Synthesis and photophysical characterisation of luminescent zinc complexes with 5-substituted-8-hydroxyquinolines

Article abstract of DOI:10.1039/b202945f

New 5-substituted-8-hydroxyquinoline ligands HL_n (1-3), in which the quinoline and the p-N,N-dimethylphenyl fragments are connected through the-N=N-(1), -CH=N- (2) or -CH₂-NH- (3) group, were designed and synthesised. Their reaction

Full text of DOI:10.1039/b202945f

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Synthesis and photophysical characterisation of luminescent zinc
complexes with 5-substituted-8-hydroxyquinolines
Mauro Ghedini,* Massimo La Deda, Iolinda Aiello and Annarita Grisolia
LASCAMM, Unità INSTM della Calabria, Dipartimento di Chimica, Università della Calabria,
I-87030 Arcavacata (CS), Italy. E-mail: m.ghedini@unical.it
Received 25th March 2002, Accepted 1st July 2002
First published as an Advance Article on the web 30th July 2002
New 5-substituted-8-hydroxyquinoline ligands HLn (1–3), in which the quinoline and the p-N,N-dimethylphenyl
fragments are connected through the –N᎐N– (1), –CH᎐N– (2) or –CH –NH– (3) group, were designed and
᎐
᎐
2
synthesised. Their reaction with a Zn() salt gave the pertinent Zn(Ln)2 complexes (4–6). The compounds 1–4 and 6
are stable in solution and their photophysical properties were explored. Zinc complexes 4 and 6 are both luminescent
(4, λem: 620 nm, Φ < 1 × 10Ϫ4; 6, λem: 475 nm, Φ = 7 × 10Ϫ3), although the states responsible for the radiative
deactivation processes were different: CT in nature for 4 and from a π–π* transition localised on the quinoline
fragment for 6. The roles exerted by both the metal and bridge on the photophysical properties are discussed.
2: –CH᎐N–; 3: –CH –NH–) were used to connect the metal
Introduction
᎐
2
chelating entity to the C6H4NMe2 group. The rationale behind
the approach above is justified by the influence the addition of
substituents to the quinoline skeleton exerts on the emission
colour of the material and by the expectation that bulky groups
surrounding the metal co-ordination sphere would prevent both
aggregation and crystallization, thus yielding molecules suitable
for the formation of homogeneous amorphous films which are
essential for good device performance.10
Organic Light Emitting Devices (OLEDs) are based on
materials which feature easily tuneable photophysical properties
and good processability. To this end, although several com-
plexes of different polycyclic aromatic ligands with Be,1 Mg,1a
Zn,1a,b,d,2 Ga,3 Ir,4 Pt,5 Eu1a,d,6 or Tb1a are currently being tested,
the most popular OLED electroluminescent material is the
aluminium co-ordination compound containing the 8-hydroxy-
quinoline (HQ) ligand, AlQ3.7
The HQ chelating ligand is an aromatic molecule whose
electronic and geometric features can be modulated through
appropriate substituents. It is indeed known that the electronic
π–π* transitions in AlQ3 are centred on the ligands and that the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) are localized on the
phenoxide and on the pyridyl rings, respectively.1a,b Thus, it has
been predicted and proved that while an electron-donating sub-
stituent in the 5 position of the quinoline causes a red-shift in
the absorption spectrum of the corresponding complexes, an
electron-withdrawing substituent needs to be in positions 2 or 4
in order to cause the same effect.1a,2a,5a,8,9 These results account
for different HOMO–LUMO gaps, and the electron transport
properties of these complexes can be properly regulated by
changing the electronic effects that the substituents exert on the
quinolinate skeleton.
The present paper reports the synthesis and photophysical
characterisation of new HQЈ–AB–C6H4NMe2 species and their
zinc complexes.
