Substituent Effects upon Protonation-Induced Red Shift of Phenyl

Steve Scheiner*, and Tapas Kar ... Daniela Otto, Michael Waidelich, Michael Towler, Rick White, Martin Albrecht, ... Mingxiu Liu , Qingzhong Li , Stev...
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J. Phys. Chem. B 2002, 106, 534-539

ARTICLES Substituent Effects upon Protonation-Induced Red Shift of Phenyl-Pyridine Copolymers Steve Scheiner* and Tapas Kar Department of Chemistry & Biochemistry, Utah State UniVersity, Logan, Utah 84322-0300 ReceiVed: May 30, 2001; In Final Form: NoVember 6, 2001

Ab initio methods are used to probe the underlying source of the red shift that arises in the absorption band of polymers that contain alternating phenyl and pyridine rings when the N atom of the latter is protonated. It is found that electronic charge transfers from the phenyl to the pyridine as an electron is excited from the HOMO to the LUMO in the protonated system but that no such transfer occurs if the N is unprotonated. Replacement of H atoms by cyano groups reinforces the idea that such an electron-withdrawing substituent facilitates the electron transfer when it is located on the pyridine ring, thereby stabilizing the excited state and thus enhancing the associated red shift; the opposite occurs when cyano is located on the phenyl ring. Unlike cyano and Cl, substitution by F and nitro groups leads to a less consistent pattern of absorption frequencies, suggesting that the effects of these substituents are not simply electrostatic in nature.

Introduction The finding that poly(p-phenylene vinylene) (PPV) had electroluminescent properties1 spurred a great deal of interest, which continues unabated, in conjugated polymers as emitting agents.2-8 The incorporation of conjugated polymers into lightemitting diodes is associated with a number of advantages including stability, ease of synthesis, low drive voltage, good efficiency, mechanical flexibility, and a wide spectral range.9-14 Recently, a group of polymers has been developed with particularly interesting and potentially useful properties. These polymers incorporate a pyridine ring,15,16 which invests the polymer with a high electron affinity that enables the use of metals as efficient electron-injecting contacts.13 Other advantages include greater ease of synthesis and resistance to oxidation and better electron-transport properties. These systems are highly luminescent, while fluorescing in the blue part of the spectrum. Moreover, the presence of a N atom, which can be protonated or have another charged group added to it, permits a certain degree of tuning of the emission wavelength. The possibility of joining the pyridines in head-to-head or head-to-tail arrangements affords additional options into the properties.17 It has been shown that the photoinduced absorption in these polymers is due to an intrachain singlet exciton and not to polaron pairs.18 One of the most interesting properties of these pyridyl polymers is the large red shift that occurs in their optical absorption upon protonation or alkylation of the pyridyl nitrogen.7,12 This shift is presumed to involve a charge transfer from the phenyl ring to the pyridine, a phenomenon that is stabilized by protonation of the N atom on the latter ring.12 The magnitude of this shift can be as large as 71 nm, depending on the nature of the particular polymer. This charge-shift idea is reminiscent of other polymer systems.19,20 * To whom correspondence should be addressed. E-mail: scheiner@ cc.usu.edu.

Figure 1. Segment of a copolymer of poly(p-pyridyl vinylene) (PPyV) and poly(p-phenylene vinylene) (PPV). R represents various groups such as C12H25, OC16H33, and COOC12H25.

A particular example of such a system, a copolymer of poly(p-pyridyl vinylene) (PPyV) and poly(p-phenylene vinylene) (PPV), is illustrated in Figure 1, where the R group refers to a number of different choices such as C12H25, OC16H33, and COOC12H25.19 The aim of the present communication is to use molecular orbital methods to probe the nature of the red shift that occurs in the absorption frequency upon protonation or alkylation of the pyridyl nitrogen. A better understanding of the underlying source of this phenomenon would enable the design of derivative systems that have superior properties of luminescence and would permit a certain degree of tuning of the absorption frequency. Methods To study this system in some detail and by methods of suitable accuracy, the polymer was modeled by just one of its subunits. This model would appear to be appropriate for a number of reasons. First, the model would certainly incorporate the possibility of charge shift from the phenyl to the pyridyl ring, which has been presumed to be the source of the observed red shift. Second, the conducting polymer’s band gap (HOMOLUMO) is largely determined by its local electronic structure, as are electrochemical oxidation and reduction potentials,7 so the localized model ought to reflect these properties in the entire polymer. The actual system studied by the calculations is illustrated in Figure 2, which also reports the atomic numbering

10.1021/jp012049c CCC: $22.00 © 2002 American Chemical Society Published on Web 12/15/2001

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J. Phys. Chem. B, Vol. 106, No. 3, 2002 535

Figure 2. Molecule studied, showing atomic numbering scheme.

