Preparation, Photophysics, and Electrochemistry of Segmented

Sep 11, 2007 - João Pina , J. Sérgio Seixas de Melo , Rosa M. F. Batista , Susana P. G. Costa and M. Manuela M. Raposo. The Journal of Physical Chem...
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J. Phys. Chem. B 2007, 111, 11407-11418

11407

Preparation, Photophysics, and Electrochemistry of Segmented Comonomers Consisting of Thiophene and Pyrimidine Units: New Monomers for Hybrid Copolymers Ste´ phane Dufresne,† Garry S. Hanan,‡ and W. G. Skene*,† De´ partement de Chimie, PaVillon J. A. Bombardier, UniVersite´ de Montre´ al, CP 6128, succ. Centre-Ville, Montre´ al, Que´ bec, H3C 3J7, Canada, and De´ partement de Chimie, PaVillon Roger-Gaudry, UniVersite´ de Montre´ al, 2900 Edouard-Montpetit, Montre´ al, Que´ bec, H3T 1J4, Canada ReceiVed: July 5, 2007

An efficient coupling route to novel π-conjugated comonomers consisting of pyrimidine, thiophene, and bithiophene units was developed. The novel π-donor-acceptor-donor and π-donor-acceptor-acceptordonor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman homocoupling reactions. Photophysical investigation of these alternating π-donor and acceptor compounds indicated that the deactivation of their singlet excited state proceeds predominately by fluorescence and results in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%. Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene produced its triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative to its metal-free counterpart.

Introduction Conjugated polymers have received much attention because of their electrochemical and photophysical properties that are ideal for the development of functional devices including field effect transistors,1-3 light emitting diodes,4-10 and solar cells,11-13 to name but a few. Preparation of π-conjugated polymers is well established including many powerful synthetic tools such as Suzuki coupling14,15 and electropolymerization16-18 of heterocyclic monomers. However, polymers prepared via electropolymerization protocols often contain defects arising from competing side reactions such as over oxidation, R,β-coupling, and cross-linking, as well as other side reactions due to mismatching of monomer redox potentials. A potentially attractive solution to this problem involves the use of comonomers because they offer the possibility to reduce such polymer defects that otherwise would limit the degree of conjugation resulting in inadequate properties. Comonomers contain the elements of the copolymer structure such as a central aromatic acceptor core inserted between electron-rich donors. This configuration simplifies the electropolymerization process that would otherwise require two or three separate monomers to obtain analogous alternating copolymers via other coupling methods. The π-donoracceptor-donor relationship further permits tailoring the redox potentials and other properties of both the monomer and the resulting polymers to match those desired for a given application. * Address correspondence to this author. E-mail: [email protected]. † De ´ partement de Chimie, Pavillon J. A. Bombardier, Universite´ de Montre´al. ‡ De ´ partement de Chimie, Pavillon Roger-Gaudry, Universite´ de Montre´al.

Comonomers consisting of electron-deficient heterocycles such as pyridines and pyrimidines19 sandwiched between electron-rich units further provide the means to tailor polymer properties and they benefit from intramolecular charge transfer. The π-deficient heterocycles can easily be reduced and polymers derived from such comonomers are equally n-dopable. Incorporation of external π-rich units into the comonomer affords polymers that can be both p- and n-doped. Such polymers are of great interest because emitting devices constructed from them require fewer layers and result in easier device assembly, in addition to increased efficiency.20-22 Unfortunately, there are only a limited number of polymers capable of sustaining both oxidation and reduction with the capacity to act as mutual charge and hole carriers; indeed, this shortage is due primarily to the complicated synthesis of such polymers. An additional advantage of polymers incorporating nitrogen-containing heterocycles is their capacity to bind metal ions, leading to hybrid polymers. The conductivity, photophysics, and redox potentials of these hybrid polymers can be easily modulated by incorporating different metals, thereby making them suitable for a variety of applications including sensors17,23,24 for different anions and small molecules and electronic uses including field effect transducers,25 solar cells,26 and spintronics.27 An added benefit of heterocyclic comonomers is that their electropolymerization can occur in two ways as represented in Scheme 1. These involve (1) electropolymerization of the pristine comonomer followed by metal ion complexation into the resulting polymer or (2) incorporating a metal ion into the comonomer to form a metal-ligand complex followed by its polymerization. Our previous endeavors in comonomer synthesis28 and our ongoing photophysical and electrochemical investigation of conjugated polymers29-35 have prompted us to prepare a new

10.1021/jp075259j CCC: $37.00 © 2007 American Chemical Society Published on Web 09/11/2007

11408 J. Phys. Chem. B, Vol. 111, No. 39, 2007

Dufresne et al.

SCHEME 1: Synthetic Representation of Possible Anodic Polymerization of Comonomers Either by Incorporating the Metal Prior To or After Electropolymerization

series of alternating π-rich and π-poor comonomers consisting of thiophenes and pyrimidines. These compounds are of particular interest because they are expected to exhibit the much sought after mutual n- and p-doping properties. Furthermore, the electron donor-acceptor arrangement leads to efficient intramolecular charge transfer providing the means to reduce the HOMO-LUMO energy levels and to tailor the spectroscopic and electrochemical properties.14,36-38An additional advantage of the nitrogen-containing heterocycles is that they can act as ligands and coordinate metal ions leading to hybrid polymers. Herein, we present the preparation of novel comonomers along with their steady-state and time-resolved photophysics, electrochemistry, and HOMO-LUMO energy gap characterization. We also discuss the influence of the number of thiophene and pyrimidine units on the photophysics and electrochemistry. The comonomer electropolymerization and n- and p-doping behavior of the resulting polymers are also investigated along with their physical properties and the capacity of a polymer model to bind ruthenium.

CHART 1: Compounds Examined and Their Analogues

Results and Discussion Synthesis. Synthesis of the novel comonomers in Chart 1 involved conventional Suzuki and Ullman couplings starting from inexpensive and commercially available reagents according to Scheme 2. The precursor 1, required for Suzuki coupling, was prepared in moderate yields by nucleophilic aromatic chlorination of 4,6-dihydropyrimidine in the presence of triethylamine. This volatile base was selected instead of N,Ndimethylaniline that is known to mediate this type of coupling merely because excess base can be easily removed making for easy purification of the product. However, this base is most likely responsible for the observed moderate product yields since similar reactions occurred in higher yields with N,N-dimethylaniline.39 The complementary 2-thiophene boronic acid (2) and 2-bithiophene boronic acid (3) precursors required for Suzuki coupling were obtained by standard aryl anion formation with n-BuLi followed by nucleophilic addition to triethylboronate ester starting from inexpensive thiophene and bithiophene, respectively. Even though the reagents for Suzuki coupling were easily synthesized, the hetero aryl-aryl coupling proved problematic with standard coupling protocols in toluene and aqueous Na2CO3 with Pd(PPh3)4. The desired heterocoupling occurred only in aqueous isopropanol most likely because this

solvent ensures good homogeneity between the catalyst and reagents. Interestingly, the reaction proceeded cleanly at 50 °C with only 3 mol % of catalyst. Product selectivity between mono- and bis-coupling to afford 4 and 6, respectively, could be marginally controlled via different reagent stoichiometries. For example, 4 was obtained in 71% with a 1:1 ratio of 1 and 2 while a 1:2 ratio afforded 86% of 6. The same trend in the product distribution was observed with 3. The Ni(0)-mediated Ullman homocoupling conversely proceeded in low yields to provide 8 and 9 from 4 and 5, respectively. Spectroscopy. The effect of the alternating π-donor and acceptor groups and the number of thiophenes and pyrimidines on the energy levels is evident from the spectroscopic data that are tabulated in Table 1. A bathochromic shift of 50 nm relative to the starting reagent 1 occurred with the substitution of one chlorine by thiophene to afford 4. Even though unsubstituted pyrimidine is a more suitable reference, 1 was chosen simply

