Femtosecond Transient Absorption Studies of Energy Transfer within

Stefano Cicchi , Pierangelo Fabbrizzi , Giacomo Ghini , Alberto Brandi , Paolo Foggi , Agnese Marcelli , Roberto Righini ... Jonathan L. Bentz , John ...
0 downloads 0 Views 72KB Size
Download PDF
J. Phys. Chem. B 2001, 105, 1307-1312

1307

Femtosecond Transient Absorption Studies of Energy Transfer within Chromophore-Labeled Dendrimers Frederik V. R. Neuwahl and Roberto Righini* Department of Chemistry and LENS, UniVersity of Florence, 50125 Florence, Italy

Alex Adronov, Patrick R. L. Malenfant, and Jean M. J. Fre´ chet* Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460 ReceiVed: October 26, 2000

Dendrimers possessing coumarin-2 dyes at the periphery, and either a coumarin-343 or a heptathiophene dye at the core, have been studied by femtosecond transient absorption spectroscopy in order to elucidate energytransfer rate constants. These measurements indicated that energy transfer occurs with average correlation times of less than 6 ps in the first three generations, and slows to a correlation time of approximately 18 ps in the fourth. The transient spectral features arising from excitation of the peripheral donor chromophores at 330 nm are described. Additionally, these measurements provide a quantitative comparison between observed and theoretical energy-transfer rate constants (kET), indicating a close correlation of the experimental results with Fo¨rster theory.

Introduction The photophysical behavior of a series of coumarin-functionalized dendrimers was previously characterized via steady state and picosecond time-resolved fluorescence spectroscopy.1,2 These macromolecules can be considered as very simple synthetic analogues of the natural light-harvesting complexes: large, highly ordered arrays of chlorophylls that are devoted, in photosynthetic organisms, to the task of collecting photons from the sun and transferring their energy to a single trap, where a charge separation is induced and ultimately leads to the formation of energy-rich ATP.3-7 The series of molecules we have characterized are composed of a peripheral array of coumarin-2 chromophores linked to an energy trap (coumarin343) located at the focal point of the structure (Scheme 1). Radiative energy can be absorbed by the peripheral shell and transferred to the energy trap.2 By varying the chemical nature of the focal chromophore, the absorbed energy can be utilized in various ways.8 In this study, the focal point of the dendrimers is labeled with a fluorophore, which is convenient for the characterization of the energy-transfer mechanism, along with its efficiency. The efficiency of the energy-transfer process can be evaluated by monitoring the fluorescence of the dendrimer, both in the absence and in the presence of the energy trap.9 This method is accurate as long as the absorption bands of the peripheral shell and of the core are well separated, so that selective excitation of only the donor can be achieved. Under these conditions, the decrease of the fluorescence quantum yield of the donor in the presence of the acceptor is a direct measure of the energytransfer efficiency. However, when such quenching is nearly quantitative, as in the present case, the sensitivity of this method * Corresponding authors. Prof. Roberto Righini, LENS, Largo E. Fermi, 2, 50125 Firenze, Italy. Phone: ++39-055-2307863. Fax: ++39-055224072. E-mail: [email protected]. Prof. Jean M. J. Fre´chet, Department of Chemistry, 718 Latimer Hall, University of California at Berkeley, Berkeley, CA 94720-1460. Phone: (510) 643-3077. Fax: (510) 643-3079. E-mail: [email protected].

