Photoisomerization by Hula twist: 2,2′-Dimethylstilbene and a ring-fused analogue

Article abstract of DOI:10.1002/anie.200351763

Let's twist again: While the photoisomerization of stilbene analogues in fluid solution occurs by a one-bond-flip mechanism and involves only configurational changes, in a solid solution (glass) a Hula-twist mechanism operates and results in conformationa

Full text of DOI:10.1002/anie.200351763

Communications
Photoisomerization
The specific role of the Hula twist process in a general
mechanistic scheme that accounts for all the photoisomeriza-
tion reactions made it possible to design specific examples to
test the validity of the concept and allowed for the possibility
to further broaden its scope.[2] Now we report experimental
results on the first set of test systems: 2,2’-dimethylstilbene (1)
and a related ring-fused analogue 2[6] (which minimizes the
number of possible conformers). The location of the alkyl
substituent was chosen to facilitate the investigation of
conformational changes occurring during photoisomeriza-
tion. The results are relevant to current thoughts on the
photoisomerization of organic systems,[1,2] although it does
not necessarily relate to the protonated Schiff bases of the
visual chromophore.[3]
We previously predicted the photochemical reactions of
2,2’-substituted stilbenes 1 and the ring-fused stilbene 2 to be
those shown in Scheme 2.[2a,b] In a fluid solution, OBF
photoisomerization should lead to a stable conformer of the
isomerized photoproduct, while in a frozen solid solution, HT
photoisomerization should give an unstable conformer of the
isomerized product. The latter should be readily converted
into the stable conformer by warming.
Photoisomerization by Hula Twist: 2,2’-
Dimethylstilbene and a Ring-Fused Analogue**
Yasushi Imamoto, Takayuki Kuroda, Mikio Kataoka,
Sergey Shevyakov, G. Krishnamoorthy, and
Robert S. H. Liu*
Recently, the case for medium-dependent photoisomerization
from the S1 state was made;[1] it was argued that the one-
bond-flip (OBF) process (or torsional relaxation)[2] dominates
in solutions, but this volume-demanding process is unlikely in
confined media. Instead, a volume-conserving reaction mech-
anism that involves the simultaneous rotation around two
adjacent single and double bonds was suggested. The net
result was a CH group rotating out of the plane of a polyene
(which is not volume demanding; Scheme 1) and the process
was termed the “Hula twist”.[3,4] The concept was first
introduced to account for the rapid isomerization of the
confined visual chromophore in rhodopsin.[3] Interest was
recently rekindled after the successful demonstration of a
Hula twist in a simple organic system: pre-vitamin D in an
organic glass.[5]
We have now investigated the irradiation of isomers of 1
and 2 in 2-methylpentane both in a fluid solution (at 08C) and
in the glassy state (À1938C). The low-temperature irradiation
apparatus used in this study was essentially that
described in the literature.[7] The progress of the
photoreactions was followed by UV/Vis spectro-
scopy (Figures 1–4).
These sets of data show that all these com-
pounds indeed exhibit different photochemical
behavior in solvent media of different rigidity.
The difference absorption curves obtained during
irradiation of trans-1 and cis-1 (Figure 1d and
2d), or (E)-2 and (Z)-2 in fluid solutions at 08C
Figure 3d and 4d) were essentially mirror images
of each other, which reflects that the same
equilibrium mixture is reached from both direc-
tions. On the other hand, the absorption spectra
obtained during low-temperature irradiation of
(E)-2 and (Z)-2 (Figure 3a and 4a; less exactly
Scheme 1. Top : cis to trans isomerization by Hula twist (HT) with simultaneous conformational
reorganization and one CH unit rotating out of the plane. Bottom: cis to trans isomerization by
one-bond-flip (OBF) with half of the molecule rotating around a double bond.
for 1 because of the low conversion of the trans
isomer making the difference spectra (Figure 1c)
rather noisy) showed no mirror image relation-
ship. At high conversion, the absorption spectrum (calculated
after subtracting the residual amount of (E)-2) of the low-
temperature product from (E)-2 was found to be different
from that of (Z)-2 cooled to 80 K. At lower conversions, the
difference spectra substantially deviate from that of the stable
isomer. Warming the low-temperature product to 08C re-
sulted in its facile conversion into the stable product (Z)-2.
Such a conversion under the mild conditions of thawing and
warming to 08C can only suggest formation of unstable
conformers (namely, the hindered s-cis form) at low temper-
ature. In other words, the observed photochemical events of
these compounds (more clearly shown for (E)-2 and (Z)-2)
paralleled the reactions outlined in Scheme 2.
[*] Prof. Dr. R. S. H. Liu, S. Shevyakov, G. Krishnamoorthy
Department of Chemistry
University of Hawaii
2545 The Mall, Honolulu, HI 96822 (USA)
Fax: (+1)808-956-5908
E-mail: rliu@gold.chem.hawaii.edu
Dr. Y. Imamoto, T. Kuroda, Prof. Dr. M. Kataoka
Graduate School of Materials Science
Nara Institute of Science and Technology
Ikoma, Nara 630-0192 (Japan)
[**] The work was supported by a grant from the National Science
Foundation (CHE-32250) and a grant from Sekisui Integrated
Research Inc. We thank Dr. Alabugin for the calculated energies of
the conformers of 2. X.-S. Wang, H.-R. Li, L. U. Colmenares, and D.
