J. Phys. Chem. B 2002, 106, 5761-5768
5761
Energy Transfer in Supramolecular Artificial Antennae Units of Synthetic Zinc Chlorins and Co-aggregated Energy Traps. A Time-Resolved Fluorescence Study†,‡ V. I. Prokhorenko,§ A. R. Holzwarth,*,§ M. G. Mu1 ller,§ K. Schaffner,§ T. Miyatake,| and H. Tamiaki⊥ Max-Planck-Institut fu¨ r Strahlenchemie, Postfach 101365, D-45413 Mu¨ lheim an der Ruhr, Germany, Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku UniVersity, Otsu, Shiga, 520-2194, Japan, and Department of Bioscience and Biotechnology, Ritsumeikan UniVersity, Kusatsu, Shiga 525-8577, Japan ReceiVed: July 9, 2001; In Final Form: January 8, 2002
Using time-resolved fluorescence, we explored the energy transfer process(es) in supramolecular zinc chlorin aggregates co-aggregated with various kinds of energy traps. The energy transfer times from the antenna aggregate to the trap are in the picosecond time range (7-9 ps) under reducing conditions (addition of dithionite to avoid oxidative quenching) and were resolved in all cases. Under nonreducing conditions substantial fluorescence quenching occurred in the antenna aggregates. We tentatively suggest that a small amount of chlorin cations acts as a quencher. We find that in the aggregates the excitation is delocalized over at least 10-15 pigments, on the basis of the corresponding strong increase of the pure radiative rate vs a monomeric chlorin. In bacteriochlorophyll-based aggregates the transfer to the energy trap is biexponential (27 and 91 ps), which is reminiscent of native isolated chlorosomes from green sulfur bacteria. For the zinc chlorinbased light-harvesting units the overall efficiency of the energy collection reaches up to 70%. We conclude that these units have suitable properties as artificial antenna systems for solar energy utilization.
Introduction In green photosynthetic bacteria the supramolecular selfassembled aggregates of bacteriochlorophylls (BChl) form the major light-harvesting antenna complexes, known as chlorosomes1-8 (see ref 9 for a review). They contain several thousands of BChls, arranged into rodlike aggregates, which are tightly packed into an organelle surrounded by a lipid monolayer.10,11 The mechanism of self-organization of the particular BChls contained in these organelles and their relative orientation is now quite well established.3,12-24 Due to a regular organization of pigments and strong excitonic interactions,25 the dipole strength is concentrated in a few allowed excitonic transitions, which is very favorable for the highly efficient light collection and energy transfer properties of these aggregates. Self-organized aggregates with spectroscopic properties very similar to those of BChls in native chlorosomes can be easily prepared also using synthetic zinc chlorins, as was demonstrated in numerous studies.19,20,26-36 Such supramolecular assemblies could provide an elegant way to attach an antenna function to an artificial reaction center unit.37,38 However, if these units were to be used for efficient light energy collection, the absorbed excitation should be transferred rapidly to an energy trap from where it could be utilized in further chemical reaction steps. In native chlorosomes the excitation energy is transferred within
a few picoseconds to the so-called baseplate containing BChl a pigments, which function as the first energy trap.39-41 Subsequently, the collected energy is transferred further to the reaction center where the primary photosynthetic reaction occurs.9 Using the principles realized in natural photosynthetic organisms, we designed the first artificial element in this chain, a self-organized light-harvesting unit containing a zinc-chlorin antenna with a linked bacteriopheophytin-based energy trap.42-44 In those reports we demonstrated by steady-state fluorescence that in these assemblies an efficient energy transfer occurs, thus providing the basis for the first step in the design and development of artificial light-harvesting devices. In the present work we provide a detailed study by timeresolved fluorescence that more thoroughly characterizes the energy transfer (ET) and light-collecting properties of these units applying various donors and acceptor pairs. The intention of this paper is not a thorough theoretical description of all the exciton relaxation and energy transfer processes in these units, which would be too early given some still open problems. Rather the present work should be viewed as a first more technical characterization of the dynamics in these systems, thus providing the data for a further development of artificial photosynthetic units. Materials and Methods
†
Dedicated to Professor George S. Hammond on the occasion of his 80th birthday. ‡ Abbreviations: bacteriochlorophyll, BChl; decay-associated fluorescence spectrum, DAS; energy transfer, ET; full width at half-maximum, fwhm; methyl, Me; single photon timing, SPT; zinc, Zn. * To whom all correspondence should be addressed. E-mail: holzwarth@ mpi-muelheim.mpg.de. Fax: (+49)208 306 3951. § Max-Planck-Institut fu ¨ r Strahlenchemie. | Ryukoku University. ⊥ Ritsumeikan University.
