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FORMATION, PHOTOPHYSICS AND PHOTOCHEMISTRY OF WATER-SOLUBLE CADMIUM(II) PORPHYRINS;
THE EFFECTS OF BROMINATION AND THE AXIAL HYDROXO LIGAND
Zsolt Valicseka, Ottó Horvátha, György Lendvaya,b, Ilijana Kikašc, Irena Škorićc
a Department of General and Inorganic Chemistry, Institute of Chemistry, University of Pannonia
 H-8201 Veszprém, P.O.B. 158., Hungary  Phone/Fax: 36(88)624548  E-mail: [email protected]
b Institute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary
c Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000 Zagreb, Croatia
Introduction
Metalloporphyrins represent one of the most significant families of compounds in bio-, coordination-, and photochemistry. Within
this group the so-called out-of-plane (OOP) or sitting-atop (SAT) metalloporphyrins are characterized by special properties originated from the
nonplanar structure, for which, first of all, the size of the metal center is responsible. In these complexes, the metal, due to its large ionic radius
(> 80-90 pm), does not fit coplanar into the cavity of the ligand, hence it is located above the porphyrin plane, distorting it [1].
The formation rate of metalloporphyrins is much slower for the in-plane or normal types than for the OOP complexes because of the
inflexibility of porphyrins. In an OOP complex the distortion of the porphyrin caused by the out-of-plane location of the metal center makes
two diagonal pyrrolic nitrogens more accessible on the other side of the ligand due to the increase of their sp3 hybridization. Thus, another
metal ion, even with smaller ionic radius can easily coordinate to them. Hence, larger metal ions such as Pb2+, Hg2+, or Cd2+ can catalyse the
formation of normal (in-plane) metalloporphyrins via generation of OOP complex intermediates. Since the deformation of the porphyrin ring
proved to be the main factor governing the acceleration of the metalloporphyrin formation, this can also be achieved, e.g., by substituents at the
peripheral ring [2].
Experimental
Formation and absorption spectra
In aqueous solution cadmium(II) reacted easily with H2P4-, forming a complex
with 1:1 composition at pH 8-12 (HOCdP5-). Using the spectrophotometric
titration method, the equilibrium constant was also determined in this system
(lgK’ = 6.90) at adjusted pH of 8.
a)
K
[HOCdP 5 ]
K'   2 
[H ] [H 2 P 4 ][CdOH  ]
According to our experiences, the complex with axial hydroxo ligand (HOCdP5-)
was quantitatively formed at pH 8. In order to confirm the coordination of
hydroxide, the solution of this complex was acidified to pH≈5. Under such a
slightly acidic condition the axial HO- ligand was removed, which was manifested
in significant blue shifts of the characteristic absorption bands (Figs. 2, 3 ).
Irradiation can also promote the dissociation of the axial hydroxo ligand, leading
to the CdP4- species. Notably, this complex does not form directly in the reaction
of Cd2+ and H2P4- below pH 6, but its formation can be detected in the range of
pH 7-8, suggesting that it is produced only through HOCdP5-. Above pH 8, where
the partial molar fraction of the aqua complex compared to the monohydroxo
complex of cadmium(II) ion becomes more negligible, CdP4- cannot form through
HOCdP5-. It is also important to note that CdP4- and CdBrP4- formed by
acidification at pH 5 or by photochemical reactions of HOCdP5- and HOCdBrP5complexes, respectively, are stable at pH 8, moreover, an increase in pH above 8
cannot promote a fast re-coordination of a hydroxide ion into axial position,
indicating that CdP4- and CdBrP4- are not so labile, otherwise the central
cadmium(II) ion would easily dissociate and its position would be occupied with
a [Cd(OH)]+ species, resulting in the formation of the original complexes,
HOCdP5- and HOCdBrP5-. However, such a phenomenon has not been observed.
Octabrominated porphyrin (H2BrP4-) forms a complex with cadmium(II) even
more readily than the unbrominated ligand does. The equilibrium constant can
only be estimated in our system (lgK’≈7.36). The formation rate constant
(lgk+(HOCdBrP5-) = 4.82) was ~19 times higher than that of the unbrominated
complex (lgk+(HOCdP5-) = 3.55).
