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Ultrafast Energy Transfer in Oligofluorene-AluminumBis(8-hydroxyquinoline)acetylacetone Coordination Polymers

Victor A. Montes, Grigory V. Zyryanov, Evgeny Danilov, NeerajAgarwal, Manuel A. Palacios, and Pavel Anzenbacher Jr.*

J. Am. Chem. Soc., 2009, 131 (5), 1787-1795

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Outline Introduction Resonance energy transfer Organic light-emitting diode Experiment Synthesis Optical properties Ultrafast energy migration Solid-state electroluminescence Conclusions

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Resonance Energy Transfer (RET)

http://micro.magnet.fsu.edu/primer/techniques/fluorescence/fret/fretintro.html

T. Förster in 1959 proposed the Förster theory of resonance energy transfer

S*+Q → S+Q*

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Resonance Energy Transfer (RET)

Energy transfer is efficient when: 1.The energy donor and acceptor are separated by a short distance.(30~100 Å) 2.Photons emitted by the excited state of the donor can be absorbed directly by the acceptor.

Emission spectra of DonorAbsorption spectra of acceptor

OverlapsEt: efficiency of energy transferR0: Förster distance

r : distance between donor and acceptor

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Principle of OLED Device Operation

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OLED v.s LCD

萬能科技大學光電系張興華 OLED 投影片

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Device structures

Electroluminescence Layer

萬能科技大學光電系張興華 OLED 投影片

Cathode : CsF:Al

Hole Injection Layer : PEDOT:PSS

Anode : Indium-tin-oxide

Electroluminescence Layer

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OLED v.s PLED

萬能科技大學光電系張興華 OLED 投影片

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Structure of Alq3-type complexes

Montes, V. A; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.-Eur. J. 2006, 12, 4523.

Complex 2

Red-shift

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Electroluminescence Spectra of Alq3-type Complexes

Montes, V. A; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.-Eur. J. 2006, 12, 4523.

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Oligofluorene :OF Alq2acac

Anzenbacher, P., Jr. Chem. Commun. 2007, 3708.

X.; Wang, Y.Appl. Phys. Lett. 2008, 92, 103305.

n

NN

OHHO

Al N

O

N O

N

O

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Synthesis of Oligofluorene

Anzenbacher, P., Jr. Chem. Commun. 2007, 3708.

a) Pd(PPh3)4, Et4N+OH- in MeOH, toluene, 60°C

b) 1,4-cyclohexadiene, Pd-C (10%), isopropanol, reflux.

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1H NMR spectra of Oligofluorene

Figure 1. 1H NMR spectra of the ditopic ligands. Residual CHCl3 signals are marked with an asterisk.

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Synthesis of Alq2(acac) and 1a-e

Scheme 1. Synthesis of Alq2(acac) and Coordination Polymers 1a-e Using Tris(acetylacetonate)aluminum(III), and X-ray Structure of Alq2(acac)

n

5 days

Yield=76% ~ 98%

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UV-vis absorption spectra of 1a-e

Figure 2. UV-vis absorption spectra of 1a-e in a CH2Cl2 solution showing contribution of both oligofluorene (OF) and AlIII quinolinolate chromophores.

Table 1. Summarized Absorption Data for Bichromophoric Systems 1a-e

340 nm 475 nm

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Emission spectra of 1a-e

Figure 3. Corrected emission spectra of the coordination polymers 1a-e in CH2Cl2 upon excitation at 340 nm

550 nm

410 nm

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Excitation spectra of 1a-e

Figure 4. Excitation spectra of the polymers when monitored at 550 nm.

UV-vis absorption spectra of 1a-e

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Transient Absorption Spectra

清華大學化學研究所 2005 陳學穎碩士論文

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TMSTMS

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Model 3

Al N

O

N O

N

O

Model 2

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Model compound 2

Figure 5. (A) Absorption and emission spectra of model compound 2. (C) Transient absorption spectra of 2 0.2 ps after pump pulse at 475 nm and its decay monitored at 750 nm (inset).

excitation at 475 nm ( - * of Alq3)

τ=9200 ps

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Model compound 3

Figure 5.(B) Absorption and emission spectra of model compound 3. (D) Transient absorption spectra of 3 0.2 ps after pump pulse at 340 nm and its decay monitored at 750 nm(inset).

excitation at 340 nm ( - * of oligofluorene )

τ=642 ps

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Transient absorption spectra of 1a-e0.2 ps after excitation at 475 nm( - * of Alq3)

520 nm 640 nm

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Transient absorption spectra of 1a-e0.2 ps after excitation at 340 nm( - * of oligofluorene )

475 nm

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Transient absorption spectra for 1d

Figure 6. Left : Transient absorption spectra for 1d after excitation at 340 nm (0.5mW) at various times . Right : Exponential fit of the kinetic profile at 750 nm.

τ=1.4 ps

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Rate Constants for Energy Transfer

kET = Obs-1 - Fl

-1

Table 2. Calculated Rate Constants for Energy Transfer in the Coordination Polymers 1c-e Monitored by Decay at 750 nm

kET is the overall rate of energy transfer

Obs is the lifetime observed for the spectral change in the transient experiment

Fl represents the fluorescence lifetime

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Mechanism for Intramolecular Energy Transfer

kET = keh+kfq

1e

keh = exciton hopping between the fluorene moieties kfq = strongly exothermic transfer from fluorene to AlIII quinolinolate

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Mechanism for Intramolecular Energy Transfer

Figure 7. Schematic representation of the mechanism for intramolecular energy transfer as proposed for the behavior of the bichromophoric systems 1c-e.Only one pathway of energy migration is shown for simplicity purposes.

1c

1d

1e

kET=6.9x1011 (s-1)

kET=7.1x1011 (s-1)

kET=3.3x1011 (s-1)

kET=keh+kfq

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Simplified OLED architectures

Cathode : CsF (10 Å) : Al (1200 Å)

Hole Injection Layer : PEDOT:PSS (500 Å)

Anode : Indium-tin-oxide

Electroluminescence Layer : 1a-c (600 Å)

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1a-OLED

Figure 8. Left: Electroluminescence spectra of 1a-OLED at a voltage of 9 V. The inset shows a photograph of the operating device. Right: I-V and luminance curves of the ITO/PEDOT:PSS/1a /CsF:Al OLED.

external quantum efficiency of 1.2%.maximum luminance was 6000 cd/m2

turn-on voltage of 6 V∼

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Conclusions

Novel coordination polymers comprising oligofluorene moieties of a varying size (n = 1-9) connected via aluminum(III) bis(8-quinolinolate)acetylacetone (Alq2(acac)) complexes were synthesized and their photophysical properties were studied.

The energy migration from oligofluorene to the quinolinolate moieties was observed proceeding at a rate order of 1011 s-1.

In the solid state, complete energy transfer from oligofluorene fragments to the quinolinolate centers was observed due to intermolecular energy transfer.