Comparative Survey of Conducting Polymers Containing Benzene

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Comparative Survey of Conducting Polymers Containing Benzene, Naphthalene, and Anthracene Cores: Interplay of Localized Aromaticity and Polymer Electronic Structures Alicia M. Fraind† and John D. Tovar*,†,‡ Department of Chemistry and Department of Materials Science and Engineering, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed: October 23, 2009; ReVised Manuscript ReceiVed: January 20, 2010

We present a systematic study to understand to what extent the localization of aromaticity in an orthogonal sense to the main polymer conjugation pathway will influence the observed optical and electrical properties as the polymers undergo oxidation and doping into conductive materials. Three classes of electropolymerizable monomers were prepared where the critical electronic unit was chosen to foster different degrees of aromatic localization pendant to the conjugation pathway: specifically, those based upon benzene, naphthalene, and anthracene cores. The expectation was that the benzene unit would foster extensive intramolecular delocalization upon adoption of the quinoidal electronic structure on account of the strong polyene character. On the other hand, resonance contributors can be rationalized for naphthalene and anthracene whereby one or two aromatic sextets evolve within the quinoidal structure thereby leading to a more localized electronic structure. Monomer and polymer electronics were probed with UV-vis spectroscopy and cyclic voltammetry as well as through in situ profiling of the conductive states of the respective polymers. A semiempirical analysis of the frontier orbital wave functions was employed to further understand the influences of competing aromaticity pendant to the polymer backbones. Our findings indicate the potential for complex and tunable π-conjugated polymers whose properties can be externally controlled through local alterations of aromatic character within units fused or cross-conjugated to polymer main chains. 1. Introduction Organic electronic materials are currently the subjects of intense synthetic, computational, and engineering investigations in the context of gaining greater understanding of their properties and realizing functional devices and applications. Charge carrier injection into these materials, the extent of their carrier mobilities once doped, and the theoretical nature of these carriers are among the many concerns one must consider. With respect to conjugated polymers bearing nondegenerate ground state electronic structures, several charge carriers have been identifiedssuch as polarons, bipolarons, and π-dimersswith physical analogies drawn to the energy carriers found in solid-state inorganic semiconductors.1 From the perspective of organic structure, the charge injection event (specifically, of holes) leads to an electronic reorganization or isomerization within the polymer in order to better accommodate the charge. The changes in solidstate lattice characteristics due to this reorganization result in the polaron state that is often described in the parlance of resonance structures as a transition from an aromatic to a quinoidal electronic structure. In the neutral ground state, the conjugated polymer has an energetic preference to maintain aromaticity locally within the repeating structural units, while upon oxidation, the newly formed radical cation (or other charged species) becomes more stabilized within the more delocalized and markedly olefinic quinoidal structure. This interplay of aromatic and quinoidal bonding geometries has been utilized to great success in the design and synthesis of several important conducting polymers.2 * Corresponding author. Fax: (410) 516-7044. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Materials Science and Engineering.

The judicious choice of substituents and bonding patterns can tune the energetics of the conjugated polymer to stabilize quinoidal forms and to decrease the energies of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps. One powerful example comes from Wudl’s design of polybenzo[c]thiophene (or polyisothianaphthene, PITN).3 This polymer has competing resonance structures where the aromaticity could be localized within the thiophene moieties or the benzene moieties. When incorporated into a conjugated polymer, resonance contributors that maintained benzene aromaticity simultaneously forced the backbone to adopt a quinoidal ground state electronic structure that dramatically lowered the band gap for PITN. Other strategies to incorporate defined quinoidal structures in the polymer ground state through arene-quinone methide alternating structures4 or through the use of donor-acceptor alternating copolymers5 have also been effective in reducing the optical band gaps of conjugated polymers. In the case of PITN, the sulfur atoms may play a role in the intra- and interpolymer transport of charge carriers within the doped material. We were curious to learn to what extent the aromatic localization would provide an energetic barrier for delocalization within polycyclic aromatic hydrocarbons inserted within the backbones of conducting polymers. The development of localized aromaticity has been argued for in the contexts of charge transport through molecular conductors,6 Diels-Alder reactivities of linear and bent acenes,7 and optimization of nonlinear optical activity through the evolution of aromaticity in the excited states of donor-acceptor molecules.8 Our interests in locally evolved aromaticity emerged from a desire to develop chemical systems whereby external stimuli might attenuate the aromaticity of units fused to conjugated polymers thereby leading to real-time alterations of electronic properties with well-