Chang participated in the early part of the work described here.
Not surprisingly, the efficiency of the isomerization of
stilbene 1 favors reaction of the cis isomer over the trans
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DOI: 10.1002/anie.200351763
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Angewandte
Chemie
Scheme 2. Predicted photoisomerization pathways. Left: 2,2’-substituted stilbenes 1; right: ring-fused stilbene analogues 2. Horizontal: photoiso-
merization in solution by OBF; vertical: isomerization in frozen solution by HT; diagonal: warming and thawing of the solid solution containing
the HT product.
Figure 3. Progress of the photoreaction (l>280 nm) of (E)-2 at 80 K
in 2-methylpentane glass (a and c) and in 2-methylpentane solvent at
273 K (b and d). Changes in the absorption curves versus time (0, 5,
10, 20, 40, 80, 160, 320, 640, and 1280 s for a and b) and difference
absorption bands (AtÀA0) for the 80 K (c) and 273 K (d) data are
shown.
Figure 1. Progress of the photoreaction (l>280 nm) of trans-1 at 80 K
in 2-methylpentane glass (a and c) and in 2-methylpentane solvent at
273 K (b and d). Changes in the absorption spectra versus time (0, 5,
10, 20, 40, 80, 160, 320, 640, 1280, 2560, and 5120 s for a; the same
intervals to 640 s for b) and difference absorption bands (AtÀA0) for
the 80 K (c) and 273 K (d) data are shown.
Figure 2. Progress of the photoreaction (l>280 nm) of cis-1 at 80 K in
2-methylpentane glass (a and c) and in 2-methylpentane solvent at
273 K (b and d). Changes in the absorption curves versus time (0, 5,
10, 20, 40, 80, 160, 320, 640, 1280, 2560, and 5120 s for a; same inter-
vals to 2560 s for b) and difference absorption bands (AtÀA0) for the
80 K (c) and 273 K (d) data are shown.
Figure 4. Progress of the photoreaction (l>280 nm) of (Z)-2 at 80 K
in 2-methylpentane glass (a and c) and in 2-methylpentane solvent at
273 K (b and d). Changes in the absorption curves versus time (0, 5,
10, 20, 40, 80, 160, 320, 640, 1280, 2560, and 5120 s for a; same inter-
vals to 2560 s for b) and difference absorption bands (AtÀA0) for the
80 K (c) and 273 K (d) data are shown.
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Communications
isomer at low temperature in organic glass. However, the
However, conformational as well as configurational changes,
that is, by HT, occur in a solid solution.[1] These results
reinforce the suggestion that all photoisomerizations of
polyenes in rigid media (pre-vitamin D in organic glass,[5]
1,3-butadiene in argon matrix,[11] 1,3,5,7-octatetraene in an n-
octane matrix,[12] and selected cases of diarylethylenes in
organic glasses[13]) proceeded by way of HT.[1] However, at the
same time, we recognize that the current results do not
necessarily rule out the possibility that the isomerization at
room temperature could also proceed by way of HT.[14] The
fluid medium and the higher temperature would complicate
detection of the unstable conformer. We consider this
possibility unlikely; however, we intend to carry out similar
irradiation experiments on samples in constrained and uncon-
strained media, but at an identical reaction temperature.
trans isomer was not entirely photochemically inert, as
claimed for many other trans-1,2-diarylethylenes.[8] (At
80 K, the trans isomer is estimated to be about 100 times
less reactive than the cis isomer.) There are, however, distinct
differences in the low-temperature photochemical behavior
of the conformationally more restricted 2 relative to that of 1.
The presence of the extra ring made (E)-2 sterically cis-
stilbene-like. Therefore, the (E)-2 to (Z)-2 photoisomeriza-
tion also proceeded efficiently. The quantum yield of the
reaction was found to be 0.18 Æ 0.02.[10] On the basis of the
relative irradiation periods, we believe that this value is a
good reflection of the ease of isomerization of cis-diaryl-
ethylenes in low-temperature glass.
Changes in the absorbances of isomers of 1 and 2 versus
time are shown in Figure 5. It is clear that compounds trans-1,
cis-1, and (E)-2 all exhibit single exponential decay, which
Received: April 28, 2003 [Z51763]
Keywords: alkenes · isomerization · photochemistry ·
.
reaction mechanisms
[1] a) R. S. H. Liu, G. S. Hammond, Proc. Natl. Acad. Sci. USA
2000, 97, 11153 – 11158; b) R. S. H. Liu, Acc. Chem. Res. 2001,
34, 555 – 562; c) R. S. H. Liu, G. S. Hammond, Chem. Eur. J.
2001, 7, 4536 – 4544.
[2] See, for example, a) J. Saltiel, D. F. Sears, Jr., D.-H. Ko, K.-M.
