Directing energy transfer within conjugated polymer thin films.

Light-harvesting organisms employ photosystems to collect and harness light energy. Photosystems use hundreds of chromophores arranged in an energy gradient to move absorbed energy unidirectionally toward the reaction center1 and the efficiency provided by this directional energy transfer has inspired intensive investigation.2 Our group has utilized energy transport within conjugated polymer (CP) films to create highly sensitive chemosensors.3 Initially, we investigated the signal enhancement observed when multiple binding sites are linked together via a conjugated backbone.4 This first-generation system featured a dilute solution of CP and relied on energy migrating along a single polymer chain. Within thin films, the polymers electronically couple, encouraging interpolymer energy transfer. The movement of energy between the CPs in three dimensions allows the film’s luminescence to be more strongly quenched by energy traps such as TNT.5 Further investigation of the energy movement within CP thin films revealed that transfer of energy was distancedependent and was limited to films 16 polymer layers thick (∼18 nm).6 The z-directional (film thickness) limitation observed in our previous work was confined to a film composed of 16 layers of 2. Herein, we present a striated multipolymer system, which utilizes directional energy transfer to overcome the z-direction limitation. This system is analogous to the antenna complex in that it encourages maximum Förster energy transfer in one direction (Figure 1).7 Three poly(p-phenylene ethynylene)s with tailored absorption and emission λmax were synthesized. Polymers 1-38 (Figure 1) were designed to have large spectral overlap between a donor emission and an acceptor absorption ranging from the blue (1) to the red (3).9 The excellent spectral overlap encourages energy transfer from 1 to 2 and from 2 to 3. Polymers 2 and 3 were also designed to be nonaggregating10 and amphiphilic, thus allowing manipulation at the air-water interface.11 Multilayer films of 2 (4-, 8-, 16-, 24-, and 32-layers) were fabricated by the Langmuir-Blodgett (LB) method.12 An LB monolayer of 3 was deposited on top of each multilayer film. As can be seen in Figure 2 efficient energy transfer occurs between 2 and 3. The emission intensity of 3 (ex. 420 nm) gradually increases up to 16 layers of 2. Above 16 layers the increase in fluorescence intensity levels off, clearly demonstrating the 16layer limitation previously observed.6 The acceptor-independent behavior suggests that the saturation observed above 16 layers is a property of thin films of 2 and is most likely due to the finite diffusion length of an exciton. A film composed of a monolayer of 3 sandwiched between 16 layers of 2 on the bottom and eight layers of 2 on the top (161-8) showed higher fluorescence intensity emission from 3 (Figure 2) than that of the nonsandwiched films 16-1, 24-1, 32-1. The higher intensity observed for the 16-1-8 film further illustrates that the 16-layer limitation is due to the film thickness and not due to an intrinsic property of the acceptor. On the other hand, the observation that the fluorescence intensity of the 16-1-8 is not a linear combination of the fluorescence from films 16-1 and 8-1 may be due to acceptor limitations (e.g., poor orientation or longer lifetime of the excited-state residing on 3 that can lead to ‡ Department of Chemistry. † Department of Materials Science and Engineering and the Center for Materials Science and Engineering. (1) Alberts, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.; Watson, J. D. Molecular Biology of the Cell, 2nd ed.; Garland Publishing: New York, 1989. (2) For leading references, see: (a) Berggren, M.; Dodabalapur, A.; Slusher, R. E.; Bao, Z. Nature 1997, 389, 466. (b) Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658. (c) Lammi, R. K.; Ambroise, A.; Balasubramanian, R.; Wagner, R. W.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 2000, 122, 7579. (d) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R., Schanze K. S. J. Am. Chem. Soc. 2000, 122, 8561. e) Hagfeldt, A.; Grätzel, M. Acc. Chem. Res. 2000, 33, 269. (f) Vuorimaa, E.; Lemmetyinen, H. Langmuir 1997, 13, 3009. (g) Chrisstoffels, L. A. J.; Adronov, A.; Fréchet, M. J. Angew. Chem., Int. Ed. 2000, 39, 2163. (h) Shortreed, M. R.; Swallen, S. F.; Shi, Z.-Y.; Tan, W.; Xu, Z.; Devadoss, C.; Moore, J. S.; Kopelman, R. J. Phys. Chem. B 1997, 101, 6318. (i) Calzaferri, G.; Devaux, A.; Pauchard, M. Chem. Eur. J. 2000, 6, 3456. (j) Haycock, R. A.; Yaartsev, A.; Michelsen, U.; Sundström, V.; Hunter, C. A. Angew. Chem., Int. Ed. 2000, 39, 3616. (3) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (b) For a recent review, see: McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (4) (a) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017. (b) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. (5) (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321. (b) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (6) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 1466. (7) Although not shown for clarity, within each polymer layer there are also small energy gradients due to disorder. Site-selective luminescence from energy transfer to low-energy traps in conjugated polymers has been previously shown. Bässler, H.; Deuben, M.; Huen, S.; Lemmer, U.; Mahrt, R. F. Zeitsch. Phys. Chem. 1994, 184, 233. (8) Polymer 1 Mn ) 73 000, PDI ) 3.0; Polymer 2 Mn ) 81 200 PDI ) 3.4, Polymer 3 Mn ) 103 000, PDI ) 1.5. (9) Polymer 2 was reported earlier (ref 4), and the synthesis of 1 and 3 will be reported in forthcoming manuscripts. (10) (a) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 5885. (b) Kim, J.; Swager, T. M. Nature 2001, 411, 1030. (11) Kim, J.; McHugh, S. K.; Swager, T. M. Macromolecules 1999, 32, 1500. (12) Polymers 2 and 3 were deposited at a surface pressure of 27 and 20 mN/m, respectively. Transfer ratios were greater than 95%. Figure 1. Energy is preferentially focused to the surface of a threelayer conjugated polymer film where the films have decreasing band gaps moving from the bottom to the top. Polymer 1 (abs./em. max 390/424 nm) overlaps with 2 (abs./em. max 430/465 nm) which overlaps with 3 (abs./em. max. 495/514 nm). 11488 J. Am. Chem. Soc. 2001, 123, 11488-11489