Semiconductor photocatalysis: Photodegradation and trans-cis photoisomerization of carotenoids

Article abstract of DOI:10.1021/jp980326i

In the presence of semiconductor CdS or ZnO particles, irradiation (>350 nm) of all-trans-β-carotene (II) in dichloromethane leads to rapid degradation of the carotenoid, which is relatively stable in the absence of the semiconductors. Canthaxanthin (I), however, undergoes significant photocatalyzed degradation only on ZnO, not on CdS. High-performance liquid chromatographic studies indicate that CdS catalyzes trans-cis photoisomerization of both I and II. As in the photoisomerization in the absence of semiconductor, the major cis isomers have the 9-cis and 13-cis configuration, but, under otherwise the same condition, the ratio of cis/trans isomers has doubled. In contrast to CdS, ZnO does not catalyze the photoisomerization of either I or II, although it enhances their rate of degradation. A photoisomerization mechanism involving carotenoid radicals formed by reaction with interstitial sulfur on the CdS surface is proposed.

Full text of DOI:10.1021/jp980326i

J. Phys. Chem. B 1998, 102, 3897-3901
3897
Semiconductor Photocatalysis: Photodegradation and Trans-Cis Photoisomerization of
Carotenoids
Guoqiang Gao, Yi Deng, and Lowell D. Kispert*
Department of Chemistry, UniVersity of Alabama, Tuscaloosa, Alabama 35487-0336
ReceiVed: NoVember 25, 1997; In Final Form: February 11, 1998
In the presence of semiconductor CdS or ZnO particles, irradiation (>350 nm) of all-trans-â-carotene (II) in
dichloromethane leads to rapid degradation of the carotenoid, which is relatively stable in the absence of the
semiconductors. Canthaxanthin (I), however, undergoes significant photocatalyzed degradation only on ZnO,
not on CdS. High-performance liquid chromatographic studies indicate that CdS catalyzes trans-cis
photoisomerization of both I and II. As in the photoisomerization in the absence of semiconductor, the
major cis isomers have the 9-cis and 13-cis configuration, but, under otherwise the same condition, the ratio
of cis/trans isomers has doubled. In contrast to CdS, ZnO does not catalyze the photoisomerization of either
I or II, although it enhances their rate of degradation. A photoisomerization mechanism involving carotenoid
radicals formed by reaction with interstitial sulfur on the CdS surface is proposed.
Introduction
SCHEME 1: Structures of All-trans-Canthaxanthin (I)
and Geometrical Isomers of â-Carotene (II) [(a)
All-trans, (b) 7-cis, (c) 9-cis, (d) 13-cis, and (e) 15-cis]
Carotenoids are widely distributed in green plants and
animals; ∼600 naturally occurring carotenoids having been
isolated and identified.1 They have the important biochemical
and biological functions of light harvesting and photoprotection
in green plants and have been found in all photosynthetic
organisms that evolve O2.2,3 Although carotenoids are generally
present in their most stable form in which all double bonds in
the backbone have the trans configuration, many mono- or di-
cis geometrical isomers, primarily at positions 7, 9, 13, 15, 13′,
and 9′ (Scheme 1) are known. These cis isomers, together with
trans isomers, are present in human as well as animal tissue
and in pigment-protein complexes of photosynthesizing plants
and bacteria, and are known to affect the biochemistry. For
example, different configurations are found in spinach leaves,
and cis-â-carotene is bound to the reaction centers (RCs) of
spinach Photosystem II and Photosystem I;4,5 cis isomers of
spheroidene, neurosporene, and spirilloxanthin are bound to the
RCs of the bacteria Rhodobacter sphaeroides 2.4.1, Rhb.