Sample Preparation. The investigated compounds can be divided into two groups (see Figure 1): Zn chlorin aggregates 1a and 1b with different ester side chains combined with either energy traps T1 or T2 and BChl d aggregates 2 with energy trap T2. Materials. Synthetic chlorophyll derivatives of antenna pigments (zinc methyl/stearyl 3-devinyl-3-hydroxymethylpyro-
10.1021/jp0125754 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/09/2002
5762 J. Phys. Chem. B, Vol. 106, No. 22, 2002
Prokhorenko et al. TABLE 1: Fluorescence Lifetimes τi and Amplitudes Ai for Aggregates Alonea reducing conditions aggregate of compound (λexc) 1ab (713 nm)
without sodium dithionite
with sodium dithionite
τ1 ) 6 ps A1 ) 0.89 τ2 ) 28 ps A2 ) 0.1 τ3 ) 146 ps A3 ) 0.01 τ4 ) 878 ps A4 ) 0.005 τav ) 13.5 ps
τ1 ) 16 ps A1 ) 0.53 τ2 ) 50 ps A2 ) 0.36 τ3 ) 114 ps A3 ) 0.11 τ4 ) 667 ps A4 ) 0.003 τav ) 41 ps τ1 ) 38 ps A1 ) 0.16 τ2 ) 158 ps A2 ) 0.5 τ3 ) 311 ps A3 ) 0.33 τ4 ) 1.03 ns A4 ) 0.01 τav ) 198 ps
2 (710 nm) not measured
a The sum of amplitudes is normalized to 1. Average lifetimes τav reflect the fluorescence quantum yields of corresponding bands. b Similar data were obtained when using R3 ) stearyl (compound 1b).
Figure 1. Structural formulas of the synthetic compound 1, BChl d (2), the metal-free bacteriochlorins T1, and the donor-acceptor diad T2.
pheophorbide a (1),20 energy traps [methyl bacteriopyropheophorbide a (T1)45 and ethylene bacteriopyropheophorbide a-zinc 3′devinyl-3′-hydroxymethylpyropheophorbide a (T2)42] were prepared according to the reported procedures. Natural farnesyl (31R)-8-ethyl-12-ethyl BChl d (2) was extracted from cultured cells of Chlorobium Vibrioforme and purified by reversed-phase HPLC.46 L-R-Lecithin (from frozen egg yolk, Sigma) was purified chromatographically by elution with 20% methanoldichloromethane from a silica gel column. Preparation of Pigment Lipid Aggregates.43 Antenna pigments (1, 2), energy traps (T1, T2) and R-lecithin were dissolved in a minimal amount of methanol/THF (9/1; 40 µL). The solution was injected into 20 mM potassium phosphate buffer (pH 7.4; 4 mL) and shaken vigorously. The final concentrations of antenna pigments, energy traps and R-lecithin were 10, 0.4, and 32 µM, respectively. The molar concentration ratio of the aggregated chlorins to energy traps (25:1) used in this work was found to be optimal for the ET process. The aqueous aggregate was incubated in the dark for 2 h at RT. To
explore the role of reducing conditions, before spectroscopic measurements sodium dithionite (20 mM) was added to some solutions as indicated in Tables 1 and 2. Single-Photon Timing (SPT) Measurements. For SPT measurements, about 12 mL of a sample with optical density of 0.3 cm-1 at the excitation wavelength were pumped through the flow cell with 1.5 mm × 1.5 mm active cross-section at a rate of 10 mL/min. To avoid possible photooxidation of samples by molecular oxygen, all measurements were conducted under a nitrogen atmosphere at RT. Samples were excited at 700-713 nm (see Tables 1 and 2) with laser pulses at a repetition rate of 4 MHz provided by a laser system consisting of a 2080-15S mode-locked argon ion pumping laser (Spectra Physics) and a dye laser (model 375 Spectra Physics, pyridine I dye) combined with a cavity