85 CHAPTER INFLUENCE OF DONOR AND ACCEPTOR SUBSTITUENTS ON THE ELECTRONIC CHARACTERISTICS OF POLY(FLUORENE-PHENYLENE) 86 5-1 Introduction Since the discovery of electroluminescence of poly(p-phenylenevinylene)1, a conjugated polymer, polymer light-emitting diodes (PLEDs) have drawn special attention because of the potential applications in developing large area and flexible displays2-4. The design of light-emitting diodes (LEDs) involving an organic conjugated system as the active layer has been the focus of a considerable amount of theoretical and experimental studies in the past few years1,5. Several strategies have been set up to improve the performances of the devices, among which derivatization of the conjugated backbone has been found to have a significant impact6. Indeed, this approach allows for optimizing the match between the frontier electronic levels of the polymer and the Fermi energies of stable metallic electrodes, and hence for a better balance of electron and hole injection rates, which is required for the achivement of high efficiencies. These considerations have also led to the fabrication of efficient multilayer LED devices including electron and/or hole transporting layers7. Here, we investigate theoretically the influence of the presence of chemically simple acceptor and/or donor groups as “side-groups” along a poly(9,9dihexylfluorene-1,4-phenylene) chain. The present quantum chemistry calculations report on the changes in the geometric, electronic properties of the poly(9,9dihexylfluorene-1,4-phenylene) unit cell that are induced by substitution with an acceptor such as a cyano group or a donor such as a methoxy or an amino group. We stress that the trends derived from this study fully represent of the properties observed at the scale of long polymer chains. 87 5-2 Theoretical methodology The chemical structures of the polymers I-VIII are shown in Figure 5.1. The polymers can be divided into four groups: (i) poly(9,9-dihexylfluorene-1,4-phenylene), which serves as reference; (ii) poly(9,9-dihexylfluorene-2,5-dimethoxyl-1,4- phenylene), a compound representing of the dialkoxy derivatives, often used in devices; (iii) 2-amino, 5-amino and 2,5-diamino derivatives in which the amino group(s) substitutes the phenylene group; and (iv) 2-cyano, 5-cyano and 2,5-dicyano derivatives, with the cyano group(s) as substituent(s) on the phenylene group. Semiempirical SCF MO calculations were performed using the AMPAC 6.51 program8 installed on a SGI Origin 2000 workstation. The optimized geometries and the one-electron structures of the polymer unit cells were obtained by using the Austin Model (AM1) method with MNDO as the Hamiltonian. 88 OCH3 n n n C6H13 C6H13 C6H13 C6H13 NH2 C6H13 C6H13 OCH3 I III II NH2 NH2 n n C6H13 C6H13 C6H13 C6H13 NH2 IV V CN n n C6H13 C6H13 CN VI Figure 5.1 5-3 CN C6H13 C6H13 n C6H13 C6H13 CN VII VIII Chemical structures of the various PDHFP derivatives Results and discussion 5-3-1 Geometry considerations The calculated dihedral angles for the various unit cells are shown in Table 5.1. We note that the dihedral angle between the plane of phenylene and fluorene for I is 41.2 º from the AM1 calculation results. Upon substitution of the hydrogen atoms at and positions of the phenylene ring with two methoxy groups in II, the dihedral angle is very similar to that of I, showing that the methoxy groups have not much effect on 89 the dihedral angle. Upon substitution of the hydrogen atom at (III) or position (IV) and two hydrogen atoms at and positions (V) of the phenylene group with amino group(s), the dihedral angle are - 62.0, - 41.4 and - 62.6, respectively. It seems that the amino group at position of the phenylene group affects the dihedral angle in a great degree while the amino group at position has not much effect on the dihedral angle. That the dihedral angle of V is very similar to that of III confirms this point. The situation is almost the same when the hydrogen atom at (VI) or position (VII) and two hydrogen atoms at and positions (VIII) of the phenylene group are substituted by cyano group(s). Table 5.1 Dihedral angles between the plane of phenylene and fluorene for the various unit cells Oligomer I II III IV V VI VII VIII Dihedral angle (º) - 41.2 - 41.1 - 62.0 - 41.4 - 62.6 - 54.8 - 41.3 - 55.3 5-3-2 One-electron structure Here, we focus our attention on the way the locations of the HOMO (highest occupied molecular orbital) level and LUMO (lowest unoccupied molecular orbital) level are affected upon derivatization with respect to the unsubstituted 9,9dihexylfluorene-1,4-phenylene; the results are shown in Figure 5.2 and Table 5.2. 