Blue emitting pentacoordinated Al(III) complexes based on 2-methylquinolin-8-olate and substituted phenolate ligands. The role of phenolate derivatives on emission and absorption properties

Article abstract of DOI:10.1039/b510463g

This paper reports the synthesis, characterisation and photophysical investigation of the homologous series of blue-emitting pentacoordinated AlQ′₂L complexes 1-6, where Q′ is 2-methylquinolin-8- olate and L is a phenolate substituted in para to the oxygen donor atom. Both electron attracting and electron releasing substituents have been considered. Also binuclear complexes of formula Q′₂Al-L′- AlQ′₂, where L′ = OC₆H₄C ₆H₄O, OC₆H₄(CH₂) ₃C₆H₄O and OC₆H₄C(CH ₃)₂C₆H₄C(CH₃) ₂C₆H₄O have been synthesised and characterised. These are interesting blue-emitting materials for OLED (organic light emitting diode) fabrication. A comparison between computational and experimental results allowed the description of the absorption and fluorescence processes. A study of the frontier orbitals involved in the electric conduction and fluorescence emission has been performed. Influences of the phenolate-based ligands on the compounds spectroscopy have been particularly taken into account. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b510463g

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Blue emitting pentacoordinated Al(III) complexes based on 2-
methylquinolin-8-olate and substituted phenolate ligands. The role
of phenolate derivatives on emission and absorption properties†
Massimo La Deda,a Iolinda Aiello,a Annarita Grisolia,b Mauro Ghedini,*a Mario Amatib and Francesco Lelj*b
Received 21st July 2005, Accepted 7th September 2005
First published as an Advance Article on the web 7th November 2005
DOI: 10.1039/b510463g
This paper reports the synthesis, characterisation and photophysical investigation of the homologous
series of blue-emitting pentacoordinated AlQꢀ2L complexes 1–6, where Qꢀ is 2-methylquinolin-8-olate
and L is a phenolate substituted in para to the oxygen donor atom. Both electron attracting and electron
releasing substituents have been considered. Also binuclear complexes of formula Qꢀ2Al–Lꢀ–AlQꢀ2,
where Lꢀ = OC6H4C6H4O, OC6H4(CH2)3C6H4O and OC6H4C(CH3)2C6H4C(CH3)2C6H4O have been
synthesised and characterised. These are interesting blue-emitting materials for OLED (organic light
emitting diode) fabrication. A comparison between computational and experimental results allowed the
description of the absorption and fluorescence processes. A study of the frontier orbitals involved in the
electric conduction and fluorescence emission has been performed. Influences of the phenolate-based
ligands on the compounds spectroscopy have been particularly taken into account.
and time dependent DFT (TD-DFT), have been applied to
Introduction
describe the emission process in AlQ3.1g
The future of organic light emitting diodes (OLEDs) is certainly
AlQ3 presents several desirable properties for OLED fabrica-
bound to their application in the technology of flat full-colour
tion, among which the stability of amorphous AlQ3 films (thermal
displays. In this sense, tuning the peak wavelength of the emitted
stability, resistance to crystallisation, relatively long life under
light through chemical structure design is an important and
voltage application) and good electroluminescence properties, as
well as low cost and easy processability during OLED fabrication.2
However, one of its drawbacks is the electroluminescence maxi-
mum at about 520 nm, in the green region of the spectrum. This
limits its applications in full-colour displays, which is the principal
demand of the market. For such displays, efficient emission in
the three primary colours spectral ranges (red, green and blue) is
needed. Efficient green and red emission can be obtained with
pure AlQ3 and AlQ3 doped with dyes, respectively.3 However,
the blue component (between 430 and 500 nm) of the AlQ3
electroluminescence band amounts to only about the 16% of the
total.4 Therefore, even the inclusion of very efficient blue dyes in
an AlQ3 matrix does not produce an efficient blue-OLED. Host
materials with emission peak under 500 nm are necessary for
efficient emission in the blue range. Thus, efforts have been
aimed to shift the luminance of AlQ3 to the blue region by, for
example, introducing donors at positions 2 or 4, or acceptors at
position 5 or 7 of the 8-hydroxyquinoline ligand.5 Aluminium
tris(4-methylquinolin-8-olato), although blue shifted with respect
to AlQ3, emits at about 515 nm, and therefore it is still unsuitable
for use as a blue emitter for display purposes.6
Possible candidates for replacing AlQ3 in blue-emitting devices
are pentacoordinated aluminium complexes, AlQꢀ2L which con-
tain two molecules of chelant 2-methylquinolin-8-olate, Qꢀ in
the following, and a monodentate ligand, L in the following.7
Promising HL ligands could be phenol and substituted phenols. In
fact, several phenolate-based AlQꢀ2L complexes exhibit sufficient
stability and useful efficiency as blue-emitting materials in double-
layered OLEDs.8 However, despite their useful properties, few
studies have addressed to the origin of the blue-shifted emission
challenging goal. Aluminium tris(quinolin-8-olate) (AlQ3) is by
far the most studied compound for OLEDs fabrication. It
finds application in the so-called molecular OLEDs. These are
OLEDs based on relatively low molecular weight molecules with
weak intermolecular interactions in the solid phase. In fact, the
emission properties of AlQ3 can be associated to intramolecular
