The photochemical isomerization of cross-linked 1,3,5-tristyrylbenzene dendrimer with hula-twist mechanism

Article abstract of DOI:10.1016/j.tetlet.2005.03.015

A cross-linked 1,3,5-tristyrylbenzene dendrimer 5 was synthesized to study the photochemical isomerization mechanism. On irradiation, 5 isomerized with the quantum yields (Φ_E→Z) of 0.063, which was not very much different from that for a model compound 1 (Φ_E→Z 0.080), supporting a volume-conserving hula-twist (HT) mechanism during the photochemical E-Z isomerization.

Full text of DOI:10.1016/j.tetlet.2005.03.015

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3021–3024
The photochemical isomerization of cross-linked
1,3,5-tristyrylbenzene dendrimer with hula-twist mechanism
Mayuko Uda,a Atsuya Momotakeb and Tatsuo Araia,*
aGraduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba-city, Ibaraki 305-8571, Japan
bResearch Facility Centre for Science and Technology, University of Tsukuba 1-1-1, Tennodai, Tsukuba-city, Ibaraki 305-8571, Japan
Received 4 February 2005; revised 1 March 2005; accepted 3 March 2005
Available online 18 March 2005
Abstract—A cross-linked 1,3,5-tristyrylbenzene dendrimer 5 was synthesized to study the photochemical isomerization mechanism.
On irradiation, 5 isomerized with the quantum yields (UE!Z) of 0.063, which was not very much different from that for a model
compound 1 (UE!Z 0.080), supporting a volume-conserving hula-twist (HT) mechanism during the photochemical E–Z
isomerization.
Ó 2005 Elsevier Ltd. All rights reserved.
Photoisomerization of small molecules has been widely
studied and the isomerization mechanism in the excited
state has been revealed.1,2 For E–Z photoisomerization
of C@C double bond, one bond flip (OBF), and hula-
twist (HT) mechanisms have been proposed. In the con-
ventional OBF motion, the one half of the molecule
turns over through the perpendicular intermediate.
The HT mechanism, proposed by Liu and Asato in
1985, is the simultaneous isomerization of C@C double
bond and adjacent single bond.3 Since the HT pathway
needs much less reaction volume than OBF process, it
can be used to explain how the retinal is able to undergo
rapid photoisomerization in confined media4 or how a
small molecule isomerize in organic glass at ꢀ196 °C.5
In the photoisomerization of the dendritic stilbenoid
compounds,6–8 we have found experimental support of
rapid photoisomerization through the HT pathway, fol-
lowed by slow conformational change of the dendritic
substituents.9
As shown in Scheme 1, compound 2 was prepared by the
deprotection of 1 with BBr3 in dichloromethane. Den-
dron 3 was prepared from the corresponding alcohol,
which has been reported by Zimmerman et al.10 Tristyr-
ylbenzene dendrimer 4 was then obtained by coupling
reaction of 2 and 3 in the presence of K2CO3 and 18-
crown-6 ether in butanone, followed by cross-linking
reaction with GrubbÕs ruthenium catalyst11,12 in benzene
to give the cross-linked tristyrylbenzene dendrimer
E,E,E-5.13 After purification by flash column chroma-
tography and by GPC, the MALDI-TOF mass spectra
showed peaks for 12 fully cross-linked dendrimer
E,E,E-5 and for 11- and 10-cross-links. The ratio of
the mass peak intensity is 1.0:3.9:2.9 for 12-, 11-, and
1
10-cross-links, respectively. All the H NMR signals of
E,E,E-5 were broaden because of the existence of 10-
and 11-cross-linked dendrimers. Since the cross-linking
reaction with GrubbÕs catalyst can take place not only
between two adjacent alkenes but also proceeds alter-
nately,10 the 11- and 10-cross-linked dendrimers were
produced and were not separable by silica gel column
chromatography or by GPC. The reaction could not
be completed even when the reaction time was extended.
In this communication, we report comparison of Z- to
E-isomerization efficiency for a cross-linked 1,3,5-tristyr-
ylbenzene dendrimer 5 and its model compound 18 in
THF solution. Whereas only the HT mechanism allows
5 to isomerize, compound 1 can undergo photoisomer-
ization with both the OBF and the HT mechanisms.
Figure 1 shows the change in the absorption spectra of 1
and E,E,E-5 upon the UV irradiation at 330 nm. The
absorption spectra of 1 and E,E,E-5 before irradiation
were similar except around 280 nm, which is attributed
to the peripheral dendrons. The spectral change of both
1 and 5 were also very similar in the region at 300–
400 nm. On UV irradiation at 330 nm, 1 in THF at
*
Corresponding author. Tel.: +81 298 53 4315; fax: +81 298 53
6503; e-mail: arai@chem.tsukuba.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.015

