Effects of the terminal-methyl position and of the length of the conjugated chain on the cis isomers produced by photo-isomerization: Analysis by HPLC and configurational determination by 1H-NMR spectroscopy of isomeric analogues of retinal and β-apo-12′-carotenal

Article abstract of DOI:10.1016/S1386-1425(96)01863-X

An analogue of retinal (β-apo-12′-carotenal), its terminal-methyl group being shifted from the 13 to 14 (from 13′ to 14′) position, was subjected to direct (iodine-sensitized) photo-isomerization. The configurations of five (seven) isomers of the retinal (β-apo-12β-carotenal) analogue were determined by ¹H-NMR spectroscopy to be all-trans, 7-, 9- and 11-mono-cis and 9,11-di-cis (all-trans, 9-, 13- and 13′-mono-cis and 9,13-, 9,13′- and 13,13′-di-cis). Comparative direct photo-isomerization in acetonitrile of the above parent and analogue aldehydes showed that the kinds and the amounts of cis isomer produced in the stationary-state mixtures are drastically affected by the terminal-methyl position and by the length of the conjugated chain. The results are discussed in terms of enhanced polarization upon excitation in the terminal C=O and the neighbouring C=C bonds in the conjugated chain.

Full text of DOI:10.1016/S1386-1425(96)01863-X

Spectrochimica Acta Part A 53 (1997) 913–926
Effects of the terminal-methyl position and of the length of the
conjugated chain on the cis isomers produced by
photo-isomerization: analysis by HPLC and configurational
1
determination by H-NMR spectroscopy of isomeric analogues
of retinal and i-apo-12%-carotenal
Ying Hu a, Akio Okumura a, Yasushi Koyama a,*, Yumiko Yamano b,
Masayoshi Ito b
a Faculty of Science, Kwansei Gakuin Uni6ersity, Uegahara, Nishinomiya 662, Japan
b Kobe Pharmaceutical Uni6ersity, Motoyamakita-machi, Higashinada-ku, Kobe 658, Japan
Received 14 November 1996; accepted 27 November 1996
Abstract
An analogue of retinal (i-apo-12%-carotenal), its terminal-methyl group being shifted from the 13 to 14 (from 13%
to 14%) position, was subjected to direct (iodine-sensitized) photo-isomerization. The configurations of five (seven)
1
isomers of the retinal (i-apo-12%-carotenal) analogue were determined by H-NMR spectroscopy to be all-trans, 7-,
9- and 11-mono-cis and 9,11-di-cis (all-trans, 9-, 13- and 13%-mono-cis and 9,13-, 9,13%- and 13,13%-di-cis). Compara-
tive direct photo-isomerization in acetonitrile of the above parent and analogue aldehydes showed that the kinds and
the amounts of cis isomer produced in the stationary-state mixtures are drastically affected by the terminal-methyl
position and by the length of the conjugated chain. The results are discussed in terms of enhanced polarization upon
excitation in the terminal CꢀO and the neighbouring CꢀC bonds in the conjugated chain. © 1997 Published by
Elsevier Science B.V.
Keywords: Photo-isomerization; H-NMR spectroscopy; Terminal-methyl position
1. Introduction
rhodopsin as protonated Schiff base and the 11-
cis to all-trans photo-isomerization triggers the
primary process in vision [1,2]. 15-cis-spheroidene
and 15-cis-i-carotene are bound to the photo-re-
action centers of a purple bacterium [3] and of
spinach photosystems II [4] and I [5]. The 15-cis
carotenoids are supposed to be involved in the
photo-protective function and to dissipate excess
Certain cis configurations of retinoids and
carotenoids play important roles in their physio-
logical functions: 11-cis-retinal is bound to
* Corresponding author. Tel.: +81 798 546389; fax: +81
798 510914.
1386-1425/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved.
PII S1386-1425(96)01862-X

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Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
Fig. 1. The structures of all-trans aldehydes having different lengths of the conjugated chain and different positions of the methyl
groups. (a) Retinal (C20 aldehyde), (b) 2,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)-2,4,6,8- nonatetraenal (C20% aldehyde), (c)
retinylideneacetaldehyde (C22 aldehyde), (d) i-apo-12%-carotenal (C25 aldehyde) and (e) 3,7,11-trimethyl-13-(2,6,6-trimethyl-1-cyclo-
hexenyl)-2,4,6,8,10,12-tridecahexenal (C25% aldehyde), and (f) i-apo-8%-carotenal (C30 aldehyde). Fragment structures, i.e. (g)
‘parallel-methyl fragment’ and ‘antiparallel methyl fragment’ are also defined with double-bond typifications. Arrows indicate the cis
configurations so far identified; the dotted arrow in C25 aldehyde indicates that the 13%-cis isomer can be produced not by direct
photo-isomerization but by I2-sensitized photo-isomerization; and the small arrow in C30 aldehyde indicates that the amount the
7-cis isomer is very small.
completely explain the absence of the 11-cis
configuration in the natural carotenoids.
triplet energy transferred from chlorophyll [6,7].
The selection of the 11-cis configuration by
rhodopsin and of the 15-cis configuration by the
photo-reaction centers is most probably due to
the extremely efficient, one-way isomerization into
the all-trans configuration [6]. An interesting ques-
tion to be addressed from this viewpoint is why
the 15-cis configuration, instead of the 11-cis
configuration, has been used in the physiological
functions of carotenoid.
The 11-cis isomer of carotenoid has never been
found to be produced from the all-trans isomer by
either photo- or thermal-isomerization [8]. Zech-
meister [9] attributed the fact to the severe steric
hindrance in the particular unmethylated-cis
configuration. However, the 7-cis [10] and 11-cis
[11] isomers of retinal can be produced by photo-
isomerization of the all-trans isomer in acetoni-
trile. Furthermore, the 7-cis isomers can be
thermally produced from the all-trans isomers in
i-carotene [12] and photolytically in i-apo-8%-
carotenal [13]. Thus, the steric hindrance does not
In relation to a more specific question why the
11-cis isomer can be produced photochemically
not in carotenoids but in retinoids, we have been
determining the cis–trans isomers which can be
produced in various aldehydes having a series of
different lengths of the conjugated chain. Fig.
1(a), (c), (d) and (f) summarizes the cis configura-
tions so far identified (those cis configurations are
indicated by arrows): the 11-cis isomer has been
found in retinal (hereafter called C20 aldehyde)
and in retinylidene aldehyde (C22 aldehyde) [14],
but not in i-apo-8%-carotenal [13]. In a previous
investigation, we searched for the 11-cis isomer in
i-apo-12%-carotenal (C25 aldehyde), but we could
not find it [15]. A discrete change has been found
between C22 aldehyde having six CꢀC and one
CꢀO double bonds and C25 aldehyde having seven
CꢀC and one CꢀO double bonds. Since all the
aldehydes have a similar structure, i.e. the i-
ionone ring at one end and the carbonyl group at

Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
915
2. Experimental
the other end, a most likely answer to the
above question has been the length of the
conjugated chain.
2.1. Synthesis of C20% aldehyde
Comparison of the aldehyde structures in
Fig. 1 shows that a definitive difference be-
tween the retinoids (C20 and C22 aldehydes)
and the apo-carotenals (C25 and C30 alde-
hyde) lies in the positions of the methyl
groups attached to the conjugated chain. The
retinoids contain only the C(CH3)ꢀCH–
CHꢀCH–C(CH3) structures (hereafter, we call
this ‘the parallel-methyl fragment’; see Fig.
1)(g)), but the carotenals additionally contain
the C(CH3)ꢀCH–CHꢀCH–CHꢀC(CH3) struc-
ture (we call this ‘the antiparallel-methyl
fragment’; Fig. 1(h)). Therefore, the terminal-
methyl position might affect the presence or
absence of the 11-cis configuration. Actually,
we have compared the ground-state and the
excited-state properties between C25 and C25%
aldehydes by means of NMR and resonance-
Raman spectroscopies, and found that the
position of the terminal-methyl group affects
not only the local electronic distribution in
the vicinity of the particular methyl group in
the ground state, but also the bond order of
the entire conjugated chain in both the sin-
glet- and the triplet-excited states [16].
In the present investigation, we first deter-
mined the cis isomers of C20% and C25% alde-
hydes produced by direct and I2-sensitized
photo-isomerization of the all-trans isomers,
respectively. (We used I2-sensitized photo-iso-
merization for C25% aldehyde, because the 13%-
cis isomer of C25 aldehyde could be obtained
only by this method.) Second, in order to
find how the terminal-methyl position affects
the kinds and the amounts of cis isomers
produced from the all-trans isomers, we com-
pared the isomeric compositions in the pho-
tostationary-state mixtures obtained by direct
photo-isomerization of the all-trans C20, C20%,
C25 and C25% aldehydes in acetonitrile. Fi-
nally, we tried to explain the results in
terms of ‘enhanced polarization of double
bonds upon excitation’ which takes place in
the conjugated chain.
All-trans C20% aldehyde, i.e. (2E, 4E, 6E, 8E)-
2,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)
-2,4,6,8-nonat etraenal, was synthesized by two
steps, i.e. (1) preparation of C20% ethyl ester from
all-trans-i-ionylidene acetaldehyde and (2) its
subsequent reduction and oxidation to the alde-
hyde; we followed the procedures in the previous
preparation of C25% aldehyde [16] with some mod-
ifications: (1) n-butyl lithium (13.6 mmol) in n-
hexane solution (8.05 ml; concentration, 1.69
mol/l) was added to a solution of ethyl 4-(di-
ethoxyphosphoryl)-2-methylbut-2-enoate
(13.6
mmol) in dry diethyl ether (40 ml). After stirring
for 20 min, a solution of all-trans-i-ionylidene
acetaldehyde (10.46 mmol) in dry diethyl ether (20
ml) was added dropwise and the mixture was
stirred for an additional 1 h. The rest of the
procedure was the same as described previously
[16] except for the eluent in silica-gel chromatog-
raphy (here, a 5:95 mixture of diethyl ether and
n-hexane was used). An isomeric mixture of C20%
ethyl ester (10.12 mmol) was obtained as orange
oil (yield, 97%). (2) A solution of C20% ethyl ester
(10.12 mmol) in dry diethyl ether (20 ml) was
added dropwise to a suspension of lithium alu-
minium hydride (10.12 mmol) in dry diethyl ether
(50 ml). The rest of the procedure was the same as
described previously [16] except for the amount of
manganese dioxide (30 g) and the eluent in silica-
gel chromatography (here, a 15:85 mixture of
diethyl ether and n-hexane). All-trans C20% alde-
hyde (8.38 mmol) was obtained as orange solid
particles (yield, 83%), whereas its isomeric mixture
(1.06 mmol) was obtained as orange oil (10.5%).
2.2. Isolation of the cis–trans isomers of C20% and
C25% aldehydes and measurements of their
electronic-absorption and NMR spectra
A set of cis–trans isomers of C20% aldehyde were
collected by means of high-pressure liquid chro-
matography (HPLC) from an isomeric mixture
which was obtained by direct photo-isomerization