deactivating processes (the exciton can be localised on a single
molecule). Furthermore, AlQ3 electric conduction is accurately
described as an electron hopping between neighbouring molecules
rather than the result of extensive electron delocalization in the
solid phase. For this reason, numerous theoretical researches
succeeded in understanding many phenomena occurring in AlQ3
solid phase although they have been applied to a single molecule
in vacuum.1a–e Density functional theory (DFT) has been used to
study the interaction between AlQ3 and metallic atoms with the
aim to model typical processes of the electrode–AlQ3 interface.1f
Configuration interaction (CI) techniques, combined with DFT
aCentro di Eccellenza CEMIF.CAL-LASCAMM, CR-INSTM Unita`
della Calabria, Dipartimento di Chimica, Universita` della Calabria,
I-87036, Arcavacata di Rende(CS), Italy. E-mail: m.ghedini@unical.it;
Fax: +39 (0)984 492066; Tel: +39 (0)984 492062
bLASCAMM, CR-INSTM Unita` della Basilicata and LaMI, Dipartimento
di Chimica, Universita` della Basilicata, I-85100, Potenza, Italy. E-mail:
lelj@unibas.it; Tel: +39 (0)971 202246
† Electronic supplementary information (ESI) available: Synthesis de-
tails of complexes 1–6, computed geometry of compounds A, B, 2–4,
computed excited states energy of compounds A, B, 2–4, parameters of
the Gaussian curves obtained from decomposition of the fluorescence
emission spectra of complexes 1–4 in solution and in KBr pellets,
fluorescence emission spectrum of Qꢀ2Al(l-O)Qꢀ2Al in solution. See
http://dx.doi.org/10.1039/b510463g
330 | Dalton Trans., 2006, 330–339
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in this new class of luminescent compounds, although some
authors attribute it to the relative weakness of the metal–nitrogen
bond.7d Furthermore, the role of the phenolate ligand has not been
sufficiently treated.
aliphatic chain (5, 6). The computational study was performed
on complexes 2–4 and on two model compounds, A (with L =
OC6H5) and B (with L = OC6H4CH3).
This paper reports on the synthesis, characterisation, photo-
physical investigation and computational studies of the homolo-
gous series of blue-emitting pentacoordinated AlQꢀ2L complexes
1–6 shown in Chart 1. The aim of the present study is to clarify
the role of the pentacoordination on the emissive properties of
aluminium quinolin-8-olates. Additionally, particular interest has
been addressed to the influences of the HL ligand on the complex
spectroscopic features; therefore, electron-donating (in 1, 4, 5 and
6) and electron-withdrawing (in 2 and 3) substituents were selected
and the possibility of producing tunable luminescent AlQꢀ2L
molecules was investigated. Moreover, considering that a useful
material for OLED application should have good charge carrier
mobility, and that charge transport can be enhanced by linking
two metal centres,2f two classes of compounds were synthesised:
mononuclear (1–3) and binuclear (4–6) aluminium complexes. In
this way, it was possible to explore the effects of the interaction
between the AlQꢀ2 emitting fragments when they are joined by
aromatic rings (4) or by aromatic rings connected through an
Results and discussion
Synthesis
For the synthesis of complexes 1–6, aluminium isopropoxide was
reacted by adapting procedures developed for the preparation of
similar complexes.7b In particular, the mononuclear complexes
1–3 were synthesised reacting aluminium isopropoxide with the
appropriate phenol and 2-methyl-8-hydroxyquinoline in a ratio
1 : 1 : 2, in toluene. The binuclear complexes 4–6 were prepared by
reacting two molar equivalents of the aluminium isopropoxide in
the presence of one molar equivalent of the appropriate bis-phenol
and four molar equivalents of 2-methyl-8-hydroxyquinoline. All
the new complexes were obtained in good yields and were
yellowish solids, exhibiting the expected stoichiometry (see ESI†)
in the elemental analyses. Moreover, these compounds were
characterised by IR and, in the case of complex 1 which
was well soluble in dichloromethane, 1H NMR spectroscopic
Chart 1 Molecular structures of synthesised complexes 1–6 and the model complexes A and B.
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characterisation was also carried out (see ESI†). According
to the X-ray crystal structure reported for the isobutylbis(2-
methylquinolin-8-olate)aluminium complex, Qꢀ2AliBu,9 in the
homologous series of pentacoordinated complexes 1–6 described
in the present study, the aluminium coordination polyhedron can
be considered a distorted trigonal bipyramid, in which four sites
are taken up by two bidentate Qꢀ ligands, and the fifth one filled
by the monodentate phenoxide ligand.
Photophysical measurements
The photophysical data are collected in Table 1; the reported rel-
ative luminescence intensities in solution were evaluated from the
area of the luminescence spectra with reference to a luminescence
standard [Ru(bpy)3]Cl2 (U = 0.028 in air-equilibrated water10).
Reference to electronic spectra of similar compounds7 suggests
that the absorption and emission bands (Fig. 1 and 2) can be
assigned to transitions localised on the Qꢀ ligands. In fact, no
appreciable differences were observed when the spectra were
compared among these complexes and with those of similar
compounds.7b Furthermore, these complexes show a relevant blue-
shifted emission with respect to AlQ3, whereas the luminescence
quantum yield remains of the same order of magnitude.11
Fig. 2 Excitation (on the left; kem = 475 nm) and emission (on the right;
kex = 375 nm) spectra of 3 in solution (solid line) and in solid (dashed
line).
