Correlation of fluorescence quenching in carotenoporphyrin dyads with the energy of intramolecular charge transfer states. Effect of the number of conjugated double bonds of the carotenoid moiety

Article abstract of DOI:10.1039/b209694c

The electrochemistry of a series of non-symmetric synthetic carotenoids, with different conjugated double bounds chain lengths (5 to 11) is reported. The values of the first oxidation potentials of the carotenoids were evaluated by digital simulation of t

Full text of DOI:10.1039/b209694c

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Correlation of fluorescence quenching in carotenoporphyrin dyads
with the energy of intramolecular charge transfer states. Effect of
the number of conjugated double bonds of the carotenoid moiety
Fernando Fungo,a Luis Otero,a Edgardo Durantini,a William J. Thompson,b Juana J. Silber,a
Thomas A. Moore,b Ana L. Moore,b Devens Gustb and Leonides Sereno*a
a
´
Departamento de Quımica, Universidad Nacional de Rıo Cuarto, 5800, Rıo Cuarto, Argentina.
E-mail: lsereno@exa.unrc.edu.ar
Center for the Study of Early Events in Photosynthesis, Department of Chemistry and
Biochemistry, Arizona State University, Tempe, AZ 85297-1604, USA
´
´
b
Received 4th October 2002, Accepted 9th December 2002
First published as an Advance Article on the web 3rd January 2003
The electrochemistry of a series of non-symmetric synthetic carotenoids, with different conjugated double
bounds chain lengths (5 to 11) is reported. The values of the first oxidation potentials of the carotenoids were
evaluated by digital simulation of the experimental cyclic voltammograms. There is a clear relationship between
calculated (AM1) HOMO energies of neutral carotenoids with their conjugated chain length, indicating that the
change of solvation energy of carotenoids is small throughout the series, and that the electron-donating ability
of carotenoids increases with the length of the conjugated chain. Carotenoids had been previously used to
design carotenoporphyrin (C–P) molecular dyads. Carotenoid oxidation potentials and the reduction potential
of the porphyrin moiety were used in order to calculate the energy of intramolecular charge transfer state in
C–P dyads. Correlation of porphyrin fluorescence quenching of these dyads with the energy of the charge
transfer state is reported, showing that effective quenching is only possible for carotenoids with more than
eight conjugated double bonds.
quenching mechanism of the chlorophyll singlet state by caro-
Introduction
tenoids has been proposed to be a consequence of electron
transfer processes in a series of carotenoporphyrin dyad
molecules.5,6
Carotenoid pigments are present in the photosynthetic appara-
tus of green plants and many others organisms, where sun light
absorption is responsible for the initiation of a number of com-
plex reactions in order to convert electromagnetic energy into
chemical potential. The role of carotenoids is usually classified
in three functions resulting from their unique structural, elec-
tronic and photophysical properties:1 (a) Antenna function.
Most of the absorbed light is supplied to the reaction center
of the photosynthetic apparatus by singlet-singlet energy trans-
fer from antenna systems, where carotenoids play a very
important role. They strongly absorb light in the 400–550
nm region, and their singlet states can transfer excitation
energy through singlet-singlet mechanism to chlorophyll mole-
cules.2 (b) Photoprotection. The generation of the chlorophyll
triplet excited state in photosynthetic organisms is a dangerous
process. This state is a very good sensitizer for the formation of
singlet oxygen, a highly reactive species that is able to destroy
the organism. Carotenoid compounds quench efficiently both
porphyrin triplet or oxygen singlet due to their low energy tri-
plet state. (c) Photosynthesis regulation. Under illumination
conditions where the rate of light absorption exceeds the turn-
over rate of the photosynthetic reaction centre, light induced
damage in the organism is possible. When some photosyn-
thetic organisms are under high illumination regime, excess
of chlorophyll excitation energy is dissipated by a mechanism
where carotenoid polyenes are involved. Details about this
process are not yet completely elucidated, but have been corre-
lated with the increase of zeaxanthin (a xanthophyll of 11 con-
jugated double bonds) concentration in the photosynthetic
apparatus at high levels of light.3,4 On the other hand, the
Another possible function of carotenoids is as electron con-
ductors. The carotenoids possess a chain of alternating double
and single C-C bonds suggesting that they can act as ‘‘molecu-
lar wires’’. Indeed, carotenoids have been found to be ꢀ106
times better conductors of electrons than equal-length satu-
rated hydrocarbons.7 In isolated photosystem II preparations
carotenoids appear to conduct electrons from cytochrome
b559 to the reaction centre P680 chlorophylls.8 Although the
exact role of the process in vivo has not been established, there
is little doubt that a molecule of b-carotene conducts electrons
9–12
˚
over a distance of ꢀ30 A.