sphaeroides GIC, and Rhodospirillum rubrum S1, respectively.6
Carotenoid cis isomers are also found in the thermophilic cyano
bacterium Synechococcus Valcunus copeland.7,8 The first cis
carotenoids observed in animals were mono cis isomers of
canthaxanthin localized in the ovaries, eggs, and hemolymph
of the female brine shrimp Artemia. The presence of these cis
isomers at specific sites of only reproductively active females
implies that they are essential to reproduction and/or embryonic
development.9
In vitro methods leading to formation of mixtures of cis and
trans isomers include refluxing in organic solvents, melting of
crystals, contact for prolonged periods with certain active
surfaces, treatment with acids, and irradiation of solutions (often
catalyzed by iodine).10,11 Recently it was found that electro-
chemical oxidation of all-trans-canthaxanthin and â-carotene,
shown in Scheme 1, in dichloromethane also leads to significant
trans-to-cis isomerization, with cis isomers accounting for ∼40%
of the carotenoid;12 the proposed mechanism involves geo-
metrical isomerization mediated by radical cations and/or
dications. The same species are also generated by chemical
oxidation; for example, treatment of all-trans carotenoids with
FeCl3, which results in the formation of cis isomers.13
During the past decade, the utilization of semiconductors as
catalysts in photochemical transformations of organic and
inorganic compounds has seen a tremendous growth.14-20 The
participation of a semiconductor in a photocatalytic process can
be either direct or indirect. Under band gap excitation,
semiconductor particles act as short-circuit microelectrodes and
directly oxidize or reduce the adsorbed substrates. Alternatively,
they can promote a photocatalytic reaction by acting as
mediators for the charge transfer between two adsorbed
molecules (Scheme 2). This process is extensively employed
in photoelectrochemistry and imaging science. In this case, the
semiconductor accepts an electron from the excited state of an
adsorbed dye and then transfers the charge to another substrate
S1089-5647(98)00326-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/22/1998

3898 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Gao et al.
SCHEME 2: Principle of Photoinduced Charge
Transfer Processes at a Semiconductor Particle
Figure 1. Schematic of the flow system used for irradiation of
carotenoid/semiconductor/CH2Cl2 mixtures and optical spectra measure-
ment.
(A f A-). The energies of the conduction and valence bands
of the semiconductor and the redox potential of the adsorbed
molecule control the course of the photochemical reaction.
Application of dye-sensitized semiconductor electrodes to a new
type of photovoltaic cell, the dye-sensitized solar cell or Graetzel
cell,21 has generated much current interest in the photochemistry
community.22 Development of new semiconductor sensitizers
that produce high conversion efficiency and low cost solar
photovoltaic dye cells is a challenging task.
Carotenoids absorb light in the range 380-520 nm, with
molar extinction coefficients >105. The photodriven electron
transport processes in synthetic triad and pentad molecules
containing carotenoids23,24 and light-induced electron polariza-
tion and separation in carotenoid/solvent25,26 have been well
studied. Carotenoid Langmuir-Blodgett (LB) films deposited
on indinum tin oxide (ITO) have been shown to be photoactive
electrodes.27 These properties, together with their photoprotect
characteristics, make certain carotenoids potential sensitizers in
solar photovoltaic cells. Because carotenoids exhibit different
stability in solution, it is essential to investigate the photochemi-
cal behavior as well as the stability of carotenoids in semicon-
ductor/carotenoid/solvent systems.
In this study, we irradiated mixtures of carotenoids and
semiconductor powders. It was found that in the presence of
certain semiconductor powders, dichloromethane solutions of
carotenoids show diverse photochemical behavior. Thus, with
CdS particles, irradiation (> 350 nm) of all-trans-canthaxanthin
(I) and â-carotene (II) in dichloromethane leads to significant
enhancement of cis isomer formation. However, ZnO does not
catalyze these photoreactions. Carotenoids I and II show
different photostability with different semiconductors. In the
presence of ZnO particles, rapid degradation of both I and II
was observed under irradiation; with CdS, however, significant
photodegradation of only II, not I, occurred.
Figure 2. Difference optical absorption spectra of 14 µM canthaxanthin
in CH2Cl2 after irradiation for 2, 4, 6, 8, 10, 15, and 20 min with a
xenon lamp (>350 nm) in the presence of (a) no semiconductor, (b)
CdS powder (3 mg/mL), and (c) ZnO powder (3 mg/mL). The mixture
was purged with N2 to remove O2 before irradiation. Arrows indicate
increase of irradiation time.
dichloromethane was purged with nitrogen for 10 min prior to
irradiation. During irradiation, the solution was stirred magneti-
cally and continuously pumped through the flow cell connected
by a 1-mm i.d. silicon rubber tube. A pump-inlet filter (stainless
steel mobile phase filter for HPLC, 2 µm) was used to prevent
particles from entering the flow cell. The pump was stopped
during the spectral measurements.
A Vydac 201TP54 polymeric C18 column (250 × 4.6 mm
i.d.) packed with 5-µm particles (Hesperia, CA) and a Shimadzu
LC-600 pump with a SPD-6AV UV-vis detector were used
for the HPLC separation and detection. Acetonitrile was used
as the mobile phase. The detector was set at 465 and 450 nm
for canthaxanthin and â-carotene, respectively, near the maxi-
mum absorbance of the neutral cis carotenoids.