The negligible effect that the various L fragments have on
absorption and emission spectra in solution could be accounted for
by low perturbations exerted by the phenolate molecular orbitals
on the Qꢀ chelants. Furthermore, no differences have been recorded
in the electronic spectra of the binuclear compounds (4–6). This
excludes relevant electronic effects that could be attributed to two
metal centres interactions, mediated through delocalised electrons
(4) or through space (5 and 6).
To clarify these crucial points a computational study has been
performed on the synthesised complexes 2, 3 and 4 and on two
model complexes A and B. Compound A has to be considered
a parent molecule for a general description of the ground and
excited states characteristics. Compound B has been used in place
of compound 1 for simplifying the geometry optimisation. It is
reasonable that substantially equivalent perturbations are induced
by the methyl and ethyl groups.
Computational study of model compound A
Fig. 1 Absorption spectrum of the complex 4 in dichloromethane
solution.
The computed geometry of A is shown in Fig. 3 and Table 2
presents some computed geometrical parameters compared with
the computed ones for the mer-AlQ3 isomer1c,e (mer-AlQ3 is the
most stable AlQ3 isomer in solution). The complete geometry is
reported in ESI (Table S1†).
The AlQꢀ2L complexes exhibited different spectral features in
CH2Cl2 solution and in solid phase, suggesting that the L fragment
affects the intermolecular interactions. In particular, the emission
band of the solid samples spectra of 1, 3 and 6 are ipsochromically
shifted with respect to their solutions spectra, while compounds
2, 4 and 5 are bathochromically shifted (Table 1).
Table 2 Selected bond lengths computed at the B3LYP/6-31G(d) level
of approximation for the complex A. The computed values of mer-AlQ3
are reported for comparison
Table 1 Photophysical data of synthesised complexes 1–6
˚
Compound
Bond
Length/A
Solution (CH2Cl2)
Solid (KBr pellets)
A
Al–O1
Al–O2
Al–O3
Al–N1
Al–N2
1.764
1.820
1.820
2.076
2.087
Compound Abs./nm Em./nm
Abs./nm
Em./nm
1
2
3
4
5
6
260, 354
260, 354
260, 356
260, 356
260, 352
260, 358
490 (14%)
480a
260, 390
260, 360
278, 400
260, 360
260, 360
260, 360
460
490
468
488
480
476
480a
480 (10%)
490 (7%)
480 (11%)
mer AlQ3
Al–O
1.881
1.884
1.885
2.125
2.064
2.084
Al–N
a Low solubility of the complexes prevents accurate quantum emission
yield determination which was in the order of 10–15%.
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reported orbitals have negligible contributions from the central
metal orbitals. This feature has also been observed in AlQ3.1
Interestingly, the HOMO is localised on the phenolate ligand,
whereas the LUMO is localised on the quinolinate moieties. Fig. 4
also shows the Kohn–Sham orbital energy levels used for assigning
the absorption and emission bands of A. The associated orbitals
can be grouped in orbitals which are localised on the phenolate
fragment and those localised on the Qꢀ fragments. Furthermore,
each Kohn–Sham orbital can be assigned to a single Kohn–Sham
orbital of the isolated and negatively-charged ligand (referred to
as fragment orbital). They are shown in Fig. 4. The occupied
orbitals 114 (HOMO), 111 and 108 and the virtual orbitals
119 and 122 are localised on the phenolate moiety, while the
remaining orbitals are localised on the Qꢀ chelants and form
couples of almost degenerate orbitals: couples 106–107, 109–110,
112–113 of occupied orbitals, couples 115–116, 117–118, 120–
121 of virtual orbitals. These orbitals are mainly delocalised on
the two Qꢀ rings, but the fragment orbitals from which these are
composed interact weakly and exhibit similar energy (calculated
from the expectation value of the Kohn–Sham operator). Thus, the
delocalization of the resulting almost degenerate couple of orbitals
should not be interpreted as the consequence of relevant electronic
interactions.
Fig. 3 The computed structure of the model complex A. The atoms of
the coordination sphere are labelled for reference in Table 2.
As shown in Table 2, the Al–O bond distances of A are shorter
when compared to those of AlQ3; in fact the Al–O bond distances
ꢀ
˚
The energies of the orbitals shown in Fig. 4 and their clas-
sification as quinolinate (qꢀ label) or phenolate localised orbital
(ph label) are listed in Table 3. The HOMO–LUMO energy
gap in A is larger when compared to that in AlQ3 (3.60 vs.
3.27 eV1e).
associated with the Q chelants are shorter by at least 0.061 A.
The Al–N distances of A are comparable to the two shortest Al–N
˚
distances of AlQ3. If the longest Al–N distance of AlQ3 (2.125 A)
is not considered, the maximum difference between the Al–N bond
˚
distances of A and AlQ3 is 0.023 A. If we consider that the Al–
Table 4 reports the principal excited states, their oscillator
strengths and composition in terms of Kohn–Sham orbitals
(mono-electron excitations). It is apparent that the computed
absorption spectrum is dominated by two transitions at 391
and 240 nm, with the second one being much more intense.
Interestingly, these two absorptions are mainly associated to qꢀ
orbitals. Furthermore, close to the intense transition at 240 nm,
two transitions at 245 and 244 nm were computed. As illustrated
in Table 4, they receive a larger contribution from the ph orbitals.
N distances are longer than the Al–O distances, it is clear that
the Al–O bonds experience the largest percentage change in A
when compared to AlQ3. This point will be discussed below when
considering the origin of the blue-shift in the emission spectra of
AlQꢀ2L complexes.
