Article pubs.acs.org/Macromolecules
Star-Shaped Oligothiophenes Containing an Isotruxene Core: Synthesis, Electronic Properties, Electropolymerization, and Film Morphology Ting-An Liu, Ch. Prabhakar, Jian-Yuan Yu, Chun-hsien Chen, Hsin-Hau Huang, and Jye-Shane Yang* Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617 S Supporting Information *
ABSTRACT: The synthesis, photophysical and electrochemical properties, and DFT calculations of four star-shaped πconjugated systems (ITTn, n = 1−4) consisting of an isotruxene (IT) core and three α-oligothiophene (OT) arms of 1−4 hexylthiophene rings (T1−T4) are reported. The electronic properties, spectroelectrochemistry, and AFM images of the electrodeposited polymer films of ITTn on ITO electrodes (i.e., poly(ITTn)) are also presented. As the OT length (i.e., the n value) increases, the electronic spectra are red-shifted, the fluorescence quantum yields are reduced, the first oxidation potentials are negatively shifted, the surface roughness of the polymer films is increased, and the formation of bipolaron dopants in the polymer becomes more favorable. The strong electronic couplings among the three OT arms render ITTn and poly(ITTn) a new class of two-dimensional π-conjugated oligomers and polymers.
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(Chart 1). To further understand how the nature of π-arms affects the ortho−para conjugation interactions, it is desired to investigate IT-cored π-stars having non-OP-based π-arms. We report herein the synthesis, photophysical and electrochemical properties, and AFM (atomic force microscopy)probed film morphology of a series of star-shaped IT−OT hybrids ITTn (n = 1−4, Chart 1) and their polymers poly(ITTn) (n = 1−4) prepared through electropolymerization on ITO (indium−tin oxide) electrodes. Our results show that the two-dimensional electronic coupling among the OT arms is also effective in ITTn, although torsion in the OT arms is not completely restricted. The OT length (i.e., the n value) affects not only the electronic properties but also the surface morphology of the electrodeposited films of ITTn. Comparison of optical properties between ITTn and the meta−meta crossconjugated truxene analogues (TTn)12 as well as a series of known OP−OT hybrids21−23 is also provided.
INTRODUCTION The potential application of oligothiophenes (OT) and polythiophenes (PT) in solar cells,1−3 electrochromic devices,4 field-effect transistors,2,5,6 and chemosensors7,8 has driven numerous researches on molecular structure−property engineering of OT and PT.9,10 Approaches include functional group substitution, incorporation of nonthiophene π-segments, and/ or new topology of the π-conjugated backbone.9−11 The latter two approaches in constructing star-shaped and dendritic πconjugated frameworks are particularly attractive because not only the intramolecular electronic nature but also the intermolecular interactions could differ significantly from the rod-shaped systems.12,13 Electrochemical polymerization (electropolymerization) provides a convenient way for redox-active monomers to deposit a conductive polymer film on the surface of electrodes.14−16 Electropolymerization of thiophenes is generally conducted on an anode at a constant potential or through several cycles of potential sweep, during which the radical cations of thiophene are generated, dimerized, polymerized, and dehydrogenated. While the topology of thiophene-based monomers should play a crucial role in determining the quality and morphology of an electrodeposited PT film, the topology−morphology relationship for PT systems remains to be established. We have recently shown that isotruxene (IT) is an excellent branching unit for constructing star-shaped π-conjugated oligophenylenes (IT−OP systems) having strong electronic couplings among the three OP arms.17−20 This IT-mediated two-dimensional electronic coupling stems from the ortho− para connectivity of the peripheral phenylene rings to the central ring and from the planarized rigid ladder-type scaffold © 2012 American Chemical Society
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RESULTS AND DISCUSSION Oligomer Synthesis. We adopted an iterative divergent method for the synthesis of ITTn (Scheme 1). The key starting material tribromoisotruxene (ITBr) was prepared according to our recently reported protocol.17,20 The reagent sodium 4hexyl-2-thienylboronate (T1) was synthesized according to literature procedures.24 The two types of reactions involved in Scheme 1 are (A) the Suzuki cross-coupling reaction and (B) Received: March 20, 2012 Revised: May 8, 2012 Published: May 21, 2012 4529
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Chart 1. Structures of ITTn and TTn (n = 1−4)a
a The isotruxene (IT) and truxene (T) scaffolds are shown in red, and the π-branching connectivity of the central phenylene ring is highlighted in bold.