Numerous efforts have been made to design molecular struc-
tures in which the electron and energy transfer processes invol-
ving carotenoids can be probed.13 In particular, they are
present in large and complex molecules, which are formed by
covalently linked chromophores, electron donors and electron
acceptors. Triad molecules consisting of porphyrin (P) cova-
lently linked to both carotenoid (C) and quinone (Q) can
achieve photodriven electron transfer in good yield.14,15
In order to analyse which mechanism is involved in the
photosynthetic regulation process (quenching of chloro-
phylls-singlet-excited states), a series of specially designed car-
otenoporphyrin (C–P) molecular dyads were synthesized.5,6 In
the case of a fluorinated porphyrin, linked to a carotenoid by
an amide linkage, transient absorption spectra after laser exci-
tation of the porphyrin showed the formation of an absorption
band at l ¼ 975 nm, which is characteristic of carꢂotenoid
cation radical. The direct observation of Cꢁ+–Pꢁ state
DOI: 10.1039/b209694c
Phys. Chem. Chem. Phys., 2003, 5, 469–475
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469

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confirms electron transfer quenching of the porphyrin excited
state in this case.6,16 On the other hand, the thermodynamics
of this process depends on the reduction potential of the por-
phyrin and the oxidation potential of carotenoid moiety, which
is expected to be a function of the number of conjugate double
bonds.17 Studies of carotenoporphyrin dyads with carotenoids
of various lengths (5 to 11 conjugated double bonds, Fig. 1),
showed efficient porphyrin singlet excited state quenching only
for those dyads with carotenoid containing nine or more dou-
ble bonds.5 Due to the overlap of the oxidation potential of
porphyrin moiety with the oxidation potentials of the carote-
noids, this study is oriented to obtain the electrochemical prop-
erties of the individual constructing blocks of the dyads. In this
work, we report the electrochemistry of a series of non-sym-
metric synthetic carotenoids with different number of conju-
gated double bonds (5 to 11, Fig, 1), in order to correlate
the observed quenching in carotenoporphyrin dyads5 with
their redox properties.
Experimental
General chemicals
Except as indicated, all the chemicals from Aldrich were used
without further purification. Hydroquinone was recrystallized
from toluene. 1,2-Dichloroethane (DCE, Sintorgan HPLC
grade), dimethylformamide (DMF), toluene, dichloromethane
(DCM) and chloroform (GR grade from Merck) were distilled
˚
and stored over 4 A molecule sieves. Tetra-N-butylammonium
hexafluorophosphate (TBAHFP) from Aldrich was purified as
previously reported.18
Fig. 1 Structure of the carotenoids studied (C5 to C11) and model porphyrin (P).The carotenoporphyrin (Cn–P) dyads (where n stands for the
number of conjugated double bonds in the carotenoid moiety) C5–P to C11–P were obtained by attaching the different moieties employing amide
bonds.
470
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Synthesis of carotenoids
The preparations of C6–C9 have been previously reported.5
Carotenoid C5: All-trans-retinoic acid (200 mg, 0.66 mmol)
was converted to the acid chloride derivative by reaction with
thionyl chloride (150 mL, 2 mmol) in toluene (15 mL) and pyr-
idine (10 mL) at room temperature. The mixture was dried
under vacuum and redissolved in a mixture of toluene (10
mL) and pyridine (5 mL). Then, 5 mL of methanol were added
and the mixture stirred for 2 h at room temperature. The sol-
vents were removed under reduced pressure and flash chroma-
tography column (silica gel, chloroform) afforded 182 mg
(88%) of C5. Carotenoid C10: A solution of 8’-apo-b-caro-
ten-8’-al (400 mg, 0.96 mmol) and triethyl phosphonoacetate
(430 mg, 1.92 mmol) in 20 mL of THF was stirred with sodium
methoxide (216 mg, 4 mmol) in 50 mL of THF for 20 min at
room temperature. The mixture was treated with water and
extracted with dichloromethane. The organic phase was dried
with MgSO4 , filtered and the solvents removed under vacuum.
