Carotenoids in Photosynthetic Antenna Designs
J. Phys. Chem. B, Vol. 108, No. 1, 2004 415
tenoids in terms of the S2 and S1 states mentioned above is
incomplete, and that additional optically dark states need to be
considered. Koyama and co-workers presented experimental
evidence concerning the existence of the theoretically predicted
apo-â-caroten-8′-oic acid and silicon tetra-tert-butylphthalocyanine
dihydroxide (2, Aldrich, mixture of regioisomers) to generate
triad 1 has been described.29 1H NMR (500 MHz, CDCl3) δ
-0.21 (6H, s, car H-19′), 1.02 (12H, s, car H-16, H-17), 1.41
(6H, s, car H-20′), 1.45-1.47 (4H, m, car H-2), 1.60-1.62 (4H,
m, car H-3), 1.71 (6H, s, car H-18), 1.81-1.83 (36H, m, Pc
H-1′), 1.98 (6H, s, car H-19), 1.99 (6H, s, car H-20), 2.01 (4H,
t, J ) 6.0, car H-4), 3.59 (2H, d, J ) 11.5, car H-10′), 5.08
(2H, d, J ) 14.5, car H-12′), 5.19 (2H, d, d, J ) 13.0, 11.5),
5.81 (2H, d, J ) 12.0, car H-14′), 6.11 (2H, d, J ) 7.0, car
H-14), 6.13-6.17 (6H, m, car H-7, H-8, H-10), 6.30 (2H, d, J
) 15.5, car H-12), 6.35 (2H, d, d, J ) 13.5, 13.5, car H-15′),
6.58 (2H, d, d, J ) 13.0, 13.0, car H-15) 6.67 (2H, d, d, J )
13.5, 12.0, car H-11), 8.44 (4H, d, J ) 8.0, Pc H-2(3), H-9(10),
H-16(17), H-23(24)), 9.58-9.74 (8H, m, Pc H-1, H-4, H-11,
H-15, H-18, H-22, H-25); (MALDI-TOF) m/z 1628 (M)+, calc.
for C108 H126 N8 O4 1628.3; UV/VIS (Toluene) 693(Pc), 662-
(Pc), 622(Pc), 478(C), 452 (C) and 366(Pc) nm.
1Bu state in carotenoids by carrying out Raman excitation
-
profile measurements.24,25 Subsequent ultrafast studies were
interpreted in terms of an internal conversion cascade from S2,
via 1Bu- to S1 on the sub-100 fs time scale.26,27 Using multicolor
femtosecond spectroscopy, we (R.v.G. and J. K.) uncovered
another new optically forbidden electronic state in carotenoids,
which we labeled S*. Surprisingly, in carotenoids bound to the
LH complexes of purple photosynthetic bacteria, this new S*
state is the precursor on an ultrafast reaction pathway to the
carotenoid triplet state.18 To explain the ultrafast triplet forma-
tion, a singlet fission mechanism was invoked by which the S*
singlet state separates into a pair of triplet states localized on
separate parts of the polyene chain, thereby conserving a total
singlet spin. In subsequent work, we obtained clear evidence
that S* is active as an excited-state energy donor to BChl in
bacterial LH complexes.23 So far, the only information we have
on the new S* state is phenomenological: we know its optical
absorption properties and its dynamical behavior. Its nature,
identity, and origin have remained elusive. Given its ability to
generate triplets, we suggested that S* may correspond to one
of the “covalent” optically dark states, like the 1Bu- state, which
exhibits a doubly excited triplet character.28
The development and study of simple artificial photosynthetic
antennas serve a number of goals. On one hand, technological
development of organic photovoltaic devices could greatly
benefit from insights and design considerations that derive from
artificial systems. As carotenoids constitute an integral part of
the natural photosynthetic machinery where they have a
multitude of functions, it would appear advantageous to
incorporate them into artificial antennas. On the other hand,
artificial light-harvesting antennas can be designed in a mini-
malistic way to exert specific functions, like maximizing
absorption cross sections, accessing certain spectral windows,
and carrying out efficient energy transport or photoprotection
in its various forms. The basic simplicity of artificial antenna
systems has important advantages over the invariably far more
complex natural photosynthetic systems. Their specific photo-
physics can be related to unambiguous energetic, electronic, and