Results and discussion
Synthesis
Ligand 1 was synthesized following the procedure described in
the literature for homologous compounds,11 while 2 was
obtained through reaction of 5-formyl-8-hydroxyquinoline12
with N,N-dimethyl-1,4-phenylenediamine and 3 was prepared
by catalytic hydrogenation of 2. Finally, the respective zinc
complexes 4–6 were obtained by treating the ligands 1–3 with
zinc chloride.
The ligands and their complexes were characterised by IR
1
and H NMR spectroscopy13 and their purity confirmed by
In this area we have focused our attention on a series of
5-substituted-8-hydroxyquinoline ligands (hereafter HQЈ–AB–
elemental analysis. Together, these data confirmed the chemical
structure of the expected products (1–3 in Fig. 1 and 4–6 in
Fig. 2).
C H NMe , Fig. 1) in which different AB bridges (i.e. 1: –N᎐N–;
᎐
6
4
2
Fig. 1 Chemical structures and proton numbering scheme for the HLn
ligands 1–3.
Fig. 2 Chemical structures and proton numbering scheme for the
Zn(Ln)2 complexes 4–6.
3406
J. Chem. Soc., Dalton Trans., 2002, 3406–3409
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b202945f

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Table 1 Photophysical spectral data for 1 and 3 and absorption and emission spectral data for 2, 4 and 6
Absorption
Emission
Compound
Solvent
λabs/nm (ε/MϪ1 cmϪ1
)
λem/nm
Φ
1
4
2
3
6
CH2Cl2
DMSO
CH2Cl2
CH2Cl2
DMSO
235(23600), 261(15800), 323(4700), 442(25500), 460(sh)
430 (sh), 490 (55600)
237(33500), 263(15300), 312 (18200), 407 (16100)
328 (2700)
n.o.
620
570
n.o.
475
—
<10Ϫ4
<10Ϫ4
—
338 (41000), 408 (28500)
0.007
The synthesised compounds are solid and stable in solution,
except for 5, whose imino group hydrolyses yielding the bis-
[(5-formyl)-8-hydroxyquinolinate]Zn complex.13
1 does not emit while 4 shows a band at 620 nm (Fig. 3).
Since the emission involves the CT energy, the observed
behaviour could tentatively be explained assuming that the
energy of the CT state of 1 is higher than that of the nNπ*
related state (Fig. 4). In such circumstances, whilst 1 undergoes
Spectroscopic properties
The photophysical properties of the 1–4 and 6 species (Table 1)
were investigated in solution and the fluorescence quantum
yields were measured with the method described by Demas and
Crosby14 using [Ru(bipy)3]Cl2 (bipy = 2,2Ј-bipyridine; Φ = 0.028
in aerated water15) as standard.
Fig. 4 States diagram for 1; wavelengths (in nm, left) and transitions
(right).
a thermal deactivation, 4 radiatively deactivates as the CT state
becomes the lowest excited one by effect of the lowering of the
LUMO energy which takes place upon zinc co-ordination.
This model seems to be supported by the well defined low
energy band, probably CT in nature, detected at 490 nm in the
absorption spectrum of 4 (Fig. 3).
Only the spectrum of ligand 2 was recorded because of the
instability of the zinc complex 5 towards hydrolysis. The elec-
tronic spectrum of 2 (Fig. 5) shows three bands (at 237, 263
Fig. 3 Absorption spectrum of 1 and absorption and emission spectra
of 4.
