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* E-mail: wushi@zju.edu.cn. Received August 11, 2010; revised November 9, 2010; accepted December 17, 2010.

888 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2011, 29, 888—892

Electronic Structures and Spectroscopic Characters of Modified Oligo(alkylenedioxypyrrole)

Yuan, Shuaia,b(原帅) Zhang, Peibina(张培斌) Huang, Xinweib(黄昕暐) Li, Xina(李歆) Teng, Qiwen*,a(滕启文)

a Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China b Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China

Polyalkylenedioxypyrrole (PADOP) exhibited an excellent conductivity experimentally. A series of oligomers for the electron-rich monomer alkylenedioxypyrrole (ADOP) were designed in order to study properties of PADOP. The structures of these oligomers were optimized using density function theory (DFT) at B3LYP/6-31G(d) level. The energy gaps and thermal stabilities of the oligomers were decreased when the chain lengths were increased. These properties were also decreased with the enlargement of the neighboring substituted rings. The 13C nuclear magnetic resonance (NMR) spectra and nucleus independent chemical shifts (NICS) of the oligomers were calcu-lated at B3LYP/6-31G(d) level. The chemical shifts at δ 96.1 of the linking carbon atoms in the dimer of 3,4-methylenedioxypyrrole (MDOP) were moved downfield relative to those at δ 89.5 of the same carbon atoms in the monomer of MDOP. The aromaticity of the central pyrrole ring in the oligomers is improved with the enlarge-ment of the neighboring substituted rings.

Keywords alkylenedioxypyrrole oligomers, conductive polymer, aromaticity, NICS scanning

Introduction

Polypyrroles are widely used in some fields such as catalysis, energy storage, fuel cell and anti-bacterial coatings owing to the various properties.1,2 Pyrrole can be used as a working electrode, the oxidation potential of which is related to the 2,5-substitution.3,4 The pyrrole structure in conjugated polymers can reduce the oxida-tion potential in the synthesis of dithienopyrrole.5 Pyr-role and thiophene can be utilized to form lock copoly-mers, in which N-methylpyrrole rings cause large rota-tional defects.6 Carboxylated pyrrole is successfully polymerized on a Pt electrode to get the deposition of the corresponding conductive polymer.7 2,5-Positions in 3-vinylpyrrole are more reactive than those in the pris-tine pyrrole ring.8 Functionalized electron-rich mono-mers such as 3,4-ethylenedioxypyrrole (EDOP) can be employed to produce highly electroactive and stable conducting polymers.9 Microns-long conducting polypyrrole nanotubes of the 6 nm average pore diame-ter can be synthesized using chemical oxidative polym-erization.10

The monomer EDOP has been synthesized,9 but the theoretical research related to EDOP or other al-kylenedioxypyrrole (ADOP) has not been reported yet. Herein, a series of ADOP oligomers are designed. The effects of the chain lengths and substituents on the elec-tronic structures and spectroscopy are investigated. These properties are helpful to explain the conductivity

and stability of polyalkylenedioxypyrrole (PADOP).

Theoretical method

The conjugation system of the oligomers is extended as the chain length increases. The energy gaps of the oligomers are decreased, which contributes to improv-ing the electrical conductivity. The solubility of the oli-gomers can be improved by changing substituents on the main chain. In view of these, compounds 1—13 were designed. On the basis of 3,4-methylenedioxy- pyrrole (MDOP) (compound 1), compounds 2—5 were respectively formed by increasing the chain length. The pyrrole rings are connected at the 2,5-linkage (Figure 1) in an alternative array of the nitrogen atoms whether on the top or on the bottom of the pyrrole rings like that in the literature.6 Compounds 6—8 were progressively designed through increasing the number of the methyl-ene groups on the substituted rings based on compound 3. Compounds 9—13 were generated by changing sub-stituents in each ring of compound 3.

