Impact of Cyano-Functional Group on Luminescence of Poly(m

Chapter 7

Downloaded by MONASH UNIV on June 17, 2013 | http://pubs.acs.org Publication Date: September 29, 2005 | doi: 10.1021/bk-2005-0916.ch007

Impact of Cyano-Functional Group on Luminescence of Poly(m-phenylenevinylene) Derivatives: Its Dependence on Conjugation Length 1

1.*

2

2

LiangLiao ,YiPang ,LimingDing ,and Frank E. Karasz 1

Department of Chemistry and Center for High Performance Polymers and Composites, Clark Atlanta University, Atlanta, GA 30314 Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003 2

Soluble cyano-substituted poly(1,3-phenylene vinylene) (8) and poly[(1,3-phenylene vinylene)-alt-tris(1,4-phenylene vinylene)] (10) derivatives have been synthesized and characterized, in comparison with a green-emitting poly[(1,3phenylene vinylene)-alt-(1,4-phenylene vinylene)] derivative (9). Chromophores in these polymers are well defined as a result of π-conjugation interruption at adjacent m-phenylene units, leading to blue-emission for film 8 (λ ~477 nm) and red-emission for film 10 (λ ~640 nm). Optical color tuning in these polymers is achieved through controlled insertion of different oligo(p-phenylene vinylene) length. Although the chromophore of 10 contains only 4.5 phenylene-vinylene units with cyano-substitution on ~50% of the vinylene units, emission λ of 10 is comparable to that of cyano-substituted PPV derivative 1. An LED based on 10 emits red-light (644 nm) with an external quantum efficiency of 1.2%. max

max

max

In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

77 1

Since the discovery of polymeric light-emitting diodes (LEDs), πconjugated polymers have attracted significant attention over the past decade because of their potential applications in display technologies. In a typical LED device, the emitting polymer layer is sandwiched between two electrodes. Electroluminescence (EL) is achieved by injecting electrons from a cathode into the conduction band and holes from an indium tin oxide (ITO) anode into the valence band of die polymer emissive layer. Combination of die injected holes and electrons leads to formation of excitons, which emit photons upon returning to ground states via radiative relaxation. To achieve highly efficient LEDs, charge carrier injection (including both electrons and holes) as well as charge transport must be balanced, and the energy barriers at the electrode-polymer interface be minimized. 2


3

For poly(p-phenylenevinylene) (PPV), injection and transport of holes is easier than that of electrons, which make this polymer useful as a hole-transport material in LEDs. The relative poor electron affinity of PPV, which leads to a low rate of electron injection, however, has been a major barrier in the development of PPV-based bright LEDs. An increase in the electron affinity of PPV would lead to improved charge injection and transport. This concept has been successfully demonstrated by comparing poly[(2-methoxy-5-ethylhexyloxyl,4-phenylene)vinylene] (MEH-PPV, la) with the cyano-substituted MEH-PPV lb (Scheme 1); the latter significantly enhanced EL efficiency than la, reaching an external quantum efficiency as high as 4%. In addition, attachment of cyano group shifts the emission color of MEH-PPV (yellow) to longer wavelength, although the unspecified chromophore length in CN-MEH-PPV somewhat hampers the estimation of each cyano group's contribution to the optical properties. Furthermore, the effect of the cyano group on the charge injection may vary with the chromophore conjugation length, which remains to be a poorly understood issue. 4

4

1 a: R= 2-ethylhexyl, R^methyl, X=H; b: R=2-ethylhexyI, R*=methyl, X=CN; c: R=R'=hexyI,X=CN Scheme 1. Chemical structures of MEH-PPV (la) and CN-MEH-PPV (lb).


78 Recent studies have shown that poly[( 1,4-phenylenevinylene)-a&-( 1,3phenylenevinylene)] (PpPVwPV) derivatives 3 are bright green-emitting materials. While replacement of the /?-phenylene in 3 with an w-phenylene leads to the blue-emitting poly( 1,3-phenylenevinylene) (P01PV) 2, extension of the /?ara-phenylenevinylene block length along the chain provides a yellowemitting material 4. To improve the electroluminescence of these polymers, a logical approach is to attach a cyano-functional group on the vinylene bond, which increases the electron affinity of the polymer backbone. On the basis of its relative position to the substituted phenyl rings, there are two possible positions (a and β on the vinylenes of 2-4) to locate the cyano-group. Since steric interaction between the cyano-substituent at the α-position and the alkoxy group on the phenyl ring (shown in 7) can cause significant twisting of a πconjugated polymer backbone, a preferred choice is to attach the cyanosubstituent at the β-position (Scheme 2). The alkoxy substituent on the phenyl ring of 5 is located at the ariAo-position relative to the vinyl bond, which permits resonance between the alkoxy and cyano groups to lead to the resonance structure 6. 5,6,7,8

9

10


4

11,12

9

5 6 7 Scheme 2. Ground-state resonance forms for β- and a-cyano-substituted compounds. Charge transfer resonance is forbidden between an alkoxy and occyano group.