Figure 4. HOMO (lower) and LUMO (upper) of the protonated system with an additional H added to N of the pyridine ring. Contours are spaced at 0.0125 au intervals.

Figure 3. HOMO (lower) and LUMO (upper) of the unprotonated system. Contours are spaced at 0.0125 au intervals.

scheme used here. This model focuses its attention on the polymer wherein the R groups on the phenyl ring represent an ether linkage rather than a hydrocarbon, although some calculations reported below explore the latter option as well. All calculations have been carried out using the Gaussian 98 (revision A.7) program.21 Geometries of the various molecules were fully optimized at the SCF level, using both the 3-21G and 6-31G* basis sets. (In some cases, the geometry was forced to remain planar.) To determine the vertical excitation energies of the first singlet states (1ππ*), the CIS method22 was applied to the geometry optimized for the ground state. Atomic charges of both ground and excited states were obtained using Mulliken and natural population analyses.23,24 Results and Discussion Pictorial Representations. One means of considering possible shifts of charge is via examination of the chief MOs that are involved. The principal photoexcitation responsible for the optical properties of the system of interest corresponds to an excitation of an electron from the HOMO to the LUMO. Figure 3 illustrates the HOMO of the unprotonated molecule and the LUMO into which this electron is deposited. There are certain differences in nodal structure that are readily apparent. One

example is that whereas there is a strong bonding interaction between C7 and C8, the two C atoms that bridge the two aromatic rings, in the HOMO, this interaction is decidedly antibonding in the LUMO with a node separating the two atoms. Of greatest concern, however, is the amount of electron density on each of the two rings of the molecule. In that respect, examination of the two MOs reveals little obvious difference. Visual inspection does not indicate any particular distinction between the HOMO and LUMO with regard to the total electron density on one ring or the other. This situation is quite different when considering the HOMO/ LUMO pair in the protonated analogue. Even a cursory inspection of the two MOs in Figure 4 leads to the conclusion that the density of the phenyl ring on the left is greater than that of the right pyridine ring in the HOMO, whereas the exact opposite is true in the LUMO. In short, the excitation of an electron from the HOMO to the LUMO of the protonated species would appear likely to result in significant transfer of electron density from left to right, whereas no such transfer would seem to occur in the unprotonated molecule. Examination of individual MOs can provide strong indications of charge shift, but a more thorough inquiry must address possible changes in all of the occupied MOs that are perturbed by the electronic excitation not just the HOMO and LUMO. For that reason, it is advisable to consider the total electron density in each system of interest. The upper part of Figure 5 illustrates the loss or gain of total density that accompanies electron excitation in the neutral (unprotonated) molecule by red and blue shading, respectively. Each ring manifests certain localized regions of either loss or gain; however, neither ring seems to have a preponderance of one color over the other. This near equality is consistent with the indication of little charge shift in the unprotonated system arising from inspection of the individual MOs. Further supporting the above

536 J. Phys. Chem. B, Vol. 106, No. 3, 2002

Scheiner and Kar TABLE 1: Natural Population Atomic Charges, Reported as Group Charges on the Two Rings, Computed at the 6-31G* Level and Changes that Are Associated with Excitation

Figure 5. Electron density shifts associated with HOMO f LUMO excitation in the (upper) unprotonated and (lower) protonated systems. Gain of electron density is shown in blue and loss in red.

