High-efficiency energy transfer from carotenoids to a phthalocyanine in an artificial photosynthetic antenna

Article abstract of DOI:10.1562/0031-8655(2002)076<0116:HEETFC>2.0.CO;2

Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S₁

Full text of DOI:10.1562/0031-8655(2002)076<0116:HEETFC>2.0.CO;2

High-efficiency Energy Transfer from Carotenoids to a Phthalocyanine in an
Artificial Photosynthetic Antenna
Author(s): Ernesto Mariño-Ochoa, Rodrigo Palacios, Gerdenis Kodis, Alisdair N. Macpherson, Tomas
Gillbro, Devens Gust, Thomas A. Moore, and Ana L. Moore
Source: Photochemistry and Photobiology, 76(1):116-121.
Published By: American Society for Photobiology
DOI: http://dx.doi.org/10.1562/0031-8655(2002)076<0116:HEETFC>2.0.CO;2
URL: http://www.bioone.org/doi/full/10.1562/0031-8655%282002%29076%3C0116%3AHEETFC
%3E2.0.CO%3B2
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Photochemistry and Photobiology, 2002, 76(1): 116–121
High-efficiency Energy Transfer from Carotenoids to a Phthalocyanine in
an Artificial Photosynthetic Antenna¶
Ernesto Marin˜o-Ochoa1, Rodrigo Palacios1, Gerdenis Kodis1, Alisdair N. Macpherson*2, Tomas Gillbro2,
Devens Gust*1, Thomas A. Moore*1 and Ana L. Moore*1
1Department of Chemistry and Biochemistry and the Center for the Study of Early Events in Photosynthesis, Arizona State
University, Tempe, AZ and
2Department of Biophysical Chemistry, Umea˚ University, Umea˚, Sweden
Received 11 April 2002; accepted 2 May 2002
ABSTRACT
photoinduced electron transfer in a system having an alter-
native molecular geometry in which the carotenoids are al-
most perpendicular to the plane of the tetrapyrrole and are
located at its center.
Two carotenoid pigments have been linked as axial li-
gands to the central silicon atom of a phthalocyanine de-
rivative, forming molecular triad 1. Laser flash studies
on the femtosecond and picosecond time scales show that
both the carotenoid S1 and S2 excited states act as donor
states in 1, resulting in highly efficient singlet energy
transfer from the carotenoids to the phthalocyanine.
Triplet energy transfer in the opposite direction was also
observed. In polar solvents efficient electron transfer
from a carotenoid to the phthalocyanine excited singlet
state yields a charge-separated state that recombines to
the ground state of 1.
INTRODUCTION
Carotenoid pigments are ubiquitous in photosynthetic organ-
isms, where the light they absorb drives a significant fraction
of the photosynthetic biomass production. In the photosyn-
thetic light harvesting pigment protein complexes examined
to date, the carotenoid antenna molecules are found elec-
tronically coupled to chlorophylls by relatively short-range
interactions that are necessary for efficient singlet energy
transfer (1). It is of interest to find ways to use these pig-
ments in artificial photosynthetic assemblies because, in ad-
dition to being powerful light absorbers, they can protect
systems from oxidative damage and participate in electron
transfer processes. In the carotenoid-containing tetrapyrrole-
based model systems reported so far, the carotenoid has been
covalently linked to the periphery of the tetrapyrrole, where
the short-range interactions necessary to promote energy
transfer arise from the proximity of the carotenoid
system to -electrons at the edge of the tetrapyrrole (2–4).
Triad 1 was designed to explore singlet energy transfer and
␲-electron
MATERIALS AND METHODS
␲
Instrumental techniques. 1H nuclear magnetic resonance (NMR)
spectra were recorded on a Unity Inova 500 (Varian, Palo Alto, CA)
spectrometer at 500 MHz. The samples were dissolved in deuter-
iochloroform with tetramethylsilane as an internal reference. Mass
spectra were obtained with a matrix-assisted laser desorption time-
of-flight spectrometer (matrix used: sulfur). Ultraviolet–visible ab-
sorption spectra were measured on a Cary 500 (Varian Techtron,
Australia) UV-VIS-NIR spectrophotometer, and corrected fluores-
cence excitation and emission spectra were obtained using a Photon
Technology International MP-1 (Photon Technology International,
Monmouth Junction, NJ) spectrofluorimeter and optically dilute
¶Posted on the web site on 16 May 2002.