is poor, since a small absolute error in the experimental data can lead to changes of orders of magnitude in the calculated energy-transfer rate constant (kET). In an attempt to overcome this sensitivity problem, picosecond time-correlated single photon counting fluorescence measurements were performed with an instrumental function of ca. 70 ps (fwhm), in order to establish if a finite rise time of the acceptor could be detected, providing a direct measurement of the energy-transfer rate.2 For all four generations, it was found that the available time resolution was not sufficient to characterize the correlation time of the energy transfer, which was determined to be faster than the temporal resolution allowed by this technique (∼30 ps after deconvolution for the instrumental function). For almost all of the samples, the observed fluorescence decay profiles were not monoexponential. This observation was ascribed to inhomogeneity of the chromophores constituting the system resulting from a distribution of different conformations and local microenvironments due to the flexible nature of the backbone of the dendrimers.2 In fact, it is known that the lifetime of coumarins is highly sensitive to the polarity of their surrounding medium, and therefore to the specificity of their local environments.10,11 Indeed, in the higher generation (G-3 and G-4) model dendrons without the energy trap at the core, extremely fast decay components (several hundred picoseconds) were observed and ascribed to local concentration quenching since the normal excited-state lifetimes of these chromophores are on the order of several nanoseconds. This is a valid conclusion since the probability of having the dendrimer assume a folded structure in which two dyes are in close proximity increases with the size of the dendrimer. Analogous short components were not observed in the lifetime of the fully functionalized dendrimers since the excitation energy is quickly transferred to the single core chromophore, for which high local concentrations are not possible. Both static and picosecond time-resolved spectroscopic measurements clearly indicated that the efficiency of energy

10.1021/jp003963t CCC: $20.00 © 2001 American Chemical Society Published on Web 01/30/2001

1308 J. Phys. Chem. B, Vol. 105, No. 7, 2001

Neuwahl et al.

SCHEME 1

transfer from the light-harvesting antenna to the focal acceptor is extremely high in all the cases considered, and that the relative time scale is faster than the limit imposed by the instrumental resolution of a picosecond experiment. Therefore, femtosecond experiments were deemed necessary in order to achieve a sufficient time resolution to extract complete information on the energy-transfer mechanism within these molecules. The excited-state dynamics of our dendrimers were therefore followed via femtosecond transient absorption spectroscopy. This pump-probe technique involves photoexcitation by a laser pulse resonant with a transition from the electronic ground state, followed by a second probe pulse that reaches the sample at a delay time controlled by a variable delay line. The intensity variation of the second pulse is recorded with respect to the pump-probe delay time. The photoexcitation causes a depletion of the ground state and a decrease of the absorbance at the wavelengths characteristic of the ground state of the sample. Concurrently, this results in an increased population of one or more excited states. Consequently, the absorbance of the sample at the characteristic wavelengths of the electronic transitions from the generated excited states increases. By monitoring variations of the absorption spectrum as a function of the delay time after excitation, precise information on the dynamics of the excited states can be obtained. In addition to analysis of our original coumarin-343 (C-343) core-functionalized samples, these experiments were also performed on dendrimers bearing a conjugated heptathiophene (T-7) focal acceptor group (Scheme 1).8 In the latter case, our aim was to demonstrate that it is possible to vary the interacting chromophores within the light-harvesting dendrimers, and therefore the wavelength of emitted light, while preserving high energy-transfer efficiencies. Experimental Section All of the dendrimer samples were synthesized as previously described.8,12 Spectrophotometric grade toluene and chloroform were purchased from Aldrich and used as received. All time-resolved data herein presented were obtained by means of transient absorption spectroscopy. The relevant

instrumentation has been described elsewhere.13 Briefly, a 1 kHz repetition rate pulse train (central wavelength ) 800 nm, pulse fwhm ) 70 fs, pulse energy ) 750 µJ) is provided by a Ti-S regenerative amplifier (BMI/CSF-Thomson Alpha-1000-S) coupled to a commercial oscillator (Spectra-Physics Tsunami). Two different configurations of the detection system were implemented to obtain the data sets, which are composed of both transient spectra (transmittance of the excited state vs the wavelength at a given pump-probe delay time) and kinetic plots (intensity of the probe vs the delay time at a fixed wavelength). Single-wavelength detection was achieved by spectrally filtering a white-light continuum: a ∼1 µJ reflection of the main beam was focused into a 2.5 mm thick calcium fluoride plate to generate the white-light continuum. The resulting spectrally broadened laser pulse spans the entire visible region and extends in the near UV to roughly 330 nm. The relative pump-probe polarization was set to magic angle (54.7°) to discriminate against the orientational dynamics that, especially in the lowest generation dendrimers, might occur on time scales short enough to interfere with the measurement of the population dynamics of interest. The desired wavelength was selected with 5 nm broad interference filters. The intensity of the probe was detected by a silicon photodiode and a lock-in amplifier synchronized to a chopper, switching the pump pulse on and off at half the repetition rate of the regenerative amplifier. In this way, the phase-locked amplifier amplifies the probe intensity modulation due to the interaction with the pump pulse. For the measurement of the transient spectra throughout the entire visible region, the white light continuum, generated as described for the previous setup, was further split into a probe pulse and a reference pulse. The sequence of interactions of the three pulses with the sample was reference, then pump, and finally probe. Both the intensities of the reference and of the probe were detected. By defining the appropriate ratios of the measured quantities, one can construct a transient transmittance spectrum, i.e., the transmittance spectrum of the excited states, over the entire detected spectral range.13 The detection system consisted of a flat-field spectrometer with a 300 GPM grating (Chromex 250), coupled to a retro-illuminated CCD camera

