˚
V (A )
Z
4455.8(9)
4
1.116
9689.9(13)
4
1.208
dcalcd(g/cm3)
F(000)
1.165
880
1564
0.15 ꢀ 0.14 ꢀ 0.11
3744
0.40 ꢀ 0.12 ꢀ 0.11
cryst size (mm)
T (K)
0.20 ꢀ 0.15 ꢀ 0.10
130
130
130
θ range (deg)
no. of unique reflns
no. of obsd reflns (I > 2σ(I))
no. of params refined
R1(I > 2σ(I))a
wR2(I > 2σ(I))b
GOF(I > 2σ(I))
1.24 e θ e 31.82
84 871
15 122
603
0.0566
1.44 e θ e 20.22
24 443
4263
487
0.0844
1.69 e θ e 25.75
15 851
7724
555
0.0792
0.1893
1.027
0.1865
1.027
0.1678
1.007
a R1 = Fo| - |Fc /|Fo|. b wR2 = [w(Fo2 - Fc2)2]/w(Fo2)2]]1/2
.
including toluene, methanol, acetone, and n-hexane.
Although some examples of oxo-bridged aluminum com-
plexes such as (Q2Al)2O have been reported,20a their stabi-
lities could not satisfy the requirement for application
because of their unfilled coordination sites. However, com-
pound 5 has extremely higher thermal stability than other
pentacoordinated systems.
Compounds 1-5 have also been investigated with respect
to thermal stability under a dinitrogen atmosphere using
TGA. TGA measurements exhibit high DT5 values (5.0 wt %
decomposition temperatures) of 220, 210, 207, 215, and
380 °C for 1-5, respectively. This data reflect that the
Lig1-Al or Lig2-Al bond is thermally robust. Most inter-
estingly, complex 5 has extremely high thermal stability
probably originated from the strong Al-O-Al bond. All
complexes 1-5 are air-stable in both the solid and solution
states, probably due to the steric protection of the Al center
by two tert-pentyl or tert-butyl groups at the R1 position as
well as the three strong Al-O bonds.
Figure 1. X-ray structures of compounds 1 and its atom label-
ing (H atoms and tert-pentyl groups were omitted for clarity).
Selected bond lengths (A) and angles (deg): Al-O1 1.7509(13),
Al-O2 1.7504(12), Al-O3 1.7531(13), Al-N1 2.0151(15),
Al-N2 2.0134(15), O1-Al-O2 117.18(6), O1-Al-O3
120.99(6), O2-Al-O3 121.81(6), O1-Al-N1 89.60(6), O2-
Al-N1 90.08(6), O3-Al-N1 91.87(6), O1-Ti-N4 90.70(6),
O2-Al-N4 89.04(6), O3-Al-N4 88.69(6), N1-Al-N4
179.11(6).
˚
Compounds 1-5 were characterized by 1H, 13C{1H}, and
27Al NMR spectroscopy, EI-mass spectrometry, and ele-
mental analysis. The structures of 1, 3, and 5 were deter-
mined by single-crystal X-ray crystallography. The electron-
impact mass spectrum (70 eV) indicates monomeric behavior
for 1-4 in the gas phase; however, the nonvolatile property
of 5 is observed in the mass spectrum. The 1H NMR spectra
of 1-5 display well-defined sharp resonances with the ex-
pected integrations. Also, well-defined resonances for the
aromatic and aliphatic carbons are observed in the 13C{1H}
NMR spectrum, and there is no indication of the presence of
higher oligomers because the resonances consist of very
sharp and intense singlets. Despite a high quadrupolar
relaxation rate for the 27Al nucleus (I = 5/2), a single broad
27Al NMR chemical shift at 82.93, 89.21, 80.80, 83.00, and
84.86 ppm for 1-5, respectively, was detected at room
temperature in chloroform-d1. These peaks can be reason-
ably assigned to a pentacoordinated aluminum atom, which
is normally observed in the range 60-100 ppm.21 These
results are consistent with X-ray structures of 1, 3, and 5 in
the solution phase.