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SCHEME 2: Synthetic Routes Used for the Synthesis of Various π-Donor-Acceptor Comonomers

TABLE 1: Spectroscopic Values of the Various Comonomers Measured in Anhydrous Dichloromethane compd

abs.a (nm)

λmax (M-1 cm-1)

Em.b (nm)

∆Ec (eV)

Egd (eV)

kre (ns-1)

knrf (ns-1)

Φflg

τh (ns)

1 4 5 6 7 8 9

253 301 372 329 396 270/333 396

3 900 26 100 27 400 36 300 54 100 48 000 55 500

269 364 450 378 479 448 474

4.7 3.7 3.0 3.5 2.9 3.4 2.9

4.5 3.5 2.9 3.4 2.7 3.3 2.7

0.13 0.37 0.31 0.31 0.06 0.25

1.49 0.03 1.24 0.09 0.40 0.20

3 × 10-4 0.08 0.92 0.20 0.77 0.13 0.55

0.63 2.47 0.64 2.45 2.30 2.17

k

Phosi (nm)

ETj (kJ mol-1)

443 507 702 518 710 512/704 712

270 236 170 230 168 234/170 168

a Absorption maximum. b Fluorescence maximum. c Absolute HOMO-LUMO spectral difference. d Spectroscopic energy gap. e k ) Φ /τ . f k r fl fl nr ) kr(1 - Φfl)/Φfl. g Fluorescence quantum yields measured at λmax, relative to naphthalene,43,44 bithiophene,45 anthracene,43,44 or fluorescein46 depending on λabs max. h Monoexponential fluorescence lifetime measured at maximum λem. i Phosphorescence maximum measured in 1:4 methanol/ethanol matrix at 77 K. j Triplet energy. k Fluorescence too weak to measure lifetime.

because it was the starting reagent used for the synthesis of the compounds studied while also allowing for easy identification of any residual starting material with the sensitive spectroscopic methods used. Additionally, the influences of the degree of conjugation and the electron donating and withdrawing groups on the physical properties are possible with 1. The same trend in the absorption shift as with 4 was also observed for 7. Further coupling of an additional thiophene results in a 71 nm bathochromic shift upon going from 4 to 5 and a similar 67 nm shift from 6 to 7. Similarly, the fluorescence spectra also show a bathochromic shift that correlates with the increased number of thiophenes resulting from the increased degree of conjugation. The emission is shifted by 100 nm with the addition of each thiophene unit relative to 4. This trend confirms an increasing degree of conjugation resulting from narrowing of the HOMOLUMO energy levels, which in turn are responsible for the observed bathochromic shifts in the spectroscopic data. The absence of vibronic transitions in the observed fluorescence spectra suggests (Figure 1) that there are no defined transitions in the singlet excited and the ground states and is consistent with other oligothiophenes.40-42 The absolute energy difference between the ground (HOMO) and excited (LUMO) states can be calculated from the intercept of the normalized absorption and emission spectra. The spectroscopically measured values show a narrowing of the HOMO-LUMO energy gap when going from 1 to both 4 and 5 and the same trend is observed

Figure 1. Normalized absorbance (9) and fluorescence (0) spectra of 4 recorded in dichloromethane measured at room temperature and the phosphorescence (b) measured in a 1:4 methanol/ethanol matrix at 77 K.

upon going from 1 to 6 and 7. This is further supported by narrowing of the energy levels determined from the absorption onset whose values parallel those measured from the intercept method and decrease from 4.5 to 2.7 eV. Accurate energy gaps can be obtained from the absorption onset because little

11410 J. Phys. Chem. B, Vol. 111, No. 39, 2007 difference between the absorbances in solution and in thin films was observed. It can be concluded that the excited state energy is lowered with the addition of each thiophene to the comonomer. Given the spectral shifts and the increased degree of conjugation found for 4-7, significant bathochromic shifts for both the absorption and fluorescence were expected for 8 and 9. Interestingly, only marginal shifts were observed and the spectroscopic properties for both 8 and 9 are similar to those for 6 and 7, respectively. This is ascribed to the limited conjugation between the two pyrimidines and illustrates the segmented nature of the compounds in which the conjugation is isolated to the thiophene-pyrimidine segments only. The conjugation degree is further limited because substitution on the pyrimidine occurs via the meta position and not via the para position, which is know to promote extended electronic delocalization. Unfortunately, para substitution of the pyrimidine eliminates a vital metal coordination site and further limits the compounds as eventual hybrid polymers. Nonetheless, the meta substitution of 6-9 affords the means to assess the donoracceptor capacity of the compounds and the effect of the thiophene/pyrimidine moieties on the spectroscopic and electrochemical properties. The limited delocalization extension over the pyrimidine-pyrimidine segment owing to the meta substitution is further supported by the molar absorption coefficients of 8 and 9 that are both ∼50 000 M-1 cm-1 and they are similar to 7. This implies the pyrimidine units do not adopt a planar configuration and that they are most likely twisted from planarity leading to two isolated segments similar to biphenyl. A slight twist from planarity is also expected to occur between the thiophene-pyrimidine units supported by the crystal data (vide infra). However, the 50 nm bathochromic shift corresponding to a 40 kJ/mol lowering of the excited state energy for 7 versus 12 suggests the twisting between the dissimilar heterocycles is not significant and that the compound is rather conjugated, which can only occur as a result of coplanarity of the thiophene-pyrimidines. Despite the deviations from coplanarity, coordination of a metal center with the pyrimidine units of 8 and 9 is expected to reinforce a planar geometry leading to increased degree of conjugation. This is expected to result in significantly different spectroscopic properties compared to its metal-free analogue. Time-resolved and steady-state fluorescence techniques were used to further investigate the excited state properties of the compounds. The fluorescence quantum yields (Φfl) and singlet excited state lifetimes (τ) were obtained by these respective methods. In all cases, the Φfl is greater than 0.1 with the exception of 1 and 4. This is not surprising since halides are known to deactivate efficiently the singlet excited state by intersystem crossing (ISC) and populate the triplet state by heavy atom induced spin-orbit coupling (SOC). The net effect is a reduced fluorescence.47-49 High fluorescence quantum yields were, however, measured for the remaining compounds. The intense emission is unexpected owing to the thiophene unit that is known to efficiently promote triplet formation resulting in suppressed fluorescence.49 This phenomenon is consistent with oligo- and polythiophenes with the exception of thiopheno azomethines that deactivate their excited state by nonradiative internal conversion (IC).29 Insight into the deactivation modes responsible for the different fluorescence quantum yields is derived from the excited state dynamics. The measured fluorescence lifetimes were all monoexponential and are consistent with unimolecular deactivation modes. Moreover, the similar radiative (kr) and nonradiative (knr) rate constants of 4 and 6

Dufresne et al.

