J. Phys. Chem. B 2001, 105, 6103-6107
6103
Structural and Electronic Characterization of Chemical and Conformational Defects in Conjugated Polymers Kim F. Wong, Munir S. Skaf,† Chao-Yie Yang, Peter J. Rossky,* Biman Bagchi,‡ Dehong Hu, Ji Yu, and Paul F. Barbara Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712-1167 ReceiVed: January 31, 2001; In Final Form: April 23, 2001
Quantum chemical analyses of poly(p-phenylenevinylene) (PPV)-based polymers with conformational and chemical defects are presented. It is shown that defects leading to tetrahedral kinks in the polymerization direction that were proposed previously to explain the optical anisotropy of polymer films (Hu et al. Nature 2000, 405, 1030) are indeed energetically stable. These defects are shown not to affect the primary electronic absorption band, but they may localize the electron-hole excitation in one segment of the polymer chain.
Introduction Conjugated polymers comprise a class of novel molecularbased semiconductors not only possessing the optoelectronic properties of Si, Ge, group III-V, and group II-VI inorganic semiconductors but also possessing the processing advantages and mechanical properties of polymers. Conjugated polymer devices are typically fabricated as films by spin casting from a solution of high concentration. In particular, much effort has been invested in exploiting poly(p-phenylenevinylene) (PPV) and related classes of polymers as templates for the fabrication of displays.1-3 By adding and modifying substituent side chains to the active conjugation backbone, one chemically controls the full range of luminescence colors. More recently, it has been shown that the luminescence and other optoelectronic properties can be controlled on a macroscopic scale by the careful choice of solvent and concentration of the precursor solution.4,5 This prospect of solution processibility and chemical control also allows opportunities, during processing, for the formation of defects. Conjugated polymers in nature need not be the idealized structures that chemists invoke for describing the observed electronic and optical properties of this type of material. The conjugation backbone of a polymer, for example, is not simply a planar monomer unit replicated to macroscopic dimensions,6 as the structure inherently includes defects. A variety of experimental studies show that defects from the idealized structure occur. These can be chemical defects due to postsynthesis photooxidation7-10 or due to the synthesis itself,11,12 conformational distortions arising from the thermal environment,13,14 or macroscopic inhomogeneities due to aggregation.15-17 Such defects can strongly influence the optical, electronic, and mechanical properties of the material. Nevertheless, the experimental absorption spectrum is recovered from theoretical calculations which consider the individual quasichromophores as consisting simply of straight chains with a small number (∼7-20) of monomer units.18-20 In contrast, the electronic properties of polymers with chemical or conformational defects have yet to be systematically addressed from a theoretical standpoint. †
Permanent address: Instituto de Quı´mica, UNICAMP, Cx. P. 6154, Campinas, SP 13083-970, Brazil. ‡ Permanent address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
Most theoretical studies of polymer conformations have concentrated on the thermodynamic collapse transition from the high temperature coil structure to the lower temperature dense globule. Off-lattice Monte Carlo simulations recover equilibrium random coil, rod, toroid, and molten globule states, depending on the choice of chain stiffness and intersegment attraction.21-23 By providing for the possibility of tetrahedral defects, where conjugated carbon-carbon bonds are replaced by tetrahedral kinks, within a lattice Monte Carlo technique, two new thermodynamically accessible states, “defect-coil” and “defectcylinder”, were shown to exist.24 Defect-coils adopt extended anisotropic conformations while defect-cylinders collapse into ordered quasi-cylindrical structures. The polymer aggregates, characterized by the aftermath of the conformational collapse brought about by torsional and/or chemical tetrahedral defects, reproduce the experimental fluorescence anisotropy distribution of poly(2-methoxy-5-(2′-ethylhexyl)oxy-1,4-phenylenevinylene) (MEH-PPV) films very well.24 Moreover, the structurally identifiable quasi-straight-chain segments common to both types of conformations can be naturally associated with the localized quasi-chromophores commonly utilized for modeling the electronic properties of this class of polymers. Clearly, the success of the Monte Carlo simulations in describing the experimental optical anisotropy of thin polymer films is conditional