Fine-tuning the luminescent properties of metal-chelating 8-hydroxyquinolines through amido substituents in 5-position

Article abstract of DOI:10.1016/S0020-1693(03)00427-4

New series of 5-substituted-8-hydroxyquinolines HL_n (1-6) bearing aliphatic or aromatic amido groups were synthesised. The chelating ability of these ligands toward the zinc(II) ion was tested and the photophysical characterisation of the resulting complexes (ZnL_n) ₂·2H₂O (7-12) is reported and compared to those of the uncomplexed ligands. The photophysical data of 1-6 revealed interesting differences between aliphatic (1-3) and aromatic (4-6) amido-substituted species which, however, are no longer evident upon metal complexation. In fact while the ligands 1-3 showed a very high quantum yield (2, λ _em=470 nm; Φ=0.22) higher than that of the unsubstituted HQ compound, the ligands 4-6 displayed low quantum yield, similar to that of the complexes 7-12, which was in turn lower than that of ZnQ₂·2H₂O. The behaviour of these compounds is discussed with particular reference to the possibility of controlling the photophysical properties of such compounds through selective modification of the amido substituents.

Full text of DOI:10.1016/S0020-1693(03)00427-4

Inorganica Chimica Acta 357 (2004) 33–40
www.elsevier.com/locate/ica
Fine-tuning the luminescent properties of metal-chelating
8-hydroxyquinolines through amido substituents in 5-position
*
Mauro Ghedini , Massimo La Deda, Iolinda Aiello, Annarita Grisolia
LASCAMM, Unitaꢀ INSTM della Calabria, Dipartimento di Chimica, Universitaꢀ della Calabria, I-87030 Arcavacata (CS), Italy
Received 5 February 2003; accepted 20 June 2003
Abstract
New series of 5-substituted-8-hydroxyquinolines HLn (1–6) bearing aliphatic or aromatic amido groups were synthesised. The
chelating ability of these ligands toward the zinc(II) ion was tested and the photophysical characterisation of the resulting complexes
(ZnLn)2 ꢀ 2H2O (7–12) is reported and compared to those of the uncomplexed ligands. The photophysical data of 1–6 revealed
interesting differences between aliphatic (1–3) and aromatic (4–6) amido-substituted species which, however, are no longer evident
upon metal complexation. In fact while the ligands 1–3 showed a very high quantum yield (2, kem ¼ 470 nm; U ¼ 0:22) higher than
that of the unsubstituted HQ compound, the ligands 4–6 displayed low quantum yield, similar to that of the complexes 7–12, which
was in turn lower than that of ZnQ2 ꢀ 2H2O. The behaviour of these compounds is discussed with particular reference to the
possibility of controlling the photophysical properties of such compounds through selective modification of the amido substituents.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Zinc complexes; 5-Amido-substituted-8-hydroxyquinoline ligands; Luminescence
1. Introduction
materials, studies involving either HQ or HQ-like spe-
cies are currently under way [4].
Photoluminescence is an intrinsic molecular property
of great interest for the preparation of advanced mate-
rials required for applications such as organic light
emitting devices (OLED) [1] or liquid crystals displays
(LCD) [2]. The requisites of a luminescent substance for
practical industrial purposes are good processability and
the possibility of fine-tuning the photophysical features
in order to adapt material performance to the desired
application. Thus, with regard to the molecular mate-
rials which are based on coordination compounds, the
role of the metal-bonded ligands is of paramount im-
portance for achieving such aims.
In this context, the role of a series of substituents
placed on position 5 of the HQ rings was considered
focusing, in particular, on the nature of different bridges
(i.e., AN@NA, ACH@NA, ACH2ANHA) for con-
necting the 8-hydroxyquinoline core to the C6H4NMe2
group [5].
Since the emission maxima of all the considered
compounds resulted, in varying extent, to be red-shifted
with reference to that of HQ, obtained data suggested a
useful way to modulate the luminescence properties of
these species: 5-amido-substituted quinolines may be
ideal for studying photophysical modulations of useful
compounds.
Here we report the synthesis of a series of different
aliphatic and aromatic 5-amido-substituted-8-hydroxy-
quinolines (Fig. 1) and their photoluminescent proper-
ties. The chelating ability of these ligands toward the
zinc(II) ion was tested and the photophysical charac-
teristics of the resulting complexes are also reported and
compared to those of the uncomplexed ligands.