Scheme 1
Figure 1. Normalized absorption (solid lines) and fluorescence (dashed lines) spectra of ITTn in hexane (black) and THF (red).
The absorption bands are resolved in ITT1 (one at 350 nm and the other at 384 and 404 nm as vibronic bands) but not in ITT2−ITT4. In contrast, the fluorescence spectra show resolved 0−0 and 0−1 vibronic bands for all cases. The significant Stokes shifts (3856−4032 cm−1) indicate a significant structural relaxation from the Franck−Condon excited state to the fluorescing state, attributable to the planarization of the π-conjugated backbone as in the stepladder IT−OP systems.19 This in turn suggests that in the ground state the molecular π-backbone is distorted from planarity due to steric interactions with the hexyl substituents on the thiophene rings. Similar observations have been reported for rod-shaped OT having alkyl substituents at the β-position of thiophenes.25,26 Both the absorption and fluorescence spectra are red-shifted as the OT arm length is increased. The magnitude of shifts through the series (n = 1→ 4) is larger for fluorescence than for absorption, which is consistent with a more planar geometry for the excited vs the ground state. The solvent effect on the spectra is small, as expected for systems lacking strong electron-donating or electron-withdrawing substituents. The spectral shift is only of 2−4 nm for the absorption maxima and of 5−10 nm for the fluorescence 0−0 bands on going from hexane to THF (Figure 1).
α-bromination of terminal thiophenes with N-bromosuccinimide (NBS).12 Since each reaction step involves three reaction sites, the observed yield of 69−89% for each reaction step corresponds to a yield of 88−96% per reaction site. The Pd catalyst loading (5−10 mmol %) for the Suzuki couplings is relatively lower than that (20 mmol %) required for the construction of the IT−OP systems.19 Typical reaction procedures and compound characterization data for the intermediates ITTnBr (n = 1−3) and the target compounds ITTn (n = 1−4) are provided in the Experimental Section. Oligomer Photophysics. The normalized absorption and fluorescence spectra of ITTn in hexane are shown in Figure 1, and the corresponding spectral data are summarized in Table 1. Whereas ITT1 displays three absorption peaks at 350, 384, and 404 nm, there is only one broad absorption band for ITT2 (384 nm), ITT3 (410 nm), and ITT4 (423 nm). On the basis of our previous studies on IT−OP systems18,19 and the results of DFT calculations on ITTn (vide infra), we conclude that all the observed spectra consist of two or more absorption bands. 4530
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Table 1. Photophysical and Electrochemical Data for ITTn and Poly(ITTn) at Room Temperature compd ITT1 ITT2 ITT3 ITT4
λabsa,b (nm) 350 384 410 423
(390) (407) (427) (447)
log εa,c 4.90 4.96 5.01 5.16
λfla,d (nm) 423 456 487 510
(510) (542) (560) (587)
Δνsta,e (cm−1) 4931 4111 3856 4032
Φfla,f 0.71 0.54 0.31 0.20
(0.02) (0.01) (0.01) ( 0.70 in general),21,28 although the exact value depends on the relative arrangement of the phenylene and thiophene rings and on the position of alkyl substituents.21−23 In this context, the star-shaped ITTn behave like rod-shaped OP−OT hybrid systems, and the relative Φfl values appear to reflect the relative weights of OP (i.e., the IT core) vs OT (the arms). The differences and analogies in electronic spectra between ITTn and the truxene analogues TTn deserve our attention. The following comparisons were based on their spectra in THF. For the absorption spectra, all four truxene systems TT1−TT4 display a single intense absorption band.12 Thus, there is a distinct difference in the absorption spectra of ITT1 (three bands at 350, 384, and 404 nm) and TT1 (one band at 341 nm). However, each of the other TTn and ITTn pairs (n = 2−4) possesses a similar absorption profile and, unexpectedly, a very close absorption maximum (Δλabs = 1−5 nm). Further analysis on their structures suggests that the similarity in λabs is an interesting coincidence due to the presence of alkyl substituents on the thiophenes of ITTn. It was reported that in rod-shaped OT systems the hexyl group substitution results in a significant blue shift (ca. 20 nm) in both absorption and fluorescence spectra.21,26 This was attributed to a steric effect of the alkyl substituents that reduces the planarity of the πbackbone. Taken this steric effect into account, the ITTn systems should display red-shifted electronic spectra than the