Experimental Section
All-trans-canthaxanthin and â-carotene were purchased from
Fluka. Anhydrous dichloromethane, HPLC grade acetonitrile,
and cadmium sulfide and zinc oxide powder (<5 µm) were
obtained from Aldrich.
Optical absorption spectra in the range of 190 to 1100 nm
were measured using a Shimadzu UV-1601 UV-vis spectro-
photometer. A 250 W xenon lamp (ILC Technology) at a
distance of 20 cm was used to irradiate the solutions. Deionized
water contained in a cylindrical glass cell (path length, 10 cm)
was used as an IR cutoff filter. An optical filter was used to
cut off most of the UV light below 350 nm. The photoenergy
at the external surface of the cuvette, measured with a Newport
power meter (model 818-SL), was 0.4 W cm-2. A closed
continuous flow system (Figure 1), consisting of a 100-mL glass
bottle with a flat glass window, a peristaltic pump (Rabbit,
RAININ Instrument Company Inc.), and a flow cell (Shimadzu,
1-cm optical path length and 3-mm diameter), was used for
irradiation of the solution and measurement of the spectra. A
mixture of carotenoid and semiconductor powder in 50 mL of
Results and Discussion
Photodegradation Catalyzed by Semiconductor Powders.
Figure 2 shows the difference optical absorption spectra (At -
A0) of 14 µM canthaxanthin (I) in CH2Cl2 after irradiation with
a xenon lamp (>350 nm) for increasing periods of time. In
the absence of semiconductor powders, irradiation of the solution
leads to the gradual disappearance of I (Figure 2a). The rate
of decrease in the concentration of I increases somewhat in the
presence of CdS powder (Figure 2b), and much more so in the
presence of ZnO powder (Figure 2c). The decrease in concen-
tration of neutral canthaxanthin shows a linear dependence on
irradiation time (Figure 3a, curves 1-3) during the early stages
(et1/2); that is, the photodegradation reaction appears to be zero-

Photocatalysis of Carotenoids
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3899
Figure 3. Photodegradation of (a) 14 µM canthaxanthin and (b) 14
µM â-carotene by irradiation for increasing periods of time, in the
presence of (1) no semiconductor, (2) CdS powder, and (3) ZnO
powder. Data from Figures 2 and 4, respectively.
Figure 5. (a) High-performance liquid chromatograms of 0.3 mM all-
trans-canthaxanthin in CH2Cl2 after irradiation for 15 min. The solution
contained (‚‚‚) no powder, and (s) CdS powder when irradiated.
Solution was filtered before injection (mobile phase, CH3CN; injection
volume, 5 µL; flow rate, 1 mL/min; temperature, 22 °C). (b) Same as
a, except with ZnO powder.
TABLE 1: Approximate cis-trans Ratios of Carotenoids
Formed by Photoisomerization from All-trans Carotenoids
in the Presence of Semiconductors
cis/trans ratio ((0.01)
carotenoid isomer
without semiconductor
ZnO
CdS
9-cis
13-cis
9-cis
13-cis
15-cis
0.07
0.03
0.05
0.04
0.03
0.07
0.03
0.05
0.04
0.03
0.15
0.12
0.12
0.09
0.06
Ia
IIb
a Irradiation for 15 min. b Irradiation for 10 min.
II than I. In the absence or presence of CdS powder, the
decrease in concentration of II also shows a linear dependence
on the irradiation time in the early stages (Figure 3b, curves 1
and 2), and the apparent zero-order rate constants kobs are 0.07
and 0.25 µM min-1, respectively. With ZnO powder, however,
the decrease of â-carotene concentration shows a logarithmic
dependence on the irradiation time, indicating that it is a first-
order reaction (k1 ) 0.05 min-1). These results show that
canthaxanthin is photochemically more stable than â-carotene
in the presence of semiconductors.
Trans-Cis Photoisomerization by Semiconductor Powders.
