Abstract:

1. Abstract: The synthesis, crystal structure, and fluorescence behavior of acetylene-bridged pentiptycene dimer (2), trimer (3), and tetramer (4) are reported. For comparison, a phenylene-pentiptycene-phenylene three-ring system (5) is also investigated. As a result of the unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, their twisted conformers are populated in polar solvents and at low temperatures, and the phenomenon of nonequilibration of excited rotational conformers is observed. Twisting of the π-conjugated backbones leads to blue-shifted absorption and fluorescence spectra and increased fluorescence quantum yields and lifetimes. The fluorescence spectra of 2-4 undergo small red shifts but large intensity variations in the 0-1 vs 0-0 bands on going from solutions to thin solid films, which can be accounted for by the reabsorption effect. However, the reduction in fluorescence quantum yields for 2-4 in films vs solutions is mainly attributed to efficient interchain exciton migration to nonfluorescent energy traps. In contrast, the behavior of nonequilibration of excited rotamers is not observed for 5 in solutions. Compound 5 forms J-type aggregates through terminal phenylene π-stackings in the solid state, resulting in a new absorption band at 377 nm and large red shifts of the structured fluorescence spectra.

2. Probing the Intrachain and Interchain Effects on theFluorescence Behavior of Pentiptycene-DerivedOligo( p-phenyleneethynylene)s Jye-Shane Yang,* Jyu-Lun Yan, Chung-Yu Hwang, Shih-Yi, Chiou, Kang-Ling Liau, Hui-Hsu Gavin Tsai, Gene-Hsiang Lee, and Shie-Ming Peng J. Am. Chem. Soc. 2006, 128, 14109-14119 Speaker：Po-yuan Chung

3. Poly(phenyleneethynylene)s (PPEs) • 高分子(polymer)，顧名思義就是分子結構龐大、分子量高的物質，換言之，在分子主鏈上具有單鍵、雙鍵(或參鍵)交替之共軛結構，使電子可沿著分子鏈或跨分子鏈運動，因而具導電性的高分子量物質，我們稱之為共軛高分子。由於具有導電能力及電激發光的性質，可應用於有機電激發光元件。而PPEs即為一種利用三鍵將單體(monomer)連結起來的共軛高分子。 PPEs

4. Intrachain Effect and Interchain Effect • Intrachain effect：Undergo fast excited-state conformational relaxation to planarize the conjugated backbone before the fluorescence is emitted. • Interchain effect：The significant reduction in fluorescence quantum yields for 3 and 4 in films vs solutions is however attributed to efficient interchain exciton migration to nonfluorescent energy traps.

5. Coplanar and Twisted Geometry Rotational barrier ＜1 kcal/mol

6. Swager’s Approach Conformations and spatial arrangements of polymers 1-4 at the air-water interface and their reversible conversions between face-on, zipper and edge-on structures. Kim, J.; Swager, T. M. Nature, 2001, 411, 1030-1034.

7. Oligo(p-phenyleneethynylene)s (OPEs)

8. Synthesis of 2 Lithium trimethylsilylacetylide

9. Synthesis of 3 Quinone

10. Synthesis of 4 Quinone

11. Synthesis of 5

12. Rotational Potential of 3’ and 5’ Barrier = 2.6 kcal/mol Barrier = 0.06 kcal/mol Rotational potential of 3’ and 5’ in the ground state were calculated with the AM1. Transition energies were calculated with theZINDOalgorithm.

14. Intrachain Effect • Undergo fast excited-state conformational relaxation to planarize the conjugated backbone before the fluorescence is emitted. • Be better investigated in dilute solutions or low-temperature solvent glasses.

15. Jablonski Diagram http://www.shsu.edu/~chemistry/chemiluminescence/JABLONSKI.html

16. Franck-Condon Theory and Mirror Image Rule 曾炳墝教授,高等無機上課講義,2006;Chapter 1, with permission from Dr. Tzeng, B.-C

17. The (a) Absorption and (b) Fluorescence Spectra of 3 λex = 302 nm 314 nm 3 Polarity：MeCN > THF > CHCl3 > hexane

18. Temperature Dependence of (a) Absorption and (b) Fluorescence Spectra of 3 in MTHF λex = 302 nm Blue-shift 20 nm 363 nm 3 Blue-shift 4 nm 5.6 ns 0.8 ns

19. Temperature Dependence of (a) Absorption and (b) Fluorescence Spectra of 4 in MTHF λex = 303 nm Blue-shift 41 nm 380 nm 4 5.5 ns 0.8 ns

20. Structure of Three-ring and Four-ring Zipper Red-shift

21. Temperature Dependence of Fluorescence Spectra of 2 in MTHF λex = 306 nm

22. (1) Σk：the sum of all nonactivated processes (fluorescence and intersystem crossing) A ：preexponential Ea：activation energy for the activated process Compound 2 Ea = 630 cm-1 DPA Ea = 610 cm-1 Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178-1179. Diphenylacetylene (DPA)

23. Temperature Dependence of (a) Absorption and (b) Fluorescence Spectra of 5 in MTHF λex = 320 nm Compound 3 5

24. Interchain effect • The significant reduction in fluorescence quantum yields for 3 and 4 in films vs solutions is however attributed to efficient interchain exciton migration to nonfluorescent energy traps. • Be better investigated in thin solid films. • Be prepared by spin casting with 5×10-3 M chloroform solutions. http://nmeg.group.shef.ac.uk/index.php?page=spincast

25. Crystal Structure of 11 35° 8 Å • the nonplanar conjugated backbones and layered packing motif. • (b) head-to-tail tilted packing, included solvent molecules, and disordered octyl chains.

26. Crystal Structure of 5 5 3.46 Å • the coplanar conjugated backbones and parallel interchain alignments. • (b) interchain offset π-stacking of the terminal phenylene rings (the octyl groups were removed for clarity).

27. Normalized Absorption and Fluorescence Spectra of 2, 3, 4, and 5 377 nm in CHCl3 in spin-cast films

29. Normalized Fluorescence Spectra for Films of 3 Mixed (a) with PMMA and (b) with 2 Molar fractions of 3 = 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 3 mixed with PMMA 3 mixed with 2 poly(methyl methacrylate)

30. Comparison of the Fluorescence Data of 3-4 with those of 18

31. Conclusions • The intrachain conformation and interchain exciton coupling effects on the fluorescence properties of PPEs have been studied. • The unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, as well as their twisted conformers and their photophysics can be characterized at low temperatures.