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M. Uda et al. / Tetrahedron Letters 46 (2005) 3021–3024
Scheme 1. Synthesis of cross-linked dendrimer E,E,E-5.
However, it can be considered that the photochemical
isomerization only between E,E,E-5 and Z,E,E-5 takes
place at least in this conditions, due to the similar spec-
tral change of 1 and 5.15 The irradiated sample of 5 was
the mixture of 12-, 11-, and 10-cross-linked dendrimers.
Therefore, the isosbestic point for irradiation of 5 indi-
cates that the absorption spectra of E,E,E- as well as
Z,E,E-isomers of 12-, 11-, and 10-cross-links are very
similar to each other.
The corresponding time profiles for the absorbance
change during the photoirradiation of 1 and 5 moni-
tored at 330 nm are shown in Figure 2a. In the same
irradiation conditions, the decrease of the absorbance
for the cross-linked dendrimer 5 was slower than that
for compound 1, probably because the isomerization
with OBF mechanism is impossible in 5. Previously,
we have determined the quantum yield for the photo-
chemical E,E,E ! Z,E,E isomerization of 1 in the same
conditions (UE!Z 0.080).8,16 The initial changes in the
absorbance at 330 nm of 1 and 5, as shown in Figure
2b, were compared to give the quantum yield for the
photochemical isomerization of 5 (UE!Z 0.063).
The obtained similar spectral change for 5 to that for 1
in the same condition indicates that the isomerization of
5 takes place only between E,E,E- and Z,E,E-isomers
probably with HT mechanism (Fig. 3). Although 5 can
take only HT pathway, the UE!Z value for 5 was not
very different from that for 1. Since HT demands at least
small conformational change, the interior of the cross-
linked dendrimer should be flexible enough to allow
the core unit can smoothly undergo the isomerization
with HT process. Meanwhile, there are theoretical re-
ports where OBF and HT can be the competing relax-
ation processes of the excited Frank–Condon species.17,18
Although it is considered that OBF is favored in solu-
tion and HT is favored in confined media,4,19 the iso-
merization of 1 in solution at room temperature may
take place not only by OBF, but also by HT mechanism.
A recent paper by Fuß et al., who concluded that HT
can take place even in small molecules such as stilbene
without any substituents in gas phase, since HT motion
does not need any external constraint but is driven by an
internal force, which is the slope on the way through a
conical intersection (CI).20
Figure 1. (a) Change in the absorption spectra of 1 upon the
irradiation at 330 nm light from a 150 W Xenon lamp through a
monochrometer in THF under argon. (b) Change in the absorption
spectra of cross-linked dendrimer 5 in the same conditions.
room temperature under argon underwent mutual pho-
tochemical isomerization only between E,E,E- and
Z,E,E-isomers.8,14 As shown in the inset in Figure 1a,
the isosbestic point at 288 nm can be observed during
the irradiation, supporting that the isomerization takes
place between E,E,E- and Z,E,E-isomer. The isosbestic
point for 5 during the photoirradiation was found at
273 nm (inset in Fig. 1b), which it is not very clear
because the absorbance around 272–275 nm for both
E,E,E-5 and its photoproduct are almost identical.