10.1021/jp9101459  2010 American Chemical Society Published on Web 02/16/2010

Conducting Polymers Containing Aromatic Cores defined observables. In this report, we describe optical and electrochemical properties of a series of conducting polymers that incorporate benzene, naphthalene, and anthracene units. Although we anticipated that the phenyl unit would encourage charge delocalization as part of a quinoidal structure, we hypothesized that the anthracenyl polymers once doped might be resistant to encouraging charge delocalization on account of the energetic stability gained through localizing aromaticity within the two arenes fused along the conjugation pathway. Monomer electronic properties, spectroelectrochemical analyses, and in situ conductivity profiles of the corresponding polymers are discussed and correlated with calculated HOMO and LUMO profiles for small oligomers in both their aromatic and quinoidal bonding geometries. 2. Experimental Methods General Information. All syntheses were performed on a Schlenk line using standard air-free techniques. Ether, tetrahydrofuran (THF), and methylene chloride (DCM) were passed through alumina columns and were air-free. Dioxane was obtained from a Sureseal bottle (Aldrich), and all other reaction solvents were degassed before use. 2-Tributylstannyl-bithiophene and 2-tributylstannyl-3,4-ethylenedioxythiophene were prepared through lithiation of bithiophene and 3,4-ethylenedioxythiophene (EDOT), respectively, followed by quenching with tributylstannyl chloride.9 Palladium catalysts were purchased from Strem Chemicals, and all other reagents and chemicals were purchased from Aldrich and used as received. The identities and purities of all compounds were verified by 1H NMR (400 MHz) and 13 C NMR (100 MHz) obtained on a Bruker Avance spectrometer, and high resolution electron impact/chemical ionization (EI/CI) mass spectrometry was obtained on a VG Instruments VG70S magnetic sector mass spectrometer. All electrochemical and spectroelectrochemical solutions were prepared in anhydrous 0.1 M n-Bu4NPF6 (TBAP) in DCM. Electrochemical films were grown on a 2 mm2 platinum button working electrode with a quasi-internal Ag/Ag+ reference electrode (silver wire immersed in 0.01 M AgNO3 and 0.1 M TBAP in acetonitrile, separated from the cell with a porous Vycor frit, obtained from BioAnalytical Systems) and a platinum wire counter electrode using a scan rate of 100 mV/s. The half-wave potential of the Fc/Fc+ couple was used as an external standard. The potentials were cycled using an Autolab PGSTAT 302 bipotentiostat. The spectroelectrochemical films were grown on glass coated with indium tin oxide (Aldrich, 70-100 Ω/square surface resistivity), and UV/ vis measurements were taken with a Varian Cary 50 Bio UV/ vis spectrophotometer. Polymer films for conductivity measurements were grown on platinum interdigitated electrode arrays with the Ag/Ag+ reference electrode mentioned above and a platinum wire counter electrode using a 5 mV/s scan rate with a potential difference of 40 mV between the two working electrodes;10 the devices were purchased from Abtech (IME 0550.5 M Pt U). Molecular orbital calculations were conducted at the semiempirical AM1 level of theory using Spartan ’04 (Wavefunction, Inc.).11 Equilibrium geometries were used in all cases, but the quinoidal HOMOs were calculated from ground states with complete quinoidal bonding geometries imposed within the model trimers in order to better reflect the minimization of the torsional angles anticipated in the solid-state polymer films as described in the discussion below. 1,4-Bis(2-thienyl)benzene (TBT).12 A solution of 1,4-dibromobenzene (0.2338 g, 0.9911 mmol) and (Ph3P)2PdCl2 (0.0366 g, 0.0521 mmol) in degassed DMF (5.00 mL) was heated to 80 °C. 2-(Tributylstannyl)thiophene (0.822 g, 2.20 mmol) was

J. Phys. Chem. B, Vol. 114, No. 9, 2010 3105 added dropwise over 2 min. The reaction mixture was left heating and stirring for a total of 5 h before being cooled and mixed with ether and then poured through Celite. The filtrate was stirred with 1 M aqueous KF for 15 min and then filtered through fresh Celite; this was performed twice. The aqueous layer was removed from the filtrate; the organic layer was extracted with saturated aqueous NH4Cl and distilled water. The organic layer was dried with MgSO4 and filtered, and the solvent was removed under reduced pressure by rotary evaporation. The product was purified by column chromatography (SiO2, elutant 5% DCM in hexane), yielding a shiny white solid (0.1242 g, 0.5132 mmol, 52%). The product had 1H NMR, 13C NMR, and high-resolution mass spectra (HR-MS) data consistent with reported values. 1,4-Bis(2-thienyl)naphthalene (TNT).13 The reaction conditions used in the preparation of TBT were followed. 1,4Dibromonaphthalene (0.2866 g, 1.002 mmol), (Ph3P)2PdCl2 (0.0348 g, 0.0496 mmol), and 2-(tributylstannyl)thiophene (0.822 g, 2.20 mmol) were added and heated as described for TBT. The reaction was left to heat and stir for 2 h, and then it was worked up identically to TBT. The residue was purified by column chromatography (SiO2, elutant hexane) to give a yellow solid (0.1127 g, 0.3860 mmol, 39%). 1H NMR and 13C NMR data were consistent with reported values. HR-MS (EI/ CI): found m/z ) 292.0375 (M+); calcd for C18H12S2: 292.0380. 9,10-Bis(2-thienyl)anthracene (TAT).14 9,10-Dibromoanthracene (0.3354 g, 0.9981 mmol) and (Ph3P)2PdCl2 (0.0366 g, 0.0521 mmol) were heated to 80 °C in a mixture of degassed DMF (5.00 mL) and toluene (2.20 mL). 2-(Tributylstannyl)thiophene (0.8225 g, 2.20 mmol) was added dropwise over 2 min. The reaction was heated and stirred for 18 h and was worked up identically to TBT. The residue was then purified by column chromatography (SiO2, elutant DCM) to give a yellow solid (0.1405 g, 0.4108 mmol, 41%). 1H NMR data were consistent with reported values. 13C NMR (100 MHz, CDCl3) δ: 139.0, 131.6, 130.4, 129.6, 127.3, 126.9, 126.8, 125.8. HRMS (EI/CI): found m/z ) 342.0543 (M+); calcd for C22H14S2: 342.0537. 1,4-Bis(2-(3,4-ethylenedioxy)thienyl)benzene (EBE).15 The reaction conditions used in the preparation of TBT were followed. 1,4-Dibromobenzene (0.2388 g, 1.0122 mmol), (Ph3P)2PdCl2 (0.0351 g, 0.0500 mmol), and 2-tributylstannyl3,4-ethylenedioxythiophene (1.0130 g, 2.4878 mmol) were added and heated as described for TBT. The reaction was heated and stirred for 2 h before being worked up identically to TBT. The residue was then purified by column chromatography (SiO2, elutant 1:1 hexane:DCM) to give a shiny white solid (0.0546 g, 0.1525 mmol, 15%). The product had 1H NMR, 13C NMR, and HR-MS data consistent with reported values. 1,4-Bis(2-(3,4-ethylenedioxy)thienyl)naphthalene (ENE). The reaction conditions used in the preparation of TBT were followed. 1,4-Dibromonaphthalene (0.2844 g, 0.9945 mmol), (Ph3P)2PdCl2 (0.0366 g, 0.0521 mmol), and 2-tributylstannyl3,4-ethylenedioxythiophene (1.0604 g, 2.6042 mmol) were added and heated as described for TBT. The reaction was heated and stirred for 66 h before being worked up identically to TBT. The residue was then purified by column chromatography (SiO2, elutant DCM) to give a yellow solid (0.1457 g, 0.3571 mmol, 36%). 1H NMR (400 MHz, CDCl3) δ: 8.07 (dd, J ) 6.6, 3.6 Hz, 2H), 7.60 (s, 2H), 7.51 (dd, J ) 6.6, 3.6 Hz, 2H), 6.49 (s, 2H), 4.29 (m, 4H), 4.22 (m, 4H). 13C NMR (CDCl3) δ: 141.5, 138.5, 132.3, 130.6, 128.5, 126.8, 126.4, 115.3, 99.4, 64.8, 64.7. HR-MS (EI/CI): found m/z ) 408.0492 (M+); calcd for C22H16O4S2: 408.0490.