*To whom correspondence should be addressed at: Prof. Thomas A.
Moore, Department of Chemistry and Biochemistry, University of
Arizona, P.O. Box 871604, Tempe, AZ 85287-1604, USA. Fax:
480-965-2747; e-mail: tmoore@asu.edu; alisdair.macpherson@
chem.umu.se; dgust@asu.edu; amoore@asu.edu
Abbreviations: NMR, nuclear magnetic resonance; THF, tetrahydro-
furan.
᭧
2002 American Society for Photobiology 0031-8655/02
$5.00
ϩ0.00
samples (AϽ0.07). The excitation spectra of silicon 1,4,8,11,15,
116

Photochemistry and Photobiology, 2002, 76(1) 117
18,22,25-octabutoxyphthalocyanine dihydroxide and triad 1 were re-
corded under identical conditions. The excitation spectrum of silicon
1,4,8,11,15,18,22,25-octabutoxyphthalocyanine dihydroxide, which
has some absorption in the carotenoid region (360–530 nm), was
used to generate an excitation correction file in the 300–750 nm
region by assuming that the absorption and fluorescence excitation
spectra were identical.
Fluorescence decays on the picosecond–nanosecond time scale
were performed on
ϳ1 ϫ
10–5 M solutions by the time-correlated
single photon counting method, using an apparatus that has been
previously described (5). Fluorescence decays on the femtosecond–
picosecond time scale were recorded using the fluorescence upcon-
version technique and a spectrometer that had been modified to give
better time response than previously described (6). Excitation light
centered at 489 nm was obtained by frequency-doubling the ca 75
fs near-IR pulses produced by a Tsunami (Spectra-Physics Lasers,
Mountain View, CA) 82 MHz mode-locked Ti:sapphire laser
pumped by an argon ion laser. Fluorescence decays were recorded
at up to 13 wavelengths in the 552–710 nm range using parallel (in
the carotenoid S2 emission region) or magic angle (in the region of
the tetrapyrrole emission) polarization. Fluorescence transients were
fitted to a sum of two or more exponential decays with a floating
background and were convoluted with a Gaussian response function,
using the Spectra program.
Transient absorption measurements on the femtosecond–picosec-
ond time scale were made using the pump–probe technique, with
systems described previously (7,8). To determine the number of life-
time components in the transient absorption data, singular value de-
composition analysis was performed and decay associated spectra
derived, as previously described (9). Nanosecond transient absorp-
tion measurements were made with excitation from an Opotek (Opo-
tek, Carlsbad, CA) optical parametric oscillator pumped by the third
harmonic of a Continuum Surelight (Continuum, Santa Clara, CA)
Figure 1. Absorption spectra of triad 1 (—), phthalocyanine 2
(- - -) and carotenoid 3 (-·-) in toluene solution and fluorescence
emission spectrum of triad 1 (······) with excitation at 610 nm in the
same solvent. The spectra of 1 and 2 are normalized at the Qy ab-
sorption band of the phthalocyanine (between 675 and 695 nm). The
spectrum of 3 is presented to illustrate peak positions and band-
shape; the optical density ordinate is arbitrary.
lines, in conjunction with molecular modeling (AM1, Gauss-
ian 98) and crystal structure data for silicon carboxylato-
phthalocyanines (14,15) was used to estimate the theoretical
changes in chemical shift (⌬␦) in the 1H resonances expected
for carotenoid protons for different conformations. The dif-
Nd:YAG laser (pulse width
ϳ5 ns, repetition rate 10 Hz). The tran-
sient absorbance was measured using a diode detector equipped with
10 nm band-pass interference filters. The detection portion of the
spectrometer has been described previously (10).