The DFT method in Gaussian 03 package11 has been widely used in studying the electronic structures of su-pramolecular systems,12 hydrogen-bonding com-plexes,13 fluorescent materials,14 conductive polymers,15

fullerene derivatives,16 carbon nanotubes,17 and other compounds.18 Full geometric optimization of com-pounds 1—13 was carried out using Becke three

Electronic Structures and Spectroscopic Characters of Modified Oligo(alkylenedioxypyrrole)

Chin. J. Chem. 2011, 29, 888—892 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 889

Figure 1 The structural scheme of compounds 1—13. Compound 1: x＝1, y＝1, R1

＝H, R2＝H; Compound 2: x＝2,

y＝1, R1＝H, R2

＝H; Compound 3: x＝3, y＝1, R1＝H, R2

＝H; Compound 4: x＝4, y＝1, R1

＝H, R2＝H; Compound 5: x＝5,

y＝1, R1＝H, R2

＝H; Compound 6: x＝3, y＝2, R1＝H, R2

＝H; Compound 7: x＝3, y＝3, R1

＝H, R2＝H; Compound 8: x＝3,

y＝4, R1＝H, R2

＝H; Compound 9: x＝3, y＝1, R1＝Me, R2

＝H; Compound 10: x＝3, y＝1, R1

＝Me, R2＝Me; Compound 11:

x＝3, y＝1, R1＝Et, R2

＝H; Compound 12: x＝3, y＝1, R1＝

n-propyl, R2＝H; Compound 13: x＝3, y＝1, R1

＝n-butyl, R2＝

H.

parameters plus Lee, Yang and Parr’s (B3LYP) method using 6-31G(d) basis set.19 Then the equilibrium ge-ometries with the minimum energies of compounds 1—13 were obtained. According to Koopmans’ theorem, vertical ionization potential (IP) is approximately de-fined as the negative value of the energy for the highest occupied molecular orbital (HOMO). Vertical electron affinity (EA) is similarly defined as the negative value of the energy for the lowest unoccupied molecular or-bital (LUMO). The absolute hardness (η) is equal to the half of the difference between IP and EA. The absolute electron negativity (χ) equals the half of the sum for IP and EA. All these values were calculated at the B3LYP/6-31G(d) level. Based on the B3LYP/6-31G(d) optimized geometries, the 13C nuclear magnetic reso-nance (NMR) spectra and nucleus independent chemical shifts (NICS) of compounds 1—13 were computed at the B3LYP/6-31G(d) level on the gauge-including atomic orbital (GIAO) method.20 The helium atom was

located at the center of the central pyrrole ring in each compound.21

Results and discussion

Geometric and electronic structures at the ground state

The optimized bond lengths of N(1)—C(2), C(2)—C(3) and C(3)—C(4) in compound 1 are 0.139, 0.137 and 0.140 nm, respectively, which are consistent with the X-ray diffraction data 0.137, 0.138 and 0.142 nm in pyrrole.5 The bond lengths of C(3)—O(6) and O(6)—C(7) in compound 1 are 0.138 and 0.144 nm, respec-tively. The energy gaps of compounds 1—5 (Table 1) are gradually decreased as the chain length increases. The pyrrole rings and substituted rings are located in the same plane, thus the conjugation systems in compounds 1—5 are gradually extended. The energy gap of the MDOP polymer is extrapolated to be 2.72 eV when x tends to be infinity, which is favorable to resulting in the high conductivity.

The energy gaps of compounds 3, 7 and 8 are pro-gressively decreased with the enlargement of the sub-stituted rings. The alkylene groups on the substituted rings act as the electron-donating groups, thus the elec-tron density on the pyrrole rings is increased and the conjugation effect is improved. Compound 6 has an ap-parently narrower energy gap than compound 3, 7 or 8. The substituted hexagons in compound 6 are more sta-ble than pentagons in compound 3, heptagons in com-pound 7 or octagons in compound 8. Thus, the cyclic tension in compound 6 is reduced, and the conjugation effect is strengthened.

The energy gaps of compounds 9, 11—13 are de-creased compared with that of compound 3. The