8-Hydroxyquinoline (HQ) is a chelating ligand with
emitting properties which has been extensively exploited
in the synthesis of luminescent metal complexes [3], and
to improve the opto-electronic output of this class of
* Corresponding author. Tel./fax: +39-984-492062.
E-mail address: m.ghedini@unical.it (M. Ghedini).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00427-4

34
M. Ghedini et al. / Inorganica Chimica Acta 357 (2004) 33–40
tion, washed with ethyl acetate, chloroform and dried in
vacuo. Yield 61%. Mp 240–243 °Cdec Anal. Calc. for
C11H10N2O2 ꢀ HCl: C, 55.36; H, 4.64; N, 11.74. Found:
C, 55.08; H, 4.63; N, 11.48%. IR (KBr, cmꢁ1): m 3227
(stretching NAH), 2914 (stretching CH3), 1689
(stretching C@O), 1598, 1530 (bending NAH), 1381,
Fig. 1. General structure of the 5-amido-substituted-8-hydroxyquino-
linate ligands.
1
1304. H NMR (CD3OD, ppm): d 9.14 (d, 1H, J 8.67
Hz, H4), 9.08 (d, 1H, J 4.32 Hz, H2), 8.10 (dd, 1H, J
8.67 and 4.32 Hz, H3), 7.77 (d, 1H, J 8.67 Hz, H6), 7.46
(d, 1H, J 8.67 Hz, H7), 2.30 (s, 3H, CH3).
The homologous compounds 2–6 were prepared
similarly.
2. Experimental
2.1. General procedures
The infrared spectra in KBr were recorded on a
Perkin–Elmer Spectrum One FT-IR spectrometer
equipped for reflectance measurements. The H NMR
2.2.2. HL2, 5-dodecinoylamino-8-hydroxyquinoline hy-
drochloride (2)
1
The green crude product was recrystallised from
methanol; the green solid which formed was collected by
filtration, washed with ethyl acetate, chloroform and
dried in vacuo. Yield 45%. Mp 193–195 °C. Anal. Calc.
for C21H30N2O2 ꢀ HCl: C, 66.56; H, 8.24; N, 7.39.
Found: C, 66.49; H, 8.11; N, 7.13%. IR (KBr, cmꢁ1): m
3242 (stretching NAH), 2914 (stretching aliphatic CH),
1677 (stretching C@O), 1596, 1557 (bending NAH),
spectra were recorded on a Bruker WH-300 spectrom-
eter in CDCl3 or DMSO solutions, with TMS as internal
standard. Elemental analyses were performed with a
Perkin–Elmer 2400 analyzer CHNS/O. The thermal
behaviour was monitored with a Zeiss Axioscope pola-
rising microscope equipped with a Linkam CO 600
heating stage. Absorption spectra were recorded with a
Perkin–Elmer Lambda 900 spectrophotometer. Cor-
rected luminescence spectra were obtained with a Per-
kin–Elmer LS-50B spectrofluorimeter, equipped with a
Hamamatsu R-928 photomultiplier tube. Photolumi-
nescent quantum yields were measured with the method
described by Demas and Crosby [6] using [Ru(bipy)3]Cl2
(bipy ¼ 2,20-bipyridine); U ¼ 0:028 in aerated water [7]
as standard. Luminescence lifetimes were obtained with
IBH single-photon counting apparatus whose operating
lamp employed nitrogen or deuterium. Selection of ex-
citation and emission wavelengths was performed with
monochromators or optical filters. The experimental
uncertainty in the band maximum for absorption and
luminescence spectra is 2 nm; that for luminescence in-
tensity is 20%. The time resolution of the single-photon
spectrometer is 200 ps, the uncertainty of the evaluated
lifetimes is 8% and that of the extinction coefficient is
10%. All emissions are confirmed by excitation spectra.
All the commercially available chemicals were used
without further purification.
