Tuning of the emission efficiency and HOMO-LUMO band gap for ester-functionalized {Al(salophen)(H2O)2}+ blue luminophors

Article abstract of DOI:10.1021/ic201208c

A series of [AlL(H₂O)₂(NO₃)] complexes, with L standing for an ester substituted salophen-type ligand, has been synthesized, and the luminescence properties have been investigated. These derivatives differ by the nature of

Full text of DOI:10.1021/ic201208c

Article
pubs.acs.org/IC
Tuning of the Emission Efficiency and HOMO−LUMO Band Gap for
Ester-Functionalized {Al(salophen)(H2O)2}+ Blue Luminophors
Virginie Bereau,*,⊥,‡,§ Carine Duhayon,⊥,‡ Alix Sournia-Saquet,⊥,‡ and Jean-Pascal Sutter*,⊥,‡
́
⊥CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, F-31077 Toulouse, France.
‡Universite
́
de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
§IUT Paul Sabatier, Av. G. Pompidou, BP 20258, F-81104 Castres, France
S
* Supporting Information
ABSTRACT: A series of [AlL(H2O)2(NO3)] complexes, with L standing for an
ester substituted salophen-type ligand, has been synthesized, and the luminescence
properties have been investigated. These derivatives differ by the nature of the ester-
R group introduced at the C5 position of their salicylidene rings (i.e., phenyl, 7a,a′;
naphthyl, 7b,b′; pentafluorophenyl, 7c,c′; and p-nitrophenyl, 7d) and by the bis-
imino bridge (i.e., 1,2- phenylene, 7a−d; and 1,2-naphthalene, 7a′−c′). All the
complexes are characterized by luminescence in the blue range, the chemical
diversity having no effect on the emission wavelength (480−485 nm). However, the
emission efficiency was found to be strongly dependent on the Schiff-base ligand
with quantum yields ranging from ϕ = 22% to 44%, the highest values being for the
salophen derivatives with the electron-withdrawing ester-R groups (7a, 34%; 7a′,
23%; 7b, 31%; 7b′, 22%; 7c, 40%; 7c′, 29%, and 7d, 44%). Both the electrochemical
data and DFT calculations show that the HOMO−LUMO band gap is modified as a
function of the ester R group (from 2.92 to 3.16 eV, based on the redox potentials). The crystal structures for the N,N′-bis(5-
(phenoxycarbonyl)salicylidene)-1,2-phenylenediamine and the N,N′-bis(5-(p-nitrophenoxycarbonyl)salicylidene)-1,2-phenyl-
enediamine aluminum complexes (7a and 7d) are reported.
For these two families of aluminum complexes (Schiff base
and quinolate derivatives), it has been shown that the emission
wavelengths can be modulated by chemical alterations at the
periphery of the core of the ligand. The electron-withdrawing
or electron-donating nature of substituents was found to
influence the color of the emitted light for a given
luminophor.11,19,20 Another parameter of importance for
potential applications as materials in OLEDs fabrication is the
emission efficiency of these molecules. The design of such
materials also requires the control of the HOMO and LUMO
energy levels (and of the corresponding band gap) for proper
alignment of these energy levels with those of other device
components (charge-injection and charge-transport layers) in
order to balance the mobility of the charge carriers and achieve
efficient charge recombination. We have reported recently that
a subtle alteration of the salophen core [i.e., the N,N′-
bis(salicylidene)-o-phenylenediamine] at its periphery (the C5
position of the salicylidene rings as shown in Scheme 1) with an
ester group was at the origin of a bright blue emission for
aluminum complexes. Noticeable differences in the quantum
yields and for the HOMO−LUMO band gaps were found as a
function of the ester moiety (methanoate, tert-butanoate, and
benzoate esters).12 In a continuation of this first observation,
we have investigated the effects of the nature of the ester R-
INTRODUCTION
■
Schiff bases, of which salen (i.e., N,N′-bis(salicylidene)-
ethylenediamine) is the most representative example of the
family, are easily prepared from condensation between an
aldehyde and a primary amine. These versatile molecules are
often used in coordination chemistry to complex metal ions of
different sort.1,2 One of the remarkable applications of Schiff-
base complexes is in catalysis with prominent examples in the
field of olefin metathesis,3 lactide polymerizations,4 and
asymmetric oxidation.5 Among the Schiff-base complexes are
those formed with Al3+ that exhibit catalytic activities for
oxiranes and ethylene polymerization, and for Michael addition
reactions.6 In spite of the great attention devoted to the Schiff
base aluminum derivatives in catalysis, there are only very few
studies devoted to their emissive properties. However,
fluorescence properties for such complexes have been reported
almost 40 years ago by Morisige.7−9 Only recently,
comprehensive studies on their use as luminophors,10−12 as
labeling sensors in biological systems,13,14 and for the
determination of Al3+ traces15 or as materials in OLEDs16,17
have been reported. Such a scarce examination of this family of
luminophors is in obvious contrast with the extensive studies
devoted to the tris(quinolato)aluminum complexes (Alq3)
which have been used as emitters, electron transporting
materials, and host materials in the fabrication of OLEDs
since 1987.18
Received: June 6, 2011
Published: January 5, 2012
© 2012 American Chemical Society
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Scheme 1. Ester Functionalized Schiff-Base Ligands
Scheme 2. Synthetic Pathways for the Salicylaldehyde
a
Derivatives 5a−d
substituent and of the extent of the π-system of the ligand. We
show herein that both parameters modulate significantly the
emission efficiency of the complexes. For this purpose, we
developed a chemical strategy for the widespread synthesis of
ester-functionalized Schiff bases. This strategy focuses on the
preparation of ester-functionalized salicylaldehyde derivatives
that has allowed us to introduce various aromatic ester-R
groups (−COOR with R = phenyl, 2-naphthyl, pentafluor-
ophenyl, and p-nitrophenyl) on the salophen core. At the same
time, we took advantage of this strategy to vary the extent of
conjugation of the Schiff base derivatives by involving either
phenylenediamine or naphthalenediamine (Scheme 1). The
AlIII complexes of the new Schiff bases, 6b−d and 6a′−c′,
exhibit the bright blue emission observed for complex of 6a,12
and enhanced quantum yields up to ϕ = 44% were obtained.
Electrochemical data and DFT calculations show that the
HOMO−LUMO band gap is also modified as a function of the
ester R group.
a
(i) 1 equiv BnCl, 1 equiv K2CO3, DMF, N2, 85°C, overnight; (ii) 1.5
equiv LiOH·H2O, MeOH/H2O (75/25), reflux, 2h; (iii) 3a 1 equiv
phenol, 1.1 equiv DCC, 0.1 equiv DMAP, CH2Cl2, reflux, 4 h; 3b 1
equiv 2-naphthol, 1.1 equiv DCC, 0.1 equiv DMAP, CH2Cl2, reflux, 4
h; (iv) 4a 0.031 equiv 10% Pd/C, H2, MeOH, RT, 3 h; 4b 0.031 equiv
10% Pd/C, H2, MeOH, RT, 24 h; (v) 5a 10.4 equiv BaMnO4, CH2Cl2,
RT, 4h; 5b 10.4 equiv BaMnO4, CH2Cl2, RT, 24 h; (vi) 5c 1 equiv
pentafluorophenol, 1.1 equiv DCC, 0.1 equiv DMAP, CH2Cl2, reflux, 3
h; 5d 1 equiv p-nitrophenol, 1.1 equiv DCC, 0.1 equiv DMAP,
CH2Cl2, reflux, 3 h.
