0
0
Table 1 Values of DG ET, DG ex, quantum yields and triplet lifetimes
for 1–3
1
2
3
0
a
a
DG ET/eV
n.a.
n.a.
0.47
0.47
0
DG ex/eV
20.22
0.012
0.79
20.22
0.020
0.91
W
W
f
0.67
0.18
T
T
t /ms
46.5
5.0
14.4
a
n.a.: not applicable.
2
SCE) and half-wave oxidation potential of pyrene in the Rehm–
7
Weller relationship (eqn 1).
0
+
2
DG ET = Eox(D/D ) 2 Ered(A /A) 2 E0,0* +
.6 eV/e 2 0.13 eV
(
1)
2
where E0,0* is the singlet energy of the pyrene chromophore
3
3.34 eV) and e is the dielectric constant of the solvent (e = 2.21 for
(
dioxane).
0
The DG ET values indicate that photoelectron transfer between
pyrene and BT is not thermodynamically feasible in dioxane
0
Table 1). However, exciplex formation (DG ex) in this solvent
(
would be favourable in the bichromophoric systems 2 and 3, as
2 3 8
estimated by eqn 2 using an average value of 0.75 eV for m /r .
0
+
2
DG2ex = Eox(D/D ) 2 Ered(A /A) 2 E0,0* 2
m /r {[(e 21)/(2e + 1) ] 2 0.19} + 0.38 eV
(
2)
3
Transient absorption studies were performed for the bichromo-
phoric compounds, and data were compared with those obtained
3
for the simpler model (Py-NH-COCH ).
Laser excitation (LFP) at 355 nm (Nd : YAG, 10 ns laser
pulse) of deaerated dioxane solutions showed for all the pyrenyl
compounds the typical transient absorption spectrum of the
Fig. 5 A: Transient absorption spectra of 3 in deaerated dioxane (A)
recorded 0.13 (&), 0.70 ($) and 3.87 (m) ms after the laser pulse. B: Triplet
decay traces (l = 355 nm, l = 520 nm) of 2 (#) and 3 (n) in
9
pyrene triplet (lmax at 420 and 520 nm) (see Fig. 5A for 3).
exc
obs
Intersystem crossing quantum yields were determined for all the
deaerated dioxane.
pyrenyl compounds by a comparative method using a deaerated
3
= 0.38) with the same
We thank the Ministerio de Educaci o´ n y Ciencia (Project
BQU2002-00377 and doctoral fellowship to M.G.B) and the
Generalitat Valenciana (Grupos 03/082) for generous support of
this work. S.-E.S. thanks also the MEC (RyC program) for support.
acetonitrile solution of pyrene (W
T
absorbance at the excitation wavelength. The obtained values
were higher for the bichromophoric compounds than for 1 (WT,
Table 1) and the triplet quantum yield for 3 approached the unity.
Since in dioxane photoelectron-transfer is thermodynamically
disfavoured, exciplex formation seems to enhance nonradiative
intersystem crossing to the pyrene triplet.
Notes and references
1 (a) C. Bohne, in Handbook of Organic Photochemistry and Photobiology,
ed. W. M. Horspool and P. S. Song, CRC Press, Boca Raton, 1994,
p. 416; (b) M. B. Rubin, in Handbook of Organic Photochemistry and
Photobiology, ed. W. M. Horspool and P. S. Song, CRC Press, Boca
Raton, 1994, p. 430; (c) J. P e´ rez-Prieto, R. E. Galian and M. A. Miranda,
Minirev. Org. Chem., in press.
In addition, decay traces were monitored at 520 nm; they
exhibited pseudo-first order kinetics. The triplet lifetime of the
bichromophores were shorter than that of the simple pyrenyl
derivative 1 (Table 1). Moreover, the intramolecular quenching
rate constants were estimated using eqn 3
2
(a) J. P e´ rez-Prieto, S. E. Stiriba, M. Gonz a´ lez-B e´ jar, L. R. Domingo and
M. A. Miranda, Org. Lett., 2004, 6, 3901; (b) J. P e´ rez-Prieto, F. Bosc a´ ,
R. E. Galian, A. Lahoz, A. L. R. Domingo and M. A. Miranda, J. Org.
Chem., 2003, 68, 5104; (c) R. S. Becker, G. Favaro, G. Poggi and
A. Romani, J. Phys. Chem., 1995, 99, 1410; (d) D. R. Arnold and
R. J. Birtwell, J. Am. Chem. Soc., 1973, 95, 4599.
T
0
T
k
q
= (1/t
T
2 1/t
)
(3)
0
T
where t
bichromophoric compounds or in Py-NH-COCH
A remarkable regiodifferentiation was found in the triplet decay
T
and t
are the pyrene triplet lifetimes in the
3
, respectively.
3
S. Murov, I. Carmichael and G. L. Hugin Handbook of Photochemistry,
2nd edn, Marcel Dekker, New York, 1993.
T
(
Fig. 5B), being the quenching rate constants ratio kq (Py-NH-SUP)
/
4 (a) N. Mataga, M. Migita and T. Nishimura, J. Mol. Struct., 1978, 47,
199; (b) J. F. Delouis, J. A. Delaire and N. Ivanoff, Chem. Phys. Lett.,
T
k
q (Py-NH-TPA) of 3.8/1.
1979, 61, 343; (c) S. Hashimoto, J. Phys. Chem., 1993, 97, 3662; (d)
In summary, the combination of the pyrene and benzoylthio-
phene units as found in compounds 2 and 3 constitutes an interest-
ing approach to design new bichromophoric photosensitizers with
increased intersystem crossing quantum yield and enhanced
selectivity. The potential of this strategy has been illustrated in
the present work by using a test reaction as a proof of the concept.
J. Kawakami and M. Iwamura, J. Phys. Org. Chem., 1994, 7, 31.
5 G. A. Crosby and J. N. Demas, J. Phys. Chem., 1971, 75, 991.
6
R. Gonz a´ lez-Luque, M. Merch a´ n, M. Rubio, L. Serrano-Andr e´ s,
B.-O. Roos and M. A. Miranda, Mol. Phys., 2003, 101, 1977.
A. Weller, Z. Isr. J. Chem., 1970, 259.
7
8
A. Weller, Z. Phys. Chem. Neue Folge, 1982, 130, 129.
9 W. E. Ford and M. A. J. Rodgers, J. Phys. Chem., 1992, 96, 2917.
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