6318
J. Phys. Chem. B 1997, 101, 6318-6322
Directed Energy Transfer Funnels in Dendrimeric Antenna Supermolecules† Michael R. Shortreed, Stephen F. Swallen, Zhong-You Shi, Weihong Tan,‡ Zhifu Xu,§ Chelladurai Devadoss,§ Jeffrey S. Moore,§ and Raoul Kopelman* Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055 ReceiVed: February 18, 1997; In Final Form: May 1, 1997X
We have studied the dynamics of directed, multistep energy transport in a class of fractal-like dendrimeric molecules. For particular forms of these highly branched phenylacetylene dendrimers, both theory and experiment put the lowest excitation energy at the center (locus) of the supermolecule. This results in a structurally symmetric and ordered exciton funnel, with a well-directed energy gradient. We have designed and synthesized a derivative of these dendrimers with a perylene moiety at the locus, which acts as an energy trap for the directed exciton funnel. Spectroscopic evidence indicates transfer efficiency of 98% from the photoabsorbing dendrimer backbone to the perylenic trap.
Introduction Most life on Earth is sustained directly or indirectly through sunlight collection by supramolecular systems. The primary step of photosynthesis consists of energy flow from a supramolecular antenna to a reaction center via a multistep transport process.1-7 The excitonic funnels in green plants5,7-9 and in their polymeric mimics3,4,10,11 are structurally and energetically disordered, and thus the energy transport must rely, at least in part, on random walk, thermal activation, exciton percolation, or a combination of these effects.7,12,13 In contrast, a large molecular antenna with an ordered geometry and ordered energetics would form an ideal energy funnel. Highly symmetric and perfectly hyperbranched macromolecules with wellcontrolled structures14-17 are good candidates for such nanostructures. It has been shown theoretically17 that ordered “Bethe trees”18 may be the optimal energy funnels. Because of their interesting structural and energetic properties, we have synthesized and examined a series of related phenylacetylene dendrimers,14-17,19 which act as ideal energy funnels. The socalled “extended” Bethe dendrimers currently under investigation include the extremely large, structurally controlled molecule D127 (C1398H1278), which contains 127 benzene rings and 2676 atoms (Figure 1a), as well as a specially designed peryleneterminated dendrimer derivative called the “nanostar” (C460H424), which is the focus of this work. The introduction of this new class of supermolecules provides a significant addition to the literature on the subject of directed intramolecular energy transfer17,20-25 and provides a quantitative measurement of directed, multistep energy transfer in supermolecular and supramolecular compounds. Experimental Section The steady-state absorption measurements were taken on a Shimadzu UV-2101PC UV-VIS Scanning Spectrophotometer, while the fluorescence and excitation spectra were recorded on † Dedicated to Robin M. Hochstrasser on the occasion of his 65th birthday. * To whom correspondence should be addressed. Email: Kopelman@ umich.edu. ‡ Department of Chemistry, University of Florida, Gainseville, Florida 32611-7200. § University of Illinois, Department of Chemistry, Urbana, Illinois 618013364. X Abstract published in AdVance ACS Abstracts, June 1, 1997.