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9,10-Bis(2-(3,4-ethylenedioxy)thienyl)anthracene (EAE). The reaction conditions for the preparation of TAT were followed. 9,10-Dibromoanthracene (0.3348 g, 0.9963 mmol), (Ph3P)2PdCl2 (0.0343 g, 0.0489 mmol), and 2-tributylstannyl3,4-ethylenedioxythiophene (0.9610 g, 2.3601 mmol) were added and heated as described for TAT. The reaction was heated and stirred for 43 h and cooled to room temperature. The mixture was diluted with methanol and the precipitate was filtered off. The precipitate was then purified by column chromatography (SiO2, elutant DCM) to give a yellow solid (0.0900 g, 0.1965 mmol, 20%). 1H NMR (400 MHz, CDCl3) δ: 7.99-7.95 (m, 4H), 7.46-7.43 (m, 4H), 6.65 (s, 2H), 4.30 (m, 4H), 4.16 (m, 4H). 13C NMR (100 MHz, CDCl3) δ: 141.4,140.0, 127.7, 126.9, 125.9, 112.6, 100.2, 66.0, 64.8. HR-MS (EI/CI): found m/z ) 458.0647 (M+); calcd for C22H18O4S2: 458.0647. 1,4-Bis(2,2′-bithiophene-5-yl)benzene (BBB). Dry 1,4-dioxane (2.00 mL) was added to 1,4-dibromobenzene (0.2352 g, 0.9970 mmol), CsF (0.6065 g, 3.9928 mmol), and Pd(P(t-Bu)3)2 (0.0092 g, 0.0452 mmol).16 The reaction mixture was stirred at room temperature as 5-(tributylstannyl)-2,2-bithiophene (0.9714 g, 2.1364 mmol) was added slowly. The reaction was allowed to proceed for 19 h, at which point an orange solid was filtered off in quantitative yield. 1H NMR (400 MHz, o-Cl2C6D4) δ: 7.60 (s, 4H), 7.36 (d, 2H), 7.26-7.20 (m, 4H), 7.16-7.11 (m, 4H). HR-MS (EI/CI): found m/z ) 405.9981 (M+); calcd for C22H14S4: 405.9978. 1,4-Bis(2,2′-bithiophene-5-yl)naphthalene (BNB). The reaction conditions for the preparation of TAT were followed. A solution of 1,4-dibromonaphthalene (0.2859 g, 0.9998 mmol), (Ph3P)2PdCl2 (0.0364 g, 0.0519 mmol), and 5-(tributylstannyl)2,2-bithiophene (1.0066 g, 2.2138 mmol) in DMF (3 mL) and toluene (3 mL) was prepared and heated as described for TAT. The reaction was heated and stirred for 18 h, and then worked up identically to TAT. The residue was purified by column chromatography (SiO2, elutant 15% DCM in hexane) to give a yellow solid (0.1547 g, 0.3390 mmol, 34%). 1H NMR (400 MHz, CDCl3) δ: 8.37 (dd, J ) 6.6, 3.3 Hz, 2H), 7.62 (s, 2H), 7.27-7.24 (m, 8H), 7.19 (d, J ) 3.6 Hz, 2H), 7.07 (dd, J ) 8.9, 3.7 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ: 140.6, 138.0 137.4, 132.6, 132.3, 128.5, 128.0, 127.6, 126.8, 126.3, 124.6, 124.5, 124.4, 124.2, 123.9. HR-MS (EI/CI): found m/z ) 456.0135 (M+); calcd for C26H16S4: 456.0135. 9,10-Bis(2,2′-bithiophene-5-yl)anthracene (BAB). The reaction conditions for the preparation of TAT were followed. A solution of 9,10-dibromoanthracene (0.3343 g, 0.9948 mmol), (Ph3P)2PdCl2 (0.0367 g, 0.0523 mmol), and 5-(tributylstannyl)2,2-bithiophene (1.0564 g, 2.3233 mmol) in DMF (3 mL) and toluene (3 mL) was prepared and heated as described for TAT. The reaction was heated and stirred for 18 h, and then worked up identically to TAT. The residue was purified twice by column chromatograpy (SiO2, elutant 20% DCM in hexane) to give a sparingly soluble yellow solid (0.0606 g, 0.1194 mmol, 12%). 1 H NMR (400 MHz, CDCl3) δ: 8.02 (dd, J ) 6.8, 3.4 Hz, 4H), 7.45 (dd, J ) 6.8, 3.3 Hz, 4H), 7.39 (d, J ) 3.5 Hz, 2H), 7.29-7.27 (m, 4H), 7.13 (d, J ) 3.6 Hz, 2H), 7.08 (dd, J ) 4.7, 3.8 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ: 139.1, 137.9, 131.5, 130.5, 130.0, 128.0, 126.8, 126.0, 124.7, 124.0, 123.9. HR-MS (EI/CI): found m/z ) 506.0283 (M+); calcd for C30H18S4: 506.0291.

