Mechanism of photoisomerization of 1-naphthyl-2-phenylethylenes in organic glasses

Article abstract of DOI:10.1111/j.1751-1097.2007.00183.x

Photoisomerization reactions of cis isomers of 1-β-naphthyl-2- phenylethylene, an o-methylated homolog and 1-α-naphthyl-2-phenylethylene in organic glasses at liquid nitrogen temperature were studied. Reactions were followed by changes in UV-absorption spectra of the irradiated samples. Formation of an unstable trans-photoproduct was detected only with the o-methylated homolog. The results are consistent with high regioselective Hula-twist photoisomerization at the benzylic sites of the three compounds examined. Calculated data on relative energies of the conformers of both the cis and the trans isomers are in agreement with the suspected conformational population of the starting materials and the photoproducts.

Full text of DOI:10.1111/j.1751-1097.2007.00183.x

Photochemistry and Photobiology, 2007, 83: 1436–1440
Mechanism of Photoisomerization of 1-Naphthyl-2-Phenylethylenes
in Organic Glasses
Lan-Ying Yang and Robert S. H. Liu*
Department of Chemistry, University of Hawaii, Honolulu, HI
Received 24 February 2007; accepted 1 June 2007; DOI: 10.1111 ⁄ j.1751-1097.2007.00183.x
ABSTRACT
Photoisomerization reactions of cis isomers of 1-b-naphthyl-2-
phenylethylene, an o-methylated homolog and 1-a-naphthyl-2-
phenylethylene in organic glasses at liquid nitrogen temperature
were studied. Reactions were followed by changes in
UV-absorption spectra of the irradiated samples. Formation
of an unstable trans-photoproduct was detected only with the
o-methylated homolog. The results are consistent with high
regioselective Hula-twist photoisomerization at the benzylic
sites of the three compounds examined. Calculated data on
relative energies of the conformers of both the cis and the trans
isomers are in agreement with the suspected confor-
mational population of the starting materials and the photo-
products.
Figure 1. Mechanisms of photoisomerization of a model cis-triene
fragment: upper, torsional relaxation or one-bond-flip (OBF); middle,
bicycle-pedal (BP); lower, Hula-twist (HT). The characteristic stereo-
INTRODUCTION
Recently, there has been a renewed interest in search of the
exact mechanism of photoisomerization in confined media
chemical consequences for the three processes are the following: one
double-bond isomerization for OBF, two double-bond isomerization for
BPandadjacentsingle-anddouble-bondisomerizationforHT. Changes
of color red to blue indicates change of sidedness of that portion of the
molecule as a result of the photochemical transformation.
(
whether a chromophore embedded within a protein binding
cavity or a simple organic molecule trapped within a rigid
host, such as an organic glass or in a crystal). (See e.g., 1,2)
The question was first raised when the photoisomerization of
rhodopsin chromophore was found to be an extremely fast
process (subpico seconds) (3,4) seemingly incompatible with
the volume demanding, albeit commonly accepted, torsional
relaxation (also known as one-bond-flip [OBF]) process
taking place within a rigid binding cavity. Two volume-
conserving processes have since been introduced: the bicycle-
pedal (BP) process in which the middle two CH fragments of
a diene unit rotate simultaneously (5) and the Hula-twist
examples showed direct formation of the more stable
trans,trans isomer from the cis,cis isomer without detectable
intermediacy of the cis,trans isomer. The first example of a pair
of a double-bond and a single-bond isomerization was
reported by Fuss and coworkers on pre-vitamin D (9). Since
then many other examples of photochemical transformation
involving such stereochemical consequences have appeared in
the literature (10–13). At the same time there was a report
questioning the need to invoke the two bond isomerization HT
process in discussing results of the only example, 1-b-naphthyl-
2-phenylethylene, 1, used in that paper (14).
Recently, in a report on low temperature photoisomeriza-
tion of stilbene derivatives (15), the need to choose and design
systems carefully for such mechanistic studies was emphasized.
It was pointed out that the only chosen system in the said
report (14) is incapable of yielding information suitable for
establishing or eliminating any of the above-mentioned reac-
tion mechanisms. In this paper, we report results of our
investigation on low temperature photochemical reactions of
isomers of 1 and related systems (2 and 3).
(
HT) process in which one CH unit transposes across the
polyene skeleton (6) (Fig. 1). The stereochemical conse-
quences for the two processes are different, BP leading to
simultaneous isomerization of two adjacent double bonds
(configurational changes only), while HT to simultaneous
isomerization of a pair of adjacent double and single bonds
(
conformational as well as configurational changes).
Definitive examples of simultaneous isomerization of two
double bonds are rare. The first example was reported by
Matsumoto and coworkers on crystals of the muconate
(cis,cis) salt (7). More recently there were similar reports on
crystals of cis,cis-1,4-diphenyl-1,3-butadienes (2,8). All these
*
ꢀ
Corresponding author email: rshl@hawaii.edu (Robert S. H. Liu)
2007 The Authors. JournalCompilation. The AmericanSocietyofPhotobiology 0031-8655/07
Figures and schemes are in color in the online version of the superscript
manuscript, but appear in shaded gray in the printed version.
1
436