The ground-state absorption spectrum of 1 (Fig. 3) shows
transitions attributable to the quinoline (235, 261 and 323 nm)
and to the azobenzene (442 nm) chromophores and could safely
be assigned taking into account the literature data.16,17 It is
known that the HQ absorption spectrum shows three bands,
the first one in the range of 220–250 nm (Φ–Φ* process), the
second at about 270 nm (σN–πpy* transition from the electron
pair of the nitrogen atom which is part of the pyridine ring to
the LUMO) and the third in the range of 300–320 nm
(HOMO–LUMO transition, corresponding to the πph
πpy*
excitation). Moreover, azobenzenes show three accessible
excited states to which three absorption bands, in the visible
and near UV spectral region, are related. In particular, the
lowest energy absorption at approximately 430–440 nm, arises
from the partially forbidden n–π* process; the band at about
310 nm (for the more stable at the photostationary equilibrium
trans –N᎐N– form) is due to the allowed π–π* transition and
᎐
the highest energy band which appears in the 230–240 nm
region involves the –N᎐N– bonded benzene rings. However, in
᎐
the present cases both the bands expected around 310 nm and
in the 230–240 nm range are superimposed on absorptions of
the HQЈ– fragment. In addition, it should be pointed out that
the shoulder observed at 460 nm in the absorption spectrum
for 1, which was absent in the spectra of azobenzenes without
substituents in the para-positions, could be assigned to a
charge transfer (CT) from the lone pair of the nitrogen atom of
the –NMe2 group to empty orbitals of the quinoline aromatic
rings.
Fig. 5 Absorption and emission spectra of 2.
and 312 nm) attributable to transitions, slightly perturbed by
the imino substituent, localised on the quinolinate rings. A
low energy band, at 407 nm, can be assigned to a CT transition
from the –NMe2 group to the quinoline rings mediated by the
–CH᎐N– π electrons. In the absorption spectrum of the amino
᎐
J. Chem. Soc., Dalton Trans., 2002, 3406–3409
3407

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in the 5 position, and the Stokes’ shift, lower than that for
ZnQ2, accounts for a small change in molecular structure when
passing from the ground to the excited state.
In solution, 4 and 6 emit at 620 nm (orange) and 475 nm
(greenish blue) respectively, so this outcome illustrates the
tunability of the emission energy in species whose molecular
structures feature the same peripheral group and different AB
bridges. Since it can be envisaged that these compounds
experience similar intermolecular interactions, the result of
this preliminary research can be used for the preparation of
stable mixed homogeneous films of broad band luminescent
molecular materials.
Experimental
General procedures
Fig. 6 Absorption spectrum of 3 and absorption and emission spectra
of 6.
The infrared spectra in KBr were recorded on a Perkin-Elmer
Spectrum One FT-IR spectrometer equipped for reflectance
measurements. The 1H NMR spectra were recorded on a
Bruker WH-300 spectrometer in CDCl3 or DMSO solutions,
with TMS as internal standard. Elemental analyses were
performed with a Perkin-Elmer 2400 analyzer CHNS/O. The
thermal behavior was monitored with a Zeiss Axioscope
polarizing microscope equipped with a Linkam CO 600 heating
stage. Absorption spectra were recorded with a Perkin-Elmer
Lambda 900 spectrophotometer. Corrected luminescence
spectra were obtained with a Perkin-Elmer LS-50B spectro-
fluorimeter. Selection of excitation and emission wavelengths
were performed with monochromators or optical filters.
All the commercially available chemicals were used without
further purification.
ligand 3, wherein the –C᎐N– bond is replaced by the saturated
᎐
–CH2–NH– linkage (Fig. 6), this band disappears. The above
mentioned CT state is the lowest excited one, at which takes
place the fluorescence of 2 (Fig. 5).
In the examined range, while the absorption spectrum of 3
shows only a broad band at 328 nm, corresponding to a
πph–πpy* transition of the quinolinate chromophore, the
spectrum of 6 shows two pronounced absorption bands,
attributable to quinoline localised transitions at 338 nm
(σN–πpy*) and 408 nm (πph–πpy*). Therefore it can be concluded
that the metal complexation preserves the identity of the
electronic states. Ligand 3 does not emit whereas 6 shows, in
DMSO solution, a fluorescence emission at 475 nm, with an
absolute quantum yield of 0.7%.
Preparation of ligands, 1–3
HL1, 5-[(4Ј-N,N-Dimethyl)phenylazo]-8-hydroxyquinoline (1).