1
1519, 1470, 1391, 1307. H NMR (DMSO-d6, ppm): d
10.26 (s, 1H, NAH), 9.04 (d, 1H, J 0.96 Hz, H2), 9.00 (d,
1H, J 8.79 Hz, H4), 8.07–8.03 (m, 1H, H3), 7.72 (d, 1H,
J 8.35 Hz, H6), 7.50 (d, 1H, J 8.31 Hz, H7), 2.48 (t, 2H,
J 7.80 Hz, O@CACH2), 1.70–0.85 (m, 21H, aliphatic
protons).
2.2.3. HL3, 5-(10-adamantanoyl)amino-8-hydroxyquino-
line hydrochloride (3)
Yellow solid. Yield 36%. Mp > 300 °C. Anal. Calc.
for C20H22N2O2 ꢀ HCl: C, 66.94; H, 6.46; N, 7.81.
Found: C, 66.80; H, 6.66; N, 7.62%. IR (KBr, cmꢁ1): m
3280 (stretching NAH), 2909 (stretching aliphatic CH),
1649 (stretching C@O), 1598, 1552 (bending NAH),
1504, 1386, 1361, 1301. 1H NMR (CD3OD, ppm): d 9.07
(d, 1H, J 4.03 Hz, H2); 8.88 (dd, 1H, J 8.72 and 1.34 Hz,
H4), 8.09 (dd, 1H, J 8.72 and 5.37 Hz, H3), 7.64 (d, 1H,
J 8.05 Hz, H6), 7.46 (d, 1H, J 8.71 Hz, H7), 2.12 (m, 9H,
aliphatic protons), 1.86 (m, 6H, aliphatic protons).
0
2.2.4. HL3 , 5-(10-adamantanoyl)amino-8-hydroxyquino-
2.2. Preparation of ligands, 1–6
line (30)
White solid obtained by heating 3 at 160 °C under
vacuum for 5 days. Mp > 300 °C. Anal. Calc. for
C20H22N2O2: C, 73.34; H, 6.81; N, 8.65. Found: C,
73.29; H, 6.86; N, 8.73%. IR (KBr, cmꢁ1): m 3300
(stretching NAH), 2905 (stretching aliphatic CH), 1640
(stretching C@O), 1598, 1583 (bending NAH), 1508,
1397, 1372, 1280 (KBr). 1H NMR (CD3OD) 8.83 (d, 1H,
J 2.01 Hz, H2); 8.15 (d, 1H, J 7.38 Hz, H4), 7.55 (dd,
1H, J 8.72 and 4.03 Hz, H3), 7.30 (d, 1H, J 8.71 Hz, H6),
7.11 (d, 1H, J 8.05 Hz, H7), 2.12 (m, 9H, aliphatic
protons), 1.86 (m, 6H, aliphatic protons).
2.2.1. HL1, 5-acetylamino-8-hydroxyquinoline hydrochlo-
ride (1)
Acetyl chloride (0.096 ml, 0.105 g, 1.341 mmol) was
added dropwise to 5-amino-8-hydroxyquinoline dihy-
drochloride (0.250 g, 1.072 mmol) in pyridine dry at 0
°C. The solution was stirred for 6 h at 0 °C. Thereafter
the solvent was removed under reduced pressure, the
residue dissolved in methanol and the insoluble white
solid filtered off. Ethyl acetate was added to the solution;
the green solid, which formed, was collected by filtra-

M. Ghedini et al. / Inorganica Chimica Acta 357 (2004) 33–40
35
2.2.5. HL4, 5-benzoylamino-8-hydroxyquinoline hydro-
chloride (4)
The homologous compounds 8–12 were prepared
similarly.
Green solid. Yield 64%. Mp 253–256 °C. Anal. Calc.
for C16H12N2O2 ꢀ HCl: C, 63.90; H, 4.36; N, 9.31.
Found: C, 63.78; H, 4.19; N, 9.13%. IR (KBr, cmꢁ1): m
3250 (stretching NAH), 1662 (stretching C@O), 1596,
2.3.2. Zn(L2)2, bis-[5-dodecinoylamino-8-hydroxyquino-
linate] zinc(II) dihydrate (8)
Green solid. Yield 84%. Mp > 300 °Cdec. Anal. Calc.
for C42H58N4O4Zn ꢀ 2H2O: C, 64.31; H, 7.97; N, 7.14.