90 E (eV) LUMO -2 -4 -6 -8 HOMO -10 I II III IV V VI VII Figure 5.2 AM1 calculated energies of HOMO and LUMO of the various unit cells Table 5.2 AM1 calculated shifts (in eV) of HOMO and LUMO for the various unit cells II-VIII with respect to I Oligomer II III IV V VI VII VIII HOMO + 0.07 + 0.17 + 0.10 + 0.63 - 0.19 - 0.21 - 0.36 LUMO + 0.02 + 0.10 + 0.04 + 0.10 - 0.20 - 0.26 - 0.72 The band gap of II is calculated to decrease by 0.05 eV with respect to that of I. This band gap lowering is due to asymmetric destabilizations of the HOMO and LUMO levels with respect to I: the energy of HOMO increases by 0.07 eV and that of LUMO increases by 0.02 eV. This destabilization effect is very clearly due to the π– electron donating character of the methoxy groups. This behavior is further rationalized by a detailed analysis of linear combination of atomic orbital (LCAO) VIII 91 coefficients for the HOMO and LUMO levels; see Figure 5.3(a). The backbone HOMO level is found to be more affected than the LUMO level upon substitution with donor groups; the asymmetry of destabilization is characterized by a stronger antibonding character of the C-O bond in the HOMO wave function. The absolute value of the shift of the HOMO level is governed by the strength of the electronic coupling between the substituting group and the backbone. The destabilization effect has been observed experimentally in electrochemical study for poly(2,5- dimethoxyparaphenylene vinylene)9. (a) Derivative II LUMO HOMO (b) Derivative V LUMO HOMO (c) Derivative VIII LUMO Figure 5.3 HOMO Sketch of the AM1 LCAO coefficients for the HOMO and LUMO levels in (a) derivative II, (b) derivative V and (c) derivative VIII. 92 In the case of monoamino substitution, an overall destabilization of the HOMO and LUMO levels is expected, due to the donor nature of the substituents. Furthermore, it is of interest to evaluate which substitution effect is stronger when the amino group is located at position or position of the phenylene ring. From the results shown in Table 5.2, we observe that the influence of substitution at position is stronger than that at position. This is in agreement with the trend of the dihedral angle change at position in III and at position in IV relative to I. With respect to I, the energy of HOMO in the monoamino derivatives increases by 0.17 and 0.10 eV for position and position substitutions, respectively; the energy of LUMO increases by 0.10 and 0.04 eV, respectively. We have also considered the diamino substitution on the phenylene ring in V. The results of the calculations, Figure 5.2 and Table 5.2, indicate a strong destabilization of the HOMO level, resulting in an energy which increases by 0.63 eV with respect to I. Relative to I, the energy of LUMO in V increases by 0.10. The combined evolution of the HOMO and LUMO levels results in the band gap that is shifted to the red, decreasing by 0.53 eV. This value is significantly higher than that in dimethoxyl and monoamino derivatives discussed above. The overall destabilization of the HOMO and LUMO levels caused by the donor nature of the amino substituents is illustrated by the analysis of the LCAO coefficients; see Figure 5.3(b). The situation is very similar to that of derivative II. The larger difference between LUMO and HOMO levels induced by diamino groups in derivative V relative to that induced by dimethoxy groups in derivative II confirms that the decrease of band gap in derivative V relative to derivative I is larger than that in derivative II. The calculation result for derivative V is in accordance with the experimental result reported before.10 In solution, the UV-visible absorption maximum 93 of poly(2,5-bis[N-methyl-N-hexylamino]phenylene vinylene) has been observed to shift about 40 nm to the red of unsubstituted poly(paraphenylene vinylene). In the case of monocyano substitution, the acceptor nature of the cyano substituents gives rise to an overall stabilization of the HOMO and LUMO levels in VI and VII. With respect to I, the energy of HOMO in the monocyano derivatives decreases by 0.19 and 0.21 eV for position and position substitutions, respectively; the energy of LUMO decreases by 0.20 and 0.26 eV, respectively. The calculation results on the dicyano substitution in VIII shows a strong stabilization of the LUMO level, with the energy decreasing by 0.72 eV with respect to I. Relative to I, the energy of HOMO in VIII decreases by 0.36. Thus, the band gap is estimated to decrease by 0.36 eV that is shifted to the red. This value is much higher than that of either of the monocyano derivatives. The overall stabilization of the HOMO and LUMO levels caused by the acceptor nature of the cyano substituents is also illustrated by the analysis of the LCAO coefficients; see Figure 5.3(c). The LUMO level is found to be more affected upon substitution with acceptor groups; the asymmetry of stabilization is rationalized by the stronger bonding character, found in the LUMO wave function, for the bond between the carbon atom of the cyano group and the adjacent carbon atom on the phenylene ring. The large shifts of the frontier levels in derivative VIII relative to derivative II reflect the stronger coupling of the cyano groups to the monomer unit with respect to the methoxy groups. Also, the larger difference between LUMO and HOMO levels induced by dicyano groups in derivative VIII relative to that induced by dimethoxy groups in derivative II confirms that the decrease of