M. Uda et al. / Tetrahedron Letters 46 (2005) 3021–3024
3023
Acknowledgments
This work was supported by a Grant-in-Aid for Scien-
tific Research on Priority Areas (417), a Grant-in-Aid
for Scientific Research (No. 16350005) and the 21st
Century COE Program from the Ministry of Education,
Culture, Sports, Science, and Technology (MEXT) of
the Japanese Government, by University of Tsukuba
Research Projects, Asahi Glass Foundation and JSR
Corporation.
References and notes
1. Saltiel, J.; DÕAgostino, J.; Megarity, E. D.; Metts, L.;
Neuberger, K. R.; Wrighton, M.; Safiriou, O. C. In
Organic Photochemistry; Chapman, O. L., Ed.; Marcel
Dekker: New York, 1973; 3, p 1.
2. Arai, T.; Tokumaru, K. Chem. Rev. 1993, 93, 23–29.
3. Liu, R. S. H.; Asato, A. E. Proc. Natl. Acad. Sci. U.S.A.
1985, 82, 259–263.
4. Liu, R. S. H.; Hammond, G. S. Proc. Natl. Acad. Sci.
U.S.A. 2000, 97, 11153–11158.
5. Krishnamoorthy, G.; Asato, A. E.; Liu, R. S. H. Chem.
Commun. 2003, 2170–2171.
6. Uda, M.; Mizutani, T.; Hayakawa, J.; Momotake, A.;
Ikegami, M.; Nagahata, R.; Arai, T. Photochem. Photo-
biol. 2002, 76, 596–605.
7. Momotake, A.; Hayakawa, J.; Nagahata, R.; Arai, T.
Bull. Chem. Soc. Jpn. 2004, 77, 1195–1200.
8. Uda, M.; Momotake, A.; Arai, T. Org. Biomol. Chem.
2003, 1, 1653–1657.
Figure 2. (a) Time dependence of the absorbance of 1 and 5 observed
at 330 nm upon the irradiation at 330 nm light in THF at room
temperature. (b) Initial change of the absorbance of 1 and 5 observed
at 330 nm.
9. Tatewaki, H.; Baden, N.; Momotake, A.; Arai, T.;
Terazima, M. J. Phys. Chem. B 2004, 108, 12783–2789.
10. Zimmerman, S. C.; Zharov, I.; Wendland, M. S.; Rakow,
N. A.; Suslick, K. S. J. Am. Chem. Soc. 2003, 125, 13504–
13518.
11. Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc.
1996, 118, 100–110.
12. Zuercher, W. J.; Hashimoto, M.; Grubbs, R. H. J. Am.
Chem. Soc. 1996, 118, 6634–6640.
13. Compound 3: 1H NMR (400 MHz, CDCl3) d 6.61 (2H, d,
J = 2.2 Hz, ArH), 6.56 (4H, d, J = 2.2 Hz, ArH), 6.53 (1H,
t, J = 2.2 Hz, ArH), 6.41 (2H, t, J = 2.2 Hz, ArH), 5.80–
6.00 (4H, m, CH2@CH), 5.08–5.18 (8H, m, CH2@CH),
4.93 (4H, s, ArOCH2Ar), 4.39 (2H, s, CH2Br), 3.99 (8H, t,
J = 7.0 Hz, ArOCH2), 2.48–2.58 (8H, m, CH2). Com-
pound 4: 1H NMR (400 MHz, CDCl3) d 7.56 (3H, s,
ArH), 7.13 (6H, s, CH@CH), 6.39–6.73 (63H, m, ArH),
5.82–5.89 (24H, m, CH2@CH), 5.05–5.15 (48H, m,
CH2@CH), 4.90–4.99 (36H, m, ArCH2OAr), 3.90–3.99
(48H, m, RCH2OAr), 2.24–2.50 (48H, m, CH2). MALDI-
TOFMS: calcd for C252H276O42Na [M+Na]+: m/z 3993.9.
Found: 3996.9. Compound 5: 1H NMR (400 MHz,
CDCl3) d 7.56 (3H, br s), 7.13 (6H, br s), 6.30–6.70
(63H, m), 5.50–5.89 (24H, m), 4.90–5.18 (36H, m), 3.95
(48H, br s), 2.47 (48H, br s). MALDI-TOFMS calcd for
C
228H228O42Na [M+Na]+ (12-cross-links): m/z 3660.6.
Found: 3664.8. C230H232O42Na [M+Na]+ (11-cross-links):
m/z 3691.2. Found: 3691.8. C232H236O42Na [M+Na]+ (10-
cross-links): m/z 3719.3. Found: 3719.8.
14. In 1H NMR spectrum of the irradiated sample of
compound 1, only Z,E,E-isomer was observed as a single
product. For example, the singlet peak d 7.14 (6H, s, trans-
CH@CH–) of compound 1 changed only to three peaks
after irradiation: d 7.04 (2H, d, J = 16 Hz, E-CH@CH),
6.91 (2H, d, J = 16 Hz, E-CH@CH), and 6.62 (2H, s, Z-
CH@CH).
Figure 3. Schematic drawing of photochemical E,E,E ! Z,E,E iso-
merization of 5 with the hula-twist (HT) mechanism.
In conclusion, we showed the clear support of HT mech-
anism during the photochemical E–Z isomerization in a
cross-linked tristyrylbenzene dendrimer.

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M. Uda et al. / Tetrahedron Letters 46 (2005) 3021–3024
15. E,E,E-1 underwent photochemical isomerization to Z,E,E-
1 giving a mixture of E,E,E- and Z,E,E-1 at the photosta-
tionary state at room temperature under argon atmo-
sphere, as observed by 1H NMR spectroscopy of olefin
protons.14 Thus, Z,E,E-1 undergoes photochemical isom-
erization to E,E,E-1 at room temperature. Z,Z,E-Isomer of
1 could be obtained when the photochemical isomerization
was carried out at high temperature (at 50 °C) or under
oxygen atmosphere. The study on the detail of the
isomerization including Z,Z,E-isomer is on going.
solution was deaerated by bubbling argon and was
irradiated to keep the conversion within 8%. The light
intensity was determined by tris(oxalato)ferrate(III) acti-
nometry. The concentration of each isomer in THF were
determined by HPLC.
17. Wlisey, S.; Houk, K. N. Photochem. Photobiol. 2002, 76,
616–621.
18. Ruiz, D. S.; Cembran, A.; Garavelli, M.; Olivucci,
M.; Fuss, W. Photochem. Photobiol. 2002, 76, 622–
633.
16. Quantum yield of E,E,E ! Z,E,E isomerization of 1 in
THF was determined on irradiation at 330 nm from a
150 W xenon lamp through a monochrometer. The sample
19. Liu, R. S. Acc. Chem. Res. 2001, 34, 555–562.
20. Fuß, W.; Kosmidis, C.; Schmid, W. E.; Trushin, S. A.
Angew. Chem., Int. Ed. 2004, 43, 4178–4182.