Found: C, 64.58; H, 7.88; N, 7.26%. IR (KBr, cmꢁ1): m
2920 (stretching aliphatic CH), 1653 (stretching C@O),
1
1557 (bending NAH), 1515, 1389, 1359, 1306, 1250. H
NMR (CD3OD, ppm): d 9.07–9.10 (m, 2H, H2 and H4);
0
0
8.05–8.10 (m, 3H, H3 and H2 ;6 ), 7.82 (d, 1H, J 8.31 Hz,
0
30;50
H6), 7.55–7.68 (m, 3H, H
8.31 Hz, H7).
and H4 ), 7.50 (d, 1H, J
1576 (bending NAH), 1542, 1504, 1468, 1387, 1389. H
NMR data are not available because of the scarce sol-
ubility.
1
2.2.6. HL5, 5-(40-tert-butyl)-benzoylamino-8-hydroxy-
quinoline hydrochloride (5)
2.3.3. Zn(L3)2, bis-[5-(10-adamantanoyl)-8-hydroxyqui-
nolinate] zinc(II) dihydrate (9)
Green solid. Yield 63%. Mp 241–243 °C Anal. Calc.
for C20H20N2O2 ꢀ HCl: C, 67.32; H, 5.93; N, 7.85%.
Found: C, 67.19; H, 5.99; N, 8.06%. IR (KBr, cmꢁ1): m
3210 (stretching NAH), 2961 (stretching aliphatic CH),
1631 (stretching C@O), 1599, 1553 (bending NAH),
1525, 1386, 1365, 1248. 1H NMR (CD3OD, ppm): d 9.04
(d, 1H, J 4.47 Hz, H20);0 8.94 (d, 1H, J 8.61 Hz, H4), 8.02
(d, 2H, J 8.61 Hz, H2 ;6 ), 7.96–8.00 (m, 1H, H3), 7.76 (d,
Green solid. Yield 70%. Mp > 300 °C. Anal. Calc. for
C40H42N4O4Zn ꢀ 2H2O: C, 64.56; H, 6.23; N, 7.53.
Found: C, 64.25; H, 6.19; N, 7.42%. IR (KBr, cmꢁ1): m
2904 (stretching aliphatic CH), 1640 (stretching C@O),
1578 (bending NAH), 1492, 1465, 1388. 1H NMR
(DMSO-d6, ppm): d 9.06 (s, 2H, NAH); 8.67 (br s, 2H,
H2), 8.10 (d, 2H, J 8.07 Hz, H4), 7.56 (m, 2H, H3), 7.13
(br s, 2H, H6), 6.81 (br s, 2H, H7), 2.04–2.00 (m, 30H,
aliphatic protons).
0
0
1H, J 8.25 Hz, H6), 7.62 (d, 2H, J 8.61 Hz, H3 ;5 ), 7.44
(d, 1H, J 8.25 Hz, H7), 1.39 (s, 9H, C(CH3)3).
2.2.7. HL6, 5-(10-naphthoyl)-amino-8-hydroxyquinoline
hydrochloride (6)
2.3.4. Zn(L4)2, bis-[5-benzoylamino-8-hydroxyquinoli-
nate] zinc(II) dihydrate (10)
Green solid. Yield 59%. Mp 272–275 °C. Anal. Calc.
for C20H14N2O2 ꢀ HCl: C, 68.48; H, 4.31; N, 7.98. Found:
C, 68.10; H, 4.19; N, 8.10%. IR (KBr, cmꢁ1): m 3227
(stretching NAH), 1661 (stretching C@O), 1598, 1558
(bending NAH), 1513, 1385, 1302, 1250. 1H NMR
(CD3OD, ppm): d 9.18 (d, 1H, J 8.76 Hz, H4); 9.09 (d, 1H,
Green solid. Yield 88%. Mp > 300 °C. Anal. Calc. for
C32H22N4O4Zn ꢀ 2H2O: C, 61.21; H, 4.17; N, 8.92.