TTn systems provided that the thiophenes in ITTn were free of alkyl substitutions. This argument is indeed supported by the red-shifted fluorescence spectra for ITTn vs TTn because the steric effect is minimized in the excited states due to backbone planarization relaxation. The differences in the fluorescence 0−0 bands (Δλfl) between each pair of ITTn and TTn are as large as 42−64 nm, and the smaller is the system, the larger is the difference. For example, the Δλfl is 64 nm between ITT1 (λfl = 428 nm) and TT1 (λfl = 364 nm) and 42 nm between ITT4 (λfl = 517 nm) and TT4 (λfl = 475 nm). The phenomenon of photoinduced planarization relaxation has recently been reported for the stepladder IT−OP systems.19 This observation demonstrates that electronic couplings through the ortho−para conjugated isotruxene are stronger than through the meta−meta conjugated truxene core. No comparison could be made on the fluorescence quantum yields, as the Φfl data for the TTn systems were not reported.12 In summary, the OT π-backbone in ITTn is more planar in the lowest singlet excited state than in the ground state. Since torsion of the backbone would hamper electronic couplings among the OT arms,29 the phenomenon of two-dimensional conjugation is more evidenced by their fluorescence than absorption spectra. DFT Calculations. To gain insights into the electronic structures of these star-shaped OP−OT hybrid π-systems, we carried out DFT calculations on structure optimization at the B3LYP/6-31G* level and TDDFT calculations on absorption energy at the B3LYP/6-31G** level. To expedite the calculations, all the ethyl and hexyl substituents were replaced with methyl groups. As one can envision, the larger is the πsystem, the more possible conformers of similar energy can exist. Thus, the electronic structures reported herein are for the optimized conformations. Figure 2 shows the optimized structures of ITT1 and ITT4, and those of ITT2 and ITT3 are provided in Figure S1. We use the ortho (o), para (p), and meta (m) bays of the isotruxene core to define the orientation of the thiophene S atoms. Accordingly, the three thiophenes in ITT1 are in the p-p-m orientation and the thiophene−phenylene dihedral angles are ca. 26°. The same p-p-m orientation is retained for the thiophenes directly attached to the IT core in ITT2−ITT4, and the outer thiophenes in each OT arm adopt an orientation opposite to the neighboring thiophenes (i.e., an anti conformation). The thiophene−thiophene dihedral angles are in the range 20°−36°, similar to those found for methyl- or hexyl-substituted OT in head-to-tail arrangements.30 More detailed data of these dihedral angles are provided in Table S1. According to the low torsion barriers (1−2 kcal mol−1) and the small energy differences (0.6 kcal mol−1) of the syn and the anti conformers reported for rod-shaped OT systems,26 we expect the presence of a spectrum of conformers of different 4531
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Table 2. B3LYP/6-31G** Level Calculated Absorption Energy, Oscillator Strength, and Description for the Lowest Three Singlet Excited States of ITTn compd ITT1
ITT2
ITT3
ITT4
Figure 2. Optimized molecular structures and orientation of the transition dipoles for the B3LYP/6-31G** level calculated three lowest energy absorptions of (A) ITT1 and (B) ITT4.