ET within Chromophore-Labeled Dendrimers

Figure 1. Evolution of the transient transmittance spectrum of the third generation, coumarin-343 cored dendrimer in toluene. The two spectra are recorded at +0.5 and +50 ps, respectively. Two major features can be identified: (a) an absorption structure due to the excited states of the donors and the ground state of the core in the 350-450 nm region; and (b) a growing emission band due to the S1 state of coumarin-343 in the 430-550 nm region.

(Princeton Instruments TE/CCD PBVTSAR). The spectral window, accessible simultaneously, was ca. 300 nm. The photoexcitation pulses, 3 µJ energy at a wavelength of 330 nm, were generated by an optical parametric generator (Light Conversion TOPAS),14,15 as the fourth harmonic of the signal at 1.32 µm. A 150 mm focal length off-axis parabolic mirror was used as a focusing element for all three pulses, the spot diameter of the pump pulse being ca. 200 µm. In this configuration, the pump-probe cross correlation was 230 fs, as measured by means of a stimulated Raman gain band of the solvent.13 In order to prevent ablation of the sample, the solution was contained in a home-built X-Y two-directionally translating cell (capacity ca. 2 mL, optical path length 2 mm) equipped with calcium fluoride windows. The optical density (OD) of the sample at the excitation wavelength was kept at ca. 1 in all cases, corresponding to concentrations ranging from 5 × 10-5 to 2 × 10-4. Results and Discussion (a) Transient Spectral Features. The multichannel transient spectra of the dendrimers with coumarin-343 core (G-1 to G-4) and heptathiophene core (G-1 to G-3) were measured in toluene solution at various delay times, ranging from 0 to 200 ps, to identify the features best defining the energy-transfer dynamics. Excitation of one of the peripheral donors with the pump pulse resulted in the occurrence of spectral features characteristic of coumarin-2. These features were observed until energy transfer caused the ground state recovery of the initially excited chromophore and the promotion of the coumarin-343 focal acceptor to the excited state. Upon increasing dendrimer generation, the evolution of the coumarin-2 spectral features into those of coumarin-343 occurred over different time scales due to slight differences in interchromophoric distance. In addition, the overall intensity of the transient spectra differed with generation, corresponding to the difference in the number of peripheral dyes and hence to the overall extinction coefficient () of the dendrimeric unit. However, the shape and position of these features were independent of dendrimer generation. It should be mentioned that the energy levels of the backbone are far from resonance at the excitation wavelength of 330 nm,1,2 so that the measured transient spectra are only due to the attached chromophores. In Figures 1 and 2, we report the transient spectra of the third generation dendrimers with coumarin-343 (C-343) core and heptathiophene (T-7) core, respectively, recorded at delay times of +0.5 and +50 ps after excitation. For the T-7 core-

J. Phys. Chem. B, Vol. 105, No. 7, 2001 1309

Figure 2. Evolution of the transient transmittance spectrum of the third generation, heptathiophene cored dendrimer in toluene. The two spectra are recorded at +0.5 and +50 ps, respectively. In this case, the larger separation of the various spectral features allows for the identification of four different structures: (a) an absorption band due to the S1 state of coumarin-2 in the 350-400 nm region; (b) an emission band due to the same state in the 400-450 nm region; (c) an absorption band due to the S1 state of the focal heptathiophene in the 450-520 nm region; and (d) a structured emission band due to the same state in the region above 520 nm.