1.72 mL of 12 M hydrochloric acid (0.75 g, 20.0 mmol) was
slowly added to a stirred solution of 0.94 g (6.89 mmol) of
N,N-dimethyl-1,4-phenylenediamine dissolved in the minimum
amount of H2O (10 mL). The resulting solution was cooled in
an ice bath and an aqueous sodium nitrite (0.51 g, 7.44 mmol)
solution (5 mL) was added dropwise. The diazonium chloride
so-formed was then coupled with 8-hydroxyquinoline (1.0 g,
6.89 mmol) dissolved in 7.92 mL of aqueous sodium hydroxide
2 N solution (0.63 g, 20.0 mmol) and the reaction mixture was
stirred for 1 h at 0 ЊC, and then allowed to reach room
temperature slowly. The resulting suspension was acidified with
dilute hydrochloric acid and the precipitate which formed was
collected by filtration, dissolved in CHCl3 and the resulting
solution dried over anhydrous Na2SO4. The crude red product,
obtained after removal of the solvent under reduced pressure,
was purified by recrystallisation from CHCl3–EtOH (1.14 g,
53%). Mp 214–216 ЊC. Anal. calc. for C17H16N4O: C, 69.84; H,
Conclusion
The photophysical properties of some 5-substituted-8-
hydroxyquinolines have been investigated. The comparison
of their absorption spectra has confirmed that the bands,
positioned in the 300–330 nm range, are sensitive to the elec-
tronic effect exerted by the different substituents. Interestingly,
from these data it can be inferred that, with respect to the
unsubstituted HQ ligand, both the –N᎐N–C H NMe and
᎐
6
4
2
–CH2–NH–C6H4NMe2 groups displayed an electron donating
ability (–CH –NH–C H NMe > –N᎐N–C H NMe ) whereas
᎐
2
6
4
2
6
4
2
the –CH᎐N–C H NMe molecular fragment exhibited an
᎐
6
4
2
electron withdrawing effect. Moreover, the emission properties
appeared to be strongly perturbed by these substituents. This
effect can be attributed to the nature of the AB bridge since the
nπ* state carried by the –N᎐N– linkage suppresses fluorescence
in 1 and the –CH᎐N– group makes 2 luminescent through a
radiative deactivation which originates from a charge transfer
from the NMe2 electron lone pair to the quinolinate fragment.
This CT process requires the –CH᎐N– double bond, so that, as
observed for 3, the replacement of the –CH᎐N– with the –CH –
NH– bridge causes the loss of luminescence.
The preparation of new zinc complexes with different 5-
substituted-8-hydroxyquinolines was successful. The known
electroluminescent ZnQ2 complex,1a,18 in dichloromethane
solution displayed absorptions at 341 and 377 nm and emission
at 540 nm. With reference to ZnQ2, some spectral modification,
induced by the two substituents for which the test was possible
᎐
᎐
5.52; N, 19.16%. Found: C, 69.40; H, 5.41; N, 19.38%. νmax
˜
/cmϪ1
3317 (stretching OH), 2900 (stretching aliphatic CH), 1600,
᎐
1575, 1566, 1519, 1502, 1444 (stretching N᎐N), 1278, 1250,
᎐
᎐
2
1210, 1151 (KBr). δH (CDCl3) 9.30 (1H, d, J 8.82 Hz, H4), 8.86
(1H, d, J 4.41 Hz, H2), 7.96 (3H, m, H6 and H2Ј,6Ј), 7.58 (1H, dd,
J 4.41 and 8.82 Hz, H3), 7.26 (1H, d, J 8.10 Hz, H7), 6.80 (2H, d,
J 8.82 Hz, H3Ј,5Ј), 3.11 (6H, s, N(CH3)2).