Carotenoid C10 was crystallized from a mixture of methanol
and dichloromethane to yield 438 mg (94%). Carotenoid
C11: Carotenoid C10 was converted to the aldehyde in two
steps. First, C10 was reduced to the alcohol with DIBAL-H
in THF at 0 ꢃC, followed by oxidation of the alcohol to the
aldehyde with activated MnO2 .5 The aldehyde (200 mg, 0.45
mmol) was treated with triethyl phosphonoacetate (215 mg,
0.96 mmol) as described for C10. Flash chromatography col-
umn (silica gel, dichloromethane) afforded 227 mg (96%) of
C11. Spectroscopic data of the carotenoid derivatives coincide
with that previously reported.5
Fig. 2 Cyclic voltammogram at platinum electrode of C5, C7, C9
and C11 carotenoids (0.5 mM) in DCE containing 0.1 M TBAHFP
as supporting electrolyte. Sweep rate ¼ 0.075 V sꢂ1. (the currents have
been normalized for comparison).
current/A; A is the working electrode area/cm2; v the sweep
rate/V sꢂ1 and c the bulk concentration/M of the correspond-
ing carotenoid. c values were compared with model com-
pounds that exchange one and two electrons (measured with
the same working electrode in similar experimental condi-
tions). Ferrocene was selected as a model for one-electron
The synthesis and spectroscopic properties of C–P dyads
have already been described.5
exchange (c ¼ 0.73 A (V sꢂ1 ꢂ1/2
)
cmꢂ2 Mꢂ1) and 70-apo-70-
(4-aminophenyl)-b-carotene was the two electron exchange
Electrochemistry
model,21 (c ¼ 1.53 A (V sꢂ1 ꢂ1/2
)
cmꢂ2
M
ꢂ1). The c value
cmꢂ2
obtained for C5 to C8 are close to 0.82 A (V sꢂ1 ꢂ1/2
)
All electrochemical measurements were performed in DCE
containing 0.1 M TBAHFP. For cyclic voltammetry (CV) an
EG & G Princeton Applied Research PAR-273 potentiostat–
galvanostat was employed. The current and potentials were
registered either on an EG & G PAR RE 0150 XY recorder
or with a Keithley 194A high-speed voltmeter, connected to
a PC. A conventional three-compartment Pyrex cell was used.
The working electrode was a Pt disc of 0.17 cm2. All the poten-
tials were referred to a saturated calomel electrode (SCE). IR
drop was corrected by a positive feedback technique. All mea-
surements were made at T ¼ 298 ꢃK. Oxygen was carefully
removed from the solutions by a stream of nitrogen, and main-
taining a nitrogen atmosphere in the cell during the experi-
ment. The electrochemical simulation was performed with
DigiSim2 software (Bioanalytical Systems, version 2.1). The
results are compared in iterative mode with the experimental
result until a good fit is obtained.19
M
ꢂ1, indicating that these compounds exchange one electron
in this condition.
On the other hand, C9 shows two separated peaks (Fig. 2),
the value of c ¼ 0.87 A (V sꢂ1 ꢂ1/2
)
cmꢂ2 Mꢂ1 for the first
peak is consistent with the exchange of one electron. The dif-
ference between the peaks potential of both oxidation waves,
which is 0.140 V for C9, decreases for C10. The peaks practi-
cally overlap for C11, which shows a broad reversible wave
with a shoulder, that corresponds to two electrons exchange.
This behaviour is characteristic of polyenes in which by
extending conjugation one electron waves coalesce to a single
two electron redox couple.22 More recently, Heinze et al.23 stu-
died the oxidation of vinylogous b-carotenoids with between 5
and 23 double bonds. They found that the oxidation of the oli-
goenes in DCM starts with one electron redox step for b-
carotenoids with five double bonds, but when the chain length
is equal or greater than ten double bonds the first oxidation
wave involves two-electron transfer.