of ester-functionalized (poly)phenol22,23 and even salicylalde-
hyde derivatives by hydrogenation.24−26 Various reaction
conditions (amount of catalyst, reaction time, nature of the
solvent) did not change the outcomes; the only products
formed were the benzyl alcohols 4a or 4b with some remaining
starting material depending on the reaction conditions. An
additional step was therefore necessary to recover the aldehyde
function. The smooth oxidation with barium manganate (step
v) led to 5a/5b. This oxidant appeared more efficient and
reliable than manganese dioxide.27 Despite the series of
protection/deprotection steps, this procedure is perfectly
reproducible and more efficient, with an overall yield of almost
40%, than that reported earlier for 5a.12
For the more acidic pentafluorophenol and para-nitrophenol,
the direct DCC-mediated esterification of 3-formyl-4-hydrox-
ybenzoic acid (Pathway 2) was straightforward with no
formation of side products, yielding 5c and 5d in quite good
yield. The formation of Schiff bases 6b−d and 6a′−c′ was
classically operated in alcohol with the appropriate stoichiom-
etry of the diamine (Scheme 3), as reported for 6a.12 Addition
of Al(NO3)3 to these Schiff bases in alcohol led to the
corresponding [AlL(H2O)2](NO3) complexes [L = N,N′-bis(5-
naphthoxy carbonylsalicylidene)-1,2-phenylenediamine, 7b;
N,N′-bis(5-pentafluorophenoxy carbonylsalicylidene)-1,2-phe-
nylenediamine, 7c; N,N′-bis(5-(p-nitro)-phenoxy carbonylsali-
cylidene)-1,2-phenylenediamine, 7d; N,N′-bis(5-phenoxycarbo-
nylsalicylidene)-1,2-naphthalenediamine, 7a′; N,N′-bis(5-naph-
thoxycarbonylsalicylidene)-1,2-naphthalenediamine, 7b′; and
N,N′-bis(5-pentafluorophenoxycarbonylsalicylidene)-1,2-naph-
thalenediamine, 7c′]. All the complexes have been fully
RESULTS AND DISCUSSION
■
Synthesis and Crystal Structures. The routes for the
synthesis of ester-functionalized salicylaldehydes derivatives are
presented in Scheme 2. The phenyl- and 2-naphthyl-ester
functionalized salicylaldehydes, respectively, 5a and 5b, were
obtained from 3-formyl-4-hydroxybenzoic acid according to
pathway 1 (a 6 step procedure). In a previous report,12 5a was
obtained through the reaction of 3-formyl-4-hydroxybenzoic
acid with phenol and POCl3 in benzene but with poor yield and
in a nonreproducible way. This was attributed to the formation
of polymeric materials resulting from the esterification reaction
between the 3-formyl-4-hydroxybenzoic acid molecules them-
selves. To circumvent this drawback, we envisaged an
alternative strategy by the introduction of protection/
deprotection steps of the phenol group in the starting material
(pathway 1). Several ether-type protecting groups have been
tested, the limiting and crucial step being the deprotection that
should not alter the newly introduced ester group. For this
reason, benzyl ether was chosen, and intermediate 1 was
obtained from the selective benzylation of the phenol group of
3-formyl-4-hydroxybenzoic acid (step i). The latter acid
function was initially protected by a methyl ester, and
deprotected (step ii)21 by hydrolysis with LiOH in a
methanol−water system leading to 2 with an overall yield of
82%. The esters 3a and 3b were obtained through a DCC-
mediated coupling with a catalytic amount of DMAP and with
yields no less than 80% (step iii). Cleavage of the benzyl groups
in a quantitative yield was achieved in very smooth conditions
by H2 (Pd/C) with concomitant reduction of the aldehyde
function (step iv). This reduction was quite surprising
regarding the literature about the deprotection of benzyl ethers
1
characterized by H NMR and elemental analyses, and are
very soluble in polar solvents such as alcohols, DMF, and
DMSO. They are stable in air in both solid and solution states.
Single crystals suitable for X-ray analysis of the molecular
structures have been obtained for the two complexes 7a and 7d
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species, the [AlL(H2O)2]+ complex, one nitrate anion, and
solvent molecules, i.e., one MeOH and two H2O molecules for
7a, and one and a half EtOH molecules for 7d. The geometries
of the two complexes are nearly identical with a distorted
octahedral environment for the Al atom. The basal plane
formed by two O atoms and two N atoms from the complexing
cavity of the Schiff base (O3, O4, N1, and N2) is quite planar
with the rms deviation from planarity of the atoms equal to
0.1094 and 0.0996, respectively, for 7a and 7d. The Al atom sits
in the [N2O2] plane in 7a and slightly above (0.012 Å) in 7d.
The deviation from the Oh geometry is evidenced by the two
Al−O bond distances (from 1.803(3) to 1.832(1) Å) shorter
than the two Al−N bond distances (from 1.982(2) to 2.002(2)
Å), and by the difference between the two equatorial bond
angles O3−Al1−O4 and N1−Al1−N2. The two apical
positions of the distorted octahedron are occupied by water
molecules with Al−O bond distances longer than the equatorial
Al−O bond distances (Figure 1).
Scheme 3. Synthetic Pathways for the Ester Functionalized
a
Schiff Bases 6a−d and 6a′−6c′ and Their Al3+ Complexes
Absorption, Emission, and Electrochemical Properties
in Solution. The absorption and emission spectra of
complexes 7a−d and 7a′−c′ were recorded in methanol at
room temperature (Figure 2). The photophysical data for
absorption and emission are gathered in Table 2. All the UV−
vis transitions observed for 7a−d and 7a′−c′ are assignable to
ligand-centered n−π and π−π* transitions. The UV−vis spectra
feature high energy bands centered at 281, 285, 290, and 286
nm for 7a, 7b, 7c, and 7d, respectively, and at 284, 284, and
291 nm for 7a′, 7b′, and 7c′, respectively. For the complexes
having the bis-iminonaphthyl bridge in their structures, i.e.,
7a′−c′, this high energy band is preceded by an additional
higher energy band at 264, 265, and 268 nm for 7a′, 7b′, and
7c′, respectively. An intermediate absorption band with low
absorbance is present around 326−332 nm; this band is not
well resolved in complexes 7a′−c′. Finally, the spectra are all
characterized by a large and quite unstructured lower energy
band. This absorption, detected between 372 and 376 nm for
7a−d, is slightly red-shifted for 7a′−c′ (380−381 nm).
a
(i) 0.5 equiv o-phenylenediamine, MeOH, 3 h, RT; (ii) 0.5 equiv o-
naphthalenediamine, MeOH, RT, 3 h; (iii) 1 equiv Al(NO3)3·9H2O,
EtOH, refux, 3 h.
by slow evaporation of alcoholic solutions (MeOH for 7a and
EtOH for 7d) at room temperature. The ORTEP representa-
tions of the metal complexes are shown in Figure 1; the
detailed crystallographic data are summarized in Table 1.
The room temperature luminescence spectra of complexes
7a−d and 7a′−c′, irradiated at 380 nm, are quite similar and
display a large, unstructured band centered between 480 and
486 nm. Obviously, the nature of the ester-R group and the
extent of conjugation have no influence on the emission
wavelength maximum, in agreement with our preliminary
observations.12 This implies that the observed emission is
salophen-centered, and involves molecular orbitals for the
ground and excited singlet states that are similar for all the
complexes. However, the found quantum yields clearly indicate
that the emission efficiency is tuned by the nature of the ester
group (Table 2). If complex 7a with R = Ph (C6H5) is taken as
the reference, an extension of the conjugated π system in the R
group appears to have little effect on the quantum yield; values
of ϕ = 34% and 31% have been obtained for, respectively, 7a
and 7b (R = 2-naphthyl). Conversely, the presence of electron-
withdrawing groups on the phenyl ring of the ester (i.e., R =
C6F5, 7c; and p-NO2−C6H4, 7d) significantly enhances the
emission efficiency with ϕ = 40% for 7c and ϕ = 44% for 7d.