As a consequence of the coordination of cadmium(II) ion, both the Soret-bands
(Fig. 2) and the Q-bands (Fig. 3) are red-shifted in the case of the unbrominated
compounds. The molar absorbances of both the main Soret- and the Q-bands of
the metalloporphyrins are higher than the corresponding values for the free-base
porphyrin. According to our earlier observations [1], this type of spectral
properties is unambiguously characteristic for OOP or SAT complexes,
confirming the expectations based on the size (95 pm ionic radius) of Cd(II).
Also axial coordination of, e.g., a hydroxo ligand to CdP4- is accompanied by
bathochromic shifts of the characteristic absorption bands because the axial ligand
pulls out the metal center from the cavity of the ligand, resulting in the increase of
dome distortion.
In the case of the brominated porphyrins, both the Soret- and the Q-bands are
significantly red-shifted compared to those of the corresponding unbrominated
compounds. While metalation of the unbrominated free-base porphyrin leads to
considerable bathochromic shifts of both the Soret- and the Q-bands, insertion of
cadmium(II) into the cavity of the brominated free base results in significant
hypsochromic shifts of these bands. This very unusual phenomenon in the case of
the OOP complexes may be attributed to the decrease of the saddle distortion of
the brominated macrocycle. Coordination of the axial hydroxo ligand may
appreciably increase the distortion again, as the red-shifts of the characteristic
bands indicate.
b)
Figure 2. Molar absorption spectra of H2P4- and its
cadmium(II) complexes, CdP4- and HOCdP5- (a), and those
of the corresponding octabromo derivatives, H2BrP4-,
CdBrP4-, and HOCdBrP5- (b) at the B- or Soret-band.
a)
360
0.075 (0.062b)
0.056
0.75 (0.83b)
10.0
7.5
9.2
8.2
CdP4980
390
0.026
0.022
0.83
3.4
7.6
28.6
33.7
While the normal (in-plane) metalloporphyrins do not undergo efficient and irreversible photoinduced ligand-to-metal
charge-transfer reactions, due to their kinetically inert, planar structure, OOP complexes display a characteristic photoredox chemistry
featured by photodegradation caused by the effective separation of the reduced metal center and the oxidized macrocycle following the LMCT
reaction, finally leading to irreversible ring cleavage of the ligand, e.g., the formation of the open-chain dioxo-tetrapyrrol derivatives. The
irradiations were carried out at both the Soret- and the Q-bands, in both aerated and deoxygenated systems (Table 2).
Photolysis of CdP4- at the B- or Soret-band results in the decrease of the
absorption at the characteristic bands, indicating an irreversible degradation of the
a)
complex, the quantum yield of which is two orders of magnitude higher than that
of the free base in aerated system. Besides the LMCT process, dissociation of
CdP4- (to the free base and the metal center) can also beobserved. Upon irradiation
of HOCdP5- at the Soret-band, both redox degradation of the complex and
dissociation of the axial hydroxo ligand take place (Fig. 4a). The overall quantum
yield at Soret-band excitation is one order of magnitude higher than that for the
reaction of CdP4- because of the pulling effect of the axial ligand, i.e., the higher
out-of-plane distance of the metal center.
The primary photochemical reactions of brominated porphyrins are very oxygen
sensitive probably because of the non-bonding electron-pairs on the peripheral
bromine atoms, which may be suitable targets of electrophile attack by oxygen
b)
molecules. Deviating from the corresponding unbrominated species, irradiation of
CdBrP4- results in a less efficient transformation than that observed for CdP4-. The
decrease of the characteristic absorption of CdBrP4- is accompanied by the
appearance of a new species, which still reserved the conjugated bond system,
probably a chlorin-like product on the basis of its individual spectrum (Fig 4b).
Similarly to the case of HOCdP5-, irradiation of the corresponding brominated
species, HOCdBrP5-, leads also to both dissociation of the axial hydroxo ligand
and irreversible ring cleavage. However, the overall quantum yield in this case is
about one order of magnitude lower than that for the unbrominated species in
aerated system.
