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Dendrimers and light: towards electroactive and photoactive dendrimers

Dendrimers and light: towards electroactive and photoactive dendrimers. Jean M.J. Fréchet, Dept. of Chemistry, UC Berkeley AFOSR MURI Program (Dr. C. Lee) Graduate Students: Alex Adronov, Adam Freeman, Stefan Hecht, Patrick Malenfant.

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Dendrimers and light: towards electroactive and photoactive dendrimers

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  1. Dendrimers and light:towards electroactive and photoactive dendrimers Jean M.J. Fréchet, Dept. of Chemistry, UC Berkeley AFOSR MURI Program (Dr. C. Lee) Graduate Students: Alex Adronov, Adam Freeman, Stefan Hecht, Patrick Malenfant. Postdoctoral Fellow: Lysander Chrisstoffels. Collaborators: Prof. M. Thompson, USC Prof. P. Prasad, SUNY Buffalo Dr. S. Gilat, Lucent Bell Labs Prof. G. Fleming, UC Berkeley \ Dr. D. Robello, Kodak

  2. Natural photosynthetic processes 1. Light Harvesting Chlorophyll Bound to Protein 2. Energy Transfer Relay Reaction Center Lipid Bilayer 3. Charge Separation Frechet@cchem.berkeley.edu

  3. The dendritic antenna • Design & synthesis of a dendritic light harvesting “antenna” • Demonstration of efficient through-space energy transfer • Study of the effect of increasing dendrimer generation on ...the energy transfer efficiency Frechet@cchem.berkeley.edu

  4. Antennas for Light Harvesting andEnergy Transfer • Efficient light harvesting and energy transfer (>90%) in dendritic systems have been demonstrated. • Synthesized molecules have been fully characterized and energy transfer was studied by both steady-state and time-resolved techniques. • The synthetic approach has a modular design that provides versatility in the choice of core acceptor and surface donor dyes (coumarins, oligothiophenes, two-photon chromophores). Frechet@cchem.berkeley.edu

  5. Summary of initial energy transfer results Donor Dye Acceptor Dye G-4 Dendrimer Frechet@cchem.berkeley.edu

  6. Frechet@cchem.berkeley.edu Synthetic strategy

  7. Peripheral laser-dye donor functionalization Frechet@cchem.berkeley.edu

  8. Core acceptor functionalization Identical reactions can be carried out to link the penta- thiophene cores to the donor dendrons Frechet@cchem.berkeley.edu

  9. Thiophene-core fully dye-labeled dendrons Frechet@cchem.berkeley.edu

  10. 3411 (3420) G-3 2075 (2079) G-2 1403 (1408) G-1 3000 3500 4000 4500 5000 2000 2500 1500 Mass (m/z) MALDI-TOF of T7-labeled dendrons Frechet@cchem.berkeley.edu

  11. Spectral properties of the models Emission Intensity (a.u.) Extinction coefficient e (x 10-4, M-1 cm-1) Wavelength (nm) Large overlap between donor emission and acceptor absorption enables efficient energy transfer. Frechet@cchem.berkeley.edu

  12. Light harvesting: G-1 to G-3 G-3 G-2 Extinction coefficient e (M-1 cm-1) G-1 T7 Wavelength (nm) “Light Harvesting” capacity doubles with generation. Frechet@cchem.berkeley.edu

  13. Energy transfer: G-1 to G-3 conc. = 5.06 x 10-6 M lexc = 343 nm G-3 Emission intensity (a.u.) G-2 G-1 Direct Core Emission lmax = 425 nm T7 Wavelength (nm) Beyond G-1, sensitized fluorescence becomes much more intense than fluorescence from direct excitation of the core. Frechet@cchem.berkeley.edu

  14. 300000 200000 100000 T5-core dendrons as antennas conc. = 3.93 x 10-6 M lexc = 343 nm • G-3 • G-3 Emission intensity (a. u.) G-2 Extinction coefficient (M-1cm-1) G-2 G-1 Direct Core Emission G-1 lexc = 425 nm T5 T5 Wavelength (nm) Wavelength (nm) The observed absorption and fluorescence emission spectra of the G-1 to G-3 pentathiophene core dendrons were very similar to those for the series of heptathiophene core dendrons. Frechet@cchem.berkeley.edu