The bis-imino bridge was also found to affect the efficiency of
the luminescence; lower quantum yields have been obtained for
the bis-iminonaphthyl-bridged compounds 7a′−c′ as compared
to their bis-iminophenyl homologues 7a−c (Table 2). Clearly,
aromatic electron-withdrawing groups on the ester unit notably
enhance the emission efficiency of the complexes whereas an
extension of the aromatic system of the salophen core has the
Figure 1. Molecular structures of the cationic units in 7a and 7d
(ORTEP representation with ellipsoids at the 50% level). The nitrate
anion, the H atoms, and the solvents molecules have been omitted for
clarity. Selected bond lengths (Å) and angles [deg]: 7a Al1−O1 =
1.927(3), Al1−O2 = 1.908(3), Al1−O3 = 1.821(2), Al1−O4 =
1.803(3), Al1−N1 = 1.991(3), Al1−N2 = 1.993(3), O1−Al1−O2 =
174.8(1), O3−Al1−O4 = 90.9(1), N1−Al1−N2 = 81.7(1), N1−Al1−
O3 = 94.1(1), N2−Al1−O4 = 93.2(1); 7d Al1−O1 = 1.936(2), Al1−
O2 = 1.912(2), Al1−O3 = 1.832(1), Al1−O4 = 1.806(2), Al1−N1 =
1.982(2), Al1−N2 = 2.002(2), O1−Al1−O2 = 175.22(8), O3−Al1−
O4 = 92.05(7), N1−Al1−N2 = 81.81(8), N1−Al1−O3 = 93.74(7),
N2−Al1−O4 = 92.55(7).
Complex 7a crystallizes in the monoclinic system and 7d in
the triclinic one. Both structures are characterized by a cationic
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Table 1. Crystallographic Data and Parameters for 7a and 7d
7a
7d
empirical formula
(C34H26AlN2O8)(NO3), CH3OH,2H2O
(C34H24AlN4O12)(NO3), 1.5CH3CH2OH
fw
747.65
838.67
crystal size, mm3
cryst syst
space group
a, Å
0.2 × 0.2 × 0.25
monoclinic
0.2 × 0.2 × 0.3
triclinic
P21/C
P1
̅
16.2719(2)
11.7907(3)
b, Å
15.3628(2)
12.5569(3)
c, Å
15.2615(2)
14.0286(4)
α, deg
β, deg
γ, deg
V, Å3
90
87.450(2)
114.579(2)
84.788(2)
90
62.541(3)
3469.38(8)
1835.4(1)
Z
4
2
−3
ρcalcd, g cm
1.431
1.517
−1
μ, mm
1.171
1.248
F(000)
1528
867
T, K
180
100
θ range, deg radiation (wavelength)
hkl range
4.1−61.3 Cu Kα (1.541 80)
−18/18, −17/17, −17/15
3.2−61.6 Cu Kα (1.541 80)
−13/13, −14/14, −15/15
no. reflns
26 290
204 56
unique reflns (Rint)
reflns used for refinement
refined params
R1 (I > 3σ(I))
wR (I > 3σ(I))
GOF on F
5317 (0.019)
4565
5633 (0.019)
4614
478
565
0.0654
0.0465
0.0649
0.0413
1.000
1.259
ρfin (max/min) (e Å−3)
1.11/−0.61
0.76/−1.31
opposite effect. It is worth noting that the quantum yields for
7c−d compare favorably with other efficient emitters based on
Al complexes.11,20,28
for the different Schiff-base complexes, a series of TD-DFT
calculations has been undertaken.
DFT Calculations. The electronic structures of the
complexes 7a−d and 7a′−b′ were investigated by quantum
chemical calculations at the TD-DFT level of theory. The
geometries of 7a and 7d were optimized from their X-ray
structures. The optimized structures show no significant
differences with the solid state structures, and the angles
between the planes of the aromatic R groups and the
salicylidene moieties remain similar (54° and 64°, respectively,
for 7a); the only salient modification is a curvature of the ligand
core for the optimized structure as compared to a perfectly
planar arrangement in the solid state (Figure S3 in Supporting
Information). Optimization of the geometry for 7a′, 7b, 7b′,
and 7c was performed from geometry 7a after the adequate
modification of the phenyl ester group and/or the bis-
iminophenyl group.
A selection of frontier molecular orbitals for 7a, 7b′, and 7d
are presented in Figure 3. The five highest occupied molecular
orbitals and the five lowest unoccupied molecular orbitals for all
compounds are given in Supporting Information (Figures S4−
S9). All the MOs are exclusively ligand centered without any
contribution of the metal, in agreement with earlier reports.11,29
For 7a and 7a′, besides contribution of the salophen core in all
MOs from HOMO to HOMO − 4, a strong involvement of the
phenyl ester group is seen from the HOMO − 1 down to the
HOMO − 4 with a stronger participation in the two orbitals
HOMO − 2 and HOMO − 3. For the LUMO and the above
orbitals, a strong contribution of the salophen core is seen; the
contribution of the ester group is only slightly visible in the
LUMO + 2 and LUMO + 3. For 7b and 7b′, the HOMO,
HOMO − 1, and HOMO − 4 involve ester-based orbitals.
The square wave voltammetric measurements have been run
in deoxygenated acetonitrile for 7a−c and 7a′−c′. 7d could not
be investigated because of its poor solubility. The voltammo-
grams (Figures S1 and S2 in Supporting Information) indicate
that the Al complexes undergo single irreversible oxidation and
reduction. The oxidation potentials are quite high compared
with those usually found (between 0.30 and 0.80 V) for neutral
complexes of the same family.11 The more difficult oxidation of
compounds 7a−c and 7a′−c′ can be related to the positive
charge they are bearing; i.e., the complexes are already one
electron deficient systems. Conversely, the reduction potentials
have been found between −1.50 and −1.75 V indicating a
rather good capability to gain an electron for 7a−c and 7a′−c′.
The experimental HOMO and LUMO energy levels have been
deduced from the redox potentials by scaling the reference
(here the ferrocene/ferrocenium couple) to the zero vacuum
level (Fermi level) (Table 2 and Experimental Section). While
the HOMO energy levels remain almost unchanged when
varying the bis-imino bridge from 1,2-phenylene (7a, 7b, 7c) to
1,2-naphthalene (7a′, 7b′, 7c′), a stabilization of the HOMO
level is found by increasing the electron-withdrawing character
of the phenyl ring in the ester group, from −6.21 eV for 7a to
−6.31 eV for 7c. The same trend is found for the LUMO
energy level, but the variations detected in the LUMO energy
levels are more significant with −3.05 eV for 7a to −3.25 eV for
7c. Another noteworthy effect is related to the extent of the
aromaticity of the bis-imino bridge which causes also a
stabilization of the LUMO level. To gain insights into the
molecular orbital diagrams and into the absorption transitions
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Figure 3. Selected frontier molecular orbitals for 7a, 7b′, and 7d.
two compounds only involve orbitals from the salophen core.