Figure 2. Transient absorption spectra of 4 recorded in deaerated and anhydrous dichloromethane at 5.4 (9) and 9.8 µs (O) after the laser pulse at 266 nm. Inset: Typical triplet decay kinetic of 4 monitored at 370 nm.

relative to 12 suggest these two comonomers undergo ISC to preferentially populate their triplet state. This manifold is assumed to be formed inefficiently by the other compounds reported in Chart 1 according to the calculated kr and knr values. Formation of the triplet manifold is supported by the phosphorescence measurements at 77 K, where a bathochromic emission relative to the fluorescence emission is seen according to Figure 1. Even though the low-temperature measurements artificially increase this triplet manifold, it nonetheless confirms triplet formation. The triplet energy (ET) values can also be derived from the 0,0 band, from which all the compounds were determined to exhibit roughly the same ET around 170 kJ/mol, and these are comparable to those of 12.50,51 Unfortunately, extensive information regarding the triplet state including quantification of this manifold is not possible from the lowtemperature measurements. Laser Flash Photolysis. Quantification of the triplet state is possible by laser flash photolysis (LFP). Direct excitation of all the comonomers yielded a transient absorption spectrum similar to that represented in Figure 2. All the comonomers investigated produced a triplet that was visible at 375 nm for the thiophene containing comonomers while the bithiophene derivatives exhibited a triplet-triplet absorption at 475 nm, recorded in Table 2. The observed transients were assigned to the triplet owing to their monoexponential decays, all of which fell within the standard microsecond range of triplet lifetimes, ranging between 5 and 15 µs. Conversely, deactivation of a radical cation reactive intermediate would follow second-order kinetics involving bimolecular radical cation coupling. Additional confirmation of the triplet nature of the observed transients is derived from the quenched signal with standard triplet deactivators including oxygen, 1,3-cyclohexadiene, and methylnaphthalene. Quantification of the produced triplets (ΦT) and their corresponding molar absorption coefficients (TT) is possible according to eq 1. These values are of interest not only because they are unknown, but also because they provide important information required to understand the excited states of the compounds while permitting an assessment of the suitability of these compounds for emitting applications. Quantification of the triplet state relative to an actinometer (xanthone) was possible; however, either ΦT or TT of the compound of interest must be known in order to extract any viable information from eq 1. Observing the growth of a common transient for both xanthone and the comonomer of the study simplifies eq 1 such that TT

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TABLE 2: Triplet State Properties of Various Comonomers Measured in Anhydrous and Deaerated Dichloromethane Excited at 355 nm molecule

λTTa (nm)

τTT (µs)

ko (104 s-1)

kq (MeNP) (106 M-1 s-1)

TTb (M-1 cm-1)

ΦTc

kISCd (107 s-1)

kICe (107 s-1)

λphosf (nm)

4g 5 6 7 8 9 12

370 470 380 475 370 470 385

5.7 10.7 12.3 9.5 6.9 14.1 1.5

18 9.3 8.1 11 14 7.1 67

6.5 33 10 13 11

15 000 9 000 12 500 13 500 17 500 29 500

0.05 0.22 0.14 0.11 0.11 0.31

2.0 34 5.7 4.8 5.1 36

1.2 91 3.7 33 16 78

507 702 518 710 512/704 712 601

a Triplet-triplet absorption maximum. b Molar absorption coefficient determined relative to xanthone in dichloromethane (TT ) 28 000 M-1 cm-1). c Triplet quantum yield measured against xanthone (ΦT ) 1) and methylnaphthalene in dichloromethane. d Intersystem crossing rate constant calculated according to kISC ) ΦT/τfl. e Internal conversion rate constant calculated according to kIC ) ΦIC/τfl ) (1 - Φfl - ΦT)/τfl. f Phosphorescence maximum measured at 77 K in 1:4 methanol/ethanol glass matrix. g Excited at 266 nm. Decomposes with laser radiation.

Figure 3. Rate constants of triplet decay for 8 measured in deaerated dichloromethane as a function of varying amounts of methylnaphthalene. Inset: Change in triplet decay lifetime of 8 monitored at 370 nm with 0 (0), 0.08 (b), 0.17 (O), and 0.42 mM (9) 2-methylnaphthalene.

becomes unity. This is possible by using a triplet quencher in sufficient quantity such that it quenches 95% of the produced comonomer and xanthone triplets. Methylnaphthalene (MeNp) is an ideal triplet quencher because of its low-lying triplet that quenches most triplets by energy transfer (ET), resulting in a visible triplet at 420 nm. The low-lying triplet results normally in rapid exothermic triplet quenching by ET, therefore only a small amount of the quencher is required. The amount of quencher required to deactivate 95% of the produced triplets can be calculated empirically according to ko > 20kq, where ko is the rate constant for triplet decay in the absence of quencher and kq is the intermolecular triplet rate constant for quenching with methylnaphthalene. The required kq values for the different comonomers are determined from the slope of Figure 3, which are obtained by observing the change in the triplet rate constant in the presence of varying amounts of quencher, Q (inset Figure 3), according to kobs ) ko + kq[Q]. The calculated rate constants reported in Table 2 are slower than diffusion controlled processes (1010 M-1 s-1) implying the energy transfer quenching process is endothermic. This implies the ET of the comonomers is lower than that of methylnaphthalene (379 kJ mol-1)52 and corroborates the spectroscopic values derived from the lowtemperature measurements. The required kq values could be calculated for all the compounds except 4, which decomposed under the photolysis conditions at 266 nm. The photoinstability of 4 was confirmed by steady-state irradiation at 254 nm resulting in dramatic changes in the ground state absorption (see the Supporting Information). Conversely, the remaining compounds from Chart 1 exhibited high photostability at 254 and

Figure 4. The maximum triplet of methylnaphthalene produced by quenching the triplet of 5 (9), 6 (b), 7 (0), 8 (2), 9 (O), 12 (1), and xanthone ([) via energy transfer measured as a function of laser power in dichloromethane. Inset: Typical triplet of methylnaphthalene monitored at 420 nm obtained by quenching a triplet donor via energy transfer.

350 nm allowing for triplet yield determination by laser flash photolysis by exciting at 355 nm.

Φunknown )

∆AbsunknownΦrefref ∆Absrefunkown hυ

comonomer 98 comonomer* ISC

comonomer *98 3comonomer 3

(1) (2) (3)

ET

comonomer + MeNp 98 comonomer + 3MeNp (4)

As shown in Figure 1, direct excitation of the comonomers generates their singlet excited state (2) that undergoes ISC to then form the triplet state (3). This manifold is then quenched quantitatively by methylnaphthalene resulting in its triplet formation (4) visible at 420 nm (inset Figure 4). Comparing the maximum signal of triplet methylnaphthalene, produced under similar conditions for both xanthone and the comonomers as a function of laser power, provides the representative plots shown in Figure 4. The triplet quantum yields are derived from the ratio of the slopes of xanthone and the comonomers according to (1) by the widely accepted method of relative actinometry with methylnaphthalene.44,53 The TT for the various comonomers were calculated from the ratio of the slopes obtained by observing the amount of xanthone triplet (ΦT ) 1 and TT ) 28 000 M-1 cm-1) measured at 630 nm relative to the triplet comonomers observed at their corresponding maxi-

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Dufresne et al. TABLE 3: Cyclic Voltammetrya Data of Various Thiophene-Pyrimidine Comonomers Measured in Anhydrous Dichloromethane

Figure 5. Cyclic voltammogram of 7 (9) and its polymer (O) obtained by anodic polymerization.