  15. Energy transfer efficiency Frechet@cchem.berkeley.edu

  16. Tunable emission Emission intensity (a. u.) Emission intensity (a. u.) Wavelength (nm) Wavelength (nm) It is possible to tune the dendrimer emission wavelength by changing the core functionality. Also, by mixing the different types of dendrimers (no dye at core, coumarin 343 at core, and oligothiophene at core), it is possible to obtain broadband emission by exciting at a single wavelength (lmax of donor dye - 343 nm). Frechet@cchem.berkeley.edu

  17. Incorporation of two-photon dyes Investigation of cooperativity effects of two-photon chromophores when arranged in a branched structure EDC/DMAP + G-1 A collaboration with Prof. Paras Prasad. Frechet@cchem.berkeley.edu

  18. Two-photon dendrimers K2CO3 + 18-Crown-6 G-2 Collaboration with Prof. Paras Prasad Frechet@cchem.berkeley.edu

  19. Future directions:dendritic energy transfer relay 380 nm 320 nm 470 nm 390 nm 480 nm 510 nm Frechet@cchem.berkeley.edu

  20. Surface-confined energy transfer • Self-assembly of individual • donor dendrons and acceptor dyes simplifies the preparation of antennas. • Energy transfer on surfaces opens avenues for the fabrication of novel photonic devices. • Variation of photoactive donors and acceptors allows for numerous applications, ranging from sensors to solar cells and new devices. Frechet@cchem.berkeley.edu

  21. Energy transfer on self-assembled surfaces O O O O O O O hn O O O O O O O O O N N N N N N N hn’ N N N N N O O O O O O O O O O O O O O O O O O O O O N H N H O O N H N H O N H N H N H N H S i S i S i S i S i S i O S i S i O O O O O O O O O O O O O O O O S i S i S i S i S i S i S i S i S i S i Frechet@cchem.berkeley.edu • Self-assembled monolayers of chromophores with different aspect ratios were prepared on silicon wafers by using siloxane chemistry. • Complete quenching of the donor emission as well as efficient energy transfer from the assembled coumarin-2 (donor) dyes to the coumarin-343 (acceptor) dyes was observed. • The photophysical properties are tuned by varying the molar ratio of assembled donor and acceptor chromophores on the surface.

  22. Synthesis of adsorbates Donor chromophore Acceptor chromophore Frechet@cchem.berkeley.edu

  23. Emission scan -1 cps ( s ) Physisorption of coumarin-343 onto amino-terminated SAMs 40000 = 420 nm l ex II 30000 20000 10000 III I 0 400 500 600 lem (nm) Frechet@cchem.berkeley.edu

  24. Photophysical properties of coumarin-derivatized SAMs Excitation spectra and Emission spectra of SAMs of 1 or 2 1 A.U. 0.75 2 1 2 0.5 0.25 1 0 320 370 470 520 570 420 l (nm) em 1 2 Frechet@cchem.berkeley.edu

  25. -1 cps (s ) Energy transfer within SAMs of mixed monolayers of coumarin chromophores Emission spectra from mixed monolayer of 1 and 2 (1:2 ratio) 80000 lex = 370 nm 60000 40000 lex = 420 nm 20000 0 350 400 450 500 550 600 1 2 (nm) l em Frechet@cchem.berkeley.edu

  26. Energy transfer on surfaces… The next step • Light harvesting event is followed by electron transfer. • The excited state of secondary donor (De) transfers an electron instead of emitting light. • Electron acceptor can inject an e- into a semi-conducting substrate. Frechet@cchem.berkeley.edu

  27. Light harvesting and electron transfer:the concept 3. Charge Transport 4. Subsequent Reaction 1. Light Harvesting & Energy Transfer 2. Charge Separation Electroactive core is capable of donating an electron to attached acceptor; this effects charge separation that may be followed by charge transport and subsequent reaction Frechet@cchem.berkeley.edu

  28. Light harvesting and electron transfer:the molecules 1. Light Harvesting 2. Energy Transfer 3. Electron Transfer Frechet@cchem.berkeley.edu