functionalized dendrimers, four different features can be identified in the reported spectral range (365-615 nm, Figure 2). Two of these, the transient absorption band at 380 nm and the stimulated emission band at 430 nm, belong to the peripheral dyes (C-2’s), and the other two, the transient absorption band at 490 nm and the stimulated emission structures at 530-620 nm, belong to the single focal dye (T-7). The transient spectrum of coumarin-2 also shows an additional absorption feature in the spectral region where the stimulated emission from the core acceptor occurs. It is evident from these spectra that the intensity of the transient absorption features is much weaker than the intensity of the features from stimulated emission. We believe this observation is not due to the intensity of the transitions, but instead to the overlap of spectral features of opposite sign. These include the 380 nm absorption band (negative transient transmittance) that lies between the bleach of the ground state absorption and the stimulated emission band at 430 nm, both giving rise to positive variations of the transmittance. The same is observed for the 490 nm transient absorption band, which is partially hidden by the overlapping 430 and 530 nm emission bands. This observation indicates that the single-channel kinetics for the energy-transfer process (see below) should be monitored at the wavelength of an emission band rather than an absorption band, since the increase in signal-to-noise ratio outweighs the disadvantage of interference due to the detection of photons arising from spontaneous emission.13 Since coumarin-343 is also a fluorescent dye, one would expect to observe four analogous spectral features for the C-343 core dendrimers. In this case, however, the bandwidth and energy separation of the electronic levels is such that identification of all the expected features is not possible. A substantial separation of the spectral features of the focal acceptor from those of the donors is not present, resulting in significant overlap. Nevertheless, this overlap does not prevent the determination of energy-transfer kinetics, which can be monitored at the red side of the emission peak of coumarin-343 (see below) where the transient transmittance change due to energy transfer is at a maximum. One other consideration regarding the above-mentioned spectra has to be addressed: the longest of the identified time scales in the energy-transfer evolution of the [G-3]/T-7 dendrimer is approximately 12 ps. Therefore, the 50 ps transient spectrum is expected to be fully relaxed with respect to the

1310 J. Phys. Chem. B, Vol. 105, No. 7, 2001 periphery-to-core energy transfer. Nevertheless, it is evident that this spectrum, whose shape remains unchanged for several hundred picoseconds, still significantly exhibits the spectral features corresponding to the presence of residual excitation on the C-2 peripheral dyes (emission band at 430 nm). This phenomenon is likely due to the relatively high illumination intensity utilized in the experiment. As soon as a significant fraction of the molecules in the sample is excited, the probability of exciting more than one peripheral dye in the same dendrimeric unit increases. Once the first excitation transfer has occurred from periphery to core, the long lifetime of such excitation on the focal chromophore acts as a “bottleneck” preventing further transfer of excitation from other excited-state chromophores in the outer shell. Under these conditions, some of the peripheral dyes remain in their excited states much longer than when they are able to release their energy to the focal acceptor. We assume that this occurrence does not affect the onset of the excitation of the focal acceptor, as long as the energy-transfer rate is much faster than the inverse lifetime of the acceptor. It is expected that the magnitude of this effect, and the long-lived spectral feature at 430 nm, should decrease upon dilution of the sample. However, the signal-to-noise ratio was found to significantly diminish before this decrease could be clearly observed. This phenomenon may also account for an observed decrease in photostability of the peripheral chromophores in highgeneration dendrimers under intense illumination. When only a single peripheral chromophore is excited, the energy is quickly transferred to the core and is no longer harmful. However, if multiple excitations occur on the same dendrimer, only one will be transferred, while the energy of subsequent excitations will remain on the peripheral dye and may lead to its degradation. This is clearly seen when the photosensitivity of model dendrons having no energy trap attached to the core is monitored. In this case the absorbed UV energy resides on the peripheral chromophores long enough to cause damage to the structure. In practice, extreme care must be taken to minimize illumination times when performing experiments on the G-3 and G-4 dendrons. The optical quality of the samples, and the integrity of their fluorescence spectrum, were checked against photodegradation upon completion of each experiment. (b) Energy-Transfer Kinetics. As energy transfer proceeds, the intensity of the spectral features arising from the peripheral dyes decreases, giving way to those of the focal acceptor. The kinetic analysis of such spectral evolution defines the energytransfer rate. Hence, after the spectral regions of interest were identified by measuring the transient spectra at several fixed delay times, the kinetics relative to the energy-transfer process were obtained by measuring the intensity of the probe as a function of the pump-probe delay time. Ideally, this should be done at a wavelength where only the excited state of the acceptor gives signal. In practice, the kinetic plots were recorded at the maximum of the stimulated emission band from the focal acceptor, i.e., 470 nm for the C-343 series of dendrimers and 580 nm for the T-7 series. Therefore, the energy-transfer kinetics were measured as the rise of the excited-state population of the acceptor rather than the decay of the donors. Since the S1 state of coumarin-2 shows a broad absorption structure throughout this spectral region, the initial effect of the photoexcitation is a decrease in transmittance. The subsequent progress of energytransfer results in predominant population of the S1 state of the focal acceptor. Consequently, radiative decay from this excited state brings the transient transmittance level above one. Overall, the recorded time-domain signal has the shape of an absorption dip at time zero, followed by a rise to a plateau. The area of