Fraind and Tovar

Figure 1. Resonance contributors to neutral conjugated polymers derived from benzene (top), naphthalene (middle), and anthracene (bottom) with aromatic sextets emphasized (aromatic form depicted at left and the quinoidal form depicted at right).

typically on the order of 10 subunits. With some notable exceptions,17 there is usually a length at which the optical properties of these longer oligomers conVerge with the corresponding polymer, an indication of conformational torsion or saturated chemical defects existing within the polymer backbone. An increase in the effectiVe conjugation length (ECL) of the polymer allows the charged carriers to have greater intramolecular delocalization that can ultimately lead to an increase in electrical conductivity.18 However, not all π-electrons are necessarily involved in the extension of the effective conjugation length. When aromatic systems such as naphthalene or anthracene are linked into a conducting polymer, it is hard to predict which π-electrons will be involved in the intramolecular conjugation and which might contribute more to localized electronic structures. It is important to determine the behavior CHART 1: Monomers with Benzene, Naphthalene, or Anthracene Cores Synthesized by Stille Couplingsa

3. Results and Discussion Increasing the conjugation length of a π-conjugated monomer typically leads to a decrease in the oxidation potential at which it polymerizes and in the HOMO-LUMO gap up to oligomers

B ) benzene; N ) naphthalene; A ) anthracene; T ) thiophene; E ) ethylenedioxythiophene (EDOT); B ) 2,2′-bithiophene. a

Conducting Polymers Containing Aromatic Cores

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TABLE 1: TXT Monomer Electrochemical and Absorption Data monomer

Eonset,m (V vs Ag/Ag+)

Epeak,m (V vs Ag/Ag+)

UV/vis λmax (nm)

TBT TNT TAT

0.92 0.97 0.92

1.25 1.22 1.12

333 337 262, 366, 383, 404

TABLE 2: PolyTXT Electrochemical, Spectroelectrochemical, and Conductivity Data

polymer

Eonset,p (V vs Ag/Ag+)

UV/vis neutral λmax (nm)

UV/vis polaron λmax (nm)

conductive potential range (V)

polyTBT polyTNT polyTAT

0.47 0.51 N/A

450 450 N/A

625 725 N/A

0.4-1.0 0.5-1.2 N/A

of these π-electrons in order to understand the electronic properties of the polymer. Predicting how the ECLs or observed electrical properties change as a function of doping for polymers built from complex aromatic components with orthogonal conjugation pathways is not straightforward given the prospects for the development of formally localized aromaticity within the quinoidal electronic structure. For example, while a paraconjugated benzene component has a dramatic loss of aromaticity in the quinoidal form (Figure 1, top), anthracene conjugated through the 9,10-positions could develop two aromatic sextets in the quinoidal form (Figure 1, bottom). Outside of isolated molecular examples, we were unable to uncover any comparative studies that involved benzene, naphthalene, and anthracene units incorporated into conducting polymers. This led us to prepare three monomer families built around 1,4-phenyl (B), 1,4-naphthyl (N), and 9,10-anthracenyl (A) cores, where each core was disubstituted with thiophene

(T), ethylene dioxythiophene (EDOT, E), or bithiophene (B, Chart 1). With this nomenclature, TBT represents the thiophenebenzene-thiophene trimer depicted in Chart 1; namely, 1,4di-(2-thienyl)benzene. The monomers were synthesized using Stille couplings, purified, and polymerized electrochemically. Monomers and polymers were characterized by UV-vis absorption, cyclic voltammetry (CV), spectroelectrochemistry, and in situ conductivity measurements as appropriate. The oxidation potentials of each monomer and respective polymer were obtained from CV. Spectroelectrochemistry was used to follow electronic changes in the polymers as they were oxidized and reduced. Monomers were polymerized electrochemically on platinum interdigitated electrode arrays for in situ conductivity determination according to procedures described by Wrighton.10 Due to differences in growth efficiency and packing densities expected among the different polymers, the absolute conductivities were not measured, but the data were used in a qualitative sense to observe the range of potentials over which the polymers were conductive. Molecular modeling and semiempirical (AM1) calculations were used to visualize the wave functions associated with the relevant frontier molecular orbitals, both as energy minimized and as artificially planarized structures. Although we attempt to correlate observed electronic properties to the extents of wave function delocalization, it is important to realize that the techniques used may be affected differently by intermolecular factors such as packing of polymer chains among themselves and at electrode surfaces. Thiophene-Arylene-Thiophene Family (TXT) (See Tables 1 and 2). The onsets of monomer oxidation (Eonset,m) are roughly the same within the TXT series, with TNTs being slightly more positive, despite the fact that naphthalene and anthracene have more π-electrons (Figure 2). Furthermore, the potentials of peak anodic current for the monomers (Epeak,m) are quite similar albeit slightly less positive for TAT. This leads us to conclude that

Figure 2. Monomer CV profiles of (a) TBT, (b) TNT, and (c) TAT. The first scan is bolded. All polymers were grown on a Pt button with a 2 mm2 surface area from 2.5 mM monomer solutions in 0.1 M TBAP/DCM using a 100 mV/s scan rate; the Fc/Fc+ half-wave potential (averaged from the three profiles) fell at 0.237 V in 0.1 M TBAP/DCM. UV/vis spectra (d) were obtained in DCM at room temperature.

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Figure 3. CVs of polyTBT (a) and polyTNT (b) films on 2 mm2 Pt buttons. Spectroelectrochemical data of polyTBT (c) and polyTNT (d) films grown on indium tin oxide (ITO) coated glass typically taken in ca. 100 mV steps. The arrows indicate trends in absorption as the film is held at increasingly more positive potentials, although reversion back to the neutral polymer is also plotted here to show reversibility. Other conditions as in Figure 2. CV and in situ conductivity profiles for polyTBT (e) and polyTNT (f) grown on Pt interdigitated electrodes from a 5 mM monomer solution in 0.1 M TBAP/DCM at room temperature; the polymer measurements were taken in 0.1 M TBAP/DCM at a scan rate of 5 mV/s with a 40 mV applied offset potential between the two working electrodes of the device.