Figure 6. Difference in electron density shifts associated with HOMO f LUMO excitation between unprotonated and protonated systems. That is, the upper part of Figure 5 is subtracted from the lower part. Thus, the gain of electron density, shown in blue, represents a greater accumulation of electron density in the protonated than in the unprotonated case.

idea that density shifts from left to right in the protonated system, it would appear that the right ring has more blue region than red in the lower part of Figure 5, while red predominates in the left ring. This fundamental difference between the unprotonated and protonated cases is perhaps more obvious in Figure 6 which illustrates the difference in charge shift between these two cases. The largely blue character of the right pyridine ring confirms the contention that this ring acquires more electron density in the protonated case, and vice versa for the predominately red character of the left ring. Quantitative Representations. The pictorial representations of charge shifts considered to this point are useful and provide some real insights but are difficult to quantify. For that reason, an additional tack was taken wherein the total electron density is assigned to individual atoms, leaving each with a nonintegral charge. By summing the charges on the appropriate atoms, it is

phenyl ring

pyridine ring

S0 charge ππ* charge change

Unprotonated -0.028 -0.014 +0.014

-0.063 -0.073 -0.010

S0 charge ππ* charge change

Protonated 0.053 0.297 +0.245

0.751 0.483 -0.268

S0 charge ππ* charge change

Methylated 0.049 0.284 +0.234

0.762 0.522 -0.235

possible to assign a group charge to each ring of the system. The first row of Table 1 indicates that in the ground electronic S0 state of the unprotonated molecule both rings bear a small negative charge, less than 0.1 in magnitude. These group charges are little changed upon excitation to the ππ* state, indicated in the second row. Indeed, the third row confirms the earlier finding that there is no appreciable charge transfer induced by electron excitation in the unprotonated system with changes of only (0.01. The situation is quite different after the system has been protonated. The fourth row of Table 1 indicates that most, 75%, of the positive charge of the system, resides on the pyridine ring in the ground electronic state. Unlike the unprotonated case, the charges of the two rings are both changed a great deal by electronic excitation to ππ*. As reported in the sixth row of Table 1, roughly 0.25 units of electronic charge are lost from the phenyl ring and appear on the pyridine ring, as a result of the excitation. This numerical data provides further confirmation of the pictorial representations of charge shifts reported above. Estimation of atomic charges requires certain arbitrary choices to be made in terms of boundaries of each atom. Hence, there are numerous ways in which one can assign atomic charges. The data listed in Table 1 were extracted using the natural population scheme. To be sure that this particular approach did not introduce an artifact into the findings, the computations were repeated and the charges computed using the Mulliken scheme. The conclusions are precisely the same; the only difference is a small quantitative change. The Mulliken excitation-induced density shifts in the unprotonated case are within 0.003 of the natural population data. Mulliken densities also shift electrons from the phenyl to the pyridine ring in the protonated system in the amount of 0.22, as compared to the 0.25 natural population shift. To determine whether the effects noted above are unique to the protonation of the nitrogen atom, similar calculations were carried out wherein the N is methylated by a -CH3+ group. The results were very much the same as in the protonated system, first of all in that the HOMO/LUMO topologies of the two are quite similar. On a quantitative level, the last three rows of Table 1 reinforce the idea that methylated and protonated systems show very similar charge patterns. The charges of the two rings in these two cases differ by very small amounts, and both show the phenomenon of charge transferred from the phenyl to the pyridine ring as a consequence of ππ* excitation. A more refined breakdown of this charge transfer can be gleaned from Table 2, which displays the change in the charge of each atom of the system that occurs upon electronic excitation. This table reports both Mulliken and natural population charges for purposes of noting artifacts that might be

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J. Phys. Chem. B, Vol. 106, No. 3, 2002 537

TABLE 2: Changes in Individual Atomic Charges Caused by ππ* Excitation unprotonated

protonated

atom

Mulliken

natural

Mulliken

natural

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 N13 C14 C15 C16 O17 O18

0.011 -0.006 -0.005 0.016 -0.005 -0.015 0.002 -0.007 0.003 -0.017 0.020 -0.024 0.008 -0.003 0.008 -0.007 0.003 0.012

0.021 -0.002 -0.021 0.037 -0.005 -0.037 0.010 -0.009 0.016 -0.034 0.027 -0.028 0.004 0.007 0.004 -0.005 0.002 0.011

0.021 0.022 -0.008 0.036 0.027 -0.013 -0.021 0.058 -0.038 -0.080 0.041 -0.088 -0.013 0.039 0.031 -0.042 0.015 0.030