Ester centered orbitals appear for higher MOs. For 7c and 7d,
the ester contribution appears from the HOMO down to the
HOMO − 4 whereas a contribution of the salophen core is
seen from the HOMO down to the HOMO − 2 for 7c and
down to HOMO − 4 for 7d. Several trends can be deduced
from these molecular orbital schemes. First, the nature of the
bis-imino bridge (going from 7a to 7a′ and from 7b to 7b′)
does not influence the distribution of the MOs. Second, a larger
π system for the ester-R group (7b versus 7a) and the
introduction of an electron-withdrawing character on the
phenyl of the ester (7c and 7d) do affect the profile of the
Figure 2. Absorption spectra (10−6 M in methanol) and emission
spectra (10−6 M in methanol and λex = 380 nm) of 7a−c (---), 7a′−c′
(), and 7d (···).
Contribution of the salophen core is seen only in the HOMO
− 2 and HOMO − 3 for 7b, and HOMO − 2, HOMO − 3,
and HOMO − 4 for 7b′. The LUMO and LUMO + 1 for these
Table 2. Photophysical and Electrochemical Data of Complexes 7a−7d and 7a′−7c′ in Solution
a
b
c
d
e
e
f
f
λabs (logε) (nm)
λem (nm)
ϕ
Eg (eV)
Eox (V)
Ered (V)
HOMO (eV)
LUMO (eV)
HOMO−LUMO (eV)
7a
7a′
7b
7b′
7c
375 (4.30)
409 (4.11)
381 (4.43)
411 (sh)
482
485
484
486
480
482
34
23
31
22
40
29
44
2.74
2.60
2.73
2.61
2.75
2.61
2.73
1.41
1.40
1.39
1.38
1.50
1.42
g
−1.75
−1.55
−1.72
−1.60
−1.55
−1.50
g
−6.21
−6.20
−6.19
−6.18
−6.31
−6.22
−3.05
−3.25
−3.08
−3.20
−3.25
−3.30
3.16
2.95
3.11
2.98
3.06
2.92
376 (4.24)
406 (4.11)
381 (4.40)
405 (sh)
372 (4.36)
402 (4.18)
380 (4.46)
406 (sh)
7c′
7d
376 (4.43)
482
406 (4.32)
a
b
c
Measured in methanol (10−6 M). Measured in methanol (10−6 M) with λex = 380 nm. ϕ in %. Quinine sulfate (ϕ = 55%) used as standard.
d
e
Estimated from the absorption edge. The oxidation and reduction E1/2 potentials measured in deoxygenated acetonitrile solution at room
f
g
temperature. Potentials are reported against the Fc/Fc+ couple. Calculated from the Eox and Ered. Not measured because of the very poor solubility
of 7d in acetonitrile.
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molecular orbitals by clearly segregating the contribution of the
ester groups and of the salophen core in MOs.
The computed electronic transitions and their energies are
given in Table 3 together with the experimental bands. The
involving similar MO profiles have almost the same energies for
all the compounds; only their positioning in the molecular
orbital diagram is different. This allows us to understand, for
instance, why the maximum emission, obtained for excitation
wavelength centered at 380 nm, is same for these compounds
despite chemical modifications of the ligand. We may suppose
that this analogy also applies to the de-excitation pathway,
hence explaining that very similar emission energies have been
found for all the complexes.
Table 3. Experimental and Calculated Absorption Maximum
Wavelength, Oscillator Strength (fcalcd), and Major
Contribution of 7a−7d and 7a′−7b′
λexptl
λcalcd
fcalcd
major contribution
The computed HOMO and LUMO energy levels are given
in Figure 4 for 7a−d and 7a′−b′. It can be noticed that the
7a
281
329
375
409
7a′
274
326
365
409
0.3814
0.1308
0.3272
0.4433
HOMO → LUMO + 2 (57%)
HOMO − 4 → LUMO (87%)
HOMO − 1 → LUMO (58%)
HOMO → LUMO (98%)
284
327
381
411
7b
280
340
383
395
0.1848
0.1478
0.8924
0.2827
HOMO − 8 → LUMO (90%)
HOMO − 3 → LUMO (79%)
HOMO − 1 → LUMO (69%)
HOMO − 1 → LUMO + 1 (91%)
285
329
376
406
7b′
309
329
363
406
0.2960
0.2140
0.3273
0.3966
HOMO − 6 → LUMO + 1 (65%)
HOMO − 6 → LUMO (90%)
HOMO − 3 → LUMO (59%)
HOMO − 2 → LUMO (87%)
284
332
381
405
7c
305
332
381
392
0.0906
0.1134
0.8821
0.2130
HOMO − 1 → LUMO + 2 (70%)
HOMO − 7 → LUMO (66%)
HOMO − 3 → LUMO (69%)
HOMO − 3 → LUMO + 1 (65%)
◆
■
Figure 4. Calculated HOMO ( ) and LUMO ( ) energy levels for
7a−d and 7a′−b′.
290
326
372
402
7d
278
329
360
406
0.3577
0.2451
0.3680
0.4782
HOMO → LUMO + 3 (85%)
HOMO − 2 → LUMO (100%)
HOMO − 1 → LUMO (64%)
HOMO → LUMO (98%)
calculated LUMO energy levels only slightly vary whereas the
energy level of the HOMO is more sensitive to the chemical
alterations. An increase of the π system for the ester-R group
(i.e., from R = phenyl, 7a, to naphthyl, 7b) lifts the HOMO
energy level whereas imposing an electron-withdrawing
character to the phenyl of the ester (in 7c and 7d) stabilizes
the energy level of the HOMO (from −6.352 eV for 7a to
−6.420 eV for 7c and −6.399 eV for 7d). Such a modulation of
the HOMO level without affecting the LUMO level has already
been observed for 5-substituted Alq3 complexes19 and 5-
substituted salophen complexes of aluminum.11 Experimentally
(Table 2), the LUMO was found to be more sensitive than the
HOMO to the chemical modifications of the ligand. However,
the HOMO−LUMO gaps suggested by the DFT calculations
and those found experimentally (Table 2) follow the same
trend as a function of the chemical features of the ligands.
286
329
376
406
302
332
363
407
0.1244
0.3676
0.3207
0.5192
HOMO − 3 → LUMO+3 (68%)
HOMO − 2 → LUMO (95%)
HOMO − 1 → LUMO (56%)
HOMO → LUMO (97%)
calculations show that the transitions with largest contributions
to the low energy absorptions (450−350 nm) all involve MOs
with a strong contribution of the salophen core. As far as the
absorption close to 380 nm (the experimental excitation
wavelength) is concerned, the largest contribution is from the
transition HOMO − 1−LUMO for all derivatives except for 7b
and 7b′ for which the HOMO − 3−LUMO transition has the
main contribution. However, in all the cases the same MO
profiles are concerned. Regarding the HOMO−LUMO
transitions, for 7a, 7c, and 7d, they are found with a very
large contribution to the band at lower energies (406−409
nm). For 7a′, 7b, and 7b′, the HOMO−LUMO transition has
no contribution to the band in the energy domain close to 380
nm; for these compounds, major contributions are, respectively,
from HOMO − 1−LUMO + 1, HOMO − 2−LUMO, and
HOMO − 3−LUMO + 1 transitions. Here again, for all the
compounds, these transitions involve closely related MO
profiles but with a different positioning in the respective
molecular orbital diagrams.
CONCLUSIONS AND PERSPECTIVES
■
The possibility to chemically tune essential features of
luminescent compounds such as emission efficiency and
HOMO−LUMO band gap is of prime importance because of
the prospective relevance of these materials in application such
as LED. For the ester functionalized salophen complexes
reported here, this is achieved simply through the ester moiety.