the additional π-electrons in TNT and TAT are not involved in extending the π-conjugation in the monomer to a dramatic extent. This is further supported by the comparable wavelengths of maximum absorption (λmax) of TBT and TNT, suggesting that the ECLs of both monomers are similar (Figure 2d). However, the lowest energy π-π* transition for TBT has a higher molar absorptivity than that for TNT, showing that this transition will take place more readily than that for TNT. The UV/vis spectrum of TAT has peaks highlighting the long-axis polarized transition at 260 nm along with the weak vibronic features associated with the short-axis transitions that are slightly red-shifted relative to anthracene itself (at ca. 360, 380, and 400 nm for TAT versus 340, 360, and 380 nm for anthracene). This is likely due to the large dihedral angles between the thiophenes and the anthracene unit that discourage extended conjugation along the short axis of the anthracene. In principle, an increasing λmax observed on account of extending π-electron conjugation can be taken as a sign of enhanced delocalization

that in the classical Hu¨ckel molecular orbital sense arises from HOMO destabilization and LUMO stabilization. Despite the similarities among the TXT monomers with respect to the electrochemical oxidation, TAT monomer did not produce an adherent thin polymer film on the platinum or ITOcoated electrode surfaces. It seems likely that, once the TAT monomer was oxidized, the large carbocycle-thiophene dihedral angle prevented the oxidized monomer (and/or subsequent oligomers) from depositing on the surface due to enhanced solubility under the measurement conditions. An alternative explanation is that the oxidized intermediate adopted an electronically stable structure that persisted during the measurement, but we place less credence on this latter idea due to the lack of any apparent electrochemical reversibility. The onset of polymer oxidation (Eonset,p) is slightly less positive for polyTBT compared to polyTNT (Figure 3a,b), and the spectroelectrochemical data show that neutral polyTBT and polyTNT have identical λmax values (Figure 3c,d). Although


Figure 4. Semiempirical (AM1) nonplanar aromatic HOMOs of the TBT trimer (a), TNT trimer (b), and TAT trimer (c).

polyTNT has more π-electrons on a per repeat unit basis, these additional electrons do not contribute to enhancing the ECL or otherwise affect the polymer electronics. Upon oxidation, new spectral signatures evolve at longer wavelengths that indicate the formation of quinoidal electronic structures. Even though TNT appeared to polymerize quite well as judged by the CV (Figure 2b), only a thin film of polyTNT could be obtained after cycling the monomer solution through 80 CV scans compared to the eight cycles used to prepare the polyTBT film. Regardless, the polyTNT polaron peak grows in at a higher wavelength than that of polyTBT, indicating that polyTNT can adopt a more coplanar conformation to encourage a more delocalized quinoidal structure. In addition, the spectroelectrochemical data of polyTNT suggest that a bipolaron peak may be growing in almost simultaneously with the polaron peak at low doping levels, leading to the creation of more charged carriers and complicating our analysis. Electrochemical oxidation of both polymers leads to conductive materials as both polyTBT and polyTNT show drain currents during the measurement of in situ conductivity; the onset of the drain current for polyTBT occurs at a slightly lower potential than that for polyTNT but polyTNT is conductive to a higher potential (Figure 3e,f). The quinoidal electronic structure necessarily requires fairly small dihedral angles among repeat units of the oligomeric units that make up the effective conjugation length, and the periprotons of TAT present steric issues that would preclude extended planarity. Furthermore, the packing of the polymers in the solid state thin film might also influence conformational dynamics and thus the observed ECL for a particular polymer. These issues motivated a semiempirical investigation of the electronic structures of the HOMO and LUMO levels for model

J. Phys. Chem. B, Vol. 114, No. 9, 2010 3109 oligomers. The correlation between UV-vis λmax and the ECL does not readily translate among dramatically different electronic structures or oligomers with strong donor-acceptor character but rather for the oligomer extrapolation model with identical repeat units.19 For example, oligophenylenes absorb in the blue while oligoenes of comparable length show much more dramatic absorption throughout the visible region. Furthermore, the extent of spatial distribution of the frontier wave functions in different electronic structures need not correlate with a particular trend in experimental data. The motivation here is to extrapolate these calculations and experimental findings to future materials targets where local electronic changes that impact the frontier wave functions lead to clear and measurable alterations of electronic observables. The TBT and TNT trimer HOMO wave functions are spread over comparable lengths (Figure 4a,b), which when considered in light of the comparable absorption energies further supports the idea that the two polymers have a similar electronic structure. Although the HOMO of the TNT trimer reveals π-electron density on both rings making up the naphthalene core, there exist nodes at the ortho fusion sites (formally on carbons 4a and 8a, Figure 4b). This keeps the peripheral π-electrons (that is, those in arenes fused to the primary polymer conjugation pathway) localized from the HOMO wave function that can be envisioned to span over the effective conjugation length of the model. Although we could not obtain polyTAT experimentally, the HOMO wave function for the TAT trimer is localized to the extreme case entirely on an anthracene moiety (Figure 4c). This is due to the average TAT carbocycle-thiophene dihedral angle of 90° compared to average dihedral angles of 61° within the TNT trimer and 24° within the TBT trimer. This is not unexpected since the anticipated steric clashing among acene peri-hydrogens and the β-hydrogens of the thiophenes within TAT is relieved partially within TNT and entirely within TBT. In many respects, the orbital distribution in the LUMO can give qualitative insight into the wave function and thus the electronic structure associated with the quinoidal charge carriers. This analogy is drawn through comparison of the radical cation carriers generated from electrochemical oxidation to the mobile excitons generated from electronic excitation that populate the LUMO. It has been well-established that polymers such as polythiophenes and polyarylenes evolve into quinoidal electronic structures upon chemical or electrochemical doping, and these quinoidal electronic structures were evident in the LUMOs found in our initial AM1 calculations. We therefore compared the LUMOs calculated for the aromatic forms of the model oligomers (which maintain substantial dihedral angles among adjacent aromatics due to the steric clashing and should restrict the ECL to some extent) to the HOMOs calculated for their quinoidal counterparts (which enforce more planarity among

Figure 5. Semiempirical (AM1) wave functions calculated for the nonplanar aromatic LUMOs of the TBT trimer (a), TNT trimer (b), and TAT trimer (c) as well as for the planar quinoidal HOMOs of the TBT trimer (d), TNT trimer (e), and TAT trimer (f).