0.041 0.047 -0.029 0.068 0.064 -0.043 -0.055 0.127 -0.068 -0.136 0.062 -0.125 -0.027 0.069 0.041 -0.070 0.014 0.028

introduced by one method or the other. Examination of the first two columns of Table 2 indicates that the atomic charges are relatively unaffected by this excitation when it occurs in the unprotonated molecule. The largest changes are associated with C10, C11, and C12, all on the pyridine ring, but these changes are all less than 0.03. (A slightly larger change is associated with the natural population charge of C6 on the other ring, but the magnitude of this change is halved in the Mulliken scheme.) Atomic charge changes are clearly magnified in the protonated system. Focusing first on pyridine, C10 and C12 both become more negative by 0.1 or more. C9 and N13 also acquire more negative charge, albeit by a smaller amount. The two other atoms of this ring, C11 and C14, become more positive by around 0.04-0.07, smaller in sum than the negative changes of the other four atoms and consistent with the overall shift of charge from phenyl to pyridine. Also consonant with this overall shift are the generally more positive charges of the atoms C1 through C6, plus O17 and O18, on the phenyl ring. Of these atoms, C4 and C5 would appear to undergo the largest changes. With regard to the two bridging atoms, C8 becomes more positive by a relatively large amount. Of ultimate concern is the energy required to excite the system from the ground to the ππ* state. Using the 6-31G* basis set, this excitation requires 4.72 eV in the unprotonated system and 4.33 eV after protonation. Thus protonation can be said to cause a 0.39 eV red shift in the absorption band, consistent with the large shift that has been observed experimentally. It is important to note at this point that the direction and even the magnitude of this shift are fairly insensitive to basis set. A 3-21G computation of the same quantity yields a red shift of 0.42 eV. Fluorosubstitutions. As a first step toward modulating the protonation-induced red shift of the excitation, a number of different H atoms located on the phenyl or pyridine rings, or the bridge between them, were replaced by fluorine. It might be thought that because the red shift appears to be associated with the observation that protonation of the N atom facilitates the transfer of electron density from the phenyl ring to pyridine upon excitation, replacement of H by the much more electronegative F atom ought to interfere with this process in some way. Atoms C2 and C6 are both located on the phenyl ring. Nonetheless, F-substitution on these two atoms has different effects upon the red shift. As may be seen in the second and third rows of Table 3, F-substitution on C2 has little effect upon

TABLE 3: Excitation Energies (eV) of Unprotonated and Protonated Species, the Magnitude of the Red Shift Caused by Protonation, and the Effect on These Quantities of Substitution of H by F at the Indicated Atomsa none C2 C6 C8 C10 C14

unprotonated

protonated

red shift

substituent effect

4.72 4.76 4.75 4.78 4.75 4.73

4.33 4.35 4.45 4.30 4.36 4.19

0.39 0.40 0.31 0.48 0.38 0.54

0.01 (-0.01) -0.08 (-0.10) 0.09 (0.09) -0.01 (0.03) 0.15 (0.19)

a Results calculated at the 6-31G* level (except for 3-21G data in parentheses)

the red shift, while a reduction by 0.08 eV arises when the replacement occurs at C6. Similarly contrasting behavior is observed when the replacement occurs at two sites on the pyridine ring, C10 and C14. It might also be noted that inclusion of a F atom on the bridge between the two rings, at C8, results in a significant enhancement of the red shift. The 3-21G values in parentheses in the last column of Table 3 reveal trends that parallel the 6-31G* data. There does not appear to be an obvious correlation between the magnitude of the substituent effect upon the red shift on one hand and the sensitivity of the atomic charge to HOMO f LUMO excitation on the other. For example, even though C10 undergoes perhaps the largest change in its charge of any atom in the system upon excitation of the protonated system (see Table 2), F-substitution at this atom results in a negligible change in the red shift. A much larger red-shift perturbation is associated with substitution at C14, yet this atom undergoes only a moderate change in its charge. The same is true for the two atoms on the phenyl ring. Whereas the charges of C2 and C6 are affected to a similar degree by excitation, substitution at the former has no effect upon the red shift, while this quantity is diminished substantially by C6-substitution. Also, one cannot draw parallels between the data in Table 3 and the nature of the MOs in Figures 3 and 4. Taking the phenyl ring as an example, C2 is associated with greater electron density than C6 in the relevant frontier MOs, yet it is the latter atom at which substitution has a greater effect upon the red shift. A like contrast occurs for the pyridine ring in which it is C10 that is more populated in the frontier MOs but C14-substitution that influences the red shift. In summary, C6, C8, and C14 appear to be “hot spots” with respect to where F-substitution will affect the absorption frequency of the protonated species. The causes of this phenomenon are not entirely clear from inspection of the MOs, density shifts, or atomic charge patterns. Cyanosubstitutions. In contrast to F-substitution, replacement of hydrogen atoms by a cyano group leads to a consistent pattern. As indicated by the first rows of Table 4, cyanosubstitution on C2, C3, or C6, all on the phenyl ring, reduces the magnitude of the red-shift significantly, by 0.16-0.26 eV. In contrast, placement of cyano groups on the pyridine ring yields the opposite effect of a red shift increase, as evident in the C10, C11, and C14 rows of Table 4. If the position chosen for replacement lies on either of the two bridging atoms, C7 or C8, the effect is intermediate between these two extremes with a small red-shift reduction of some 0.11 eV. (It should be noted from the entries in parentheses in Table 4 that the results are not sensitive to choice of basis set.) It is worth noting that the effects of multiple substitution are cumulative. That is, cyanosubstitution on both C10 and C14 increases the red shift by more than monosubstitution at either site alone. These effects are not