HL2,
5-[(4Ј-N,N-Dimethyl)phenyliminomethyl]-8-hydroxy-
quinoline (2). 0.79 g (5.77 mmol) of N,N-dimethyl-1,4-phenyl-
enediamine in 5 mL of EtOH was added to a suspension of
5-formyl-8-hydroxyquinoline (1.0 g, 5.77 mmol) in EtOH,
(10 mL). The resulting mixture was stirred at reflux for 4 h and
then slowly cooled to room temperature. The crude green
product, collected by filtration, was purified by recrystallisation
from CHCl3–n-hexane (1.24 g, 74%). Mp 173–175 ЊC. Anal.
calc. for C18H17N3O: C, 74.20; H, 5.88; N, 14.42%. Found: C,
(i.e. –N᎐N–C H NMe and –CH2–NH–C6H4NMe2) were
᎐
6
4
2
noteworthy. Thus, while in 4 a new CT state involving the
quinolinate core was responsible for low energy bands in the
absorption spectrum and luminescent properties, in 6 the
substituent did not modify the identity of the ZnQ2 electronic
states. Its absorption spectrum showed a red-shift of the band
at lower energy, as expected for an electron donating substituent
3408
J. Chem. Soc., Dalton Trans., 2002, 3406–3409

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73.97; H, 5.72; N, 14.61%. νmax
˜
/cmϪ1 2850 (stretching aliphatic
CH3OH (v/v 1/1) as solvent. The compound was obtained as a
CH), 1609, 1504, 1476, 1190 (KBr). δH (CDCl3) 9.93 (1H, dd,
green solid (0.24 g, 70%). Mp 320–323 ЊC (dec.). Anal. calc. for
C36H36N6O2Znؒ2H2O: C, 63.02; H, 5.88; N, 12.25%. Found: C,
J 1.83 and 8.55 Hz, H4), 8.83 (1H, s, CH᎐N), 8.82 (1H, m, H2),
᎐
7.85 (1H, d, J 8.55 Hz, H6), 7.58 (1H, dd, J 4.26 and 8.55 Hz,
H3), 7.30 (2H, d, J 9.15 Hz, H2Ј,6Ј), 7.22 (1H, d, J 8.40 Hz, H7),
6.80 (2H, d, J 9.15 Hz, H3Ј,5Ј), 3.00 (6H, s, N(CH3)2).
63.65; H, 5.45; N, 12.13%. ν˜ max
/cmϪ1 3346 (stretching N–H),
2837 (stretching aliphatic CH), 1597, 1573, 1505, 1463, 1326
(KBr). As 6 was insoluble in common solvents, no NMR data
are available.
HL3, 5-[(4Ј-N,N-Dimethyl)phenylaminomethyl]-8-hydroxy-
quinoline (3). A catalytic amount of Pd/C and then 0.04 g (1.03
mmol) of NaBH4 were added to 0.50 g (1.72 mmol) of 2 in
EtOH (10 mL) The resulting mixture was stirred at room
temperature for 4 h. The solvent was removed under reduced
pressure and the crude product was dissolved in CHCl3 and
then filtered on Celite. The crude yellow product, obtained after
removal of the solvent under reduced pressure, was purified by
recrystallisation from CHCl3–n-hexane (0.19 g, 64%). Mp
190–193 ЊC. Anal. calc. for C18H19N3O: C, 73.70; H, 6.53; N,
Acknowledgements
This work was partly supported by CIPE grants (Clusters 14
and 26) from the MIUR and by CNR, PF-MSTAII, Progetto
DEMO.
References and notes
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433.
14.32%. Found: C, 73.70; H, 6.00; N, 14.10%. νmax
/cmϪ1 3363
˜
(stretching N–H), 2838 (stretching aliphatic CH), 1505, 1476,
1214, 1194 (KBr). δH (CDCl3) 8.78 (1H, d, J 4.02 Hz, H2), 8.44
(1H, d, J 8.52 Hz, H4), 7.48–7.42 (2H, m, H3 and H6), 7.10 (1H,
d, J 7.62 Hz, H7), 6.77–6.69 (4H, m, H2Ј,6Ј and H3Ј,5Ј), 4.55 (2H, s,
NCH2), 2.84 (6H, s, N(CH3)2).
3 A. Elschner, H. W. Heuer, F. Jonas, S. Kirchmeyer, R. Wehrmann
and K. Wussow, Adv. Mater., 2001, 13, 1811.
