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Excimer luminescence from single crystals and films of a cyano-substituted phenylene–vinylene model compound
Chemical Physics 241 Ž1999. 139–154
Excimer luminescence from single crystals and films of a cyano-substituted phenylene–vinylene model compound P.F. van Hutten, V.V. Krasnikov, H.-J. Brouwer, G. Hadziioannou
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Department of Polymer Chemistry and Materials Science Centre, UniÕersity of Groningen, Nijenborgh 4, NL-9747 AG Groningen, Netherlands Received 14 September 1998
Abstract In the context of luminescence from the solid state of polyŽ para-phenylene vinylene.-type materials, we report a study of three octyloxy-substituted five-ring oligoŽ para-phenylene vinylene.s, two of which bear cyano substituents on the innermost vinylene bonds. For each compound, the molecular arrangement in the single crystal has been derived from a crystallographic analysis. For the compound which has the cyano group not directly adjacent to the central ring, synthesis and structure are described here in detail. This compound, in contrast to the other two, has molecular p-stacking in the crystal lattice. Only for this compound do we observe a strongly red-shifted, long-lived emission, which is therefore ascribed to an excimer-type intermolecular excitation. The observations of excimer-type emission from cyano-substituted alkoxy–PPV’s and in related copolymers fit in very well with the molecular picture derived from this study of model compounds. q 1999 Elsevier Science B.V. All rights reserved. Keywords: PolyŽ para-phenylene vinylene.; Oligomers; Single-crystal structure; Excimer; Photoluminescence
1. Introduction PolyŽ para-phenylene vinylene. ŽPPV. and its derivatives are the most extensively studied p-conjugated organic materials for the fabrication of polymeric light-emitting diodes ŽLEDs. w1–3x. A diversity of synthetic procedures has been developed, and, although enhancements are still pursued, a variety of materials that have good processability, high luminescence efficiency and adequate stability and durability are now available w4x. Although most of the
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Corresponding author. Fax: q31-50-3634400; E-mail: [email protected]
visible spectrum is covered by this class of materials, full-colour display applications would require a much better spectral purity Žcolour saturation. of the emitted light in the appropriate wavelength ranges. The localization, mobility and decay mechanisms of the excitations generated by charge injection are the key issue, and time-resolved spectroscopic techniques are the primary tools for investigating it. Optical and electrical properties of solids of conjugated polymers depend in an indirect, and therefore complicated way on the chemical structure: molecular conformation, arrangement and density of packing and degree of order are all important. An example relevant to this paper is the observation of longlived luminescence in cyano-substituted PPV by
0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 4 1 3 - 3
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Samuel et al. w5,6x, which has been ascribed by the authors to interchain excitations. Cyano substitution of the vinylene linkages in PPV’s was introduced with the objective of decreasing the barrier for injection of electrons into the emissive polymer; it was found to shift optical energy gap to a lower value w7x. It is likely that the presence of the cyano group will strongly affect the molecular organization, due to its size as well as its polar nature. Monte-Carlo calculations of the arrangement of neighbouring chain segments have been presented by Conwell et al. w8x, and these suggest that cyano-substitution leads to inter˚ which is considered by chain distances below 3.5 A, the authors to be in favour of the formation of interchain excitations that luminesce rather than that they constitute a non-radiative trap. Unfortunately, the molecular organization in a polymeric solid is not accessible in detail, and, moreover, it is very inhomogeneous a priori. It is therefore not possible to confirm the calculated structures and to relate these to luminescence properties in a straightforward manner. The objective of this paper is to demonstrate that insight can be gained by using suitable model compounds. These exhibit very much the same phenomena but the results can be interpreted in a much more convincing manner since the structures are known in detail. In this work, the optical properties of the solid state of three related five-ring oligoŽ para-phenylene vinylene.s ŽOPV5’s. are compared, and crystallographic data are presented to support the occurrence of intermolecular luminescence in single crystals and thin films of one of the cyano-substituted compounds. We demonstrate the effects of the molecular packing and show that p-stacking gives rise to behaviour which is different from that resulting from dipolar interactions only.
films on a KBr pellet on a Mattson Instruments FT-IR spectrometer. Elemental analyses were carried out at the Microanalytical Department of the University of Groningen. 2.2. Materials Ooct–OPV5 was synthesized via a Wittig-reaction as described previously in Ref. w9x. Ooct– OPV5–CNX was prepared via a Knoevenagel condensation as described in Ref. w10x. Ooct–OPV5– CNY was obtained by a similar Knoevenagel condensation of 2,5-di-n-octyloxyterephthaldialdehyde and Ž4-styrylphenyl.acetonitrile, as outlined below ŽScheme 1.. Triethylamine was dried over potassium hydroxide and THF was distilled from lithium aluminium hydride before use. All other starting materials and solvents were used as received Žp.a. grade., unless stated otherwise. All reactions were performed under a dry argon atmosphere. 2.3. 1,4-Di-n-octyloxybenzene A mixture of hydroquinone Ž50.0 g, 0.450 mol., potassium hydroxide Ž55.0 g, 1.00 mol. and 1bromo-n-octane Ž183.0 g, 0.950 mol. in ethanol Ž450 ml. was refluxed for 4 h. After hot filtration, the filtrate was stirred and the product crystallized upon cooling. The solid was filtered off and washed with cold ethanol. Recrystallization from ethanol afforded white flaky crystals Ž131.0 g.. Yield: 131.0 g, 87%; m.p.: 49–508C. 1 H-NMR: d 0.90 Žt, C H3 , 6H., 1.31 Žm, C H2 , 20H., 1.77 Žm, b-C H2 , 4H., 3.91 Žt, a-C H2 , 4H., 6.84 Žs, arom C H, 4H. ppm. 13 C-NMR: d 13.8 Ž C H 3 ., 22.4, 25.8, 29.0, 29.2 Ž2 = ., 31.6 Ž C H 2 ., 68.4 ŽO–C H 2 ., 115.2 Žarom C H., 153.1 Žarom C–O. ppm. 2.4. 1,4-Diiodo-2,5-di-n-octyloxybenzene
2. Experimental 2.1. Measurements NMR spectra Žchloroform-d solutions. were recorded on a Varian spectrometer at 200 or 300 MHz Ž1 H., and at 50, 75 or 125 MHz Ž13 C.. All chemical shifts reported were externally referenced to TMS Ž0 ppm.. IR spectra were recorded from thin
A mixture of 1,4-di-n-octyloxybenzene Ž6.72 g, 20.0 mmol., iodine Ž4.62 g, 18.2 mmol., HIO 3 Ž2.23 g, 12.7 mmol., acetic acid Ž35 ml., 40% H 2 SO4 Ž5 ml. and CCl 4 Ž8.0 ml. is heated at 758C for 3 h. Cooling with an ice bath while stirring resulted in crystallization. The solid was filtered off and washed with methanol. Recrystallization from ethanol afforded the title compound as beige flakes. Yield: 8.5
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
141
Y
Scheme 1. Reaction scheme for the synthesis of Ooct–OPV5–CN .
g, 86%; m.p.: 428C. 1 H-NMR: d 0.90 Žt, C H3 , 6H., 1.30 Žm, C H2 , 20H., 1.81 Žm, b-C H2 , 4H., 3.94 Žt, a-C H2 , 4H., 7.19 Žs, arom C H, 2H. ppm. 13 C-NMR: d 13.9 Ž C H 3 ., 22.4, 25.8, 28.9, 29.0 Ž2 = ., 31.6 Ž C H 2 ., 70.1 ŽO–C H 2 ., 86.1 Žarom C–I., 122.6 Žarom C–H., 152.7 Žarom C–O. ppm. 2.5. 2,5-Di-n-octyloxyterephthaldialdehyde (1) 1,4-Diiodo-2,5-di-n-octyloxybenzene Ž5.00 g, 8.53 mmol. was dissolved in dry THF Ž100 ml. and cooled to below y708C. After 30 min., tert-BuLi Ž24 ml, 36 mmol, 1.7 M solution in pentane. was added dropwise, keeping the temperature below y508C. Stirring was continued for 30 min. at y508C, after which DMF Ž5 ml. was added all at once and stirring was continued for 15 min at y308C. The mixture was poured into a 1% aqueous solution of HCl Ž500 ml.. During stirring of the aqueous solution for several hours, the pentane evaporated and the yellow product separated. The solid was filtered off and washed with methanol. The pure, yellow product was obtained by recrystallization from etherrethanol Ž1:1.. Yield: 3.04 g, 90%; m.p.: 608C. 1 H-NMR: d 0.86 Žt, C H3 , 6H., 1.29 Žm, C H2 , 20H., 1.80 Žm, b-C H2 , 4H., 4.08 Žt, a-C H2 , 4H., 7.42 Žs, arom C H, 2H., 10.52 Žs, C HO, 2H. ppm. 13 C-NMR: d 13.9 Ž C H 3 ., 22.4, 25.8, 28.9, 29.0, 29.1, 31.8 Ž C H 2 ., 69.1, ŽO–C H 2 ., 111.5 Žarom
C H., 129.2 Žarom C H., 155.2 Žarom C–O., 189.5 Ž C HO. ppm. 2.6. (4-Styrylphenyl)acetonitrile (2) A mixture of styrene Ž1.30 g, 12.5 mmol., 4bromophenylacetonitrile Ž2.45 g, 12.5 mmol., PdŽOAc. 2 Ž0.057 g, 0.26 mmol., tri-o-tolylphosphine Ž0.37 g, 1.2 mmol. and triethylamine Ž5 ml. was placed in a heavy-wall pressure tube. The tube was degassed, closed ŽTeflon brushing., and heated to 1008C in an oil bath. After stirring for 20 h, the reaction mixture was cooled down to room temperature and poured into methanol Ž50 ml.. The precipitate was filtered off, washed with methanol three times and dried in vacuum. The crude reaction product was dissolved in dichloromethane, filtered and crystallized from dichloromethaneracetone Ž30:70. to give the title product. Yield: 0.90 g, 33%; m.p.: 928C. 1 H-NMR: d 3.80 Žs, C H2 , 2H., 7.10 Žd, C H s C H, 2H., 7.2–7.4 Žm, arom C H, 5H., 7.52 ŽAB q, arom C H, 4H. ppm. 13 C-NMR: d 23.1Ž C H 2 ., 118.9, 126.5, 127.0, 127.5, 127.8, 128.1, 128.6, 128.8, 129.4, 136.8, 137.1 ppm. 2.7. 3-{ 4-[2-Cyano-2-(4-styrylphenyl)Õinyl]-2,5bis(n-octyloxy)phenyl}-2-(4-styrylphenyl)-acrylonitrile (3, Ooct–OPV5–CN Y ) 2,5-Di-n-octyloxyterephthaldialdehyde Ž1. Ž586 mg, 1.5 mmol. and Ž4-styrylphenyl.acetonitrile Ž2.