916
Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
(light intensity, 4000 lux) of the all-trans iso-
mer in acetonitrile (5.0×10−5 M) at 0°C for
6 h under N2 atmosphere. HPLC conditions
were as follows: column, 4 mm i.d.×300 mm
for analysis and 8 mm i.d.×300 mm for sam-
ple collection; adsorbate, Daisogel Si sp-60, 5
mm; eluent, 7% diethyl ether in n-hexane; flow
rate, 1.0 ml min−1; and detection, 360 nm (for
analysis) and 380 nm (for collection). The pu-
rity of each isomer for NMR measurements
was estimated (by assuming the same m value
at the detection wavelength) as follows: all-
trans, 99%; 7-cis, 100%; 9-cis, 98%; 11-cis,
100%; and 9,11-cis, 96%.
A set of cis–trans isomers of C25% aldehyde
were collected by HPLC from an isomeric mix-
ture which was obtained by iodine-sensitized
photo-isomerization (light intensity, 2300 lux)
of the all-trans isomer (5.5×10−5 M) at room
temperature for 30 min under N2 atmosphere.
Exactly the same HPLC conditions were used
as in the case of C25 aldehyde [15] except for
the separation of the first three peaks (here,
the eluent, 5% diethyl ether in n-hexane was
used). The purity of each isomer for NMR
measurements was estimated as follows: all-
trans, 100%; 9-cis, 97%; 13-cis, 94%; 13%-cis,
98%; 9,13-cis, 98%; 9,13%-cis, 88%; and 13,13%-
cis, 93%.
3. Results and discussion
3.1. Isolation by HPLC and configurational
1
determination by H-NMR spectroscopy of the
cis–trans isomers of C20% aldehyde
Fig. 2(a) shows an HPLC elution profile for the
cis–trans isomers of C20% aldehyde. Peaks 1, 2, 3, 4
and 6 will be assigned to the 11-cis, 9-cis, 9,11-cis,
7-cis and all-trans isomers (vide infra); Peak 5 is
left unknown due to its low yield, and Peak 7
turned out to be a degradation product. C20%
aldehyde exhibits the order of elution of the
mono-cis and all-trans isomers as follows: 11-cis,
9-cis, 7-cis and all-trans. It is interesting that the
order of elution (among the isomers identified)
follows the positional order of the cis bend from
the carbonyl end to the i-ionone end. The order
of elution is 13-cis, 11-cis, 9-cis, 7-cis and then,
all-trans in C20 [17] and C22 [14] aldehydes. It is
9-cis, 7-cis and all-trans in C15 and C17 aldehydes
[14], whereas it is 13%-cis, 15-cis, 13-cis, 9-cis and
all-trans for C30 aldehyde [13]. It is a general trend
that the retention time becomes longer when the
all-trans fragment having the carbonyl end be-
comes longer. Most probably, the order is due to
the van der Waals interaction between this all-
trans fragment and the microscopically-flat sur-
face of the adsorbate, silica-gel.
For comparative direct photo-isomerization
of C20, C20%, C25 and C25% aldehydes in acetoni-
trile, the following conditions were used: tem-
perature, 0°C; time of irradiation,
6
h;
concentration, 5.0×10−5 M for C20 and C20%
aldehydes and 6.0×10−5 M for C25 and C25%
aldehydes.
Exactly the same apparatus and conditions
as used in the previous investigations [15,16]
were used for the measurements of electronic
absorption and NMR spectra. The electronic
absorption spectra of C20 and C20% aldehydes
1
were recorded in n-hexane. The H-NMR spec-
tra of the set of cis–trans isomers of C20% and
C25% aldehydes were recorded in chloroform-d1
(CEA, 99.6%) and benzene-d6 (CEA, 99.6%),
respectively. The 1H- and 13C-NMR spectra of
all-trans C20 and C20% aldehydes were also
recorded in benzene-d6 for comparison.
Fig. 2. The elution profiles of isomeric mixtures of (a) C20%
aldehyde obtained by direct photo-isomerization in acetoni-
trile, and (b) C25% aldehyde obtained by I2-sensitized photo-iso-
merization in n-hexane, both starting from the all-trans
isomer.

Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
917
Table 1
concave side due to increase in this interaction.
Each peak can be assigned as follows: Peak 4 can
be assigned to the 7-cis isomer because of the hfs
of 7H; Peak 2 can be assigned to the 9-cis isomer
on the basis of the hfs of 10H and the lfs of 8H
and 11H; Peak 1 can be assigned to the 11-cis
isomer based on the hfs of 11H and 12H as well
as the lfs of 10H and 13H; and finally, Peak 3 can
be assigned to the 9,11-cis isomer, because it
exhibits the isomerization-shift characteristics of
both the 9-cis (the lfs of 8H) and 11-cis (the hfs of
11H and 12H and the lfs of 10H and 13H)
configurations.
H-H COSY and NOE correlations detected in the 2D spectra
of all the all-trans isomer
Table 1 lists the COSY correlations for all-trans
C20% aldehyde which were used for the assignment
1
1
of its H signals (hereafter, H will be abbreviated
as H); the case of the all-trans isomer is shown,
for example. The COSY peaks correlate the
neighbouring olefinic Hs, whereas the long-range
COSY peaks correlate the methyl H (hereafter,
abbreviated as Me) and the neighbouring olefinic
Hs. The carbonyl 15H in the low-field region was
used to start the set of assignments. Table 2 lists
the chemical shifts of the olefinic Hs for this
all-trans configuration, which will be used as the
reference in the definition of the isomerization
shifts (vide infra). The NOE correlations found in
the NOESY spectrum (see both Table 1 and Fig.
3(a)) were used to establish the all-trans configu-
ration of this isomer. The correlations between
1Me and 7H as well as between 5Me and 8H
guarantee the s-cis configuration around the 6C–
7C axis and the rest of the correlations prove the
trans-zigzag configuration of the conjugated
chain.
Table 2 also lists the chemical shifts of the
olefinic Hs for the set of cis isomers. (The H
signals were assigned by the use of COSY correla-
tions as in the case of the all-trans isomer.) ‘The
isomerization shifts’, which are defined by the
changes in the H chemical shifts on going from
the all-trans isomer to a particular cis isomer, are
listed in parentheses; larger shifts are underlined.
They have been used for configurational determi-
nation of the cis isomers on the following basis:
when a cis configuration is introduced, the high-
field-shifts (hfs) take place in the convex side due
to decrease in the steric interaction between Hs
and the low-field-shifts (lfs) take place on the
Fig. 3 depicts the NOE correlations which have
been used to confirm the above configurational
assignments (see the structure of each cis isomer):
(b) The 7-cis configuration of Peak 4 is confirmed
by the NOE correlation between 5Me and 9Me;
(c) the 9-cis configuration of Peak 2 is confirmed
by the NOE correlation between 8H and 11H; (d)
the 11-cis configuration of Peak 1 is confirmed by
the NOE correlation between 10H and 13H; and
(e) the 9,11-di-cis configuration of Peak 3 is confi-
rmed by the NOE correlations between 8H and
11H as well as between 10H and 13H. The s-cis
configuration around the 6C–7C axis is proved
for all the isomers by the NOE correlations be-
tween 1Me and 7H as well as between 5Me and
8H (5Me and 9Me in the case of the 7-cis isomer).
The rest of the NOE correlations in each cis
isomer establishes the configuration of its all-trans
part(s).
Table 3 (upper part) lists the vicinal coupling
constants for the olefinic Hs. All of the vicinal
coupling constants through the conjugated single
bonds are 12 Hz. Concerning the vicinal coupling
constants through the trans double bonds, those
of 7Hꢀ8H are 16 Hz and those of 11Hꢀ12H are
14ꢀ15 Hz; this difference indicates increased
conjugation in the central part. The vicinal cou-
pling constants for the unmethylated-cis double
bonds, i.e. those of the 7Hꢀ8H bond in the 7-cis
isomer and of the 11Hꢀ12H bond in both the
11-cis and the 9,11-cis isomers, are all 12 Hz.
These values approximately agree with those of
7-cis [10] and 11-cis-retinal [18]; and 7-cis-i-
carotene [12]. All of the above values of coupling

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Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
Table 2
Chemical shifts (isomerization shiftsa) of the olenic H signals of isomeric C20% aldehyde in chloroform-d1 (ppm)
Peak 6 (all-trans)
Peak 4 (7-cis)
Peak 2 (9-cis)
Peak 1 (11-cis)
Peak 3 (9,11-cis)
7H
6.33
6.15
6.21
7.03
6.64
6.95
9.43
5.98
(−0.35)
6.12
(−0.03)
6.26
(+0.05)
6.96
(−0.07)
6.61
(−0.03)
6.94
(−0.01)
9.42
6.32
(−0.01)
6.67
(+0.52)
6.12
(−0.09)
7.12
(+0.09)
6.58
(−0.08)
6.93
(−0.02)
9.41
6.34
(+0.52)
6.22
(−0.07)
6.62
(+0.41)
6.82
(−0.21)
6.41
(−0.23)
7.39
(+0.44)
9.49
6.35
(+0.02)
6.68
(+0.53)
6.53
(+0.32)
6.91
(−0.12)
6.36
(−0.28)
7.39
(+0.44)
9.50
8H
10H
11H
12H
13H
15H
(−0.01)
(−0.02)
(+0.06)
(+0.07)
a Isomerization shifts larger than 0.08 are underlined.
constants support the set of assignments of the H
signals and of the cis–trans configurations.
unique CHꢀC(CH3)–CHꢀO structure of C25 alde-
hyde (instead of the C(CH3)ꢀCH–CHꢀO struc-
ture of C25% aldehyde) may enhance the
intermolecular interaction between 13%-cis C25
aldehyde (see Fig. 3(e) of [15] for its structure)
and the flat surface of silica-gel, and give rise to a
longer retention time.
3.2. Isolation by HPLC and configurational
1
determination by H-NMR spectroscopy of the
cis–trans isomers of C25% aldehyde
Table 4 lists the chemical shifts of the olefinic
Hs and the isomerization shifts (in parentheses)
for isomeric C25% aldehyde. Here also, COSY cor-
relations were used to establish the assignments of
the H signals (data not shown). The following
configurational assignments of the cis isomers
have been made on the basis of the isomerization
shifts: Peak 6 can be assigned to the 9-cis isomer
on the basis of the hfs of 10H as well as the lfs of
8H and 11H; Peak 4 can be assigned to the 13-cis
isomer based on the hfs of 14H and the lfs of 12H
and 15H; and Peak 2 can be assigned to the
13%-cis isomer based on the hfs of 13%H and the lfs
of 12%H and 15%H. Concerning the di-cis isomers,
Peak 5 can be assigned to the 9,13-cis isomer,
because it exhibits the isomerization shifts charac-
teristic of both the 9-cis configuration (the hfs of
10H and the lfs of 8H and 11H) and the 13-cis
configuration (the hfs of 14H and the lfs of 12H
and 15H). Peak 3 can be assigned to the 9,13%-cis
isomer; it shows the isomerization-shift character-
istics of both the 9-cis configuration (the hfs of
Fig. 2(b) shows the HPLC elution profile for
the cis–trans isomers of C25% aldehyde. Peaks 1, 2,
3, 4, 5, 6 and 8 will be assigned to the 13,13%-cis,
13%-cis, 9,13%-cis, 13-cis, 9,13-cis, 9-cis and all-trans
isomers (vide infra). 1H-NMR spectroscopy of
Peak 7 showed that it is a degradation product.
The order of elution concerning the mono-cis and
all-trans isomers is: 13%-cis, 13-cis, 9-cis and all-
trans. In C25 aldehyde [15], it was 15-cis, 13-cis,
9-cis, all-trans and then 13%-cis. The position of
the terminal-methyl group drastically affects both
the kind of cis isomers produced (vide infra) and
the order of elution (see Fig. 1 for the chemical
structures of these aldehydes). Concerning the
order of elution, the general trend that an isomer
having a cis bend on the carbonyl side elutes
faster than an isomer having it on the i-ionone
side (vide supra) holds in the above pair of alde-
hydes except for the case of the 13%-cis isomer.
Namely, the order of elution is 13-cis, 9-cis and
all-trans for C25% aldehyde, whereas it is 15-cis,
13-cis, 9-cis and all-trans for C25 aldehyde. The

Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
919
Fig. 3. The configurations of a set of cis–trans isomers of C20% aldehyde determined in the present investigation: (a) all-trans (Peak
6), (b) 7-cis (Peak 4), (c) 9-cis (Peak 2), (d) 11-cis (Peak 1), and (e) 9,11-cis (Peak 3). NOE correlations which were identified in the
NOESY spectra are indicated by arrows.
10H and the lfs of 8H and 11H) and the 13%-cis
configuration (the hfs of 13%H and 15H, and the
lfs of 12%H and 15%H). Peak 1 can be assigned to
the 13,13%-cis isomer, because it exhibits the char-
acteristics of both the 13-cis configuration (the hfs
of 14H and the lfs of 12H and 15H) and the
13%-cis configuration (the hfs of 13%H and the lfs of
12%H and 15%H).
Fig. 4 depicts the NOE correlations which have
been used to confirm the above configurational
assignments: (a) The all-trans configuration of
Peak 8 is confirmed by the NOE correlations
between each methyl group attached to the conju-
gated chain and the neighbouring olefinic Hs on
both sides of the conjugated chain as well as those
between a pair of gem-type olefinic Hs. (c) The
13-cis and (d) the 13%-cis configurations of Peaks 4
and 2 are confirmed by the NOE correlations
between 12H and 15H and between 15%H and
12%H, respectively. (e) The 13-cis configuration in
the 9,13-cis isomer of Peak 5 is confirmed by the
NOE correlation between 12H and 15H; (f) the
9,13%-cis configuration of Peak 3 is confirmed by
the NOE correlations between 8H and 11H as
well as between 15%H and 12%H; and (g) the 13,13%-
cis configuration of Peak 1 is confirmed by NOE
correlations between 12H and 15H as well as
between 15%H and 12%H. (The NOE correlation
between 8H and 11H to confirm the 9-cis configu-
ration could be identified in neither the 9-cis nor
the 9,13-cis isomer (Fig. 4(b) and (e)): these corre-
lation peaks are supposed to appear in the diago-
nal region in the 2D spectra.) The s-cis
configuration around the 6C–7C axis for all the
cis-trans isomers is confirmed by the NOE corre-
lation between 1Me and 7H as well as between
5Me and 8H. The rest of the NOE correlations
shown in the figure confirm the configurations of
the all-trans parts of each cis isomer.
Table 3 (the lower part) lists a set of vicinal
coupling constants obtained for isomeric C25%
aldehyde. The coupling constants of 11Hꢀ12H (15
Hz) are smaller than those of 7Hꢀ8H (16 Hz),
indicating increased conjugation in the central
part of the conjugated chain. Those values indi-
cate also that no unmethylated-cis configurations

920
Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
are present in the set of cis isomers. The coupling
constants through the single bonds are in the
range of 11–12 Hz.
3.3. Effects of the terminal-methyl position and
the length of the conjugated chain on the
photostationary-state isomeric composition
Table 5 (above the broken line) compares the
amounts of the all-trans and mono-cis isomers in
the photostationary-state mixtures of C20, C20%,
C22, C25% and C25 aldehydes, which were obtained
by direct photo-isomerization of the all-trans iso-
mer in acetonitrile. Comparison of the chemical
structures of these aldehydes in Fig. 1 shows that
C20 and C25% aldehydes (C20% and C25 aldehydes)
can be classified into the same group, because the
former aldehydes (the latter aldehydes) contain
the parallel-methyl (antiparallel-methyl) fragment
on the carbonyl end. The difference in the posi-
tion of the terminal-methyl group, causing these
two different type of fragments, does affect the
kind of mono-cis isomers produced (see Table 5):
C20 aldehyde produces the 7-cis, 9-cis, 11-cis and
13-cis isomers, whereas C20% aldehyde does not
produce the 13-cis isomer and produces only the
7-cis, 9-cis and 11-cis isomers. Concerning the
longer pair of aldehydes, C25% aldehyde produces
the 9-cis, 13-cis and 13%-cis isomers, whereas C25
aldehyde produces the 9-cis, 13-cis and 15-cis
isomers; the 13%-cis isomer in the former is re-
placed by the 15-cis isomer in the latter. (Note
that I2-sensitized photoisomerization of C25 alde-
hyde additionally produces the 13%-cis isomer; see
below the broken line of the table.)
Fig. 5 depicts the compositions of the all-trans
and mono-cis isomers of C20, C20%, C25% and C25
aldehydes for the photostationary-state mixtures
in acetonitrile. If we consider the above-men-
tioned grouping of the aldehydes concerning the
fragments adjacent to the carbonyl group, we
notice the following systematic changes in compo-
sition on going from the former group (i.e. C20
and C25% aldehydes) to the latter group (i.e. C20%
and C25 aldehydes). Namely, (1) increase in the
amount of the 9-cis isomer; (2) increase in the
amount of the 11-cis isomer; and (3) disappear-
ance of the 13-cis isomer on going from C20 to

Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
921
Table 4
Chemical shifts (isomerization shiftsa) of the olefinic H signals of isomeric C25% aldehhyde in benzene-d6 (ppm)
Peak 8 (all-
trans)
Peak 6 (9-cis)
Peak 4 (13-cis) Peak 2 (13%-cis) Peak 5 (9,13-
cis)
Peak 3 (9,13%-
cis)
Peak 1 (13,13%-
cis)
7H
6.37
6.42
6.32
6.82
6.37
6.40
(+0.03)
7.08
(+0.66)
6.18
(−0.14)
7.09
6.39
(+0.02)
6.39
(−0.03)
6.26
(−0.06)
6.81
6.40
(+0.03)
6.42
6.41
(+0.04)
7.06
(+0.64)
6.13
(−0.19)
7.09
6.40
(+0.03)
7.08
(+0.66)
6.19
(−0.13)
7.10
6.40
(+0.03)
6.40
(−0.02)
6.26
(−0.06)
6.82
8H
10H
11H
12H
6.34
(+0.02)
6.83
(+0.01)
6.37
(+0.27)
6.33
(−0.01)
6.91
(+0.27)
6.87
(+0.28)
6.35
6.91
(−0.04)
10.02
(+0.54)
10.02
(+0.50)
10.02
(−0.02)
10.15
(+0.54)
10.17
12%H 10.03
10.17
(−0.01)
5.99
(−0.02)
6.03
(−0.01)
6.00
(−0.01)
5.86
(−0.17)
7.60
(+0.82)
5.97
(+0.14)
5.78
(−0.23)
6.06
(+0.03)
6.70
(−0.08)
7.10
(−0.01)
5.99
(−0.02)
5.82
(−0.21)
7.05
(+0.27)
5.94
(+0.12)
5.76
(−0.25)
6.06
(+0.03)
6.65
(−0.13)
7.07
(+0.14)
5.77
(−0.24)
5.88
(−0.15)
6.99
(+0.21)
7.01
13%H
14H
15H
15%H
6.01
6.03
6.78
6.03
6.74
(−0.04)
6.01
(−0.02)
(−0.06)
(+1.07)
(−0.09)
(+1.04)
(+0.98)
a Isomerization shifts larger than 0.09 are underlined.
C20% aldehyde correspond nicely to (1%) increase in
the amount of the 13-cis isomer, (2%) appearance
of (or in other words, increase in the amount of)
the 15-cis isomer, and (3%) disappearance of the
13%-cis isomer on going from C25% to C25 aldehyde.
These systematic changes indicate that the shift of
the terminal-methyl group closer to the carbonyl
end (changing the parallel-methyl fragment into
the antiparallel-methyl fragment) stabilizes the cis
configuration around ‘double bonds a and b’ in
the resultant antiparallel-methyl fragment (see
Fig. 1(h)); on the other hand, it destabilizes the cis
configuration around ‘double bond c’ in the par-
ticular fragment.
Fig. 1 summarizes all the cis configurations so
far detected; the results of the present investiga-
tion concerning C20% and C25% aldehydes are added
to those described in the Introduction section. It
was known that C20 and C22 aldehydes produce
the 11-cis isomer, but C25 and C30 aldehyde do
not. Present investigation has shown that C20%
aldehyde produces the 11-cis isomer, but C25%
aldehyde does not. Now it is clear that the posi-
tion of the terminal-methyl group is not relevant
to the presence or absence of the 11-cis isomer;
the key factor in the generation of the 11-cis
configuration (in a parallel-methyl fragment) must
be the length of the conjugated chain.
3.4. A possible explanation of the effects of the
terminal-methyl position on the
photostationary-state composition in terms of
‘enhanced polarization of double bonds upon
excitation’
3.4.1. General consideration
Presence or absence of a cis configuration in a
photostationary-state isomeric mixture, which is
obtained starting from the all-trans isomer, can be
determined by two factors: (1) whether isomeriza-
tion from the all-trans to the particular cis
configuration can take place upon photo-excita-
tion; and (2) whether the particular cis configura-
tion is stable enough against additional photo- or
thermal-isomerization back to the all-trans iso-
mer. Concerning factor (1), photo-isomerization
can take place via either the singlet- or the triplet