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TABLE 3: EXE Monomer Electrochemical and Absorption Data monomer

Eonset,m (V vs Ag/Ag+)

Epeak,m (V vs Ag/Ag+)

UV/vis λmax (nm)

EBE ENE EAE

0.58 0.80 0.78

0.83 1.03 1.29

349 260, 339 262, 374, 394, 409

TABLE 4: PolyEXE Electrochemical, Spectroelectrochemical, and Conductivity Data

polymer

Eonset,anodic (V vs Ag/Ag+)




polyEBE polyENE polyEAE

-0.55 0.33 0.30

525 442 400

700 735 700

-0.5 to 1.0 0.2-1.0 N/A

repeat units therefore giving a better picture of the electronic influences by attenuating steric concerns, and more representative for the packing-induced planarization20 expected in the solid state). It is reasonable to expect that the structures most likely operative in the disordered polymer thin films fall at an intermediate regime between these two extremes. Calculations of model radical cation species would help to better understand this issue in the future, but care must be taken in the choice of assumptions or levels of theory employed for spin-bearing substrates.21 The TBT wave functions in both cases appear quite similar (Figure 5a,d). The TNT trimer aromatic LUMO shows nodes on the fusion atoms in naphthalene cores that again isolate some

of the peripheral π-electron density in the fused arene from the rest of the conjugation path, and the greatest density is localized on the bithiophene repeat unit in between naphthalene units (Figure 5b). This energy-minimized structure still indicates substantial dihedral angles between aromatic repeat units. This is not found in the TNT quinoidal HOMO, where planarity throughout the structure is enforced by the olefinic bonding between adjacent aromatic rings (Figure 5e). The peripheral fused naphthalene rings within TNT do not contribute at all to this frontier wave function, but the orbital densitites are more evenly distributed over several aromatic units that comprise the conjugation length. Similarly, the TAT quinoidal HOMO reveals a more diffuse wave function that reflects the propensity to delocalize through the anthracene core in the absence of steric deplanarization (Figure 5f). There is a notable lack of π-electron density on both ortho fusion sites of anthracene. Although the thiophene-carbocycle dihedral angles are similar among all three quinoidal architectures (5° for TBT, 6° for TNT and TAT), the latter two quinoidal models indicate more dramatic extensions in the delocalization of the frontier wave functions. This is because the naphthalene and anthracene systems are forced significantly out of planarity in the aromatic form, whereas the benzene-containing system is not. EDOT-Arylene-EDOT Family (EXE) (See Tables 3 and 4). In the EXE family, there is a noticeable geometric influence on the Eonset,m value for the EBE monomer that places this process over 200 mV less positive than ENE and EAE (Figure 6a-c). The CVs after continued sweeping reveal quite different growth patterns for all three monomers, although all were able to react and polymerize after electrochemical oxidation. While

Figure 6. Monomer CV profiles of EBE (a), ENE (b), and EAE (c). The first scan is bolded. The Fc/Fc+ half-wave potential was determined to be 0.237 V in 0.1 M TBAP/DCM. UV/vis spectra of the thiophene monomers (d). Other conditions are listed in Figure 2.



Figure 7. CV and spectroelectrochemical data of polyEBE (a, b), polyENE (c, d), and polyEAE (e, f). The experimental conditions are listed in Figures 2 and 3.

EBE shows very ill-defined low potential current increases in subsequent scans reminiscent of PEDOT growth, ENE shows the evolution of a fairly reversible, or perhaps localized, polymer electronic structure forming on the electrode. On the other hand, EAE shows rather unusual anodic activity, with two low potential anodic peaks and a more positive irreversible oxidation that ultimately led to the formation of an electroactive polymer. The lowest energy absorption profiles for EBE and ENE appear qualitatively similar in terms of their energies, with EBE having a more structured profile and greater extinction coefficients (Figure 6d). For EAE, the long-axis polarized anthracene absorption remains constant at 260 nm while the vibronic structure of the short-axis polarized transitions were only slightly red-shifted. As the central monomer core was changed from benzene to naphthalene, the corresponding Eonset,p shifted to more positive

potentials (Figure 7a,c). PolyEBE has a film profile that resembles PEDOT itself, but an electrochemically irreversible redox behavior is found for polyENE. Although the naphthalene core contains more π-electrons than benzene, the data suggest that this does not enhance the conjugation pathway and perhaps even contributes to a more localized electronic structure in the neutral polymer. The EAE polymer is much more difficult to interpret (Figure 7e). The major anodic peak is narrow and identical to that which grew in during the monomer CV cycling, and we speculate that this activity corresponds to local oxidation of the bis-EDOT fragment within polyEAE with little interpolymer delocalization to the anthracene. Severe changes in conformation to stabilize this charge carrier would be responsible for the lack of reversible redox behavior. All three members of the EXE family could be electropolymerized to allow for a more inclusive comparison of their

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Figure 8. CV and in situ conductivity data profiles for polyEBE (a) and polyENE (b) on Pt interdigitated electrodes. The experimental conditions are listed in Figure 3.

polymer spectroelectrochemical properties (Figure 7b,d,f). The λmax of neutral polyENE and of polyEAE are both shorter than that of polyEBE, which would indicate less extent of conjugation due to steric concerns regarding EDOT and acene coplanarity. PolyEBE has classical electrochromic signatures, with the neutral λmax at 525 nm decreasing in intensity and accompanied by a lower energy peak centered at 700 nm emerging with continued polymer oxidation. The neutral absorption of polyENE also decreased in intensity as the film was held at progressively more positive potentials. PolyEAE has a fairly high energy neutral transition that (as with the EAE monomer) displays substantial vibronic character centered around 400 nm, with a low energy shoulder at ca. 450 nm. Upon oxidation of polyEAE, the neutral band persists along with the emergence of two welldefined peaks (λmax of 500 and 700 nm). The persistence of the neutral polymer peak and the lack of significant near-IR absorption as the polymer is being oxidized suggests a relatively localized electronic structure, and we speculate that the bisEDOT segment is being locally oxidized as hinted at above. Unfortunately, EAE did not polymerize well enough on platinum (or on gold) interdigitated electrodes for conductivity data to be obtained, but polyEBE and polyENE became conductive upon oxidation (Figure 8). PolyEBE has a conductivity profile very similar to that of PEDOT, with a slight hysteresis in the forward and reverse scans, while the peak in the drain current for polyENE is coincident with the fairly reversible redox wave at ca. 0.5 V. PolyEBE has a larger window of conductivity than polyENE, and no signs of decomposition are seen in either polymer film during the measurements, indicating environmentally stable materials. Semiempirical calculations performed on each neutral trimer suggest that there is a difference in the extent of linear conjugation between the three trimers at the HOMO levels (Figure 9). This difference may be in part due to a larger deviation from planarity in the polyENE and polyEAE films on account of the greater steric clash imposed by the EDOT relative to the TXT family, thus limiting the spatial distribution of the wave function. The average dihedral angle between the different carbocycles and EDOT was calculated from the energyminimized models to be 18° for the EBE trimer, 57° for the ENE trimer, and 89° for the EAE trimer. The ENE trimer has a substantial portion of the orbital density spread over only two rings (the central EDOT-naphthalene linkage, Figure 9b), with nodes at the naphthalene fusion atoms similar to the TNT case. The severe localization due to steric concerns is again evident in the EAE trimer, with the HOMO wave function restricted completely to the anthracene ring (Figure 9c). To visualize the orbital distribution expected for the quinoidal structures, a comparison of the aromatic LUMO levels and the