dumper (model 344, Spectra Physics). The full width of half-maximum (fwhm) of the laser autocorrelation function was ∼10 ps. The excitation energy density was (2-4) × 1011 (photons/cm2)/pulse. It was checked that under these conditions no annihilation occurs (cf. Figure 2). The fluorescence signal was detected, under magic angle polarization, with a fast microchannel plate photomultiplier R3809U-51 (Hamamatsu). The system response fwhm was 3034 ps, allowing a time resolution for lifetimes down to 3 ps by the deconvolution procedure. The decay traces were fitted using the global analysis method as described in ref 47, resulting in decay-associated fluorescence spectra (DAS), i.e., a plot of the associated lifetime amplitudes Ai(λ) vs wavelength. Average lifetimes were calculated as
τav )
∑Aiτi/∑Ai
where τi and Ai are the lifetimes and their amplitudes. These average lifetimes are directly proportional to the quantum yield of fluorescence. Steady-State Spectroscopy. Absorption spectra were measured in a Shimadzu UV-1601 spectrophotometer in a standard 1-cm fluorescence cell. Stationary fluorescence emission spectra were recorded using the above SPT equipment in steady-state mode, and they were corrected for the wavelength-dependent sensitivity of the detection system. Relative quantum yields for different samples were determined by normalizing to the excitation intensity and the absorbance of the samples. Results and Discussion Aggregates. Figure 3 shows the absorption and steady-state emission spectra of aggregates without trap. The absorption
Artificial Self-Assembled Light-Harvesting Units
J. Phys. Chem. B, Vol. 106, No. 22, 2002 5763
TABLE 2: Fluorescence Lifetimes, Corresponding Amplitudes, and Averaged Lifetimes τav (at Indicated Wavelengthsa) of Investigated Light-Harvesting Devices reducing conditions without sodium dithionite
with sodium dithionite
amplitudesb
amplitudesb
Co-aggregate (λexc)
lifetimes
@745 nm
@825 nm
lifetimes
@750 nm
@840 nm
1a + T1 (710 nm)
4 ps 40 ps 129 ps 437 ps 2 ns
0.87 0.125 0.004 0.001 0.0003 τav ) 10 ps
0.55 0.45 0.24 0.01 0.0006 τav ) 45 ps
8 ps 40 ps 131 ps 243 ps 955 ps
0.89 0.07 0.03 0.01 0.001 τav ) 17 ps
-1 0.07 1.32 0.95 0.014 τav ) 178 ps
c
reducing conditions without sodium dithionite
with sodium dithionite
amplitudes
amplitudes
Co-aggregate (λexc)
lifetimes
@745 nm
@875 nm
lifetimes
@750 nm
@840 nm
1a* + T2 (710 nm)
3 ps 20 ps 60 ps 159 ps 696 ps
0.97 0.02 0.006 0.002 0.0002 τav ) 4 ps
-3.34 1.12 2.06 1.14 0.013 τav ) 78 ps
7 ps 32 ps 109 ps 215 ps 722 ps
0.91 0.06 0.02 0.006 0.0005 τav ) 13 ps
-0.32 0.49 0.79 0.25 0.005 τav ) 104 ps
reducing conditions without sodium dithionite
with sodium dithionite amplitudes
Co-aggregate (λexc)
lifetimes
@750 nm
@820 nm
2 + T2 (700 nm)
27 ps 91 ps 213 ps 431 ps 1.17 ns
0.31 0.36 0.28 0.05 0.007 τav ) 130 ps
-0.16 -0.065 0.88 0.33 0.011 τav ) 281 ps
not measuredd
a For the 750-nm region this corresponds to the maximum of ET-component; for the 820-870 nm region this corresponds to the maximal amplitude of negative component. b The sum of amplitudes is normalized to 1 at the wavelengths indicated. c When methyl was replaced by stearyl as R3, essentially the same data was obtained. d Without sodium dithionite this compound was very instable under light illumination.
