Effects of polyene chain length and acceptor substituents on the stability of carotenoid radical cations

Article abstract of DOI:10.1021/jp994436g

The stability of radical cations of three series of carotenoids substituted with terminal ester, aldehyde, and cyano groups and with different numbers of backbone double bonds was studied by electrochemical and optical methods. The ethyl esters are 8′-apo-β-caroten-8′-oate (I), 6′-apo-β-caroten-6′-oate (II), and 4′-apo-β-caroten-4′-oate (III); the aldehydes are 8′-apo-β-caroten-8′-al (IV), 6′-apo-β-caroten-6′-al (V), and 4′-apo-β-caroten-4′-al (VI); and the cyano compounds are 8′-caroten-8′-nitrile (VII), 6′-apo-β-caroten-6′-nitrile (VIII), and 4′-apo-β-caroten-4′-nitrile (IX). Cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) results indicate that the stability of carotenoid radical cations depends on the number of conjugated chain double bonds. For the esters, the longer the olefin chain, the more unstable the radical cations. In contrast, for the aldehydes and the nitriles, the stability of the radical cations is similar or varies slightly with backbone chain length. The half-lives, determined by stop-flow, and the decay rate of optical absorption of the radical cations generated by reaction with ferric chloride are: I, 202; II, 125; III, 2.35; IV, 149; V, 167; VI, 257; VII, 227; VIII, 158; and IX, 133 (s). AM1 molecular orbital calculations predict a large decrease in the dipole moments between radical cations and the neutral aldehydes and nitriles, but an increase for the esters as the number of chain double bonds increases. Radical cations with larger dipole moments have shorter lifetimes. It is likely that stronger interactions of the radical cation dipoles with the solvent dipoles result in enhanced decay of the radical cations. For esters, aldehydes, and nitriles, the shorter the olefin chain, the more difficult is the oxidation. The UV-vis optical absorption spectra of the carotenoids containing aldehyde groups in solvents of different polarity exhibit intramolecular charge transfer (ICT) phenomena, and their optical spectra are sensitive to the polarity of the solvents. In contrast, the esters do not show this behavior.

Full text of DOI:10.1021/jp994436g

J. Phys. Chem. B 2000, 104, 5651-5656
5651
Effects of Polyene Chain Length and Acceptor Substituents on the Stability of Carotenoid
Radical Cations
Yi Deng, Guoqiang Gao, Zhangfei He, and Lowell D. Kispert*
Department of Chemistry, The UniVersity of Alabama, Tuscaloosa, Alabama 35487-0336
ReceiVed: December 20, 1999; In Final Form: March 24, 2000
The stability of radical cations of three series of carotenoids substituted with terminal ester, aldehyde, and
cyano groups and with different numbers of backbone double bonds was studied by electrochemical and
optical methods. The ethyl esters are 8′-apo-â-caroten-8′-oate (I), 6′-apo-â-caroten-6′-oate (II), and 4′-apo-
â-caroten-4′-oate (III); the aldehydes are 8′-apo-â-caroten-8′-al (IV), 6′-apo-â-caroten-6′-al (V), and 4′-apo-
â-caroten-4′-al (VI); and the cyano compounds are 8′-apo-â-caroten-8′-nitrile (VII), 6′-apo-â-caroten-6′-
nitrile (VIII), and 4′-apo-â-caroten-4′-nitrile (IX). Cyclic voltammetry (CV) and Osteryoung square wave
voltammetry (OSWV) results indicate that the stability of carotenoid radical cations depends on the number
of conjugated chain double bonds. For the esters, the longer the olefin chain, the more unstable the radical
cations. In contrast, for the aldehydes and the nitriles, the stability of the radical cations is similar or varies
slightly with backbone chain length. The half-lives, determined by stop-flow, and the decay rate of optical
absorption of the radical cations generated by reaction with ferric chloride are: I, 202; II, 125; III, 2.35; IV,
149; V, 167; VI, 257; VII, 227; VIII, 158; and IX, 133 (s). AM1 molecular orbital calculations predict a
large decrease in the dipole moments between radical cations and the neutral aldehydes and nitriles, but an
increase for the esters as the number of chain double bonds increases. Radical cations with larger dipole
moments have shorter lifetimes. It is likely that stronger interactions of the radical cation dipoles with the
solvent dipoles result in enhanced decay of the radical cations. For esters, aldehydes, and nitriles, the shorter
the olefin chain, the more difficult is the oxidation. The UV-vis optical absorption spectra of the carotenoids
containing aldehyde groups in solvents of different polarity exhibit intramolecular charge transfer (ICT)
phenomena, and their optical spectra are sensitive to the polarity of the solvents. In contrast, the esters do not
show this behavior.