Table 2. The overall photochemical quantum yields of porphyrins in air-saturated
Figure 4. Spectral changes during the irradiation
and deoxygenated solution.
of HOCdP5- (a), CdBrP4- (b).
a: c(H2P4-)=2.16×10-6 M, c(Cd2+)=9.99×10-6 M, lirr=429 nm,
species
H2P4- CdP4- HOCdP5- H2BrP4- CdBrP4- HOCdBrP5I0=7.98×10-6 M photon s-1, irradiation time 65 s.
0.60
39
730
15.6
16.3
104
(B) /10-5
4-5
2+
-5
b: c(H2BrP )=2.70×10 M, c(Cd )=3.31×10 M, lirr=669 % structural
3%
20%
79%
23%
nm, I0=3.55×10-5 M photon s-1, irradiation time 230 min.
(B-Ar) /10-5 0.33
44
610
1.65
8.8
12.3
%structural
5%
15%
86%
22%
70
114
5.4
23
77
(Q) /10-5
% structural
100%
81%
-5
(Q-Ar) /10
76
108
1.5
4.4
2.3
% structural
100%
85%
H2TPP
H2TPPBr8
CdTPP
CdTPPBr8
HOCdTPPBr8-
HOCdTPP-
Figure 3. Molar absorption spectra of H2P4- and its
cadmium(II) complexes, CdP4- and HOCdP5- (a), and those
of the corresponding octabromo derivatives, H2BrP4-,
CdBrP4-, and HOCdBrP5- (b) at the Q-bands (the dotted
blue line represents the average of energy of Qy(0,0) and
Qx(0,0) of free-base porphyrin).
Table 1. The S1-fluorescence data of porphyrins .
H2P4-
Primary photochemistry
b)
Metalation of the porphyrin results in a hypsochromic effect in the fluorescence (Table 1), in contrast to the red-shift in the absorption.
This blue-shiftred-shift anomaly is virtual, because the absorption shift is referred to the average of Qy(0,0)- and Qx(0,0)-bands of the freebase ligand, while the emission derives not from a hypothetical average level, but from the energetically lower S1x-state (populated in Qx(0,0)
absorption). The Stokes-shift is a bit larger in CdP4- than in H2P4-, although the complex is slightly nonplanar already in the ground state, while
the free base is quite planar. Out-of-plane monoporphyrin complexes of numerous different metal ions were found to have very similar vibronic
overtones, thus in the S1-excited-state these metalloporphyrins can be assumed to have the same degree of ring deformation [1]. Coordination of
the axial hydroxo ligand significantly increases the Stokes-shift, indicating a more nonplanar distortion in the S1-excited state compared to that of
the ground state.
species
S1-shift (metalation) /cm-1
S1-shift (bromination) /cm-1
S1-Stokes /cm-1
(S1)
(S1-B)
(IC)
(S1) /ns
kr(S1) /106s-1
knr(S1) /107s-1
kr(Strickler-Berg) /106s-1
Figure 3. The analysed fluorescence spectra of H2P4- and its
cadmium(II) complexes, CdP4- and HOCdP5- (a), and those
of the corresponding octabromo derivatives, H2BrP4-,
CdBrP4-, and HOCdBrP5- (b).
Electronic Structure Calculations
Photophysics
HOCdP5450
H2BrP4-3360
450
1170
0.010
2.7×10-3
0.0027
4.4×10-4
0.26
0.17
0.36 (0.26a)
0.15a
28.8
279
39.5
17
CdBrP41460
-2880
1350
1.5×10-3
3.9×10-4
0.27
0.062a
HOCdBrP5199
-3620
1690
1.0×10-2
1.1×10-3
0.10
0.35a
25
29
Φ(S1B) = φ(IC S2S1)×φ(S1) and kr(S1) = φ(S1)/τ(S1);. a estimated by the Strickler-Berg-equation; b from Qy-state.
References
1.
2.
3.
Coordination of cadmium(II) ion to the unbrominated porphyrin significantly
decreases both the estimated quantum yield and lifetime of S1-fluorescence.
