Zinc porphyrin with phenoxy-bridged pentacoordinate bis(8- hydroxyquinaldinate)gallium lateral pendants: Synthesis and photophysical characterization

Article abstract of DOI:10.1016/j.inoche.2004.09.025

New polymetallic species formed by a zinc-tetraphenol porphyrinate core and peripheral gallium-quinaldinate fragments were synthesized and characterized. These polynuclear complexes are luminescent and photophysical analysis showed that the emitted colour can be tuned through selection of the excitation energy. This communication reports the synthesis of polymetallic species formed by a zinc-tetraphenolporphyrinate core (ZnTPP(OH)₄, 1) and peripheral gallium-quinaldinate (Q2′Ga) fragments. The photophysical analysis of 1 and of the model compound Q2′GaOPh, R, shows an appreciable overlap between the absorption spectra of 1 and the emission spectra of R, suggesting the presence of a donor-acceptor energy-transfer process. Contrarily, the obtained results indicated that energy transfer does not occur, hence these polymetallic complexes work as double emitting species, for which the colour of luminescence depends on the excitation energy.

Full text of DOI:10.1016/j.inoche.2004.09.025

Inorganic Chemistry Communications 7 (2004) 1273–1276
www.elsevier.com/locate/inoche
Zinc porphyrin with phenoxy-bridged pentacoordinate
bis(8-hydroxyquinaldinate)gallium lateral pendants:
synthesis and photophysical characterization
*
Massimo La Deda, Mauro Ghedini , Iolinda Aiello, Irene De Franco
` `
Centro di Eccellenza CEMIF.CAL, LASCAMM, Unita INSTM della Calabria, Dipartimento di Chimica, Universita della Calabria,
I-87030 Arcavacata (CS), Italy
Received 28 July 2004; accepted 24 September 2004
Available online 30 October 2004
Abstract
This communication reports the synthesis of polymetallic species formed by
a zinc-tetraphenolporphyrinate core
(ZnTPP(OH)4, 1) and peripheral gallium-quinaldinate ðQ0 GaÞ fragments. The photophysical analysis of 1 and of the model
2
compound Q02GaOPh, R, shows an appreciable overlap between the absorption spectra of 1 and the emission spectra of R, sug-
gesting the presence of a donor–acceptor energy-transfer process. Contrarily, the obtained results indicated that energy transfer
does not occur, hence these polymetallic complexes work as double emitting species, for which the colour of luminescence
depends on the excitation energy.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Metalloporphyrin; Gallium quinaldinate complexes; Photophysics; Energy transfer; Fluorescence
The pentacoordinated gallium complexes formed by
two equivalents of the chelating 8-hydroxyquinaldine
(HQ0) and a monodentate ligand, such as substituted
phenols or carboxylic acids, are chemically stable
coordination compounds which have been studied
with reference to their potential applications as molec-
ular emitting materials [1]. The available literature
data concerning these species show that their photo-
physical properties are fairly insensitive to the nature
of the monodentate ligand [1a,c], being the active elec-
tronic states localized on the quinaldinate fragment
[2].
In order to better exploit both the chemistry and the
photophysical properties of the ‘‘Q02Ga’’ fragment, a
protocol for the preparation of polymetallic complexes
was established wherein the ‘‘Q02Ga’’ emitting features
can be combined with those carried by an additional me-
tal centre such as metalloporphyrins. Porphyrins are
well known chromophores, which are used in a number
of light-operating materials [3]; moreover, several supra-
molecular architectures, built with porphyrin cores,
have been reported [4]. Thus, ZnTPP(OH)4 (1), the zinc
derivative which quantitatively forms (IR, NMR and
MALDI/TOFMS data) upon addition of the stoichio-
metric amount of zinc acetate to the symmetrically
substituted 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,
23H-porphyrin, H2TPP(OH)4, has been selected to be
tested in reactions for the synthesis of mixed Ga/Zn
complexes.
*
Corresponding author. Tel.: +39 984 492062; fax: +39 984 492066.
E-mail address: m.ghedini@unical.it (M. Ghedini).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.09.025