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P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
Ž658 mg, 3.0 mmol. were dissolved in a mixture of tert-butanol Ž8 ml. and THF Ž2 ml. at 508C. Potassium-tert-butoxide Ž16.8 mg, 0.15 mmol. and tetran-butylammonium hydroxide Ž0.1 N solution in 2propanolrmethanol, 1.5 ml, 0.15 mmol. were added quickly. After 15 min the reaction mixture was poured into acidified methanol. The red precipitate was filtered off and washed with methanol three times. Pure Ž3. was obtained as red needles after repeated crystallization from a chloroformrn-hexane mixture. Yield: 0.973 g, 82%; m.p.: 2008C. 1 H-NMR: d 0.87 Žt, J s 6.6 Hz, C28., 1.3–1.5 Žm, C24–C27., 1.51 Žm, J s 7.8 Hz, C23., 1.86 Žm, J s 7.3 Hz, C22., 4.12 Žt, J s 6.6 Hz, C21., 7.12 and 7.14 ŽAB q, J s 16 Hz, C7,C8,C10,C11., 7.27 Žt, J s 7.7 Hz, C17., 7.36 Žt, J s 7.5 Hz, C15,C19., 7.51 and 7.53 Žm, C16,C18., 7.56 Žd, J s 8.4 Hz, C13., 7.67 Žd, J s 8.4 Hz, C12., 7.89 Žs, C1., 8.03 Žs, C4. ppm. 13 C-NMR: d 13.96, 22.53, 26.10, 29.05, 29.16, 29.20, 31.64, 69.33, 111.18, 118.29, 125.79, 126.30, 126.63, 127.04, 127.50, 128.00, 128.72, 128.74, 129.95, 133.66, 135.26, 136.92, 138.31, 151.55 ppm. IR ŽKBr.: 3027 Žm., 2922 Žs., 2853 Žs., 2209 Žw., 1603 Žm., 1583 Žm., 1492 Žm., 1466 Žm., 1436 Žm., 1368 Žm., 1306 Žw., 1251 Žm., 1211 Žs., 1057 Žw., 1024 Žw., 964 Žm., 911 Žw., 864 Žw., 818 Žm., 753 Žm., 720 Žw. cmy1 . Elem. anal. Calcd: C: 84.47; H: 7.86; N: 3.58. Found: C: 84.66; H: 7.65; N: 3.62. 2.8. Single crystals
˚ sy1 . The as-deposited posited at a rate of 2–4 A layer showed a fine-grained polycrystalline structure when observed in a polarizing optical microscope. 2.10. X-ray crystallography of Ooct–OPV5–CN Y A needle-shaped crystal of Ooct–OPV5–CNY Ž3., of approximate size 0.04 = 0.04 = 0.5 mm3 , was mounted on top of a glass fibre and transferred into the cold nitrogen cold stream of the low temperature
Table 1 Y Crystallographic and experimental data for Ooct–OPV5–CN Crystal data Formula Formula weight Žg moly1 . Crystal system Space group ˚. a ŽA
C 56 H 60 N2 O 2 793.11
˚. b ŽA
12.664Ž4. 18.35Ž1. 92.39Ž6. 91.67Ž5. 99.06Ž2. 1115.9Ž7. 1 426 1.180 0.7 0.04=0.04=0.5
˚. c ŽA a Ž8. b Ž8. g Ž8. ˚ 3. V ŽA Z F Ž000. Dcalc Žg cmy3 . m ŽMo K a . Žcmy1 . Crystal size Žmm3 .
triclinic P1 4.870Ž1.
Data collection
Single crystals were obtained by precipitation of the respective compounds from a solution in a mixture of solvents, either THFrmethanol or chloroformrn-hexane, which was slowly evaporated at room temperature. Ooct–OPV5 crystallized into yellow needle-shaped crystals; orange plate-shaped crystals were obtained for Ooct–OPV5–CNX . In the case of Ooct–OPV5–CNY , orange-red crystal needles were obtained. 2.9. Thin-film deposition Films on glass substrates were prepared by evaporation from a molybdenum boat under high vacuum Ž- 1 = 10y6 mbar. at a temperature just above the melting point of the oligomer. The film was de-
T ŽK. u range: min., max. Ž8. Data collected No. of unique data No. of rflns. obsd. Ž Fo G 4.0 s Ž Fo ..
130 1.11, 25.0 4423 3920 1087 Structure and refinement
No. of reflections, n Ž Fo2 ) 0. No. of parameters, p RŽ F . s S < < Fo
2873 272 0.119 0.310
˚ y3 . Residual r Že A ŽMaximum shift.r s Žfinal cycle.
y0.53, 0.36 Ž10. - 0.001
1.005
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
unit mounted on an Enraf-Nonius CAD-4F diffractometer Žmonochromatized Mo-K a radiation, 50 kV, ˚ .. Unit cell parameters and 40 mA, l s 0.71073 A orientation matrix were determined from a leastsquares treatment of the setting angles of 25 reflections in the range 5.048 - u - 10.398. The intensities of two standard reflections, monitored every three hours of X-ray exposure time, showed no greater fluctuations during data collection than those expected from Poisson statistics. Intensity data were corrected for Lorentz and polarization effects, scale variation, but not for absorption, and reduced to Fo2 . The structure was solved by direct methods with SHELXS86 w11x. Neutral atom scattering factors and anomalous dispersion corrections were taken from International Tables of Crystallography w12x. The positional and anisotropic thermal displacement parameters for the non-hydrogen atoms were refined. The hydrogen atoms were included in the final refinement riding on their carrier atoms with U s c = Uequiv of their parent atom, where c s 1.2 for the aromaticrnon-methyl hydrogen atoms and c s 1.5 for the methyl hydrogen atoms and where values Uequiv are related to the atoms to which the H atoms are bonded. The methyl group was refined as a rigid group, which was allowed to rotate free. A few atoms converged to non-positive definite thermal displacement parameters when allowed to vary anisotropically and some atoms showed unrealistic thermal displacement parameters suggesting some
143
Fig. 2. Molecular packing of Ooct–OPV5 in the crystal lattice. Left: oblique view of the unit cell of Ooct–OPV5. Right: projection of the unit cell on a plane perpendicular to the a-axis.
degree of disorder, which is in line with the weak scattering power of the crystals investigated. In the final refinement, the non-positive definite atoms were restrained so that their Ui j components approximate to isotropic behaviour. The positional and anisotropic thermal displacement parameters for the non-hydrogen atoms and isotropic thermal displacement parameters for hydrogen atoms were refined on F 2 with full-matrix least-squares procedures w13x. Final refinement converged at wRŽ F 2 . s 0.310 for 2873 reflections with Fo2 G 0 and 272 parameters and RŽ F . s 0.119 for 1087 reflections with Fo G 4.0 s Ž Fo .. The final difference map showed no significant peak having chemical meaning above the general background. Crystal data and numerical details on data collection and refinement are given in Table 1. 2.11. Thin-film X-ray diffraction Wide-angle X-ray diffraction patterns of 2 = 2 cm2 thin Žca. 300 nm. films on glass plates were recorded in reflection geometry Ž u –2 u scan. on a Rigaku horizontal goniometer mounted on a Rotaflex RU-200B rotating-anode generator operated at 40 kV and 40 mA. 2.12. Optical characterization
Fig. 1. The five-ring oligoŽphenylene vinylene.s considered in this study.
UV-vis absorption spectra were obtained on an SLM Aminco 3000 Array spectrophotometer. Photoluminescence spectra were recorded on a Perkin Elmer LS50-B spectrofluorometer. The excitation
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P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
wavelength was 10 nm below the absorption maximum. The absolute photoluminescence quantum
X
yields of our crystals and films were determined using a calibrated integrating sphere with an Arq
Fig. 3. A: packing arrangement of Ooct–OPV5–CN in the crystal lattice Žview at a slight angle with respect to the long molecular axis.; B: X top-view Žtop. and side-view Žbottom. of two nearest-neighbour Ooct–OPV5–CN molecules in the crystal, related by a translation along the a-axis. See Fig. 5 for details.
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
Ž488 nm. laser as excitation source w14,15x. Luminescence lifetime measurements were made by timecorrelated single-photon counting. Excitation at approx. 400 nm was provided by the second harmonic of a ; 100 fs mode-locked Ti-sapphire laser ŽMira, Coherent.. To match the photon counting electronics, the laser repetition rate was reduced by an acoustooptical pulse-picker ŽCAMAC. to 2 MHz. The excitation energy density did not exceed 1 nJrcm2 per pulse. The instrumental response time at half-width of full maximum was approx. 40 ps. All optical measurements were made under vacuum better than 10y4 mbar.
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contained in layers parallel to the Ž010. plane Žinter˚ .; the molecules are not layer distance br2 s 3.77 A p-stacked. The octyloxy side chains are not fully stretched in an all-trans configuration, but curved towards the periphery of the conjugated backbone. Strong disorder is found in one of the two octyloxy tails. 3.1.2. Ooct–OPV5–CN X The unit cell is triclinic, space group P1, containing one discrete molecule, which has a crystallographically imposed centre of inversion w10x. The molecule has a remarkable, wave-like shape ŽFig. 3.. Such distortions indicate the presence of consider-
3. Results and discussion 3.1. Single-crystal structures Since PPV has become the focus of research on light-emitting polymers, a variety of OPV’s have been synthesized and electrically and optically characterized. Although the molecular organization in PPV-type materials is highly relevant to their luminescence properties, surprisingly few crystallographic studies have been carried out to date. Early crystallographic work on derivatives of distyrylbenzene Žreferred to as OPV3 in this paper. was presented by Geise and coworkers w16–18x. Data for cyano-substituted OPV3’s have been presented recently by Hohloch et al. w19x. Our group has focused on five-ring OPV’s; those considered in this paper are shown in Fig. 1. We have reported the syntheses and the single-crystal structures for Ooct–OPV5 w9x as well as for Ooct– OPV5–CNX w10x. This paper presents detailed structural data for Ooct–OPV5–CNY , which was obtained by a Knoevenagel condensation as described in Section 2. 3.1.1. Ooct–OPV5 The unit cell is monoclinic, space group I 2ra, and contains eight discrete molecules separated by normal van der Waals distances w9x ŽFig. 2.. The conjugated backbones lie parallel to each other and are lined up into strings along the a-axis and separated along the c-axis by a layer accommodating the aliphatic octyloxy side chains. The molecules are
Table 2 Final fractional atomic coordinates and equivalent isotropic therY mal displacement parameters for Ooct–OPV5–CN with estimated standard deviations in parentheses Žnon-H atoms of the . asymmetric unit; Ueq s1r3Ý i Ý j Ui j ai) a ) j a i .a j
O N CŽ1. CŽ2. CŽ3. CŽ4. CŽ5. CŽ6. CŽ7. CŽ8. CŽ9. CŽ10. CŽ11. CŽ12. CŽ13. CŽ14. CŽ15. CŽ16. CŽ17. CŽ18. CŽ19. CŽ20. CŽ21. CŽ22. CŽ23. CŽ24. CŽ25. CŽ26. CŽ27. CŽ28.
x
y
z
˚2. Ueq ŽA
0.1889Ž14. 0.625Ž2. y0.150Ž2. 0.0854Ž19. 0.2456Ž19. 0.491Ž2. 0.671Ž2. 0.905Ž2. 0.996Ž2. 1.218Ž2. 1.3705Ž19. 1.289Ž2. 1.059Ž2. 1.605Ž2. 1.684Ž2. 1.917Ž2. 1.968Ž2. 2.203Ž3. 2.378Ž3. 2.324Ž3. 2.110Ž2. 0.639Ž2. 0.016Ž2. 0.179Ž2. 0.010Ž2. 0.174Ž2. 0.022Ž2. 0.197Ž2. 0.036Ž2. 0.217Ž3.
0.0037Ž5. 0.2678Ž7. y0.0666Ž7. y0.0007Ž7. 0.0706Ž7. 0.1416Ž7. 0.2168Ž7. 0.2823Ž7. 0.2557Ž7. 0.3137Ž8. 0.4051Ž7. 0.4315Ž8. 0.3720Ž7. 0.4701Ž7. 0.4655Ž7. 0.5262Ž8. 0.5084Ž9. 0.5615Ž10. 0.6405Ž11. 0.6628Ž9. 0.6069Ž9. 0.2442Ž8. y0.0604Ž8. y0.0413Ž8. y0.0993Ž7. y0.0877Ž8. y0.1502Ž7. y0.1449Ž8. y0.2075Ž8. y0.2015Ž9.
0.1433Ž3. y0.1078Ž5. 0.0491Ž5. 0.0727Ž5. 0.0265Ž5. 0.0567Ž5. 0.0286Ž5. 0.0692Ž5. 0.1382Ž5. 0.1760Ž5. 0.1466Ž5. 0.0777Ž5. 0.0402Ž5. 0.1845Ž6. 0.2536Ž6. 0.2941Ž6. 0.3685Ž6. 0.4085Ž7. 0.3750Ž8. 0.3030Ž8. 0.2629Ž6. y0.0490Ž6. 0.1940Ž5. 0.2675Ž5. 0.3278Ž5. 0.3992Ž5. 0.4594Ž5. 0.5321Ž5. 0.5905Ž5. 0.6615Ž5.
