480
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 10 (2007) 479–481
While water which is always present in small amounts and
ethanol provide the protons, ethanol or other suitable sub-
+
strates serve as reductants. Ag ions are further photo-
products. Since they are too large for the porphyrin
cavity they are released. The overall stoichiometry can be
expressed by a simple equation:
II
2
Ag TPP þ H
2
O þ CH
3 2
CH OH
!
Ag O þ 2H TPP þ CH CHO
ð3Þ
2
2
3
In summary, under reducing conditions AgTPP undergoes
a photodemetalation induced by LMCT excitation. At this
point it should be mentioned that a light-induced demeta-
lation of a silver(II) porphyrin has been previously noticed
ꢀ
5
II
Fig. 1. Spectral changes during the photolysis of 4.50 · 10 M Ag TPP
in a mixture of dmso/ethanol (99:0.5) at room temperature after 0 min (a)
0 and 40 min (b) irradiation times with kirr > 360 nm (Osram HBO
2
2
00 W/2 lamp; cut-off filter Schott GG 375/4), 1-cm cell.
[
27]. However, in this case very peculiar reaction conditions
have been applied and the origin of the photochemical
reaction has not been discussed.
[
21]. Longer irradiation times and higher AgTPP concentra-
tions led to the deposition of a dark-brown precipitate which
apparently consists of Ag O. This is easily soluble in dilute
HNO or aqueous ammonia. The presence of Ag in these
As mentioned above, the photolysis of silver(II) porphy-
rins may be used to enhance the benefit of photodynamic
therapy with porphyrins. Metal-free porphyrins serve as
sensitizers for the generation of singlet oxygen which, in
turn, attacks and destroys cancer cells [24–26]. While
2
+
3
solutions was confirmed by simple qualitative tests (e.g. pre-
cipitation of silver halides). Moreover, it was suspected that
ethanol serves as a photoreductant. Indeed, acetaldehyde
was formed as a further photoproduct. It was identified as
an addition compound (kmax = 560 nm) of 4-hydroxydiphe-
nyl [23]. An extended irradiation of AgTPP was associated
with a secondary photolysis which led to the disappearance
of H TTP. It is well known that the irradiation of H TPP
II
1
Ag porphyrin does not generate O , its photoproduct,
2
+
the demetalated porphyrin, does. Ag ions which are fur-
ther photoproducts of Ag porphyrin are well known to
II
suppress infections [17–20] which could result from inflam-
mations as deleterious side effects of PDT.
2
2
is accompanied by the formation of singlet oxygen [24–26]
which oxidatively destroys the porphyrin.
References
Generally, the photoreactivity of metalloporphyrins
depends on the nature of their lowest excited states which
are frequently of the porphyrin IL type, as indicated by
the appearance of the typical IL luminescence [1–3]. In con-
[
1] K. Kalyanasundaram, Photochemistry of Polypyridine and Porphy-
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[2] D.M. Roundhill, Photochemistry and Photophysics of Metal Com-
plexes, Plenum Press, New York, 1994.
[3] J. Sima, Struct. Bond. 84 (1995) 135.
II
trast, Ag TPP is not luminescent under any conditions
[
[
4] H. Kunkely, A. Vogler, Inorg. Chim. Acta 254 (1997) 417.
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854.
[
16,22]. It has been concluded that in this case the excited
state properties are determined by low-energy CT states
which quench the IL emission [22]. Originally it has been
[6] L.R. Robinson, P. Hambright, Inorg. Chem. 31 (1992) 652.
[7] S.L. Bailey, P. Hambright, Inorg. Chim. Acta 344 (2003) 43, and
reference therein.
II
*
suggested that this CT state is of the MLCT (Ag ! p
porphyrin) type. We assume that the CT takes place in
the opposite direction. A low-energy LMCT (p porphy-
[
8] M. Tabata, A.K. Sarker, E. Nyarko, J. Inorg. Biochem. 94 (2003) 50,
and reference therein.
II
rin ! Ag ) transition is not only consistent with the gen-
[9] F.R. Longo, E.M. Brown, W.G. Rau, A.D. Adler, in: D. Dolphin
(Ed.), The Porphyrins, vol. 5, Academic Press, New York, 1978, p.
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[14] A. Giraudeau, A. Louati, H.J. Callot, M. Gross, Inorg. Chem. 20
1981) 769.
eral redox behavior of Ag(II), but also explains the
II
photoreactivity of Ag TPP. Silver(II) is certainly not a
[
[
strong reductant but a strong oxidant facilitating low-
energy LMCT excitation. Accordingly, irradiation is asso-
ciated with a reduction of Ag(II) and oxidation of the por-
phyrin ligand.
[
LMCT=hm
!
II
þ
Ag TPP
Ag þ TPP radical anion
ð1Þ
(
The low quantum yield of this photolysis should be caused
by two competing processes. A simple charge recombina-
tion regenerates AgTPP. Moreover, the TPP radical anion
[15] K.M. Kadish, X.Q. Lin, J.Q. Ding, Y.T. Wu, C. Araullo, Inorg.
Chem. 25 (1986) 3236.
[
16] M.A. Oar, W.R. Dichtel, J.M. Serin, J.M.J. Fr e´ chet, J.E. Rogers, J.E.
Slagle, P.A. Fleitz, L.-S. Tan, T.Y. Ohulchanskyy, P.N. Prasad,
Chem. Mater. 18 (2006) 3682.
(
or deprotonated TPP radical cation) must be intercepted
by a reductant and two protons in order to form the
free-base porphyrin.
[17] A. Melaiye, Z. Sun, K. Hindi, A. Milsted, D. Ely, D.H. Reneker, C.A.
Tessier, W.J. Youngs, J. Am. Chem. Soc. 127 (2005) 2285.
[18] V. Sambhy, M.M. MacBride, B.R. Peterson, A. Sen, J. Am. Chem.
ꢀ
ꢀ
þ
ðTPP radicalÞ þ e þ 2H ! H
2
TPP
ð2Þ
Soc. 128 (2006) 9798.