Neuwahl et al.

Figure 3. Intensity vs time plots of the G1-G4 series of coumarin343 cored dendrimers in chloroform, monitored at 470 nm.

Figure 4. Intensity vs time plots of the G1-G3 series of heptathiophene cored dendrimers in toluene, monitored at 580 nm.

the initial absorption dip is proportional to the time elapsed between the photoexcitation and the energy transfer: the slower the energy transfer, the longer the duration of the donor’s excited state, and the larger the area of the dip. In Figures 3 and 4, we report the relative kinetic plots of the two series of dendrimers: G-1 through G-4 with a C-343 core in chloroform (monitored at 470 nm), and G-1 through G-3 with a T-7 core in toluene (monitored at 580 nm), respectively. In addition, measurements of the C-343 series were performed in toluene up to G-3. The decreased solubility of the G-4 dendrimer in toluene precluded the collection of adequate data in this solvent. For the other samples, no major difference was observed between the kinetics in toluene and those in chloroform. In all cases the data were fitted by functions of the form

S(t) ) H(t)[1 - (1 + C)(A1e-t/τ1 + A2e-t/τ2)]

(1)

where H(t) is a step function forbidding any dynamics prior to the photoexcitation, C is the magnitude of the instantaneously generated absorption dip, and the evolution due to energytransfer is modeled as a biexponential rise function. Similarly to the fit of the decay profiles in the previous picosecond experiment,2 the choice of this particular mathematical expression is somewhat arbitrary. The function was chosen such that it would utilize the minimum number of exponential components to reproduce, within the measured signal-to-noise ratio, the entire experimental time profile. A biexponential function proved sufficient to fit all the data herein presented. In Table 1, we report the fitting parameters for all the data sets, along with the average rise times in order to illustrate the trend of the dynamic behavior with varying generation. It is evident from the table that the slower components gain relative intensity and the average rise time increase for the higher generations. This observation confirms the earlier analysis of the time-resolved fluorescence measurements: the flexible nature of the dendrimers allows for both a distribution of different conformations of the dendrimers in solution and a distribution of different interchromophore distances within a

ET within Chromophore-Labeled Dendrimers

J. Phys. Chem. B, Vol. 105, No. 7, 2001 1311

TABLE 1: Energy Transfer Components, Average Transfer Times, and Fitting Parameters for the C-343 Cored Dendrimers (in Toluene and Chloroform), and the T-7 Cored Dendrimers (in Toluene) compd

C

τ1 (ps)

A1

τ2 (ps)

A2

τaverage (ps)