An aromatic electron-withdrawing ester group (−COOR with
R = C6F5 or p-NO2−C6H4) was found to increase the quantum
yield by 30% (from ϕ = 34% to 44%) with respect to the
corresponding R = Ph derivative. The reason for the increased
emission efficiencies with electron-withdrawing R groups
These calculations reveal that the chemical modifications of
the ligand may induce some noticeable variations in the
molecular orbital diagram. However, the electronic transitions
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Inorganic Chemistry
Article
remains to be established but it might be related to charge
separation considerations in the excited states.
of CH2Cl2. Phenol or 2-naphthol (5.86 mmol) was then added
followed by the DCC (1.33 g; 6.45 mmol) leading to a clear
solution. DMAP (71.6 mg; 0.586 mmol) was then added to the
solution. Rapidly, a white precipitate started to appear (DCU).
The mixture was refluxed for 4 h. After it was cooled down to
room temperature, the solvent was evaporated. The residue was
triturated with 5 mL of acetone, and the insoluble DCU was
filtered. After evaporation to dryness, the yellow solid was
purified by column chromatography (SiO2, CHCl3 as eluent).
Data for phenyl-(4-benzyloxy-3-formyl)benzoate (3a) follow.
Yield 79%. Elemental analysis calcd (%) for C21H16O4: C 75.89;
H 4.85. Found: C 76.13; H 4.96. 1H NMR (250 MHz,
(CD3)2CO) δ 10.59 (1H, s, HCO), 8.58 (1H, d, Ar-H), 8.42
(1H, dd, Ar-H), 7.61−7.30 (11H, m, Ar-H), 5.51 (2H, s, CH2).
IR (ATR) νCO (ester) 1720 cm−1, νCO (aldehyde) 1686
cm−1 . Data for 2′-naphthyl-(4-benzyloxy-3-formyl)benzoate
(3b) follow. Yield 84%. Elemental analysis calcd (%) for
C25H18O4: C 78.52; H 4.74. Found: C 78.79; H 5.03. 1H NMR
(250 MHz, (CD3)2CO) δ 10.59 (1H, s, HCO), 8.62 (1H, s,
Ar-H), 8.47 (1H, d, Ar-H), 8.00 (3H, m, Ar-H), 7.83 (1H, s, Ar-
H), 7.53 (9H, m, Ar-H), 5.52 (2H, s, CH2). IR (ATR) νCO
The HOMO−LUMO band gap is also subtly modulated by
the R group and by the extended aromaticity. A narrow band
gap span could thus be achieved. Importantly, for this family of
compounds the emission wavelength is not affected by the
chemical modification at the periphery of the ligand core. For
other luminophors a modification of the electronic character of
the substituents has a dramatic effect on the emission
wavelength.
For this study, we have also settled a straightforward
synthesis strategy for ester-functionalized salicylaldehydes that
allows the introduction of virtually any R at the ester function.
Future work will concern the possibility to modulate the
color of emitted light by modifications of the Schiff-base core
with still in mind the objective of reaching high quantum yields
and adjustable HOMO−LUMO band gaps.
EXPERIMENTAL SECTION
■
Chemicals and General Procedures. Reagents and
solvents were obtained commercially and used without further
purification. (3-Formyl-4-hydroxy)benzoic acid, methyl-(3-
formyl-4-hydroxy)benzoate, Schiff base 6a, and AlIII complex
7a were prepared as previously reported.12 When specified,
reactions were monitored using silica gel 60 Å analytical TLC
plates by UV detection (254 nm). Silica gel (60 Å, 70−200 μm)
was used for column chromatography. Elemental C, H, and N
analyses were performed on a Perkin-Elmer 2400 II analyzer on
freshly prepared and isolated samples
(ester) 1717 cm−1, νCO (aldehyde) 1684 cm−1
.
General Procedure for the Synthesis of the 4-Hydroxy-3-
(hydroxymethyl)benzoate Derivatives 4a and 4b. To a
stirred solution of 3a or 3b (2.12 mmol) in 100 mL of
MeOH (heating with a water bath around 50 °C is necessary to
complete the solubilization) was added 10% Pd/C (70 mg;
0.066 mmol: 3.10% mol). The reaction vessel was air free
pumped and then hydrogenated under a balloon pressure of
hydrogen for 3 h for 4a and 24 h for 4b at room temperature.
After decantation of the mixture, the supernatant was filtered
through a celite pad. The solvent was then evaporated to
dryness to give an oily residue which was dried under air to give
a creamy white solid. Data for phenyl-(4-hydroxy-3-
Preparations. Methyl-(3-formyl-4-benzyloxy)benzoate
(1). BnCl (1.74 g; 13.72 mmol) and K2CO3 (1.90 g; 13.72
mmol) were added to the solution of methyl-(3-formyl-4-
hydroxy)benzoate (2.47 g; 13.72 mmol) in 20 mL of dry DMF.
The resulting suspension was heated under N2 at 85 °C
overnight. After cooling down at room temperature, water was
added. The orange solution was extracted with CH2Cl2. The
organic phases were washed with brine, dried over Na2SO4, and
evaporated to dryness to give a yellow solid (3.53 g; 13.07
mmol). Yield 95%. Elemental analysis calcd (%) for C16H14O4:
C 71.10; H 5.22. Found: C 70.92; H 5.06. 1H NMR (250 MHz,
(CD3)2CO) δ 10.54 (1H, s, HCO), 8.40 (1H, s, Ar-H), 8.24
(1H, d, Ar-H), 7.61−7.39 (6H, m, Ar-H), 5.44 (2H, CH2), 3.90
(3H, s, CH3). IR (ATR) νCO (ester) 1716 cm−1, νCO
(aldehyde) 1678 cm−1.
1
(hydroxymethyl))benzoate (4a) follow. Yield 100%. H NMR
(250 MHz, (CD3)2CO) δ 8.18 (1H, s, Ar-H), 7.97 (1H, d, Ar-
H), 7.46 (2H, m, Ar-H), 7.28 (3H, m, Ar-H), 6.99 (1H, d, Ar-
H), 4.81 (2H, s, CH2). IR (ATR) νCO (ester) 1728 cm−1
(very broad). Data for 2′-naphthyl-(4-hydroxy-3-
1
(hydroxymethyl))benzoate (4b) follow. Yield 100%. H NMR
(250 MHz, (CD3)2CO) δ 8.25 (1H, s, Ar-H), 8.02 (4H, m, Ar-
H), 7.78 (1H, s, Ar-H), 7.54 (2H, m, Ar-H), 7.45 (1H, d, Ar-
H), 7.04 (1H, d, Ar-H), 4.76 (2H, s, CH2). IR (ATR) νCO
(ester) 1724 cm−1.
These creamy solids were used in the next step without
further purification.
3-Formyl-4-benzyloxybenzoic Acid (2). Methyl-(3-formyl-4-
benzyloxy)benzoate 1 (0.35 g; 1.3 mmol) was solubilized in 8
mL of MeOH followed by addition of 3 mL of H2O, leading to
a cloudy solution. LiOH·H2O (0.082 g; 1.9 mmol) was then
added as solid. The suspension was refluxed until the
disappearance of the precursor (around 2 h). A clear yellow
solution was obtained. After evaporation of the MeOH, 35 mL
of H2O was then added. Acidification until pH 1 by addition of
concentrated HCl afforded the precipitation of a white solid
which was filtered and air-dried (0.285 g; 1.1 mmol). Yield
86%. Elemental analysis calcd (%) for C15H12O4: C 70.31; H
4.72. Found: C 70.66; H 4.42. 1H NMR (250 MHz,
(CD3)2CO) δ 10.54 (1H, s, HCO), 8.45 (1H, s, Ar-H),
8.27 (1H, d, Ar-H), 7.63−7.39 (6H, m, Ar-H), 5.45 (2H, CH2).