Figure 2. Excitation intensity dependency of fluorescence (measured at the maximum of the emission spectra). For better comparison, for 2 the intensity of fluorescence is reduced by a factor 3.3.
maxima are strongly red-shifted as compared to the monomeric forms of these compounds, and they are quite similar to those of isolated natural chlorosomes from green bacteria. The fluorescence maxima are red-shifted typically by 8-15 nm with respect to the absorption maximum of the Qy band. The shape of the fluorescence corresponds well with the calculated spectra using the Stepanov relationship48 (not shown), which indicates
that the Qy exciton band in the aggregates is thermally equilibrated. The first measurements have been performed without the addition of dithionite to the solutions. However, it was found later that, e.g., for aggregate 1a, the quantum yield of fluorescence was approximately 3-5 times higher in the presence of sodium dithionite (this factor varied somewhat from preparation to preparation). The same behavior was observed also for the other aggregates. We subsequently performed all the measurements in the presence of a very small amount of dithionite, which we consider to result in more reliable and consistent data sets. We nevertheless provide also the data obtained under nonreducing conditions, since such data have been reported earlier in the literature for various (bacterio)chlorin aggregates. Although the exact nature of the quencher is unknown so far, we presume from our observations that this might actually be due to the presence of a small amount of chlorin cations, which could be formed by photoautooxidation upon illumination in the presence of molecular oxygen. Once formed, these chlorin cations would remain in the aggregates and could be very efficient quenchers. Addition of dithionite bars this possibility (i) by removing the oxygen and (ii) by reducing any chlorin cations already present. The redox sensitivity of the fluorescence of the aggregates is actually reminiscent of the pronounced redox sensitivity of the fluorescence yield of natural chlorosomes49,50 and might have the same origin. In all aggregates investigated the fluorescence kinetics can be characterized by four lifetimes, stretching from ∼3 ps up to
5764 J. Phys. Chem. B, Vol. 106, No. 22, 2002
Prokhorenko et al.
Figure 3. Absorption (solid) and fluorescence (dashed) spectra of aggregates of (a) compound 1a, (b) compound 1b, and (c) 2 (see also Figure 1). Fluorescence intensity is given in arbitrary units. The spectra (a) and (c) were recorded in the presence of sodium dithionite, and (b) was recorded without dithionite.
Figure 4. Decay-associated fluorescence spectra of aggregates (a) 1a without sodium dithionite, (b) 1a with sodium dithionite, and (c) 2 with sodium dithionite.