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M. La Deda et al. / Inorganic Chemistry Communications 7 (2004) 1273–1276
cies ZnTPPðOGaQ20 Þ ; ZnTPPðOHÞðOGaQ02Þ ; ZnTPP
OH
4
3
ðOHÞ ðOGaQ02Þ ; and ZnTPPðOHÞ ðOGaQ02Þ, whereas
2
2
3
elemental analysis gave experimental values which
indicated that the product was not a pure species [7].
In order to investigate the influence that the gallium
and zinc centres mutually exert on the photophysical
properties of 2, the pentacoordinate reference complex
Q02GaOPh, R, was synthesized [8], characterized and
compared with 1 and 2. Table 1 presents the photophys-
ical data, in which the relative luminescence intensities
were evaluated from the area (on an energy scale) of
the luminescence spectra and with reference to a lumi-
nescence standard [Ru(bpy)3]Cl2 (/ = 0.028 in air-equil-
ibrated water [9]).
N
Ga
N
O
O
N
N
N
N
O
Zn
HO
OH
OH
R
1
O
O
The absorption spectrum of 1 (Fig. 1 and Table 1)
displayed a strong transition to the second excited state
(S0 ! S2) at about 420 nm (the Soret or B band) and
two weak vibronic transitions to the first excited state
(S0 ! S1) at about 560 and 600 nm (the Q bands).
Remarkably, such a spectrum closely resembles that
of the unsubstituted zinc porphyrin [10], thus indicating
that, probably because of the steric influence [11], the
phenols at the meso positions form a large dihedral an-
gle with the porphyrin ring, preventing significant p-
electron delocalization. With reference to the emitting
properties of 1, it should be pointed out that the inter-
nal conversion from S2 to S1 is rapid so fluorescence
can only be detected from S1 (kmax = 610 nm, /
N
Ga
O
N
N
Ga
N
N
O
O
O
O
N
N
N
O
Zn
O
Ga
N
N
O
N
Ga
N
O
O
0.20
0.16
0.12
0.08
0.04
0.00
100
80
60
40
20
0
ZnTPP(OGaQ'2)4
We hereby describe the product which was formed
upon reacting HQ0, Ga(NO3)3 Æ xH2O and 1 in a
8:4.5:1 molar ratio, respectively [5]. The dark blue so-
lid residue, 2, recovered after solvent distillation and
repeated washing with water, ethanol and chloroform,
was highly insoluble in most of the common solvents.
The IR spectrum (KBr pellets) of the product dis-
played the expected absorptions in the 1610–1390
cmꢀ1 range, which are typical for the metal chelated
Q0 [1a]. Moreover, MALDI/TOFMS data [6] identified
a mixture of ZnTPP(OH)4 and of the polymetallic spe-
450
500
550
600
650
Wavelength/nm
Fig. 1. Overlap of the absorption spectrum of 1 (solid line) with the
emission spectrum of R (dashed line).
Table 1
Photophysical dataa of complexes 1, 2 and R
Compound
Absorption, kmax/nm(e/Mꢀ1 cmꢀ1
)
Emissionb
kmax/nm
/
s/ns
1
2
R
a
424(555000), 559(18500), 600(11500)
256(210000), 352(33000), 424(931000), 559(29500), 600(18500)
256(67000), 358(4500)
610
500, 605
510
0.018
0.040
0.106
1.2
12.4c; 1.2 (64%)d, 11.7 (36%)d
12.5
In methanol at room temperature.
b
c
k
k
k
ex = 350 nm.
em = 500 nm.
em = 605 nm.
d