Table 1 The optimized variables of compounds 1—13 at the B3LPY/6-31G(d) level

Compound 1 2 3 4 5 6 7

Energy gap/eV 6.413 4.842 4.015 3.628 3.346 3.859 3.982

IP/eV 5.299 4.588 4.294 3.985 3.960 3.772 3.893

EA/eV －1.115 －0.254 0.278 0.358 0.613 －0.087 －0.089

η/eV 3.207 2.421 2.008 1.814 1.673 1.929 1.991

χ/eV 2.092 2.167 2.286 2.172 2.287 1.842 1.902

NICS －1.424 －0.009 1.205 0.454 1.299 －2.749 －1.575

Compound 8 9 10 11 12 13

Energy gap/eV 3.974 4.003 4.027 4.001 4.003 4.004

IP/eV 3.928 4.171 3.987 4.157 4.147 4.140

EA/eV －0.046 0.168 －0.040 0.155 0.144 0.136

η/eV 1.987 2.001 2.014 2.001 2.001 2.002

χ/eV 1.941 2.170 1.973 2.156 2.146 2.138

NICS －1.401 0.752 0.505 0.963

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symmetries of compounds 9, 11—13 are lowered be-cause of the presence of the substituents, which leads to the splitting of the molecular orbitals and the decrease in the energy gaps. On the contrary, the energy gap of compound 10 is increased relative to that of compound 3. Actually, compound 3 is not a planar molecule. The presence of the two symmetrical methyl groups in compound 10 elevates the molecular planarity, and then the symmetry is increased. The molecular orbitals in compound 10 are degenerate, and thus the energy gap is widened.

Some important variables

The IP values of compounds 1—5 are decreased, and the EA values are increased progressively. These oli-gomers are likely injected with holes and electrons si-multaneously as the chain length increases. The IP val-ues of compounds 6, 7 and 8 are lower than that of compound 3, thus the enlargement of the substituted ring favors injecting the holes. Compound 6 has an ex-tremely low IP value. The hexagons in compound 6 are beneficial to the delocalization of the positive charge. The experiment shows that the oxidative potential of EDOP is significantly low compared with that of other ADOP,9 which supports our results. The IP values of compounds 9, 11—13 relative to that of compound 3 are decreased. The electron-donating groups on the substi-tuted ring increase the electron density on the pyrrole rings, which benefits the injection of the holes. The IP and EA values of compound 10 in contrast to those of compound 9 are decreased remarkably, thus compound 10 is likely injected with the holes. On one hand, the presence of the two methyl groups promotes the copla-narity between the substituted rings and pyrrole rings; on the other hand, the electron-donating effect is inten-sified with the increase in the number of the methyl groups. The experiment also shows that the oxidative potential of pyrrole is decreased in the presence of the methyl group.3

The η values of compounds 1—5 are gradually de-creased, thus thermal stabilities are decreased with the increase in the chain length. The η values of compounds 6—8 in light of that of compound 3 are decreased be-cause of the enlargement of the substituted rings. The η values of compounds 9, 11—13 are decreased compared with that of compound 3, thereby the thermal stabilities of the compounds are decreased in the presence of the substituents. The χ values of compounds 1, 2, 3, 5 are progressively increased. Thus, the hardness of these compounds as a base is elevated, and the ability of the resisting oxidation is improved with the increase in the chain length. The χ values of compounds 3, 9 and 11—13 are successively decreased, and the χ value of compound 10 is lower than that of compound 9. The ability of the resisting oxidation for these compounds decreases as either the length or the number of the sub-stituents increases.

13C NMR spectra

The chemical shifts of C(2), C(3) and C(7) in com-pound 1 are situated at δ 89.5, 134.7 and 105.9, which are basically in agreement with the experimental results at δ 71.4, 137.5 and 105.4 in propylenedioxypyrrole respectively.9 The chemical shifts of normal sp3-C on C—O bonds are usually at δ 50—80. The chemical shift of C(7) in compound 1 is changed downfield compared with the above values, therefore C(7) serves as an elec-tron-donating group. The chemical shifts of normal sp2-C on C＝N bonds are generally at δ 130—160. The chemical shift of C(2) is shifted upfield in view of the above values. According to the chemical shifts of C(7) and C(2), it is concluded that the electron cloud on the substituted ring is driven to the pyrrole ring in the pres-ence of the oxygen atoms.