mum as a function of laser power. The triplet molar absorption coefficients were determined from such plots by using the calculated ΦT from Table 2 according to eq 1.54-56 The calculated values in Table 2 show the TT values for all the compounds are greater than 12 000 M-1 cm-1 confirming the conjugated nature of the transients. The similar TT observed for all the compounds corroborates the segmented degree of conjugation due to the meta substitution for the compounds since higher values would be observed for 8 and 9 if these were more conjugated than the other comonomers. The measured ΦT range from 0.1 to 0.2 with the exception of 5. The lower value observed for 5 relative to the other comonomers is consistent with its high fluorescence quantum yield. This result suggests there are only two possible deactivation modes for these compounds: (i) fluorescence and (ii) ISC. The low values for 6-9 are, however, surprising since thiophenecontaining compounds are known to have high triplet quantum yields.45,49 To validate our LFP results, we measured the ΦT for 12. Our measured value of 0.31 is in agreement with other spectroscopic studies57,58 and differs from the previously measured values by photoacoustic calorimetry.47 The discrepancy between these values can be in part due to the photoacoustic method, whose accuracy relies heavily on the triplet lifetime being longer than the detector’s response time. This notwithstanding, the low yield of triplet formation for the comonomers implies that their major pathway for deactivation occurs predominately by fluorescence according to the following energy conversation equation: Φfl + ΦISC + ΦIC ≈ 1, where IC is the nonradiative deactivation by internal conversion. From the combined fluorescence and ΦT data from Tables 1 and 2, respectively, deactivation by IC occurs in only a small amount according to the above energy conversion equation. The exception is 8 where IC plays a major role in dissipating the singlet excited state energy according to the above energy conservation equation and is further supported by the large knr. The observed long-lived singlet excited state and high fluorescence yields concomitant with the high singlet state energies make the comonomers ideal ligands for metal complexes and they are expected to exhibit light harvesting properties making them suitable for solar cell applications. Furthermore, the pristine blue emission displayed by the comonomers further makes them suitable candidates for use in emitting devices. Electrochemistry. The thiophene-pyrimidine’s capacity to undergo oxidation is evident from the cyclic voltammetric measurements represented in Figure 5. Only a one-electron

compd

Epa (V)

Epc (V)

HOMO (eV)

LUMO (eV)

Egb (eV)

1 4 5 6 7 8 9 10f 11g

1.2 1.3 1.5 1.2 1.4 1.6 1.5 1.4 1.5

-c -c -1.5 -c -1.7 -1.6 -1.5 -1.9 -2.3

5.3 5.5 5.7 5.3 5.4 5.7 5.6 5.7 5.7

0.8d 2.0d 2.5 1.9d 2.3 2.4 2.5 2.5 2.3

4.5e 3.5e 3.2 3.4 3.1 3.3 3.1 3.3 3.7

a Values reported against SCE. b Electrochemical energy gap calculated from the difference between the oxidation and reduction potential onsets. c Undefined reduction peak. d Approximated from the spectroscopically measured HOMO-LUMO energy gap (∆E) from Table 1. e HOMO-LUMO energy gap measured spectroscopically. f Literature value.22 g Literature value.59

process was observed for all the compounds, corresponding to oxidation of the thiophene unit to form a radical cation. Even through the specific thiophene unit that undergoes oxidation cannot be unambiguously assigned, the irreversible oxidation process consistently occurred at ca. 1.2 V for all the compounds reported in Table 3. Similarly, a one-electron reduction process was observed for all the compounds and is ascribed to the radical anion formation located on the pyrimidine moiety. The measured oxidation potentials are higher than those for similar thiophene compounds16,28 in part owing to the electron-deficient pyrimidine. The nitrogen-containing heterocycle is responsible for increasing the ionization potential of the compounds and further confirms the strong donor-acceptor nature of these compounds in which the electron-deficient pyrimidine exerts its influence on the thiophene core. By comparison, the oxidation potential of 8 is 250 mV higher than its bipyridine analogue 10, illustrating the difficulty in oxidizing the comonomer as a result of the additional nitrogen. Conversely, the reduction of 8 is much easier as evidence by the 200 mV lower reduction potential relative to 10.22 The same trend of more difficult reduction and easier oxidation holds true for 6 and 7 relative to their pyridine analogue 11. The difference in the redox potentials for these compounds is a result of the additional nitrogen that perturbs the HOMO and LUMO energy levels owing to its electrondeficient character. Determination of the HOMO and LUMO energy levels is possible from the oxidation and reduction potentials, respectively. The oxidation onset (Eox onset) provides the ionization potential (IP) according to IP ) Eox onset(SCE) + 4.4, where Eonset(SCE) is the oxidation potential onset in volts versus the SCE electrode. Similarly, the LUMO energy level can be calculated from the electron affinity (EA) according to the red reduction potential onset (Ered onset) by EA ) Eonset(SCE) + 4.4. The energy gap (Eg) between the two levels is the difference between the EA and IP. From the calculated values reported in Table 3, it can be concluded that the HOMO and the LUMO energy levels vary by 39 and 58 kJ/mol, respectively. The oxidation potential onsets taken together with the spectroscopic data provide an accurate representation of the influence of the adjacent electron-rich and electron-poor aryl groups on the energy levels and the electronic effect, schematically depicted in Scheme 3. It is obvious from this scheme that the adjacent aryl units affect the LUMO more than the HOMO energy level. As expected, an adjacent pyrimidine decreases the LUMO level more than an adjacent thiophene because of its electron-

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SCHEME 3: Schematic Representation of the HOMOLUMO Energy Gaps Calculated According To the Oxidation and Reduction Onset Potentials, Respectively

Figure 6. HOMO (left) and LUMO (right) of 6 calculated by DFTB3LYP with the 6-31g* basis set using the crystal data as the optimized geometry.

withdrawing capacity. Conversely, the HOMO level is influenced by the electron-rich thiophene. The increased degree of conjugation moreover decreases the energy difference between the HOMO and LUMO energy levels. Confirmation of the pyrimidine-deficient character and the electronic-rich nature of thiophene are evidenced in Figure 6. This figure shows the orbital densities calculated by DFT-B3LYP for 6, using the 6-31g* basis set from our crystallographic data. It is clear from this figure that the LUMO density is concentrated predominately on the nitrogen centers while the HOMO is localized on the thiophene. This allows for intramolecular charge transfer between the electron-poor and the electron-rich segments and further contributes to the donor-acceptor charge transfer and narrowing of the HOMO-LUMO energy levels. The charge transfer further promotes the distribution of the electronic density across the entire conjugated compound resulting in broad absorption and emissions spectra.60 This is supported by the absence of discrete vibronic transitions observed in the absorption and fluorescence spectra. Since the LUMO and the HOMO levels are controlled by the pyrimidine and the thiophene units, respectively, tailoring of the oxidation and the reduction potentials is possible by adjusting the modular segments. This affords the means to customize the electronic properties for a given application. Electropolymerization. The irreversible reduction of the radical cation observed with scan rates ranging from 100 to 1000 mV/s confirms the reactive nature of this transient. Its reactivity was exploited to promote anodic polymerization via homocoupling according to two methods: (i) by applying a constant potential 100 mV more positive than the observed oxidation potential and (ii) by repeated cycling of the potential between 0 and 1700 mV. Polymer deposition onto the platinum working electrode was possible with the former method. Electrochemical and spectroscopic characterization of the resulting polymer deposited onto the electrode was, however, problematic. Anodic polymerization of the comonomers was subsequently done on a transparent ITO electrode to circumvent these short comings. Anodic polymerization of 6-9 on ITO afforded the results recorded in Table 4. A significant reduction in the oxidation potential is normally associated with polymers obtained by anodic polymerization as a result of the high degree of conjugation occurring from R-R homocoupling. However, the electron-withdrawing pyrimidine leads to acceptor terminated segments with an A-D-D-A arrangement for P6 and P8 and a A-D-D-D-D-A configuration for the polymers P7 and P9. The strong electron acceptor group in both cases counter affects any HOMO-LUMO energy reduction and any lowering of the