The solvents used for the spectroscopic work were toluene
(Ͼ99%), tetrahydrofuran (THF; Ͼ99.8%) and pentane (95%) from
Merck (Darmstadt, Germany) and EM Science (Gibbstown, NJ) and
benzonitrile (99.9%, high-performance liquid chromatography
grade) from Aldrich (Milwaukee, WI). The solvents were purified
by column chromatography with neutral activated Al2O3 (Merck,
Type I) or by distillation. THF was refluxed with sodium metal and
benzophenone before distillation.
1
ferences between the observed H chemical shifts in 1 and
the corresponding 1H resonances in model carotenoid 3 were
used to derive the experimental ⌬␦ values. An extended (all-
trans configuration and s-trans conformation) carotenoid
structure positioned above or below the phthalocyanine ring
gave the best agreement between the experimental and the
theoretical ⌬␦ for the five hydrogen atoms of the carotenoid
moiety proximal to the silicon atom (two methyl group hy-
drogens and three hydrogens on the conjugated chain).
Synthesis. Methyl 8
cording to published procedures and 8
Ј
-apo-
␤
-caroten-8
Ј
Ј
-oate (3) was prepared ac-
-apo- -caroten-8 -oic acid
␤
Ј
was prepared by base-catalyzed hydrolysis of 3 (11). The carotenoic
acid (182 mg, 0.42 mmol) was dissolved in 4 mL of toluene and 1
mL of pyridine. One drop of thionyl chloride was added to the
mixture, which was stirred under argon for 5 min. The solvent was
distilled under vacuum, and the acid chloride was kept under high
Absorption and fluorescence excitation spectra
The absorption spectrum of model phthalocyanine 2 in tol-
uene, with
␭
max
at 679, 649, 610 and 359 nm, is shown as
the dashed line in Fig. 1. This spectrum is typical of a metal
phthalocyanine, and only weak absorption is observed in the
region where the solar irradiation is highest (between 450
and 550 nm). In triad 1 (solid line, Fig. 1), the addition of
the two carotenoid pigments results in a large increase in
absorption in the center of the spectrum and a substantial
vacuum for
ϳ15 min. A suspension of 16 mg (0.02 mmol) of silicon
tetra-tert-butylphthalocyanine dihydroxide (2, Aldrich, mixture of
isomers) in 3 mL of 2-picoline was mixed with the carotenoid acid
chloride and stirred under argon for 2 h. A portion of 4-dimethylam-
inopyridine (30 mg) was added to the mixture, which was kept well
stirred under argon for an additional period of 36 h at 60ЊC. The
solvent was evaporated under reduced pressure, and the product was
purified by column chromatography (silica gel–CH2Cl2) to yield 9.6
mg (19%) of triad 1. Matrix-assisted laser desorption–ionization
time-of-flight mass spectrometry; m/z: 1628 (M)ϩ; the NMR spec-
trum is consistent with the expected structure.
redshift in all the phthalocyanine bands (
␭
at 693, 662,
max
622, 478, 452 and 366 nm). In contrast, the absorption max-
ima of the carotenoid moiety of 1 are slightly blueshifted
compared with model 3 in toluene (
␭
max
at ϳ482 nm [shoul-
der] and 456 nm). The fluorescence emission spectrum of
the phthalocyanine moiety of 1 (dotted line, Fig. 1) has a
maximum at 704 nm. Figure 2 shows the absorption spec-
trum and the corrected excitation spectrum for the phthalo-
cyanine fluorescence of 1 with emission monitored at 790
nm. The excitation spectrum was normalized to the absorp-
tion spectrum at the red-most bands. This spectrum clearly
indicates efficient singlet–singlet energy transfer from carot-
enoid to phthalocyanine.
RESULTS
Conformation of triad 1 by NMR
Silicon phthalocyanines are known to induce large shielding
effects on the NMR chemical shifts of their axial ligands on
account of the ring-current of the macrocycle
The isoshielding lines for this effect have been previously
calculated (12,13). In this work, a map of the isoshielding
␲-electrons.

118 Ernesto Marin˜o-Ochoa et al.
Figure 3. Normalized fluorescence decay kinetics at 578 nm of an
10Ϫ4 M solution of triad 1 (
) and carotenoid 3 ( ) in toluene
ϳ1
ϫ
⅜
᭞
Figure 2. The absorption spectrum of triad 1 (—) and the corrected
after excitation with parallel polarization at 489 nm. The lines are
fits with decay time constants of 55 fs (98% amplitude) and 81 fs
(97%), the carotenoid S2 lifetimes of 1 and 3, respectively. A 128
fs Gaussian response function is also shown (—).