Fig. 4 shows the Kohn–Sham orbitals of A whose energy levels
are close to that of the HOMO (the highest-in-energy occu-
pied molecular orbital) and the LUMO (the lowest-in-energy
unoccupied molecular orbital). It is worthy to note that all the
Fig. 4 The energy scheme of the Kohn–Sham orbitals computed for the complex A. The fragment orbitals with larger contribution have been reported.
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Table 3 One-electron energies and composition of the highest occupied
and the lowest unoccupied Kohn–Sham orbitals of A. The phenolate-
localised orbitals (ph) and the couple of almost degenerate orbitals
localised on the 2-methylquinolin-8-olate (qꢀ) moieties have been labelled
for easiest reference in the following. The fragment orbitals and their
energy levels are shown in Fig. 4. B3LYP/6-31G(d) level of approximation
Computational study of the compounds B, 2, 3 and 4
DFT and TD-DFT computations were performed on the B, 2,
3 and 4 compounds. The geometrical structures were optimised
at the B3LYP/6-31G(d) level of approximation. The complete
structures of B, 2, 3 and 4 are presented in Table S1 (see ESI†).
They all result to be asymmetric.
Label
Orbital
Energy/eV MO main composition
Information regarding the frontier orbitals of these compounds
is presented in Table 5, with the orbitals ordered according to their
energy and, when possible, labelled as in Table 3. In the case of
2 the HOMO−1 and HOMO−2 are delocalised on L (Chart 1)
and Qꢀ ligands, whereas in the case of the binuclear complex 4,
which contains four Qꢀ chelants, four almost degenerate orbitals
are localised on Qꢀ.
From Table 5, it is possible to verify that, in comparison to
the complex A, the CN and NO2 electron withdrawing groups
(compounds 2 and 3) lower (make more negative) the energy of
the L-localised orbital ph3. Consequently, the HOMO becomes
localised on the Qꢀ chelants. Orbitals qꢀ3 and qꢀ4 are weakly affected
by the substituent carried by the L ligand and the energy gap
between them is the same of A (3.74 eV). As a consequence, the
HOMO–LUMO gap increases from 3.60 eV (in A) to 3.74 eV
(in 2 and 3). On the contrary, the presence of electron-donating
groups (in 4 and B) destabilises of the ph3 HOMO, reducing the
HOMO–LUMO energy gap to 3.34 eV (complex B) and to 2.85 eV
(binuclear complex 4); interestingly, the energy gaps between qꢀ3
and qꢀ4 in B and 4 are practically the same as A (3.74 eV). Some
relevant computed electronic transitions up to the most intense
absorption around 240 nm are presented in Table 6.
qꢀ1
106
107
108
109
110
111
112
113
−7.95056
−7.91166
−7.29776
−6.84869
−6.83373
−6.25246
−5.63883
−5.4536
2-Methylquinolin-8-olate
2-Methylquinolin-8-olate
Phenolate
2-Methylquinolin-8-olate
2-Methylquinolin-8-olate
Phenolate
2-Methylquinolin-8-olate
2-Methylquinolin-8-olate
Phenolate
2-Methylquinolin-8-olate
2-Methylquinolin-8-olate
2-Methylquinolin-8-olate
2-Methylquinolin-8-olate
Phenolate
ph1
qꢀ2
ph2
qꢀ3
ph3 (HOMO) 114 (HOMO) −5.31107
qꢀ4 (LUMO) 115 (LUMO) −1.71496
116
117
118
119
120
121
122
−1.64696
−0.4945
−0.46131
0.51136
0.769216 2-Methylquinolin-8-olate
0.804032 2-Methylquinolin-8-olate
0.952544 Phenolate
qꢀ5
ph4
qꢀ6
ph5
From the above analysis, it is possible to conclude that the
absorption spectrum calculated for A is dominated by transitions
between orbitals belonging to the Qꢀ chelants. Very interestingly,
S1 and S2 singlet states (Table 4) are mainly associated to charge-
transfer transitions between L-localised occupied orbitals (ph3
orbitals) and Qꢀ-localised unoccupied orbitals (qꢀ4 orbitals). In
the following paragraph, compounds B, 2, 3 and 4 will be
compared to the parent compound A, with the aim to under-
line differences induced by the substituents on the phenolate
ligand.
The case of the complex A has been reported for comparison.
It is possible to note that the first intense absorption band falls
at about 392 nm in all the compounds. This transition principally
consists in the qꢀ3-qꢀ4 excitation (Table 4), thus it is associated to
Table 4 B3LYP/6-31G(d) singlet–singlet excitation energies (eV and nm), oscillator strengths (f ) and composition of the excited state-function (threshold
of reported coefficients = 0.001) for the complex A. Only transitions with oscillator strength higher than 0.01 are reported for excitation 7 and the successive
ones. The complete list of excitations is reported in Table S2 (ESI†)
Excited state
Composition
Energy/eV (wavelength/nm)
f
S1
S2
ph3 → qꢀ4; 0.50 (HOMO–LUMO)
ph3 → qꢀ4; 0.43
qꢀ3 → qꢀ4; 0.05
qꢀ3 → qꢀ4; 0.40
ph3 → qꢀ4; 0.05
qꢀ3 → qꢀ4; 0.44
qꢀ3 → qꢀ4; 0.47
qꢀ3 → qꢀ4; 0.47
qꢀ3 → qꢀ5; 0.48
qꢀ2 → qꢀ4; 0.01
qꢀ2 → qꢀ4; 0.14
ph2 → ph5; 0.03
qꢀ3 → qꢀ5; 0.09
ph3 → ph4; 0.12
qꢀ2 → qꢀ4; 0.07
ph2 → ph5; 0.06
qꢀ3 → qꢀ5; 0.05
ph3 → ph4; 0.24
qꢀ1→ qꢀ5; 0.01
3.01 (412)
3.12 (398)
0.0004
0.0098
S3
3.17 (391)
0.0738
S4
S5
S6
S14
3.25 (382)
3.39 (366)
3.44 (360)
4.63 (268)
0.0047
0.0019
0.0058
0.0062
S19
S20
S21
5.07 (245)
5.09 (244)
5.16 (240)
0.1193
0.1159
0.7270
qꢀ2 → qꢀ4; 0.19
qꢀ3 → qꢀ5; 0.14
ph3 → ph4; 0.02
a The molecular orbitals are labelled according to Table 3.