Photochemistry and Photobiology, 2007, 83 1437
reactions of these frozen samples. Sample cooling was slow
(ꢀ30 min to reach equilibrium) by way of heat conduction
through the metal framework of the sample holder.
RESULTS AND DISCUSSION
In addition to compound 1, we have now prepared the
o-methylated homolog 2 and the related a-naphthyl analog 3
and examined their photochemical reactions in EPA glass at
liquid nitrogen temperature. The results are shown below.
Low temperature photochemical reactions of 1 were first
reported some time ago (17,18). Photoisomerization in organic
glasses was described. However, formation of the unstable
product conformer(s) was interpreted in terms of the OBF
concept, the only recognized photoisomerization process avail-
able at that time. We have now repeated the low temperature
irradiation of 1 in an EPA glass with progress of reactions
followed by changes of UV absorption spectra. The trans isomer
of 1 (and that of the other two analogs) was found to be
photostable in low temperature glasses. Figure 2 summarizes
results obtained during irradiation of cis-1 in EPA glass at 77 K.
The immediate increase in absorption with a simultaneous
red-shift and the appearance of fine structures (Fig. 2a) is
characteristic of formation of the trans isomer. Upon warming
of the product mixture followed by recooling to 77 K, the
absorption spectrum remained virtually the same (Fig. 2b) (only
minor changes of relative heights of some of the fine vibrational
structures). The obvious conclusion of formation of stable
conformer(s) of the trans isomer (based on UV absorption
spectra) is similar to that reported earlier (based on fluorescence
studies) (14). However, formation of such a stable product(s) is
consistent with any one of the three reaction pathways: OBF
around the double bond, HT of the benzylic C–H unit or HT of
the C–H unit near the naphthyl group if a conformer mixture
similar to that of the trans isomer is obtained.
EXPERIMENTAL
Synthesis
Compounds 1–3 were prepared following standard Wittig
coupling reactions (16) of the corresponding Wittig salt and
arylaldehyde (obtained from Aldrich). In general, the trans
isomer was obtained in slight excess. Conditions for chromato-
graphic separation of the isomers were essentially those des-
cribed (15). The H NMR spectra, obtained on a Nicolet
3
00 MHz spectrometer, were most indicative of the structure
with expected larger vicinal coupling constants for the trans
isomers (ꢀ16 Hz) and smaller coupling constants for the cis
isomers (ꢀ12 Hz). UV spectra were recorded on a Varian 50
spectrometer.
Compound 1. H NMR (acetone-d