IR (ATR) νCO (acid) 1678 cm−1, νCO (aldehyde) 1600
cm−1.
General Procedure for the Synthesis of the Salicylalde-
hyde Derivatives 5a and 5b. The corresponding 4-hydroxy-3-
(hydroxymethyl)benzoate derivative 4a or 4b (0.4 mmol) was
solubilized in 15 mL of CH2Cl2 (clear cream color). BaMnO4
(1.07 g; 4.17 mmol) was then added, and the suspension was
stirred at room temperature for 4 h for 5a and for 24 h for 5b.
After decantation of the mixture, the supernatant was isolated.
The residual solid was washed twice with 10 mL of CH2Cl2.
These washing solutions were filtered through a celite pad. The
solutions were combined and evaporated to dryness to give a
brown oil. Purification of this oil was accomplished by column
chromatography (SiO2, CHCl3 as eluent). Data for phenyl-(4-
hydroxy-3-formyl)benzoate (5a) follow. Yield 55%. Elmental
analysis calcd (%) for C14H10O4: C 69.42; H 4.16. Found: C
1
General Procedure for the Synthesis of the 4-Benzyloxy-3-
formylbenzoate Derivatives 3a and 3b. 3-Formyl-4-benzylox-
ybenzoic acid 2 (1.5 g; 5.86 mmol) was suspended in 100 mL
69.65; H 4.30. H NMR (250 MHz, (CD3)2CO) δ 10.23 (1H,
s, HCO), 8.66 (1H, s, Ar-H), 8.35 (1H, dd, Ar-H), 7.50 (2H,
m, Ar-H), 7.32 (3H, m, Ar-H), 7.21 (1H, d, Ar-H). IR (ATR)
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νCO (ester) 1727 cm−1, νCO (aldehyde) 1657 cm−1. Data for
2′-naphthyl-(4-hydroxy-3-formyl)benzoate (5b) follow. Yield
52%. Elemental analysis calcd (%) for C18H12O4: C 73.97; H
4.14. Found: C 74.13; H 3.96. 1H NMR (250 MHz,
(CD3)2CO) δ 10.30 (1H, s, HCO), 8.70 (1H, d, Ar-H),
8.40 (1H, dd, Ar-H), 8.01 (3H, m, Ar-H), 7.83 (1H, d, Ar-H),
7.56 (2H, m, Ar-H), 7.48 (1H, dd, Ar-H), 7.23 (1H, d, Ar-H).
IR (ATR) νCO (ester) 1731 cm−1, νCO (aldehyde) 1678
(%) for C46H30N2O6·H2O: C 76.23; H 4.45; N 3.87. Found: C
1
75.91; H 4.31; N 4.05. H NMR (300 MHz, (CD3)2CO) δ
13.55 (2H, s, Ar-OH), 9.35 (2H, s, HCN), 8.69 (2H, d, Ar-
H), 8.29 (2H, dd, Ar-H), 8.02 (10H, m, Ar-H), 7.85 (2H, d, Ar-
H), 7.59 (6H, m, Ar-H), 7.50 (2H, dd, Ar-H), 7.23 (2H, d, Ar-
H). IR (ATR) νCO (ester) 1722 cm−1, νCN (imine) 1622
cm−1.
N,N′-Bis(5-(pentafluorophenoxycarbonyl)salicylidene)-
1,2-phenylenediamine (6c). Yield: 89%. Elemental analysis
calcd (%) for C34H14F10N2O6: C 55.45; H 1.92; N 3.80. Found:
cm−1
.
General Procedure for the Synthesis of the Salicylalde-
hyde Derivatives 5c and 5d. 3-Formyl-4-hydroxybenzoic acid
(0.5 g; 3.01 mmol) was solubilized in 25 mL of CH2Cl2. The
corresponding phenol derivative (3.01 mmol) was then added,
followed by the DCC (0.684 g; 3.31 mmol), leading to a clear
solution. DMAP (37 mg; 0.303 mmol) was then added to the
solution. Rapidly, a white precipitate started to appear (DCU).
The mixture was refluxed for 3 h. After it was cooled down to
room temperature, the insoluble DCU was filtered and the
solvent removed by evaporation. Data for pentafluorophenyl-
(4-hydroxy-3-formyl)benzoate (5c) follow. The yellow solid
was purified by column chromatography (SiO2, CHCl3 as
eluent). Yield 65%. Elemental analysis calcd (%) for
1
C 55.58; H 1.57; N 3.89. H NMR (300 MHz, (CD3)2CO) δ
13.55 (2H, s, Ar−OH), 9.21 (2H, s, HCN), 8.60 (2H, d, Ar-
H), 8.32 (2H, dd, Ar-H), 7.59 (4H,m, Ar-H), 7.22 (2H, d, Ar-
H). IR (ATR) νCO (ester) 1743 cm−1, νCN (imine) 1615
cm−1.
N,N′-Bis(5-(pentafluorophenoxycarbonyl)salicylidene)-
1,2-naphthalenediamine (6c′). Yield: 76%. Elemental analysis
calcd (%) for C38H16F10N2O6: C 58.03; H 2.05; N 3.56. Found:
1
C 58.04; H 1.84; N 3.68. H NMR (300 MHz, (CD3)2CO) δ
13.55 (2H, s, Ar−OH), 9.33 (2H, s, HCN), 8.65 (2H, d, Ar-
H), 8.32 (2H, dd, Ar-H), 8.09 (4H, s + m, Ar-H), 7.55 (2H, m,
Ar-H), 7.25 (2H, d, Ar-H). IR (ATR) νCO (ester) 1748 cm−1,
νCN (imine) 1614 cm−1.
1
C14H5F5O4: C 50.62; H 1.52. Found: C 50.86; H 1.38. H
NMR (250 MHz, CDCl3) δ 10.04 (1H, s, HCO), 8.52 (1H,
d, Ar-H), 8.36 (1H, dd, Ar-H), 7.18 (1H, d, Ar-H). IR (ATR)
νCO (ester) 1750 cm−1, νCO (aldehyde) 1666 cm−1. Data for
p-nitrophenyl-(4-hydroxy-3-formyl)benzoate (5d) follow. The
yellow solid was washed several times with acetone until there
is no more precipitation of DCU. The yellow oil was washed
with CH2Cl2 to yield a cream solid. Yield 65%. Elemental
analysis calcd (%) for C14H9NO6: C 58.54; H 3.16; N 4.88.
Found: C 58.41; H 3.42; N 4.89. 1H NMR (250 MHz, CDCl3)
δ 10.25 (1H, s, HCO), 8.70 (1H, d, Ar-H), 8.42 (3H, m, Ar-
H), 7,67 (2H, d, Ar-H), 7.22 (1H, d, Ar-H). IR (ATR) νCO
(ester) 1731 cm−1, νCO (aldehyde) 1656 cm−1.
N,N′-Bis(5-(p-nitrophenoxycarbonyl)salicylidene)-1,2-phe-
nylenediamine (6d). The above procedure was slightly
modified: ethanol was used as the solvent, and the reaction
medium was refluxed for 4 h 30 min. After filtration, the orange
solid was washed with ethanol and dried with diethyl ether.
Yield: 82%. Elemental analysis calcd (%) for
C34H22N4O10·0.5H2O: C 62.29; H 3.54; N 8.55. Found: C
1
62.28; H 3.14; N 8.60. H NMR (300 MHz, (CD3)2CO) δ
13.55 (2H, s, Ar-OH), 9.20 (2H, s, HCN), 8.66 (2H, d, Ar-
H), 8.37 (4H, d, Ar-H), 8.17 (2H, dd, Ar-H), 7.62 (6H, d, Ar-
H), 7.50 (2H, m, Ar-H), 7.12 (2H, d, Ar-H). IR (ATR) νCO
(ester) 1728 cm−1, νCN (imine) 1613 cm−1.