0.027Ž2. 0.055Ž4. 0.018Ž3. 0.019Ž3. 0.016Ž3. 0.021Ž3. 0.020Ž3. 0.024Ž3. 0.028Ž4. 0.023Ž3. 0.021Ž3. 0.029Ž4. 0.022Ž3. 0.032Ž4. 0.036Ž4. 0.032Ž4. 0.042Ž4. 0.056Ž5. 0.067Ž7. 0.051Ž5. 0.038Ž4. 0.033Ž4. 0.027Ž4. 0.030Ž4. 0.023Ž3. 0.021Ž3. 0.020Ž3. 0.023Ž3. 0.038Ž4. 0.051Ž5.
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P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
Table 3 Ya Selected data on the geometry of Ooct–OPV5–CN ŽStandard deviations in the last decimal place are given in parentheses. Interatomic Distances (A˚) O–CŽ2. O–CŽ21. N–CŽ20. CŽ1. –CŽ2. CŽ1. –CŽ3. a CŽ2. –CŽ3. CŽ3. –CŽ4. CŽ4. –CŽ5. CŽ5. –CŽ6. CŽ5. –CŽ20. CŽ6. –CŽ7. CŽ6. –CŽ11. CŽ7. –CŽ8. Bond angles (deg) CŽ2. –O–CŽ21. CŽ2. –CŽ1. –CŽ3.a O–CŽ2. –CŽ1. O–CŽ2. –CŽ3. CŽ1. –CŽ2. –CŽ3. CŽ2. –CŽ3. –CŽ4. CŽ2. –CŽ3. –CŽ1.a CŽ4. –CŽ3. –CŽ1.a CŽ3. –CŽ4. –CŽ5. CŽ4. –CŽ5. –CŽ6. CŽ4. –CŽ5. –CŽ20. CŽ6. –CŽ5. –CŽ20. CŽ5. –CŽ6. –CŽ7. CŽ5. –CŽ6. –CŽ11. CŽ7. –CŽ6. –CŽ11. CŽ6. –CŽ7. –CŽ8. CŽ7. –CŽ8. –CŽ9. Torsion angles (deg) CŽ21. –O–CŽ2. –CŽ1. CŽ21. –O–CŽ2. –CŽ3. CŽ2. –O–CŽ21. –CŽ22. CŽ3.a–CŽ1. –CŽ2. –O CŽ3.a–CŽ1. –CŽ2. –CŽ3. CŽ2. –CŽ1. –CŽ3.a–CŽ2.a CŽ2. –CŽ1. –CŽ3.a–CŽ4.a O–CŽ2. –CŽ3. –CŽ4. O–CŽ2. –CŽ3. –CŽ1.a CŽ1. –CŽ2. –CŽ3. –CŽ4. CŽ1. –CŽ2. –CŽ3. –CŽ1.a CŽ2. –CŽ3. –CŽ4. –CŽ5. CŽ1.a–CŽ3. –CŽ4. –CŽ5. CŽ3. –CŽ4. –CŽ5. –CŽ6. CŽ3. –CŽ4. –CŽ5. –CŽ20. CŽ4. –CŽ5. –CŽ6. –CŽ7. CŽ4. –CŽ5. –CŽ6. –CŽ11. CŽ20. –CŽ5. –CŽ6. –CŽ7.
1.372Ž11. 1.458Ž11. 1.135Ž14. 1.356Ž13. 1.447Ž13. 1.422Ž13. 1.458Ž13. 1.321Ž13. 1.464Ž13. 1.489Ž14. 1.401Ž13. 1.393Ž13. 1.361Ž14.
115.7Ž7. 120.8Ž8. 123.0Ž8. 114.1Ž8. 122.9Ž9. 119.5Ž8. 116.2Ž8. 124.2Ž8. 133.1Ž9. 124.9Ž9. 120.6Ž9. 114.5Ž8. 122.0Ž8. 122.1Ž8. 115.9Ž8. 123.0Ž9. 120.7Ž9.
y5.3Ž12. 174.2Ž8. 179.8Ž7. y178.0Ž8. 2.5Ž14. y2.3Ž13. 177.9Ž9. y1.7Ž12. 178.1Ž8. 177.9Ž9. y2.4Ž14. y177.6Ž10. 2.7Ž17. 178.5Ž9. 1.2Ž17. 14.2Ž15. y168.9Ž10. y168.4Ž9.
CŽ8. –CŽ9. CŽ9. –CŽ10. CŽ9. –CŽ12. CŽ10. –CŽ11. CŽ12. –CŽ13. CŽ13. –CŽ14. CŽ14. –CŽ15. CŽ14. –CŽ19. CŽ15. –CŽ16. CŽ16. –CŽ17. CŽ17. –CŽ18. CŽ18. –CŽ19.
1.410Ž13. 1.384Ž13. 1.441Ž13. 1.394Ž14. 1.321Ž15. 1.435Ž14. 1.414Ž16. 1.426Ž15. 1.403Ž17. 1.385Ž19. 1.39Ž2. 1.345Ž18.
CŽ8. –CŽ9. –CŽ10. CŽ8. –CŽ9. –CŽ12. CŽ10. –CŽ9. –CŽ12. CŽ9. –CŽ10. –CŽ11. CŽ6. –CŽ11. –CŽ10. CŽ9. –CŽ12. –CŽ13. CŽ12. –CŽ13. –CŽ14. CŽ13. –CŽ14. –CŽ15. CŽ13. –CŽ14. –CŽ19. CŽ15. –CŽ14. –CŽ19. CŽ14. –CŽ15. –CŽ16. CŽ15. –CŽ16. –CŽ17. CŽ16. –CŽ17. –CŽ18. CŽ17. –CŽ18. –CŽ19. CŽ14. –CŽ19. –CŽ18. N–CŽ20. –CŽ5. O–CŽ21. –CŽ22.
117.4Ž9. 123.1Ž9. 119.5Ž9. 121.1Ž9. 121.9Ž9. 126.9Ž9. 129.6Ž9. 121.0Ž9. 123.Ž1. 115.9Ž9. 122.5Ž10. 118.1Ž12. 120.1Ž13. 121.9Ž12. 121.2Ž11. 177.1Ž11. 104.8Ž7.
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
147
Table 3 Žcontinued. Torsion angles (deg) CŽ20. –CŽ5. –CŽ6. –CŽ11. CŽ5. –CŽ6. –CŽ7. –CŽ8. CŽ11. –CŽ6. –CŽ7. –CŽ8. CŽ5. –CŽ6. –CŽ11. –CŽ10. CŽ7. –CŽ6. –CŽ11. –CŽ10. CŽ6. –CŽ7. –CŽ8. –CŽ9. CŽ7. –CŽ8. –CŽ9. –CŽ10. CŽ7. –CŽ8. –CŽ9. –CŽ12. CŽ8. –CŽ9. –CŽ10. –CŽ11. CŽ12. –CŽ9. –CŽ10. –CŽ11. CŽ8. –CŽ9. –CŽ12. –CŽ13. CŽ10. –CŽ9. –CŽ12. –CŽ13. CŽ9. –CŽ10. –CŽ11. –CŽ6. CŽ9. –CŽ12. –CŽ13. –CŽ14. CŽ12. –CŽ13. –CŽ14. –CŽ15. CŽ12. –CŽ13. –CŽ14. –CŽ19. CŽ13. –CŽ14. –CŽ15. –CŽ16. CŽ19. –CŽ14. –CŽ15. –CŽ16. CŽ13. –CŽ14. –CŽ19. –CŽ18. CŽ15. –CŽ14. –CŽ19. –CŽ18. CŽ14. –CŽ15. –CŽ16. –CŽ17. CŽ15. –CŽ16. –CŽ17. –CŽ18. CŽ16. –CŽ17. –CŽ18. –CŽ19. CŽ17. –CŽ18. –CŽ19. –CŽ14.
8.5Ž13. 178.1Ž9. 1.0Ž14. y177.2Ž9. y0.1Ž14. y0.2Ž15. y1.4Ž14. 179.3Ž10. 2.2Ž14. y178.4Ž9. y11.2Ž16. 169.4Ž10. y1.5Ž15. 178.7Ž10. y179.4Ž10. 0.5Ž17. 176.0Ž11. y3.9Ž16. 179.3Ž11. y0.9Ž16. 5.4Ž18. y2.Ž2. y3.Ž2. 4.1Ž19.
a
Atoms labelled ‘a’ are generated from the unlabelled ones by the symmetry operation: yx, yy, yz; The sign of the torsion angle is positive when, looking from atom 2 to atom 3, a clockwise motion of atom 1 would superimpose it on atom 4.
able packing forces. The dense packing Žthe crystal density of 1.210 g cmy3 is 10% higher than that of Ooct–OPV5. may be driven both by side-chain arrangement and by electrostatic interactions between the polar groups. Although the shortest interatomic distance between nearest Ž a-axis. neighbours is ap˚ one does not expect p – p interaction to prox. 3.5 A, occur for these molecules, since the p-systems are both laterally and axially displaced, in a view perpendicular to the plane of the central ring. The aromatic systems face the octyloxy substituents of the neighbour molecules. The conformation around the double bond that bears the cyano group is different from that found in other OPV crystals studied so far. The introduction of the cyano group has caused an almost 1808 rotation around the single bond to the central ring and results in the molecular backbone and octyloxy tail being in close proximity. 3.1.3. Ooct–OPV5–CN Y This compound, which has its cyano substituents at the outer position of the central vinylene linkages,
has a P1 crystal packing which is very different from that of Ooct–OPV5–CNX Žcrystallographic data are in Tables 1–3, Fig. 4 contains the atom labelling, Fig. 5 shows the molecular packing.. As Fig. 5 illustrates, nearest-neighbour Ž a-axis related . molecules exhibit only an axial displacement of approximately half a phenylene–vinylene unit. In a view perpendicular to the central ring, the aromatic systems overlap to a large extent. Since the distance between the conjugated backbones is approximately ˚ significant p – p interactions between neigh3.5 A, bouring molecules are to be expected Žp-stacking.. The configuration of the vinylene linkages is such that the molecule assumes an S-type shape in the plane of the aromatic backbone. The octyloxy chains are extended in this plane, nearly perpendicular to the molecular axis. In Table 3, values for the dihedral angles along the backbone are listed. The central and end rings are nearly in the same plane, while the penultimate ring is somewhat tilted with respect to this plane. The cyano group is almost in the plane of the central ring, and as a result of its steric require-
148
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Y
Fig. 4. Perspective ORTEP drawing of the non-hydrogen atoms of Ooct–OPV5–CN with the atom labelling scheme. All non-hydrogen atoms are represented by their thermal displacement vibrational ellipsoids drawn to encompass 50% of the electron density; the hydrogen atoms are drawn with an arbitrary radius.