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Scheiner and Kar

TABLE 4: Excitation Energies (eV) of Unprotonated and Protonated Species, the Magnitude of the Red Shift Caused by Protonation, and the Effect on These Quantities of Substitution of H by -CtN at the Indicated Atomsa unprotonated

protonated

red shift

substituent effect

4.90 4.73 4.87 4.70 4.76 4.72 4.71 4.87 4.78 4.62 4.53 4.88

4.48 4.53 4.61 4.54 4.45 4.41 3.96 4.34 4.17 3.78 4.95 4.20

0.42 0.20 0.26 0.16 0.31 0.31 0.75 0.53 0.61 0.85 0.42 0.68

-0.22 (-0.22) -0.16 -0.26 -0.11 -0.11 0.33 (0.32) 0.11 0.19 (0.23) 0.43 0.00

none C2 C3 C6 C7 C8 C10 C11 C14 C10 + C14 Me/OH N at 14

a Results calculated at 3-21G level (except for 6-31G* data in parentheses)

TABLE 5: Natural Population Group Charges of the Two Rings in the Protonated and Unprotonated C2-Cyanosubstituted System phenyl ring

pyridine ring

S0 charge ππ* charge change

Unprotonated -0.056 -0.103 -0.048

-0.052 -0.027 0.025

S0 charge ππ* charge change

Protonated 0.026 0.213 +0.187

0.762 0.573 -0.189

TABLE 6: Natural Population Group Charges of the Two Rings in the Protonated and Unprotonated C14-Cyanosubstituted System phenyl ring

pyridine ring

S0 charge ππ* charge change

Unprotonated -0.007 0.053 0.061

-0.115 -0.205 -0.090

S0 charge ππ* charge change

Protonated 0.075 0.388 +0.313

0.703 0.282 -0.421

strictly additive, however, as the 0.43 eV red shift observed for C10 + C14 substitution is smaller than the sum of 0.33 + 0.19, the shift connected with each substitution individually. This particular pattern is understandable on the basis of the charge shifts alluded to earlier, wherein the excitation causes a good deal of electron density to shift from the phenyl to the pyridine ring in the protonated system. The presence of a very electronegative cyano group on the latter ring could serve as an effective sink for this electron density, facilitating the charge shift, and hence magnify the red shift. Conversely, the presence of the cyano group on the phenyl ring would work against its loss of charge, thereby diminishing the associated red shift of the absorption band. This supposition was confirmed by examination of the atomic charges. It was mentioned earlier, and reported in Table 1, that the unprotonated molecule shows very little transfer of electron density from one ring to the other as a result of electronic excitation, only around 0.01. However, the situation is quite different following protonation of the nitrogen, which facilitates a transfer of some 0.25 units of charge from the phenyl ring to pyridine. The data reported in Tables 5 and 6 illustrate the effects of cyanosubstitution on atoms C2 and C14, respectively. In the