S1089-5647(97)00598-1 CCC: $14.00
a Shimadzu RF 5000U Spectrofluorophotometer. Dichloromethane (99.9+% HPLC grade, Aldrich) was used without further purification as the solvent for the quantitative measurements. Spectroscopic measurements were performed in highquality matched quartz cuvettes with 1 cm inner diameter. The synthesis of the dendrimer samples has been described in detail elsewhere.14,19 Dendrimer concentrations were typically 10-7 M. This was low enough to prevent the formation of dimers or excimers in solution, as well as to prevent intermolecular energy transfer between dendrimer molecules. Results and Discussion Two families of Bethe dendrimers, referred to as the “compact” and “extended” forms, have been investigated. The relationship between the structural and energetic properties of these dendrimers, and comparisons between the two forms, was the subject of another manuscript,17 although a brief description will be given here for clarity. A fundamental characteristic of all these molecules is the composition of the legs or spokes. The compact Bethe dendrimers are perfectly structurally symmetric molecules, with each of three main legs composed of many independent diphenylacetylene (DPA) units. The phenyl group at the ends of each of these moieties have 3-fold coordination at the meta positions, serving as branching points to additional phenylacetylene groups, giving rise to an exponentially expanding structure. This molecular configuration helps create two of the fundamental characteristics of these dendrimer systems. First, this branching pattern allows the maximum structural flexibility, thus minimizing steric hindrance. This enables the synthesis of much larger dendrimeric systems. Second, and more importantly, the meta branching positions disrupt the local π-electron excitation conjugation between adjacent aromatic ring systems.17 As a result, the individual diphenylacetylene chains are electronically decoupled from the resonative conjugation between neighboring benzene rings. This decoupling allows each DPA chain to act as the site of a localized excitation, with a well-defined vibrationless electronic excitation energy. Due to the well-prescribed symmetry and the identical chain length of all the legs of the compact dendrimers, these molecules do not act as energy funnels. An exciton which may initially be localized on a particular DPA chain will not experience an energy gradient, and thus any movement to adjacent chains occurs via random hopping transfer events. In contrast to these characteristics of the compact dendrimers, © 1997 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6319
Figure 1. (A) D127, referred to as an extended Bethe dendrimer. It contains 127 benzene rings and a total of 2676 atoms. The peripheral benzene rings are terminated with tert-butyl groups. (B) The structure of the nanostar supermolecule.
the extended Bethe dendrimers are unique in that they have unequal legs, composed of linear chains of DPA units of
consecutively increasing length toward the center of the molecule (e.g., Figure 1a). Because these legs are of varying
6320 J. Phys. Chem. B, Vol. 101, No. 33, 1997 lengths, these localized excitations are found to be of varying energies, with an inverse correlation between chain length and excitation energy. The single-unit DPA chain states (found at the periphery, or tree canopy) have the greatest excitation energy, while this value monotonically decreases toward the center (locus) of the molecule as the chain lengths increase. This is an expected observation, as the ground state excitation energy of a molecule typically decreases with an increase of the extent of conjugation.26,27 As a result, intramolecular energy transfer should be well directed toward the locus of these extended Bethe dendrimers (e.g., D127, Figure 1a) and the associated dendrimer derivatives (e.g., nanostar, Figure 1b). For these molecules, we show below that energy transport is highly efficient, multistepped, and unidirectional from the canopy to the locus. The electronic absorption spectra of both the compact and extended dendrimers both exhibit an identical high-energy peak, which has been assigned17 as an electronic excitation localized on the shortest DPA chains: those of unit length. For the compact molecules, all of the electronically active absorbing states are identical, thus giving rise only to this single spectroscopic peak. Interestingly, however, the case is quite different for the extended dendrimers. In addition to the same high-energy peak due to single-unit DPA chains found at the periphery of the molecule, the longer DPA chains found in the interior of the molecule give rise to well-defined lower energy peaks. If the terminal benzene rings are at least partially included in these localized states, then, for