Found: C, 60.95; H, 4.21; N, 8.68%. IR (KBr, cmꢁ1): m
1647 (stretching C@O), 1579 (bending NAH), 1507,
1
1487, 1387, 1094 (KBr). H NMR (DMSO-d6, ppm): d
10.13 (s, 2H, NAH); 8.76 (br s, 2H, H2), 8.31 (d, 2H, J
0
0
80
J 4.77 Hz, H2), 8.35–8.39 (m, 1H, H ), 8.03–8.12 (m, 3H,
8.22 Hz, H4), 8.07 (d, 4H, J 7.14 Hz, H2 ;6 ), 7.52–7.64
0 0
0
0
0
0
20
40
H3, H and H4 or 5 ), 7.96–8.01 (m, 2H, H6 and H5 or 4 ),
(m, 8H, H3, H and H3 ;5 ), 7.37 (d, 2H, J 8.10 Hz, H6),
6.85 (d, 2H, J 8.10 Hz, H7).
0
0
0
7.58–7.67 (m, 3H, H3 , H6 and H7 ), 7.53 (d, 1H, J 8.04 Hz,
H7).
2.3.5. Zn(L5)2, bis-[5-(40-tert-butyl)benzoylamino-8-hy-
droxyquinolinate] zinc(II) dihydrate (11)
2.3. Preparation of complexes, 7–12
The green crude product was purified by recrystalli-
sation from pyridine/water. Yield 80%. Mp > 300 °C.
Anal. Calc. for C40H38N4O4Zn ꢀ 2H2O: C, 64.91; H,
5.72; N, 7.57. Found: C, 65.15; H, 5.53; N, 7.61%. IR
(KBr, cmꢁ1): m 2963 (stretching aliphatic CH), 1647
(stretching C@O), 1576 (bending NAH), 1491, 1465,
1387. 1H NMR (DMSO-d6, ppm): d 10.07 (s, 2H,
2.3.1. Zn(L1)2, bis[5-acetylamino-8-hydroxyquinolinate]
zinc(II) dihydrate (7)
A hot solution of 1 (0.1 g, 0.419 mmol) in methanol (10
ml) was added dropwise to a stirred solution containing
zinc chloride (0.029 g, 0.209 mmol) and ammonium ace-
tate (0.468 g, 6.075 mmol), dissolved in 30 ml of H2O.
After 5 h of vigorous stirring at reflux, the resulting green
solid was filtered off, washed with water and methanol,
filtered and then dried in vacuo. Yield 69%. Mp > 300 °C.
Anal. Calc. for C22H18N4O4 Zn ꢀ 2H2O: C, 52.45; H, 4.40;
N, 11.12. Found: C, 52.71; H, 4.27; N, 10.96%. IR (KBr,
cmꢁ1): m 2920–2860 (stretching CH3), 1654 (stretching
C@O), 1576 (bending NAH), 1553, 1504, 1469, 1388,
1296. 1H NMR (DMSO-d6, ppm): d 9.55 (s, 2H, NAH),
8.59 (br s, 2H, H2), 8.28 (d, 2H, J 7.98 Hz, H4), 7.51–7.47
(m, 2H, H3), 7.23 (br s, 2H, H6), 6.84 (br s, 2H, H7).
NAH); 8.76 (br s, 2 H, H2),08.29 (d, 2H, J 8.25 Hz, H4),
0
8.00 (d, 4H, J 8.25 Hz, H2 ;6 ), 7.61 (m, 2H, H3), 7.56 (d,
0
0
4H, J 8.25 Hz, H3 ;5 ), 7.34 (d, 2H, J 7.71, Hz, H6), 6.84
(d, 2H, J 7.71 Hz, H7), 1.34 (s, 18H, C(CH3)3).
2.3.6. Zn(L6)2, bis-[5-(10-naphthoyl)amino-8-hydrox-
yquinolinate] zinc(II) dihydrate (12)
Green solid. Yield 76%. Mp > 300 °C. Anal. Calc. for
C40H26N4O4Zn ꢀ 2H2O: C, 65.99; H, 4.15; N, 7.70.
Found: C, 65.94; H, 4.13; N, 7.77%. IR (KBr, cmꢁ1): m

36
M. Ghedini et al. / Inorganica Chimica Acta 357 (2004) 33–40
1650 (stretching C@O), 1578 (bending NAH), 1493, 1470,
1407. 1H NMR (DMSO-d6, ppm): d 10.55 (s, 2H, NAH);
9.06 (br s, 2H, H2), 8.78 (d, J 7.80 Hz, 2H, H4), 8.61–8.64
The characterisation of the reaction products 1–6
(HLn ꢀ HCl in Scheme 1) was performed by elemental
analyses, which confirmed the reported stoichiometry,
0
0
0
0
(m, 2H, H8 ), 8.24–8.39 (m, 6H, H2 , H04 and H5 ), 7.46–
IR and H NMR spectroscopy. In particular, the for-
1
30
60
7.95 (m, 10H, H3, H6, H , H and H7 ), 7.21 (br s, 2H,
H7).
mation of the ANHCOA amido linkage is apparent
from the IR spectra (Section 2) which showed the NAH
stretching frequency (in the 3280–3210 cmꢁ1 region,
diagnostic of secondary amides) and the C@O absorp-
tion band (at about 1689–1631 cmꢁ1).