on the existence of tetrahedral defects within the structure. In this work, we show through quantum chemical calculations that the conformational and/or chemical tetrahedral defects are energetically stable, thus providing a compelling microscopic basis for the underlying assumption implicit in the simulation. Using the AM125 and PM326 semiempirical Hamiltonians, we have performed ground-state geometry optimization on model polymers of PPV and MEH-PPV in which torsional and chemical defects are introduced. The influence of such defects on the absorption spectrum of the polymer is examined by calculating the singlet excitation energies from the corresponding optimized geometries using the Pariser-Parr-Pople (PPP) Hamiltonian27,28 coupled with single configuration interaction.29 Computational Framework We have considered isolated oligomers of PPV and a model of MEH-PPV in which the (2-ethylhexyl)oxy substituent is replaced by an ethyloxy group [hereafter denoted as ME-PPV
10.1021/jp010392b CCC: $20.00 © 2001 American Chemical Society Published on Web 06/01/2001
6104 J. Phys. Chem. B, Vol. 105, No. 26, 2001
Wong et al. cessfully to related conjugated systems, including conjugated ring heterocycles.19,31,32,36 Results and Discussion
Figure 1. Molecular structures for PPV representing the AM1 optimized geometries for the linear chain (top), cis-defect (middle), and sat-defect (bottom).
for poly(2-methoxy-5-ethyloxy-1,4-phenylenevinylene)]. It is generally agreed that the alkyl chains terminating the substituent side groups in MEH-PPV influence the solubility of the polymer and the spatial morphology of films but have little or no impact on the optoelectronic properties of an isolated polymer, because alkyl groups do not contribute to the π-conjugation network. Two types of defects, which can give rise to a physical kink in the polymer backbone, were individually introduced in the middle of a polymer consisting of 10 aromatic units (Figure 1): The “cis-defect” is a conformational isomer of the linear chain in which one vinylene group takes on a predominantly cis conformation. A “sat-defect” results from the saturation of a CdC vinylene bond followed by rotation around the resulting single bond. The sat-defect introduced here represents an idealized model defect that effectively interrupts the conjugation of the polymer chain. We have also considered saturation defects containing carbonyl groups7,10 and find similar structural and electronic features; we do not report those calculations here. Our choice of system size was made on the basis of previous theoretical and experimental analyses which have indicated that the conjugation length of these polymers is of the order of six or seven repeat units.19,20 The ground-state optimized geometries and heats of formation of the isolated oligomers with and without defects were obtained within the semiempirical SCF (single determinant) AM1 and PM3 models.25,26,30 These Hamiltonians have been shown to provide an accurate description of the geometry of conjugated molecules, with AM1 providing somewhat superior results.19,31 From the ground-state optimized geometries, the electronic structure of the π-conjugated systems was determined using the PPP Hamiltonian. One-center repulsion and core parameters were derived from the spectroscopic data of Hinze and Jaffe.32,33 The two-electron repulsion matrix elements were obtained from the Mataga-Nishimoto relationship,34 while the resonance integrals follow directly from the Linderberg equation35 using Slater-type 2p atomic orbitals. Electronic transition energies to excited states and the corresponding oscillator strengths were obtained from configuration interaction with all possible single excitations (SCI). Similar procedures have been applied suc-
Defects that alter the spatial direction of backbone polymer propagation ultimately determine the macroscopic conformational state of the chain and the final morphology of the material film. While the experimental anisotropy measures an ensemble distribution of quasi-chromophores delineated by these structural defects, the microscopic nature of these sites is largely unknown. In this section, we begin with an identification of these tetrahedral sites that define the boundary between neighboring quasi-chromophores on the same chain. The stability of these sites is addressed, and implications of their existence on the primary electronic transition are quantitatively assessed. Figure 1 displays the optimized molecular conformations for the 10-ring PPV chain obtained using the AM1 model. The fully trans conformation (top) corresponds essentially to the idealized planar structure. In the cis-defect PPV (middle), the optimized geometry is such that the 5-ring subunits twist in opposite directions