Figure 5a shows the HPLC results after irradiating a 0.3 mM
solution of I for 15 min, in the absence (dotted line) and the
presence (solid line) of CdS powder in the solution. The two
new major components were identified to be the 9-cis and 13-
cis isomers by comparison with previous results.28,29 Although
in both cases cis isomers were formed during irradiation of
trans-canthaxanthin, the ratio of cis/trans products formed in
the presence of CdS powder is several times larger than that
obtained in its absence during the same time of irradiation. The
9-cis/all-trans ratio increased from 0.07 to 0.15, and the 13-
cis/all-trans ratio increased from 0.03 to 0.12 (Table 1). For
â-carotene, similar results were obtained. After irradiation three
new components were identified to be the 9-cis, 13-cis, and
15-cis isomers. The ratio of cis/trans products formed in the
presence of CdS powder increased from 0.05 to 0.12 (9-cis),
Figure 4. Difference optical absorption spectra of 14 µM â-carotene
in CH2Cl2 after irradiation for 2, 4, 6, 8, 10, 15, 20 min with xenon
lamp (>350 nm), in the presence of (a) no semiconductor, (b) CdS
powder (3 mg/mL), and (c) ZnO powder (3 mg/mL). The mixture was
purged with N2 to remove O2 before irradiation. Arrows indicate
increase of irradiation time.
order in carotenoid. The apparent rate constants, kobs, are 0.05,
0.08, and 0.23 µM min-1 in the presence of no powder, CdS
powder, and ZnO powder, respectively. It may be noted that
changes in absorption due to concurrent formation of cis isomers
were ignored because both λmax and extinction coefficients of
cis and trans isomers are similar (∆λmax ) 5-7 nm). The
difference spectrum after irradiation for 20 min approaches that
of pure trans-canthaxanthin, which shows no fine structure at
its maximum near 475 nm in CH2Cl2. The spectral results of
similar experiments with â-carotene (II) are shown in Figure
4. The difference spectrum after irradiation for 20 min shows
the fine structure characteristic of II in CH2Cl2. Comparison
of Figures 2 and 4 leads to the conclusion that both CdS and
ZnO exert a greater catalytic effect on the photodegradation of

3900 J. Phys. Chem. B, Vol. 102, No. 20, 1998
Gao et al.
SCHEME 3: Energy Levels of the Conduction and
Valence Bands of CdS and ZnO, and Oxidation and
Excited States of Canthaxanthin (I) and â-Carotene (II)
SCHEME 4: CdS-Catalyzed cis-trans
Photoisomerization of Carotenoids
the driving force (∆E) of a particular semiconductor/carotenoid
pair. Thus, it is expected that under the same irradiation
conditions, II/ZnO would give the fastest rate of the photo-
chemical reactions, and I/CdS would give the slowest. The
experimental results bear out this expectation and lend support
to the proposed mechanism.
0.04 to 0.09 (13-cis), and 0.03 to 0.06 (15-cis; Table 1). These
results indicate that the extent of carotenoid geometrical
photoisomerization is enhanced significantly in the presence of
CdS powder. In our experiments, in the absence of CdS powder,
the cis/trans ratio of products of photoisomerization of caro-
tenoids is relatively low; such isomerization is believed to follow
a triplet mechanism.30
On the other hand, the HPLC result after irradiating a 0.3
mM solution of I for 15 min in the presence of ZnO powder
(Figure 5b, solid line) showed no observable difference in the
cis/trans ratio of isomers compared with that obtained in the
absence of ZnO powder (Figure 5b, dotted line; Table 1).
However, two as yet unidentified components with much shorter
retention times (between 5 and 8 min) were generated upon
irradiation in the presence of ZnO (Figure 5b, solid line).
Similar results were obtained for â-carotene isomerization (Table
1). These observations lead to the conclusion that ZnO exerts
little, or no, catalysis on the photoisomerization of carotenoids,
although it obviously catalyzes other reactions (e.g., photodeg-
radation). The possibility that the two new components are
carotenoid geometrical isomers is unlikely because of their very
short retention times. Further investigations need to be carried
out to identify these new products.
Semiconductor-Catalyzed Photodegradation and Photo-
isomerization Mechanism. It has been demonstrated previ-
ously that the degradation of carotenoids oxidized by FeCl3
occurs by formation of carotenoid radical cations and that the
decay rate of these radical cations is greatly enhanced by
irradiation.28 It is also known that large-band gap semiconduc-
tors such as ZnO can be sensitized with various dye molecules
that became oxidized in the process.31,32 The semiconductor-
catalyzed photodegradation mechanism could involve photo-
chemical formation of carotenoid radical cations by a sensiti-
zation process. In such a process, an electron from the excited
carotenoid molecule is injected into the conduction band of the
semiconductor to form a carotenoid radical cation that, upon
further irradiation, undergoes rapid degradation. Such an
electron transfer to the semiconductor process is feasible
according to the relative energies of the semiconductor band
edges and the carotenoid excited states. Scheme 3 shows the
energy levels of the conduction and valence bands of CdS and
ZnO33,34 and the excited states of carotenoids I and II, as well
as the oxidation potentials of I and II which were determined
by cyclic voltammetry using SCE as the reference electrode.35
In the excited states, these oxidation potentials correspond to
-1.98 and -2.21 V for I and II, respectively. Thus, the energy
gained from the light excitation provides sufficient driving force
(∆E, the potential difference between carotenoid excited states
and semiconductor conduction bands) to inject electrons from
Car* into the conduction band of the semiconductor particles.