The efficiency of nonradiative decay increases with the deformation, especially
with the out-of-plane position of metal center. The observation that the
fluorescence lifetimes are hardly affected by the nature of the metal center
implies that probably the geometry is the determining factor in the balance of
excited-state processes. Coordination of the axial hydroxo ligand dramatically
decreases both the quantun yield and the lifetime of the fluorescence.
The fluorescence spectra of the brominated porphyrins could not be measured
in the range of wavelength beyond 900 nm (Fig. 3b) because of the detection
limit of our equipment. Even so the emission bands belonging to the S1(0,0)
and partly the S1(0,1) transitions were recorded and analysed (Table 1). In these
cases the resolution by Gaussian curves can compensate, to some extent, the
truncation of the spectra. Compared to the emission bands of the corresponding
unbrominated species, those of the brominated porphyrins display very large
red-shifts, predominantly due to the strongly distorted structure of the
macrocycle. Moreover, the Stokes-shifts in these cases are much higher than
those for the corresponding unbrominated porphyrins. This phenomenon
indicates that, in spite of the considerably nonplanar ground state, much
stronger structural distortions occur upon excitation of these species than in the
case of the unbrominated compounds.
b)
Preparation of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H6TSPPBr8, hereinafter H6BrP, Fig.1 ) was realized by a modified procedure of
Tabata
[2].
The
analytical
grade
tetrasodium
5,10,15,20-tetrakis(4sulfonatophenyl)porphyrin Na4H2TSPP.12H2O (hereinafter Na4H2P) and CdCl2.2.5H2O
(Sigma-Aldrich) were used for the experiments. The pH of each solution was adjusted to 8
by application of borate buffer, also keeping the ionic strength at constant value of 0.01 M.
The absorption spectra were recorded and the photometric titrations were monitored using
a Specord S-600 diode array spectrophotometer. For the measurement of fluorescence
spectra a Perkin ELMER LS 50-B spectrofluorimeter was applied. The spectrum analyses
were carried out by fitting Gaussian and Lorentzian curves in MS Excel. Rhodamine-B
and Ru(bpy)3Cl2 were used as references for correction of the detector sensitivity and for
determination of the fluorescence quantum yields. Luminescence lifetime measurements
were performed using a laser flash photolysis system. A Quantel Brilliant Nd:YAG laser
yielding 355- and 532-nm pulses of about 5 ns duration served as a light source. The
measurement data were recorded by a Tektronix DPO 4034 digital oscilloscope. Since the
fluorescence lifetimes of the compounds studied are comparable with the laser half-width,
Figure 1. Structures of 5,10,15,20-tetrakis(4a deconvolution method was applied for their determination. For continuous irradiations an
sulfonatophenyl)porphyrin, H2P4-, and its
AMKO LTI photolysis equipment (containing a 200-W Xe-Hg-lamp and a
4-.
octabromo
derivative,
H
BrP
2
monochromator) was applied. All measurements were carried out at room-temperature.
Electronic structure calculations involved molecular geometry optimization and the necessary vibrational frequency analysis as well as the
determination of vertical electron excitation energies. For both purposes we used density functional theory, in particular, the B3LYP
combination of functionals, and time-dependent density functional. In the geometry optimizations we used the Hay-Wadt valence double-zeta
(LANL2DZ) basis set. In the calculations we modeled H2TSPP4- (H2P4-) and H2TSPPBr84- (H2BrP4-) by H2TPP and H2TPPBr8, respectively. In
our test calculations we found that the sulfonato substituent has a negligible effect on the coordination site. All calculations were performed
using the Gaussian 03 suite of programs.
H2P4- + CdOH+  HOCdP5- + 2 H+
a)
O. Horváth, R. Huszánk, Z. Valicsek, G. Lendvay, Coord. Chem. Rev. 2006, 250, 1792-1803.
M. Tabata, J. Nishimoto, A. Ogata, T. Kusano, N. Nahar, Bull. Chem. Soc. Jpn. 1996, 69, 673-677.
Z. Valicsek, O. Horváth, G. Lendvay, I. Kikaš, I. Škorić, J. Photochem. Photobiol. A. 2011, 218, 143-155.
Figure 5. The calculated structure of porphyrins.