Introduction
more difficult it is to oxidize the compound, and the more stable
is the radical cation. In addition to the electrochemical properties,
the photophysical properties of donor/acceptor-substituted poly-
enes and carotenoids (push-pull polyenes) have been studied.24-28
However, studies of the effect of the conjugated backbone length
on the electrochemical properties of carotenoids containing
donor/acceptor substituents are scarce, although the electronic
structures and photophysical properties of polyenes and carot-
enoids with 3-19 conjugated double bonds have been
investigated.29-31 In this study, the properties of three series of
carotenoids substituted with terminal ester, aldehyde, and cyano
groups with 8-10 conjugated polyene double bonds were
investigated by voltammetry and optical spectroscopy. The effect
of the olefin chain length on the radical cation stability was
ascertained. AM1 calculations of the dipole moments of the
neutral species and radical cations were carried out, and the
dependence of the radical cation stability on its (gas-phase)
dipole was explored.
It is well established that carotenoids play important roles in
photosynthetic systems because they serve as photoprotecting
agents and light-harvesting antenna pigments.1-3 Certain ca-
rotenoids have important biochemical and biological functions,
such as providing the essential provitamin A compounds.4
Carotenoids are also good antioxidants5 and have been impli-
cated in cancer prevention and treatment,6 the efficacy of which
may be due to their free-radical-quenching activity,7 their
immune enhancement effect,8 and their in vivo function as
antioxidants.7,9 Other research has shown that carotenoid radical
cations can play an active role in photodriven electron transport
processes,10-12 and that they are present in the photosystem II
reaction center.13 It is therefore of fundamental interest to study
the formation and decomposition of carotenoid radical cations
as well as their stability and the factors that influence it. Methods
of generating carotenoid radical cations include chemical,14-18
photochemical,19 acid-induced,20 and electrochemical oxida-
tion.21,22 Modification of carotenoids by substitution with
electron-accepting and -donating terminal groups provides
interesting molecules for this purpose. Studies by electrochemi-
cal oxidation in dichloromethane solution22,23 of more than 10
carotenoids containing various donor/acceptor substituents but
the same number of chain double bonds indicate that, in general,
the stronger the electron-accepting nature of the substituent, the
Experimental Section
All-trans ethyl 8′-apo-â-caroten-8′-oate (I) and 8′-apo-â-
caroten-8′-al (IV) were generously provided by Hoffmann-La
Roche (Basel) and Roche Vitamins and Fine Chemicals (Nutley,
NJ), respectively (for structures, see Table 1). Ethyl 6′-apo-â-
caroten-6′-oate (II) was prepared by Wittig reaction of IV with
(Ph3PCH2CO2Et)+Cl- (CH2Cl2, aq 10% K2CO3, 3 d, rt); 6′-
apo-â-caroten-6′-nitrile (VIII) was formed from IV and diethyl
* To whom correspondence should be addressed.
10.1021/jp994436g CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/20/2000

5652 J. Phys. Chem. B, Vol. 104, No. 23, 2000
Deng et al.
TABLE 1: Data Derived from Simulation of Cyclic
Voltammogramsa
SCHEME 1
All measurements were carried out at room temperature (ca.
23 °C). For CV and OSWV measurements, dichloromethane
solutions of 0.1 M TBAHFP and 1.0 mM carotenoids were used.
Dichloromethane solutions of 15 µM carotenoids were used for
optical spectral measurements. Saturated stock solutions of ferric
chloride in dichloromethane (≈45 µM) were diluted to 15 µM.
All solutions were prepared in a drybox under a nitrogen
atmosphere at room temperature.