7
fluorescence excitation spectrum (- - -) of an
ϳ1
ϫ
10Ϫ M solution
of 1 in toluene. The excitation spectrum has been normalized to the
absorption spectrum at the phthalocyanine Qy band (693 nm). The
fluorescence excitation spectrum was corrected in the range of 300–
750 nm using a file generated from the absorption and fluorescence
excitation spectra of silicon 1,4,8,11,15,18,22,25-octabutoxyphthal-
ocyanine dihydroxide. The apparent energy transfer efficiency in
Triplet state dynamics in triad 1
Figure 5 presents the transient absorption spectrum of phtha-
locyanine 2 (dashed line) in its lowest excited triplet state in
argon-saturated toluene solution measured 400 ns after ex-
citation by a laser flash at 620 nm. The transient spectrum
excess of 100% in the region of the shoulder at
caused by errors in the correction process.
ϳ480 nm is likely
decayed uniformly with a time constant of 150
Ϯ 15 ␮s and
Carotenoid singlet state dynamics
could be quenched by oxygen. The transient spectrum of
triad 1 taken after excitation by a laser flash at 680 nm is
shown by the solid line in Fig. 5. It decayed with a time
Using the fluorescence upconversion technique, the lifetime
of the strongly allowed second excited singlet state (S2,
ϩ
11Bu ) of the carotenoid, which gives rise to the absorption
constant of 4.7
oxygen.
Ϯ 0.5 ␮s and could also be quenched by
maximum at
ϳ450 nm, was measured. The S2 lifetime of
model carotenoid 3 in toluene was determined to be 82
Ϯ
3
fs (average of 20 measurements). The emission decay curve
at 577 nm is shown in Fig. 3, along with the instrument
response function. The average lifetime of the S2 state of the
carotenoid moieties of triad 1 in toluene was found to be 53
Charge transfer state dynamics in triad 1
The decay-associated absorption spectrum of triad 1 dis-
solved in THF and excited at 680 nm is shown in Fig. 6.
The prominent absorption in the near-IR (ϳ860 nm) is char-
acteristic of a carotenoid radical cation species having 10
conjugated double bonds (including the carbonyl group). In
spectral regions where the transient absorption is positive,
decaying species are indicated by positive amplitudes and
Ϯ
3 fs. The lifetime of the lowest excited singlet state (S1,
Ϫ
21Ag ) of carotenoid 3 was determined in toluene by pump
(489 nm)–probe (555 nm) techniques and found to be 24.4
Ϯ
0.6 ps. The major component of the corresponding life-
time for triad 1 was only 2.6 0.2 ps.
Ϯ
Phthalocyanine singlet state dynamics
To determine the formation kinetics of the S1 excited state
of the phthalocyanine moiety of triad 1 after excitation of
the carotenoids at 489 nm, fluorescence upconversion mea-
surements were made at wavelengths longer than 674 nm in
two solvents, toluene and THF. As shown in Fig. 4, the rise
time of the phthalocyanine fluorescence in toluene at 693
nm is clearly composed of two components, 500
Ϯ 50 fs
and 2.3 0.2 ps, and the emission does not decay over the
Ϯ
20 ps time scale recorded. In the more polar solvent, THF,
the fluorescence is strongly quenched; it decays with a time
constant of 6.2 ps. Using the time-correlated single photon
counting method, the fluorescence lifetime of 1 was found
Figure 4. Isotropic formation kinetics of the excited S1 state of the
phthalocyanine moiety of triad 1 detected by upconversion of the
phthalocyanine fluorescence at 693 nm, after carotenoid excitation
2
2
to be 2.84 ns (␹ ϭ 1.18) in toluene, 0.011 ns (␹ ϭ 1.08)
at 488 nm with magic angle polarization of an
of triad 1 in toluene ( ) and in THF ( ). The lines are fits with
ϳ1 ϫ
10Ϫ4 M solution
2
2
in THF, 0.004 ns (␹ ϭ 1.13) in benzonitrile and 5.43 ns (
␹
⅜
᭞
ϭ
1.14) in pentane, whereas the fluorescence lifetime of 2
time constants of 500 fs (0.22) and 2.3 ps (0.78) for the rise of the
signal in toluene (the values in parentheses indicate relative ampli-
tudes). The decay of the signal in THF was fitted with a 6.2 ps time
constant.