anodic potential that are otherwise gained from the increased degree of conjugation occurring from extended delocalization. Even though the deposited products on the ITO electrode exhibit higher oxidation potentials relative to their corresponding comonomers, their polymeric nature is supported by the spectroscopic data. A bathochromic shift between 80 and 150 nm in the absorption spectra occurs for all polymers relative to their corresponding comonomers. Similarly, fluorescence bathochromic shifts corresponding to lowering of the excited state energy by 26-84 kJ/mol occur, leading to HOMO-LUMO energy gaps for the homocoupled products that are lower than their corresponding comonomers. The net decrease in the energy levels is a result in the increased degree of conjugation that can only come from polymer formation. Both the absorption and the fluorescence bathochromic shifts of the polymers relative to their corresponding comonomer decrease less upon going from 6 to 9 owing to the limited degree of conjugation from the meta substitution. The same decreasing trend was also observed for the absolute HOMO-LUMO energy difference in the series 6 to 9. This trend further demonstrates that the degree of conjugation is confined to the thiophene-thiophenepyrimidine portion of the molecule for 7 and 9 and it does not extend across adjacent pyrimidines. Similarly, the conjugation is confined to the thiophene-pyrimidine segment for 6 and 8. The spectroscopic data and the bathochromic trend nonetheless provide evidence that R-R homocoupled products are obtained by anodic polymerization. Anodic coupling of 4 and 5 were additionally examined to further confirm that 6-9 afforded homocoupled products via radical cation coupling of the terminal thiophenes. Since these two compounds possess only one vacant terminal site that is capable of sustaining homocoupling, anodic R-R coupling of the resulting radical cations would result only in soluble dimers. Conversely, undesired R-β coupling would lead to insoluble and unconjugated polymers deposited as discolored films on the ITO electrode. The absence of insoluble material deposited on the ITO electrode after anodic coupling of 4 and 5 in addition to no visible changes in the absorption and fluorescence spectra of the transparent electrode further confirm that these compounds do not undergo any decomposition reactions or any unwanted R-β coupling defects. Undeniable proof of anodic homocoupling of 6-9 affording higher ordered polymers is derived from the large-scale anodic polymerization of 8. A large surface area mesh electrode was used to obtain sufficient material for conventional molecular weight characterization. A molecular weight of 900 g/mol and a polydispersity index of 1.1 were measured by conventional GPC for only the soluble fraction (oligo-8). Even though the calculated average molecular weight corresponds to a product with 12 aromatic units, this represents a lower limit for the molecular weight of the soluble coupled products, while the molecular weight of the bulk insoluble material deposited on the ITO electrode by similar anodic polymerization is much higher.29,31,35,61 It is clear from Table 4 that the absorption and the fluorescence spectra for the soluble oligo-8 are bathochromically shifted by approximately

11414 J. Phys. Chem. B, Vol. 111, No. 39, 2007

Dufresne et al.

TABLE 4: Electrochemical and Spectroscopic Data of Polymers Deposited on an ITO Electrode by Anodic Polymerization polymera

abs.b (nm)

Em.c (nm)

Epa (V)

Epc (V)

HOMO (eV)

LUMO (eV)

Eg(echem.)d (eV)

∆E(spec.)e (eV)

P6 P7 P8 P9 oligo-8f Ru-oligo-8f

475 533 479 484 378 513

503 565 506 508 427 606

1.4 1.9 2.1 2.0 1.5 1.3

-1.1 -0.5 -0.6 -0.8 -1.5 -1.3

5.7 6.2 6.4 6.3 5.8 5.6

3.2 3.8 3.6 3.5 2.8 3.0

2.5 2.4 2.8 2.8 3.0 2.6

2.5 2.2 2.5 2.5 3.1 2.3

a Thin film measurements were done directly on the ITO electrode. b Absorption maximum. c Fluorescence maximum. d HOMO-LUMO energy gap calculated from the oxidation and reduction onset differences. e Absolute HOMO-LUMO difference taken from the absorption and fluorescence spectral intercept. f Measured in dichloromethane.

CHART 2: Possible Heteroleptic and Homoleptic Ruthenium Complexes of Comonomer 8, Respectively

50 nm relative to 8. This results in a 0.3 eV narrowing of the HOMO-LUMO energy gap that can result only from an enhanced degree of conjugation arising from R-R homocoupling via the terminal thiophenes. Given the more pronounced bathochromic shifts of 100 nm of P8 relative to oligo-8 concomitant with the lower HOMO-LUMO energy gap of 0.6 eV observed for P8, it can be confirmed that the insoluble film deposited on the ITO possesses a higher degree of conjugation and therefore is indeed a polymer with DP > 10. The spectroscopic data provide additional evidence that P8 is a conjugated polymer with substantially higher molecular weight than oligo-8. It can thus be concluded that the anodic polymerization of 6-9 occurs by R-R type coupling via the unsubstituted 5-thiophene position leading to conjugated polymers. Since pyrimidines can coordinate with metal ions, we examined the possibility of forming a ruthenium complex with 8 to afford hybrid polymers. Such polymers can be achieved by two possible routes: either polymerization followed by metal complexation or direct polymerization of the metal-ligand complex depicted in Scheme 1. We first examined the latter method using ruthenium heteroleptic and homoleptic complexes 13 and 14 shown in Chart 2. These two compounds were examined because their physical properties are well studied and therefore would allow for easy investigation of anodic polymerization leading to hybrid alternating copolymers. Unfortunately, anodic polymerization of both ruthenium complexes failed to yield any detectable polymers despite countless efforts with various reaction conditions and is consistent with other reports.23,24,62 The absence of homocoupled products is most likely a result of the steric hindrance of the terminal thiophenes and the incorrect geometry of the metal-ligand complex required for R-R coupling. Electropolymerization of the comonomer followed by metal ion coordination was alternatively pursued. Oligo-8 was examined for metal coordination because its metal complex readily lends itself to subsequent photophysical and electrochemical characterization owing to its

TABLE 5: Details of Crystal Structure Determination for 6 formula MW; F(000) crystal color and form crystal size (mm3) T (K); dcalc. (g/cm3) crystal system space group unit cell: a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3); Z θ range (deg); completeness reflections: collected / independent; Rint µ (mm-1) abs corr R1(F); wR(F2) [I > 2σ(I)] R1(F); wR(F2) (all data) GoF(F2) max residual e- density

C12H8N2S2 244.32; 1008 yellow plate 0.28 × 0.24 × 0.04 150(2); 1.450 monoclinic P21/c 19.194(4) 11.297(2) 10.366(2) 90.000 95.35(3) 90.000 2237.8(8); 8 2.31-71.88; 0.982 26249/4310; 0.041 4.065 semiempirical 0.0390; 0.1088 0.0451; 0.1123 1.061 0.300 e-‚Å-3

solubility in most organic solvents, contrary to P8. Complexation of RuCl3 with the pyrimidine oligo-8 ligand in dichloromethane at room temperature led to significant spectroscopic and electrochemical changes. Even though a weak metal-to-ligand charge-transfer (MLCT) band was observed at 513 nm, characteristic of Ru(II) complexes,63,64 strong evidence supporting Ru-oligo-8 complexation is the intense emission at 606 nm, also indicative of Ru(II) complex formation.65-67 The observed emission is shifted bathochromically by 169 nm from that of the free oligo-8 corresponding uniquely to the Ru-oligo-8 complex. The sufficiently different absorption and emission changes associated with metal ion complexation compared to the free ligand and Ru(III) confirm formation of the Ru-oligo-8 complex. The voltammogram of the Ru complex revealed that both the oxidation and the reduction potentials are decreased by 200 mV relative to oligo-8 while no redox processes corresponding to the uncomplexed oligo-8 were observed. The spectroscopic and electrochemical data prove unprecedented metal ion complexation postpolymerization is possible. The resulting physical properties of the hybrid polymers are comparable to those of highly conjugated metal-free polymers of high molecular weight. Therefore, hybrid polymers that are metal-containing conjugated materials provide the means to tailor the spectroscopic and electrochemical properties of polymers. Crystallography. Suitable crystals for X-ray diffraction were obtained by slow evaporation of 6 in dichloromethane. As shown in Figure 7, the structure of 6 consists of two different aryl units: two thiophenes and one pyrimidine. The molecule appears to be almost perfectly symmetric, with two crystallographically different molecules in the unit cell. The major difference

New Monomers for Hybrid Copolymers

J. Phys. Chem. B, Vol. 111, No. 39, 2007 11415 The measured distances for these interactions are within the limit of the sum of van der Waals radii calculated to be 3.00 and 2.75 Å, respectively. The aryl-aryl and the donor-acceptor interactions contribute to a close-packed crystal structure arrangement. Conclusion

Figure 7. ORTEP representation of the crystal structure of 6 (top) with the atom numbering scheme. Schematic representation of 6 seen along the a-c plane (bottom).