AM1 (Austin Model 1) semiempirical molecular orbital
calculations33 were carried out using HyperChem software34 on
a Gateway 2000 P5-60 (Pentium) personal computer. Digital
simulations of the CVs were performed using the DigiSim
program35 on the same PC.
a E1, E2, and E3 are the redox potentials for the first, second, and
third waves, respectively. ∆E is the difference between E2 and E1.
cyanomethyl phosphonate (THF, NaH, 10 min, rt); 6′-apo-â-
caroten-6′-al (V) was obtained by Wittig reaction of IV with
Ph3PdCHC(O)N(OCH3)CH3 (PhH, 3 d, 65 °C) followed by
reductive cleavage with DIBAL-H (THF, -78 °C, 1 h). Ethyl
4′-apo-â-caroten-4′-oate (III), 4′-apo-â-caroten-4′-al (VI), and
4′-apo-â-caroten-4′-nitrile (IX) were similarly prepared from
V. 8′-Apo-â-caroten-8′-nitrile (VII) was obtained by dehydration
of the 8′-aldoxime with (CF3CO)2O (THF, Et3N, rt, 10 min;
then 60 °C, 3 h). According to NMR analysis, purification of
the crude products by treatment with MeOH and/or column
chromatography (silica gel) of the insolubles gave pure all-E
isomers of II, III, V, and VIII. Compounds VI and IX were
mixtures of the all-E and the 5′-Z isomers: VI, 85% all-E; IX,
two fractions: 85% all-E and 85% 5′-Z.32 Tetra-n-butylammo-
nium hexafluorophosphate (TBAHFP) of polarographic grade
was purchased from Fluka. Anhydrous dichloromethane (99+%),
cyclohexane, acetonitrile, ethanol, and ferric chloride were
obtained from Aldrich.
Cyclic voltammetry (CV) and Osteryoung square wave
voltammetry (OSWV) were carried out using the Bio Analytical
Systems BAS-100W electrochemical analyzer. A platinum disk
electrode (diameter: 1.6 mm) was used as the working electrode,
the auxiliary electrode was a platinum wire, and the reference
electrode was a saturated calomel electrode (SCE).
Optical absorption spectra in the range of 190 to 1100 nm
were measured using a Shimadzu UV-1601 spectrophotometer.
Quartz cuvettes (1-cm optical path length) were used for spectral
measurements. Optical absorbance decrease of the carotenoid
radical cations, generated by oxidation with ferric chloride, was
measured using a flow cell (Shimadzu, 1-cm optical path length
and 3-mm diameter) by stop-flow. A carotenoid solution (2 mL,
15 µM) and a ferric chloride solution (2 mL, 15 µM), contained
in two separate syringes, were simultaneously injected into the
flow cell through a tee, and the optical absorbance change of
the carotenoid radical cation was recorded.
Results
Cyclic and Square Wave Voltammetry. Earlier studies of
carotenoids revealed that during an electrochemical oxidation-
reduction cycle, not only radical cations, but also dications,
cations (loss of one H+ from dications), and neutral radicals
can be formed, and the electrode and homogeneous reactions
shown in Scheme 1 have been established.17,21-23,36-38
The electrochemical oxidation-reduction reactions of many
carotenoids are quasi-reversible or reversible, so that the CVs
display comparable, symmetrical anodic and cathodic waves.
For example, the cyclic voltammogram in dichloromethane of
canthaxanthin consists of two pairs of separate anodic and
cathodic waves (∆E° ) E°2 - E°1 ≈ 200 mV). As the stability
of the radical cation decreases, the CV displays only a small or
no second anodic wave (wave 2); the same is true for the
reversal cathodic waves (waves 1′ and 2′).
The CVs of the esters, compounds I-III, are shown in Figure
1a. The CV of I, Figure 1a (solid line), exhibits two separate
waves (waves 1 and 2) during the anodic scan, which correspond
to the oxidation of the neutral species and the resulting radical
cations, respectively. During the cathodic scan, in addition to
the two reverse waves (waves 2′ and 1′) which correspond to
the reduction of the dications and the radical cations, an
additional reduction wave (wave 3) near 0.1 V is present, and
this is attributed to the reduction of Car+ (eq 3, Scheme 1).
#
The CV of II (Figure 1a, chained line) shows that the relative
magnitude of wave 2, as well as those of waves 2′ and 1′,
decreases. Decreases in wave 2 and its reverse wave 2′ indicate
that these radical cations are more unstable. The decrease in
wave 2 is even more pronounced in the CV of III (Figure 1a,
dotted line) in which waves 2, 2′, and 1′ are absent, i.e., an
irreversible CV occurs. The decrease of wave 2 of I-III can
be more clearly observed in OSWVs (Figure 1b). The voltam-
metry results show that the stability of the radical cations of

Stability of Carotenoid Radical Cations
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5653
Figure 2. (a) Cyclic voltammograms of compounds IV (solid line),
V (chained line), and VI (dotted line). Dichloromethane solutions of
0.1 M TBAHFP and 1.0 mM carotenoids. Scan rate: 100 mV/s. (b)
Osteryoung square wave voltammograms of compounds IV (solid line),
V (chained line), and VI (dotted line). The arrows indicate the potential
scanning direction.