The degradation rate of the carotenoids will be determined by
the formation rate of their radical cations, which is related to
Depending on the method used to effect cis-trans isomer-
ization of carotenoids, several mechanisms have been proposed;
for examples, triplet energy transfer30 and formation of singlet
biradicals39 and radical cations.12,13 The proposed radical cation
mechanism is based in part on AM1 calculations that show that
the barrier to rotation about certain bonds in the carotenoid
radical cations, compared to the neutral species, is greatly
reduced. The isomerized radical cations are then transformed
to neutral cis-carotenoids by exchanging one electron with
neutral carotenoids (which are converted to radical cations).
Such reactions could also be involved in the photoisomerization
because, as already discussed, carotenoid radical cations can
be photoproduced by CdS and ZnO semiconductor particles.
However, if this were the only mechanism, both CdS and ZnO
should catalyze trans-cis photoisomerization of carotenoids, but
only CdS, not ZnO, particles enhance the photoisomerization.
An additional mechanism must occur, presumably at the surface
of CdS.
It has been shown that CdS powder can induce efficient cis-
trans photoisomerization of alkenes.31,33,36-38 Two mechanisms
have been invoked previously. For the photoisomerization of
styrene derivatives in the presence of CdS, the intermediacy of
styrene radical cations, formed by electron transfer from styrene
to valence band holes in the irradiated CdS, was proposed.37 In
a later investigation of the photoisomerization of nonaromatic
monoalkenes catalyzed by CdS, it was proposed that the active
sites for the cis-trans photoisomerization are photoproduced
radicals from surface states, and that surface defects such as
interstitial sulfur (Is) play important roles in their formation.33
A trapped hole would be produced if Is donates an electron to
the photoformed hole in the valence band before the recombina-
tion. The trapped holes arising from Is can be regarded as sulfur
radical cations (Is‚+). Addition (and subsequent elimination)
reactions of alkenes should occur to sulfur radicals on the surface
of catalysts, leading to the catalytic cis-trans isomerization. A
similar mechanism fits the CdS-catalyzed photoisomerization
of carotenoids. As shown in Scheme 4, upon irradiation of CdS
particles, surface defects such as interstitial sulfur (Is) form sulfur
radical cations (Is‚+) by donating electrons to the photoformed
holes in the valence band.33 An electron from a double bond
in the carotenoid backbone could combine with Is‚+ to form a
S+sC bond and carotenoid radicals that can easily undergo
geometrical isomerization to form various cis configurations.
According to AM1 molecular orbital calculations, the energy
barriers of configurational transformation in the gas phase from
trans to cis are much lower in the radicals than in the neutral
molecule.12
As discussed above, in the CdS/carotenoid case, there exist
two competing reactions; that is, photoisomerization via forma-
tion of attached carotenoid radicals on CdS surface-active sites,

Photocatalysis of Carotenoids
J. Phys. Chem. B, Vol. 102, No. 20, 1998 3901
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It has been shown that canthaxanthin and â-carotene can
undergo charge transfers on ZnO and CdS particle surfaces,
leading to significant photodegradation of â-carotene on both
ZnO and CdS, and for canthaxanthin only on ZnO. The
photodegradation rate is related to the energy difference between
the semiconductor conduction bands and the excited states of
carotenoids. CdS also catalyzes geometrical photoisomerization
of all-trans-canthaxanthin and â-carotene, to form mainly 9-cis
and 13-cis isomers. Active sites, such as interstitial sulfur (Is),
of the CdS particles have been proposed to play an essential
role in the isomerization.
Acknowledgment. Dr. Elli Hand is thanked for helpful
discussions and critically reading the manuscript. This work was
supported by the Division of Chemical Sciences, Office of Basic
Energy Sciences, Office of Energy Research of the U.S.
Department of Energy under Grant No. DE-FG05-86ER13465.
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