excited state
λabs,cala (nm)
fb
configc
wtd (%)
S1 S2
412 375
0.948 0.030
S3
360
1.214
S1 S2 S3 S1 S2
462 421 411 500 464
1.398 0.867 0.772 1.896 1.793
S3
447
0.379
S1 S2
530 503
2.381 2.428
S3
479
0.353
H→L H-1 → L H→L+1 H-1 → L H→L+1 H→L H-1 → L H→L+1 H→L H-1 → L H→L+1 H-2 → L H-1 → L H → L+1 H→L H-2 → L + 1 H-1 → L H→L+1 H-2 → L H-1 → L H→L+1
99 60 35 37 61 98 96 93 95 80 14 7 16 71 90 4 68 20 34 22 37
a
Calculated absorption energy. bOscillator strength. cH and L stand for HOMO and LUMO, respectively. The second and third highest occupied molecular orbitals are denoted as H-1 and H-2 and the second and third lowest unoccupied molecular orbitals as L+1 and L +2, respectively. dOnly configurations with 4% or greater contribution are included.
thiophene−thiophene dihedral angles for ITTn at room temperature. TDDFT calculations on the optimized conformations of ITTn suggest that electronic transitions to the lowest three singlet excited states (S1−S3) have different magnitude and orientation of transition dipoles (Figure 2). The calculated absorption wavelength (λabs,cal), oscillator strength (f), and the configuration description of S1−S3 are shown in Table 2. For the S0 → S1 transition, it is highly allowed for all four cases with oscillator strengths of 0.95−2.38, which increases with increasing the OT length. This transition is mainly contributed by the HOMO → LUMO configuration. The transition dipole is aligned approximately parallel to the para chain. For the S0 → S2 and S0 → S3 transitions, the HOMO−1, HOMO−2, LUMO +1, and LUMO+2 orbitals are also involved. Both of the transition dipole vectors are aligned approximately parallel to the ortho branch. As represented by the case of ITT4 (Figure 3), the HOMO, LUMO, HOMO−2, and LUMO+2 cover all the π-backbone with larger electron density in the central vs terminal regions for the former two orbitals, but an opposite electron distribution is observed for the latter two orbitals. In contrast, the molecular orbitals HOMO−1 and LUMO+1 have electron density localized on the two OT arms that are metarelated with respect to the central phenylene ring. These frontier molecular orbitals possess the same features as recently observed molecular orbitals for ladder-type two-dimensionally conjugated IT−OP systems.18 Evidently, the optimized conformations of ITTn allow effective two-dimensional πconjugation interactions among the OT arms. This is consistent with the large red shifts of the fluorescence spectra for ITTn vs TTn. Correlation Plots. The correlations of electronic transition energies against the reciprocal of the number of repeat unit (1/ n)31 or shortest-path double bonds (1/N)32 have often been
Figure 3. B3LYP/6-31G** level calculated structure and the frontier molecular orbitals of ITT4. Only atomic charge densities with 1% or higher contribution are included.
employed to elucidate the effective conjugation length (ECL) and limiting bandgap of rod-shaped conjugated polymers. Although such correlations are originally aimed for homooligomers and homopolymers, it might also be informative for hybrid systems.33 In this context, we have adopted the fluorescence 0−0 band energy and the parameter 1/N for the analysis of ECL or degree of exciton delocalization in ITTn. For the purpose of discussion, previously reported OT (Tn),26 ladder-type OP (LPn),34 and OP−OT hybrids, including TTn,12 FTn,23 FTnF,22 and TnFTn23 (n = 1−4, Chart 2), are included. By definition, the N value for each phenylene and 4532
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Chart 2. Structures of OP, OT, and OP−OT Hybrid Systems Discussed in the Text
Figure 4. Linear plots of the fluorescence 0−0 band energy against reciprocal of the number of double bonds along the shortest path of the molecular backbone (1/N) for (A) LPn (black, square), Tn (red, circle), ITLPn (purple, triangle), FTn (brown, star), FTnF (blue, invert triangle), and TnFTn (green, hexagon) and for (B) LPn (black, square), Tn (red, circle), ITTn-all (brown, triangle), ITTn-para (blue, triangle), ITTn-ortho (orange, triangle), TTn-all (green, rhombus), and TTn-one (purple, rhombus).