t, 2H), 7.40 (d, 1H), 7.43 (d, 1H) (J = 16.2 Hz from a sample
in toluene-d ), 7.48 (m, 2H), 7.65 (d, 2H), 7.86–7.91 (m, 4H),
6
): trans, 7.28 (t, 1H), 7.40
(
8
7
.99 ppm (s, 1H); UV (ether:isopentane:ethanol=5:5:2 [EPA])
max = 223.8, 272.3, 281.8 and 316.9 nm. cis, 6.73 (d, 1H,
k
J = 12.2 Hz), 6.80 (d, 1H, J = 12.1 Hz), 7.20–7.26 (m, 3H),
7.27 (d, 1H), 7.46 (m, 2H), 7.66 (d, 1H), 7.70–7.79 (m, 3H),
7.77 (s, 1H), 7.81–7.86 ppm (m, 1H). UV (EPA) kmax = 225.2,
268.9 and 301.0 nm.
Compound 2. H NMR (acetone-d ): trans, 2.47 (s, 3H), 7.21
6
The slight difference in the relative heights of the finer
structures between 330 and 350 nm is likely due to perturbed
population of the two conformers of the trans isomer from HT
photoisomerization of the two conformers of the cis isomer.
To clarify the situation, we investigated the low temperature
photochemical reaction of homolog 2, in which the additional
(
m, 4H), 7.30 (d, 1H, J = 16.4 Hz), 7.48 (m, 2H), 7.62 (d, 1H,
J = 16.3 Hz), 7.25 (d, 1H), 7.90 (m, 3H), 8.01 ppm (s, 1H);
UV (EPA) kmax = 223.9, 274.9 and 316.0 nm. cis, 2.28 (s, 3H),
6
(
7
(
.78 (d, 1H, J = 12.1 Hz), 6.85 (d, 1H, J = 12.2 Hz), 7.04
t, 1H), 7.11 (d, 1H), 7.18 (d, 1H), 7.17 (d, 1H), 7.22 (d, 1H),
.44 (m, 2H), 7.62 (d, 1H), 7.68 (s, 1H), 7.70 (m, 1H), 7.78 ppm
m, 1H); UV (EPA) kmax = 226.3 and 301.0 nm.
Compound 3. H NMR (acetone-d ): trans, 7.28 (d, 1H,
J = 16.2 Hz), 7.31 (t, 1H), 7.41 (t, 2H), 7.50–7.61 (m, 3H),
.74 (d, 2H), 7.87 (d, 2H), 7.93 (d, 1H), 8.08 (d, 1H,
6
7
J = 16.2 Hz), 8.39 ppm (d, 1H), UV (EPA) kmax = 227.3
and 321.6 nm; cis, 6.90 (d, 1H, J = 12.0 Hz), 7.08 (s, 5H), 7.09
(
2
d, 1H, J = 12.0 Hz), 7.33 (d, 1H), 7.38 (t, 1H), 7.47–7.56 (m,
H), 7.85 (d, 1H), 7.91–7.97 (m, 1H), 8.06–8.09 ppm (m, 1H);
UV (EPA) kmax = 225.5, 267.9 and 310 (sh) nm.
Irradiation Procedure
The Oxford low temperature cell holder (Optistat DN), used in
conjunction with a Varian 50 spectrometer, was the same as
that described before (13,15). A 100 W Xenon Xe–Hg arc
lamp was used as the irradiation light source. Corning O-54
filter was inserted for obtaining light of >310 nm for the more
red-shifted compound 3. Samples were not deoxygenated
because oxygen was found not to have a noticeable effect on
Figure 2. Low temperature photochemistry of 1. (a) Irradiation
(
tane ⁄ ethanol = 5:5:2) in a quartz cell at 77 K showing progressive
changes of UV–Vis absorption spectra; the final spectrum being in
blue. (b) The same final product absorption before (blue solid line) and
after warming to 200 K and recooling to 77 K (red dashed line).
100 W Xe–Hg lamp, no filter) of cis-1 in EPA glass (ether ⁄ isopen-