Figure 1. (a) Cyclic voltammograms of compounds I (solid line), II
(chained line), and III (dotted line). Dichloromethane solutions of 0.1
M TBAHFP and 1.0 mM carotenoids. Scan rate: 100 mV/s. (b)
Osteryoung square wave voltammograms of compounds I (solid line),
II (chained line), and III (dotted line). The arrows indicate the potential
scanning direction.
I-III decreases in the sequence of I, II, and III; i.e., the longer
the conjugated backbone, the less stable the radical cations.
The CVs of the aldehydes, IV-VI, are shown in Figure 2a.
In contrast to those of the esters, the CVs of these aldehydes
show similar magnitudes of waves 2. OSWVs of IV-VI (Figure
2b) show the same behavior. These results indicate that there
is little difference or only a small increase in the stabilities of
radical cations of IV-VI (see Discussion). Therefore, the
stability of the carotenoid radical cations depends on both the
length of the conjugated backbone and the type of terminal
substituent.
CVs of another series of carotenoids, VII-IX, are shown in
Figure 3. The CVs of nitriles VII-IX display features very
similar to those of aldehydes IV-VI, (see Figure 2a), indicating
that the stabilities of radical cations of VII-IX are similar to
those of aldehydes.
Figure 3. Cyclic voltammograms of compounds VII (solid line), VIII
(chained line), and IX (dotted line). Dichloromethane solutions of 0.1
M TBAHFP and 1.0 mM carotenoids. Scan rate: 100 mV/s. The arrow
indicates the potential scanning direction.
Digital simulations of the CVs using the DigiSim program35
resulted in the redox potentials and other relevant data listed in
Table 1. Note that the oxidation potentials for the first and
second waves increase as the length of the conjugated chain
decreases; in other words, the shorter the olefin chain, the more
difficult the oxidation.
Optical Absorption Spectra in Solvents of Different
Polarity. Optical absorption spectra of compounds I-VI in
three solvents of increasing polarityscyclohexane (relative
polarity 0.04), acetonitrile (0.65), and ethanol (0.88)sare shown
in Figures 4 and 5. For I-III, the spectra show no large
variation except that considerable fine structures occur only in
the least polar solvent, cyclohexane; the absorption maxima are
red shifted by about 16 nm with each additional double bond.
It is evident that the spectra of the esters, which are substituted
with weakly electron accepting groups, are not sensitive to
solvents. In contrast, the spectra of the aldehydes IV-VI (Figure
5) show considerable broadening of the bands with increasing
solvent polarity. The presence of a conjugated, strongly electron-
accepting terminal substituent greatly increases the sensitivity
of carotenoid photophysical parameters to solvent polarity.
Lifetimes of Carotenoid Radical Cations Measured by the
Stop-Flow Method. It is known that carotenoid radical cations
can be generated by oxidation with ferric chloride and detected
by optical absorption in the near-infrared region.17,18 Examples
of the spectra of the neutral species of II (solid line) and its
radical cation generated by oxidation with ferric chloride
(chained line) are shown in Figure 6. The neutral compound II
does not absorb above 600 nm. Upon addition of ferric chloride,
the absorption of the radical cation centered at 902 nm appears
immediately and then rapidly decreases to about one-half of its

5654 J. Phys. Chem. B, Vol. 104, No. 23, 2000
Deng et al.
Figure 4. Optical absorption spectra of neutral carotenoids (a) I, (b)
II, and (c) III in cyclohexane (solid line), acetonitrile (chained line),
and ethanol (dotted line).
Figure 5. Optical absorption spectra of neutral carotenoids (a) IV,
(b) V, and (c) VI in cyclohexane (solid line), acetonitrile (chained line),
and ethanol (dotted line).
intensity. The spectrum measured 10 min after addition of ferric
chloride is also shown in Figure 6 (dotted line). In addition to
the residual absorption at 902 nm, a new band appears near
720 nm. The lifetimes of different carotenoid radical cations,
which vary from a few seconds to a few minutes, were evaluated
by measurements of the initial decreases of the optical absor-
bances. The time courses of the reactions of radical cations of
I-III are shown in Figure 7a-c, respectively. Relevant data,
listed in Table 2, show that the stability of the radical cations
of the esters decreases considerably with increasing number of
conjugated double bonds. In contrast, the results obtained for
the aldehydes and nitriles IV-IX (Table 2) indicate much less
variation in lifetimes of the radical cations: small increases for
the aldehydes and small decreases for the nitriles with increasing
number of conjugated double bonds. The optical results obtained
by the stop-flow method are consistent with the voltammetry
results discussed above.