∼1 ns (Table 1). The DAS spectra (Figure 4) show that these components have very similar spectral shapes and maxima. Due to strong overlapping of spectral shapes no ET components (with negative amplitude) were observed in these aggregate lifetimes, as expected. However, the shortest lifetimes in population kinetics seem to be due to ET between different excitonic levels characterized by different radiative lifetimes. For aggregates of the synthetic Zn chlorins (1a and 1b) the fastest lifetime component carries the largest amplitude (Figure 4a,b), whereas in the BChl d (2) aggregates the second shortest (158 ps) component has the maximal amplitude (Figure 3c). The BChl d (2) aggregate also has a substantially longer average lifetime (∼200 ps). The average lifetimes of the other aggregates in the presence of sodium dithionite are ∼40 ps, and approximately 3 times faster without. The fastest lifetime components are at least ∼2 times longer in the presence of sodium dithionite, and the corresponding amplitudes in the DAS spectra are dominant
(Figure 4b). In contrast, the lifetimes of the long-lived components are not changed significantly by addition of dithionite. This may indicate that the longer-lived components arise from unquenched aggregates; i.e., there may occur a heterogeneous composition of aggregates that either contain a quencher or not. Due to the large size of the aggregates (we estimate at least several thousand chlorins per aggregate)43 and the efficient energy transfer and excitonic coupling within these units, a single quencher per aggregate could have a drastic quenching effect. Aggregates with a Trap. Figure 5 shows some typical absorption/fluorescence spectra of the co-aggregates with a trap. As demonstrated in our previous work,42-44 upon co-aggregation of the energy traps T1 and T2 with the chlorins a red shoulder appears in the absorption spectrum (at ∼800 nm), the amplitude of which is approximately proportional to the concentration of the trap molecules. This shoulder reflects the absorption of the
Artificial Self-Assembled Light-Harvesting Units
Figure 5. Absorption (solid) and fluorescence (dashed) spectra of coaggregates (a) 1a + T1 without sodium dithionite and (b) in its presence and (c) 2 + T2 with sodium dithionite.
metal-free bacteriochlorin trap that is substantially red-shifted (as compared to the absorption of the free monomers with a maximum around 750 nm). This long-wavelength band is most likely due to excitonic interaction with the aggregated pigments. Assuming nearly degenerate energies for the separate donor and acceptors, we can roughly estimate the excitonic interaction V from the observable split to be ∼400 cm-1 (the splitting is 2V). This indicates a strong excitonic coupling between the trap molecule and the chlorin aggregates, which should provide a favorable condition for ET. We observed two emission bands independent of the presence or absence of sodium dithionite (Figure 5a,b). The 750-nm band reflects the fluorescence of the chlorin aggregate (cf. Figure 3), but with a significantly lower intensity than for the pure aggregate fluorescence. A broader second band located at 820-830 nm corresponds to the fluorescence from the attached energy trap. The area ratio of these bands depends to some extent on the presence of sodium dithionite. However, the quantum yield of trap fluorescence is up to 5 times higher in the presence of sodium dithionite. For
J. Phys. Chem. B, Vol. 106, No. 22, 2002 5765 the BChl d-based co-aggregate 2 + T2 the relative amplitude of the fluorescence band corresponding to the emission from T2 is ∼3 times smaller than the aggregate band, which indicates a low ET efficiency in this case. Figure 6 shows the DAS for co-aggregates 1a + T1, 1a + T2 and, for comparison, the BChl d-based co-aggregate 2 + T2. The energy transfer component (negative amplitude) in the co-aggregates is clearly resolved in all cases and is more pronounced in the presence of sodium dithionite. The important point to discuss here is whether the experimentally resolved aggregate to trap energy transfer lifetime is indeed the dominant one or not. This can be checked by comparing the corresponding amplitudes of the rise and decay of the trap fluorescence (for details, see ref 47). If the predominant lifetime component is indeed resolved, their magnitudes in the overlapping region should be roughly equal but their sign should be opposite. This is clearly the case for Figure 6a, while for Figure 6b this is a borderline case. For BChl d-based co-aggregates (Figure 6c) the negative amplitude is too small, and the corresponding lifetime is quite long as compared to other cases. Thus we have to assume that some faster but unresolved lifetime component (with lifetime 1000 cm-1) the Boltzmann factor is sufficiently small β ≈ 6 × 10-3, and the uphill ET can be nearly neglected at RT. Note that the following equations assume that the uphill ET is zero. This is not entirely correct in our case; however, this assumption simplifies tremendously the equations and is quite sufficient to make the relevant point. The intensity of fluorescence is proportional to the radiative rate constant and to the concentration of excited donors/ acceptors:52