example, the D127 molecule has one, two, three, and four-membered chains, present in consecutively decreasing numbers (see Figure 1a). As mentioned above, the localized lowest excited state decreases in energy as the length of the linear chain increases. In this way, the extended dendrimers exhibit a localized excited state gradient, thus creating an ideal energy (exciton) funnel (see Figure 2a). The absorption spectra of the family of extended dendrimers have been studied previously, and the electronic energy assignments have been identified.17 Of particular current interest, however, are a set of molecular derivatives, which contain a perylene pendant at the locus. These molecules contain the identical molecular backbone as the parent dendrimer, but, in effect, are given a perylene substituent in place of one of the three main legs of the dendrimer. As a result, the electronic absorption spectrum can be given the identical energy assignments as the parent dendrimers, with an additional peak to the red due to the perylene ground state excitation. Figure 2 provides a schematic summary of the energy funnel models for both D127, the largest extended dendrimer synthesized thus far, and the nanostar, the largest perylene-substituted derivative. This is based on the absorption spectra of these two molecules, shown in Figure 3A. The energy diagram for one leg of the D127 dendrimer (one-third of the entire molecule) is given in Figure 2a, for which we make the following observations: (a) counting diphenylacetylene fragments gives 24 single-chain (periphery) localized states, all at about 32 000 cm-1; (b) 4 two-membered linear chain states at 28 400 cm-1, each of which is collecting the energy from six periphery states; (c) 2 three-membered linear chain states at 26 500 cm-1, each collecting the energy from 2 two-membered chain states; (d) 1 four-membered chain acceptor state at 25 600 cm-1, collecting the energy from the 2 threemembered linear chain states. Similar observations are made in Figure 2b, which gives the energy diagram of the entire nanostar molecule: (a) counting diphenylacetylene fragments gives 24 single-chain (periphery) localized states, all at about 32 000 cm-1; (b) 4 two-membered linear chain states at 28 400
Letters
Figure 2. (A) Energy funnel schematics of one of the three arms of the D127 molecule (comprising one-third of the entire supermolecule; see Figure 1A). The lowest energy state is the diphenylacetylene chain with four acetylenic bands and five benzoic rings, which terminates at the locus. (B) The energy funnel of the entire nanostar supermolecule (see Figure 1B). The lowest energy “trap” state is the ethynyl perylene moiety. In both figures, the uppermost energy step consists of 24 localized energy sites, the next of 4, then 2, and finally the lowest energy state comprised of one very compact acceptor.
cm-1, each of which is collecting the energy from 6 periphery states; (c) 2 three-membered linear chain states at 26 500 cm-1, each collecting the energy from 2 two-membered chain states; (d) one perylenic acceptor state at 21 000 cm-1, collecting the energy from the 2 three-membered linear chain states. This lowest state is a “supertrap” state.28,29 (It should be noted that while these energies exhibit a noticeable solvent shift, the relative change is effectively constant with respect to each of the localized excitation states.) In order to clarify the mechanism of energy transfer from these dendrimeric localized states to the trap, we have synthesized donor and acceptor portions of the nanostar as separate molecules and studied the absorption, emission, and excitation spectra of each. The direct parent of the excitation donor moiety consists of a dendrimer segment containing 39 phenyl groups and a silyl pendant and is referred to as M39-SiH3 (see Figure 3B, left-hand side). The parent molecule of the acceptor fragment is 1-ethynylperylene (Figure 3B, center), which, when used to replace the silyl group of the M39-SiH3 molecule, gives rise to the nanostar itself (Figure 3B, right-hand side). As can be seen for the nanostar in Figure 3A, the absorption spectrum of the antenna fragment (M39) is effectively unchanged by the perylenic fragment addition, except for the weak perylenic group absorption above 400 nm. Analysis of the steady-state spectral
Letters
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6321
Figure 4. Excitation (dashed line) and absorption spectra (solid line) of the nanostar in dichloromethane, each normalized to the maximum absorption peak around 310 nm. The excitation spectrum was detected at 515 nm. From left to right, the arrows indicate the vibrationless electronic excitations by diphenylacetylene chains with one, two, and three units, and the ethynylperylene, respectively. (Assigned according to ref 17.)