structural features through the use of advanced spectroscopic
methods, and this may yield important insights into many aspects
of natural photosynthesis.29-32
Dicarotenophthalocyanine 2 (Triad 2). Methyl 6′-apo-â-
caroten-6′-oate (4′) was synthesized according to previous
methods (same as above). The corresponding acid, 6′-apo-â-
caroten-6′-oic acid, was obtained by basic hydrolysis. This acid
(94.8 mg, 0.21 mmol) was transformed to the corresponding
acid chloride by dissolving it in 5 mL of a 4:1 toluene:pyridine
solution and adding 6 drops of thionyl chloride. The reaction
mixture was stirred under a nitrogen atmosphere for 10 min,
and then the solvents were removed under high vacuum. A
solution of silicon tetra-tert-butyl-phthalocyanine dihydroxide
(2, Aldrich, mixture of regioisomers) (20.4 mg, 0.025 mmol)
in 2-picoline (4 mL, distilled from CaH2) was added to the
carotenoid acid chloride. Stirring continued under a nitrogen
atmosphere for 4 h at 60 °C. At this time, 30 mg of
4-(dimethylamino)pyridine (DMAP) was added, and the mixture
was allowed to react for another 24 h. Purification was done
by column chromatography (silica, 6:4 CH2Cl2:hexanes) to yield
16.2 mg of the final product (37.8% yield). 1H NMR (500 MHz,
CDCl3) δ 0.81 (6H, s, car H-19′), 1.01 (12H, s, car H-16, H-17),
1.42-1.45 (4H, m, car H-2), 1.60-1.63 (4H, m, car H-3), 1.68
(6H, s, car H-20′), 1.70 (6H, s, car H-18), 1.81-1.82 (36H, m,
Pc H-1′), 1.95 (6H, s, car H-19), 1.96 (6H, s, car H-20), 1.99-
2.00 (4H, m, car H-4), 2.92 (2H, d, J ) 15.5, car H-7′), 4.44
(2H, d, J ) 15.5, car H-8′), 5.19-5.21 (2H, m, car H-10′),
5.97-5.98 (2H, m, car H-11′), 6.09-6.19 (10H, m, car H-7,
H-8, H-10, H-12, H-15), 6.31 (2H, d, J ) 15.0, car H-14′), 6.57
(2H, d, J ) 12.5, car H-12′), 6.61-6.62 (2H, m, car H-11),
6.65 (2H, d,d, J ) 13.5, J ) 12.0, car H-15′), 8.44 (4H, d, J )
8.5, Pc H-2(3), H-9(10), H-16(17), H-23(24)), 9.58-9.73 (8H,
m, Pc H-1, H-4, H-8, H-11, H-15, H-18, H-22, H-25); MALDI/
TOF m/z 1679 (M)+, base peak 1238 (M - 441 (carotene))+,
calc. for C112 H130 N8 O4 Si 1680.4; UV/VIS (CH2Cl2) 693-
(Pc), 663(Pc), 623(Pc), 468(C), 361(Pc) nm.
In this report, we have investigated the light-harvesting and
photoprotective function of carotenoids in simple artificial
photosynthetic antennas that consist of a phthalocyanine (Pc)
moiety, to which a pair of carotenoids have been covalently
linked in an axial way (Figure 1). By varying the number of
carbon-carbon conjugated double bonds of the carotenoid
moieties in the triads, we have studied the influence of the
relative energy levels of carotenoid and Pc on their designed
functions. Spectroscopic studies show that these simple antennas
are capable of performing light harvesting, photoprotective, and
electron transfer processes, and the mechanisms, pathways, and
electronic states involved are highly reminiscent of those in
natural photosynthesis.
Instrumental Techniques. Ultraviolet-visible absorption
spectra were measured on a Cary 500 UV-vis-NIR spectro-
photometer, and corrected fluorescence excitation and emission
spectra were obtained using a Photon Technology International
MP-1 spectrofluorometer and optically dilute samples (A <
0.07). The excitation spectrum of silicon 1,4,8,11,15,18,22,25-
octabutoxyphthalocyanine dihydroxide 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.
2. Materials and Methods
Synthesis. Dicarotenophthalocyanine 1 (Triad 1). Methyl 8′-
apo-â-caroten-8′-oate (3) was prepared by published proce-
dures,33 and 8′-apo-â-caroten-8′-oic acid was prepared by base-
catalyzed hydrolysis of 3. The coupling reaction between 8′-
Femtosecond transient absorption measurements in the visible
were carried out with an amplified Ti:sapphire laser system as
described earlier.14 In brief, part of the output of a 1 kHz