(2)
Solving the system eq 1 for the stationary case gives a relationship for the intensity ratio of donor/acceptor fluorescence:
IDfl
|
IAfl stat
)
kDrad kAFL kA kET
IDfl (t) ∝ kDradσDN0 exp[-(kET + kDFL)t] IAfl (t) ∝ kAradσDN0
(3)
rad
Since in the present case the fluorescence rate constant of acceptor is much smaller than the ET rate constant (see Table 2), owing to the efficient ET (large kET), similar fluorescence intensities of the donor and acceptor bands can be obtained only for a very large difference in the radiative rate constants, i.e., if kDrad . kArad. This is more apparent for the time-resolved data. Solving eqs 1 and 2 for impulsive excitation gives the following
kET kET + kDFL - kAFL
{exp(-kAFLt) exp[-(kET + kDFL)t]} (4)
and we see that the ratio between the amplitudes A of the ET component for the donor and acceptor is
ADfl
|
AAfl ET
IDfl (t) ∝ kDradND(t) IAfl (t) ∝ kAradNA(t)
expressions for the time-resolved fluorescence:
)-
(
kDrad kArad
1+
)
kDFL - kAFL kDrad ≈- A kET k
(5)
rad
Taking into account the difference in spectral widths for the donor/acceptor fluorescence bands (∼1:2 on a wavenumber scale), both eqs 3 and 5 can be reconciled with our experimental data only if kDrad g 10kArad. This means that the excitation is delocalized over at least 10-15 molecules.53 This number actually represents a lower limit, since depending on the structure of the aggregates and the nature of the resulting exciton states, the actual number of coupled pigments might be substantially larger. Lacking any further information on the excitonic structure of the co-aggregates, we can at present only provide a lower limit of coupled pigments rather than give a precise number of actually coupled chlorins. An increase of the pure radiative rate due to excitonic coupling (generally known as superradiance54) was observed earlier in chlorophyll a dimers55 and in small chlorin aggregates.56 This effect plays an important role, e.g., in the excited-state kinetics of the so-called J aggregates.57
Artificial Self-Assembled Light-Harvesting Units In the co-aggregate 2 + T2 we observed two ET components with lifetimes of 27 and 91 ps, the negative bands of which are located at 820 and 830 nm, respectively. Such a biexponential ET process has also been observed in isolated chlorosomes from Chlorobium tepidum,58,59 and in isolated BChl d/BChl ccontaining chlorosomes from Chlorobium limilola,60 with very similar lifetimes (31/78 and 27/74 ps, respectively). Two possible explanations are to be considered for two ET lifetime components to occur. One is that they have some trivial origin such as, e.g., a heterogeneity of the aggregates (see above) and/ or some heterogeneity of the trap location in the aggregates. Alternatively, and particularly in view of the long persistence of the superradiant state at room temperature, the two components may also be attributable to ET from different excitonic levels that are highly isolated, hence only interchange very slowly. A final evaluation of these alternative possibilities is beyond the scope of this paper. It will demand detailed exciton model calculations, which may also require additional experimental data. All results of the time-resolved fluorescence study of lightharvesting devices are collected in Table 2. For better comparison, the average lifetimes corresponding to the “blue” and “red” bands (i.e., fluorescence from the aggregate and from the energy trap) are also shown. The ratios of average lifetimes for the aggregate and energy trap fluorescence regions are in excellent agreement with the ratio between the areas of the corresponding fluorescence bands. This means that all components in the population kinetics are properly resolved and we did not “lose” any ultrafast components with large amplitude due to insufficient time resolution. The fluorescence from the energy trap (820-850 nm region) is in all cases characterized by several lifetime components as well (Figure 6 and Table 2). This can be attributed to heterogeneity of the co-aggregates, probably due to co-aggregation of energy traps at different positions with respect to the aggregate body. This is also supported by the different DAS of the energy trap fluorescence, which are located at somewhat different wavelengths. It is important to note that the same lifetimes appear in all cases in the aggregate emission region