  29. Why use dendrimers in OLEDs? Preorganization of active components and building blocks A collaboration with Prof. M. Thompson, USC Site-isolated light emitting chromophore Hole or electron transporting moiety “Insulating” building block

  30. Dendrimers in single layer OLED’s Metal cathode ITO anode Glass substrate A collaboration with Prof. M. Thompson, USC Frechet@cchem.berkeley.edu

  31. Synthesis of Dye-Labeled Cores

  32. Synthesis of HT-Labeled Monodendrons

  33. Synthesis of Reactive Monodendrons

  34. Synthesis of Fully-Labeled Dendrimers

  35. Single-Layer OLEDs A collaboration with Prof. M. Thompson, USC

  36. Device Fabrication A collaboration with Prof. M. Thompson, USC

  37. Photo- and electroluminescenceof C343 labeled dendrimers TAA = TAA2-[G-1]-OH PBD = A collaboration with Prof. M. Thompson, USC

  38. Photo- and Electroluminescenceof T5 Labeled Dendrimers PBD = A collaboration with Prof. M. Thompson, USC

  39. Color Tunable OLEDs by Mixing Dendrimers The small dendrimer affords partial site-isolation T5 C343

  40. Towards enhanced properties… Surface (HT) chromophores Central lumophore Interior “insulating” monomer layers Increasing the size of the dendrimer will increase site isolation of the central lumophore; this should lead to enhanced color tunability of devices containing mixtures of dendrimers. Frechet@cchem.berkeley.edu

  41. Increasing dendrimer size for enhanced site-isolation Frechet@cchem.berkeley.edu

  42. Current status of project • A new family of fully-labeled dendrimers has been successfully prepared via a modular approach. • Photoluminescence studies indicate that efficient Forster energy transfer between peripheral and core chromophores occurs within these dendrimers. • Analogous energy conveyance processes occur in single-layer OLEDs containing these dendrimers and exclusive emission from the core chromophores is observed. • Site isolation of chromophoric dyes within the dendrimer affords some degree of color tunability.

  43. Porphyrin-core stars as photo- and electroactive polymers peripheral functionalities (chromophores or solubilizing groups) polymer backbone (UV-transparent and redox-stable) porphyrin-core unit (energy sink and catalytic site) advantages: ease and flexibility of preparation/modification efficient shielding of the core (site isolation) solvent-induced change of shape and size potential: photoresponsive devices and sensors, catalysts

  44. Toward Light-driven Supermolecular Catalysis substrate product water-soluble micellar structureÞsolvophobically-driven catalysis Solar Energy Concentration and Conversion

  45. Encapsulated porphyrins - a retrospective Myoglobin Mimics - Oxygen Binding in Artificial Enzymes: Þ dioxygen binding affinity: 1500 times higher than hemoglobin (T-state) Collman, Fu, Zingg, and Diederich Chem. Commun.1997, 193 see also: Jiang and Aida Chem. Commun.1996, 1523 (polyether dendrimers)

  46. Encapsulated porphyrins - a retrospective Hemeprotein Mimics - Tuning of Redox properties by Isolation of the Core: Pollak, Leon, Fréchet, Maskus, Abruña Chem. Mater.1998, 10, 30 Jin, Aida, Inoue Chem. Commun.1993, 1260 Aida and coworkers Macromolecules1996, 29, 5236 see also: Diederich and coworkers Angew. Chem. Int. Ed. Engl.1994, 33, 1739; Angew. Chem. Int. Ed. Engl.1995, 34, 2725; Helv. Chim. Acta1997, 80, 1773

  47. Synthesis of branched porphyrin-core star polymers branched initiators star polymers

  48. Characterization of porphyrin-core star polymers 1H NMR Analysis:

  49. Modification of porphyrin-core star polymers core modification (metalation): end-group modification (esterification):

  50. Accessibility of the zinc porphyrin core fluorescence quenching experiments: employing methyl viologen in acetonitrile Stern-Volmer Analysis Þ Þ enhanced shielding of the core moiety by the polymer backbone Þ degree of site isolation is a function of the chain length

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