A synthesis of novel conjugated thiophene-pyrimidine comonomers was presented. The singlet excited state of these novel π-donor-acceptor-donor and π-donor-acceptor-acceptor-donor conjugated comonomers preferentially deactivate by radiative means leading to high fluorescence quantum yields, confirmed by laser flash photolysis in which the triplet state was formed in only 10%. Both the spectroscopic and the electrochemical properties are dependent upon the number of π-donors and acceptors in the comonomer. Anodic polymerization on ITO plates afforded conjugated polymers that exhibit narrow HOMO-LUMO energy gaps and their absorption and fluorescence spectra are bathochromically shifted relative to their corresponding comonomers. The pendant pyrimidines from the polymer backbone act as potential π-acceptor sites for metals such as ruthenium, ultimately leading to conjugated polymers with incorporated metals providing new properties with the potential for functional devices such as field effect transistors and photovoltaic devices.25 Experimental Section

Figure 8. Crystal packing of 6 showing eight molecules per unit cell and the π-stacking occurring between alternating thiophene-pyrimidine units.

between the two molecules is the 3° deviation in the torsion angle between the planes of the aromatic units. Although the aryl units are coplanar, there exists a slight twist between the different aryl units as illustrated in Figure 7. The mean angles between the planes of the two terminal thiophenes relative to the central pyrimidine plane are 19.7(1)° and 6.1(1)° for one of the molecules in the lattice. Mean angles of 15.9(1)° and 3.2(1)° were found for the corresponding mean planes for the other molecule in the crystal lattice. The observed deviation from planarity is required to prevent steric hindrance between H13H16 and H16-H19. The distance between these hydrogens is approximately 2.35 Å compared to 2.40 Å for the sum of the van der Waals radii. The aryl group twisting limits the degree of conjugation as evidenced by the spectroscopic data. Therefore, no increase in the degree of conjugation is possible with 4 aryl units such as for 8 or 9 owing also to the meta substitution effect. Two π-interactions take place between two molecules within the lattice. π-Stacking occurs between the electron-poor pyrimidine and the electron-rich thiophene. These interactions take place between the same molecules within the lattice, represented in Figure 8. The distance between the aryl group is 3.89(1) Å. In addition to the aryl π-stacking, weak hydrogen-bonding interactions between S‚‚‚H-C and N‚‚‚H-C were also found.

Materials and General Experimental Details. All reagents were commercially available from Aldrich and were used as received unless otherwise stated. Anhydrous and deaerated solvents were obtained via a Glass Contour solvent purification system. Isopropanol was dried over activated molecular sieves and stored under nitrogen prior to use. 1H and 13C NMR spectra were recorded on a Bruker 400 spectrometer with the appropriate deuterated solvents. Spectroscopic Measurements. Absorption measurements were done on a Cary-500 spectrometer and fluorescence studies were carried out on an Edinburgh Instruments FLS-920 fluorimeter after deaerating the samples thoroughly with nitrogen for 20 min. Fluorescence quantum yields were measured at 10-5 M by exciting the corresponding compounds near the maximum absorption wavelength in spectroscopic grade dichloromethane using relative actinometry by exciting the references at the same wavelength. Naphthalene (φ275nm ) 0.21),44 bithiophene (φ303nm ) 0.013),45 anthracene (φ356nm ) 0.26),44 or fluorescein (φ494nm ) 0.92)46 were used as fluorescence references. The actinometer absorbances, and those of the compounds, were matched at the excitation wavelength to within 5%. Since the fluorescence quantum yields of naphthalene and anthracene in dichloromethane are unknown, these values were determined relative to the reported values in cyclohexane (φcyclohexane ) 0.23) and dichloromethane (φethanol ) 0.27), respectively.43,44 Phosphorescence measurements were done on a Cary Eclipse at 77 K, using a 4:1 mixture of ethanol/methanol. Steady-state irradiations were performed with a Luzchem photoreactor, using 8 lamps emitting at 254 nm and irradiating for 12 h at room temperature. Laser Flash Photolysis Measurements (LFP). Laser flash photolysis experiments were done on a Luzchem mini-LFP system excited at 355 nm with the third harmonic from a continuum YAG:Nd Sure-lite laser with the exception of 4 that was excited at 266 nm owing to an absence of absorption at 355 nm. The selected excitation wavelength was 355 nm for every analysis and the solutions were prepared with absorbances around 0.3 at this wavelength. The triplet quantum yields (ΦT)

11416 J. Phys. Chem. B, Vol. 111, No. 39, 2007 and molar absorption coefficients (TT) were measured by relative actinometry according to known methods,34 using xanthone in dichloromethane as the reference (ΦT ) 1 and TT ) 28 000). Since the required ΦT and TT values of xanthone in dichloromethane are unknown, these were measured relative to the reported values for xanthone in benzene (TT ) 5 300).54,56,68 Electrochemical Measurements. Cyclic voltammetric measurements were performed on a Bio Analytical Systems EC Epsilon potentiostat with scan rates varying from 100 to 1000 mV/s. The compounds of study were dissolved in anhydrous and deaerated dichloromethane at 10-5 M with 0.1 M NBu4PF6. Either a glassy carbon electrode or a platinum electrode was employed as the working electrode while a platinum wire was used as the auxiliary electrode. A saturated Ag/AgCl electrode was used as the reference electrode and the values were corrected to SCE in dichloromethane by using ferrocene as an internal reference (0.46 mV).69 ITO plates were purchased from Delta Technologies and were coated on one surface with RS ) 5-10 Ω. Rectangular shapes were cut to serve as the working electrode and they were used after cleaning according to standard protocols involving washing with water, acetone, and dichloromethane for 20 min each followed by ultrasonication. The cross-coupled products were electrochemically deposited onto the ITO Corning 1737 plates by 50 repeated anodic scans from 0 to 1.4 V at 100 mV/s. Alternatively, the anodic polymerization was done by applying an anodic potential 100 mV greater than the corresponding radical cation and held at that potential for 10 min. A final potential of -100 mV was applied for 2 min to ensure the resulting products were in the neutral form. Sufficient material of the cross-coupled oligo-8 derived from 8 required for standard polymer characterization methods was obtained according to the same experimental procedures described above with the exception of the working electrode. The standard platinum wire working electrode was replaced by a large area platinum mesh gauze. Several milligrams of 8 were dissolved in 75 mL of anhydrous dichloromethane and a potential 100 mV greater than its radical cation (i.e., 1.7 V) was applied for 20 min under a blanket of nitrogen followed by the usual 2 min at -100 mV. The solvent was removed under reduced pressure and the resulting oligo-8 was dissolved in a mixture of AcOEt:Et2O (50:50) where the undesired NBu4PF6 electrolyte precipitated and was removed by filtration. Molecular Weight Analysis of Oligo-8. The molecular weight was determined by standard gel permeation chromatography relative to polystyrene standards, using a Breeze system from Waters equipped with a 717 plus autosampler, a 1525 Binary HPLC pump, and a 2410 refractive index detector. Three Styragel columns HR3, HR4, and HR6 (7.8 × 300 mm2) in series (from Waters) at 33 °C were used to resolve the sample in THF with a flow rate of 1 mL/min. A calibration curve required to interpolate the molecular weight of the sample was created by using 10 polystyrene standards from Shodex. An average number molecular weight (Mn) of 900 g/mol and a polydispersity index of 1.1 were determined for oligo-8. Crystal Structure Determination. Diffraction data for 6 were collected on a Bruker Smart 6000 diffractometer, using graphite-monochromatized Cu KR radiation with 1.54178 Å. The structure was solved by using direct methods (SHELXS97). All non-hydrogen atoms were refined based on Fobs2 (SHELXS97), while the hydrogen atoms were refined on calculated positions with fixed isotropic U, using riding model techniques.

