Published on Web 06/05/2004
Self-Assembly of Supramolecular Light-Harvesting Arrays from Covalent Multi-Chromophore Perylene-3,4:9,10-bis(dicarboximide) Building Blocks Michael J. Ahrens,† Louise E. Sinks,† Boris Rybtchinski,† Wenhao Liu,† Brooks A. Jones,† Jovan M. Giaimo,† Alexy V. Gusev,† Andrew J. Goshe,‡ David M. Tiede,‡ and Michael R. Wasielewski*,† Contribution from the Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly, Northwestern UniVersity, EVanston, Illinois 60208-3113, and Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 Received November 26, 2003; E-mail:
[email protected] Abstract: We report on two multi-chromophore building blocks that self-assemble in solution and on surfaces into supramolecular light-harvesting arrays. Each building block is based on perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophores. In one building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 3 and 6 carbons of pyromellitimide to form a cross-shaped molecule (PI-PDI4). In the second building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 1 and 7 carbons of a fifth PDI to produce a saddle-shaped molecule (PDI5). These molecules self-assemble into partially ordered dimeric structures (PI-PDI4)2 and (PDI5)2 in toluene and 2-methyltetrahydrofuran solutions with the PDI molecules approximately parallel to one another primarily due to π-π interactions between adjacent PDI chromophores. On hydrophobic surfaces, PDI5 grows into rod-shaped nanostructures of average length 130 nm as revealed by atomic force microscopy. Photoexcitation of these supramolecular dimers in solution gives direct evidence of strong π-π interactions between the excited PDI chromophore and other PDI molecules nearby based on the observed formation of an excimer-like state in 3 ns, as well as for (PDI5)2, τ ) 17 ps and >3 ns. The transient spectra and kinetics are independent of whether 400 or 550 nm excitation is used, so that the fast component is not due to relaxation of an upper excited state, such as the upper exciton state. In both (PI-PDI4)2 and (PDI5)2, a significant fraction of the excited-state population decays to the ground state with τ ) 19 and 17 ps, respectively. The spectra of the ground-state bleaches do not evolve in time, indicating that the ground-state product of the photoinduced event is the same as the initial state, i.e., the PDI chromophores remain aggregated throughout the excited-state decay process. It is well-known that ultrafast laser excitation of dye aggregates including photosynthetic antenna proteins having large absorption cross sections often leads to multiple excitations of the chromophore array resulting in exciton annihilation processes.81-85 To probe whether the rapid initial transient absorption changes observed for (PI-PDI4)2 and (PDI5)2 are due to this process, transient absorption kinetics as a function of the laser pump pulse energy were measured with 400 nm excitation, which are illustrated for (PDI5)2 in Figure 11. The kinetics at 520 nm, normalized at the maximum ∆A, clearly show that the fraction of molecules that undergo fast deactivation increases as the pulse energy increases, which is typical of singlet-singlet exciton annihilation. Laser excitation of large arrays of chromophores frequently results in the formation of several excited chromophores within (81) Bittner, T.; Irrgang, K.-D.; Renger, G.; Wasielewski, M. R. J. Phys. Chem. 1994, 98, 11821-11826. (82) Bittner, T.; Viogt, J.; Irrgang, K.-D.; Renger, G. Photochem. Photobiol. 1993, 57, 158-162. (83) van Grondelle, R. Biochim. Biophys. Acta 1985, 811, 147-195. (84) Gillbro, T.; Sandstrom, A.; Spanfort, M.; Sundstrom, V.; van Grondelle, R. Biochim. Biophys. Acta 1988, 934, 369-374. (85) Geacintov, N. E.; Breton, J. In Biological EVents Probed by Ultrafast Laser Spectroscopy; Alfano, R. R., Ed.; Academic Press: New York, 1982; pp 158-191.
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Figure 11. Transient absorption kinetics for (PDI5)2 in toluene monitored at 520 nm following excitation with 400 nm, 80 fs laser pulses having 0.25 (s), 0.5 (- - -), 2.0 (‚‚‚‚‚), and 2.5 (-‚-‚-) µJ per pulse focused to a 200 µm diameter spot size.
