Cation-recognized photosensitization in E-Z isomerization of 1,2-dichloroethylene by crowned benzophenones

Article abstract of DOI:10.1021/ja017764e

Photosensitized isomerization of 1,2-dichloroethylene with 15- and 18-membered-ring crowned benzophenones (1a and 1b) as triplet sensitizers resulted in the cation-dependent increase in the Z/E isomer ratios up to ca. 10 by addition of alkali and alkaline earth metal ions in comparison with the metal-free reactions (Z/E = 1.5-2.1). The highest Z/E values were attained for 1a with Li⁺ (2.9) and Mg²⁺ (5.3), and for 1b with Na⁺ (8.2) and Ca²⁺ (9.8), respectively, among each family of alkali and alkaline earth metal ions. It was found that the cation recognized sensitization brought about the change of the inherent triplet energy E_T as well as the Φ_E/Φ_z ratio to raise the Z/E isomer ratio of 1,2-dichloroethylene. Copyright

Full text of DOI:10.1021/ja017764e

Published on Web 05/15/2002
Cation-Recognized Photosensitization in E-Z Isomerization of
1,2-Dichloroethylene by Crowned Benzophenones
Ken Kokubo, Hidenobu Kakimoto, and Takumi Oshima*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan
Received December 14, 2001
The triplet photosensitizer opens a new pathway for many
attractive photoreactions by transferring the excited triplet energy
T
(E ) into a ground-state substrate to produce the active triplet
1
substrate. In connection with the development of a new versatile
sensitizer capable of varying E continuously, molecular recogni-
T
tion2 is one of the most promising techniques in that such
noncovalent interaction will produce the delicate electronic and
structural perturbation upon the conjunct chromophore function.3
Here, we report that the photosensitized isomerization of 1,2-
dichloroethylene (DCE) as a model olefin by using 15- and 18-
4
membered crowned benzophenones 1a and 1b causes the marked
increase of the Z/E ratio on photostationary state (PSS) up to ca.
10 by complexation of alkaline earth or alkali metal ions in
comparison with metal-free reaction as well as the reference
dimethoxybenzophenone 2 (eq 1).
Figure 1. Dependence of the Z/E ratio of DCE (10 mM) in the
photostationary state (λ >300 nm) on the concentration of Mg(ClO4)2 for
photosensitizer 1a (b) and reference 2 (O) (10 mM) in acetonitrile-d3 under
argon atmosphere. Time course of the photosensitized irradiation (λ ) 365
nm) of Z- (0) or E-DCE (9) by 1a with 5 equiv of Mg(ClO4)2 is shown in
the inset.
plays a crucial role in the rise of the Z/E value in PSS in light of
the well-known cation complexation of crown compounds.
We extended the cation-recognized photosensitization to all the
combinations of 1a or 1b (10 mM) with alkali and alkaline earth
metal perchlorates under the presence of a 5-fold excess of metal
+
salts (50 mM) except for less soluble alkali metal ions K (7 mM),
+
+
Rb (9 mM), and Cs (10 mM). The PSS for the isomerization
DCE is transparent for the Pyrex-filtered light and can be easily
1
1
was attained within 4-24 h on monitoring by H NMR. In these
identifiable by H NMR for each isomer: the Z-form resonates at
.60 ppm and the E-form at 6.54 ppm in acetonitrile-d , respectively.
A preliminary experiment with a solution of either Z- or E-DCE
10 mM) with equimolar 1a in a NMR tube was carried out by
irradiation of the filtered monochromated light (365 nm) from a
00 W ultrahigh-pressure mercury lamp in the absence or presence
of Mg(ClO under argon atmosphere. Both isomers reached
essentially the same value of Z/E ) 1.5 in the metal-free PSS, while
the presence of 5 equiv of Mg(ClO (50 mM) raised the isomer
+
+
+
cation excess conditions (except above K , Rb , and Cs ), most
of the crown-metal systems with high binding constants are thought
to be in the significant 1:1 complexation of crown function with
metal ion.5
The saturated Z/E values for 1a and 1b were plotted against the
metal ion radius along with the metal-free values ()1.5 and 2.1,
respectively) for convenience at ion radius zero (Figure 2). On the
6
3
(
,6
5
4 2
)
+
4
)
2
photosensitization by 15-membered 1a, the similar sized Li (0.60
Å) and Mg2 (0.65 Å) gave the highest Z/E values of 2.9 and 5.3,
respectively, among each family of alkali and alkaline earth metal
ions. These ions are rather smaller than the reported cavity size
+
ratio up to the asymptotic value of 5.3 (inset in Figure 1).