thiophene ring is 2, since each ring contributes two double bonds to the shortest path of π-conjugated backbone. Accordingly, the N values are 14, 20, 26, and 32 for ITT1, ITT2, ITT3, and ITT4, respectively, when all the thiophene and phenylene rings are under consideration (denoted as ITTnall). The N values and the fluorescence 0−0 band energies of the other discussed systems are provided in Table S2. Figure 4 shows the linear correlation plots between fluorescence 0−0 band energy and 1/N. The correlations are excellent for all cases (R2 = 0.982−0.999, Table S2). The slopes for the two linear homooligomers LPn (6.14) and Tn (5.78) are similar, indicating a similar chain length effect on exciton delocalization (particle-in-a-box effect). The difference in the predicted limiting bandgaps between LPn (2.45 eV) and Tn (2.01 eV) reflects the inherent difference in the electronic nature of the phenylene and thiophene rings. Since an OP−OT hybrid would possess an electronic character between OP and OT of the same number of rings (also the same N values), it is expected to see the OP−OT data points located within the region (the zone) defined by the two nearly parallel lines of LPn and Tn. This is indeed the case for FTn, FTnF, and TnFTn
(Figure 4A). The larger slopes for the OP−OT hybrids vs the OP or OT homooligomers reveal that as the weight of OT (i.e., the n value of the series) is increased the electronic nature (bandgap) of the OP−OT hybrid is shifted from OP-like to OT-like. With this in mind, the degree of exciton delocalization for star-shaped OP−OT hybrid systems such as ITTn and TTn could be qualitatively evaluated with different π-domains, corresponding to different N values. Specifically, the data points would locate within the zone if the selected π-domain is close to the real ECL for exciton delocalization. Otherwise, they would locate above the zone if the ECL is overestimated (i.e., the adopted 1/N values are smaller than what they should be) and below the zone if the ECL is underestimated (i.e., the adopted 1/N values are larger than what they should be). For the ECL analysis, we have independently considered the parachain domain (ITTn-para) and the ortho-branch domain (ITTn-ortho) of ITTn, and one of the three symmetrical branches in TTn (TTn-one) as well as the whole π-backbone (ITTn-all and TTn-all). The N values through the series (n = 1 → 4) are thus 14−20−26−32 for ITTn-all and TTn-all, 10− 4533
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lower the oxidation potential. The significantly lower Eox1 values (vs Fc/Fc+) for ITT2 (0.51 V) vs α-bithiophene (0.91 V), ITT3 (0.40 V) vs α-terthiophene (0.69 V), and ITT4 (0.37 V) vs α-tetrathiophene (0.60 V) also show the presence of extended π-conjugation interactions through the IT core in ITTn. The electrochemical behavior of TTn was not reported, and thus comparison of redox potentials between ITTn and TTn cannot be made. Electropolymerization of ITTn occurred readily during the potential scans. For the purpose of photophysical characterization of the polymer films, the experiment of electropolymerization was carried out with transparent ITO electrodes (working area of 1.0 × 1.0 cm2). The substrate concentration is 1.0 mM in CH2Cl2, the reference electrode is Ag/AgNO3, the counter electrode is a Pt wire, the electrolyte is Bu4NPF6, and the scan rate is 100 mV s−1. The polymer films of ITO were prepared by 10 cycles of potential scans for all cases (Figure 6).
14−18−22 for ITTn-para, and 4−6−8−10 for ITTn-ortho and TTn-one. As shown in Figure 4B, the data points of ITTn-all, ITTn-para, and TTn-one fall nicely within the zone, but those of TTn-all and ITTn-ortho are located above and below the zone, respectively. Accordingly, the ECL for TTn should not cover all the aromatic rings, and the exciton is more likely localized in one of the three branches. This conclusion is consistent with the cross-conjugated (meta−meta) linkage of the truxene core. For ITTn, the observed plots suggest that the exciton should not be localized in the ortho branch but delocalized either on the two-dimensional π-backbone or on the para chain alone. The propensity of exciton delocalization along the para chain is consistent with the orientation of transition dipole of S0 → S1 (Figure 2). An example of effective two-dimensional conjugation has recently been demonstrated by the IT-cored ladder-type OP systems ITLPn (Chart 2), which display essentially the same linear plot as the rod-shaped LPn when the whole π-backbone is considered as the ECL (Figure 4A). In conjunction with the spectral features and DFT calculations, we can conclude that there exist strong electronic couplings between the para chain and the ortho branch of ITTn, particularly for the planar conformers. Electrochemistry and Electropolymerization. The oxidative electrochemical behavior of ITTn was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) on Pt electrodes in CH2Cl2 with Bu4NPF6 as the supporting electrolyte and Ag/AgNO3 (0.01 M) as the reference electrode. The oxidation potentials (Eox) relative to the ferrocene redox couples (Fc/Fc+) based on the DPVs are shown in Table 1. As shown in Figure 5, the CV waves are
Figure 6. The first 10 CV cycles (black) of ITTn (1.0 mM) and the CV (red) of the resulting poly(ITTn) films in monomer-free solutions. The solvent is CH2Cl2, electrolyte is 0.10 M Bu4NPF6, and scan rate is 100 mV s−1.