ments, the valence angle C3–C4–C5 is bent outwards and amounts to 133.18. Overall then, the whole molecule looks rather planar. In the structural studies of OPV3’s carrying hexyl chains on the central ring, Hohloch et al. w19x have also presented data for both types of cyano substitution ŽCNX and CNY in our notation.. For their compounds, the twist angles between the rings are much larger, in the range 458 to 508. Moreover, the cyano groups are rotated further away from the ring planes. Most importantly, this leads to highly non-planar molecules, and the P 2 1rc unit cells found for both compounds contain molecules in two different orientations. The p-overlap between two molecules related by a cell edge translation is limited to the ultimate phenylene–vinylene units, which face each ˚ other at a distance of 3.6 A. The striking differences in molecular packing between the compounds studied by Hohloch et al. and ours should be ascribed to the nature of the substituents on the middle ring, rather than to the length difference of the aromatic systems. Our conclusion is based on the finding that a p-stacking arrangement very similar to the one described above for Ooct– OPV5–CNY is found for the analogous three-ring
compound Ooct–OPV3–CNY w20x. ŽThe crystal structure of the latter has been fully resolved but not published in detail.. The larger steric requirement of the first methylene unit of alkyl tails compared to the ether oxygen in alkoxy tails results in a much stronger non-planarity of the aromatic backbones in alkylsubstituted OPV’s Žthe more so when cyano groups are present.. This seems to be in agreement with our finding that octyloxy-substituted compounds have a higher crystal density than the corresponding octylsubstituted ones Žin spite of the fact that the octyloxy tail is longer.: Ooct–OPV3: 1.162 g cmy3 ; oct– OPV3: 1.078 g cmy3 ; Ooct–OPV3–CNY : 1.171 g cmy3 ; oct–OPV3–CNY : 1.104 g cmy3 . Admittedly, this is a limited number of data for such a comparison. For the CNY-type compounds Žcyano group not directly adjacent to the central ring., alkoxy substitution allows a cofacial arrangement of neighbouring aromatic systems, with a large projected overlap and at a close distance, to be realized. 3.2. Optical properties of crystals and films For the three octyloxy-substituted five-ring oligomers, the normalized photoluminescence spec-
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
149
Y Fig. 5. A: packing arrangement of Ooct–OPV5–CN in the crystal lattice Žview at a slight angle with respect to the long molecular axis.; B: Y top-view Žtop. and side-view Žbottom. of two nearest-neighbour Ooct–OPV5–CN molecules in the crystal, related by a translation along ˚ the a-axis. The distance of 3.5 A corresponds to the shortest distance between two atoms on different molecules. Atom colours: light and dark grey: carbon atoms of the front and rear molecule respectively; white: oxygen atoms; black: nitrogen atoms. Atom size drawn decreases with increasing distance from point of observation.
tra of the single crystals are depicted in Fig. 6. Spectroscopic data are collected in Table 4. Due to the large absorption coefficient Žmore than 10 5 cmy1 at the maximum. we were not able to measure the
absorption spectra of the relatively thick Ž20–30 mm. single crystals. For Ooct–OPV5 and Ooct–OPV5–CNX , the PL spectra show vibrational structure characteristic of
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
150
Fig. 6. Normalized photoluminescence spectra of Ž1. Ooct–OPV5, Ž2. Ooct–OPV5–CNX and Ž3. Ooct–OPV5–CNY single crystals. Inset: corresponding PL-decay curves Žsemi-log plot..
single-molecule luminescence. In addition, the PL lifetimes for the single crystals, 1.0 and 2.2 ns respectively, are typical values reported for small organic dyes. On the basis of the molecular packing in the crystal structures, one does not expect significant p – p interaction for either of the two compounds. The observation of luminescence originating from an intramolecular excitation agrees with these conclusions.
For Ooct–OPV5–CNY , the molecular organization in the single crystal lattice is such that p – p interactions between neighbouring molecules are likely to occur ŽFig. 5.. This is in line with the emission properties of the Ooct–OPV5–CNY single crystals, which differ significantly from those of the other two oligomers. The luminescence spectrum is featureless and red-shifted in comparison with that of its isomer Ooct–OPV5–CNX . Secondly, the PL lifetime, f 8 ns, is significantly longer. This behaviour is similar to that reported for cyano-PPV films, which was attributed to excimer luminescence originating from interchain excitations w5,6x. An excimer is a pair of identical planar molecules in a cofacial arrangement, formed as a result of attractive interactions arising upon excitation of one of the molecules, which have a repulsive Žclosed-shell. interaction in the ground state. Typically, excimer formation occurs when the aromatic planes of the molecules are ˚ w21,22x. In the excimer, there is separated by 3–4 A p-orbital overlap between the molecules and a single wave function describes the electronic state of the pair. The absence of vibronic features is one of the characteristics of excimer luminescence. Another is the long radiative lifetime, caused by the symmetryforbidden nature of the transition. In the case of conjugated polymers, the excimer is thought to be formed by planar chain segments of two or more polymer chains which are parallel to each other and
Table 4 Solid-state absorption and emission data for OPV5’s Compound
Sample type a
b lAb s,max Žnm.
EAbs,max ŽeV.
l PL,max Žnm.
E PL,max ŽeV.
Shift c
d ŽeV.F PL
e t PL,1 Žns.
t PL,2 Žns.
Ooct–OPV5
sc as an sc sc as an
yf 442 442 yf yf 468 475
– 2.81 2.81 – – 2.65 2.61
539, 563 529 539 560 630 609 615
2.30, 2.20 2.34 2.30 2.21 1.97 2.04 2.02
– 0.46 0.50 – – 0.61 0.59
0.5 0.5 0.7 0.4 0.5 0.4 0.6
1.0 1.1 1.7 2.2 8.0 1.4 3.4
– – – – – 7.0 8.0
X
Ooct–OPV5–CN Y Ooct–OPV5–CN
a
sc: single crystal; as: film as-deposited; an: annealed film. lAb s,max is the longest wavelength absorption maximum, EAbs,max the corresponding energy; lPL,max represents the photoluminescence maxima, EPL ,max the corresponding energies; italic values denote the absolute maxima. c Shift is defined here as the difference between EAb s,max and E PL,max . d F PL is the photoluminescence quantum yield; estimated accuracy approx. 0.2. e PL decays were measured at the wavelength of the PL maximum; estimated accuracy approx. 0.1 ns. f No absorption spectra could be measured for the single crystal because the optical density was too high. b
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
˚ to allow separated by a distance smaller than 4 A p-orbital interaction. To obtain additional insight concerning the solid state of Ooct–OPV5–CNY , we have studied thin films of this compound obtained by sublimation and subsequent deposition onto a glass substrate at 10y6 mbar. These films are essentially similar to the ones used as active layer in light-emitting devices. The absorption and emission spectra and PL-decay of Ooct–OPV5–CNY in ŽA. solution, ŽB. as-deposited film, ŽC. annealed film and ŽD. single crystal, are shown in Fig. 7, along with optical micrographs of the as-deposited and annealed films. Annealing an as-deposited film at 1608C for 5 min.
151
resulted in the formation of small needle-like crystallites, suggesting an enhanced molecular orientation and increase of the crystalline fraction. X-ray diffraction data obtained on the thin films confirm that annealing results in a higher degree of orientation ŽFig. 8.. In general, the optical spectra of films depend, to varying degrees for different compounds, on the morphology of the films, and hence they change with annealing. The values in Table 4 should therefore be considered as approximate. The absorption spectra of the Ooct–OPV5–CNY films are somewhat red-shifted relative to the solution spectrum. Film spectra have a low-energy shoulder, which is further red-shifted and slightly more
Y Fig. 7. Top: Normalized absorption Žleft. and emission Žmiddle. spectra and PL-decay curves Žright. of Ooct–OPV5–CN in ŽA. solution, ŽB. as-deposited film, ŽC. annealed film Ž1608C, 5 min. and ŽD. single crystal. Bottom: polarized-light micrographs of the two thin-film morphologies; scale bar: 100 mm.
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P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
Fig. 8. XRD Ž u –2 u scan. of a thin Ž300 nm. film of Ooct– Y OPV5–CN vacuum-deposited on glass. Top: as-deposited; bottom: annealed at 1608C for 5 min.
pronounced for the annealed film. The emission spectra of the Ooct–OPV5–CNY films are intermediate between the single-molecule luminescence in solution and the excimer luminescence associated with the crystal lattice. Apparently, for the as-deposited and annealed films there are contributions from both a disordered and an ordered Žcrystalline. morphology. The PL spectrum of the as-deposited film ŽB. shows vibrational features, common for single-molecule luminescence, indicating that the contribution from the disordered fraction is dominant. For the annealed film ŽC., a broad and featureless composite spectrum is observed, due to a larger contribution from the crystalline fraction. With this interpretation of our data, we do not imply that the mechanisms usually associated with aggregates, are not operative w23x. By all means, a
crystal is an aggregate state. In a crystal, packing may be somewhere in between J-type and H-type Žwith regard to dipole–dipole interactions.; only Htype would lead to long lifetimes, because of the forbidden nature of the lowest-energy transition. If, in the H-type arrangement, the effect of dipolar interactions would be large, absorption would be blue-shifted relative to that of single molecules. Since we have no indication of a blue shift, we conclude that the very different behaviour of solids of Ooct– OPV5–CNY as compared to the other OPV5’s is caused by the p-stacked molecular arrangement, which leads to an excited state shared by two or more molecules. We emphasize that this p-stacking is the major difference between the molecular arrangements considered. The disorder-to-order transition is also clearly reflected in the PL-decay curves. For Ooct–OPV5– CNY in solution and the single crystal, single-exponential decay curves were observed corresponding to PL lifetimes of 1.4 and 8 ns, respectively. They are representing the two extremes: single-molecule and excimer luminescence. The PL-decay curves for the as-deposited and annealed films show double-exponential dependencies with lifetimes of 1.4r7.0 ns Žt 1rt 2 . and 3.4r8.0 ns, respectively. The contribution of the fast decay associated with single-molecule luminescence decreases upon annealing, which is in agreement with the observed increase of the crystalline fraction. Closer inspection of the diffraction patterns has revealed that the structure in the film is not identical with the single-crystal structure. While this does prevent us from giving a clear-cut interpretation as in the case of the single crystal, it does point out that an essential feature of organic compounds, polymorphism of the crystalline form, has to be taken into account. Our recent results indicate that different
Fig. 9. Structure of the octyloxy- and cyano-substituted copolymer of mixed PPP–PPV-type which shows long-lived, red-shifted luminescence w25x.
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
crystallization procedures may lead to different morphologies as well as different crystalline structures. This will have a strong influence on both optical and electrical properties of films, and affect their behaviour in electroluminescent devices. This aspect is currently under investigation. In our earlier studies on electroluminescence, we have studied a variety of alternating copolymers with regularly interrupted conjugation w24,25x. The polymer shown in Fig. 9 was found to behave similar to Ooct–OPV5–CNY and the cyano-PPV reported by Samuel et al. w5,6x. This compound shows an appreciable red-shift of the absorption and luminescence spectra of the film relative to those of the solution, a broad and featureless luminescence spectrum and a PL lifetime of 2.6 ns, relatively long in comparison with related copolymers investigated, but still short in comparison with 5.6 ns reported for cyano-PPV w5,6x. Possible reasons for the difference in PL lifetime could be the intermolecular distance or displacement Žthe axial register is likely to have a strong influence in the case of the copolymer., which together determine the magnitude of p-orbital overlap, or the degree of symmetry of the ensemble of interacting chain segments, which also affects the emission probability. In spite of this difference, we suggest that also in the case of the copolymer, the molecular packing gives rise to luminescent interchain excitations.
4. Conclusion Our study of model compounds indicates that the probability of the occurrence of interchain excitations will depend strongly on the juxtaposition of neighbouring chains Žsegments.; this, in turn, will be strongly influenced by the nature of the substituents and their pattern on the chain. The correspondence between p-stacked molecular organization and long-lived emission, as found for the model compound Ooct–OPV5–CNY , has led to an interpretation in terms of excimer-like excitations. This picture permits us to explain previous experimental observations for homopolymers that carry cyano substituents on the vinylenes and alkoxy substituents on the rings, and also for copolymers consisting of structurally similar base units.
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Acknowledgements The authors are grateful to R.E. Gill, M.P.L. Werts and J. Wildeman for their contributions in chemical synthesis, and to A. Hilberer and A. Meetsma for the crystal structure analysis. This research was financially supported by the Netherlands Organization for Scientific Research ŽNWOSONrSTWrFOM..