Figure 6. Optical absorption spectrum of 8 µM compound II in CH2Cl2
(solid line), spectrum measured immediately after mixing with same
amount of 8 µM FeCl3 (chained line), and spectrum measured 10 min
later (dotted line).
A possible reason for the observation that the absorbances
of radical cations do not go to zero is that radical cations rapidly
convert to other species which are much more stable and have
a similar absorption band to those of their parent molecules (see
Figure 6). The decay kinetics of the radical cations do show an
initial fast step and a second slow step.
AM1 Molecular Orbital Calculations. AM1 semiempirical
molecular orbital calculations of the gas-phase dipole moments

Stability of Carotenoid Radical Cations
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5655
Figure 8. AM1-calculated dipole moments of neutral species and
radical cations of nine carotenoids. Open columns are for neutral
species, shaded columns are for radical cations, and solid columns are
the differences between dipoles of radical cations and neutral species.
differences between the magnitudes of radical cations and
neutral species. It is interesting to note the following: (1) For
the esters, positive changes between the dipoles of radical cations
and neutral species occur; in contrast, for the aldehydes and
nitriles, large negative changes between the dipoles of the radical
cations and the neutral species occur. (2) For the neutral species,
the dipole moments of the aldehydes and nitriles are greater
than those of the esters; however, the dipole moments of the
radical cations of the aldehydes and nitriles are smaller than
those of the esters. (3) For the radical cations, an increase in
the backbone length results in an increase in the dipole moments
for the esters and a decrease for the aldehydes, but only a small
increase for the nitriles.
Discussion
Some effects of electron-donating and -accepting substituents
on the stability of carotenoid radical cations have been
discussed.22,23 From a study of a series of carotenoids terminally
substituted by electron-donating and -accepting groups, it was
concluded that the stronger the electron-accepting nature of the
substituent, the more stable the radical cations in dichloro-
methane solutions. The magnitudes of the EPR parameters, ∆Hpp
and g, for the carotenoid radical cations suggest a polyene
π-radical cation structure, with little unpaired electron density
present on the donor or acceptor substituent.22,39 However, the
influence of the conjugated double-bond chain length on the
stability of carotenoid radical cations has not been studied
before.
In solution, radical cations undergo decomposition via depro-
tonation (eq 6, Scheme 1) and other reactions.23,38 Stronger
interaction of the solvent dipoles with the radical cation dipoles
appears to result in enhanced decay of the radical cations;
evidently, the larger the dipoles of the radical cations, the less
stable the radical cations. The voltammetric and kinetic results
for radical cation stability parallel the AM1 calculated dipole
moments in that (1) for ester radical cations, the species with
the shortest backbone chain has the longest lifetime and the
smallest dipole moment; (2) for aldehyde radical cations, an
extra double bond in the backbone results in a decrease in the
dipole moment, and therefore the lifetime increases slightly;
radical cations of IV-VI, which show smaller dipole moments,
are relative more stable than those of the esters; and (3) for the
nitrile radical cations, AM1-calculated dipole moments increase
slightly with increasing number of chain double bonds, so the
stability also decreases slightly.
Figure 7. The change with time of optical absorbance of radical cations
generated by rapidly mixing equal amounts of 8 µM FeCl3 and 8 µM
compounds (a) I, (b) II, and (c) III, by stop-flow method. Note that
the time scales differ.