922
Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
Fig. 4. The configurations of a set of cis–trans isomers of C25% aldehyde determined in the present investigation: (a) all-trans (Peak
8), (b) 9-cis (Peak 6), (c) 13-cis (Peak 4), (d) 13%-cis (Peak 2), (e) 9,13-cis (Peak 5), (f) 9,13%-cis (Peak 3) and (g) 13,13%-cis (Peak 1).
NOE correlation which were identified in the NOESY spectra are shown by arrows.
excited state. However, the triplet-state isomeriza-
tion is unlikely under the present experimental
conditions (the solutions were not degassed and
no triplet sensitizers were added): actually, triplet-
sensitized photo-isomerization of C15, C17, C20
and C22 aldehydes produced only small amount of
cis isomers in the photo-stationary state, because
the cis-to-trans isomerization is extremely efficient
and the trans-to-cis isomerization is inefficient
[19]. The triplet-state isomerization has been as-
cribed to the formation of ‘the triplet-excited re-
gion’ located in the central part of the conjugated
chain, where drastic decrease in the bond order of
the central double bonds takes place [6,20]. Here,
in this paper, we focus our attention to isomeriza-
tion caused by excitation to the singlet-excited
state and in particular, to the excited-state poten-
tial barrier which the trans-to-cis isomerization
process to overcome. (The relative energies in the
S1 potential minima at various cis and trans posi-
tions are another factor to be examined in the
future.) We are going to propose an idea of
‘polarization of the conjugated double bonds
upon excitation’ which originates from the ex-
cited-state polarization of the terminal carbonyl
group as a factor to reduce the potential barrier
around each double bond in the process of photo-
isomerization in the singlet-excited state.
3.4.2. Comparison of the electronic structure
between C20 and C20% aldehydes
Fig. 6 compares the electronic-absorption spec-
tra of all-trans (a) C20 and (b) C20% aldehydes.
They exhibit the same absorption maximum but
different profiles. C20% aldehyde having an antipar-
allel-methyl fragment shows clearer vibrational
structures as C25 aldehyde does [16]. On the other
hand, the vibrational structure of C20 aldehyde
having a parallel-methyl fragment is not clear as
in the case of C25% aldehyde [16]. Thus, the elec-
tronic structures of all-trans C20 and C20% alde-

Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
923
Table 5
The amounts of mono-cis isomers (%) in stationary-state mixtures obtained by photo-isomerization of all-trans C20, C20%, C22, C25%
and C25 aldehydesa
Compounds
C20 aldehyde
C20% aldehyde
C22 aldehyde
all-trans
7-cis
9-cis
11-cis
13-cis
15-cis
13%-cis
Others
44.2
(40.3)
40.2
(33.8)
40.0
(33.4)
38.1
52.6
41.4
56.1
1.4
(1.3)
5.2
(4.4)
0.1
(0.1)
n.d.
n.d.
n.d.
n.d.
18.1
(19.3)
29.3
(28.9)
6.7
(6.6)
9.5
2.8
16.8
(26.0)
23.0
(32.9)
14.9
(21.6)
n.d
n.d.
n.d.
n.d.
12.2
7.3
(−)
2.3
(−)
1.1
(−)
13.1
2.7
19.9
8.6
(13.1)
n.d.b
37.2
(38.3)
34.1
37.2
16.2
11.3
C25% aldehyde
C25 aldehyde
C25% aldehyde
C25 aldehyde
n.d.
4.7
n.d.
2.1
5.2
n.d.
8.6
9.5
13.9
12.4
a Direct reading of the area under peak in the elution profiles detected at 360 nm (C20 and C20% aldehydes), 400 nm (C22 aldehyde)
and 420 nm (C25 and C25% aldehydes). Corrected values of the fraction of all-trans and mono-cis isomers, the sum of which are
normalized to 100%, are shown in parentheses. Direct photoisomerization in acetonitrite and I2-sensitized isomerization in n-hexane,
are shown above and below the broken line.
b n.d. indicates ‘not detectable’.
hydes as probed by the Ag“Bu transition are
almost the same.
mesomeric effect in the terminal CꢀO and the
neighbouring CꢀC bonds, (2) the inductive effect
of the terminal methyl group, and (3) the steric
effects between the methyl and olefinic Hs. Ex-
actly the same discussion applies to the present
case of C20 and C20% aldehydes; however it will not
be repeated here. In the present investigation, we
will use each value of C chemical shift as a
measure of the electron density on the relevant
carbon atom; in other words, we will use a pair of
chemical-shift values in a carbon–carbon double
bond as a measure of its polarization. The values
in Fig. 7(a) lead us to the following conclusions:
Fig. 7 compares the values of 13C (hereafter
abbreviated as C) and H chemical shifts between
all-trans C20 and C20% aldehydes as functions of
atomic position. Changes in both the C and H
chemical shifts on going from C20 aldehyde (open
circles) to C20% aldehyde (closed circles) parallel
very closely to those on going from C25% aldehyde
to C25 aldehyde (see Fig. 3 [16]). The changes in
the latter pair of aldehydes were discussed in
detail, in a previous paper [16], in terms of (1) the
Fig. 5. The stationary-state isomeric compositions in the direct
photo-isomerization in acetonitrile starting from the all-trans
isomers of C20, C20%, C25% and C25 aldehydes. Open spaces
indicate the rest of the isomers.
Fig. 6. The electronic absorption spectra of (a) C20 and (b) C20%
aldehydes.