The resulting polymer films on ITO are designated as poly(ITTn). Since each of the three thiophene terminals in ITTn could undergo the oxidative coupling reaction, the resulting polymers are expected to possess a three-dimensional network, where the neighboring IT groups are separated by αbithiophene, α-tetrathiophene, α-hexathiophene, and α-octathiophene segments in poly(ITT1), poly(ITT2), poly(ITT3), and poly(ITT4), respectively. Figure 7 shows a model structure of poly(ITT3). However, the possibility of branching through the β-position of thiophene during electropolymerization cannot be completely excluded.35 The first CV cycle of each compound displays a current intensity similar to one another among ITTn, and their CV profiles are similar to those (Figure 5) with Pt electrodes. In addition, the growth of the current from the previous cycle appears to be similar for all four ITTn systems. Thus, we might conclude that the efficiency of electropolymerization of ITTn is barely sensitive to the OT length. The efficient electrochemical reactivity of ITTn indicates that the radical cations are favorably located at the terminal thiophene rings. Polymer Properties. The fresh electrodeposited polymer films were rinsed with CH2Cl2 and transferred to a monomerfree solution for electrochemical characterization. The polymers cannot dissolve in common organic solvents such as toluene, THF, dichloromethane, and acetonitrile. Such a poor solubility
Figure 5. Cyclic voltammogram (black) and differential pulse voltammogram (red) for oxidation of ITTn in CH2Cl2 with electrolyte 0.10 M Bu4NPF6 at a scan rate of 25 mV s−1.
quasi-reversible for all four cases, indicating the occurrence of chemical reactions of the oxidized intermediates. Indeed, electropolymerization of all four systems occurred readily on ITO electrodes (vide infra). As the OT length is increased, the CV and DPV waves become broader and/or split into two or three peaks. In addition, the first oxidation potential (Eox1) undergoes negative shifts. Both phenomena have been observed for rod- and star-shaped π-conjugated oligomers and are a consequence of increased conjugation length.19,34 The Eox1 value for ITT1 is 0.54 V, which is lower than that for isotruxene (0.79 V) and a series of stepladder IT−OP derivatives (0.60 V). Evidently, substitution of each OP arm in the latter systems with a single thiophene can effectively 4534
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Figure 7. Model structures of poly(ITT3) in the neutral and doped forms. Only the OT segments are shown in the doped forms for clarity.