In Table 3 are summarized the distances of the atoms from the
plane that passes through only the diagonally situated N1 and N2
atoms and is perpendicular to the axis that is the remnant of the
fourfold axis of the undistorted porphyrin ring. The degree of outof-plane distortion of the porphyrin ring is characterized by these
data as well as by the torsion angles of the phenyl rings relative to
the mean plane of the macrocycle (defined by the C- Cm- C1- C2
dihedral angle, where C1 and C2 are the corresponding carbon
atoms of the phenyl group).
In the CdTPPX8 (X=H or Br, Fig. 5) complex, the diagonal N-N
distances (i.e., the size of the coordination cavity) are noticeably
longer than in the free bases; this significant expansion is
neccessary for the approximately in-plane position of the metal
center. In the CdTPPBr8 the saddle distorsion decreases, which is
manifested in the slight blue-shift of the Q-band compared to that
of Qx-band of the free-base porphyrin, deviating from the tendency
observed in the case of the unbrominated species.
The hydroxo ligand in the axial position (forming HOCdTPPX8-)
pulls the Cd2+ ion out of the ligand center so that it is located about
100 pm above the plane of the nitrogen atoms. Thus, the N-N
distances decrease, the expansion disappears.
There is a correlation between the magnitude of distortion of the porphyrin ring and the red-shift of the absorption maxima. The increasing twist
of the phenyl rings towards the plane of the macrocycle promotes its orbitals to contribute to the conjugated bond system of the porphyrin to a
larger extent, resulting in red-shifts of the absorption bands.
Table 3. The calculated structural and spectral data of porphyrins.
distance/pm
H2TPP CdTPP HOCdTPPCd,H
2
4
99
1
2
11
C
13
10
44
C
Cm
2
3
9
d(N1-N2)
409
431
422
d(N3-N4)
424
431
425
Phenyl twisting /deg
65.5
63.8
60.5
λmax(B) /nm
367
384
414
λmax(Q) /nm
565
566
652
H2TPPBr8
41
54
138
8
422
427
55.3
433
661 (697)
CdTPPBr8 HOCdTPPBr84
102
30
42
106
133
4
3
437
431
431
425
55.5
51.5
454
459
688
734
Conclusion
In slightly alkaline solution (pH = 8) Cd(II) ion and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2P4-) form a kinetically labile
complex (HOCdP5-), in which the metal center is located out of the ligand plane, due to the effects of the axial hydroxo ligand and the relatively
large radius of Cd2+. Both acidification and irradiation at the Soret-band can result in the dissociation of the axial ligand, reducing the out-ofplane distance of the metal center and the distortion of the macrocycle (CdP4-). Besides, irradiation of both types of metalloporphyrins promotes
an irreversible ligand-to-metal charge transfer leading to the oxidative degradation of the coordinated porphyrin. Under the same conditions, in
the case of the octabromo derivative of this water-soluble porphyrin (H2BrP4-), the distorted structure accelerates the formation of the
corresponding complex with cadmium(II) compared to its reaction with the parent, unbrominated ligand. The structure of this porphyrin
(HOCdBrP5-), similarly to the free base and CdBrP4-, strongly distorted by the Br substituents, significantly affects the characteristic features of
the absorption and emission spectra, red-shifting the position of the main bands of these porphyrins compared to those of the unbrominated
species. Also the emission quantum yields and lifetimes are dramatically dimished by bromination. Deviating from the unbrominated species,
photodegradation of the brominated derivatives proved to be very oxygen sensitive. DFT calculations of the geometrical structures and the
absorption bands show good correlations with the observed photophysical and photochemical properties, due to the drastic distortions of the
macrocyclic ligand [3].
Acknowledgement
This work was supported by the National Development Agency (TÁMOP 4.2.2.-08/1/2008-0018, Livable environment and healthier people
– Bioinnovation and Green Technology research at the University of Pannonia, the project is being co-financed by the European Social Fund
with the support of the European Union) and also in the frame of the Hungarian–Croatian Intergovernmental S&T Cooperation Program for
2009-2010 jointly financed by the Hungarian National Office of Research and Technology (OMFB-01247/2009).