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Table 5 The Kohn–Sham orbital energies (eV) and the HOMO–LUMO
gaps (eV) of 2, 3, 4, A and B. The orbitals have been labelled according to
Table 3, where possible
has been discussed in precedence. The experimental spectra of 1,
2 and 4 show two principal features at about 355 and 260 nm
irrespective of the withdrawing and releasing properties of the
substituents on the phenolate moiety (Table 1).
2
3
4
A
B
The good agreement between computed and experimental
absorption spectra allows to describe with good confidence the
main absorption processes. Furthermore, it gives suggestions
about the excited states kinetic after light absorption. Table S3
(see ESI†) collects more detailed information about the lowest-
energy excited states which are associated to the first absorption
band and are involved in emission of light. As said before,
on the basis of computed oscillator strengths and experimental
analysis, light absorption mostly involves transitions associated
to the Qꢀ chelants. Absorption mostly leads to the first singlet
excited state in the case of electron-withdrawing groups (R =
CN and NO2 in 2 and 3, respectively). In these cases, we can
expect that also the emission process takes place from the same
excited state, in other words, both absorption and emission are
processes mostly involving the Qꢀ chelants, and in particular the
lowest excited state S1. The other compounds seem to be different.
A, B and 4 absorption mostly leads to the lowest-energy excited
state which involves orbitals localised in the Qꢀ chelants, that is
S3 for A and B, and S5 in the case of 4, owing to their larger
oscillator strengths. Thus, after absorption, there are in principle
two deactivation paths. The first one is the occurrence of internal
conversion with consequent population of S1 and emission from
this state. If this is the case, a red-shifted emission should have
been observed. The second possibility is that the rate of the
internal conversion to S1 is too low in comparison to the rate of
emission from the above mentioned S3 and S5 singlet states. Thus,
emission takes place directly from these states. Although this
contradicts the Kasha principle,12 this behaviour could be due
to the almost orthogonality of fragment orbitals localised on
L and Qꢀ.
Experimental emission fluorescence data are collected in Table 1
and Fig. 2. As said above, a simple glance allows noting an
unexpected similarity between the emission fluorescence spectra
of the studied complexes. Only a small red shift has been detected
in the case of 1 and 5. We have analysed in a quantitative way
the experimental emission spectra by decomposing their profile
by three Gaussian functions. We have used a common value for
their width. Fig. 5 presents a pictorial view of the results, whereas
Table S4 (see ESI) lists the parameters characterising the Gaussian
curves.
From Table S4† and Fig. 5, it can be noted that the four
emission spectra might result from a vibronic progression of three
overlapping peaks, or at least two asymmetric peaks whose tail at
longer wavelengths has been described by the weakest Gaussian
curve at longer wavelengths (Gaussian 3 in Table S4†). A finding
to be highlighted is the high similarity in the kmax of the three
Gaussian curves. The largest energy difference among the studied
compounds is 0.02 eV, to be compared to the HOMO–LUMO
gaps reported in Table 5, whose differences fall in a range of
0.89 eV. Furthermore, the separation in energy between Gaussian
1 and Gaussian 2 curves goes from 0.24 to 0.21 eV, and the
separation between Gaussian 2 and Gaussian 3 ranges from 0.20
to 0.22 eV. It can be concluded that fluorescence emission mostly
involves Qꢀ chelants, with little influence from the L ligand. As said
about the similarity among absorption spectra, the weak influence
−1.60 (qꢀ4)
−1.60 (qꢀ4)
−1.67 (qꢀ4)
−1.88 (qꢀ4)
−1.92 (qꢀ4)
−1.65 (qꢀ4)
−1.71 (qꢀ4)a
−5.45 (qꢀ3)
−5.64 (qꢀ3)
−1.64 (qꢀ4)
−1.72 (qꢀ4)a
−5.46 (qꢀ3)
−5.64 (qꢀ3)
−1.95 (qꢀ4)a −1.99 (qꢀ4)a −1.67 (qꢀ4)a
−5.69b.c
−5.84d
−5.73 (qꢀ3)b −4.52 (ph3)b −5.31 (ph3)b −5.06 (ph3)b
−5.91 (qꢀ3)
−5.40 (qꢀ3)
−5.40 (qꢀ3)
−5.89d
−6.11 (ph3) −5.59 (qꢀ3)
−5.59 (qꢀ3)
3.74e
3.74e
3.74f
2.85e
3.74f
3.60e
3.34e
3.74f
3.74f
3.74f
a LUMO. b HOMO. c Orbital localised on the two Qꢀ chelants. d Orbitals
delocalised on both Qꢀ and L. e HOMO–LUMO energy gap. f qꢀ3–qꢀ4
energy gap.