is consistent with the expectation of a cross-linked polymeric network. The CV waves recorded at a scan rate of 100 mV s−1 are shown as the red curves in Figure 6. For all cases, the CV waves are broad, as commonly observed for amorphous electropolymerized organic thin films.16,36,37 A linear dependence of current density on the scan rate up to 100 mV s−1 indicates that the counterion diffusion in the film is reasonably fast. In addition, the polymer films display reasonably good electrochemical stability, as indicated by the unchanged current intensity upon continued switching of potential between 0.9 and 0.0 V (vs Ag/AgNO3) on poly(ITT3) (Figure S2). Furthermore, the intrinsic CV behavior of poly(ITTn) is different from that in the presence of monomers (i.e., the final cycle of electropolymerization, Figure 6). This might indicate a significant contribution of monomer redox reactions to the overall redox activity in the latter condition, although a change in polymer film after solvent rinse might play a role in the difference (vide infra). All these observations suggest that poly(ITTn) are electrochemically active and conductive materials. At higher applied potentials and lower scan rate of 10 mV s−1, the CV of poly(ITTn) shows more waves and fine structures, albeit ill-defined (Figure 8). While poly(ITT1) displays a single broad wave with fine structures before overoxidation at a potential larger than 1.0 V vs Ag/AgNO3, poly(ITT4) displays two distinct waves. The situation of poly(ITT2) and poly(ITT3) lies in-between the cases of poly(ITT1) and poly(ITT4). It is noted that the width of the first wave is monotonically decreased on going from poly(ITT1) to poly(ITT4). In conjunction with the spectroelectrochemistry (vide infra), we conclude that the first and the
Figure 8. Cyclic voltammogram of poly(ITTn) in CH2Cl2 with electrolyte 0.10 M Bu4NPF6 at a scan rate of 10 mV s−1.
second waves result from the formation of radical cations (polarons) and dications (bipolarons), respectively (Figure 7). The presence of fine structures indicates that the initially formed polarons or bipolarons affect the generation of new polarons or bipolarons due to interpolaron or interbipolaron electronic interactions. The negligible formation of bipolarons in poly(ITT1) is consistent with the recent reports by Roncali and Perepichka and their co-workers that short OT segments such as bithiophenes do not favor the formation of bipolarons.37 Our results further show that formation of bipolarons becomes favorable with longer OT segments such as hexathiophene and octathiophene. This conforms to the 4535
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Figure 9. (A) Absorption and (B) normalized fluorescence spectra of (a) poly(ITT1), (b) poly(ITT2), (c) poly(ITT3), and (d) poly(ITT4).
observations of bipolarons for hexathiophene and nonathiophene oligomers upon chemical oxidation in solutions.38 Figure 9 shows the absorption and normalized fluorescence spectra of poly(ITTn), and the spectral maxima data are provided in Table 1. Compared to the corresponding monomer solution spectra, the polymer film spectra are broader, less structured, and red-shifted. For example, the resolution of electronic transitions observed for ITT1 (Figure 1) completely disappears in poly(ITT1). This can be attributed to an elongation of ECL and to interchain interactions in polymer thin films. The absorption and emission spectra of poly(ITTn) continue to move to red as the n value increases. Similar behavior has been observed for FTnF (Chart 2).22 It should also be noted that the absorbance (Abs) of poly(ITT1) is much lower (Abs = 0.40) than those of poly(ITT2)−poly(ITT4) (Abs = 1.46−1.82). This is more likely from a poorer adhesive of the ITT1 polymer to the ITO surface so that a part of the film was rinsed off with CH2Cl2 rather than from a higher solubility or lower electropolymerization reactivity of ITT1 (vide supra). The film defect can be visibly detected for poly(ITT1) but not for poly(ITT2)−poly(ITT4). Shorter OT segments and/or higher density of IT branching units in the polymer of ITT1 might account for the poorer adhesive property. The fluorescence quantum yield is lower than 1% for all four systems of poly(ITTn). The spectroelectrochemistry of poly(ITTn) was investigated to complement the CV studies on the redox behavior of these polymer films (Figure 10). For poly(ITT1), increasing the oxidation potential from 0.8 to 1.0 V leads to an increased intensity of two distinct absorptions at ca. 700 and 1600 nm. These two absorption bands can be attributed to polaron
formations due to SOMO → LUMO and HOMO → SOMO transitions, respectively, according to linear OT and PT systems.38 In addition, there is an isosbestic point at 464 nm, corresponding to interconversion between the neutral and the polaron states. The absorption band at ca. 700 nm becomes broader as the oxidation potential was raised to 1.1 V, attributable to an effect of interpolaron interactions. The same phenomenon was also previously observed for highmolecular-weight IT−OP derivatives.19 While an isosbestic point (476 nm) is also present in the case of poly(ITT2), the 750 nm band formed at lower potentials (0.6−0.8 V) disappears and the 1600 nm band shifts to ca. 1400 nm at 1.2 V. An even larger blue shift for the 1600 nm band was found for poly(ITT3) (∼1300 nm) and poly(ITT4) (∼1150 nm), and their spectra no longer show isosbestic points. In conjunction with their CV behavior (Figure 8), the formation of bipolarons might account for the loss of isosbestic points, disappearance of the 600−800 nm band, and the blue shift of the 1600 nm band. Accompanied with the potential scan is a visible color change for the film from pale yellow to dark green. A typical example with poly(ITT3) at 0.0 and 0.9 V is provided in Figure S3. Poly(ITTn) provides a unique opportunity to understand the effect of star-shaped monomer topology on the morphology of electrodeposited films of OT. Figure 11 shows their threedimensional AFM images, and the corresponding two-dimensional images are shown in Figure S4. The surface morphology of poly(ITT1) shows nanobumps with a depth around 50 nm and width of 500 nm or less. The surface roughness increases to a larger extent as the n value is increased. For example, small
Figure 10. Spectroelectrochemistry of poly(ITTn) in CH2Cl2 with electrolyte 0.10 M Bu4NPF6 at 0.60−1.2 V.