Dufresne et al. Synthesis. 4,6-Dichloropyrimidine (1). Triethylamine (6.34 mL, 46 mmol) and phosphorus oxychloride (13.0 mL, 182 mmol) were mixed dropwise in a 50 mL round-bottomed flask. Then 4,6-dihydroxypyrimidine (3.00 g, 26.8 mmol) was added dropwise. The reaction was refluxed for 1 h and then poured onto crushed ice. The precipitate was filtered and then purified by flash chromatography eluted with hexanes/ethyl acetate (50/ 50% v/v) to yield the title compound as a light yellow solid (1.74 g, 44%). Mp 58-60 °C. 1H NMR (CDCl3) δ 8.90 (s, 1H), 7.85 (s, 1H). 13C NMR (CDCl3) δ 162.5, 159.2, 122.4. HRMS(+) calcd for [C4H2N2Cl + H]+ 148.96733, found 148.96723. Thiophen-2-yl-2-boronic Acid (2). Thiophene (5.31 g, 63 mmol) and 2.5 M n-BuLi in hexanes (26.5 mL, 66 mmol) were mixed in THF (100 mL) at 0 °C under nitrogen. Trimethylborate (21.5 mL, 189 mmol) was added dropwise to the solution at -70 °C. The temperature was raised gradually and the mixture was then kept at room temperature for 1 h. A 10% H2SO4 (50 mL) solution was added followed by dichloromethane (50 mL). The organic layer was extracted and poured into 1 M NaOH (50 mL). The basic aqueous layer was extracted and 1 M HCl was added until pH 1-2 was obtained. This acidic layer was extracted with dichloromethane (50 mL) to afford a white-gray product (3.45 g, 43%). The compound was used without any further purification. Mp 137-139 °C. 1H NMR (acetone-d6) δ 7.71 (d, 1H, J ) 4.4 Hz), 7.31 (s, 1H), 7.18 (t, 1H, J ) 4.1 Hz), 5.79 (s, 2H). 13C NMR (acetone-d6) δ 136.1, 131.7, 128.4, 128.3. 5-(Thiophen-2-yl)thiophen-2-yl-2-boronic Acid (3). Bithiophene (3.00 g, 18 mmol) and 2.5 M n-BuLi in hexanes (7.56 mL, 19 mmol) were mixed together in THF (50 mL) at 0 °C under nitrogen. Trimethylborate (6.15 mL, 54 mmol) was added dropwise to the solution while keeping the temperature at -70 °C. The mixture was allowed to warm to room temperature and then stirred for 1 h before 10% H2SO4 (50 mL) was added. An extraction with dichloromethane was done before 1 M NaOH (50 mL) was added. The basic aqueous layer was extracted before 1 M HCl was added to give pH 1-2. A final extraction of the acidic layer with dichloromethane (50 mL) gave a whitegreenish product (3.06 g, 81%). The compound was used without further purification. Mp 159-161 °C. 1H NMR (acetone-d6) δ 7.62 (d, 1H, J ) 3.6 Hz), 7.45 (dd, 1H, J ) 5.2 and 1.1 Hz), 7.40 (s, 2H), 7.33 (dd, 1H, J ) 3.7 and 1.2 Hz), 7.32 (d, 1H, J ) 3.6 Hz), 7.10 (dd, 1H, J ) 5.2 and 3.6 Hz). 13C NMR (acetone-d ) δ 143.1, 137.7, 137.1, 128.5, 125.4, 6 125.3, 125.2, 124.5. HRMS(+) calcd for [C8H7BO2S2 + H]+ 211.0053, found 211.00489. 4-Chloro-6-(thiophen-2-yl)pyrimidine (4). 2 (0.564 g, 4.4 mmol) and 1 (0.653 g, 4.4 mmol) were dissolved in deoxygenated isopropanol (20 mL) to which was added 2 M aqueous Na2CO3 (5 mL). Tetrakis(triphenylphosphine)palladium (0.261 g, 0.24 mmol) was added to the solution, which was then refluxed for 16 h. The solution was extracted with dichloromethane and the organic layer was then purified by flash chromatography eluted with neat hexanes then the polarity was increased with hexanes/ethyl acetate (90/10% v/v) to yield a light yellowish solid (0.451 g, 52%). Mp 105-107 °C. 1H NMR (CDCl3) δ 8.89 (s, 1H), 7.79 (dd, 1H, J ) 3.8 and 0.9 Hz), 7.59 (dd, 1H, J ) 5.0 and 0.9 Hz), 7.58 (s, 1H), 7.18 (dd, 1H, J ) 5.0 and 3.8 Hz). 13C NMR (CDCl3) δ 162.0, 160.8, 159.3, 140.9, 131.8, 129.1, 128.9, 115.5. HRMS(+) calcd for [C8H5ClN2S + H]+ 196.99347, found 196.99418. 4-Chloro-6-(5-(thiophen-2-yl)thiophen-2-yl)pyrimidine (5). In deoxygenated isopropanol (40 mL) along with 2 M aqueous