with respect to the central vinylene bond. The dihedral angle between the aromatic rings defining the cis-defect site is ∼144°. To accommodate the defect, the structure of the 5-ring segments undergo distortions from the planar conformation of the parent chain. A similar distortion from planarity is also observed in the sat-defect (bottom), where rotation about the saturated C-C bond occurs with low energy cost. Most importantly, the angles formed between the two 5-ring branches in the optimized defect chain are approximately 99° and 115° for the cis- and sat-defects, respectively. These values fall within the range of tetrahedral geometries that allows for the formation of the equilibrium “defect-coil” and “defect-cylinder” structures previously identified as responsible for the observed optical anisotropy of thin polymer films.24 In contrast to PPV, the linear ME-PPV chain (Figure 2, top) exhibits a mild continuous helical progression along the long chain axis due to steric hindrance of the methoxy and ethyloxy side groups. This twist is characterized by an approximately 3° dihedral angle between the planes of adjacent aromatic rings. Although the dihedral angles for the cis- (middle) and sat-defect (bottom) sites of ME-PPV are similar to those of PPV, the 5-ring segments in ME-PPV deviate much less from planarity than their respective PPV counterparts. This enhancement in polymer stiffness reflects the role of the side chains in stabilizing the polymer backbone in the presence of structural or chemical defects and further suggests that the effective π-conjugation length is longer in MEH-PPV than in PPV. Performing ground state geometry optimization based on the PM3 model provided results that are very similar to those discussed above. A structurally identifiable difference is the less prominent deviation from planarity of the half-chains defining the cis- and sat-defects in the PM3 than in the AM1 geometries. Although the vinylene bond is shorter than the conjugated bonds joining it to adjacent phenylene units, the out-of-plane distortion in the PM3 optimized structure is distributed more evenly among the three chemical bonds. The AM1 optimized structure, on the other hand, maintains the planarity of the vinylene units and distributes the distortion only between the two longer conjugated bonds. This differentiation between rotation about the stronger vinylene bond (higher energy barrier) and rotation about a weaker conjugated bond (lower barrier) agrees with physical intuition. While an artificially created twist in the middle of a linear polymer chain relaxes to an optimized structure in which the
Defects in Conjugated Polymers
J. Phys. Chem. B, Vol. 105, No. 26, 2001 6105
Figure 2. Molecular structures for the ME-PPV representing the AM1 optimized geometries for the linear chain (top), cis-defect (middle), and sat-defect (bottom).
TABLE 1: Heats of Formation for 10-Ring Oligomers of PPV and ME-PPV Polymers with and without Conformational Defects Obtained from AM1 and PM3 Semiempirical Methods and First Optically Allowed Transition Energies and the Corresponding Oscillator Strengths (in Parentheses) Obtained from a SCI-PPP Calculation at the AM1 and PM3 Geometries ∆Hf/kcal mol-1 AM1
PM3
linear cis-defect
370.4 372.8
PPV 361.5 365.8
sat-defect
345.6
339.9
linear cis-defect
-413.6 -410.2
ME-PPV -413.8 -410.7
sat-defect
-437.6
-438.4
transition energy/eV AM1
PM3
3.13 (7.1) 3.37 (4.7) 3.47 (2.4) 3.44 (5.0) 3.48 (1.7)
3.20 (6.9) 3.31 (4.2) 3.37 (2.8) 3.37 (4.8) 3.40 (1.7)
3.11 (5.7) 3.16 (3.7) 3.24 (2.0) 3.20 (2.3) 3.24 (2.8)
3.14 (6.7) 3.24 (3.9) 3.30 (2.7) 3.25 (3.0) 3.30 (3.4)
defect site structurally resembles a tetrahedral defect, it is essential to assess the relative stability of these newly created defects. The stability of the cis- and sat-defect oligomers relative to the linear polymer can be gauged through the heats of formation (Table 1) of these different species. The results show that the oligomers with a single saturation defect are stabilized by approximately 25 kcal/mol relative to the unsaturated linear chain. This large energy difference reflects the formation of two new carbon-hydrogen bonds at the expense of breaking a carbon-carbon double bond and accounts for the exothermicity associated with alkene hydrogenation. Clearly, saturation is a global minimum among these species. The cis-defect and linear chains, on the other hand, have naturally comparable heats of formation. The interesting observation is that the cost of cis-
Figure 3. First allowed band of the optical absorption spectra for the AM1 optimized PPV (top) and ME-PPV (bottom) chains obtained from the PPP Hamiltonian with single CI. Solid, dashed, and dotted lines correspond to linear, cis-defect, and sat-defect chains, respectively.