General Procedure for the Preparation of Schiff Bases 6a′,
6b−b′, 6c−c′, and 6d. The corresponding salicylaldehyde
derivative 5a−d (0.330 mmol) was solubilized in 4 mL of
MeOH. To this clear colorless solution was dropwise added the
diamine solution (0.165 mmol in 4 mL of MeOH). The
resulting solution turned bright yellow to orange. A yellow
precipitate started to appear during the stirring of the solution
at room temperature. After 3 h, the yellow solid was filtered and
dried under vacuum.
General Procedure for the Preparation of AlIII Complexes
7a′, 7b−b′, 7c−c′, and 7d. The corresponding Schiff base
(0.1 mmol) was suspended in 5 mL of EtOH. To this yellow
suspension was slowly added the solution of Al(NO3)3·9H2O
(1 equiv in 5 mL of EtOH). The resulting solution became
clearer. After being stirred for 3 h at reflux temperature, the
resulting solution without any solid in suspension was
evaporated to dryness to obtain a dry yellow-orange oil. This
oil was triturated with diethyl ether. A solid as powder was
recovered by filtration and dried under vacuum.
N,N′-Bis(5-(phenoxycarbonyl)salicylidene)-1,2-naphthale-
nediamine (6a′). Yield: 80%. Elemental analysis calcd (%) for
C38H26N2O6·0.5H2O: C 74.14; H 4.42; N 4.55. Found: C
N,N′-Bis(5-(phenoxycarbonyl)salicylidene)-1,2-naphthale-
nediamine Aluminum(III) Nitrate (7a′). Yield: 97%. Elemental
analysis calcd (%) for C38H28N3O11Al·4H2O: C 56.93; H 4.53;
N 5.24. Found: C 56.88; H 4.54; N 5.54. 1H NMR (300 MHz,
(CD3)2SO) δ 9.78 (2H, s, HCN), 8.78 (2H, br s, Ar-H), 8.71
(2H, br d, Ar-H), 8.24 (2H, dd, Ar-H), 8.00 (2H, m, Ar-H),
7.66 (2H, m, Ar-H), 7.52 (4H, m, Ar-H), 7.34 (6H, m, Ar-H),
7.17 (2H, d, Ar-H). IR (ATR) νCO (ester) 1709 cm−1, νCN
(imine) 1609 cm−1.
Synthesis of N,N′-Bis(5-(2′-naphthoxycarbonyl)-
salicylidene)-1,2-phenylenediamine Aluminum(III) Nitrate
(7b). Yield: 92%. Elemental analysis calcd (%) for
C42H30N3O11Al·2H2O: C 61.84; H 4.20; N 5.15. Found: C
61.51; H 4.25; N 5.54. 1H NMR (300 MHz, (CD3)2SO) δ 9.62
(2H, s, HCN), 8.75 (2H, br s, Ar-H), 8.28 (4H, br m, Ar-H),
8.04 (6H, m, Ar-H), 7.85 (2H, br s, Ar-H), 7.60 (6H, m, Ar-H),
7.49(2H, d, Ar-H), 7.18 (2H, d, Ar-H). IR (ATR) νCO (ester)
1723 cm−1, νCN (imine) 1617 cm−1.
1
74.09; H 4.51; N 4.61. H NMR (300 MHz, (CD3)2CO) δ
13.65 (2H, s, Ar−OH), 9.29 (2H, s, HCN), 8.57 (2H, d, Ar-
H), 8.24 (2H, dd, Ar-H), 8.07 (4H, m, Ar-H), 7.58 (2H, m, Ar-
H), 7.50 (4H, m, Ar-H), 7.32 (6H, m, Ar-H), 7.21 (2H, d, Ar-
H). IR (ATR) νCO (ester) 1723 cm−1, νCN (imine) 1610
cm−1.
N,N′-Bis(5-(2′-naphthoxycarbonyl)salicylidene)-1,2-phe-
nylenediamine (6b). Yield: 92%. Elemental analysis calcd (%)
for C42H28N2O6·0.5H2O: C 75.78; H 4.39; N 4.21. Found: C
1
75.67; H 4.33; N 4.52. H NMR (300 MHz, (CD3)2CO) δ
13.68 (2H, s, Ar−OH), 9.20 (2H, s, HCN), 8.59 (2H, d, Ar-
H), 8.32 (2H, dd, Ar-H), 8.00 (6H, m, Ar-H), 7.83 (2H, d, Ar-
H), 7.56 (10H, m, Ar-H), 7.21 (2H, d, Ar-H). IR (ATR) νCO
(ester) 1728 cm−1, νCN (imine) 1616 cm−1.
N,N′-Bis(5-(2′-naphthoxycarbonyl)salicylidene)-1,2-naph-
thalenediamine (6b′). Yield: 80%. Elemental analysis calcd
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Synthesis of N,N′-Bis(5-(2′-naphthoxycarbonyl)-
salicylidene)-1,2-naphthalenediamine Aluminum(III) Nitrate
(7b′). Yield: 80%. Elemental analysis calcd (%) for
C46H32N3O11Al·5H2O: C 60.07; H 4.60; N 4.57. Found: C
60.24; H 4.56; N 4.87. 1H NMR (300 MHz, (CD3)2SO) δ 9.81
(2H, s, HCN), 8.78 (4H, d, Ar-H), 8.28 (2H, dd, Ar-H), 8.04
(8H, m, Ar-H), 7.87 (2H, d, Ar-H), 7.59 (8H, m, Ar-H), 7.20
(2H, dd, Ar-H). IR (ATR) νCO (ester) 1720 cm−1, νCN
(imine) 1609 cm−1.
Synthesis of N,N′-Bis(5-(pentafluorophenoxycarbonyl)-
salicylidene)-1,2-phenylenediamine Aluminum(III) Nitrate
(7c). Yield: 78%. Elemental analysis calcd (%) for
C34H16F10N3O11Al: C 47.51; H 1.88; N 4.89. Found: C
47.68; H 1.76; N 5.00. 1H NMR (300 MHz, (CD3)2SO) δ 9.61
(2H, s, HCN), 8.75 (2H, d, Ar-H), 8.24 (4H, m, Ar-H), 7.64
(2H, m, Ar-H), 7.18 (2H, d, Ar-H). IR (ATR) νCO (ester)
1746 cm−1, νCN (imine) 1619 cm−1.
2
⎛
⎞
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞
⎟
⎠
I
∫
∫
ηs
Astd
As
s
⎜
⎜
⎝
⎟
⎟
⎠
⎜
⎜
⎟
⎟
ϕ = ϕ
s
std
η
I
std
std
(1)
1H NMR spectra were recorded using a ARX250 or a
DPX300 Bruker spectrometers with working frequencies,
respectively, at 250 and 300 MHz for H. Chemical shifts
were referenced to the residual proton resonance of the
deuterated solvents. IR spectra were recorded in the 4000−600
cm−1 region with a Perkin-Elmer Spectrum 100 FTIR using the
ATR mode.
̈
1
Electrochemistry Measurements. Square wave voltam-
metric measurements were carried out with a potentiostat
Autolab PGSTAT100. Experiments were performed at room
temperature in an homemade airtight three-electrode cell
connected to a vacuum/argon line. The reference electrode
consisted of a saturated calomel electrode (SCE) separated
from the solution by a bridge compartment. The counter
electrode was a platinum wire of ca. 1 cm2 apparent surface.