TABLE 2: Optical Absorption Maxima and Half-lifetimes of
Radical Cations for Nine Carotenoids
λ1 (nm)a ∆λ1
λ2 (nm)a ∆λ2
t1/2
a
a
b
compounds
(2
(nm)
(2
(nm)
(s)
8′-CO2Et (I)
6′-CO2Et (II)
4′-CO2Et IIII)
8′-CHO (IV)
6′-CHO (V)
4′-CHO (VI)
8′-CN (VII)
6′-CN (VIII)
4′-CN (IX)
453
471
485
468
486
496
456
474
488
855
902
948
848
895
944
841
890
939
202 ( 6
125 ( 1
2.35 ( 0.18
149 ( 1
167 ( 3
257 ( 5
227 ( 5
158 ( 2
133 ( 1
18
14
47
46
18
10
47
49
18
14
49
49
a λ1 and λ2 are the wavelengths of the optical absorption maxima of
b
the neutral and radical cation species, respectively. t1/2 is the half-
lifetime of radical cations generated by ferric chloride determined by
the stop-flow method.
of the neutral species and radical cations of carotenoids I-IX
were carried out. The structures of the species were optimized
by the AM1-RHF method prior to a single-point calculation.
As illustrated in Figure 8, the open, shaded, and solid columns
represent the magnitudes of the calculated dipole moments of
the neutral species, those of the radical cations, and the

5656 J. Phys. Chem. B, Vol. 104, No. 23, 2000
Deng et al.
It is relevant to mention that the dependence of the lifetimes
of the carotenoid lowest excited singlet states on solvent polarity
has been investigated.40 As the dielectric constant of the solvent
increases from 1.9 to 36, the lifetime of the lowest excited singlet
state of all-trans 7′-apo-7′,7′-dicyano-â-carotene decreases from
11.7 to 1.9 ps. AM1 molecular orbital calculations predict a
large change in dipole moments between the lowest excited
singlet and the ground states (a 5.1 D increase); interaction of
the excited-state dipole with solvent dipoles results in enhanced
charge-transfer character within the excited state, which in turn
enhances nonradiative decay of the excited state.
The photophysical properties of donor-acceptor molecules,
“push-pull” polyenes, and carotenoids bearing various acceptor
and donor groups and linked together by chains of different
length and structure have been studied by absorption and
fluorescence spectroscopy.25 The position of the absorption and
fluorescence maxima and their variation in solvents of increasing
polarity are in agreement with an intramolecular charge transfer
(ICT) nature, which will greatly enhance the sensitivity of the
polyene photophysical parameters to solvent polarity. In this
study, the aldehydes IV-VI, which are substituted with a
strongly electron-accepting group, display ICT properties be-
cause their absorption spectra exhibit solvent-sensitive behavior
as shown in Figure 5. In contrast, the esters I-III, substituted
with a weaker electron-accepting group, do not show ICT
properties because their spectra are not sensitive to solvents
(Figure 4). This behavior is also in agreement with the
magnitudes of the dipole moments of these carotenoids. As
shown in Figure 8, the dipoles of the neutral species of the
aldehydes IV-VI are much larger than those of the esters I-III.
The larger the dipole of a species, the more intense the
interaction between the species and the solvent, resulting in
enhancement of the sensitivity of the polyene photophysical
parameters to solvent polarity.
Acknowledgment. We thank Dr. Elli Hand for synthesizing
the carotenoids. This work was supported by the Division of
Chemical Sciences, Office of Basic Energy Sciences of the U.S.
Department of Energy under Grant No. DE-FG02-86-ER13465.
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Conclusions
57.
From voltammetry and optical spectroscopic studies of three
series of carotenoids substituted with terminal ester, aldehyde,
and cyano groups with different lengths of the backbone chain,
we have drawn the following conclusions: (1) For carotenoids
substituted with weak terminal electron acceptors (ester groups),
the stability of the radical cations is relatively low, and the
radical cations with shorter length are more stable than those
with longer length. (2) In contrast, for carotenoids substituted
with a strong terminal electron acceptor (aldehyde group), the
stability of the radical cations is relatively high, and the stability
of radical cations increases slightly with increasing conjugated
backbone length. (3) For carotenoids substituted with cyano
terminal groups, the stability of the radical cations is relatively
high, and the stability of the radical cations decreases slightly
with increasing conjugated backbone length. (4) For all esters,
aldehydes, and nitriles, the shorter the olefin chain, the more
difficult the oxidation. (5) The aldehydes display intramolecular
charge transfer (ICT) properties because their absorption spectra
exhibit solvent-sensitive behavior; in contrast, the esters do not.
(6) It is evident that the radical cation stability is related to the
change in dipole moment between radical cations and the neutral
species; AM1 calculations predict large decreases in dipole
moment between radical cations and neutral species for the
aldehydes and nitriles, but increases for the esters. (7) Interaction
of the radical cation dipoles with the solvent dipoles results in
enhanced decay of the radical cations.
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