To elucidate the nature of the metal-ligand bonding and
the solid-state structural nature of 1, 3, and 5, we carried out
a single-crystal X-ray diffraction study. Single crystals sui-
table for X-ray structural determination were obtained by
cooling of a solution of either methylene chloride/n-hexane
for 1 and 3 or n-hexane for 5 at -20 °C. The X-ray structures,
selected bond distances, and selected bond angles for 1, 3,
and 5 are shown in Figures 1-3, respectively. The complexes
1, 3, and 5 crystallized in space group P1, P21/c, and P22a,
respectively. In order to examine the distortion of the co-
ordination geometry in 1, 3, and 5, the parameters τ (τ =
(β - R)/60, where R and β are the largest and next-largest
(21) (a) Potapov, A. G.; Terskikh, V. V.; Zakharov, V. A.; Bukatov,
G. D. J. Mol. Catal. A: Chem. 1999, 145, 147. (b) Potapov, A. G.; Terskikh,
V. V.; Bukatov, G. D.; Zakharov, V. A. J. Mol. Catal. A: Chem. 1997,
122, 61.

Article
Organometallics, Vol. 29, No. 2, 2010 351
ming from the benzotriazole ligands, lending slightly
distorted trigonal-bipyramidal geometry around the alumi-
num. The equatorial oxygens form a regular triangle with
˚
˚
˚
sides of 3.033(2) A for 1, 3.003(4) A for 3, and 3.009(2) A for
5. The average distances between these oxygens and the axial
˚
˚
nitrogens are 2.669(2) for 1, 2.657(7) A for 3, and 2.686(2) A
for 5. The sum of the Oeq-Al-Oeq angles is 359.98(6)° for 1,
360.0(4)° for 3, and 359.97(20)° for 5 [average Oeq-Al-Oeq
angle is 119.99(6)° for 1, 120.0(4)° for 3, and 119.99(19)° for
5], and the Naxial-Al-Naxial angles (179.11(16)° for 1,
176.5(4)° for 3, and 172.3(2)° for 5) are nearly linear. The
average Al-O [1.751(2) A for 1, 1.733(3) A for 3, and
1.737(4) A for 5] and Al-N [2.0142(15) A for 1, 2.015(9) A
for 3, and 2.050(6) A for 5] bond distances are similar to the
˚
˚
˚
˚
˚
˚
Figure 2. X-ray structures of compounds 3 and its atom label-
ing (H atoms and two tert-butyl groups were omitted for
clarity). Selected bond lengths (A) and angles (deg): Al-O1
average distances observed for other structurally character-
ized aluminum alkoxides or aluminum amides, respecti-
vely.6a,24
˚
1.742(7), Al-O2 1.737(7), Al-O3 1.721(7), Al-N1 2.022(9),
Al-N4 2.008(8), O1-Al-O2 118.6(3), O1-Al-O3 123.7(4),
O2-Al-O3 117.7(4), O1-Al-N1 87.7(4), O2-Al-N1 89.8(3),
O3-Al-N1 92.8(4), O1-Ti-N4 91.7(3), O2-Al-N4 87.5(3),
O3-Al-N4 90.3(3), N1-Al-N4 176.5(4).
To examine the optical properties of 1-5, UV-vis and PL
experiments were carried out, and the absorption (left) and
emission (right) spectra of 1-5 in chloroform are shown in
Figure 4. All complexes 1-5 feature major absorption bands
at around 374-386 nm assigned to ligand-centered π-π*
transitions. As the ancillary ligand is varied from OPh to
OC6H4-p-Ph (from 1 to 2 or from 3 to 4), the absorption
maxima wavelength (λabs) is slightly blue-shifted. ε values of
complexes 1-5 are in the range (1.7-4.5) ꢀ 104/M-1 cm-1
,
which are considerably higher than for other Al systems.2a,24
Particularly, complex 5 gives a 2-fold higher ε value than
other complexes 1-4 due to its dinuclear structure.