2
2
was 5.40 ns (␹ ϭ 1.01) in benzonitrile, 5.65 ns (␹ ϭ 1.19)
in toluene and 6.11 ns (
2
␹ ϭ 1.10) in THF (components
having amplitudes less than 7% omitted).

Photochemistry and Photobiology, 2002, 76(1) 119
Figure 5. Transient absorption spectrum of an
triad 1 (—) in toluene 4.3 ns after excitation at 680 nm with a laser
pulse of 100 fs. Transient absorption spectrum of an argon-satu-
rated
10Ϫ M solution of phthalocyanine 2 (- - -) in toluene 400 ns
after excitation at 620 nm with a laser pulse of 5 ns.
ϳ
10Ϫ3 M solution of
Figure 6. Decay-associated spectra of the transient absorption ob-
3
tained with an
ϳ
10Ϫ M solution of triad 1 in THF after an
ϳ
100
ϳ
fs laser pulse at 680 nm: a 6.2 ps component(—), a 25 ps component
(- - -) and a nondecaying component (···).
5
ϳ
ϳ
Singlet energy transfer
rising species by negative amplitudes. This situation is re-
versed in spectral regions where the transient absorption is
negative, e.g. decaying species are indicated by negative am-
plitudes and rising species (increases in negative absorption)
are indicated by positive amplitudes. The transient absorp-
tion intensity is large and positive in the near-IR and it is
As shown in Fig. 2, essentially complete (Ͼ95%) energy
transfer from the carotenoid to the phthalocyanine of 1 is
indicated by the match between the corrected excitation
spectrum for phthalocyanine fluorescence and the absorption
spectrum. In order to determine in detail which carotenoid
excited states are involved in the flow of energy to the phtha-
locyanine, the lifetimes of the carotenoid S2 and S1 excited
states were measured in model carotenoid 3 and in triad 1.
Figure 3 shows that the S2 level of the carotenoid is
quenched from 82 fs in 3 to 53 fs in 1. This quenching is
attributed to energy transfer and may be used to calculate an
seen to rise and decay with time constants of 6.2
Ϯ 0.6 and
25 3 ps, respectively. Similarly, the positive transient ab-
Ϯ
sorption in the spectral region from 525 to 640 nm rises and
decays with the same pair of time constants. At 490 nm the
situation is reversed—a bleaching signal grows with a 6.2
ps time constant and recovers in 25 ps. The negative ampli-
tude at 680 nm has the same time constant, 25 ps, and is
likewise associated with the recovery of a bleaching signal.
Transient spectra of similar shapes were observed when triad
1 was dissolved in toluene. The amplitudes were much
smaller, and the time constants for the decay-associated
efficiency of 35%, which is substantially less than
Ͼ95%
measured by steady state fluorescence excitation. Thus, the
contribution of a second path to the flow of energy is sus-
pected and, indeed, indicated by the results of pump–probe
measurements of the S1 state lifetime of the carotenoid in
triad 1 and model 3 and by the rise time of phthalocyanine
fluorescence presented in Fig. 4.
spectra were 2.6
Ϯ 0.3 ns and 20 Ϯ 2 ps. In pentane, the
Focusing on the phthalocyanine data in toluene, two com-
ponents are found, with time constants of 500 fs and 2.3 ps.
