Unusual Fluorescence of Eu(III)Porphyrin Entrapped
In Sol-gel Silica Matrix
St. Radzkia, J.Dargiewicz-Nowickaa, M. Makarskaa and J. Legendziewiczb
a
Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland
b
Faculty of Chemistry, Wrocław University, 50-383Wrocław, Poland
e-mail: radzki@hermes.umcs.lublin.pl
The chemistry of porphyrins and related compounds are one of the most interesting
and promising subjects of chemistry, because of their photoactive properties and the ability to
electron transfer. The unique spectroscopic, magnetic, luminescence properties, and what is
the most important, the ability of porphyrins to photoconduction and photoemission have
been being utilised in the nature for a long time. Natural porphyrins and their metal
complexes, such as chlorophyll, haemoglobin and cytochromes, as well as their derivatives,
for example cobalamin and bilirubin, perform the pivotal biological functions in fauna and
flora. The compounds mentioned above are responsible for photosynthesis, cell respiration,
transport, accumulation and exchange of gases, processes of blood cells formation and
pigmentation [1]. The porphyrins have become an indispensable component in the evolution
of living organisms, due to many types of chemical reactions, characteristic of this group of
compounds, such as: coordination, polymerisation, aggregation, oxidation and reduction,
catalysis, sorption and photochemical changes.
Following both the porphyrins properties mentioned above and variety of chemical
reactions they are involved in, new kinds of macrocyclic compounds have been synthesised in
order to their future potential applications. The research conducted for many years proved
versatility of porphyrin applications,
including often the different areas of
life. These extraordinarily interesting compounds can act for example
as catalysts of many chemical
reactions, they also play the role of
pigments and dyes, photoconductors
and semiconductors, analytical reagents, as well as sensitises in
photodynamic therapy (PDT). In the
future they will be probably used as
active elements of biosensors,
molecular switches, elements of
selective electrodes, non-linear
optical materials, parts of electrochromic
displays
or
special
equipment cumulating solar and
conventional energy, also in
synthesis of new types of chemical
structures as dendrimers.
Fig. 1. Molecular structure of H2TMePyP - 5,10,15,20-Tetrakis(1-methyl-4-pyridyl)-21H,
23H-porphine, tetra-p-tosylate salt
The sol-gel process involves the evolution of inorganic networks through the
formation of a colloidal suspension (sol) and gelation of the sol to form a network in a
continuous liquid phase (gel). The precursors for synthesising these colloids consist of a metal
surrounded by various reactive ligands. Metal alkoxides are most popular because they react
readily with water. The most widely used metal alkoxides are the alkoxysilanes, such as
tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides such as
aluminates, titanates, zirconates or borates may be also used in the sol-gel process, often
mixed with TEOS. In sol-gel process the following steps are involved:
1. Hydrolysis of the precursor, catalysed either in acid or in basic conditions
Si(OR)4 + nH2O → (OR)4-nSi-(OH)n + nROH
2. Condensation reactions, which may occur with the production of alcohol:
(RO)3Si-OR + HO-Si(OR)3 → (RO)3Si-O-Si(OR)3 + ROH
or with the production of water:
(RO)3Si-OH + HO-Si(OR)3 → (RO)3Si-O-Si(OR)3 + H2O
The possibility of mixing organic and inorganic compounds in a new unique hybrid
material is realised by sol-gel method. The entrapment of organic reagents into sol-gel
matrices and coatings has been the objective of much research since Avnir and co-workers
pointed out the role of such systems for sensing purpose [2]. Sol-gel monoliths and sol-gel
thin films are very useful to encapsulate various guests such as inorganic clusters, lanthanide
complexes, laser dyes and bioactive molecules [3, 4]. The sol-gel immobilisation of
porphyrins in suitable matrices has been reported in literature due to the importance of these
systems from many points of interest, such as: chemical and biochemical sensing optical
limiting, hole-burning and catalysis [5, 6].
Here we present our study on the encapsulation water-soluble free-base H2TMePyP and
its complex with Eu(III) in xerogels prepared by the sol-gel method.
CH3
CH3
O
Eu
H3C
+N
N
CH3
N
+
O
N
N
N
CH3
+
N
N
+
CH3
Fig. 2. EuTMePyP(acac)
Europium(III) acetyloacetonate 5,10,15,20-tetrakis (1-methyl-4-piryfyl) porphine
The Eu(III)TMePyP(acac) complex (Fig. 2) was prepared by the method described
earlier [7]. The monolith samples with the diameter of about 10 x 10 x 15 mm were prepared
by tetraethoxysilane (TEOS) hydrolysis. We investigated their absorption and emission
spectroscopic properties compare with the spectra of the same compounds in various solvents.
The spectra of europium complex were compared with those of free-base porphyrins. Uv-vis
absorption spectra of monolithic gels are shown in Fig. 3.
1
.
1
A
S
o
r
e
t
b
a
n
d
4
2
8
n
m
Q
b
a
n
d
~
1
5
x
5
2
0
n
m
1
.
0
H
T
M
e
P
y
P
2
5
5
3
n
m
5
9
0
n
m
0
.
9
0
.
8
5
4
2
n
m
6
4
6
n
m
4
1
7
n
m
0
.
7
E
u
T
M
e
P
y
P
(
a
c
a
c
)
0
.
6
5
5
0
n
m
4
2
6
n
m
0
.
5
E
u
T
M
e
P
y
P
(
a
c
a
c
)
0
.
4
(
p
a
r
t
l
y
d
e
c
o
m
p
o
s
e
d
)
0
.
3
0
.
2
0
.
1
3
0
0
3
2
5
3
5
0
3
7
5
4
0
0
4
2
5
4
5
0
4
7
5
5
0
0
5
2
5
5
5
0
5
7
5
6
0
0
6
2
5
6
5
0
6
7
5
7
0
0