New Monomers for Hybrid Copolymers Na2CO3 (5 mL) were dissolved 3 (1 g, 4.8 mmol) and 1 (0.563 g, 3.8 mmol). Tetrakis(triphenylphosphine)palladium (0.300 g, 0.28 mmol) was added to the solution, which was then warmed to 50 °C. This temperature was maintained for 15 h. The product was extracted with dichloromethane and the organic layer was purified by flash chromatography with hexanes/ethyl acetate (90/ 10% v/v) to yield a yellow-orange solid (0.750 g, 71%). Mp 153-155 °C. 1H NMR (CDCl3) δ 8.90 (s, 1H), 7.72 (d, 1H, J ) 4.0 Hz), 7.58 (s, 1H), 7.34 (d, 2H, J ) 4.3 Hz), 7.26 (d, 1H, J ) 4.0 Hz), 7.09 (dd, 1H, J ) 4.2 Hz). 13C NMR (CDCl3) δ 161.9, 160.4, 159.3, 143.9, 138.8, 136.8, 129.8, 128.6, 126.5, 125.6, 125.3, 115.1. HRMS(+) calcd for [C12H7ClN2S2 + H]+ 278.98110, found 278.98119. 4,6-Di(thiophen-2-yl)pyrimidine (6). In deoxygenated ethanol (20 mL) and 2 M Na2CO3 (5 mL) in distilled water were dissolved 2 (0.346 g, 2.70 mmol) and 1 (0.200 g, 1.35 mmol). Pd(PPh3)4 (0.300 g, 0.27 mmol) was added to the solution, which was then refluxed for 6 h. The product was extracted with dichloromethane and the organic layer was then purified by flash chromatography eluted first with neat hexanes then the polarity was increased with hexanes/ethyl acetate (90/10% v/v) to yield the product as a yellow solid (0.283 g, 86%). Mp 143-145 °C. 1H NMR (CDCl ) δ 9.07 (d, 1H, J ) 1.3 Hz), 7.85 (dd, 2H, J 3 ) 3.7 and 1.0 Hz), 7.81 (d, 1H, J ) 1.4 Hz), 7.56 (d, 2H, J ) 5.0 and 1.1 Hz), 7.20 (dd, 2H, J ) 5.0 and 3.7 Hz). 13C NMR (CDCl3) δ 159.6, 159.5, 142.6, 130.7, 128.9, 127.9, 109.2. HRMS(+) calcd for [C12H8N2S2 + H]+ 245.02017, found 245.02078. 4,6-Bis(5-(thiophen-2-yl)thiophen-2-yl)pyrimidine (7). In deoxygenated ethanol (30 mL) and 2 M Na2CO3 in distilled water (5 mL) were dissolved 1 (0.100 g, 0.68 mmol) and 3 (0.284 g, 1.35 mmol). Tetrakis(triphenylphosphine)palladium (0.074 g, 0.067 mmol) was added to the solution and it was then warmed to 50 °C for 18 h. The solution was extracted with dichloromethane and the organic layer was then purified by flash chromatography eluted first with hexanes then the polarity was increased with hexanes/ethyl acetate (90/10% v/v) to yield an orange solid (0.212 g, 77%). Mp dec 299 °C. 1H NMR (CDCl3) δ 9.05 (d, 1H, J ) 1.2 Hz), 7.79 (d, 2H, J ) 4.0 Hz), 7.77 (d, 1H, J ) 1.2 Hz), 7.34 (d, 2H, J ) 3.6 Hz), 7.33 (d, 2H, J ) 5.1 Hz), 7.28 (d, 2H, J ) 4.1 Hz), 7.09 (dd, 2H, J ) 5.1 and 3.6 Hz). 13C NMR (CDCl3) δ 159.2, 159.0, 142.8, 140.4, 137.2, 128.9, 128.6, 126.2, 125.4, 125.2, 108.5. HRMS(+) calcd for [C20H12N2S4 + H]+ 408.99561, found 408.99391. 4-(Thiophen-2-yl)-6-(6-(thiophen-2-yl)pyrimidin-4-yl)pyrimidine (8). Nickel chloride hexahydrate (0.048 g, 0.20 mmol) and triphenylphosphine (0.225 g, 0.86 mmol) were mixed in deoxygenated DMF (10 mL). Zinc dust (0.050 g, 0.76 mmol) was added and the blue solution was warmed to 60 °C until it turned red. Then 4 (0.200 g, 1.02 mmol) was added to the mixture and the reaction was poured into 10% NH4OH (50 mL) after 5 h. The precipitated solid was filtered and was then purified by flash chromatography first with neat hexanes then the polarity was increased with neat ethyl acetate. The product was isolated as a yellow solid (0.074 g, 45%). Mp 245247 °C. 1H NMR (CDCl3) δ 9.30 (s, 2H), 8.77 (s, 2H), 8.03 (d, 2H, J ) 3.7 Hz), 7.63 (d, 2H, J ) 5.0 Hz), 7.25 (dd, 2H, J ) 4.8 and 3.9 Hz). 13C NMR (CDCl3) δ 161.2, 161.2, 159.2, 142.3, 131.6, 129.2, 129.2, 112.3. HRMS(+) calcd for [C16H10N4S2 + H]+ 323.04196, found 323.04192. 4-(5-(Thiophen-2-yl)thiophen-2-yl)-6-(6-(5-(thiophen-2-yl)thiophen-2-yl)pyrimidin-4-yl)pyrimidine (9). Nickel chloride hexahydrate (0.169 g, 0.70 mmol) and triphenylphosphine (0.764 g, 2.92 mmol) were mixed in deoxygenated DMF (15 mL). Zinc

J. Phys. Chem. B, Vol. 111, No. 39, 2007 11417 dust (0.120 g, 1.82 mmol) was added and the blue solution was warmed to 60 °C until it turned red. Then 5 (0.400 g, 1.44 mmol) was added to the mixture and the reaction was poured into 10% NH4OH (50 mL) after an additional 4 h of stirring. The precipitate was then washed with ethyl acetate and dichloromethane. The product was isolated as an orange powder (0.150 g, 43%). Mp 290 dec °C. 1H NMR (CDCl3) δ 9.27 (s, 2H), 8.73 (s, 2H), 7.93 (d, 2H, J ) 4.0 Hz), 7.37 (d, 2H, J ) 3.5 Hz), 7.35 (d, 2H, J ) 5.1 Hz), 7.31 (d, 2H, J ) 3.9 Hz), 7.11 (dd, 2H, J ) 4.8 and 3.8 Hz). HRMS(+) calcd for [C24H14N4S4 + H]+ 487.01744, found 487.01788. Acknowledgment. The authors acknowledge financial support from the Natural Sciences and Engineering Research Council Canada, Fonds de Recherche sur la Nature et les Technologies, and the Canada Foundation for Innovation. Prof. D. Zargarian is thanked for helpful discussions and S.D. extends appreciation to the Universite´ de Montre´al for a graduate scholarship. Johnson Matthey PLC is thanked for the gift of RuCl3. Supporting Information Available: 1H and 13C NMR spectra of compounds 1-9 and the absorption, fluorescence, and cyclic voltammograms of 1 and 4-9. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kraft, A. Chem. Phys. Chem. 2001, 2, 163-165. (2) Katz, H. E.; Dodabalapur, A.; Bao, Z. Oligo- and polythiophene field effect transistors. In Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-VCH Verlag GmbH: Weinheim, Germany, 1999; pp 459489. (3) Horowitz, G. AdV. Mater. 1998, 10, 365-377. (4) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402-428. (5) Leclerc, M. J. Polym. Sci. Part A: Polym. Chem. 2001, 17, 28672873. (6) Parthasarathy, G.; Liu, J.; Duggal, A. R. Electrochem. Soc. Interface 2003, 12, 42-47. (7) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471-1507. (8) Veinot, J. G. C.; Marks, T. J. Acc. Chem. Res. 2005, 38, 632-643. (9) Perepichka, I. F.; Perepichka, D. F.; Meng, H.; Wudl, F. AdV. Mater. 2005, 17, 2281-2305. (10) Williams, E. L.; Haavisto, K.; Li, J.; Jabbour, G. E. AdV. Mater. 2007, 19, 197-202. (11) Gratzel, M. M.; Jacques-E. Electron Transfer Chem. 2001, 589644. (12) Segura, J. L.; Martı´n, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34, 31-47. (13) Smith, G. B. Sol. Energy Mater. Sol. Cells 2004, 84, 395-409. (14) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules 2006, 39, 8712-8719. (15) Bellina, F.; Carpita, A.; Rossi, R. Synthesis 2004, 2419-2440. (16) Roncali, J. Chem. ReV. 1992, 92, 711-738. (17) Wolf, M. O. AdV. Mater. 2001, 13, 545-553. (18) Handbook of Conducting Polymers; Marcel Dekker, Inc.: New York, 1986; Vol. 1. (19) Ernst, S.; Kaim, W. J. Am. Chem. Soc. 1986, 108, 3578-3586. (20) Moliton, A.; Hiorns, R. C. Polym. Int. 2004, 53, 1397-1412. (21) Hughes, G.; Bryce, M. R. J. Mater. Chem. 2005, 15, 94-107. (22) Jenkins, I. H.; Rees, N. G.; Pickup, P. G. Chem. Mater. 1997, 9, 1213-1216. (23) Kingsborough, R. P.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 8825-8834. (24) Kingsborough, R. P.; Swager, T. M. Prog. Inorg. Chem. 1999, 48, 123-231. (25) Kojima, T.; Nishida, J.-I.; Tokito, S.; Tada, H.; Yamashita, Y. Chem. Commun. 2007, 1430-1432. (26) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. ReV. 2007, 107, 1233-1271. (27) Bodenthin, Y.; Pietsch, U.; Mohwald, H.; Kurth, D. G. J. Am. Chem. Soc. 2005, 127, 3110-3114. (28) Hansford, K. A.; Guarin, S. A. P.; Skene, W. G.; Lubell, W. D. J. Org. Chem. 2005, 70, 7996 -8000.

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