and in the trap region. The typical ratio between their amplitudes is about 1:3 to 1:5 for the Zn-based compounds (see, e.g., Figure 6a). Given all the complications that are possible due to the fact that already the pure aggregate decay is multiexponential, there appear to exist several possible explanations. The most straightforward one would be the presence of some energy back-transfer from the trap to the antenna aggregate. At first glance this appears unlikely, given the large energy difference between the two levels (about 1100 cm-1). In particular, we might a priori have expected a much smaller amplitude (with a ratio of ca. 1:15) for such a component in the aggregate emission region than is actually observed (Figure 6). However, this explanation in fact appears to be quite plausible when it is taken into account that the aggregate fluorescence is characterized by superradiant states, which will substantially increase the amplitude of the aggregate fluorescence in the lifetime components. In the following we derive some semiquantitative relations for a simple quasi-two-state model that may grossly oversimplify the situation in order of the complex nature of the aggregates studied here, but it provides the essentials that allow us to explain the large relative amplitude of this back-transfer component in the time-resolved data. Taking into account that the back ET occurs at least to 25 excitonic states (in agreement with the concentration ratio 1:25 of trap and donor molecules)
J. Phys. Chem. B, Vol. 106, No. 22, 2002 5767 TABLE 3: Efficiency of the Excitation Energy Collection in Co-Aggregates Co-aggregate
efficiencya (%)
reducing conditions
1a + T1
26 59 70 68 53
no yes no yes no
1a + T2 1b + T1 a
Accuracy (5%.
corresponding to the first exciton zone of the aggregate, the contribution of back-transfer in the kinetic balance equations for the acceptor population (eq 1, term -βkETNA) should be multiplied by this factor. In other words, due to the quasiisoenergetic location of these states, we can consider the donor state in eq 1 as 25 times degenerate. Taking further into account the increased effective radiative rate of the donor, which increases the amplitude of the donor fluorescence in the timeresolved data (eq 5), we can rationalize the relatively large amplitudes of the components that appear in both the donor and the trap fluorescence with the same lifetimes. Energy Transfer Efficiency. For artificial light-harvesting devices the most important property is the efficiency of the energy collection. This efficiency can be defined in various ways. In the most simple fashion we can define it as the ratio of the emission intensities or quantum yields (which are proportional to the average lifetimes) of the aggregates without trap and with trap. A change in quantum yield corresponds to a change in the average fluorescence lifetimes of the aggregate itself (τDav) and of the co-aggregate (τD+A av ). Thus, the efficiency can be defined as D Eff ) (τDav - τD+A av )/τav
(6)
The results are given in Table 3. The efficiency defined in this way is about 60-70%. The highest efficiency (70%) is shown by the co-aggregate 1a + T2 independent of the presence of sodium dithionite. However, the overall “technical” efficiency that characterizes a total amount of stored energy absorbed in the light-harvesting device, depends also on the absolute quantum yield of fluorescence and is about 5 times higher when sodium dithionite is added. For various reasons a more suitable criterion for the transfer efficiency is the population of acceptor state after the ET process. This definition assesses the conversion of the collected light energy into chemical energy, e.g., in the form of charge separation. When the kinetic scheme developed above (eq 1) is used, the maximal population of energy trap is estimated to be ∼75% in the case of 1a + T2. Conclusions This time-resolved study of the ET processes in Zn chlorin aggregates demonstrates the perspectives of this system to be used in an approach to potential solar energy utilization in artificial photosynthesis. The efficiency of the excitation energy collection in co-aggregated energy traps is very high, reaching 70%. Due to a large energetic split between the donor (aggregate) and acceptor (energy trap) levels the collected excitation energy can be effectively stored and used for further conversion to chemical energy, e.g., by charge separation in an attached reaction center. This next step in development of an artificial photosynthetic device will be dealt with in a forthcoming paper. An interesting aspect of the data presented here is the possibility to monitor the delocalization of excitation in aggregates by means of time-resolved fluorescence using an
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