band gap in derivative VIII relative to derivative I is larger than that in derivative II. The calculation result for derivative VIII is in agreement with the dicyano substitution in poly(2,5-dicyano-1,4- 94 phenylene vinylene) experimently. The experimental band gap of MEH-PPV and MDCN-11 have been reported to be 2.10 and 1.92 eV, respectively11. M-DCN-11 is a copolymer of MEH-PPV and poly(2,5-dicyano-1,4-phenylene vinylene), its band gap value is intermediate (mid-way) between that of MEH-PPV and poly(2,5-dicyano-1,4phenylene vinylene). Therefore we can infer that the band gap of poly(2,5-dicyano1,4-phenylene vinylene) is less than 1.92 eV. The band gap of poly(paraphenylene vinylene) is 2.4 eV deduced from optical absorption experiments on well-ordered samples12. Thus, the band gap is decreased by more than 0.48 eV upon dicyano substitution on poly(paraphenylene vinylene). Note that the stronger shift is observed for the LUMO level than the HOMO level in the acceptor group(s) substituted derivatives, while the trend is reverse in the donor group(s) substituted derivatives. Similar trends have been explained by a threelevel model including the frontier levels of the PPV repeat unit and the occupied (unoccupied) molecular orbital of the donor (acceptor) group13. We stress that the extent to which the frontier levels are shifted upon derivatization does depend on the strength of the coupling between the backbone and the substituents; changing the nature of the side groups can modulate the band gaps, hence, allows for a fine tuning of the colour of the emitted light. 5-4 Conclusions To summarize, we have investigated the influence of electron acceptors and donors on the geometric and electronic properties of the poly(9,9-dihexylfluorene-1,4phenylene) unit cell. We have shown that sizable effects can be obtained. In particular, 95 the use of diamino or dicyano substituents on the phenylene ring allows to decrease significantly the band gaps. This information is of prime importance in the design of new chemical structures aimed at a fine tuning of the emitted colour and at a significant improvement in quantum efficiency. 96 References 1. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L. and Holmes, A. B., Nature, 1990, 347, 539. 2. Hide, F.; Díaz-García, M. A.; Schwartz, B. J. and Heeger, A. J., Acc. Chem. Res., 1997, 30, 430. 3. Greenham, N. C.; Friend, R. H. in Solid State Physics; Enhrenreich, H.; Spaepen, F. Eds.; Academic Press: San Diego, CA, 1995; Vol. 49, pp 1–149. 4. Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N. and Heeger, A. J., Nature, 1992, 357, 477. 5. Braun, D.; Brown, A. R.; Staring, E. G. J. and Meijer, E. W., Synth. Met., 1994, 65, 85. 6. Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H. and Holmes, A. B., Nature, 1993, 365, 268. 7. Brown, A. R.; Bradley, D. D. C.; Burroughes, J. H.; Friend, R. H.; Greenham, N. 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Staring, E G J and Meijer, E W., Synth Met., 1994, 65, 85 6 Greenham, N C.; Moratti, S C.; Bradley, D D C.; Friend, R H and Holmes, A B., Nature, 1993, 3 65, 268 7 Brown, A R.; Bradley, D D C.; Burroughes, J H.; Friend, R H.; Greenham, N C.; Burn, P L.; Holmes, A B and Kraft, A., Appl Phys Lett., 1992, 61, 23 8 AMPAC 6 .51 Program, Semichem, 7128 Summit, Shawnee, KS 66216 9 Helbig, M and Hörhold, H H.,... Burn, P L and Holmes, A B., Nature, 1990, 347, 53 9 2 Hide, F.; Díaz-García, M A.; Schwartz, B J and Heeger, A J., Acc Chem Res., 1997, 30, 430 3 Greenham, N C.; Friend, R H in Solid State Physics; Enhrenreich, H.; Spaepen, F Eds.; Academic Press: San Diego, CA, 19 95; Vol 49, pp 1–149 4 Gustafsson, G.; Cao, Y.; Treacy, G M.; Klavetter, F.; Colaneri, N and Heeger, A J., Nature, 1992, 357 , 477 5 Braun,... 95 the use of diamino or dicyano substituents on the phenylene ring allows to decrease significantly the band gaps This information is of prime importance in the design of new chemical structures aimed at a fine tuning of the emitted colour and at a significant improvement in quantum efficiency 96 References 1 Burroughes,... Yee, R Y and Schwartz, B J., J Photochem Photobiol A, 2001, 144, 21 11 Xiao, Y.; Yu, W L.; Chua, S J and Huang, W., Chem Eur J., 2000, 6(8), 1318 12 Halliday, D A.; Burn, P L.; Bradley, D D C.; Friend, R H.; Gelsen, O M.; Holmes, A B.; Kraft, A.; Martens, J H F and Pichler, K., Advan Mater., 1993, 5, 40 97 13 Cornil, J.; Beljonne, D.; dos Santos, D A and Brédas, J L., J Phys Chem., 19 95, 99, 56 04 . 85 CHAPTER 5 INFLUENCE OF DONOR AND ACCEPTOR SUBSTITUENTS ON THE ELECTRONIC CHARACTERISTICS OF POLY(FLUORENE-PHENYLENE) 86 5- 1 Introduction Since the discovery of electroluminescence. levels of the polymer and the Fermi energies of stable metallic electrodes, and hence for a better balance of electron and hole injection rates, which is required for the achivement of high. representing of the dialkoxy derivatives, often used in devices; (iii) 2-amino, 5- amino and 2 ,5- diamino derivatives in which the amino group(s) substitutes the phenylene group; and (iv) 2-cyano, 5- cyano