Figure 9. Semiempirical (AM1) nonplanar aromatic HOMOs of EBE trimer (a), ENE trimer (b), and EAE trimer (c).

quinoidal HOMO levels was made (Figure 10). The quinoidal HOMOs artificially minimize the contribution from steric deplanarization in the gas-phase structure, allowing us to understand better the inherent electronic influence of the different acene rings. As with the TXT family, the biggest differences were found among the ENE trimers and EAE trimers, while the qualitative wave functions for EBE trimers were quite comparable. In line with TNT trimer, the orbital densities in the quinoidal HOMO of ENE vanish on the fused aromatic ring of naphthalene not directly involved with the conjugation pathway (Figure 10e), and the wave function has sizable densities on more component rings of the model. The EAE trimer also reveals a more extended wave function in the quinoidal HOMO (Figure 10f), with the notable attenuation of density on four fusion atoms of the central anthracene ring that link the benzene rings attached orthogonal to the conjugation path. This leaves in the quinoidal HOMO two distinct “packets” of orbital density localized over the bis-EDOT repeat units of the trimer and helps to rationalize the extreme localization observed in the spectroelectrochemical profile for polyEAE. Bithiophene-Arylene-Bithiophene Family (BXB) (See Tables 5 and 6). Both the Eonset,m and the Epeak,m for the BXB monomers are fairly close, differing over only about 100 mV



Figure 10. Semiempirical (AM1) wave functions calculated for the nonplanar aromatic LUMOs for EBE trimer (a), ENE trimer (b), and EAE trimer (c) as well as for the planar quinoidal HOMOs for EBE trimer (d), ENE trimer (e), and EAE trimer (f).

TABLE 5: BXB Monomer Electrochemical and Absorption Data Eonset,m Epeak,m monomer (V vs Ag/Ag+) (V vs Ag/Ag+) BBB BNB BAB

0.74 0.82 0.70

0.93 1.00 0.90

UV/vis λmax (nm) 397 373 262, 322, 373, 397, 407

TABLE 6: PolyBXB Electrochemical, Spectroelectrochemical, and Conductivity Data

polymer

Eonset,anodic (V vs Ag/Ag+)




polyBBB polyBNB polyBAB

0.40 0.44 N/A

525 475 N/A

700 750 N/A

0.35-1.2 0.4-1.0 N/A

similar to the situation for the TXT series. Unlike the case for BBB and BNB, the CV of BAB is strongly electrochemically reversible, and this stability again precluded the formation of

any electrodeposited polymer on the platinum or ITO surfaces (Figure 11a-c). We speculate that the extended π-system once oxidized leads to the formation of two localized centers of aromaticity in the anthracene core that is facilitated by the presence of the longer and lower potential bithiophene units providing additional electrochemical stability. The extent of delocalization and aromatic stability available to this intermediate renders it too stable to polymerize under the experimental conditions. BBB has a slightly higher λmax than BNB, but their absorption onsets are the same. Furthermore, BBB mirrors the low energy absorption of BAB but without the vibronic fine structure (Figure 11d). The BAB vibronic peaks are almost coincident with the other anthracene monomers studied here, indicating that this is simply the short-axis transition that has been minimally perturbed by the extended bithiophene conjugation (the lowest energy vibronic peak falls at 407 nm). Unlike the other two families, the extent of the bithiophene conjugation through the core leads to a fair discrimination in the extent of intermolecular delocalization judging by the 24 nm red shift in

Figure 11. Monomer CV profiles of BBB (a), BNB (b), and BAB (c). The first scan is bolded. CVs of BBB were acquired from an unknown concentration of the sparingly soluble monomer dissolved in 1.00 mL of o-dichlorobenzene and added to 2.00 mL of 0.1 M TBAP/DCM, with the Fc/Fc+ half-wave potential (averaged from the three profiles) at 0.233 V in 0.1 M TBAP/DCM. UV/vis spectra of the bithiophene monomers (d). Other experimental conditions as listed in Figure 2.

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Figure 12. CVs of polyBBB (a) and polyBNB (b) films on 2 mm2 Pt button electrodes. Spectroelectrochemical data of polyBBB (c) and polyBNB (d) films on ITO-coated glass electrodes (the polyBBB film was grown from a solution of unknown monomer concentration in a solution of 1.00 mL of o-dichlorobenzene and 2.00 mL of 0.1 M TBAP/DCM with all other conditions as listed in Figure 2). CV and in situ conductivity profiles of polyBBB (e) and polyBNB (f) films on Pt interdigitated microelectrodes. The polyBBB film was grown as described above. All other conditions as in Figure 3.

the λmax of the BBB monomer when compared to BNB. The BAB monomer also has the characteristic long-axis transition at 262 nm along with an intermediate absorption at 322 nm that presumably arises from the localization of π-electrons on the bithiophene chromophore rather than more extensive intermolecular communication. In many respects, the properties of polyBBB and polyBNB are quite similar. PolyBBB has a more pronounced hysteresis on the Pt button electrode (Figure 12a,b), but the two polymers look comparable on the Pt interdigitated arrays (Figure 12e,f). Spectroelectrochemical studies reveal that the λmax of polyBNB is lower and broader than that of polyBBB (Figure 12c,d), but the polyBNB polaron peak has a higher λmax than that of polyBBB. Otherwise, the fine structure such as noticeable low energy shoulders is similar for both neutral polymer absorptions, and both present a broad polaron peak that later drifts into the near-IR. PolyBBB and polyBNB are both conductive, but the onset of polyBBB conductivity falls at a less positive potential and has a much more pronounced hysteresis on the return sweep (Figure 12e,f).