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w20x R.E. Gill, A. Hilberer, P.F. van Hutten, G. Berentschot, M.P.L. Werts, A. Meetsma, J.-C. Wittmann, G. Hadziioannou, Synth. Met. 84 Ž1997. 637. w21x S.A. Jenekhe, J.A. Osaheni, Science 265 Ž1994. 765. w22x M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals, Clarendon Press, Oxford, 1982. w23x D. Oelkrug, A. Tompert, H.-J. Egelhaaf, M. Hanack, E.
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Excimer luminescence from single crystals and films of a cyano-substituted phenylene–vinylene model compound P.F. van Hutten, V.V. Krasnikov, H.-J. Brouwer, G. Hadziioannou
)
Department of Polymer Chemistry and Materials Science Centre, UniÕersity of Groningen, Nijenborgh 4, NL-9747 AG Groningen, Netherlands Received 14 September 1998
Abstract In the context of luminescence from the solid state of polyŽ para-phenylene vinylene.-type materials, we report a study of three octyloxy-substituted five-ring oligoŽ para-phenylene vinylene.s, two of which bear cyano substituents on the innermost vinylene bonds. For each compound, the molecular arrangement in the single crystal has been derived from a crystallographic analysis. For the compound which has the cyano group not directly adjacent to the central ring, synthesis and structure are described here in detail. This compound, in contrast to the other two, has molecular p-stacking in the crystal lattice. Only for this compound do we observe a strongly red-shifted, long-lived emission, which is therefore ascribed to an excimer-type intermolecular excitation. The observations of excimer-type emission from cyano-substituted alkoxy–PPV’s and in related copolymers fit in very well with the molecular picture derived from this study of model compounds. q 1999 Elsevier Science B.V. All rights reserved. Keywords: PolyŽ para-phenylene vinylene.; Oligomers; Single-crystal structure; Excimer; Photoluminescence
1. Introduction PolyŽ para-phenylene vinylene. ŽPPV. and its derivatives are the most extensively studied p-conjugated organic materials for the fabrication of polymeric light-emitting diodes ŽLEDs. w1–3x. A diversity of synthetic procedures has been developed, and, although enhancements are still pursued, a variety of materials that have good processability, high luminescence efficiency and adequate stability and durability are now available w4x. Although most of the
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Corresponding author. Fax: q31-50-3634400; E-mail: [email protected]
visible spectrum is covered by this class of materials, full-colour display applications would require a much better spectral purity Žcolour saturation. of the emitted light in the appropriate wavelength ranges. The localization, mobility and decay mechanisms of the excitations generated by charge injection are the key issue, and time-resolved spectroscopic techniques are the primary tools for investigating it. Optical and electrical properties of solids of conjugated polymers depend in an indirect, and therefore complicated way on the chemical structure: molecular conformation, arrangement and density of packing and degree of order are all important. An example relevant to this paper is the observation of longlived luminescence in cyano-substituted PPV by
0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 4 1 3 - 3
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Samuel et al. w5,6x, which has been ascribed by the authors to interchain excitations. Cyano substitution of the vinylene linkages in PPV’s was introduced with the objective of decreasing the barrier for injection of electrons into the emissive polymer; it was found to shift optical energy gap to a lower value w7x. It is likely that the presence of the cyano group will strongly affect the molecular organization, due to its size as well as its polar nature. Monte-Carlo calculations of the arrangement of neighbouring chain segments have been presented by Conwell et al. w8x, and these suggest that cyano-substitution leads to inter˚ which is considered by chain distances below 3.5 A, the authors to be in favour of the formation of interchain excitations that luminesce rather than that they constitute a non-radiative trap. Unfortunately, the molecular organization in a polymeric solid is not accessible in detail, and, moreover, it is very inhomogeneous a priori. It is therefore not possible to confirm the calculated structures and to relate these to luminescence properties in a straightforward manner. The objective of this paper is to demonstrate that insight can be gained by using suitable model compounds. These exhibit very much the same phenomena but the results can be interpreted in a much more convincing manner since the structures are known in detail. In this work, the optical properties of the solid state of three related five-ring oligoŽ para-phenylene vinylene.s ŽOPV5’s. are compared, and crystallographic data are presented to support the occurrence of intermolecular luminescence in single crystals and thin films of one of the cyano-substituted compounds. We demonstrate the effects of the molecular packing and show that p-stacking gives rise to behaviour which is different from that resulting from dipolar interactions only.
films on a KBr pellet on a Mattson Instruments FT-IR spectrometer. Elemental analyses were carried out at the Microanalytical Department of the University of Groningen. 2.2. Materials Ooct–OPV5 was synthesized via a Wittig-reaction as described previously in Ref. w9x. Ooct– OPV5–CNX was prepared via a Knoevenagel condensation as described in Ref. w10x. Ooct–OPV5– CNY was obtained by a similar Knoevenagel condensation of 2,5-di-n-octyloxyterephthaldialdehyde and Ž4-styrylphenyl.acetonitrile, as outlined below ŽScheme 1.. Triethylamine was dried over potassium hydroxide and THF was distilled from lithium aluminium hydride before use. All other starting materials and solvents were used as received Žp.a. grade., unless stated otherwise. All reactions were performed under a dry argon atmosphere. 2.3. 1,4-Di-n-octyloxybenzene A mixture of hydroquinone Ž50.0 g, 0.450 mol., potassium hydroxide Ž55.0 g, 1.00 mol. and 1bromo-n-octane Ž183.0 g, 0.950 mol. in ethanol Ž450 ml. was refluxed for 4 h. After hot filtration, the filtrate was stirred and the product crystallized upon cooling. The solid was filtered off and washed with cold ethanol. Recrystallization from ethanol afforded white flaky crystals Ž131.0 g.. Yield: 131.0 g, 87%; m.p.: 49–508C. 1 H-NMR: d 0.90 Žt, C H3 , 6H., 1.31 Žm, C H2 , 20H., 1.77 Žm, b-C H2 , 4H., 3.91 Žt, a-C H2 , 4H., 6.84 Žs, arom C H, 4H. ppm. 13 C-NMR: d 13.8 Ž C H 3 ., 22.4, 25.8, 29.0, 29.2 Ž2 = ., 31.6 Ž C H 2 ., 68.4 ŽO–C H 2 ., 115.2 Žarom C H., 153.1 Žarom C–O. ppm. 2.4. 1,4-Diiodo-2,5-di-n-octyloxybenzene
2. Experimental 2.1. Measurements NMR spectra Žchloroform-d solutions. were recorded on a Varian spectrometer at 200 or 300 MHz Ž1 H., and at 50, 75 or 125 MHz Ž13 C.. All chemical shifts reported were externally referenced to TMS Ž0 ppm.. IR spectra were recorded from thin
A mixture of 1,4-di-n-octyloxybenzene Ž6.72 g, 20.0 mmol., iodine Ž4.62 g, 18.2 mmol., HIO 3 Ž2.23 g, 12.7 mmol., acetic acid Ž35 ml., 40% H 2 SO4 Ž5 ml. and CCl 4 Ž8.0 ml. is heated at 758C for 3 h. Cooling with an ice bath while stirring resulted in crystallization. The solid was filtered off and washed with methanol. Recrystallization from ethanol afforded the title compound as beige flakes. Yield: 8.5
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
141
Y
Scheme 1. Reaction scheme for the synthesis of Ooct–OPV5–CN .
g, 86%; m.p.: 428C. 1 H-NMR: d 0.90 Žt, C H3 , 6H., 1.30 Žm, C H2 , 20H., 1.81 Žm, b-C H2 , 4H., 3.94 Žt, a-C H2 , 4H., 7.19 Žs, arom C H, 2H. ppm. 13 C-NMR: d 13.9 Ž C H 3 ., 22.4, 25.8, 28.9, 29.0 Ž2 = ., 31.6 Ž C H 2 ., 70.1 ŽO–C H 2 ., 86.1 Žarom C–I., 122.6 Žarom C–H., 152.7 Žarom C–O. ppm. 2.5. 2,5-Di-n-octyloxyterephthaldialdehyde (1) 1,4-Diiodo-2,5-di-n-octyloxybenzene Ž5.00 g, 8.53 mmol. was dissolved in dry THF Ž100 ml. and cooled to below y708C. After 30 min., tert-BuLi Ž24 ml, 36 mmol, 1.7 M solution in pentane. was added dropwise, keeping the temperature below y508C. Stirring was continued for 30 min. at y508C, after which DMF Ž5 ml. was added all at once and stirring was continued for 15 min at y308C. The mixture was poured into a 1% aqueous solution of HCl Ž500 ml.. During stirring of the aqueous solution for several hours, the pentane evaporated and the yellow product separated. The solid was filtered off and washed with methanol. The pure, yellow product was obtained by recrystallization from etherrethanol Ž1:1.. Yield: 3.04 g, 90%; m.p.: 608C. 1 H-NMR: d 0.86 Žt, C H3 , 6H., 1.29 Žm, C H2 , 20H., 1.80 Žm, b-C H2 , 4H., 4.08 Žt, a-C H2 , 4H., 7.42 Žs, arom C H, 2H., 10.52 Žs, C HO, 2H. ppm. 13 C-NMR: d 13.9 Ž C H 3 ., 22.4, 25.8, 28.9, 29.0, 29.1, 31.8 Ž C H 2 ., 69.1, ŽO–C H 2 ., 111.5 Žarom
C H., 129.2 Žarom C H., 155.2 Žarom C–O., 189.5 Ž C HO. ppm. 2.6. (4-Styrylphenyl)acetonitrile (2) A mixture of styrene Ž1.30 g, 12.5 mmol., 4bromophenylacetonitrile Ž2.45 g, 12.5 mmol., PdŽOAc. 2 Ž0.057 g, 0.26 mmol., tri-o-tolylphosphine Ž0.37 g, 1.2 mmol. and triethylamine Ž5 ml. was placed in a heavy-wall pressure tube. The tube was degassed, closed ŽTeflon brushing., and heated to 1008C in an oil bath. After stirring for 20 h, the reaction mixture was cooled down to room temperature and poured into methanol Ž50 ml.. The precipitate was filtered off, washed with methanol three times and dried in vacuum. The crude reaction product was dissolved in dichloromethane, filtered and crystallized from dichloromethaneracetone Ž30:70. to give the title product. Yield: 0.90 g, 33%; m.p.: 928C. 1 H-NMR: d 3.80 Žs, C H2 , 2H., 7.10 Žd, C H s C H, 2H., 7.2–7.4 Žm, arom C H, 5H., 7.52 ŽAB q, arom C H, 4H. ppm. 13 C-NMR: d 23.1Ž C H 2 ., 118.9, 126.5, 127.0, 127.5, 127.8, 128.1, 128.6, 128.8, 129.4, 136.8, 137.1 ppm. 2.7. 3-{ 4-[2-Cyano-2-(4-styrylphenyl)Õinyl]-2,5bis(n-octyloxy)phenyl}-2-(4-styrylphenyl)-acrylonitrile (3, Ooct–OPV5–CN Y ) 2,5-Di-n-octyloxyterephthaldialdehyde Ž1. Ž586 mg, 1.5 mmol. and Ž4-styrylphenyl.acetonitrile Ž2.