The working electrode was a Pt microdisk (0.5 mm diameter).
The supporting electrolyte (nBu4N)[PF6] (Fluka, 99% puriss
electrochemical grade) and acetonitrile were used as received.
The solutions used during the electrochemical studies were
typically 10−3 mol L−1 in complex and 0.1 mol L−1 in supporting
electrolyte. Before each measurement, the solutions were
degassed by bubbling Ar and the working electrode was
polished with a polishing machine (Presi P230). Potentials are
given vs the Fc+/Fc couple as internal standard (E1/2 = 0.4 V/
SCE)
Estimation of the HOMO/LUMO Energy Levels. The
energies of the HOMO and LUMO were calculated by using a
reported procedure.30 According to this reference, the value for
ferrocene (Fc) with respect to the zero vacuum level is
estimated as −4.8 eV, determined from −4.6 eV for the
standard electrode potential E° of a normal hydrogen electrode
(NHE) on the zero vacuum level, and 0.2 V for Fc versus NHE.
The values for the HOMO and LUMO levels were then,
respectively, obtained through eqs 2 and 3 as follows:
Synthesis of N,N′-Bis(5-(pentafluorophenoxycarbonyl)-
salicylidene)-1,2-naphthalenediamine Aluminum(III) Nitrate
(7c′). Yield: 74%. Elemental analysis calcd (%) for
C38H18F10N3O11Al·H2O: C 49.21; H 2.17; N 4.53. Found: C
48.89; H 2.13; N 4.72. 1H NMR (300 MHz, (CD3)2CO) δ 9.80
(2H, s, HCN), 8.78 (4H, d, Ar-H), 8.27 (2H, dd, Ar-H), 8.01
(2H, m, Ar-H), 7.69 (2H, m, Ar-H), 7.21 (2H, d, Ar-H). IR
(ATR) νCO (ester) 1753 cm−1, νCN (imine) 1609 cm−1.
Synthesis of N,N′-Bis(5-(p-nitrophenoxycarbonyl)-
salicylidene)-1,2-phenylenediamine Aluminum(III) Nitrate
(7d). Yield: 86%. Elemental analysis calcd (%) for
C34H24N5O15Al·3H2O: C 49.58; H 3.67; N 8.50. Found: C
49.18; H 3.35; N 8.79. 1H NMR (300 MHz, (CD3)2SO) δ 9.65
(2H, s, HCN), 8.75 (2H, d, Ar-H), 8.42 (4H, d, Ar-H), 8.33
(4H, m, Ar-H), 7.66 (6H, m, Ar-H), 7.21 (2H, d, Ar-H). IR
(ATR) νCO (ester) 1729 cm−1, νCN (imine) 1614 cm−1.
Spectroscopic Measurements. UV−vis spectra were
obtained from a Perkin-Elmer Lambda 35 spectrophotometer.
Measurements were made in 1 cm path length quartz cells at
293 K. Emission spectra were measured using a Horiba-Jobin
Yvon FluoroMax-4 spectrofluorimeter, equipped with three-slit
double-grating excitation and emission monochromators. The
steady-state luminescence was excited by unpolarized light from
a 150 W continuous-wave xenon lamp and detected at an angle
of 90° for diluted solution measurements (10 mm quartz cell)
by a red-sensitive Hamamatsu R228 photomultiplier tube.
Spectra were reference-corrected for both the excitation source
light-intensity variation (lamp and grating) and the emission
spectral response (detector and grating). The quantum yields
for fluorescence in solution were determined using an aqueous
solution of quinine sulfate in 1 N H2SO4. Concentrations of the
solutions, including solutions of standard, were adjusted so that
the absorbance at λex was between 0.04 and 0.05. In this case,
intensity of the measured emission can be considered to be
proportional to the concentration of the species in solution.
The emission quantum yields were then calculated using eq 1,
where ϕs is the emission quantum yield of the sample, ϕstd is
the emission quantum yield of the standard, Astd and As
represent the absorbance of the standard and the sample at
the excitation wavelength, while ∫ Istd and ∫ Is are the integrals
of the corrected emission envelopes of the standard and the
sample, respectively, and η is the refractive index of the solvents
used for the samples and standard solutions.
HOMO = − (E + 4.8) eV
(2)
(3)
ox
LUMO = − (E + 4.8) eV
red
Data Collection and Structure Determination. Intensity
data were collected at low temperature on a Gemini Oxford
Diffraction diffractometer using a graphite-monochromated Cu
Kα radiation source and equipped with an Oxford Cryosystems
cryostream cooler device. Structures were solved by direct
methods using SUPERFLIP,31 and refined by full-matrix least-
squares procedures on F using the programs of the PC version
of CRYSTALS.32 Atomic scattering factors were taken from the
International Tables for X-ray Crystallography.33 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were located in a difference map (those attached to carbon
atoms were repositioned geometrically), and then refined using
a riding model. For compound 7a, a disorder was observed for
an ethanol molecule and a NO3− anion, with occupancy of 50%
for each part. CCDC-823562 (7a) and 823563 (7d) contain
the supplementary crystallographic data for this paper. These
cif data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_
request/cif.
Computational Details. All the calculations described here
were carried out with the Gaussian 09 package.34 The 6-31G(d)
1317
dx.doi.org/10.1021/ic201208c | Inorg. Chem. 2012, 51, 1309−1318

Inorganic Chemistry
Article
basis set was used for all calculations.35,36 Molecular geometries
of 7a−d and 7a′-b′ were optimized in a vacuum without
symmetry restraint using DFT method with the B3LYP37,38
hybrid exchange correlation functional implemented in the
Gaussian suite of program. For all schemes the ground state
minima have been confirmed by determination of the
vibrational frequencies. The electronic transition energies,
oscillator strengths, and excited-state compositions were
computed by the time-dependent density functional theory
method (TD-DFT)39 using the same functional and basis sets
as for the geometry optimization. Solvent effects on transition
energies were evaluated by means of the polarizable continuum
model (PCM) in its integral equation formalism,40 and the
default parameters were taken from the literature.34 In the
PCM model, the calculation is performed in the presence of a
solvent (here MeOH) by placing the solute in a cavity within
the solvent reaction field.
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(23) Dodo, K.; Minato, T.; Noguchi-Yachide, T.; Suganuma, M.;
Hashimoto, Y. Bioorg. Med. Chem. 2008, 16, 7975.
(24) Maes, D.; Riveiro, M. E.; Shayo, C.; Davio, C.; Debenedetti, S.;
De Kimpe, N. Tetrahedron 2008, 64, 4438.
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Tetrahedron Lett. 2008, 49, 1619.
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(27) BaMnO4 is commercially available and easy to handle and
requires no activation; which makes it particulary useful for large-scale
reactions.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Square wave voltamogramms for 7a−c and 7a′−c′. Optimized
structures for compounds 7a and 7d. Frontier molecular
orbitals for geometries 7a, 7a′, 7b, 7b′, 7c, 7d calculated by TD-
DFT at the B3LYP/6-31G(d) level of theory using Gaussian
09. Cartesian coordinates of the optimized geometries for 7a,
7a′, 7b, 7b′, 7c, 7d. Supplementary X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(30) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R.; Bassler,
̈
H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551.
(31) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786.
(32) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.;
Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.
(33) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, U.K., 1974; IV.
(34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sutter@lcc-toulouse.fr (J.-P.S); virginie.bereau@iut-
tlse3.fr (V.B.).
ACKNOWLEDGMENTS
■
The authors thank Dr. Christine Lepetit (LCC Toulouse,
France) for assistance with the DFT calculations and for helpful
discussions.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.1; Gaussian: Wallingford, CT, 2009.
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