Table 6 The most important electronic transitions (nm) computed for
the series of compounds 2, 3, 4, A and B. A threshold of 0.01 has been
used for the reported oscillator strength (in parenthesis). Given the large
size of the molecule, for the complex 4 only the first 26 excited states have
been computed. They correspond to the first absorption band
2
3
4
A
B
392 (0.08)
356 (0.01)
267 (0.01)
253 (0.01)
245 (0.44)
392 (0.08)
383 (0.01)
338 (0.01)
301 (0.39)
266 (0.01)
253 (0.01)
246 (0.01)
245 (0.05)
244 (0.15)
240 (0.67)
392 (0.17)
383 (0.02)
361 (0.01)
398 (0.01)
391 (0.07)
360 (0.01)
268 (0.01)
245 (0.12)
244 (0.12)
240 (0.73)
392 (0.08)
360 (0.01)
268 (0.01)
250 (0.01)
249 (0.01)
245 (0.22)
240 (0.69)
the Qꢀ chelants. As seen before, the energy gaps between occupied
and virtual orbitals localised on Qꢀ do not change in the series of
studied compounds, and this could be reflected in the invariance
of the predicted first absorption. The computed transition at
392 nm is followed at shorter wavelengths by the most intense
band, predicted at 240 nm in all the cases, apart from 2 where it
is computed at 245 nm. The latter transition is mainly localised
on the Qꢀ chelants and it is equivalent to the transition associated
to the state S21 in the case of A (Table 4). Other transitions of
significant oscillator strengths have been computed close to the
most intense band, at slightly longer wavelengths. It is reasonable
to suppose that such bands are not easily seen in the experimental
spectra because they could be too close to the most intense band.
Hence, the computations describe the presence of two significant
features, one at longer wavelength (392 nm) and one, more intense,
at about 240 nm. This is true for all the compounds but 3 for which
an intense band at 301 nm is computed. It can be assigned to the
monoelectronic excitation from the ph3 orbital to a p* orbitals
mostly localised on the NO2 group.
Comparison with the experimental spectra
The computational description of the previous paragraphs well
agrees with the experimental spectra. The substantial invariance of
the experimental absorption spectra among the studied complexes
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Fig. 5 Decomposition of the experimental fluorescence emission spectra for the complexes 1–4. All the emission spectra have been recorded in CH2Cl2
solution, with an excitation wavelength of 355 nm. The highest curve in each graph is the experimental spectrum. The one beneath is the corresponding
combination of the Gaussian curves. The experimental curve has been shifted at higher values to be distinguished from the fitted curve.
of the L ligand on the Qꢀ-localised orbitals can be considered
a good explanation of the great similarity among emission
spectra.
The influence of the L ligands on the emission process has
been definitely ruled out by comparing the fluorescence emission
spectra of the four studied compounds with the binuclear complex
[AlQꢀ2]2(l-O). This is widely described in the ESI.†
Fig. 6 summarises the computational results about absorption
and fluorescence emission of the four studied compounds in
CH2Cl2 solution, considering B as a plausible model for 1. The
arrows between S0 and the excited states have been reported to
describe the main absorption and fluorescence emission processes.
As said before, the most important absorption process for 2 and
3 takes place between S0 and S1; thus, S1 should be considered
the emitter state. For 1 and 4, the most probable absorption leads
to S3 and S5 excited states, respectively. In both the cases, they
appear to be the most probable emissive states in fluorescence
emission.
In fact, according to the computations, there are excited states
at lower energy (S1 and S2 for 1 and S1, S2, S3 and S4 for 4) which
do not take part in the fluorescence process. We tried to find an
experimental confirm of the existence of these low energy states, in
which the L ligand plays an important role. This investigation has
been performed on complex 4. Fig. 7(a), shows a portion of the
emission fluorescence spectra recorded at an excitation wavelength
of 355 nm.
Fig. 7 Fluorescence emission spectra of 4 obtained by exciting at 355 nm
(a) and 490 nm (b) in CH2Cl2 solution.
This corresponds to the maximum of the first absorption
band, which can be assigned to Qꢀ localised electronic transitions.
Fig. 7(b) shows the same portion of the emission spectra when an
excitation wavelength of 490 nm was used. At this wavelength,
absorption from Qꢀ is far less intense and, according to the
computations, in this zone of the spectrum weak absorption bands
should exist for excitations toward excited states in the range from
S1 to S4 (Table S3, ESI†). In this case, at about 660 nm, a weak
Fig. 6 Level scheme of the computed ground and excites states in the
series of studied compounds. The most important absorption and emission
transitions are represented by arrows.
336 | Dalton Trans., 2006, 330–339
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emission peak has been detected. This is shifted of about 0.70 eV in
comparison to the emission peak exciting at 355 nm (emission from
Qꢀ chelants at 480 nm). This energy shift is to be compared with
the computed 0.77 eV energy difference between the computed
HOMO–LUMO gap and the energy gap between occupied and
virtual orbitals on Qꢀ chelants (2.85 and 3.74 eV, as reported in
Table 5). Furthermore, the LUMO orbital is an anti-bond orbital,
whereas the highest occupied orbitals, both on L and on Qꢀ,
are substantially non-bonding. Thus, it can be predicted that the
largest part of the geometry relaxation of the excited state is due to
geometry deformations of Qꢀ, with probable changes in the coor-
dination sphere as a consequence of charge transfers between lig-
ands, a phenomenon suggested for AlQ3.1g Given the nature of the
metal–ligand interaction in this class of compound, which is essen-
tially electrostatic, the deformation of the Qꢀ chelants may be pre-
dicted to contribute significantly to the energy stabilisation. Hence,
a similar energy stabilisation for the lower energy excited states can
be expected, irrespective of the contribution of the L ligand.