The 500 fs component is assigned to energy arriving from
the S2 level of the carotenoid. The delay (53 fs vs 500 fs)
must be associated with relaxation to the phthalocyanine S1
state before fluorescence. It is reasonable to assume that in-
ternal conversion in the carotenoid from S2 state to S1 state
is the dominant process competing with energy transfer from
S2, and a yield of 65% may be assigned to this process. The
2.3 ps component in the rise time of phthalocyanine fluo-
rescence in 1 matches well the S1 state carotenoid lifetime
measured by pump–probe (2.6 ps), and it is assigned to en-
ergy transfer from the carotenoid S1 level. Quenching of the
carotenoid S1 level from 24.4 ps in 3 to 2.6 ps in 1 gives an
energy transfer efficiency of 89%. Taking into account that
the quantum yield from the S1 state is only 65%, the quan-
tum yield of energy transfer from S1, based on light absorbed
to populate S2, is calculated to be 58%. Therefore, the over-
all quantum yield of energy transfer from carotenoid to
phthalocyanine from both S2 (35%) and S1 (58%) pathways
fluorescence lifetime of triad 1 was 5.43 ns, and there was
no evidence of transient absorption in the near-IR.
DISCUSSION
Conformation and absorption spectra
1
The fits of observed shifts in the carotenoid H NMR of 1
to the calculated values indicate that the carotenoids are dis-
posed axially from the phthalocyanine with average confor-
mations in which the angle between the carotenoid long axis
and the phthalocyanine plane is
ϳ77Њ. As presented in Fig.
1, the absorption spectrum of triad 1 clearly displays the
contributions of the two carotenoids to the spectrum of the
phthalocyanine. The spectrum of 1 is not a simple super-
position of the model phthalocyanine 2 and carotenoid 3
spectral components. The red-most band at 679 nm in 2 is
shifted to 693 nm in triad 1, and the carotenoid band at 456
nm in model carotenoid 3 is shifted to 452 nm in triad 1. It
is clear from Fig. 1 that a small degree of interchromophore
interaction occurs in 1.
is
ϳ93%, in excellent agreement with the value estimated
from steady state fluorescence excitation measurements. En-

120 Ernesto Marin˜o-Ochoa et al.
ergy flow to the phthalocyanine from both the carotenoid S2
and S1 excited states in 1 models very well the same phe-
nomenon found in natural systems (8,16–18).
earlier times (not shown) when the population of phthalo-
cyanine singlet species is greater, the Qy bleach signal is
larger; it is composed mostly of depleted ground state pop-
ulation and stimulated emission. On a time scale of 100 ns
to several microseconds, long after all the excited and ionic
species have decayed except the carotenoid triplet, a similar
but smaller Qy bleach is observed. This apparent decrease in
the absorption coefficient of the phthalocyanine Qy band is
associated with a perturbation caused by the attached triplet
carotenoid. Such spectral behavior has been observed in nat-
ural antenna systems and in other model systems and is fur-
ther evidence that the chromophores are tightly coupled; e.g.
there is sufficient electronic coupling to mediate energy
transfer on the femtosecond time scale (22–24).
The observation that both the strongly electric dipole al-
lowed S2 state and the extremely electric dipole forbidden
S1 state make major contributions, albeit on different time
scales, to the singlet energy transfer in 1 has mechanistic
implications. To the extent that the carotenoids are perpen-
dicular to the plane of the phthalocyanine, and located at its
center, coupling of donor S2 state and acceptor state transi-
tion dipole moments in the Fo¨rster sense would be small
because of the orientation factor. Moreover, the carotenoid
S1 state transition dipole moment is exceedingly small. Re-
cent theoretical treatment of the electronic coupling consid-
ers point dipoles generated by the electronic transition lo-
cated on each atom of the two chromophores. The interac-
tions between the point dipoles are then summed over the
volume of the donor and acceptor to give a coulombic cou-
pling term (17,19). It will be interesting to apply this tech-
nique to 1 and to carry out a similar calculation on a caro-
tenophthalocyanine in which the carotenoid is linked to the
periphery of the macrocycle and extends away from it.
Theoretical predictions and experimental results have lo-
cated a carotenoid excited singlet state above S1 and near S2
as defined herein. Under the conditions of our experiments
(temporal resolution, wavelength range, etc.), there is no ev-
idence that this state plays a role in the singlet energy trans-
fer in 1 (20,21).