The PL emission spectra of 1-5 exhibit emission maxima
(λem) ranging from 475 to 497 nm (Figure 4). Unlike the
UV-vis absorption spectra, the alteration of a phenoxy-type
ancillary ligand did not cause a blue-shift of λem. Interest-
ingly, 1, 2, and 5, containing a Lig1 ligand, emit a sky-blue
light around 480 nm, which is 20 nm blue-shifted compared
with 495 nm of Al(Meq)2OPh.2a The pyridylphenol ligand, a
widely used ligand in the synthesis of aluminum complexes
having six-membered chelating rings, has 3.84 eV of π-π*
transition energy,25 which is blue-shifted compared with that
(4.0 eV)26 of 8-hydroxyquinoline due to a weak conjugation,
supporting our experiments. Furthermore, the emission
maxima for 3 and 4, with a Lig2 ligand, are observed around
495 nm and are 20 nm red-shifted compared with complexes
1 and 2. The negligible effect of phenolato groups on the
absorption and emission spectra may be attributed to their
low contribution in the HOMO and LUMO.6a,24 Complexes
1-5 showed sufficient quantum efficiencies of 0.11-0.17 for
OLED application. Also, they have very strong emissions
and broad emission areas with a half-width of 150 nm in the
emission spectra. These are probably influenced by the
rigidity of coordinated ligand Lig1 or Lig2, which shows very
strong absorption bands itself, caused by its steric bulkiness
and π-conjugation.
Figure 3. X-ray structures of compounds 5 and its atom label-
ing (H atoms and tert-pentyl groups were omitted for clarity).
Selected bond lengths (A) and angles (deg): Al-O1 1.762(4),
Al-O2 1.760(4), Al-O3 1.6900(17), Al-N1 2.051(6), Al-N4
2.050(6), O1-Al-O2 120.5(2), O1-Al-O3 119.00(17), O2-
Al-O3 120.47(16), O1-Al-N1 88.0(2), O2-Al-N1 87.8(2),
O3-Al-N1 94.2(3), O1-Al-N4 89.0(2), O2-Al-N4 87.6(2),
O3-Al-N4 93.5(3), N1-Al-N4 172.3(2), Al-O3-Al0
176.3(5).
˚
interligand bond angle, respectively)22 and Δ (dihedral angle
method)23 were calculated. For the regular trigonal-bipyr-
amidal complexes, the trigonality parameter τ will be 1.0, and
it decreases to zero as the square-pyramidal distortion
increases. Similarly, Δ is zero for trigonal-bipyramidal com-
pounds and 1.0 for square-pyramidal complexes. The calcu-
lated τ and Δ values are 0.96 and 0.06 for 1, 0.88 and 0.07 for
3, and 0.86 and 0.21 for 5, respectively, indicating nearly
ideal trigonal-bipyramidal geometry. Figures 1-3 show that
the above calculations fit well for complexes 1, 3, and 5.
Thus, in addition to the three “anionic” oxygens, the alumi-
num atom is ligated by means of two nitrogen atoms stem-
To investigate the electronic transition and the electronic
structures for complex 1, time-dependent density functional
theory (TD-DFT) calculations were carried out at the
B3LYP/6-31G(d) approximation level, which has been used
in previous literature.6a The geometry of 1 for the calculation
(24) Hwang, K. Y.; Lee, M. H.; Jang, H.; Sung, Y.; Lee, J. S.; Kim,
S. H.; Do, Y. Dalton Trans. 2008, 1818.
(25) Kaczmarek, L.; Balicki, R. J. Chem. Soc., Perkin Trans. 1994,
1603.
(22) (a) Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37.
(b) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349.
(23) Meutterties, E. L.; Guggenberger, L. J. J. Am. Soc. Chem. 1974,
96, 1748.
(26) (a) Halls, M. D.; Schlegel, H. B. Chem. Mater. 2001, 13, 2632.
(b) Perkampus, H. H.; Kortum, K. Z. Anal. Chem. 1962, 190, 111.

352 Organometallics, Vol. 29, No. 2, 2010
Lee et al.
Figure 4. UV-vis absorption spectra (left) and PL emission spectra (right) of 1-5 in chloroform.
Figure 5. Simulated TD-DFT/B3LYP UV/vis absorption spec-
trum of complex 1 as a function of the 6-31G basis set employed
in both the geometry optimization and the excitation energy
determination.