Now we are in the position to give some suggestions about the
origin of the blue shift in the absorption and emission processes
of the studied pentacoordinated complexes. We know that these
processes should be associated to the highest occupied and the
lowest unoccupied orbitals localised on the Qꢀ chelants. These are
the qꢀ3 and qꢀ4 orbitals of Table 3, respectively. The qꢀ3 energy of A
is −5.45 eV. The corresponding orbital of AlQ3 (the HOMO) has
been computed at −5.00 eV at the same level of computation.1e
The qꢀ4 orbital is associated to an energy of −1.71 eV, to be
compared to the −1.73 eV of the AlQ3 LUMO.1e It is clear that
the qꢀ3 orbital is more stable in comparison to the AlQ3 HOMO,
so that the energy gap between occupied and virtual orbitals on
the Qꢀ fragments increases mostly by virtue of this effect. In this
respect, great emphasis should be assigned to the shortened Al–
O bond distance in the five-coordinated compound (Table 2).
In fact, the qꢀ3 orbital shows a large contribution from the Qꢀ
oxygen donor atoms, with consequent significant energy changes
associated to the Al–O bond distance variations. Thus, it seems
that a correlation between geometry and blue shifted absorption
and emission should be referred to the Al–O distances rather than
the Al–N distances, as suggested in the literature.5a–7d
decomposition. Actually, the integral of the emission band located
under 500 nm determines the fraction of the emission spectrum
in the range of wavelengths associated to the blue colour. This
fraction amounts to 48, 55, 55 and 60%, respectively for 1, 2, 3
and 4. In general, in CH2Cl2 solutions, the higher is the intensity of
the first Gaussian curve, the higher the percentage of the emission
spectrum in the blue-colour region, that is, more saturated is the
emitted blue light. From the above values, and from the spectra
decomposition (Table S4, ESI† and Fig. 5), it is not possible to find
a simple correlation between the percentage emission in the blue
region and the nature of the monodentate ligand in the structure.
In fact, the complex bearing the strongest electron-donating L
ligand (4) shows a fluorescence emission spectrum which is more
similar to the cases of the complexes with the electron-withdrawing
groups (2 and 3) rather than the cases of complexes with the weaker
electron-donating L ligand (model compound B).
Emission spectra in solid phase
Packing effects in solid phase can change the characteristics
of the emission spectra in comparison to solution phases. The
different intermolecular interactions can change the nature of
the electronic states, and consequently the characteristics of the
absorption and emission bands, such as absorption and emission
peak wavelengths, absorption intensity, emission quantum yield
and band shape. A study about emission from solid phases gains
particular importance when the object of the research is materials
for OLEDs fabrication. Furthermore, a comparison between
solution and solid phase emission can outline the environmental
influences on the electronic transitions. In this light, complexes 1
and 4 are particularly interesting; the lower energy excited states,
which do not seem to take part to electronic transitions (both
absorption and emission) in solution phase, could become more
effective in solid phase, with consequent neat change in emission
properties.
The emission fluorescence spectra of the same set of compounds
in solid phase have been reproduced in Fig. S4 (see ESI†). Also
in this case, a decomposition based on three Gaussian curves of
identical broadness has been performed and shown in Fig. S4
(ESI†). Furthermore, only three Gaussian curves were used and
their parameters are reported in Table 7. In the same table, we
An interesting point to be discussed concerns the relative
intensities of the three Gaussian curves used for the spectra
Table 7 Results from the decomposition of fluorescence spectra in solid phase of 1, 2, 3 and 4. The spectra and their decomposition are displayed in
Fig. S4 (see ESI†). Each curve is described in terms of kmax, relative intensity and full width at half maximum (FWHM). The collected data deal with
solid samples in KBr pellets, and the shifts relative to the CH2Cl2 solutions (Table S4, ESI†) are reported in parentheses
1
2
3
4
Gaussian 1
Gaussian 2
Gaussian 3
kmax
454 nm (−23);
2.73 eV (+0.13)
4.09 (−0.96)
55.6 (+6.2)
472 nm (+1);
2.63 eV (0.00)
2.70 (−1.53)
61.2 (+4.2)
463 nm (−9);
2.68 eV (+0.06)
4.59 (−0.18)
55.6 (−3.0)
470 nm (0);
2.63 eV (0.00)
6.18 (−3.83)
54.0 (+3.2)
Relative Imax
FWHM/nm
kmax
486 nm (−32);
2.55 eV (+0.16)
1.91 (−3.41)
55.6 (+6.2)
517 nm (−0.1);
2.39 eV (−0.01)
2.40 (−0.44)
61.2 (+4.2)
505 nm (−14);
2.45 eV (+0.06)
2.66 (−0.66)
55.6 (−3.0)
513 nm (−2);
2.42 eV (+0.01)
5.83 (−1.18)
54.0 (+3.2)
Relative Imax
FWHM/nm
kmax
520 nm (−47);
2.39 eV (+0.20)
1.00
562 nm (−0.03);
2.21 eV (+0.02)
1.00
552 nm (−19);
2.25 eV (+0.08)
1.00
55.6 (−3.0)
564 nm (−1);
2.20 eV (+0.01)
1.00
Imax
FWHM/nm
55.6 (+6.2)
61.2 (+4.2)
54.0 (+3.2)
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reported the differences in the same parameters in comparison to
spectra recorded in CH2Cl2 solution.