The absorption and fluorescence spectra of the porphyrin
were obtained with a Hewlett-Packard 5082 diode array spec-
trophotometer and a SPEX Fluoromax instrument respec-
tively.
AM1 (Austin model 1) semiempirical molecular orbital cal-
culations20 were carried out using HyperChem software.
The electrochemical behaviour of a variety of different long
chain carotenoid compounds has been extensively studied by
Kispert et al.24–35 They showed that the electroxidation in
aprotic solvents of b-carotene and other long-chain carote-
noids, including synthetic derivatives, results in at least three
heterogeneous reactions and two homogeneous ones. A similar
mechanism has been used by us to explain the electrochemical
behaviour of b-carotene,36 70-apo-70-(4-carboxyphenyl)-b-
carotene18 and 70-apo-70-(4-aminophenyl)-b-carotene.21
Thus, in order to evaluate the first oxidation potential of the
long chain carotenoids studied in this work, C9–C11, by digital
simulation of the voltammograms for the mechanism proposed
by Kispert24–35 was used as shown in eqns. (1)–(5)
Results and discussion
Cyclic voltammetry
Typical cyclic voltammograms of carotenoids are shown in
Fig. 2. As can be seen, the compounds C5 and C7 show only
an irreversible peak in the accessible range of potentials. The
same is observed for C6 and C8 (not shown). The ‘‘apparent’’
number of electrons exchanged in the overall process of these
peaks were estimated by using the experimental current func-
tion, defined as c ¼ Ip/(Av1/2c), where Ip is the value of peak
ꢁ
RH2 Ð RH2 þ þ e ðE1ꢃÞ
ð1Þ
ð2Þ
ꢁ
þ
RH2 Ð RH22þ þ e ðE2ꢃÞ
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RHþ þ e Ð RHꢁ ðE3ꢃÞ
*
2RH2
ð3Þ
ð4Þ
Table 1 Oxidation potential (CV) of C5 to C11 Carotenoids and cal-
culated (AM1) HOMO energies
Kc
ꢁ
þ
RH22þ þ RH2 )
Number of
double
Kd
)
RHþ þ Hþ
ð5Þ
2þ
E1ꢃ/V
experimental experimental energy/eV
E2ꢃ/V
HOMO
*
RH2
Compound bonds
where RH2 represents the carotenoid compounds.
The E0 values of the electrochemical reactions 1 and 2 are
strongly dependent on the presence in the carotenoid of elec-
tron donating and accepting substituents. The carotenoids
bearing terminal electron-accepting groups show two one-elec-
tron oxidation waves,28,33,37 and as the electron-withdrawing
power increased, the separation between the two-oxidation
potentials increased. The comproportionation constant (Kc ,
eqn. (4)) was calculated for a series of different carotenoid
compounds from the SEEPR data.28 It increased as the elec-
tron withdrawing capacity of the terminal groups in the caro-
tenoid increased. Reaction 3 is observed in the reverse scan of
the oxidation voltammogram of carotenoids after the produc-
tion of dication species,25 and has been attributed to the reduc-
tion of the deprotonation product (RH+), eqn. (5).
C5
C6
5
6
1.05
0.96
0.91
0.76
0.72
0.64
0.63
—
—
8.46
8.31
8.12
8.05
7.91
7.81
7.80
C7
C8
7
—
—
8
C9
9
0.86
0.78
0.70
C10
C11
10
11
unique set of values will fit the data equally well.44 Therefore
we used the DigiSim program only as a tool to obtain more
accurate values of electrochemical potentials than those esti-
mated from a CV. Some typical comparison between simulated
and experimental CV are showed in Fig. 3. The simulation
provides good agreement with the experimental data and all
the results are summarized in Table 1.