3. Results and discussion
Two molecules of 1–6, treated with ZnCl2, formed the
corresponding dihydrate ZnðL1–6Þ2 ꢀ 2H2O complexes
(7–12 in Fig. 2) in good yields (69–88%). These com-
pounds are green solids which were characterised by IR
and H NMR spectroscopy and their purity confirmed
by elemental analyses.
Compared with those of the respective ligands, the IR
spectra of the zinc complexes showed a shift of the C@O
stretching frequencies, and the appearance of two strong
bands in the 1510–1460 cmꢁ1 region which was attribut-
able to the coordination of the quinoline fragment to the
metallic centre. Elemental analyses of all the species 7–12
confirmed the general formula ZnðL1–6Þ2 ꢀ 2H2O. Along
with the X-ray crystallographic data reported in the lit-
erature concerning the structure of the parent
3.1. Synthesis
The preparation of ligands (1–6 in Scheme 1) was ac-
complished by adapting the syntheses reported in the
literature [8]. The reactions between 5-amino-8-hy-
droxyquinoline dihydrochloride and each of the selected
acyl chlorides gave the desired hydrochloride ligands,
HLn ꢀ HCl, which were yellow–green solids, with yields
ranging from 36% to 64%. Several attempts devoted to
prepare species without hydrochloric acid were per-
formed, but the only compound obtained without hy-
drochloric acid was the ligand containing the adamantane
derivative, HL03 (30), through the heating of the parent
HL3 ꢀ HCl (3) species under vacuum (Section 2).
1
Scheme 1. Synthesis of HL1–6 ligands 1–6.

M. Ghedini et al. / Inorganica Chimica Acta 357 (2004) 33–40
37
the equatorial positions and two water molecules in the
axial positions.
3.2. Spectroscopic properties
The photophysical properties of the novel species were
investigated in DMSO at room temperature (Table 1).
As the ligand containing the adamantane amido
substituent was available in two forms, 3 and 30, the
absorption spectra of these species (Fig. 3) will be
compared and discussed first. Both compounds exhib-
ited an absorption maximum at 332 nm; moreover, 3
showed a shoulder at 390 nm which disappeared in very
dilute solutions; on the contrary, upon the addition of
hydrochloric acid, this feature increased in intensity
while the band at 332 nm decreased. These outcomes
indicate the presence of an equilibrium between two
species absorbing at 332 and at 390 nm, respectively.
Fig. 2. Chemical structures and proton numbering scheme for the
ZnðL1–6Þ2 complexes 7–12.
ZnQ2 ꢀ 2H2O complex [9], it is possible to suggest that in
7–12 the zinc also adopts an octahedral coordination
geometry with two mutually trans quinoline fragments in
Fig. 3. Absorption spectra of compounds 3 and 30 in DMSO solution
(1.35 ꢂ 10ꢁ4 M).
Table 1
Photophysical properties of the synthesised compounds 1–12 in DMSO solution
Compound
Absorptiona
Emission
k (nm) (e (Mꢁ1 cmꢁ1))
k (nm)
U
D (cmꢁ1
)
s (ns)
HQ
1
316 (3190)
334 (1780)
416
467
470
468
450
452
440
534
598
592
597
584
584
602
0.005
0.14
7607
8526
8310
8753
7807
7638
6684
6273
7432
7204
7462
6914
6741
7312
1.4b
14
2
334 (1760)
332 (1870)
0.22
0.14
15
12
3
4
333 (3055)
334 (4000)
340 (3600)
2.0 E-3
2.0 E-3
6.0 E-3
0.04
<1
<1
<1
5
6
ZnQ2 ꢀ 2H2O
340 (3112), 400 (3700)
344 (4100), 414 (5170)
344 (4200), 415 (5300)
344 (4600), 413 (5460)
344 (6500), 417 (6900)
344 (6570), 418 (6800)
344 (6200), 418 (6200)
7
8
0.9 E-3
1.8 E-3
0.8 E-3
1.0 E-3
3.0 E-3
1.5 E-3
<1
<1
<1
<1
<1
<1
9
10
11
12
a The e values for 1–6 were calculated for a molecular weight corresponding to HLn.
b In DMF; see [10].