79 It should be noted that the cyano-substituent in PPV l b is at the ex position relative to the methoxy on one phenyl, but at the β-position relative to the alkoxy on the other phenyl ring. Steric interaction of cyano-substituent with the adjacent methoxy (or alkoxy) in lb, which prevents the co-planarity, reduces its electronic interaction with the PPV backbone. To achieve the maximum/optimum effect of a cyano-group on the optical properties of PPV, it is thus desirable to place the cyano-group at the β-position as shown in 8-10. Our recent study on 9 has shown that the polymer emits yellow light (k = 578 nm), which is red-shifted by -45 nm from its parent polymer 3. To further evaluate the optical impact of cyano-substituent, we now report the synthesis and optical properties of two specific examples of 8 and 10. 13


mttK

10

(R = n-hexyl)

Polymer synthesis and characterization The monomer, 2-hexyloxy-5-methylbenzene-l,3-dicarbaldehyde 12, was prepared from dibromide l l . Knoevenagel condensation of monomer 12 with 1,3-phenylenediacetonitrile proceeded smoothly in the presence of potassium tbutoxide to afford a cyano-substituted poly(l,3-phenylenevinylene) derivative 8. The polymer 10 was synthesized similarly by condensation of dialdehyde 17 with 1,3-phenylenediacetonitrile. Solubility of 10 in THF was lower than that of 8, partially attributing to the lower w-phenylene linkage in the former. The infrared spectra of polymer films of 8 and 10 showed the characteristic cyano (-CsN) stretch band at -2207 cm" in a medium intensity. Absence of carbonyl absorption at -1700 cm" in the polymer films indicated that the reaction was complete. Degree of polymerization (DP) of 8-10 was estimated to be 23, 54, and 4, respectively (Table 1). Lower Mw of 10 was partially attributed to its low solubility in solvents. Elemental analysis results of 8 and 10 were satisfactory. 9

10

1

14

1

15



80

Absorption and photoluminescence (PL) of 8. UV-vis absorption and fluorescence of 8 in tetrahydrofuran (THF) are shown in Figure 1, and the results are summarized in Table 1. The absorption spectrum of 8 at 25 °C exhibited a broad band with λ , ^ « 319 nm, which was slightly red-shifted from that of its parent polymer 2 (R=n-hexyl, X « 308 nm). Lowering the temperature to -108 °C caused a slight bathochromic shift (by -3 nm) without resolving the hidden vibronic band structure. 9

max

Fluorescence spectrum of 8 in THF solution exhibited a broad emission band at 25 °C (Figure 1). As the polymer solution was cooled, the emission X (451 nm at 25 °C) was slightly red-shifted to 455 nm at -108 °C, but notably blue-shifted to 427 nm at -198 °C. The initial small bathochromic shift was attributed to the molecular conformational response to the low temperature. As the temperature was further lowered to -198 °C, the polymer chromophore was frozen into a rigid solid matrix, in which the molecules no longer moved and rotated freely. This rigid environment reduced or eliminated the solvent effect on the excited state, which require reorientation of solvent molecules, thereby causing the emission spectra to shift to the lower wavelength.

m&x

16


81 Absorption and photoluminescence (PL) of 10. UV-vis absorption of 10 in THF at 25 °C (Figure 2) exhibited a broad absorption band with X = 464 nm, which was about 14 nm red-shifted from that of its parent polymer 4. As the temperature was lowered to -108 °C, the absorption band was further red-shifted by about 18 nm (k = 482 nm), attributing to the adoption of more planar conformation at the low temperature. The temperature-induced bathochromic shift from the solution of 10 was notably larger than that from 8, due to the longer conjugation length in the chromophore of the former. It should be noted that the true chromophores for polymers 8, 9, and 10 can be represented by the molecular fragments 18, 19, and 20, respectively, as a result of the effective πconjugation interruption at w-phenylene units. While the fragment 18 contains one cyano substitution, the fragments 19 and 20 include two cyano-substituents. The bathochromic shift in the solution absorption for each cyano substitution can be estimated to be, therefore, ~11 nm for 18, -20 nm for 19, and 7 nm for 20. In other words, the effect of cyano-substitution on the optical absorption appeared to be weakly dependent on the chromophore conjugation length. m a x


max

13

Fluorescence of 10 in THF at 25 °C exhibited a broad band (X =566 nm) and a shoulder at -608 nm. The emission shoulder at -608 nm became slightly more pronounced as the solution temperature was decreased to -108 °C, attributing to die reduced molecular motion and vibration at the low temperature. The emission λ,,^ at -108 °C was red-shifted by -13 nm to 579 and 624 nm, as max



82

Wavelength (nm)

Figure 1. UV-vis absorption (solid line) andfluorescence (broken line) spectra of polymer 8 in THF at 25

Impact of Cyano-Functional Group on Luminescence of Poly(m

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