Electron transfer
As shown in Fig. 4, the excited singlet state of the phtha-
locyanine moiety of 1 in toluene does not decay on the pi-
cosecond time scale but is quenched to 6.2 ps in THF. This
behavior is characteristic of electron donor–acceptor dyads
in which a charge transfer state is close in energy to the
local excited singlet state. Therefore, in nonpolar solvents
the charge transfer state is slightly higher than the local ex-
cited singlet and not accessible, whereas in polar solvents it
is lower and populated by electron transfer quenching of the
local excited singlet level. In order to verify relaxation of
the phthalocyanine S1 level to a charge transfer state, spectra
were taken in the region where the radical ions absorb. In
the decay-associated spectra of 1 in THF (Fig. 6), the rise
and decay of a strong signal in the near-IR is evident. Con-
comitantly, the carotenoid ground state bleach at 490 nm
rises and decays. The signal at 860 nm is assigned to the
formation and decay of the carotenoid radical cation moiety
of 1; it forms with a time constant of 6.2 ps and decays in
25 ps. Also present in the decay-associated spectra are rise
and decay components characteristic of the sum of the phtha-
locyanine S1 state (decaying), the radical anion species (ris-
ing and decaying) and the recovery of the ground state. The
match of the rise of the carotenoid radical cation spectrum
with the decay of the phthalocyanine S1 level (6.2 ps) estab-
lishes electron transfer from the carotenoid to the phthalo-
cyanine S1 state as a fluorescence quenching mechanism.
The charge transfer state forms with a quantum yield near
unity, living for 25 ps and decaying to the ground state of
triad 1.
Triplet energy transfer
The dashed line in Fig. 5 displays the transient absorption
spectrum of phthalocyanine 2 in argon saturated toluene so-
lution taken 400 ns after the laser flash at 620 nm. The long-
lived signal is assigned to the phthalocyanine triplet species
on the basis of positive absorption in the 450 to 600 nm
region, ground state bleaching and oxygen-dependent decay
dynamics. Evidence for fast triplet–triplet energy transfer in
triad 1 is shown by the transient spectrum taken 4.3 ns after
flash excitation at 680 nm. The ground state bleach in the
spectral region
to 600 nm region are characteristic of the carotenoid triplet
species. The lifetime of 4.7 s confirms the assignment of
ϳ420 to 470 nm and positive ⌬A in the 475
␮
this transient to the carotenoid triplet species. Because ex-
citation at 680 nm does not populate carotenoid singlet
states, and in any case intersystem crossing in carotenoids
is very inefficient, the high yield of triplet carotenoid must
be because of intramolecular triplet energy transfer from the
phthalocyanine triplet. The carotenoid triplet rise time (data
not shown) is the same as the fluorescence lifetime of 1 in
toluene, which means that the rate constant for triplet–triplet
energy transfer is much larger than that for intersystem
crossing in the phthalocyanine moiety of 1. Presumably, trip-
let–triplet energy transfer is mediated by Dexter electron ex-
change terms that require orbital overlap between the carot-
enoid and phthalocyanine moieties to be effective. There-
fore, the bridging Si atom must be effective in providing this
orbital interaction.
Spectra with similar shapes having reduced amplitude
were observed in toluene and were fitted to two character-
istic time constants, 2.6 ns and 20 ps. On the basis of the
2.8 ns fluorescence lifetime of the phthalocyanine moiety of
1 in toluene, it is tempting to assign the 2.6 ns component
in the decay-associated spectra to a pathway for formation
of the charge separated state; the 20 ps component would
then be the lifetime of that state. However, a 20 ps decay
time taken with a 2.6 ns formation time should reduce the
amplitude of the transient spectra in toluene by
compared with THF. The observed amplitude is reduced by
20-fold. Thus, the assignment of electron transfer dynam-
ϳ150-fold
ϳ
Note that at 4.3 ns after an excitation flash of
(Fig. 5) the transient absorption shows a strong bleach in the
region of the phthalocyanine Qy band. In spectra taken at
ϳ
100 fs
ics in toluene awaits further experiments. On the basis of
the 2.6 ns formation time, the quantum yield of this pathway
is
ϳ50%. Triad 1 dissolved in pentane had a 5.4 ns fluores-

Photochemistry and Photobiology, 2002, 76(1) 121
in tetrathiafulvalene–porphyrin–fullerene molecular triads. Helv.