Figure 3. (A) Absorption spectra of the nanostar and silicon-substituted dendrimer M39-SiH3 in dichloromethane under ambient conditions. The insert gives the absorption spectrum of D127 in hexane. (The slight red shift relative to the dichloromethane dendrimer absorption is due to different solvent-solute interactions.) The absorption spectra of these molecules show that all features below 385 nm belong solely to the dendrimeric portion. This absorbance at any wavelength below 385 nm increases nearly linearly with the total number of phenylacetylene units but does not shift. However, the absorbance features above 385 nm belong to the attached perylene pendant, with the total molar absorptivity completely unaffected by the number of phenylacetylene units. (B) Emission spectra of separate solutions of the siliconsubstituted dendrimer (M39-SiH3), 1-ethynylperylene, and the nanostar, each excited at 312 nm under ambient conditions. Sample concentrations were 3 × 10-7 M. Note the reduced scale for the ethynylperylene. When studied as part of the nanostar (right-hand side), the dendrimeric fluorescence (at 370 nm) is totally quenched, and the energy is funneled into the nanostar’s acceptor ethynylperylene group. In this case, the perylenic fluorescence is now 3 orders of magnitude more intense than that of the isolated ethynylperylene molecule.
data provides the most important result, namely, that there is efficient nonradiative intramolecular energy transfer from the dendrimer antenna to the perylenic acceptor in the nanostar molecule. The emission intensity of the M39-SiH3 molecule is 3 orders of magnitude greater than that of isolated 1-ethynylperylene when both molecules are excited at 312 nm (Figure 3B). In comparison, the nanostar’s fluorescence photon yield is of the same order as its parent dendrimer M39-SiH3 but occurs almost entirely in the perylenic region. In fact, the structure of the nanostar fluorescence spectrum is quite similar to that of isolated ethynylperylene, except for an expected shift to the red due to a slight delocalization of the tail of the
electronic excitation dipole into the dendrimer. This near equality in number of emitted photons suggests a high intramolecular energy transfer efficiency in the nanostar. (While the absorbed and emitted photon flux are of roughly similar values, the total energy of emission is significantly decreased in the nanostar due to the frequency shift. This energy loss goes into vibrational degrees of freedom, and eventually into heat.) The emission by the ethynylperylene moiety attached to the dendrimer donor is 600 times brighter than that from an equal concentration solution of isolated 1-ethynylperylene when each solution is equally excited at 312 nm (the absorption peak of the dendrimer). Even when the 1-ethynylperylene molecule is excited directly into its most absorbing peak, the emission intensity is nearly three times less than when 312 nm is used to excite the peripheral fragments of the nanostar. In addition, there is a nearly complete absence of emission from the nanostar’s dendrimeric donor fragment. The fluorescence intensity in this region decreases by a factor of 60 or more compared to the M39-SiH3 molecule (and what is left may be due to an impurity species, the dimer (M39)2). Of course, this decrease could be accounted for by an additional excited state quenching mechanism, but the simultaneous vast increase in the perylenic region emission and the excitation spectrum data presented below help to conclusively support the excitation transfer mechanism. The nanostar’s normalized excitation spectrum (detected at 515 nm, solely in the perylenic emission region) and absorption spectrum are given in Figure 4 and quantify the efficiency of energy transfer within this molecule. The arrows indicate the vibrationless electronic excitations observed in dichloromethane: a peak at 312 nm due to single-unit diphenylacetylene chains found at the periphery of the dendrimer, peaks at 353 and 372 nm due to DPA chains with two and three units, respectively, found in the interior of the molecule, and a perylenic absorption at 472 nm. The ratio of the dendrimer peak relative to the perylene peak in the excitation spectrum divided by the similar ratio for the absorption spectrum gives the efficiency of intramolecular energy transfer. Specifically, the ratio of the excitation curve intensities at 312 and 472 nm divided by the absorption intensity ratio at those same wavelengths indicates a transfer efficiency of 96% between the dendrimer periphery and the perylenic trap at the core. Even higher values, approaching unity, are obtained for transfer from the two and three-unit DPA chains in the interior of the
6322 J. Phys. Chem. B, Vol. 101, No. 33, 1997
Letters
TABLE 1: Intensity Ratios and Transfer Efficiencies (310 nm)/ (353 nm)/ (372 nm)/ (472 nm)/ (472 nm) (472 nm) (472 nm) (472 nm) absorption ratio excitation ratio transfer efficiency (%)
17.8 17.0 96
7.57 7.50 99
2.0 2.0 100
1 1 100
a The intensity ratios for the absorption and excitation spectra, given in Figure 4, are given for each 0-0 electronic peak of the nanostar. The ratio of relative intensities between the absorption and excitation curves gives the efficiency of energy transfer from each type of DPA chain to the perylenic trap at the molecular locus. The single-unit DPA chains at the peripery, which absorb at 312 nm, exhibit an energy transfer efficiency of 96%, while the longer DPA chains in the interior of the molecule at 353 and 372 nm transfer with near unit efficiency.