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Ž658 mg, 3.0 mmol. were dissolved in a mixture of tert-butanol Ž8 ml. and THF Ž2 ml. at 508C. Potassium-tert-butoxide Ž16.8 mg, 0.15 mmol. and tetran-butylammonium hydroxide Ž0.1 N solution in 2propanolrmethanol, 1.5 ml, 0.15 mmol. were added quickly. After 15 min the reaction mixture was poured into acidified methanol. The red precipitate was filtered off and washed with methanol three times. Pure Ž3. was obtained as red needles after repeated crystallization from a chloroformrn-hexane mixture. Yield: 0.973 g, 82%; m.p.: 2008C. 1 H-NMR: d 0.87 Žt, J s 6.6 Hz, C28., 1.3–1.5 Žm, C24–C27., 1.51 Žm, J s 7.8 Hz, C23., 1.86 Žm, J s 7.3 Hz, C22., 4.12 Žt, J s 6.6 Hz, C21., 7.12 and 7.14 ŽAB q, J s 16 Hz, C7,C8,C10,C11., 7.27 Žt, J s 7.7 Hz, C17., 7.36 Žt, J s 7.5 Hz, C15,C19., 7.51 and 7.53 Žm, C16,C18., 7.56 Žd, J s 8.4 Hz, C13., 7.67 Žd, J s 8.4 Hz, C12., 7.89 Žs, C1., 8.03 Žs, C4. ppm. 13 C-NMR: d 13.96, 22.53, 26.10, 29.05, 29.16, 29.20, 31.64, 69.33, 111.18, 118.29, 125.79, 126.30, 126.63, 127.04, 127.50, 128.00, 128.72, 128.74, 129.95, 133.66, 135.26, 136.92, 138.31, 151.55 ppm. IR ŽKBr.: 3027 Žm., 2922 Žs., 2853 Žs., 2209 Žw., 1603 Žm., 1583 Žm., 1492 Žm., 1466 Žm., 1436 Žm., 1368 Žm., 1306 Žw., 1251 Žm., 1211 Žs., 1057 Žw., 1024 Žw., 964 Žm., 911 Žw., 864 Žw., 818 Žm., 753 Žm., 720 Žw. cmy1 . Elem. anal. Calcd: C: 84.47; H: 7.86; N: 3.58. Found: C: 84.66; H: 7.65; N: 3.62. 2.8. Single crystals
˚ sy1 . The as-deposited posited at a rate of 2–4 A layer showed a fine-grained polycrystalline structure when observed in a polarizing optical microscope. 2.10. X-ray crystallography of Ooct–OPV5–CN Y A needle-shaped crystal of Ooct–OPV5–CNY Ž3., of approximate size 0.04 = 0.04 = 0.5 mm3 , was mounted on top of a glass fibre and transferred into the cold nitrogen cold stream of the low temperature
Table 1 Y Crystallographic and experimental data for Ooct–OPV5–CN Crystal data Formula Formula weight Žg moly1 . Crystal system Space group ˚. a ŽA
C 56 H 60 N2 O 2 793.11
˚. b ŽA
12.664Ž4. 18.35Ž1. 92.39Ž6. 91.67Ž5. 99.06Ž2. 1115.9Ž7. 1 426 1.180 0.7 0.04=0.04=0.5
˚. c ŽA a Ž8. b Ž8. g Ž8. ˚ 3. V ŽA Z F Ž000. Dcalc Žg cmy3 . m ŽMo K a . Žcmy1 . Crystal size Žmm3 .
triclinic P1 4.870Ž1.
Data collection
Single crystals were obtained by precipitation of the respective compounds from a solution in a mixture of solvents, either THFrmethanol or chloroformrn-hexane, which was slowly evaporated at room temperature. Ooct–OPV5 crystallized into yellow needle-shaped crystals; orange plate-shaped crystals were obtained for Ooct–OPV5–CNX . In the case of Ooct–OPV5–CNY , orange-red crystal needles were obtained. 2.9. Thin-film deposition Films on glass substrates were prepared by evaporation from a molybdenum boat under high vacuum Ž- 1 = 10y6 mbar. at a temperature just above the melting point of the oligomer. The film was de-
T ŽK. u range: min., max. Ž8. Data collected No. of unique data No. of rflns. obsd. Ž Fo G 4.0 s Ž Fo ..
130 1.11, 25.0 4423 3920 1087 Structure and refinement
No. of reflections, n Ž Fo2 ) 0. No. of parameters, p RŽ F . s S < < Fo
2873 272 0.119 0.310
˚ y3 . Residual r Že A ŽMaximum shift.r s Žfinal cycle.
y0.53, 0.36 Ž10. - 0.001
1.005
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
unit mounted on an Enraf-Nonius CAD-4F diffractometer Žmonochromatized Mo-K a radiation, 50 kV, ˚ .. Unit cell parameters and 40 mA, l s 0.71073 A orientation matrix were determined from a leastsquares treatment of the setting angles of 25 reflections in the range 5.048 - u - 10.398. The intensities of two standard reflections, monitored every three hours of X-ray exposure time, showed no greater fluctuations during data collection than those expected from Poisson statistics. Intensity data were corrected for Lorentz and polarization effects, scale variation, but not for absorption, and reduced to Fo2 . The structure was solved by direct methods with SHELXS86 w11x. Neutral atom scattering factors and anomalous dispersion corrections were taken from International Tables of Crystallography w12x. The positional and anisotropic thermal displacement parameters for the non-hydrogen atoms were refined. The hydrogen atoms were included in the final refinement riding on their carrier atoms with U s c = Uequiv of their parent atom, where c s 1.2 for the aromaticrnon-methyl hydrogen atoms and c s 1.5 for the methyl hydrogen atoms and where values Uequiv are related to the atoms to which the H atoms are bonded. The methyl group was refined as a rigid group, which was allowed to rotate free. A few atoms converged to non-positive definite thermal displacement parameters when allowed to vary anisotropically and some atoms showed unrealistic thermal displacement parameters suggesting some
143
Fig. 2. Molecular packing of Ooct–OPV5 in the crystal lattice. Left: oblique view of the unit cell of Ooct–OPV5. Right: projection of the unit cell on a plane perpendicular to the a-axis.
degree of disorder, which is in line with the weak scattering power of the crystals investigated. In the final refinement, the non-positive definite atoms were restrained so that their Ui j components approximate to isotropic behaviour. The positional and anisotropic thermal displacement parameters for the non-hydrogen atoms and isotropic thermal displacement parameters for hydrogen atoms were refined on F 2 with full-matrix least-squares procedures w13x. Final refinement converged at wRŽ F 2 . s 0.310 for 2873 reflections with Fo2 G 0 and 272 parameters and RŽ F . s 0.119 for 1087 reflections with Fo G 4.0 s Ž Fo .. The final difference map showed no significant peak having chemical meaning above the general background. Crystal data and numerical details on data collection and refinement are given in Table 1. 2.11. Thin-film X-ray diffraction Wide-angle X-ray diffraction patterns of 2 = 2 cm2 thin Žca. 300 nm. films on glass plates were recorded in reflection geometry Ž u –2 u scan. on a Rigaku horizontal goniometer mounted on a Rotaflex RU-200B rotating-anode generator operated at 40 kV and 40 mA. 2.12. Optical characterization
Fig. 1. The five-ring oligoŽphenylene vinylene.s considered in this study.
UV-vis absorption spectra were obtained on an SLM Aminco 3000 Array spectrophotometer. Photoluminescence spectra were recorded on a Perkin Elmer LS50-B spectrofluorometer. The excitation
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wavelength was 10 nm below the absorption maximum. The absolute photoluminescence quantum
X
yields of our crystals and films were determined using a calibrated integrating sphere with an Arq
Fig. 3. A: packing arrangement of Ooct–OPV5–CN in the crystal lattice Žview at a slight angle with respect to the long molecular axis.; B: X top-view Žtop. and side-view Žbottom. of two nearest-neighbour Ooct–OPV5–CN molecules in the crystal, related by a translation along the a-axis. See Fig. 5 for details.
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
Ž488 nm. laser as excitation source w14,15x. Luminescence lifetime measurements were made by timecorrelated single-photon counting. Excitation at approx. 400 nm was provided by the second harmonic of a ; 100 fs mode-locked Ti-sapphire laser ŽMira, Coherent.. To match the photon counting electronics, the laser repetition rate was reduced by an acoustooptical pulse-picker ŽCAMAC. to 2 MHz. The excitation energy density did not exceed 1 nJrcm2 per pulse. The instrumental response time at half-width of full maximum was approx. 40 ps. All optical measurements were made under vacuum better than 10y4 mbar.
145
contained in layers parallel to the Ž010. plane Žinter˚ .; the molecules are not layer distance br2 s 3.77 A p-stacked. The octyloxy side chains are not fully stretched in an all-trans configuration, but curved towards the periphery of the conjugated backbone. Strong disorder is found in one of the two octyloxy tails. 3.1.2. Ooct–OPV5–CN X The unit cell is triclinic, space group P1, containing one discrete molecule, which has a crystallographically imposed centre of inversion w10x. The molecule has a remarkable, wave-like shape ŽFig. 3.. Such distortions indicate the presence of consider-
3. Results and discussion 3.1. Single-crystal structures Since PPV has become the focus of research on light-emitting polymers, a variety of OPV’s have been synthesized and electrically and optically characterized. Although the molecular organization in PPV-type materials is highly relevant to their luminescence properties, surprisingly few crystallographic studies have been carried out to date. Early crystallographic work on derivatives of distyrylbenzene Žreferred to as OPV3 in this paper. was presented by Geise and coworkers w16–18x. Data for cyano-substituted OPV3’s have been presented recently by Hohloch et al. w19x. Our group has focused on five-ring OPV’s; those considered in this paper are shown in Fig. 1. We have reported the syntheses and the single-crystal structures for Ooct–OPV5 w9x as well as for Ooct– OPV5–CNX w10x. This paper presents detailed structural data for Ooct–OPV5–CNY , which was obtained by a Knoevenagel condensation as described in Section 2. 3.1.1. Ooct–OPV5 The unit cell is monoclinic, space group I 2ra, and contains eight discrete molecules separated by normal van der Waals distances w9x ŽFig. 2.. The conjugated backbones lie parallel to each other and are lined up into strings along the a-axis and separated along the c-axis by a layer accommodating the aliphatic octyloxy side chains. The molecules are
Table 2 Final fractional atomic coordinates and equivalent isotropic therY mal displacement parameters for Ooct–OPV5–CN with estimated standard deviations in parentheses Žnon-H atoms of the . asymmetric unit; Ueq s1r3Ý i Ý j Ui j ai) a ) j a i .a j
O N CŽ1. CŽ2. CŽ3. CŽ4. CŽ5. CŽ6. CŽ7. CŽ8. CŽ9. CŽ10. CŽ11. CŽ12. CŽ13. CŽ14. CŽ15. CŽ16. CŽ17. CŽ18. CŽ19. CŽ20. CŽ21. CŽ22. CŽ23. CŽ24. CŽ25. CŽ26. CŽ27. CŽ28.
x
y
z
˚2. Ueq ŽA
0.1889Ž14. 0.625Ž2. y0.150Ž2. 0.0854Ž19. 0.2456Ž19. 0.491Ž2. 0.671Ž2. 0.905Ž2. 0.996Ž2. 1.218Ž2. 1.3705Ž19. 1.289Ž2. 1.059Ž2. 1.605Ž2. 1.684Ž2. 1.917Ž2. 1.968Ž2. 2.203Ž3. 2.378Ž3. 2.324Ž3. 2.110Ž2. 0.639Ž2. 0.016Ž2. 0.179Ž2. 0.010Ž2. 0.174Ž2. 0.022Ž2. 0.197Ž2. 0.036Ž2. 0.217Ž3.
0.0037Ž5. 0.2678Ž7. y0.0666Ž7. y0.0007Ž7. 0.0706Ž7. 0.1416Ž7. 0.2168Ž7. 0.2823Ž7. 0.2557Ž7. 0.3137Ž8. 0.4051Ž7. 0.4315Ž8. 0.3720Ž7. 0.4701Ž7. 0.4655Ž7. 0.5262Ž8. 0.5084Ž9. 0.5615Ž10. 0.6405Ž11. 0.6628Ž9. 0.6069Ž9. 0.2442Ž8. y0.0604Ž8. y0.0413Ž8. y0.0993Ž7. y0.0877Ž8. y0.1502Ž7. y0.1449Ž8. y0.2075Ž8. y0.2015Ž9.