924
Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
nal) upon excitation to the Bu state. The dipole
moment increased by 13.290.6 D upon excita-
tion, and the solution-phase dipole moment in the
excited state was estimated to be 19.890.7 D. On
the other hand, increase in the long-axis polariz-
3
˚
ability upon excitation was 6009100 A . The
results suggest that strong polarization can take
place upon excitation of the present aldehydes to
the Bu state, especially in a polar solvent. On the
other hand, the generation of the 11-cis isomer by
direct photo-excitation of the all-trans isomer
takes place not in a non-polar solvent, n-hexane,
but in a polar solvent, acetonitrile [10,11]. This
observation strongly supports the idea that the
polarization of the conjugated chain must play a
key role in isomerization 6ia the singlet-excited
state. Here, we propose a hypothetical mechanism
of photo-isomerization, which includes (i) strong
polarization of the carbonyl group upon excita-
tion in the conjugated double bonds, and (ii) its
propagation to the neighbouring conjugated dou-
ble bonds toward the i-ionone ring.
Now, we try to explain, in terms of this polar-
ization mechanism, the effect of the terminal-
methyl position on the photostationary-state
isomeric composition. Here, (1) we correlate both
the magnitude and the direction of polarization in
a conjugated double bond (in the ground state) to
the magnitude of its enhanced polarization upon
excitation, and then, (2) we correlate this excited-
state polarization to the amounts of cis isomers
produced. Explanations are as follows (see Figs.
1, 5 and 7(a)): (a) the polarization of the 13Cꢀ14C
bond, which is parallel to that of the 15CꢀO
bond, is increased on going from C20% to C20
aldehyde (vide supra). An assumption that this
trend is enhanced upon singlet excitation can
explain the fact that the 13-cis isomer can be
produced not in C20% aldehyde but in C20 alde-
hyde, because stronger polarization must be more
advantageous for the trans to cis isomerization.
Similarly, the polarization of the 14%Cꢀ13%C bond
is increased on going from C25 to C25% aldehyde
(vide supra), a fact which explains that the 13%-cis
isomer can be produced not in C25 aldehyde but
in C25% aldehyde. (2) The parallel polarization of
the 11Cꢀ12C bond in reference to the terminal
CꢀO bond in the case of C20% aldehyde (in contrast
to the antiparallel polarization in C20 aldehyde)
Fig. 7. The positional dependence of the (a) C and (b) H
chemical shifts in C20 and C20% aldehydes. The chemical shifts
which are unique to C20 and C20% aldehydes are shown in open
and closed circles, respectively; those which are in agreement
between the two aldehydes are shown in shadowed circles.
(a) the polarization in the 13Cꢀ14C bond is much
larger in C20 aldehyde than in C20% aldehyde,
because the difference in the values of C chemical
shift between 13C and 14C is larger in the former
than in the latter. Similarly (see Fig. 3(a) of ref.
[16]), the polarization in the 14%Cꢀ13%C bond is
much larger in C25% aldehyde than in C25 aldehyde.
Further, the directions of the above polarizations
are in accord with the polarization anticipated for
the terminal carbonyl group (CꢀO“C+ –O−),
i.e. the 15CꢀO bond in C20 and C20% aldehydes and
the 12%CꢀO bond in C25% and C25 aldehydes. (b)
The polarization of the 11Cꢀ12C bond in C20%
aldehyde is in the same direction as that of the
15CꢀO bond, whereas the polarization of the
11Cꢀ12C bond in C20 aldehyde is in the opposite
direction. Similarly [16], the polarization of the
15Cꢀ15%C bond in C25% (C25) aldehyde is in the
same (opposite) direction in relation to the polar-
ization of the terminal 12%CꢀO bond. These char-
acteristics in the double-bond polarization, i.e. (a)
and (b), will be related to the amounts of isomers
which are produced by direct photo-excitation of
the all-trans isomer.
3.4.3. Polarization of conjugated double bonds as
a possible factor determining the barrier to
photo-isomerization from the trans to the cis
configuration
Ponder and Mathies [21] experimentally evi-
denced large increase in the dipole moment and
the polarizability of all-trans C20 aldehyde (reti-

Y. Hu et al. / Spectrochimica Acta Part A 53 (1997) 913–926
925
explains that the larger amount of the 11-cis
isomer can be produced in C20% aldehyde than in
C20 aldehyde. Similarly, the parallel (antiparallel)
polarization of the 15Cꢀ15%C bond in C25 alde-
hyde (C25% aldehyde) explains the presence (ab-
sence) of the 15-cis isomer in C25 aldehyde (C25%
aldehyde). (3) The polarization of the 9Cꢀ10C
bond are almost the same in both C20 and C20%
aldehyde in the ground state. However, the direc-
tion of polarization in the neighboring 11Cꢀ12C
bond may affect a cooperative enhancement of
polarization upon excitation in the conjugated
double bonds. The 9Cꢀ10C and 11Cꢀ12C parallel
(antiparallel) polarization in C20% aldehyde (C20
aldehyde) may enhance (suppress) the excited-
state polarization, and as a result, it produces a
larger (smaller) amount of the 9-cis isomer in C20%
aldehyde (C20 aldehyde). Similarly, the parallel
(antiparallel) polarization in C25 aldehyde (C25%
aldehyde) should increase (decrease) the amount
of the 13-cis isomer produced. Thus, the effect of
the terminal-methyl position on the stationary-
state composition can be qualitatively explained
in terms of the enhancement of the ground-state
polarization upon excitation. However, theoreti-
cal calculations concerning the singlet-excited
state and more quantitative experimental evidence
are necessary to establish this hypothetical mecha-
nism.
of the particular methyl group closer to the car-
bonyl end caused the following systematic
changes in the isomeric composition: (1) increase
in the amount of the 9-cis isomer; (2) increase in
the amount of the 11-cis isomer; and (3) disap-
pearance of the 13-cis isomer on going from C20
to C20% aldehyde correspond, on the basis of the
structural similarity in the carbonyl side of the
conjugated chain, to (1%) increase in the amount of
the 13-cis isomer, (2%) appearance of the 15-cis
isomer, and (3%) disappearance of the 13%-cis iso-
mer on going from C25% to C25 aldehyde. These
changes can be explained in terms of enhancement
of the ground-state polarization upon excitation.
The position of the terminal-methyl group is
not relevant to the presence or absence of the
11-cis isomer in the set of aldehydes examined.
The key factor in the generation of the 11-cis
configuration (in a parallel-methyl fragment) must
be the length of the conjugated chain.
Acknowledgements
The authors thank Dr Stanislav L. Bondarev
for critical reading of the manuscript. This work
has been supported by grants from the Ministry
of Education, Science and Culture (c06239101)
and from Human Frontier Science Program.
4. Conclusion
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