2226 Short Articles
Bull. Chem. Soc. Jpn. Vol. 80, No. 11, 2226–2228 (2007)
Results and Discussion
Isomer Specific Solvent Effect
on Photoisomerization of
Diels–Alder reaction of alkyne-substituted stilbene 315 with
four molar amounts of tetraphenylcyclopentadienone 416 re-
sulted in the corresponding stilbene-cored polyphenylene den-
drimer trans-1 in 18% isolated yield. The structure was iden-
Photoresponsive Polyphenylene
Dendrimers with a Stilbene Core
tified by H and 13C NMR and MALDI-TOF-MS spectrome-
1
try. After photolysis of trans-1 in hexane (Scheme 1), pure
cis-1 was separated from trans-1 by column chromatography.
trans- and cis-1 were soluble not only to benzene, chloro-
form, and THF, but also in hexane, due to the presence of
tetradodecyl groups. UV absorption spectra of trans- and
cis-1 were measured both in hexane and in benzene. The
spectra for trans-isomer were characterized by broad absorp-
tion bands tailing to 360 nm, whereas those of cis-isomer
around 300–360 nm were less intense due to its loss of planar-
ity at the core (Fig. 1a). Figure 1b shows the fluorescence and
fluorescence excitation spectra of trans-1 in hexane and ben-
zene. Fluorescence emission from pure cis-1 was not observed.
The UV absorption, fluorescence, and fluorescence excitation
spectra of trans-1 are similar to those of previously reported
trans-SPD, indicating that the peripheral alkyl groups in
trans-1 do not affect the absorption and fluorescence proper-
ties of the both dendrons and stilbene parts in the dendrimers.
The fluorescence quantum yield (ꢀf) for trans-1 was deter-
mined to be 0.69 in hexane and 0.70 in benzene, respectively
(Table 1), which is also similar to that of trans-SPD
(ꢀf ¼ 0:69).15
Tomoko Okamoto,1 Atsuya Momotake,1
Yoshihiro Shinohara,2 Ritsuko Nagahata,3
ꢀ1
and Tatsuo Arai
1Graduate School of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571
2Research Facility Center for Science and Technology,
University of Tsukuba, Tsukuba 305-8571
3Research Center of Macromolecular Technology,
National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba 305-8565
Received April 20, 2007; E-mail: arai@chem.tsukuba.ac.jp
The efficiency of cis-to-trans isomerization of stilbene-
cored polyphenylene dendrimer with alkyl chain at the pheri-
phery was one order of magnitude lower in hexane than in
benzene, whereas that of trans-to-cis isomerization was
slightly higher in hexane than in benzene.
Stilbene-cored polyphenylene dendrimer trans-1 exhibited
different photoisomerization behaviors depending on solvents.
Irradiation of trans-1 both in benzene and hexane resulted in
mutual trans–cis photoisomerization, as evidenced in Figs. 2a
and 2b by a decrease in the absorbance between 300–360 nm,
ascribed to the stilbene part. An isosbestic point in the absorp-
tion spectra during photoisomerization was clearly seen at
288 nm in benzene and at 271 nm in hexane in Figs. 2a and
2b, respectively. Inset in Figure 2 shows the time dependence
of the ratio of the trans-isomer during photoisomerization.
Under the experimental conditions used, a photostationary
state (PSS) was reached within 1–2 h of irradiation. The
amount of trans-isomer at the PSS was calculated in relation
to the absorbance of an unirradiated solution of pure trans-
and cis-isomers and those of the irradiated benzene solution
at 345 nm and the hexane solution at 325 nm. Thus, the values
of the trans- and cis-isomer ratio at the photostationary state
([t]/[c])pss were determined to be 12:88 in benzene and 3:97
in hexane. The ratio of the quantum yield for trans-to-cis
and cis-to-trans photoisomerization (ꢀt!c=ꢀc!t) were calcu-
lated by following equation:
The photochemical properties of stilbene and its analogous
have been the subject of intense research.1–10 As a part of our
study on the photochemistry of stilbenes, we have reported
that higher generation stilbene-cored dendrimers undergo
photoisomerization with moderate efficiency in solution.11–13
Furthermore, even a cross-linked 1,3,5-tristyrylbenzene den-
drimer, where the outside of the dendrons are cross-linked to
inhibit isomerization, has a photoisomerization quantum yield
(ꢀE!Z) of 0.063, which is similar to that for a model com-
pound (ꢀE!Z ¼ 0:080).14 We have also reported that stilbene-
cored polyphenylene dendrimers (SPD)15 undergo cis–trans
photoisomerization on photoirradiation in a benzene solution.
These compounds do not dissolve in an aliphatic nonpolar sol-
vent, such as hexane. Here, we report the synthesis and solvent
dependence of the photoisomerization behaviour of stilbene-
cored polyphenylene dendrimers 1 having a long alkyl chain.
Dendrimer 1 is soluble in an aliphatic nonpolar solvent, such
as hexane. In addition, we found that the efficiency of cis-to-
trans isomerization of 1 was strongly solvent-dependent and
one order of magnitude higher in benzene than in hexane.
The origin of this pronounced solvent effect on photoiso-
merization should be the introduction of long alkyl group at
the periphery, which makes the compounds soluble even in
hexane. Hexane may interact with this alkyl chain to induce
molecular aggregation among the less interactive polyphenyl-
ene groups and may suppress the efficiency of cis-to-trans
isomerization.
ð½cꢁ=½tꢁÞpss ¼ ð"t="cÞðꢀt!c=ꢀc!t
Þ
ð1Þ
where "t and "c are molar extinction coefficient of the trans-
and the cis-isomer, respectively, at the irradiated wavelength.
The values for "t="c were 22 in benzene and 10 in hexane.
The quantum yield for trans-to-cis photoisomerization of
trans-1 (ꢀt!c) in benzene was assumed to be the same as that
of trans-SPD (ꢀt!c ¼ 0:14 in benzene).15 The initial changes
in the absorbance at 345 nm of trans-1 upon photoirradiation at
325 nm in benzene and in hexane were compared to give the