38
M. Ghedini et al. / Inorganica Chimica Acta 357 (2004) 33–40
Such results were further confirmed upon the addition
of hydrochloric acid to 30, since the absorption maxi-
mum at 390 nm appeared and progressively increased
with acid concentration while the 332 nm peak corre-
spondingly decreased (Fig. 4). Such observations indi-
cate that when 3 is dissolved in DMSO, an equilibrium
between the protonated (kmax ¼ 390 nm) and the neutral
form (kmax ¼ 332 nm) takes place. At the low solution
concentration required for spectroscopic measurements,
this equilibrium was almost completely shifted toward
the neutral form. Worthy of note, HQ in DMSO dis-
played similar behaviour since, with the addition of
concentrated HCl, the band centred at 316 nm (HQ
form) decreased while the band at 364 nm (H2Qþ form
[10]) increased.
Fig. 5. Absorption and fluorescence spectra of 1 and 4 in DMSO so-
lution.
As the absorption spectra of all the ligands 1–6 in
dilute DMSO solution showed comparable features with
30 it is possible to assume that the entire series of hy-
drochloride compounds are in equilibrium between the
cationic and the neutral forms and that the recorded
spectra can actually be assigned, for all the ligands un-
der examination, to the HLn species.
The ground-state absorption spectra of 1–6 (Table 1)
show substantial uniformity with a sharp band at about
335 nm (Fig. 5) which was almost insensitive to solvent
variation. According to the literature data [4c,11], this
feature can be assigned to the p–pꢃ transition from a
filled orbital localised on the quinoline phenoxide ring
towards an antibonding molecular orbital localised on
the pyridyl ring. This excitation, observed at 316 nm for
HQ, is red-shifted, as expected from the electron-do-
nating effect [12] exerted by the ANHCOA linkage, and
appears to be unaffected by the nature of the A group
(Fig. 1).
ima ranging from 440, 6, to 470 nm, 2) can be ascribed
to the HLn species only.
The investigation of substituent effects on the
photophysical properties of the ligands highlights a
different behaviour between aliphatic and aromatic
amido-substituted compounds. The emission maximum
of the ligands (1–3) bearing aliphatic substituents was
red-shifted with respect to those with aromatic substit-
uents (4–6) (Table 1); moreover, the values of the pho-
toluminescent quantum yield (U), the Franck–Condon
shift (energy difference between absorption and emission
maximum; D) and of the lifetime of the emitting state (s)
for 1–3 were considerably greater than those measured
for 4–6.
In the whole series of zinc complexes 7–12 no differ-
ences could be attributed to the nature of group A, as
revealed by the absorption or emission spectra (Table 1)
with all complexes showing two absorption maxima at
344 and ꢄ415 nm, and an emission band peaking within
the 590–600 nm range (Fig. 6 for 7 and 10).
Regarding the luminescence properties, while HQ
emits, protonated form H2Qþ does not [10,13]; likewise,
in spite of the previously invoked equilibria, it is plau-
sible that the emissions observed in DMSO (with max-
Fig. 4. Spectral changes induced by the addition of HCl to 30 in DMSO
solution.
Fig. 6. Absorption and fluorescence spectra of 7 and 10 in DMSO
solution.