defect formation is only about 2-4 kcal/mol. Therefore, a fraction of polymer chains with cis-defects is not unlikely if, for example, their formation were coupled to the formation of attractive intrapolymer contacts. Because the observed absorption spectra of conjugated polymers can be recovered simply by treating the polymer as a linear chain of a few monomer units without defects,19 an important aspect that needs to be resolved is the influence of these defects upon the electronic absorption spectra. The first few calculated transitions from the ground-state AM1 geometry for all compounds are shown in Figure 3, where the sticks represent the actual values of the dipole oscillator strengths and the bands are obtained by convoluting the stick spectra with a Gaussian of width 0.3 eV. This width has been used in earlier spectral fitting by others.18,19 The first absorption for all trans PPV is in very good agreement with experimental measurements and theoretical calculations available in the literature.19,37-39 For trans-ME-PPV, the excitation energies are in good agreement with previous calculations for a similar oligomer performed at a similar level of theory,19 although the experimental principal absorption band of actual MEH-PPV is centered at 2.5 eV for both film and solution.4,40,41 Upon the inclusion of defects, the spectra for both types of polymers appear slightly blue shifted with respect to those for the longer linear chains, in agreement with the expected length dependence of conjugated polymers.31,42 The blue shift is somewhat smaller in ME-PPV, because the minimized distortion from planarity of the conjugation backbone by the side chains provides for better π-electron delocalization. For both materials, the primary transition is split into two transitions of lower intensity separated by less than 0.1 eV. This behavior is explained simply by the fact that the defect redefines the π-network of the long chain in two smaller subunits with each segment contributing to the excitation, thus giving rise to the splitting.43 Differences between the spectra
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Wong et al. that their presence in reasonable numbers does not alter the optical absorption properties of the material. From semiempirical quantum chemistry calculations of the structural cis-defect and the chemical sat-defect, both of which produce a similar structural kink in the polymer backbone, we have shown that these defects are reasonably thermodynamically accessible during synthesis or film preparation. We note that the formation of the cis-defect from an existing all trans structure on the electronic ground state surface will necessarily involve a very large barrier to rotation associated with disruption of π-bonding. Therefore, such a route is unlikely to be kinetically significant via thermal processes. Calculations at the same level as the calculations reported here confirm this expectation of a high barrier (∼56 kcal/mol). However, the possibility exists that these cis structures could be reached via photoexcitation pathways or during polymer synthesis. Furthermore, the features of the absorption spectrum are tolerant of defects as long as the defect sites are not so frequent as to delineate quasi-chromophores having less than the characteristic conjugation length. As a result, the defect structures hypothesized in our earlier work24 appear well justified.
Figure 4. Graphical representation of the electron-hole two-particle distribution for the PPV linear (top), cis-defect (middle), and sat-defect (bottom) chains, indicating the probability of finding the electron, assuming the hole was created at the marked site.