1 2 3 1 2 3 4 5 6 7

0.08 0.12 0.3 0.4 0.2 0.25 0.7 0.08 0.1 0.1

0.62 1.5 1 0.74 1.2 3.4 5.3 1.2 2 1.86

0.83 0.65 0.4 0.77 0.6 0.73 0.55 0.45 0.61 0.48

7 7 8 5.0 7.7 13.4 30 4 12.5 11.7

0.17 0.35 0.6 0.23 0.4 0.27 0.45 0.55 0.39 0.52

1.7 3.42 5.2 1.72 3.8 6.1 16.4 2.47 6.09 7.0

a

(c) Mechanism of Energy Transfer. It has been postulated that the mechanism of energy transfer in our chromophorelabeled dendrimers involves a long-range Coulombic interaction. Previously, we were unable to quantitatively demonstrate a close correlation between observed energy-transfer rate constants (kET) and those calculated from Fo¨rster theory due to inadequately resolved acceptor fluorescence rise times. As mentioned above, we have now been able to accurately determine the energytransfer rate constants allowing a comparison of these values to the theoretical ones obtained previously. From Fo¨rster theory, the rate constant for energy transfer is given by the equation

kET )

b

Measured in toluene. Measured in chloroform.

TABLE 2: Comparison of Theoretical and Observed kET Values, and Energy Transfer Efficiencies for the Synthesized Dendrimers compd 1 2 3 4 5 6 7

theoretical kET (×10-10)

observed kET (×10-10)

ET efficiency (%)

66 28 20 5.3

58 26 16 6.1 40 16 14

99.9 99.8 99.7 99.1 99.8 99.7 99.7

single dendrimer unit. As the generation number, and hence size, of the dendrimers increases, the distribution of possible distances between the core and the array of donors also increases, giving rise to slower components in the overall energy-transfer process. The longest of the energy-transfer components that were determined by the present experiment is 30 ps (G-4 in chloroform). Given the sensitivity of the employed technique, we can be confident that any residual evolution, possibly characterized by slower kinetic components, cannot involve a fraction of the total molecules greater than a few percent. This is in slight disagreement with the previously published picosecond measurements, which suggested the existence of more significant fractions of molecules undergoing energy transfer with a time constant of the order of 50 ps or more. It is noteworthy, however, that the sensitivity of a picosecond time-correlated single photon counting measurement to such time scales is not optimal, and the measurement might have been affected by a possible instability of the instrumental function. The above measurements of the acceptor rise times allow for the direct determination of energy-transfer rate constants (kET) for the dendrimers. The observed multiexponentiality in the rise of acceptor excited-state population indicates an inhomogeneity of the energy-transfer rates of different chromophores and dendrimeric units. As a result, one could define a distribution of rate constants, directly accounting for the different donor-acceptor geometric conformations allowed by these systems. However, given the purely phenomenological method (biexponential rise) that was used to model inhomogeneity, we prefer to report only average rate constants, which are less sensitive to the particular model adopted. These rate constants are presented in Table 2 for each of the dendrimers studied. In addition, accurate energy-transfer efficiencies based on the current energy-transfer lifetimes can be calculated by considering the previously reported donor fluorescence lifetimes in the absence of an acceptor.2

9000(ln 10)κ2φDJ 128π5n4NτDR6

(2)

where κ2 is the orientation factor, related to the relative orientation of the donor and acceptor transition dipole moments. In our calculations, κ2 can be given a value of 2/3, corresponding to the average of all possible orientations within the flexible dendrimer structure. φD is the donor quantum yield in the absence of the acceptor, J is the overlap integral, n is the index of refraction of the solvent, N is Avogadro’s number, τD is the donor lifetime in the absence of the acceptor, and R is the interchromophoric distance in cm (determined from molecular modeling). The overlap integral J (cm6/mol) is given by

J)

∫fD(ν)A(ν)ν-4 dν

(3)