M. La Deda et al. / Inorganic Chemistry Communications 7 (2004) 1273–1276
1275
= 0.018, and s = 1.2 ns). Moreover, it should be noted
0.5
0.4
0.3
0.1
0.0
100
that the molecular conformation of 1 prevents the
intermolecular p–p interactions among porphyrin cores
which, for unsubstituted porphyrins, usually quench the
fluorescence emission, as demonstrated by the fact that,
at high concentrations, 1 showed appreciable fluores-
cence [12].
The absorption spectrum of the reference complex
R (Table 1) parallels that of previously reported simi-
lar quinaldinate gallium derivatives [13]. In particular,
an intense band centred at 256 nm and a less intense
absorption at 358 nm were observed and both were as-
75
50
25
0
300
400
500
600
700
Wavelength/nm
signed to pp transitions localized on the quinaldinate
*
fragment [13]. The luminescence spectrum, reported in
Fig. 2. Absorption (solid line) and emission spectra (kex = 256 nm,
dashed line; kex = 424 nm, dotted line) of complex 2 in methanol
solution.
Fig. 1, exhibits a maximum at 510 nm, that can be
safely attributed to
a
fluorescence deactivation
(s = 12.5 ns) from the lowest excited electronic state
[13].
This study shows a possible synthetic protocol for the
preparation of new Ga/Zn polymetallic complexes. In
the experimental conditions here described the main
product is Ga4/Zn. As Ga4/Zn is highly insoluble and
very difficult to purify completely, traces of the homolo-
gous polymetallic species (e.g., Ga3/Zn, Ga2/Zn or Ga/
Zn) cannot be effectively removed. However, the above
results suggest that the photophysical properties of the
whole Gan/Zn series (n = 1–4) should be unaffected by
the number of the gallium centres which surround the
zinc-containing core. In particular, as reported for 2, a
double emission wavelength, at 500 and 606 nm, was de-
tected (Table 1). Therefore, as excitation on the quinal-
dinate fragment produces a bluish green emission and
excitation on the porphyrin levels produces a red orange
emission, the luminescence colour can be modulated by
the selected excitation wavelength. In conclusion, these
preliminary results suggest that polynuclear complexes
formed by HQ0 and H2TPP(OH)4 ligands and contain-
ing different metal ions may actually modulate the emis-
sion colour and/or improve the j2 factor. Further
studies are therefore currently underway to investigate
these complexes for both fundamental and applicative
purposes.
Comparing the spectral pattern of 1 and R, it is wor-
thy of note that the absorption spectrum of 1 at the
wavelengths corresponding to the excitation of the low-
est excited state overlaps closely with the emission spec-
trum of R (Fig. 1). Such features suggest that energy
transfer (ET) from the donor (D) R to the acceptor
(A) 1 could be expected in 2, as it is a polymetallic spe-
cies wherein the D and A chromophores are covalently
linked.
Two mechanisms are generally operating in ET proc-
esses: the Dexter ET, which is a coherent transfer of
exciton, from D to A sites, at a rate which is propor-
tional to the orbital overlap of D and A. The second
ET process is the Fo¨rster resonance energy transfer
(RET), which consists of a through-space induced di-
pole energy exchange that is dependent on the D–A dis-
tance and on the orientation of the emission–transition
dipole of D and the absorption–transition dipole of A
(orientation factor, j2) [14]. In principle, the Dexter
ET should be ineffective in 2, since, as previously men-
tioned, with respect to the porphyrin ring, the phenol
rings adopt a tilted conformation which prevents large
orbital overlap between the D and A sites. Indeed, as
the luminescence spectrum of 2 shows a band at 500
nm (kex = 256 nm; Table 1 and Fig. 2), which can be
attributable to the emission of the D component (e.g.,
the ‘‘Q02Ga’’ fragment), a complete ET process from D
to A can be excluded. Alternatively, taking into account
the Fo¨rster mechanism,the transfer efficiency can be cal-
culated according to the formula ERET = 1 ꢀ (sDA/sD),
wherein sDA and sD are the lifetimes of D, respectively,
with and without A [14]. The lifetimes measured at the
emission maxima, 500 nm for 2 (sDA = 12.4 ns) and
510 nm for R (sD = 12.5 ns) are reported in Table 1
and from these data ERET was found to be 0.008. This
low value suggests that, probably because of the unfa-
vourable j2 factor, the RET mechanism practically does
not work in 2.
Acknowledgement
This research was supported by the Italian Ministero
`
dellꢀIstruzione, dellꢀUniversita e della Ricerca (MIUR)
through the Centro di Eccellenza CEMIF.CAL grant.
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