the array.86 Dependent on the size of the chromophore array and the degree of electronic interaction between the chromophores, migration of the singlet excitons within the array leads to singlet-singlet exciton annihilation as an important decay pathway for the excitons. When two excitons collide, they can form a variety of products,85,87,88 the most typical being an upper excited singlet state and a ground singlet state.81,89 The upper excited singlet state quickly relaxes back to the lowest excited singlet state, so the net effect is the loss of one exciton. These dynamics exhibit classic “predator-prey” type behavior; i.e., the more excitons that are available, the more annihilations occur.89 However, this quickly depletes the number of excitons available, so fewer annihilations can occur. Increased pump power creates more excitons and, thus, more annihilation events, leading to an increase in the ∆A for this process.81,84 It is important to note that the conditions for annihilation, multiple excitons in an array, generally only occur when very high intensity laser pulses are employed. This behavior is usually not seen in conventional steady-state measurements at low photon fluxes, where it would manifest itself as a decrease in fluorescence quantum yield. Singlet-singlet annihilation is a second-order kinetic process, so that the simplest description of excited singlet state decay in the presence of annihilation is given by eq 1:87,90
d∆A 1 ) γ1∆A + γ2(∆A)2 dt 2
-
(1)
where γ1 is the overall rate constant for unimolecular decay of the excited singlet state and γ2 is the time independent rate constant for singlet-singlet annihilation defined as the annihilation rate per pair of excitons in a domain and has the dimensions of s-1.84,91,92 The annihilation rate constant γ2 depends critically on the number of electronically connected chromophores in the aggregate (the domain size), as well as the strength of the (86) Den Hollander, W. T. F.; Bakker, J. G. C.; van Grondelle, R. Biochim. Biophys. Acta 1988, 725, 492-507. (87) Swenberg, C. E.; Geacintov, N. E. Org. Mol. Photophys. 1973, 18, 489565. (88) Khairutdinov, R. F.; Serpone, N. J. Phys. Chem. B 1997, 101, 2602-2610. (89) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999; Vol. 56, pp 157-161. (90) Paillotin, G.; Swenberg, C. E.; Breton, J.; Geacintov, N. E. Biophys. J. 1979, 25, 513-534. (91) Paillotin, G.; Swenberg, C. E.; Breton, J.; Geacintov, N. E. Biophys. J. 1979, 25, 513-534. (92) Barzda, V.; Gulbinas, V.; Kananavicius, R.; Cervinskas, V.; van Amerongen, H.; van Grondelle, R.; Valkunas, L. Biophys. J. 2001, 80, 2409-2421. J. AM. CHEM. SOC.
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coupling. For example, studies on the LH2 antenna protein from the purple bacterium Rhodospirillum rubrum found an annihilation rate constant, τa, of 1 × 1012 s-1 for a domain of 30 chromophores.93 More recent studies on this system used a domain size of 12 and found a rate of 2 × 1012 s-1.94 Workers studying the Fenna-Matthews-Olsen antenna protein from the green sulfur bacterium Chlorobium tepidum found a rate of 1.4 × 1011 s-1 for a domain of 21 units.95 Bittner et al.81 have shown that τa for the trimeric LHCII light-harvesting protein from photosystem II of green plants, which contains 21 chlorophyll a molecules, is 25 ps, which was corroborated by the more recent study of Bardza et al..92 For both (PI-PDI4)2 and (PDI5)2 in which the exciton can migrate between a small number of sites, the observed time constant for the annihilation process, τa, can be related to the site-to-site hopping time constant, τhop, using eq 2:96
τa ) 2γ2-1 ) (π-1N ln N + 0.20N - 0.12)τhop
(2)
where N is the number of sites. Equation 2 assumes that every collision of two excitons results in rapid annihilation, so that τa is limited by the exciton hopping rate τhop, not by the trapping rate. In addition, eq 2 assumes the exciton hops between nearest neighbor sites on a square lattice with equal probability. Since the excimer-like state in both (PDI4-PI)2 and (PDI5)2 forms within the 130 fs instrument response function of the experiment, we will assume that the migrating exciton is initially delocalized between two adjacent π-stacked PDI molecules within each dimer. In that case, the exciton hops between four sites within (PDI4-PI)2 and five sites within (PDI5)2. Substituting N ) 4 and 5 and the 19 and 17 ps annihilation time constants for (PDI4-PI)2 and (PDI5)2, respectively, into eq 2 yields τhop ) 7.8 ps and τhop ) 4.9 ps for (PDI4-PI)2 and (PDI5)2, respectively. These hopping times are very similar to those estimated for the trimeric LHC-II photosynthetic light harvesting antenna complex containing 21 chromophores, where typically τhop = 4-5 ps (for a recent summary, see ref 92). It is most likely that the value of τhop differs for exciton hops between sites that are covalently bonded versus those that are not. Moreover, different covalent connections between the PDI molecules will also result in different values of τhop. In the work presented here our experimental data cannot distinguish between these routes. In future studies we will explore specific structures designed to differentiate between these pathways. (93) Deinum, G.; Aartsma, T. J.; van Grondelle, R.; Amesz, J. Biochim. Biophys. Acta 1989, 976, 63-69. (94) Valkunas, L.; Trinkunas, G.; Liuolia, V.; van Grondelle, R. Biophys. J. 1995, 69, 1117-1129. (95) Gulbinas, V.; Valkunas, L.; Kuciauskas, D.; Katilius, E.; Liuolia, V.; Zhou, W.; Blankenship, R. E. J. Phys. Chem. 1996, 100, 17950-17956.
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Conclusions
Two different multi-chromophoric arrays PI-PDI4 and PDI5 based on the PDI chromophore were synthesized and characterized. These arrays self-assemble into partially ordered supramolecular dimers (PI-PDI4)2 and (PDI5)2 in solution as indicated by both SAXS and GPC data, where the PDI molecules are approximately parallel and cofacial to one another. Photoexcitation of these supramolecular dimers gives direct evidence of strong π-π interactions between the excited PDI chromophore and other PDI molecules nearby based on the observed formation of an excimer-like state in