1
438 Lan-Ying Yang and Robert S. H. Liu
differ in their steric environments, making cis-2 (or cis-2¢) more
stable (hence dominant in an equilibrated mixture) than cis-2
0
0
1
(
by 1.13 kcal mol) as revealed from results from ab initio
calculations)* (20). This unequal conformer population should
lead to HT products enriched in the high energy conformer
(
more twisted, hence blue-shifted), which upon thawing of the
organic glass, reverses to one enriched in the stable conformer
more planar, red-shifted). (Conformer trans-2 was found to be
(
0
0
)1
more stable than trans-2 by 1.69 kcal mol )* (20).
Figure 3. Low temperature photochemistry of 2. (a) Irradiation
100 W Xe–Hg lamp, no filter) of cis-2 in EPA glass at 77 K showing
(
progressive changes of UV–Vis absorption spectra; the final spectrum
being in blue. (b) Final product absorption before (blue solid line) and
after warming to 200 K and recooling to 77 K (red dashed line). The
absorption spectrum of trans-2 (solid red line) is shown on top.
methyl group allows detection of conformational changes
nearby the phenyl group, following the thought process
outlined in earlier papers (13,15). Irradiation of cis-2 in EPA
glass also led to an immediate increase in intensity of the
absorption spectrum, however, producing a new structureless
broadband (Fig. 3a). Warming the product mixture followed
by recooling to 77 K resulted in a further increase in
absorption and the display of some vibrational structures
It should be equally clear that formation of the unstable
photoproduct does not agree with OBF photoisomerization
from the stable conformer of cis-2. Such a process can only
lead to the stable conformer of trans-2.
The combined results of cis-1 and cis-2 revealed a high
regioselectivity for HT photoisomerization favoring the ben-
zylic site. Similar selectivity was, in fact, detected in low
temperature photoisomerization of isomers of DPB (13) and a
cinnamate ester (21).
(
Fig. 3b). In fact, the final spectrum was the same as that of the
authentic sample of trans-2. This sequence of spectral changes
is reminiscent of octatetraene in n-octane crystal reported by
Ackerman et al. (19), trienes in pre-vitamin D (9) and that of
o,o¢-dimethyl-cis,trans-1,4-diphenyl-1,3-butadiene (DPB) in
EPA glass (13). Hence, similarly we interpret the result to
mean photochemical formation of an unstable conformer of
trans-2 followed by thermal equilibration upon thawing of the
glass to the stable conformer.
We have also examined low temperature photochemistry of
an a-substituted naphthalene analog. Earlier we showed that for
1
,1-dicyano-2-a-naphthylethylene (5), because of the expected
naphthyl-alkene conformational preference, photoirradiation
readily led to enrichment of the unstable product conformer. It
might be of interest to see whether such a conformational
preference will be revealed in low temperature photochemical
studies of an a-naphthylphenylethylene analog, such as 3.
This result can be readily rationalized by consideration of
conformational population controlled by relative stability of
conformers. There are four possible conformers for cis-2.
Earlier we showed in a low temperature photochemical
study of 1,1-dicyano-2-b-naphthylethylene (4) that two con-
formers close in energy are likely to be present (11).
Result of such a study of cis-3 is shown in Fig. 4. Low
temperature irradiation led to an immediate increase in
intensity and the red-shift of the UV absorption spectrum
(
Fig. 4a) reflecting formation of the trans isomer. However,
the absorption spectrum of the product did not change upon
warming and recooling, in fact, being identical to that of
stable trans-3 (Fig. 4b). In contrast to the same spectral
changes in Fig. 2b, there were no discernible changes of
relative heights of minor peaks. The apparent conversion to a
simple product conformer is consistent with the above-noted
conformational preference in such a-naphthyl analogs. These
For cis-2, conformers around the naphthyl single bond (e.g.,
cis-2 vs cis-2¢) have identical steric environment as in 4. Hence
they should both be present as an equilibrated mixture, again
making it difficult to detect (by the UV absorption method) an
increase of the high energy conformer formed from HT
photoisomerization. On the other hand, conformers around
*
We might mention that at ꢀ90 K, the approximate setting temperature of the
)1
EPA glass, an energy difference of 1.0 kcal mol should result in less than 1%
of the high energy conformer based on Boltzmann distribution.
0
0
000
the tolyl single bond (e.g. cis-2 vs cis-2 or cis-2¢ vs cis-2 )

Photochemistry and Photobiology, 2007, 83 1439
revealed in a recent crystal structure of bathorhodopsin, the
primary photoproduct (26).
Acknowledgement—The work was supported by a grant from the
National Science Foundation (CHE-0132250). We thank Professor J.
Head for his assistance in obtaining the calculated values of energy
differences between conformers.
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