The experimental results for the neutral polymers agree with the semiempirical aromatic HOMOs, where the BBB and BNB trimers share a similar extent of wave function delocalization (Figure 13a,b). The charge transport properties of polyBNB in practice may be more affected by sterics than polyBBB, which leads to a higher oxidation potential even though the conjugation lengths appear similar. The average carbocycle-thiophene dihedral angle of the BNB trimer is 59° compared to 24° for the BBB trimer. The same dihedral angle was found to be 86° for the BAB trimer, which would not polymerize. BBB and BNB HOMO orbital densities are spread predominantly over nine aromatic units, but they appear in both cases to be most intense on the oligothiophene fragments. The fused benzene orthogonal to the conjugation path of the BNB trimer also contributes minimally to the overall wave function. The anthracene trimer BAB has density localized on the carbocycle as was found for TAT and EAE anthracene models (Figure 13c). The steric issues are relieved in the quinoidal HOMOs with both polyBBB and polyBNB displaying an average thiophenecarbocycle dihedral angle of 4°. The planarization of polyBNB


Figure 13. Semiempirical (AM1) nonplanar aromatic HOMOs of the BBB trimer (a), BNB trimer (b), and BAB trimer (c).

J. Phys. Chem. B, Vol. 114, No. 9, 2010 3115 fused carbocyclic units perturbs main chain electronic properties. The fused lateral unit can become more or less “aromatic” during the evolution of quinoidal structure. This conceptual valencebond resonance structure depiction of π-electron distribution was probed with benzene, naphthalene, and anthracene building blocks. The experimental and computational data revealed that the naphthalene building blocks (bearing one lateral π-conjugated moiety) behaved in many ways as did the simple benzene substituents. This behavior indicates that the naphthalene is able to accommodate extended delocalization necessary for longer ECLs despite the potential for steric clashes that would enhance the dihedral angles relative to the naphthalene plane. On the other hand, severe steric and electronic influences were at play within polymers built from the anthracene core that severely restricted the effective conjugation lengths of the XAX families of polymers. Therefore, formally quinoidal model systems were also examined computationally to alleviate influences from torsional angle enhancements of a steric origin. We attribute the electronic origins of the restricted conjugation lengths to the strong aromatic localization within the two lateral π-conjugated moieties of anthracene that evolve into formally aromatic sextets upon electrochemical oxidation and adoption of the quinoidal electronic structure. This competition for aromaticity and subsequent stabilization thus limits the extended delocalization along the polymer as was observed in one case for the EAE polymer that successfully polymerized. Although steric concerns also influence the results, this comparative study suggests that there should be observable and measurable differences in electronic properties of conjugated polymers where local aromaticity is perturbed on fragments orthogonal or otherwise cross-conjugated to the main chain π-electron pathway. To study this in more detail, we are now studying model systems and their associated charged radical cations to better understand the steric and electronic influences leading to restricted conjugation lengths. Furthermore, we are currently designing new chemical systems where such changes can be executed externally and in real time in order to create tunable conductive polymers attenuated by disruption or evolution of localized aromaticity on these fused fragments. Acknowledgment. The authors thank Johns Hopkins University and the National Science Foundation (CAREER, DMR0644727) for support of this work.

Figure 14. Semiempirical (AM1) wave functions calculated for the nonplanar aromatic LUMOs of the BBB trimer (a), the BNB trimer (b), and the BAB trimer (c) as well as the more planar quinoidal HOMOs of the BBB trimer (d) and the BNB trimer (e). The BAB trimer quinoidal LUMO could not be calculated at semiempirical or DFT levels of theory.

as imposed by the quinoidal bonding leads to a more dramatic change in the extent of the wave function delocalization than that of polyBBB (see Figure 14a,d vs Figure 14b,e). The quinoidal naphthalene model in this frontier level has larger orbital coefficients on more aromatic components when compared to the BBB model (Figure 14 d,e). The calculation of the quinoidal structure for BAB failed at both the semiempirical and the DFT levels of theory. 4. Conclusions This report describes a combined computational and experimental effort to understand how lateral π-conjugation within

Supporting Information Available: Proton and carbon NMR data for all monomers, along with full computational results from the AM1 calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bre´das, J.-L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309–315. Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. ReV. 1988, 88, 183–200. Hill, M. G.; Mann, K. R.; Miller, L. L.; Penneau, J.-F. J. Am. Chem. Soc. 1992, 114, 2728–2730. (2) Kertesz, M.; Choi, C. H.; Yang, S. J. Chem. ReV. 2005, 105, 3448– 3481. (3) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382–3384. (4) Boudreaux, D. S.; Chance, R. R.; Elsenbaumer, R. L.; Frommer, J. E.; Bredas, J. L.; Silbey, R. Phys. ReV. B 1985, 31, 652–655. Jenekhe, S. A. Nature 1986, 322, 345–347. Hanack, M.; Hieber, G.; Mangold, K. M.; Ritter, H.; Rohrig, U.; Schmid, U. Synth. Met. 1993, 55, 827–832. Chen, W. C.; Jenekhe, S. A. Macromolecules 1995, 28, 465–480. (5) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Polym. Bull. 1992, 29, 119–126. Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 5065– 5066. (6) Perlstein, J. H. Angew. Chem., Int. Ed. Engl. 1977, 16, 519–534. Wudl, F. Pure Appl. Chem. 1982, 54, 1051–1058.

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JP9101459

Comparative Survey of Conducting Polymers Containing Benzene

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