of the linear and defect chains diminish considerably with increasing conjugation length. The primary singlet excitation energies calculated for 8-ring and 20-ring PPV linear systems, as well as for 20-ring defect chains (with branches containing 8 and 12 repeat units), differ by less than 0.04 eV. Understanding the nature of the electronic excitations in these materials is critical to controlling their optoeletronic properties. Although the nature of the excited states in conjugated polymers has been a topic of debate, there is now general agreement that the excited state responsible for the primary transition in PPV, MEH-PPV, and other PPV derivative polymers can be characterized as a singlet exciton.44 Within the SCI formalism, the spatial representation of the exciton can be conveniently characterized in terms of the electron-hole two-particle distribution, ΨSCI(xe,xh), which describes the probability amplitude for finding an electron at atomic orbital site xe when the hole is created on site xh.42,45,46 The two-particle function ΨSCI(xe,xh) corresponding to the primary excitation, assuming the hole was created in the middle of the chain near the point of defect,47 is displayed in Figure 4. For the linear conjugated polymer, the electron density corresponding to the primary optical absorption remains bound to the hole and delocalizes over approximately two to three monomer units, in agreement with the previous analysis.40 For a cis-defect chain, the excitation also delocalizes the density across multiple phenylene units, extending to both sides of the defect. In contrast, excitation on a sat-defect chain localizes the wave function on the same side of the defect where the hole was created, because conjugation is effectively terminated by the formation of this type of defect. The vinylene saturation localizes and more clearly defines distinct quasichromophores within the polymer material than does the cisdefect. Concluding Statements What we have shown in this study is that defects are thermodynamically plausible within conjugated polymers and
Acknowledgment. This work has been supported by a grant from the National Science Foundation. M.S. thanks the Brazilian agency FAPESP (99/09879-6) for support and the UT Institute for Theoretical Chemistry for its hospitality. C.-Y.Y. acknowledges support by an NSF-CISE postdoctoral grant. We thank J. F. Stanton and D. M. Lockwood for helpful discussions. References and Notes (1) Bross, M.; Mu¨ller, D. C.; Nothofer, H.-G.; Scherf, U.; Neher, D.; Bra¨uchle, C.; Meerholz, K. Nature 2000, 405, 661. (2) Kido, J. Phys. World 1999, 12, 27. (3) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (4) Nguyen, T.-Q.; Doan, V.; Schwartz, B. J. J. Chem. Phys. 1999, 110, 4068. (5) Nguyen, T.-Q.; Martini, I. B.; Liu, J.; Schwartz, B. J. J. Phys. Chem. B 2000, 104, 237. (6) Mao, G.; Fischer, J. E.; Karasz, F. E.; Winokur, M. J. J. Chem. Phys. 1993, 98, 712. (7) Yan, M.; Rothberg, L. J.; Papadimitrakopoulos, F.; Galvin, M. E.; Miller, T. M. Phys. ReV. Lett. 1994, 73, 744. (8) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock, E. W.; Miller, T. M. Synth. Met. 1996, 80, 41. (9) English, D. S.; Furube, A.; Barbara, P. F. Chem. Phys. Lett. 2000, 324, 15. (10) Cumpston, B. H.; Parker, I. D.; Jensen, K. F. J. Appl. Phys. 1997, 81, 3716. (11) Hale, G. D.; Oldenburg, S. J.; Halas, N. J. Appl. Phys. Lett. 1997, 71, 1483. (12) Zyung, T.; Kim, J. Appl. Phys. Lett. 1995, 67, 3420. (13) Chen, L. X.; Ja¨ger, W. J. H.; Niemczyk, M. P.; Wasielewski, M. R. J. Phys. Chem. 1999, 103, 4341. (14) Huser, T.; Yan, M.; Rothberg, L. J. Proc. Natl. Acad. Sci. 2000, 97, 11187. (15) Nguyen, T.-Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000, 288, 652. (16) Nguyen, T.-Q.; Kwong, R. C.; Thompson, M. E.; Schwartz, B. J. Appl. Phys. Lett. 2000, 76, 2454. (17) Blatchford, J. W.; Gustafson, T. L.; Epstein, A. J.; Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F.; Fu, D.-K.; Swager, T. M.; MacDiarmid, A. G. Phys. ReV. B 1996, 54, R3683. (18) Garstein, Yu. N.; Rice, M. J.; Conwell, E. M. Phys. ReV. B 1995, 52, 1683. (19) Cornil, J.; Beljonne, D.; Friend, R. H.; Bre´das, J. L. Chem. Phys. Lett. 1994, 223, 82. (20) Pilcher, K.; Halliday, D. A.; Bradley, D. D. C.; Burn, P. L.; Friend, R. H.; Holmes, A. B. J. Phys.: Condens. Matter 1993, 5, 7155. (21) Kuznetsov, Yu. A.; Timoshenko, E. G. J. Chem. Phys. 1999, 111, 3744. (22) Noguchi, H.; Yoshikawa, K. J. Chem. Phys. 1998, 109, 5070. (23) Doye, J. P. K.; Sear, R. P.; Frenkel, D. J. Chem. Phys. 1998, 108, 2134.
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