dendrimer to the trap. The precise values of these ratios and the relative transfer efficiences are given in Table 1. Thus, nearly all excitations which are initially localized anywhere on the dendrimer backbone are rapidly funneled to the locus. In marked contrast to these results, the excitation spectrum (not shown here) of a 1:1 dilute [10-7 M] solution of isolated donor (silyl-substituted dendrimer, M39-SiH3) and isolated acceptor (1-ethynylperylene) reveals that the intermolecular radiative donor-acceptor energy transfer is quite inefficient, as expected. In addition, it is interesting to note that the overall energy yield of these dendrimers is quite high: the fluorescence quantum yield of perylene is effectively unity.30,31 Thus, nearly all of the energy that is funneled to the ethynylperylene trap is emitted as visible light. In addition to the energetic bias, it is expected that the macromolecular structure of these dendrimers in solution may play a role in the efficiency and rate of intramolecular energy transfer. Rather than the purely planar structure displayed for simplicity in Figure 1, a rather bowllike structure is adopted by the extended dendrimers in solution. (On the basis of molecular modeling calculations, the compact series are arranged in a tighter, more globular structure.) With a sufficiently large increase in generation number, these molecules may develop a more convoluted, self-intertwined structure. In this event, successively larger dendrimers may exhibit decreased efficiency and slower trapping rates. However, for the extended dendrimer molecules thus far synthesized, of which the nanostar is the largest, this effect has not been observed. For these large, highly branched systems, energy transfer from the dendrimer periphery to the locus appears to occur via a rapid, directed, multistepped mechanism, with no significant competing relaxation pathways. Conclusions The steady-state spectroscopic data presented above provide conclusive evidence of directed, multistep exciton transfer within this new class of dendrimeric molecules. Preliminary studies of time-dependent fluorescence decays have provided additional data which supports these conclusions. Further experiments on the extended Bethe dendrimers, including time-resolved absorption bleaching and more detailed time-dependent fluorescence emission studies, will help to further quantify both the rate and efficiency of intramolecular energy transfer. The nanostar represents a new class of “designer” molecules, tailor made for single-molecule light and exciton sources. We note that the large size (125 Å for D127), excellent photostability, and high photon efficiency will allow these supermolecules to be used as “supertips” for optical nanoprobes and nanosensors,32,33 as