Keeping this in mind, we hereinafter carried out the photosen-
sitized isomerization using a 1:1 isomer mixture of DCE (10 mM)
and a Pyrex-filtered light (λ > 300 nm). First, we investigated the
2
(0.86-1.1 Å) of 15-membered-ring benzocrown ethers like 1a. A
similar photosensitization by 18-membered 1b provided the more
dependence of Z/E ratios on the concentration of added Mg(ClO
)
4 2
and found that the apparent saturation of Z/E value (∼5.3) was
enhanced and more acute peaks (Z/E ) 8.2 and 9.8, respectively)
satisfactorily attained at 2-5 equiv excess of Mg2 with respect to
+
+
2+
at almost the same size but smaller Na (0.95 Å) and Ca (0.99
Å). Thus, the observed ion-size selectivities in Z/E isomerization
are inconsistent with the expectation from the ion-in-the-hole
1
(
a (10 mM), while 2 brought about no increase in the isomer ratio
1.6-1.7) in the presence of Mg2 (5 equiv) (Figure 1). These
+
7
observations obviously indicate that the cation recognition of 1a
concepts. This is probably because in the present cation excess
3a
conditions the Z/E ratios are governed by not the binding constants
*
Corresponding author. E-mail: oshima@ch.wani.osaka-u.ac.jp.
but the efficiency of the differential energy transfer to each isomer.
6
548 J. AM. CHEM. SOC. 2002, 124, 6548-6549
9
10.1021/ja017764e CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
Figure 3. Saltiel plot of the Z/E ratio on PSS of 1,2-DCE vs various triplet
energies of sensitizers: common sensitizers 1-13 (b), 1a metals (4), and
Figure 2. Plots of Z/E ratios of DCE (10 mM) at the PSS vs the ion radius
of alkali (O, 4) and alkaline earth metal (b, 2) ions added (each 50 mM;
except [K ] ) 7, [Rb ] ) 9, [Cs ] ) 10 mM) in the presence of
photosensitizer 15-membered-ring 1a (solid line) and 18-membered-ring
1b metals (0).
+
+
+
the Saltiel plots in which DCE was found to suffer the most
1
b (dotted line) (10 mM) in acetonitrile-d3 under argon atmosphere.
-1
pronounced differential isomerization around 290 kJ mol , al-
though some sensitizers deviated from the ideal line (Figure 3).
Table 1. Quantum Yield (Φ) for the Photoisomerization of DCE
and Triplet Energy (ET) in the Presence of Sensitizer 1a and 1b
The present cation-recognized sensitizers which scattered only at
with or without Mg2 or Na
+
+
-1 9
a narrow range of E (276-289 kJ mol ) are rather separated
T
Φa
into each family of 1a and 1b on the left side slope, but both show
b/kJ mol-1
the abrupt increasing tendency of Z/E values with the increase of
sensitizer
cation
Z
E
Φ
E
/Φ
4.2
12
6.2
34
Z
E
T
Z/E
E
T
by cation complexation (see also Table 1).10
1a
1a
1b
1b
-
Mg
0.0010
0.015
0.0011
0.0014
0.0043
0.17
0.0067
0.047
276
289
271
275
1.5
5.3
2.1
8.2
2
+
In summary, photosensitization of 1,2-dichloroethylene with the
-
Na
crowned benzophenones 1a and 1b resulted in the cation-dependent
increase in the Z/E isomer ratios up to ca. 10 by addition of alkali
and alkaline earth metal ions compared with the metal-free reactions
+
a
ΦZ: Quantum yield for the isomerization from Z to E. Irradiated with
monochromated light (365 nm) using K3[Fe(C2O4)3] as an actinometer; the
(Z/E ) 1.5-2.1).
1
b
conversion of the each alkene was determined by H NMR. Estimated by
Acknowledgment. We are grateful to Prof. T. Ohno and Dr.
K. Nozaki (Osaka University) for phosphorescence spectra and
lifetime measurements.