with substituents which are able to perturb the phenolate-localised
orbitals, because this perturbation is substantially confined on
the L ligand. However, the L ligand can change the shape of the
emission band in the solid phase, probably because of influences
on the crystal packing. As a consequence, it could increase the
percentage of emission under 500 nm, and thus the intensity of
the blue emission. In this sense, this effect can be considered as a
sort of indirect tuning of the emitted light. In solution phase, this
indirect tuning is less effective, but still observed and sufficient to
distinguish emission of 1 from that of the other complexes.
From Fig. S4 (ESI†) and Table 7, it is evident that for 2 and 4
the position of the three Gaussian curves remains almost the same
as in solutions, whereas a blue-shift has been detected for 3 and
mostly for 1. For 2 and 4, the relative intensities of the Gaussian
1 curve decreases more than that of the Gaussian 2 one. As a
consequence, the emission spectra in solid phase appears flatter,
and the fraction of the spectra at wavelengths lower than 500 nm
passes from 55 and 60% in solution to 50 and 56% (for 2 and 4,
respectively). On the contrary, the Gaussian 1 curve becomes more
important in comparison to solution phases in the case of 1 and
3. This fact, together with the aforementioned blue-shift, leads to
an increase of the fraction of emission under 500 nm, which are
66 and 79% to be compared with 55 and 48% in solution (for 3
and 1, respectively). The blue-shifted emission in the case of 1 is
noticeable. The worst blue-emitter in solution becomes the best
one in solid phase. This is not in line with the predicted trend in
HOMO–LUMO gap. However, also in the case of solid samples,
it can be underlined a substantial similarity between the emission
spectra of the series of compounds. Thus, it can be concluded
that also in the solid phase the emission process can be assigned
to Qꢀ, with a small contribution from the L ligand. In a simple
monoelectronic description of the observed excitation processes,
the L-localised orbitals do not play a relevant role in the emissive
states. However, this does not mean that the emission spectra are
not influenced by L in an absolute sense. The L ligand contributes
to the percentage emission under 500 nm. This is particularly true
in the solid phase, where larger differences between the emission
fluorescence spectra have been found, a probable consequence of
changes in molecular packing and/or intermolecular interactions
induced by the substituent on the L ligands.
Experimental
Synthesis
The preparation of complexes 1–6 is available in detail as ESI.†
Physical measurements
The apparatus and the techniques used to check the purity of the
synthesised compounds were described elsewhere.15 The photo-
physical properties of the synthesised species were investigated in
solution by using the equipment previously described.15
Computational methods
All the computations have been performed using the Gaussian98
package.16a All geometries have been optimised using the standard
Gaussian98 threshold for SCF and geometry optimisation. Kohn–
Sham molecular orbital and their energies have been computed
by the DFT (density functional theory)16b approach using the
B3LYP16c hybrid exchange–correlation functional and the 6-
31G(d) basis set. Analytical evaluation of the energy second
derivative matrix w.r.t. Cartesian coordinates (Hessian matrix) at
the same level of approximation confirmed the nature of minima
of the potential energy surface stationary points associated to
the optimised structures. The time dependent density functional
theory (TD-DFT)16d–f allowed the computation of excitation
energies, oscillator strengths and excited state compositions in
terms of monoelectronic excitations between occupied and virtual
orbitals. The weight of the monoelectronic excitation has been
reported as the square of the coefficient of the Slater determinant
associated to the monoelectronic excitation.
Conclusions
Pentacoordinated aluminium (phenolate)-bis(2-methylquinolin-
8-olate) compounds are good blue emitting molecular materials
easy to prepare. This characteristic is retained regardless of
the electronic effects exerted by the substituents placed on the
phenolate ligand, a useful trait that can be utilised, for example,
to graft the AlQꢀ2 fragment to macromolecular systems such as
polymers13 or properly substituted porphyrins.14
Absorption and fluorescence emission of the series of com-
pounds 1–6 can be assigned to monoelectronic excitations on
orbitals localised on Qꢀ chelants. In this respect, it can be concluded
that absorption and emission phenomena mostly involve the Qꢀ
fragment, with weak contribution from the L ligand, as has been
observed both in solution and in the solid phase.
By comparing the obtained results with the data referred to the
six-coordinated AlQ3, it seems that the origin of the blue-shifted
emission in pentacoordinated compounds does not arise from the
lengthening of the Al–N bond, as reported elsewhere, instead it
could result from the shortening of the Al–O bond lengths with
consequent stabilisation of the highest energy occupied orbitals
localised on the 2-methylquinolin-8-olate chelants and increased
energy gap with the unoccupied orbitals.
Acknowledgements
This research was supported by the Italian Ministero
dell’Istruzione, dell’Universita` e della Ricerca (MIUR) through
FIRB and the Centro di Eccellenza CEMIF.CAL grants. F. L.
and M. A. thank PRIN 2002, CIPE CLUSTER 14 SP 5 and CIPE
CLUSTER 26 SP 8 for financial support. M. A. thanks Consorzio
INSTM for the contribution to his fellowship.
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