exciton sources for near-field and scanning exciton microscopy,33,34 and possibly as material for organic light-emitting diodes2 or sensitization of photovoltaics.23 Acknowledgment. We acknowledge support from the National Science Foundation, Division of Material Sciences, Grant DMR-9410709. One of us (J.S.M.) gratefully acknowledges the generous support of the Office of Naval Research and the National Science Foundation. References and Notes (1) Knox, R. S. Primary Processes of Photosynthesis; Barber, J., Ed.; Elsevier: Amsterdam, 1977; p 55. (2) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals; Oxford University Press: Oxford, 1982. (3) Webber, S. E. Chem. ReV. 1990, 90, 1469. (4) Fox, M. A.; Jones, W. E.; Watkins, D. M. Chem. Eng. News 1993, 71 (11), 38-48. (5) France, L. L.; Geacintov, N. E.; Breton, J.; Valkunas, L. Biochim. Biophys. Acta 1992, 1101, 105-119. (6) Fauman, E. B.; Kopelman, R. Comments Mol. Cell. Biophys. 1989, 6, 47-61. (7) Somsen, O. J. G.; Mourik, F. v.; Grondel, R. v.; Valkunas, L. Biophys. J. 1994, 66, 1-7. (8) Valkunas, L. J. Photochem. Photobiol. B 1992, 15, 159-170. (9) Valkunas, L.; Geacintov, N. E.; France, L. L. J. Lumin. 1992, 51, 67-78. (10) Berberan-Santos, M. N. J. Phys. Chem. 1993, 97, 11376. (11) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105-1111. (12) Francis, A. H.; Kopelman, R. In Laser Spectroscopy of Solids; Yen, W. M., Selzer, P. M., Eds.; Springer-Verlag: Berlin, 1986; p 241. (13) Kopelman, R. J. Phys. Chem. 1976, 80, 2191-2195. (14) Xu, Z.; Moore, J. S. Acta Polym. 1994, 45, 83-87. (15) Xu, Z.; Moore, J. S. Angew. Chem., Int. Ed. Engl. 1993, 32, 13541357. (16) Xu, Z.; Shi, Z.-Y.; Tan, W.; Kopelman, R.; Moore, J. S. Polym. Prepr. 1993, 34, 130-131. (17) Kopelman, R.; Shortreed, M.; Shi, Z.-Y.; Tan, W.; Bar-Haim, A.; Klafter, J. Phys. ReV. Lett. 1997, 78, 1239-42. Bar-Haim, A.; Klafter, J.; Kopelman, R. J. Am. Chem. Soc. 1997, 26, 6197. (18) Mandlebrot, B. B. The Fractal Geometry of Nature; Freeman: San Francisco, CA, 1983. (19) Bharathi, P.; Patel, U.; Kawaguchi, T.; Pesak, D. J.; Moore, J. S. Macromolecules 1995, 28, 5955-5963. (20) Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759. (21) Amadelli, R.; Argazzi, R.; Bignozzi, C. A.; Scandola, F. J. Am. Chem. Soc. 1990, 112, 7099-7103. (22) Balzani, V.; Campagna, S.; Denti, G.; Alberto, J.; Serroni, S.; Venturi, M. Sol. Energy Mater. Sol. Cells 1995, 38, 159-173. (23) Bignozzi, C. A.; Argazzi, R.; Schoonover, J. R.; Meyer, G. J.; Scandola, F. Sol. Energy Mater. Sol. Cells 1995, 38, 187-198. (24) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 435460. (25) Fox, M. A. Acc. Chem. Res. 1992, 25, 569-74. (26) Klessinger, M. Excited States and Photochemistry of Organic Molecules; VCH: New York, 1993. (27) Levin, I. N. Quantum Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1991. (28) Gentry, S. T.; Kopelman, R. Phys. ReV. B: Condens. Matter 1983, 27, 2579-82. (29) Kopelman, R. In Modern Problems in Condensed Matter Sciences; Agranovich, V. M., Hochstrasser, R. M., Eds.; North-Holland: Amsterdam, 1983; pp 139-184. (30) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (31) McMahon, D. H.; Soref, R. A.; Franklin, A. R. Phys. ReV. Lett. 1965, 14, 1060. (32) Kopelman, R.; Tan, W. Appl. Spectrosc. ReV. 1994, 29, 39. (33) Tan, W.; Kopelman, R. In Fluorescence Imaging Spectroscopy and Microscopy; Wang, X. F., Herman, B., Eds.; Wiley: New York, 1996; pp 407-475. (34) Kopelman, R. In Physical and Chemical Mechanisms in Molecular Radiation Biology; Glass, W. A., Varma, M., Eds.; Plenum Press: New York, 1991; pp 475-502.