9
phosphorescence spectra.
The quantum yields were obtained for the photoisomerization
) in the presence or absence of the
peak-top Mg2 for 1a and Na for 1b, respectively, together with
of Z- (Φ
z
) and E- DCE (Φ
E
+
+
References
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T
(Table 1). A survey of Table 1 gives some
values
(
1) (a) Turro, N. J.; Dalton, J. C.; Weiss, D. S. In Organic Photochemistry;
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observations: (1) both 1a and 1b yielded almost equal Φ
Z
(
∼0.001) for the Z-isomer in the absence of metal ion (2) but the
E-isomer suffered 4- (for 1a) and 6-times (for 1b) more efficient
photoisomerization than the Z-isomer, reflecting the higher Z/E ratio
for 1b than 1a, (3) interestingly, the addition of Mg2 into a solution
of 1a dramatically enlarged the quantum yield ca. 40-times for the
E-isomer and only 15-times for the Z-isomer, leading to the
considerably increased isomer ratio of 5.3, (4) however, the 1b-
(2) Steed, J. W.; Atwood, J. L. In Supramolecular Chemistry; John Wiley &
+
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3) (a) Tsuda, A.; Oshima, T. J. Org. Chem. 2002, 67, 1282. (b) Kokubo, K.;
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(
(4) Prepared by the method in the following: Stott, P. E.; Bradshaw, J. S.;
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+
Na sensitizer raised the quantum yield slightly 1.3- and moderately
-times, respectively, for Z- and E-isomers, although the highest
Z/E ratio ()8.2) is a result of the largest Φ /Φ ()34). With respect
to the increase of Φ in the presence of metal ions, possible reduction
of the radiationless process due to cation complexation can be ruled
out by almost the same lifetime of the triplet state of 1a (0.1 mM):
τ ) 24.7 µs for metal free and 23.7 µs for the presence of Mg2+ (3
mM) in acetonitrile, respectively. As expected, the addition of DCE
10 mM) to the above Mg2 solution shortened the lifetime (9.5
µs) due to the efficient energy transfer to DCE, although the lack
of Mg2 ion resulted in almost the same lifetime (23.5 µs) as in
the above metal free experiment.
Hammond and Saltiel et al. argued that the photosensitizer having
an appropriate E value causes the most enhanced increase of Z/E
T
ratio on account of the far more discriminating energy transfer on
7
(6) The alkali metal ions, K
+
-Cs , do not fit in the small cavity of 1a and
+
are to be appreciably sandwiched between two crown rings. However, no
E
Z
noticeable effect of such complexation was found in the Z/E ratios (Figure
+
and Mg2+ are also poorly complexed by larger 1b.
2). The smaller Li
(
7) Pedersen, C. J.; Frensdorff, H. K. Angew. Chem., Int. Ed. Engl. 1972, 11,
6.
1
(
8) (a) Hammond, G. S.; Saltiel, J.; Lamola, A. A.; Turro, N. J.; Bradshaw,
J. S.; Cowan, D. O.; Counsell, R. C.; Vogt, V.; Dalton, C. J. Am. Chem.
Soc. 1964, 86, 3197. (b) Herkstroeter, W. G.; Hammond, G. S. J. Am.
Chem. Soc. 1966, 88, 4769.
+
(
(9) Phosphorescence spectra were measured at 77 K in frozen acetonitrile
containing 1 (0.1 mM) in the absence or presence of 30 equiv of metal
perchlorates by 300 nm excitation. These measurements gave the
structureless broad spectra regardless of the absence or presence of metal
+
T
ions, so the adopted E values were obtained from the onset ones by
-1
subtracting 10.5 kJ mol , which was the average energy difference
between the peak onset and the apparent 0-0 band of the above common
sensitizers (1, 5, 7, and 9 in Figure 3).
(
10) This means that the present sensitizer 1 with the E around 280 ( 10 kJ
T
-1
mol is not useful for the differential isomerization of olefins with
1
relatively too large or too small E
T
value like stilbene (206 kJ mol- ).
8
each isomer. A preliminary experiment with 13 common triplet
-
1
sensitizers with a wide range of E
T
(227-311 kJ mol ) provided
JA017764E
J. AM. CHEM. SOC.
9
VOL. 124, NO. 23, 2002 6549