The chemical shifts of the linking atom C(2) in compounds 1, 2, 3 and 5 are respectively located at δ 89.5, 96.1, 96.7 and 97.0, which are slightly changed downfield as the chain length increases. The electron density on C(2) tends to be decreased and delocalized on each monomer. The chemical shifts of C(2) in com-pounds 3, 6, 7 and 8 are δ 96.7, 101.6, 105.2 and 107.7, respectively, which are also moved downfield progres-sively. The electron density on the pyrrole rings is scat-tered owing to the enlargement of the substituted rings. The chemical shifts δ 96.8, 96.9, 97.0 and 97.1 of C(2) in compounds 9, 11—13 compared with that in com-pound 3 are shifted downfield successively with the increase in the length of the substituents. The chemical shift δ 95.8 of C(2) in compound 10 compared with that in compound 9 is changed upfield owing to the increase in the number of the methyl groups on the substituted rings. The number of peaks in compound 10 is de-creased, and thus the symmetry is increased relative to that of compound 3.

Aromaticity

The NICS value at the center of the pyrrole ring in compound 1 is negative, thus this pyrrole ring is aro-matic. The aromaticity of the single pyrrole ring is also proved by the NICS calculation at B3LYP/6-311G level.21 The NICS values at the central pyrrole ring cen-ter in compounds 2, 3 and 5 are gradually elevated compared with that in compound 1. Thus the aromatic-ity of the central pyrrole ring in each compound is de-creased with the increase in the chain length, and the anti-aromaticity even appears. The NICS value at δ － 10.10 of the benzene ring center in the benzo-heterocyclic dimer relative to that at δ －10.49 of the same benzene ring in the monomer is shifted downfield,21 supporting our conclusion.

The NICS value at the central pyrrole ring center in compound 3 is positive, thus this pyrrole ring is anti-aromatic. The NICS values at the central pyrrole ring center in compounds 6, 7 and 8 are negative, thus the aromaticity of the central pyrrole ring is generated with the enlargement of the substituted rings. The NICS

Electronic Structures and Spectroscopic Characters of Modified Oligo(alkylenedioxypyrrole)

Chin. J. Chem. 2011, 29, 888—892 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 891

value at the central pyrrole ring center in compound 6 is the lowest owing to the stabilized effect of the adjacent hexagon on the pyrrole ring. It is also supported by the calculation result that the NICS value at δ －16.21 of the pyrrole ring in the presence of the neighboring hexagon is changed upfield in comparison with that at δ －13.62 of the single pyrrole ring.21 The NICS values at the central pyrrole ring center in compounds 9, 10 and 13 are decreased compared with that of the same pyrrole ring in compound 3. Therefore, the anti-aromaticity of the central pyrrole ring can be reduced in the presence of the electron-donating substituents. The B3LYP/ 6-311G calculation shows that the NICS value at δ －8.15 of the benzene ring center in o-dimethylbenzene is decreased in contrast to δ －8.03 of benzene,21 which supports the above results.

The NICS scanning in compound 1 was performed in the direction vertical to the plane of the pyrrole ring (Figure 2) like that in benzene.21 The helium atom was firstly located in the geometrical centre of the pyrrole ring. Then the scanning was carried out by gradually moving the helium atom along the vertical line away from the pyrrole ring. When the distance between the helium atom and the center of the pyrrole ring is 0.09 nm above the pyrrole ring center or 0.11 nm below the

Figure 2 The dependence of NICS values in compound 1 on the distance.

pyrrole ring center, the NICS values are decreased to the minima of δ －6.80. The electron density is increased to the maximum, and the shielding effect is the strongest on these two levels. After the minima, the NICS values tend to increase in both curves, which arises from the decreased electron cloud. This NICS scanning result on the pyrrole ring is similar to that on benzene.21 The dif-ference is that the symmetrical plane of the electron cloud on the pyrrole ring is drifted away from the pyr-role plane. The bending structure of the substituted ring squeezes the electron cloud downwards from the plane.

Conclusion

The electronic structures and chemical shifts of the oligo(alkylenedioxypyrrole) are affected by the chain length of the pyrrole rings and the size of the substituted rings as well as the substituents. The energy gaps of the oligomers are decreased in the presence of the substitu-ents. The chemical shifts of the carbon atoms on the linkage of the pyrrole rings are moved downfield by increasing the chain lengths and adding substituents. The aromaticity of the central pyrrole ring in the oli-gomers disappears with the increase of the chain length, whereas it is improved in the presence of the neighbor-ing hexagon. The trimer of EDOP possesses remarkable properties of the relatively narrow energy gap and high aromaticity. The relevant material is promised to be-come an excellent conductive moiety.

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