M. Ghedini et al. / Inorganica Chimica Acta 357 (2004) 33–40
39
The absorption bands are due to ligand-localised
transitions slightly perturbed by the metal, as described
previously for the ZnQ2 ꢀ 2H2O complex [4c,4d,4j,4k,4m]
(kabs ¼ 340, 400 nm; kem ¼ 534 nm; Table 1); with respect
to this reference compound, the lowest energy transition
of 7–12 (which is responsible for the observed fluores-
cence) exhibited a bathochromic shift due to the electron-
donating effect of the amido substituents [5]. The emission
quantum yield of 7–12 was about 10ꢁ3 (i.e., one order of
magnitude lower than that of ZnQ2 ꢀ 2H2O), irrespective
of the nature of the amido group.
associated with the observed absorption and emission
bands or (ii) introduce some n–pꢃ character from a keto
C@O-localised transition [15]. In fact, while the aromatic
ring of compounds 4–6 could easily delocalise electron
density on the pꢃC@O orbital, thus increasing its energy, in
compounds 1–3 which contain an aliphatic substituent
such delocalisation could be more difficult. Consequently,
with respect to compounds 4–6 an inversion of states
could take place in 1–3, and the high quantum yield
emission, originating from the lowest excited state, would
in this case have a partial n–pꢃ character. Instead, in 4–6
the emitting state is similar to that of the HQ system
[13] and because of the ESPT, fluorescence is partially
quenched.
4. Conclusion
In the zinc complexes, the n–pꢃ related state probably
remains at higher energy, irrespective of the nature of
the substituent, and fluorescence may thus originate
from a p–pꢃ transition which is localised on the quino-
line rings; in this case, the low quantum yield observed
with respect to ZnQ2 ꢀ 2H2O could be attributed to an
increased energy loss throughout the vibrating manifold
in the excited state [16] as evidenced by the greater D
values exhibited by these complexes with respect to the
reference compound.
In conclusion, the spectroscopic studies carried out on
the newly synthesised HQ derivatives clearly demon-
strate that fine control of the photophysical properties is
possible through the modification of substituents in po-
sition 5 of the 8-hydroxyquinoline rings. In particular,
although the bridge connecting the HQ core to an aro-
matic group was found to be crucial for luminescence [5],
we found that the aliphatic 5-amido substituents further
enhanced photoluminescence, overcoming both the HQ
and the ZnQ2 ꢀ 2H2O quantum yield.
A new series of 5-amido-substituted-8-hydroxyquin-
olines were synthesised by reaction of 5-amino-8-hy-
droxyquinoline with aliphatic or aromatic acylchlorides.
These ligands preserved the metal chelating ability of the
unsubstituted 8-hydroxyquinoline, allowing for the
preparation of the corresponding zinc(II) derivatives.
Both the ligands (1–6) and the complexes (7–12) are
stable compounds exhibiting interesting emission prop-
erties. The photophysical data obtained for 1–6 revealed
differences between aliphatic (1–3) and aromatic amido-
substituted species (4–6) which disappeared upon metal
complexation. Indeed while the ligands 1–3 showed very
high quantum yields which were higher than those of the
unsubstituted HQ compound [14], the ligands 4–6 dis-
played very low quantum yields similar to those of all
their complexes, which were in turn lower than that of
ZnQ2 ꢀ 2H2O (Table 1). In addition, comparing the ab-
sorption and emission spectra of such compounds, the
following differences were observed:
1. with respect to HQ, the main emission bands of 1–3
showed a more consistent red-shift than for 4–6;
2. the values of U and s of the excited state of 1–3 were
in the range 0.14–0.22 and 12–15 ns, respectively,
while for 4–6 these values ranged from 0.003 to
0.006 and less than 1 ns, respectively;
Acknowledgements
We thank Dr. F. Barigelletti (ISTEC-CNR, Bologna,
Italy) for fruitful discussions. This work was partly
supported by CIPE grants (Clusters 14 and 26) from the
MIUR and by CNR, PF-MSTAII, Progetto DEMO.
3. all the complexes 7–12 (irrespective of the different
nature of the substituents) showed absorption and
emission features which were only slightly influenced
by solvent polarity, with the values of U and s of the
excited state being nearly 10ꢁ3 and 1 ns, respectively.
It is well established that the lack of fluorescence in
HQ-related compounds is due to the occurrence of ex-
cited-state proton transfer (ESPT) which, involving the
OH phenoxide group and the pyridyl nitrogen atom,
transforms the neutral enolic form into its ketotautomer,
which mainly de-excitates non-radiatively [13]. The in-
teresting behaviour observed in the present cases might
therefore be explained taking into account the occurrence
of extended electronic delocalisation which is created by
the addition of the amido substituent to HQ. Such an
addition may either (i) alter the properties of the state
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