Park in CRC Handbook of Organic Photochemistry and Photo-
biology (Eds.: W. M. Horspool, P.-L. Song), CRC, Boca Raton,
1994, pp. 1 – 15; b) W. J. Leigh in CRC Handbook of Organic
Photochemistry and Photobiology (Eds.: W. M. Horspool, P.-L.
Song), CRC Press, Boca Raton, 1994, pp. 123 – 142; c) H. J. C.
Jacobs, W. H. Laarhoven in CRC Handbook of Organic Photo-
chemistry and Photobiology (Eds.: W. M. Horspool, P.-L. Song),
CRC, Boca Raton, 1994, pp. 143 – 154.
Figure 5. Changes in the absorbance maxima during irradiation of
trans-1 (a), cis-1 (b), (E)-2 (c), and (Z)-2 (d) at 80 K in 2-methylpentane
glass and 273 K in 2-methylpentane solution. The closed and open cir-
cles represent absorbance changes at 80 and 273 K, respectively.
[3] a) R. S. H. Liu, A. E. Asato, Proc. Natl. Acad. Sci. USA 1985, 82,
259 – 263; b) R. S. H. Liu, D. T. Browne Acc. Chem. Res. 1986,
19, 42 – 48.
[4] For selected papers on the photoisomerization pathways, tor-
sional relaxation, and the Hula twist, see a) R. S. H. Liu,
Photochem. Photobiol. 2002, 76, 580 – 583; b) M. Uda, T.
Mizutani, J. Hayakawa, A. Momotake, M. Ikegami, R. Naga-
hata, T. Arai, Photochem. Photobiol. 2002, 76, 596 – 605; c) S.
Wilsey, K. N. Houk, Photochem. Photobiol. 2002, 76, 616 – 621;
d) M. Squillacote, T. Semple, J.-W. Chen, F. Liang, Photochem.
Photobiol. 2002, 76, 634 – 639.
indicates the reactions occur from a single species and that the
reactant is conformationally homogeneous at liquid-nitrogen
temperature. However, the decay of (Z)-2 is clearly not a
single exponential. In fact, the curve in Figure 5d suggests the
involvement of two species. Calculated results indeed support
that notion: the two s-E and s-Z conformers of (Z)-2 differ by
1.03 kcalmolÀ1 while for (E)-2, there was no other stable
conformer within 1.5 kcalmolÀ1 of the s-E conformer.[9]
[5] A. M. Muller, S. Lochbrunner, W. E. Schmid, W. Fuss, Angew.
Chem. 1998, 110, 520 – 522; Angew. Chem. Int. Ed. 1998, 37, 505 –
507.
1
[6] (E)-2: H NMR (300 MHz, CDCl3): d = 1.82 (dd, 2H), 2.30 (s,
3H), 2.58 (t, 2H), 2.87 (t, 2H), 7.01 (brs, 1H), 7.1–7.3 (m, 7H),
7.73 ppm (m, 1H); UV/Vis (hexane): lmax 273 nm, e =
9500 molÀ1 dm3 cmÀ1. (Z)-2: 1H NMR (300 MHz, CDCl3) : d =
2.02 (q, 2H); 2.23 (s, 3H), 2.58 (q, 2H), 2.90 (dd, 2H), 6.43 (m,
1H), 6.76 (dd, 1H), 6.88 (d, 1H), 6.9–7.3 ppm (m, 6H); UV/Vis
(hexane): lmax 275 nm, e = 10700 molÀ1 dm3 cmÀ1. As expected,
the Z isomer has a shorter retention time than the E isomer on
silica gel.
[7] T. Yoshizawa, Y. Shichida, Methods Enzymol. 1982, 81, 333 –
354.
[8] U. Mazzucato, F. Momicchioli, Chem. Rev. 1991, 91, 1679 – 1719.
[9] Calculations were performed by Drs. Manoharan and Alabugin
by using B3LYP6-31G* the density functional (DFT) method.
In conclusion, the results presented above demonstrate
conclusively that photoisomerization in fluid solution
involves only configurational changes, that is, by OBF.
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[10] The actinometer system (trans-stilbene, f = 0.49) was run at
room temperature, see J. Saltiel, A. Marinari, D. W.-L. Chang,
J. C. Mitchener, E. D. Megarity, J. Am. Chem. Soc. 1979, 101,
2982 – 2996.
[11] M. E. Squillacote, R. S. Sheridan, O. L. Chapman, F. A. L. Anet,
J. Am. Chem. Soc. 1979, 101, 3657 – 3659.
[12] J. R. Ackerman, S. A. Forman, M. Hussain, B. Kohler, J. Chem.
Phys. 1984, 80, 39 – 44.
[13] a) M. V. Alfimov, V. F. Razumov, A. G. Rachinsky, V. N. Lisvan,
Yu. B. Scheck, Chem. Phys. Lett. 1983, 101, 593 – 597; b) N.
Castel, E. Fischer, J. Mol. Struct. 1985, 127, 159 – 166.
[14] D. Sempedro Ruiz, A. Cembran, M. Garavelli, M. Olivucci, W.
Fuss, Photochem. Photobiol. 2002, 76, 622 – 633.
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