0.1433Ž3. y0.1078Ž5. 0.0491Ž5. 0.0727Ž5. 0.0265Ž5. 0.0567Ž5. 0.0286Ž5. 0.0692Ž5. 0.1382Ž5. 0.1760Ž5. 0.1466Ž5. 0.0777Ž5. 0.0402Ž5. 0.1845Ž6. 0.2536Ž6. 0.2941Ž6. 0.3685Ž6. 0.4085Ž7. 0.3750Ž8. 0.3030Ž8. 0.2629Ž6. y0.0490Ž6. 0.1940Ž5. 0.2675Ž5. 0.3278Ž5. 0.3992Ž5. 0.4594Ž5. 0.5321Ž5. 0.5905Ž5. 0.6615Ž5.
0.027Ž2. 0.055Ž4. 0.018Ž3. 0.019Ž3. 0.016Ž3. 0.021Ž3. 0.020Ž3. 0.024Ž3. 0.028Ž4. 0.023Ž3. 0.021Ž3. 0.029Ž4. 0.022Ž3. 0.032Ž4. 0.036Ž4. 0.032Ž4. 0.042Ž4. 0.056Ž5. 0.067Ž7. 0.051Ž5. 0.038Ž4. 0.033Ž4. 0.027Ž4. 0.030Ž4. 0.023Ž3. 0.021Ž3. 0.020Ž3. 0.023Ž3. 0.038Ž4. 0.051Ž5.
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Table 3 Ya Selected data on the geometry of Ooct–OPV5–CN ŽStandard deviations in the last decimal place are given in parentheses. Interatomic Distances (A˚) O–CŽ2. O–CŽ21. N–CŽ20. CŽ1. –CŽ2. CŽ1. –CŽ3. a CŽ2. –CŽ3. CŽ3. –CŽ4. CŽ4. –CŽ5. CŽ5. –CŽ6. CŽ5. –CŽ20. CŽ6. –CŽ7. CŽ6. –CŽ11. CŽ7. –CŽ8. Bond angles (deg) CŽ2. –O–CŽ21. CŽ2. –CŽ1. –CŽ3.a O–CŽ2. –CŽ1. O–CŽ2. –CŽ3. CŽ1. –CŽ2. –CŽ3. CŽ2. –CŽ3. –CŽ4. CŽ2. –CŽ3. –CŽ1.a CŽ4. –CŽ3. –CŽ1.a CŽ3. –CŽ4. –CŽ5. CŽ4. –CŽ5. –CŽ6. CŽ4. –CŽ5. –CŽ20. CŽ6. –CŽ5. –CŽ20. CŽ5. –CŽ6. –CŽ7. CŽ5. –CŽ6. –CŽ11. CŽ7. –CŽ6. –CŽ11. CŽ6. –CŽ7. –CŽ8. CŽ7. –CŽ8. –CŽ9. Torsion angles (deg) CŽ21. –O–CŽ2. –CŽ1. CŽ21. –O–CŽ2. –CŽ3. CŽ2. –O–CŽ21. –CŽ22. CŽ3.a–CŽ1. –CŽ2. –O CŽ3.a–CŽ1. –CŽ2. –CŽ3. CŽ2. –CŽ1. –CŽ3.a–CŽ2.a CŽ2. –CŽ1. –CŽ3.a–CŽ4.a O–CŽ2. –CŽ3. –CŽ4. O–CŽ2. –CŽ3. –CŽ1.a CŽ1. –CŽ2. –CŽ3. –CŽ4. CŽ1. –CŽ2. –CŽ3. –CŽ1.a CŽ2. –CŽ3. –CŽ4. –CŽ5. CŽ1.a–CŽ3. –CŽ4. –CŽ5. CŽ3. –CŽ4. –CŽ5. –CŽ6. CŽ3. –CŽ4. –CŽ5. –CŽ20. CŽ4. –CŽ5. –CŽ6. –CŽ7. CŽ4. –CŽ5. –CŽ6. –CŽ11. CŽ20. –CŽ5. –CŽ6. –CŽ7.
1.372Ž11. 1.458Ž11. 1.135Ž14. 1.356Ž13. 1.447Ž13. 1.422Ž13. 1.458Ž13. 1.321Ž13. 1.464Ž13. 1.489Ž14. 1.401Ž13. 1.393Ž13. 1.361Ž14.
115.7Ž7. 120.8Ž8. 123.0Ž8. 114.1Ž8. 122.9Ž9. 119.5Ž8. 116.2Ž8. 124.2Ž8. 133.1Ž9. 124.9Ž9. 120.6Ž9. 114.5Ž8. 122.0Ž8. 122.1Ž8. 115.9Ž8. 123.0Ž9. 120.7Ž9.
y5.3Ž12. 174.2Ž8. 179.8Ž7. y178.0Ž8. 2.5Ž14. y2.3Ž13. 177.9Ž9. y1.7Ž12. 178.1Ž8. 177.9Ž9. y2.4Ž14. y177.6Ž10. 2.7Ž17. 178.5Ž9. 1.2Ž17. 14.2Ž15. y168.9Ž10. y168.4Ž9.
CŽ8. –CŽ9. CŽ9. –CŽ10. CŽ9. –CŽ12. CŽ10. –CŽ11. CŽ12. –CŽ13. CŽ13. –CŽ14. CŽ14. –CŽ15. CŽ14. –CŽ19. CŽ15. –CŽ16. CŽ16. –CŽ17. CŽ17. –CŽ18. CŽ18. –CŽ19.
1.410Ž13. 1.384Ž13. 1.441Ž13. 1.394Ž14. 1.321Ž15. 1.435Ž14. 1.414Ž16. 1.426Ž15. 1.403Ž17. 1.385Ž19. 1.39Ž2. 1.345Ž18.
CŽ8. –CŽ9. –CŽ10. CŽ8. –CŽ9. –CŽ12. CŽ10. –CŽ9. –CŽ12. CŽ9. –CŽ10. –CŽ11. CŽ6. –CŽ11. –CŽ10. CŽ9. –CŽ12. –CŽ13. CŽ12. –CŽ13. –CŽ14. CŽ13. –CŽ14. –CŽ15. CŽ13. –CŽ14. –CŽ19. CŽ15. –CŽ14. –CŽ19. CŽ14. –CŽ15. –CŽ16. CŽ15. –CŽ16. –CŽ17. CŽ16. –CŽ17. –CŽ18. CŽ17. –CŽ18. –CŽ19. CŽ14. –CŽ19. –CŽ18. N–CŽ20. –CŽ5. O–CŽ21. –CŽ22.
117.4Ž9. 123.1Ž9. 119.5Ž9. 121.1Ž9. 121.9Ž9. 126.9Ž9. 129.6Ž9. 121.0Ž9. 123.Ž1. 115.9Ž9. 122.5Ž10. 118.1Ž12. 120.1Ž13. 121.9Ž12. 121.2Ž11. 177.1Ž11. 104.8Ž7.
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
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Table 3 Žcontinued. Torsion angles (deg) CŽ20. –CŽ5. –CŽ6. –CŽ11. CŽ5. –CŽ6. –CŽ7. –CŽ8. CŽ11. –CŽ6. –CŽ7. –CŽ8. CŽ5. –CŽ6. –CŽ11. –CŽ10. CŽ7. –CŽ6. –CŽ11. –CŽ10. CŽ6. –CŽ7. –CŽ8. –CŽ9. CŽ7. –CŽ8. –CŽ9. –CŽ10. CŽ7. –CŽ8. –CŽ9. –CŽ12. CŽ8. –CŽ9. –CŽ10. –CŽ11. CŽ12. –CŽ9. –CŽ10. –CŽ11. CŽ8. –CŽ9. –CŽ12. –CŽ13. CŽ10. –CŽ9. –CŽ12. –CŽ13. CŽ9. –CŽ10. –CŽ11. –CŽ6. CŽ9. –CŽ12. –CŽ13. –CŽ14. CŽ12. –CŽ13. –CŽ14. –CŽ15. CŽ12. –CŽ13. –CŽ14. –CŽ19. CŽ13. –CŽ14. –CŽ15. –CŽ16. CŽ19. –CŽ14. –CŽ15. –CŽ16. CŽ13. –CŽ14. –CŽ19. –CŽ18. CŽ15. –CŽ14. –CŽ19. –CŽ18. CŽ14. –CŽ15. –CŽ16. –CŽ17. CŽ15. –CŽ16. –CŽ17. –CŽ18. CŽ16. –CŽ17. –CŽ18. –CŽ19. CŽ17. –CŽ18. –CŽ19. –CŽ14.
8.5Ž13. 178.1Ž9. 1.0Ž14. y177.2Ž9. y0.1Ž14. y0.2Ž15. y1.4Ž14. 179.3Ž10. 2.2Ž14. y178.4Ž9. y11.2Ž16. 169.4Ž10. y1.5Ž15. 178.7Ž10. y179.4Ž10. 0.5Ž17. 176.0Ž11. y3.9Ž16. 179.3Ž11. y0.9Ž16. 5.4Ž18. y2.Ž2. y3.Ž2. 4.1Ž19.
a
Atoms labelled ‘a’ are generated from the unlabelled ones by the symmetry operation: yx, yy, yz; The sign of the torsion angle is positive when, looking from atom 2 to atom 3, a clockwise motion of atom 1 would superimpose it on atom 4.
able packing forces. The dense packing Žthe crystal density of 1.210 g cmy3 is 10% higher than that of Ooct–OPV5. may be driven both by side-chain arrangement and by electrostatic interactions between the polar groups. Although the shortest interatomic distance between nearest Ž a-axis. neighbours is ap˚ one does not expect p – p interaction to prox. 3.5 A, occur for these molecules, since the p-systems are both laterally and axially displaced, in a view perpendicular to the plane of the central ring. The aromatic systems face the octyloxy substituents of the neighbour molecules. The conformation around the double bond that bears the cyano group is different from that found in other OPV crystals studied so far. The introduction of the cyano group has caused an almost 1808 rotation around the single bond to the central ring and results in the molecular backbone and octyloxy tail being in close proximity. 3.1.3. Ooct–OPV5–CN Y This compound, which has its cyano substituents at the outer position of the central vinylene linkages,
has a P1 crystal packing which is very different from that of Ooct–OPV5–CNX Žcrystallographic data are in Tables 1–3, Fig. 4 contains the atom labelling, Fig. 5 shows the molecular packing.. As Fig. 5 illustrates, nearest-neighbour Ž a-axis related . molecules exhibit only an axial displacement of approximately half a phenylene–vinylene unit. In a view perpendicular to the central ring, the aromatic systems overlap to a large extent. Since the distance between the conjugated backbones is approximately ˚ significant p – p interactions between neigh3.5 A, bouring molecules are to be expected Žp-stacking.. The configuration of the vinylene linkages is such that the molecule assumes an S-type shape in the plane of the aromatic backbone. The octyloxy chains are extended in this plane, nearly perpendicular to the molecular axis. In Table 3, values for the dihedral angles along the backbone are listed. The central and end rings are nearly in the same plane, while the penultimate ring is somewhat tilted with respect to this plane. The cyano group is almost in the plane of the central ring, and as a result of its steric require-
148
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Y
Fig. 4. Perspective ORTEP drawing of the non-hydrogen atoms of Ooct–OPV5–CN with the atom labelling scheme. All non-hydrogen atoms are represented by their thermal displacement vibrational ellipsoids drawn to encompass 50% of the electron density; the hydrogen atoms are drawn with an arbitrary radius.