Table 2. Computed Absorption Wavelengths (A in nm) and
Oscillator Strengths (fcalc) at the TD-B3LYP/6-31G(d) Level of
Theory Using the HF/6-31G(d) Geometry for Complex 1
state
λ (nm)
fcalc
nature
contribution
1
2
3
4
5
6
7
8
411.79
396.71
375.82
365.66
353.67
347.32
309.68
299.64
299.44
298.34
295.23
0.0026
0.0088
0.0499
0.1180
0.1359
0.0544
0.0032
0.1704
0.0240
0.3203
0.1474
HOMO f LUMO
HOMO f LUMO
94%
94%
93%
93%
93%
94%
91%
67%
81%
76%
78%
16%
þ1
HOMO-1 f LUMO
HOMO-1 f LUMO
HOMO-2 f LUMO
HOMO-2 f LUMO
HOMO-3 f LUMO
HOMO-4 f LUMO
HOMO-3 f LUMO
HOMO-5 f LUMO
HOMO-4 f LUMO
HOMO-4 f LUMO
þ1
þ1
9
10
11
þ1
þ1
was optimized from its X-ray structure. The computed
optical data are listed in Table 2, and the simulated
TD-DFT/B3LYP UV/vis absorption spectrum of complex
1 is shown in Figure 5, which matches roughly with actual
UV/vis spectra of 1 in Figure 4. The optimized geometry
and molecular orbitals for the first excited state of 1 are
shown in Figure 6. Among the calculated absorptions and
emissions, the absorption peak at 353.67 nm and the emis-
sion peak at 453.37 nm, the transition mainly from HOMO-2
Figure 6. Frontier molecular orbitals for the first excited state
of complex 1 from the B3LYP/CIS calculation.
to LUMO and from LUMO to HOMO, respectively, are
considered to be dominant according to Tables 2 and 4 and

Article
Organometallics, Vol. 29, No. 2, 2010 353
Table 3. Absorption Wavelengths (Amax in nm) Computed at the TD-B3LYP Method Using the HF/6-31G(d)-Optimized Structures and
the Emission Wavelengths (λmax in nm) Computed at the TD-B3LYP Method Using the CIS/6-31G-Optimized Structuresa
experiment
calculation
compound
UV/vis
PL
TD-B3LYP//HF
TD-B3LYP//CIS
453.37 (0.1512)
1
320, 376
475
295.23 (0.1474), 298.34 (0.3203)
353.67 (0.1359), 365.66 (0.1180)
a Oscillator strengths are in parentheses.
Table 4. Computed Emission Wavelengths (λ in nm) and Oscil-
lator Strengths (fcalc) at the TD-B3LYP/6-31G(d) Level of
Theory Using the CIS/6-31G Geometry for Complex 1
units are attached, a blue-shifted emitting material will be
obtained. A trend of similar red- or blue-shift was observed
in various homoleptic aluminum complexes containing sub-
stituents with high electron-donating or electron-withdraw-
ing abilities at the 5,7-positions of the 8-hydroxyquinolinate
ligand, respectively.2a,3
state
1
λ (nm)
fcalc
nature
contribution
476.53
0.0085
HOMO-1 f LUMO
HOMO f LUMO
HOMO f LUMO
HOMO-1 f LUMO
HOMO-2 f LUMO
HOMO-2 f LUMO
73%
25%
50%
21%
14%
82%
94%
97%
2
453.37
0.1512
Conclusion
Novel aluminum complexes 1-5 having Lig1 or Lig2
ligands have been synthesized as blue-emitting materials
and characterized by X-ray crystallography for 1, 3, and 5.
On the basis of their photophysical properties, they are
strong candidates for blue-emitting materials for OLED
application. Using DFT study, a strategy for fine-tuning
the emission was established. Detailed studies on their tuning
ability are in progress.
3
4
5
422.59
404.37
394.37
0.0954
0.0403
0.0059
HOMO f LUMO
þ1
HOMO-1 f LUMO
þ1
Figure 5, which is in good agreement with the experimental
data of 1 in solution. As expected, there is little orbital
contribution from the ancillary phenoxy group and Al metal
center. This observation is well matched with the result that
complex 5 showed a similar pattern in absorption and
emission.
Because the HOMO and LUMO orbitals are likely loca-
lized on the phenoxy group and benzotriazole group of
ligand Lig1, respectively, this calculation supported our
hypothesis that 10 and 20 nm red-shifted wavelengths of
absorption and emission in complexes 3 and 4, respectively,
may result from attachment of chloride atom, an electron-
withdrawing substituent, in the LUMO. If electron-donating
Acknowledgment. We gratefully acknowledge the Korea
Research Foundation (KRF-2008-313-C00450).
Supporting Information Available: Frontier molecular orbi-
tals for the ground state of complex 1 from the B3LYP//HF
calculation and cif file for complexes 1, 3, and 5 are available free
of charge via the Internet at http://pubs.acs.org.