Preparation of complexes, 4–6
4 M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000, 403,
750.
Zn(L1)2, Bis{5-[(4Ј-N,N-dimethyl)phenylazo]-8-hydroxyquino-
linate}zinc(II) (4). A hot solution of 1 (0.30 g, 1.03 mmol) in
acetic acid (20 mL) was added dropwise to a stirred solution
containing zinc chloride (0.07 g, 0.51 mmol) and ammonium
acetate (1.15 g, 14.90 mmol), dissolved in 30 mL of H2O. After
5 h of vigorous stirring at reflux, the resulting red solid was
filtered off, washed with water, suspended in hot chloroform,
filtered and then dried in vacuo (0.32 g, 95%). Mp 290–292 ЊC
(dec.). Anal. calc. for C34H30N8O2Znؒ2H2O: C, 59.70; H, 5.01;
5 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151;
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and M. E. Thompson, Chem. Mater., 1999, 11, 3709; (c) V. Cleave,
G. Yahioglu, P. Le Barny, R. H. Friend and N. Tessler, Adv. Mater.,
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11 I. Aiello, M. Ghedini and C. Zanchini, Inorg. Chim. Acta, 1997, 255,
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N, 16.38%. Found: C, 59.56; H, 4.82; N, 16.29%. νmax
/cmϪ1 2894
˜
(stretching aliphatic CH), 1462 (stretching N᎐N), 1387, 1362,
᎐
1257, 1227, 1198 (KBr). δH (DMSO) 9.77 (2H, br s, H2), 9.13
(2H, br s, H4), 8.40 (2H, br s, H6), 8.16 (6H, m, H3 and H2Ј,6Ј),
7.20 (6H, m, H7 and H3Ј,5Ј), 3.73 (12H, s, N(CH3)2).
Zn(L2)2,
Bis{5-[(4Ј-N,N-dimethyl)phenyliminomethyl]-8-
hydroxyquinolinate}zinc(II) (5). The same procedure for obtain-
ing 4 was used to synthesise 5 using CH3OH as solvent. The
compound was obtained as a red solid (0.21 g, 59%). Mp
238–241 ЊC (dec.). Anal. calc. for C36H32N6O2Zn: C, 66.93; H,
12 R. Clemo and R. Howe, J. Chem. Soc., 1955, 3552.
1
13 In the H NMR spectrum of 5, low intensity signals attributable to
the bis[(5-formyl)-8-hydroxyquinolinate]Zn complex, as confirmed
by comparison with the spectrum of authentic sample, were indeed
detected.
4.99; N, 13.01%. Found: C, 66.50; H, 4.84; N, 13.12%. νmax
/cmϪ1
˜
14 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 93, 2841.
15 K. Nakamaru, Bull. Soc. Chem. Jpn., 1982, 5, 2697.
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2885 (stretching aliphatic CH), 1600, 1561, 1502, 1465, 1336
(KBr). δH (DMSO) 10.12 (2H, br s, H4), 8.82 (2H, s, CH᎐N),
᎐
8.75 (2H, m, H2), 7.95 (2H, d, J 7.83 Hz, H6), 7.76 (2H, m, H3),
7.25 (4H, d, J 7.80 Hz, H2Ј,6Ј), 6.85 (2H, m, H7), 6.77 (4H, d, J
7.80 Hz, H3Ј,5Ј), 2.85 (12H, s, N(CH3)2).
Zn(L3)2,
Bis{5-[(4Ј-N,N-dimethyl)phenylaminomethyl]-8-
hydroxyquinolinate}zinc(II) (6). The same procedure for
obtaining 4 was used to synthesise 6 using a mixture H2O–
18 G. Giro, M. Cocchi, P. Di Marco, E. Di Nicolò, V. Fattori,
J. Kalinowski and M. Ghedini, Synth. Met., 1999, 102, 1018.
J. Chem. Soc., Dalton Trans., 2002, 3406–3409
3409