ments, the valence angle C3–C4–C5 is bent outwards and amounts to 133.18. Overall then, the whole molecule looks rather planar. In the structural studies of OPV3’s carrying hexyl chains on the central ring, Hohloch et al. w19x have also presented data for both types of cyano substitution ŽCNX and CNY in our notation.. For their compounds, the twist angles between the rings are much larger, in the range 458 to 508. Moreover, the cyano groups are rotated further away from the ring planes. Most importantly, this leads to highly non-planar molecules, and the P 2 1rc unit cells found for both compounds contain molecules in two different orientations. The p-overlap between two molecules related by a cell edge translation is limited to the ultimate phenylene–vinylene units, which face each ˚ other at a distance of 3.6 A. The striking differences in molecular packing between the compounds studied by Hohloch et al. and ours should be ascribed to the nature of the substituents on the middle ring, rather than to the length difference of the aromatic systems. Our conclusion is based on the finding that a p-stacking arrangement very similar to the one described above for Ooct– OPV5–CNY is found for the analogous three-ring
compound Ooct–OPV3–CNY w20x. ŽThe crystal structure of the latter has been fully resolved but not published in detail.. The larger steric requirement of the first methylene unit of alkyl tails compared to the ether oxygen in alkoxy tails results in a much stronger non-planarity of the aromatic backbones in alkylsubstituted OPV’s Žthe more so when cyano groups are present.. This seems to be in agreement with our finding that octyloxy-substituted compounds have a higher crystal density than the corresponding octylsubstituted ones Žin spite of the fact that the octyloxy tail is longer.: Ooct–OPV3: 1.162 g cmy3 ; oct– OPV3: 1.078 g cmy3 ; Ooct–OPV3–CNY : 1.171 g cmy3 ; oct–OPV3–CNY : 1.104 g cmy3 . Admittedly, this is a limited number of data for such a comparison. For the CNY-type compounds Žcyano group not directly adjacent to the central ring., alkoxy substitution allows a cofacial arrangement of neighbouring aromatic systems, with a large projected overlap and at a close distance, to be realized. 3.2. Optical properties of crystals and films For the three octyloxy-substituted five-ring oligomers, the normalized photoluminescence spec-
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
149
Y Fig. 5. A: packing arrangement of Ooct–OPV5–CN in the crystal lattice Žview at a slight angle with respect to the long molecular axis.; B: Y top-view Žtop. and side-view Žbottom. of two nearest-neighbour Ooct–OPV5–CN molecules in the crystal, related by a translation along ˚ the a-axis. The distance of 3.5 A corresponds to the shortest distance between two atoms on different molecules. Atom colours: light and dark grey: carbon atoms of the front and rear molecule respectively; white: oxygen atoms; black: nitrogen atoms. Atom size drawn decreases with increasing distance from point of observation.
tra of the single crystals are depicted in Fig. 6. Spectroscopic data are collected in Table 4. Due to the large absorption coefficient Žmore than 10 5 cmy1 at the maximum. we were not able to measure the
absorption spectra of the relatively thick Ž20–30 mm. single crystals. For Ooct–OPV5 and Ooct–OPV5–CNX , the PL spectra show vibrational structure characteristic of
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
150
Fig. 6. Normalized photoluminescence spectra of Ž1. Ooct–OPV5, Ž2. Ooct–OPV5–CNX and Ž3. Ooct–OPV5–CNY single crystals. Inset: corresponding PL-decay curves Žsemi-log plot..
single-molecule luminescence. In addition, the PL lifetimes for the single crystals, 1.0 and 2.2 ns respectively, are typical values reported for small organic dyes. On the basis of the molecular packing in the crystal structures, one does not expect significant p – p interaction for either of the two compounds. The observation of luminescence originating from an intramolecular excitation agrees with these conclusions.
For Ooct–OPV5–CNY , the molecular organization in the single crystal lattice is such that p – p interactions between neighbouring molecules are likely to occur ŽFig. 5.. This is in line with the emission properties of the Ooct–OPV5–CNY single crystals, which differ significantly from those of the other two oligomers. The luminescence spectrum is featureless and red-shifted in comparison with that of its isomer Ooct–OPV5–CNX . Secondly, the PL lifetime, f 8 ns, is significantly longer. This behaviour is similar to that reported for cyano-PPV films, which was attributed to excimer luminescence originating from interchain excitations w5,6x. An excimer is a pair of identical planar molecules in a cofacial arrangement, formed as a result of attractive interactions arising upon excitation of one of the molecules, which have a repulsive Žclosed-shell. interaction in the ground state. Typically, excimer formation occurs when the aromatic planes of the molecules are ˚ w21,22x. In the excimer, there is separated by 3–4 A p-orbital overlap between the molecules and a single wave function describes the electronic state of the pair. The absence of vibronic features is one of the characteristics of excimer luminescence. Another is the long radiative lifetime, caused by the symmetryforbidden nature of the transition. In the case of conjugated polymers, the excimer is thought to be formed by planar chain segments of two or more polymer chains which are parallel to each other and
Table 4 Solid-state absorption and emission data for OPV5’s Compound
Sample type a
b lAb s,max Žnm.
EAbs,max ŽeV.
l PL,max Žnm.
E PL,max ŽeV.
Shift c
d ŽeV.F PL
e t PL,1 Žns.
t PL,2 Žns.
Ooct–OPV5
sc as an sc sc as an
yf 442 442 yf yf 468 475
– 2.81 2.81 – – 2.65 2.61
539, 563 529 539 560 630 609 615
2.30, 2.20 2.34 2.30 2.21 1.97 2.04 2.02
– 0.46 0.50 – – 0.61 0.59
0.5 0.5 0.7 0.4 0.5 0.4 0.6
1.0 1.1 1.7 2.2 8.0 1.4 3.4
– – – – – 7.0 8.0
X
Ooct–OPV5–CN Y Ooct–OPV5–CN
a
sc: single crystal; as: film as-deposited; an: annealed film. lAb s,max is the longest wavelength absorption maximum, EAbs,max the corresponding energy; lPL,max represents the photoluminescence maxima, EPL ,max the corresponding energies; italic values denote the absolute maxima. c Shift is defined here as the difference between EAb s,max and E PL,max . d F PL is the photoluminescence quantum yield; estimated accuracy approx. 0.2. e PL decays were measured at the wavelength of the PL maximum; estimated accuracy approx. 0.1 ns. f No absorption spectra could be measured for the single crystal because the optical density was too high. b
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
˚ to allow separated by a distance smaller than 4 A p-orbital interaction. To obtain additional insight concerning the solid state of Ooct–OPV5–CNY , we have studied thin films of this compound obtained by sublimation and subsequent deposition onto a glass substrate at 10y6 mbar. These films are essentially similar to the ones used as active layer in light-emitting devices. The absorption and emission spectra and PL-decay of Ooct–OPV5–CNY in ŽA. solution, ŽB. as-deposited film, ŽC. annealed film and ŽD. single crystal, are shown in Fig. 7, along with optical micrographs of the as-deposited and annealed films. Annealing an as-deposited film at 1608C for 5 min.
151
resulted in the formation of small needle-like crystallites, suggesting an enhanced molecular orientation and increase of the crystalline fraction. X-ray diffraction data obtained on the thin films confirm that annealing results in a higher degree of orientation ŽFig. 8.. In general, the optical spectra of films depend, to varying degrees for different compounds, on the morphology of the films, and hence they change with annealing. The values in Table 4 should therefore be considered as approximate. The absorption spectra of the Ooct–OPV5–CNY films are somewhat red-shifted relative to the solution spectrum. Film spectra have a low-energy shoulder, which is further red-shifted and slightly more
Y Fig. 7. Top: Normalized absorption Žleft. and emission Žmiddle. spectra and PL-decay curves Žright. of Ooct–OPV5–CN in ŽA. solution, ŽB. as-deposited film, ŽC. annealed film Ž1608C, 5 min. and ŽD. single crystal. Bottom: polarized-light micrographs of the two thin-film morphologies; scale bar: 100 mm.
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Fig. 8. XRD Ž u –2 u scan. of a thin Ž300 nm. film of Ooct– Y OPV5–CN vacuum-deposited on glass. Top: as-deposited; bottom: annealed at 1608C for 5 min.
pronounced for the annealed film. The emission spectra of the Ooct–OPV5–CNY films are intermediate between the single-molecule luminescence in solution and the excimer luminescence associated with the crystal lattice. Apparently, for the as-deposited and annealed films there are contributions from both a disordered and an ordered Žcrystalline. morphology. The PL spectrum of the as-deposited film ŽB. shows vibrational features, common for single-molecule luminescence, indicating that the contribution from the disordered fraction is dominant. For the annealed film ŽC., a broad and featureless composite spectrum is observed, due to a larger contribution from the crystalline fraction. With this interpretation of our data, we do not imply that the mechanisms usually associated with aggregates, are not operative w23x. By all means, a
crystal is an aggregate state. In a crystal, packing may be somewhere in between J-type and H-type Žwith regard to dipole–dipole interactions.; only Htype would lead to long lifetimes, because of the forbidden nature of the lowest-energy transition. If, in the H-type arrangement, the effect of dipolar interactions would be large, absorption would be blue-shifted relative to that of single molecules. Since we have no indication of a blue shift, we conclude that the very different behaviour of solids of Ooct– OPV5–CNY as compared to the other OPV5’s is caused by the p-stacked molecular arrangement, which leads to an excited state shared by two or more molecules. We emphasize that this p-stacking is the major difference between the molecular arrangements considered. The disorder-to-order transition is also clearly reflected in the PL-decay curves. For Ooct–OPV5– CNY in solution and the single crystal, single-exponential decay curves were observed corresponding to PL lifetimes of 1.4 and 8 ns, respectively. They are representing the two extremes: single-molecule and excimer luminescence. The PL-decay curves for the as-deposited and annealed films show double-exponential dependencies with lifetimes of 1.4r7.0 ns Žt 1rt 2 . and 3.4r8.0 ns, respectively. The contribution of the fast decay associated with single-molecule luminescence decreases upon annealing, which is in agreement with the observed increase of the crystalline fraction. Closer inspection of the diffraction patterns has revealed that the structure in the film is not identical with the single-crystal structure. While this does prevent us from giving a clear-cut interpretation as in the case of the single crystal, it does point out that an essential feature of organic compounds, polymorphism of the crystalline form, has to be taken into account. Our recent results indicate that different
Fig. 9. Structure of the octyloxy- and cyano-substituted copolymer of mixed PPP–PPV-type which shows long-lived, red-shifted luminescence w25x.
P.F. Õan Hutten et al.r Chemical Physics 241 (1999) 139–154
crystallization procedures may lead to different morphologies as well as different crystalline structures. This will have a strong influence on both optical and electrical properties of films, and affect their behaviour in electroluminescent devices. This aspect is currently under investigation. In our earlier studies on electroluminescence, we have studied a variety of alternating copolymers with regularly interrupted conjugation w24,25x. The polymer shown in Fig. 9 was found to behave similar to Ooct–OPV5–CNY and the cyano-PPV reported by Samuel et al. w5,6x. This compound shows an appreciable red-shift of the absorption and luminescence spectra of the film relative to those of the solution, a broad and featureless luminescence spectrum and a PL lifetime of 2.6 ns, relatively long in comparison with related copolymers investigated, but still short in comparison with 5.6 ns reported for cyano-PPV w5,6x. Possible reasons for the difference in PL lifetime could be the intermolecular distance or displacement Žthe axial register is likely to have a strong influence in the case of the copolymer., which together determine the magnitude of p-orbital overlap, or the degree of symmetry of the ensemble of interacting chain segments, which also affects the emission probability. In spite of this difference, we suggest that also in the case of the copolymer, the molecular packing gives rise to luminescent interchain excitations.
4. Conclusion Our study of model compounds indicates that the probability of the occurrence of interchain excitations will depend strongly on the juxtaposition of neighbouring chains Žsegments.; this, in turn, will be strongly influenced by the nature of the substituents and their pattern on the chain. The correspondence between p-stacked molecular organization and long-lived emission, as found for the model compound Ooct–OPV5–CNY , has led to an interpretation in terms of excimer-like excitations. This picture permits us to explain previous experimental observations for homopolymers that carry cyano substituents on the vinylenes and alkoxy substituents on the rings, and also for copolymers consisting of structurally similar base units.
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Acknowledgements The authors are grateful to R.E. Gill, M.P.L. Werts and J. Wildeman for their contributions in chemical synthesis, and to A. Hilberer and A. Meetsma for the crystal structure analysis. This research was financially supported by the Netherlands Organization for Scientific Research ŽNWOSONrSTWrFOM..
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