The data in Table 1 indicates that as the number of conju-
gated double bonds increases, the oxidation potentials shift
to less positive values,45 making the carotenoid a better elec-
tron donor.17,23,38 The electron transfer processes occur in
one-electron steps, with the second charge transfer potential
being higher than the first one, in agreement with previous
reports when electron withdrawing groups are present in the
carotenoid molecule.ꢃ25,46 Fig. 4 shows that there is a clear rela-
tionship between E1 and the number of conjugated double
bonds, as the carotene conjugated backbone increases the oxi-
dation becomes easier, making the carotene a better electron
donor. Moreover, the dication becomes easier to form and
between 8 and 9 conjugated double bonds its formal potential
On the other hand, for short chain carotenoids C5–C8,ꢃit is
assumed that the radical cation is formed (eqn. (1)), at E1 , in
agreement with observed current function and as was well
documented with structural related compounds.23,38 On the
reverse scan no complementary reduction peak is observed in
all the range of sweep rate studied (0.05 to 0.5 V sꢂ1). This
behaviour is typical for a fast irreversible chemical reaction
couple to the charge transfer.39–41 Thus, radical cation decom-
pose by a fast chemical reaction to a product, Pr (eqn. (6)).
ꢁ
þ
RH2 * Pr
ð6Þ
Park42 observed similar behaviour in the one electron oxida-
tion of retinol (a five double bond isoprenoid).
In the digital simulation it is assumed that the electrochemi-
cal reactions (eqns. (1)–(3)) are fast,43 with ke ¼ 0.05 cm sꢂ1
,
ꢃ
ꢃ
(E2 ) becomes closer to E1 . As is shown below, at least 8 or
more conjugated double bonds are needed for an effective
quenching of the porphyrin singlet state in C–1P dyads.
and their transfer coefficient a ¼ 0.5. The electrode geometry
was planar and the diffusion coefficient for all the species is
D ¼ 1 ꢄ 10ꢂ5 cm2
s
ꢂ1, according with previous studies.25,32
The simulation also provides the magnitude of other para-
meters, for example equilibrium constants and forward rate
constants of homogeneous reactions, but the analysis of the
significance of these values requires a more complete knowl-
edge of the mechanisms, and data obtained from other experi-
mental source. It is known that in the case of complex
mechanism, where the chemical reaction are very fast, no
Calculation of the molecular and electronic structure
Using the Hartree–Fock semi-empirical Austin model 1 (AM1)
technique, we have performed full geometry optimisations for
all the carotenoids in three cases: the neutral molecule, the sin-
gle charged molecule (spin one-half cation), and the doubly
charged molecule (spin zero dication). The calculations give
information on bond length, charge distributions and orbital
energies.
In all the carotenoids, spin one-half cations exhibit the dis-
tribution of the positive charge over the whole conjugated
Fig. 3 Experimental (full lines) and simulated (dotted lines) cyclic
voltammogram of C5 and C9 carotenoids in the same conditions of
Fig. 2. (the currents have been normalized for comparison).
Fig. 4 Correlation between the number of conjugated double bonds
and the first oxidation potential of carotenoids.
472
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Fig. 5 Theoretical (AM1) molecular parameters of: neutral species (S); radical cations (ꢅ) and dications (X). Calculated charge excess (a), and
bond length (b) for C5 carotenoid. Calculated charge excess (c), and bond length (d) for C10 carotenoid. The numeration of carbon atoms is shown
in Fig. 1.
chain.46,47 Consequently, single-charged ions exhibit bond
Calculated HOMO energies of neutral carotenoids decrease
relaxation over the whole p chain, meanwhile, in the neutral
species, the bond lengths between backbone carbon atoms
alternate in a regular manner. Typical results showing the dif-
ference between the calculated charge excess of the monoca-
tions and the neutral species are depicted for the C5 and C10
carotenoids in Fig. 5, as well as the bonds length for the same
species. More significant changes occur in the dications, com-
pared with neutral species. In short-chain carotenoids (C5–C7)
charge excess and bond relaxation indicate that both positive
charge would be extended in the whole conjugated p back-
bone, (Fig. 5a and b). Moreover, if the dication is formed in
C5 carotenoids, the charge excess should be located at single
carbon atoms at both end of the molecule. Meanwhile in
long-chain carotenoids (C9–C11) the situation is different.