former case, it may be noted that the placement of the electronegative cyano group on the phenyl ring has a tendency to make this ring slightly more negatively charged, and the pyridine more positive, not only in the ground state but also in ππ*. Precisely the opposite is found in comparison of Table 1 with Table 6 where the substitution has occurred on the pyridine ring. Most important, though, are the changes noted in the final rows of Tables 5 and 6. Substitution, whether on phenyl or pyridine, has little effect on the observation that little charge is transferred from the former to the latter ring upon excitation in the unprotonated molecule. Protonation permits the excitation to transfer charge density; the amount of this transfer is 0.25 units in the unsubstituted case. When the cyano group is located on the phenyl ring, Table 5 indicates the amount of this charge shift is reduced to 0.19 units, consistent with the idea that the charge shift is connected with the diminished red shift for C2substitution. Conversely, the larger red shift for C14-substitution can be reconciled with the increased charge shift of 0.31-0.42 units reported in the final row of Table 6. As might be expected, there is some correlation between the red shift that occurs upon protonation and the energies of particular molecular orbitals. The HOMO-LUMO gap is an excellent indicator of the excitation energies, calculated and displayed in Table 4. The gap and excitation energies are linearly related with correlation coefficients for the unprotonated and protonated cases of 0.99 and 0.98, respectively. Moreover, the red shifts may be well-predicted by the difference in the HOMO-LUMO gaps between the protonated and unprotonated systems with a correlation coefficient of 0.99. With focus next on the orbital energies themselves, the HOMO of the system, whether protonated or unprotonated, is rather insensitive to the site of cyanosubstitution, leaving most of the HOMO-LUMO gap sensitivity to the LUMO. The latter orbital is relatively unaffected by cyanosubstitution in the unprotonated molecule. On the other hand, the LUMO is significantly stabilized as the site shifts from the phenyl to the pyridine, particularly so when disubstitution occurs on the latter. This pattern is consistent with the electrostatic arguments presented above for cyanosubstitution, in that it is the LUMO that has the greatest concentration of electron density on pyridine in the protonated system. Consequently, placement of the electrophilic cyano group on this ring can stabilize this MO. Other Substituents. In addition to F and CN, other substituents were examined as well. Chlorosubstitution appears to underscore the idea that the anomalies encountered with F may be unique to this substituent. When Cl was placed at the C2 and C10 sites, the results were consistent with the electrostatic arguments that explain cyanosubstitution. That is, substitution at the former site on the phenyl ring reduced the red shift (by 0.07 eV), whereas this shift was enlarged (by 0.10 eV) when Cl was placed on pyridine. The situation with other substituents was more complicated. The presence of methyl, amino, or nitro groups in certain locations results in a strong force for the molecule to lose its planarity. When located on C10, the nitro-substituted molecule remains planar in both its neutral and its protonated forms, so the results can be fairly compared to the fluoro and cyano data. In the C10-nitro entity, a red shift of 0.35 eV is computed at the 3-21G level, a reduction in this quantity by 0.07 eV when compared to the unsubstituted molecule. This trend is opposite to that observed for the cyano group for which substitution anywhere in the pyridine ring increases the red shift. The

Phenyl-Pyridine Copolymers discrepant behavior between cyano (and chloro) on one hand and fluoro and nitro on the other argues that the effects of the latter groups are not limited to simple electrostatic factors, as would appear to be the case for cyanosubstitution. Another sort of substitution was made to make contact with experiment. Results described by Fu et al.12 indicate that the identity of the R group in Figure 1 influences the amount of the red shift. The presence of an ether group (R ) OC16H33) yields the largest shift. Replacement of R by an alkyl group (R ) C12H25) cuts this shift nearly in half. To model this change from ether to alkyl, the pair of OH groups in Figure 2 was replaced by a methyl at each site. In line with the experimental findings, this replacement indeed lowered the amount of the computed red shift, from 0.42 to 0.38 eV. One last point needs to be made. In the full copolymer of Figure 1, the two rings in Figure 2 would alternate with one another. That is, after the phenyl ring, double-bond connection, pyridine ring, double bond shown, one would next find another phenyl ring and pyridine ring, and so on. Our particular model system considers the phenyl ring to the left of the pyridine. Placing the phenyl on its right would be very similar, except that because of the asymmetry of the pyridine, the N13 and C14 atoms would effectively switch positions. A calculation of this system, that is, placing the pyridine N atom adjacent to C9, yields a 3-21G red shift of 0.68 eV, somewhat larger than the 0.42 eV computed for the original system. This difference should serve as an additional caution about taking any of the values calculated here at face value. The results would be affected not only by the particular segment of the entire copolymer chain taken for study but also by other factors such as effects of neighboring molecules. What is stressed here is thus not the quantitative values but rather the trends that are seen as a result of various substitutions. Acknowledgment. We are grateful to Prof. T. Swager for helpful discussions. This work was supported by Grant DAAD1999-1-0206 from the Army Research Office. References and Notes (1) Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539.

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