[
n
m
]
Fig. 3. Absorption spectra of the: H2TMePyP (solid), Eu(III)TMePyP(acac) (dotted), and
partially decomposed Eu(III)TMePyP(acac) (dashed lined) in monlithic sol-gel material
The uv absorbance spectra illustrate the characteristic spectral changes that accompany
porphyrin metallation. When we compare spectra of free base porphyrin with spectra of their
Eu(III) complexes we can observed only 10 nm shift of Soret band, while dramatical changes
in the Q band could be noticed. The Q band of the free base porphyrin consists of four
components: Qx(0,0), Qx(1,0), Qy(0,0) and Qy(1,0) which are associated with D2h (mmm)
symmetry while in the spectra of Eu(III) porphyrins [symmetry D4h (4 mmmm)] only one
component Qy(0,0) is observed.
The luminescence properties of porphyrin complexes with rare earth metals have been
reported, but strong emission was observed only for Sc, Y, Gd, Lu and Yb. Here we report for
the first time strong emission of Eu(III) porphyrin in monolithic silica. The fluorescence
spectra of H2TMePyP and Eu complex are shown in Fig. 4. They are compared with the
spectra of silica doted with europium chloride and with the complex which was partially
decomposed. The compounds were excited with different wavelength. It can be noticed that
strong fluorescence of europium porphyrin is observed under excitation in Soret band, while
at the same time emission neither free-base porphyrin nor europium chloride does not occur.
It can be explained by the strong interaction of the Eu(III)TMePyP(acac) wit the silica.
4 [a.u]
4
.
0
0
3
.
7
5
3
.
5
0

4
2
3
n
m
e
x
c
.=
6
0
4
n
m
6
5
3
n
m
7
1
6
n
m
3
.
2
5
6
5
4
n
m
3
.
0
0
2
.
7
5
E
u
T
M
e
P
y
P
(
a
c
a
c
)
2
.
5
0
p
a
r
t
l
y
d
e
c
o
m
p
.
2
.
2
5
Relativeintensityx10
2
.
0
0
1
.
7
5
E
u
C
l
3
H
T
M
e
P
y
P
2
1
.
5
0
1
.
2
5
E
u
T
M
e
P
y
P
(
a
c
a
c
)
1
.
0
0
0
.
7
5
6
5
0
n
m
0
.
5
0
0
.
2
5
0
.
0
0
4
7
5
5
0
0
5
2
5
5
5
0
5
7
5
6
0
0
6
2
5
6
5
0
6
7
5
7
0
0
7
2
5
7
5
0
7
7
5
8
0
0

[
n
m
]
4 [a.u]
4
.
0

4
4
3
n
mE
e
x
c
.=
u
T
M
e
P
y
P
(
a
c
a
c
)
3
.
5
3
.
0
5
5
6
n
m
6
5
8
n
m
2
.
5
H
T
M
e
P
y
P
2
Relativeintensityx10
2
.
0
1
.
5
E
u
T
M
e
P
y
P
(
a
c
a
c
)
7
1
9
n
m
6
2
0
n
m
p
a
r
t
l
y
d
e
c
o
m
p
.
6
5
2
n
m
1
.
0
0
.
5
E
u
C
l
3
0
.
0
4
7
5
5
0
0
5
2
5
5
5
0
5
7
5
6
0
0
6
2
5
6
5
0
6
7
5
7
0
0
7
2
5
7
5
0
7
7
5
8
0
0

[
n
m
]
6 [a.u]
1
.
0
5
5
4
n
m
0
.
9
0
.
8
u
T
M
e
P
y
P
(
a
c
a
c
)

5
3
0
n
mE
e
x
c
.=
p
a
r
t
l
y
d
e
c
o
m
p
.
E
u
T
M
e
P
y
P
(
a
c
a
c
)
0
.
7
E
u
C
l
3
6
5
5
n
m
e
m
i
s
s
i
o
n
n
o
o
b
s
e
r
v
e
d
0
.
6
Relativeintensityx10
0
.
5
7
1
5
n
m
0
.
4
0
.
3
0
.
2
H
T
M
e
P
y
P
2
0
.
1
0
.
0
4
7
5
5
0
0
5
2
5
5
5
0
5
7
5
6
0
0
6
2
5
6
5
0
6
7
5
7
0
0
7
2
5
7
5
0
7
7
5
8
0
0

[
n
m
]
Fig. 4. Fluorescence spectra of the: H2TMePyP (solid), Eu(III)TMePyP(acac) (dotted),
partially decomposed Eu(III)TMePyP(acac) (dashed lined) and EuCl3 (dash-dot line)
in monolithic sol-gel material, excited at the various wavelengths.
References
1. Milgrom L. R., “The colors of life: an introduction to the chemistry of porphyrins and
related compounds”, Oxford University Press, UK, 1997.
2. R. Zusman, C. Rotman, and D. Avenir, J. Non-Cryst. Solids, 122 (1990) 107.
3. V. Bekiari, P. Lianos and P. Judenstein, Chem. Phys. Lett., 307 (1999) 310.
4. J.M Krüger and H.D. Breuer, Ber. Bunsenges. Phys. Chem., 11 (1998) 1554.
5. S.-K. Lee and I. Okura, Anal. Chim. Acta, 342 (1997) 181.
6. S.M. Arabei, Veret-Lamarnier and J.P. Galaup, Chem. Pys., 216 (1997) 163.
7. S. Radzki , P. Krausz, S. Gaspard, C. Giannotti, Inorg. Chimica Acta, 138, (1987) 139.