Chim. Acta 84, 2765–2783.
10. Davis, F. S., G. A. Nemeth, D. M. Anjo, L. R. Makings, D.
Gust and T. A. Moore (1987) Digital back off for computer
controlled flash spectrometers. Rev. Sci. Instrum. 58, 1629–
1631.
cence lifetime, and no charge-separated species were ob-
served.
CONCLUSION
Triad 1 exemplifies a compact molecular architecture in
which two carotenoid pigments are coupled to a single tet-
rapyrrole as axial ligands to a centrally coordinated Si atom.
This construct demonstrates highly efficient singlet energy
transfer from the carotenoids to the tetrapyrrole and fast trip-
let energy transfer in the reverse direction. In polar solvents,
photoinduced electron transfer from one of the attached ca-
rotenoids to the S1 level of the phthalocyanine is efficient
and yields a relatively long-lived charge-separated state. It
is anticipated that this new class of artificial photosynthetic
structures will form the basis of antenna systems and charge
relays for artificial reaction centers.
11. Cardoso, S. L., D. E. Nicodem, T. A. Moore, A. L. Moore and
D. Gust (1996) Synthesis and fluorescence quenching studies of
a series of carotenoporphyrins with carotenoids of various
lengths. J. Braz. Chem. Soc. 7, 19–30.
12. Janson, T. R., A. R. Kane, J. F. Sullivan, K. Knox and M. E.
Kenney (1969) Ring-current effect of the phthalocyanine ring.
J. Am. Chem. Soc. 91, 5210–5214.
13. Koyama, T., T. Suzuki, K. Hanabusa, H. Shirai and N. Kobay-
ashi (1994) A comparison of the loop-current effect of silicon
phthalocyanine and silicon naphthalocyanine rings on their axial
ligands. Inorg. Chim. Acta 218, 41–45.
14. Silver, J., C. S. Frampton, G. R. Fern, D. A. Davies, J. R. Miller
and J. L. Sosa-Sanchez (2001) Novel seven coordination ge-
ometry of Sn(IV): crystal structures of phthalocyaninato
bis(undecylcarboxylato)Sn(IV), its Si(IV) analogue, and phthal-
ocyaninato bis(chloro)silicon(IV). The electrochemistry of the
Si(IV) analogue and related compounds. Inorg. Chem. 40,
5434–5439.
15. Silver, J., J. L. Sosa-Sanchez and C. S. Frampton (1998) Struc-
ture, electrochemistry, and properties of bis(ferrocenecarbo-
xylato)(phthalocyaninato)silicon(IV) and its implications for (Si
(Pc)O)n polymer chemistry. Inorg. Chem. 37, 411–417.
16. Walla, P. J., P. A. Linden, C.-P. Hsu, G. D. Scholes and G. R.
Fleming (2000) Femtosecond dynamics of the forbidden carot-
enoid S1 state in light-harvesting complexes of purple bacteria
observed after two-photon excitation. PNAS 97, 10808–10813.
17. Krueger, B. P., G. D. Scholes, R. Jimenez and G. R. Fleming
(1998) Electronic excitation transfer from carotenoid to bacte-
riochlorophyll in the purple bacterium Rhodopseudomonas aci-
dophila. J. Phys. Chem. B 102, 2284–2292.
18. Shreve, A. P., J. K. Trautman, H. A. Frank, T. G. Owens and
A. C. Albrecht (1991) Femtosecond energy-transfer processes
in the B800–850 light-harvesting complex of Rhodobacter
sphaeroides 2.4.1. Biochim. Biophys. Acta 1058, 280–288.
19. Damjanovic´, A., T. Ritz and K. Schulten (1999) Energy transfer
between carotenoids and bacteriochlorophylls in light-harvest-
ing complex II of purple bacteria. Phys. Rev. E 59, 3293–3311.
Acknowledgments This work was supported by the U.S. Depart-
ment of Energy (DE-FG03-93ER14404), the Swedish Natural Sci-
ence Research Council (NFR) and the Kempe Foundation. We thank
Roche for the generous gift of carotenoid samples. This is publica-
tion 506 from the ASU Center for the Study of Early Events in
Photosynthesis.
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