where fD(ν) is the fluorescence intensity of the donor, A(ν) is the molar extinction coefficient of the acceptor, and the integral is calculated over the entire spectrum with respect to the frequency expressed in wavenumbers. This integral represents the overlap between the donor emission spectrum and the acceptor absorption spectrum, and is closely related to the probability of energy transfer from the donor to the acceptor. Using these parameters, the theoretical kET values were calculated, and these are reproduced in Table 2. A comparison of the observed kET values with the theoretical ones (Table 2) reveals a surprisingly close correlation, indicating that Fo¨rster theory can quantitatively describe the energy-transfer interaction in these molecules. This result supports the previous hypothesis2 that a Coulombic transition dipole moment coupling mechanism is the dominant energy-transfer pathway in our system. Also, the close agreement between the observed and calculated kET values lends some validity to the molecular modeling method we utilized to obtain the donor-acceptor distances within the dendrimers. Such close agreement between observed rate constants and those calculated from Fo¨rster theory is unprecedented in any previous examples of dendrimer-based energy transfer. Conclusions Transient absorption spectroscopy has been utilized for the accurate characterization of extremely fast photophysical processes occurring in dendrimers that exhibit efficient energy transfer from peripheral coumarin-2 donors to coumarin-343 or heptathiophene core acceptors. This process occurs with average correlation times of less than 6 ps for the first three dendrimer generations, and slows to an average correlation time of approximately 18 ps in the fourth generation. Considering that the radiative lifetime of the donors lies in the nanosecond range,10 the energy-transfer efficiencies are nearly quantitative in all cases. In addition, it has been shown that, under high-

1312 J. Phys. Chem. B, Vol. 105, No. 7, 2001 flux illumination, there is a nonnegligible probability of exciting multiple peripheral chromophores on a single, high-generation dendrimer. Such an event leads to energy transfer from one peripheral chromophore to the core, while the other peripheral chromophores return to the ground state via normal, radiative decay. The ability to excite multiple donor chromophores on a single dendrimer may allow for simultaneous two-photon energy transfer to an appropriately chosen acceptor chromophore, which would result in an energy up-conversion process. Finally, we have found that the measured rate constants for energy transfer correlate extremely well with the theoretical rate constants that were previously calculated. This observation indicates that energy transfer indeed occurs through the Fo¨rster mechanism in these dendrimers. Acknowledgment. Financial support of this research by the AFOSR-MURI program, NSF (DMR-9796106) and The Commission of the European Union under contracts ERBFMGE-CT950017 and HPRI-CT1999-00111 is gratefully acknowledged. This work was also supported by the Office of Naval Research, Order No. N00014-98-F-0402 through the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Financial support from Kodak in the form of a graduate fellowship is also gratefully acknowledged (A.A.).

Neuwahl et al. References and Notes (1) Gilat, S. L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. Engl. 1999, 38, 1422-1427. (2) Adronov, A.; Gilat, S. L.; Fre´chet, J. M. J.; Ohta, K.; Neuwahl, F. V. R.; Fleming, G. R. J. Am. Chem. Soc. 2000, 122, 1175-1185. (3) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517-521. (4) Ku¨hlbrandt, W.; Wang, D. N. Nature 1991, 350, 130-134. (5) Ku¨hlbrandt, W.; Wang, D. N.; Fujiyoshi, Y. Nature 1994, 367, 614-621. (6) Ku¨hlbrandt, W. Nature 1995, 374, 497-498. (7) Lehninger, A. L.; Nelson, D. L.; Cox, M. M. Principles of Biochemistry; Worth Publishers Inc.: New York, 1993. (8) Adronov, A.; Malenfant, P. R. L.; Fre´chet, J. M. J. Chem. Mater. 2000, 12, 1463-1472. (9) Mugnier, J.; Pouget, J.; Bourson, J.; Valeur, B. J. Lumin. 1985, 33, 273-300. (10) Jones, G., II.; Jackson, W. R.; Choi, C.; Bergmark; W. R. J. Phys. Chem. 1985, 89, 294-300. (11) Arbeloa, T. L.; Arbeloa, F. L.; Tapia, M. J.; Arbeloa, I. L. J. Phys. Chem. 1993, 97, 4704-4707. (12) Gilat, S. L.; Adronov, A.; Fre´chet, J. M. J. J. Org. Chem. 1999, 64, 7474-7484. (13) Neuwahl, F. V. R.; Bussotti, L.; Foggi, P. Res. AdV. Photochem., Photobiol., in press. (14) Bayanov, I. M.; Danielius, R.; Heinz, P.; Seilmeier, A. Opt. Commun. 1994, 113, 99. (15) Danielius, R.; Piskarskas, A.; Di Trapani, P.; Andreoni, A.; Solcia, C.; Foggi, P. Appl. Opt. 1996, 35, 5336.