Charge distribution shows that, in extended carotenoids, cen-
tral carbons bear a small positive charge and alternating car-
bons away from the centre carry the bulk of the positive
charge. This charge is distributed in the two halves of the mole-
cule over at least four carbon atoms (Fig. 5c). In addition,
bond length calculations of the p chain show that the bond
orders are inverted for most of the backbone bonds in compar-
ison with the neutral species, and bond relaxation are observed
only near to the ends of molecules (Fig. 5d). These observa-
tions reinforce the idea that long-chain carotenoids form dica-
tions relatively easy, because the charges are distributed
throughout the long polyenic skeleton, which minimize Cou-
lombic repulsion. Also, it has been observed46 that the dispro-
portionation constant (reverse eqn. (4)) decreases as the
number of conjugated double bonds increases, meaning that
the second electron transfer becomes easier as compared to
the first, like in the present case. On the other hand, it has been
shown33 that in non-symmetric carotenoids the asymmetric
charge distribution in dications leads to decreased stability.
In our case, the half of the chain near the ester group bears
greater positive charge than the other half (Fig. 5c). The dica-
tion is unstable33 and its facile deprotonation (eqn. (5)) leads
to nearly irreversible redox behaviour.
monotonically with the conjugated chain length, as is shown in
ꢃ
Fig. 6. A plot of experimental E1 and theoretical HOMO
energies show a clear relationship, as expected (Fig. 7). The
good correlation indicates that the change of solvation energy
of carotenoids is small throughout the series, and that the elec-
tron-donating ability of carotenoids increases with the length
of the conjugated chain.
Correlation of carotenoids oxidation potentials with fluorescence
quenching in carotenoporphyrin dyads
As has been already mentioned, quenching of porphyrin fluor-
escence by electron transfer from carotenoids to the porphyrin
excited singlet state is a possible mechanism for excited por-
phyrin deactivation.5,6 The already reported time resolved
fluorescence emission studies for carotenoporphyrin dyads,5
showed that the quenching is observed only with carotenoids
Fig. 6 Correlation between calculated HOMO energies and the num-
ber of conjugated double bonds of carotenoids.
Phys. Chem. Chem. Phys., 2003, 5, 469–475
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Fig. 7 Correlation between calculated HOMO energies and experi-
mental first oxidation potential of carotenoids.
having more than seven conjugated double bonds. This effect
increases with increasing solvent polarity, as is shown in Fig.
8. In the present work we observed that the first oxidation
potential of carotenoids decreases with an increase in the num-
ber of double bonds in the conjugated chain. With the oxida-
Fig. 9 Energy diagram of C–P dyads showing the porphyrin first
exited singlet state (C–1P) and the energy levels of the charge separated
states (Cꢁ+–Pꢁꢂ) with carotenoid moieties having different number of
conjugated double bonds.
ꢃ
tion E1 values of the carotenoid and the reduction potential of
the porꢂphyrin moiety (ꢂ1.19 V vs SCE)48 the energy of the
Cꢁ+–Pꢁ states in the different C–P dyads can be evaluated.
The energy of the porphyrin moiety (P) first excited singlet
states (1.9 eV) has been calculated from the average of the fre-
quencies of the longest wavelength absorption maxima and the
shortest wavelength emission maxima.48 Fig. 9 shows that
Cꢁ+–Pꢁꢂstates lie below P first excited singlet state (C–1P) only
for dyads in which the carotenoid moiety has eight or more
conjugated double bonds.
In summary, the results support the idea that an electron
transfer mechanism can be operative in the quenching of por-
phyrin singlet state in C–P dyads, at least eight double bonds
being necessary for it to be effective. It is possible that in vivo
the photosynthesis regulation mechanism also involves charge
transfer quenching by long chain carotenoids when the organ-
ism is under high illumination conditions.
´
Nacional de Investigaciones Cientıficas y Tecnicas, Consejo
de Investigaciones Cientıficas y Tecnologicas de la Provincia
´
´
´
´
de Cordoba, Agencia Nacional de Promocion Cientıfica y
Tecnologica, and Fundacion Antorchas.
´
´
´
´
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Fig. 8 Relative quantum yields of fluorescence from the porphyrin
moiety in C5–P to C11–P dyads in (K) toluene and (S) acetonitrile.
lexc ¼ 590 nm. Fluorescence of porphyrin (P) was used as reference.
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