Figure 11. Three-dimensional AFM images of poly(ITT1) and poly(ITT2) in 3 × 3 μm2 and poly(ITT3) and poly(ITT4) in 10 × 10 μm2. 4536
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Macromolecules
Article
electropolymerization was performed by sweeping the voltage at a scan rate of 100 mV s−1 by 10 cycles, against Ag/AgNO3 as a reference electrode and platinum wire as a counter electrode, for deposition of the polymers onto ITO slides. The resulting films were then washed with copious amounts of CH2Cl2 and stored in the absence of light. Spectroelectrochemistry. Spectroelectrochemical data were recorded on a Jasco V-570 spectrometer. A three-electrode cell was constructed in which each polymer was deposited onto ITO-coated glass and used as the working electrode. The counter- and pseudoreference electrodes consisted of a platinum and a silver wire, respectively, using 0.10 M Bu4NPF6 in CH2Cl2 solution. The polymers that were deposited onto the ITO electrodes were analyzed using UV−vis−NIR spectroscopy (300−1600 nm) while increasing the voltage in stepwise intervals of 0.10 V. AFM. AFM measurements were carried out with Tapping Mode (NanoScope IIIa controller, Veeco Metrology Group, Santa Barbara, CA) to minimize the film deformation. Images were acquired using a 10 μm scanner and monolithic silicon cantilevers (NCHR, NanoWorld, Neuchatel, Switzerland) with the force constant and the typical tip radius of curvature of 42 nN/m and 10 nm, respectively. The microscope was housed in a Plexiglas chamber through which dry N2 was purged throughout the experiments and the humidity was kept lower than 2%. DFT Calculation. The ground-state geometry optimizations of all the molecules have been optimized for different conformations with density functional theory (DFT) methods by using the software Gaussian 09 package.40 The Becke three-parameter hybrid exchange functional and the Lee−Yang−Parr correlation functional (LYP) were utilized in the calculation.41 Initially, ground-state geometries were optimized with 3-21G (d) basis set and frequencies were evaluated for the optimized geometries to ensure the structure obtained was a minimum on the potential energy surface at same level (B3LYP/3-21G (d)). All the conformations obtained at the B3LYP/3-21G (d) level were again fully reoptimized using the 6-31G (d) basis set. The final geometries reported here have been obtained with the higher basis set, B3LYP/6-31G (d) and lowest energy conformation. To get the absorption spectrum, excitation energies were calculated using the time-dependent density functional theory (TDDFT) method at the B3LYP/6-31G (d, p) level. The vertical excitation energy obtained at B3LYP/6-31G (d) level optimized ground-state geometry was taken to be the absorption maximum. Materials. All commercially available materials were used as received. Solvents for photochemical and electrochemical measurements were HPLC grade. CH2Cl2 was dried by calcium hydride and distilled before use. Compound ITBr and T124 were prepared according to the literature procedures. Typical synthetic procedures and characterization data for new compounds are shown in the following. Typical Procedures for the Suzuki Coupling Reaction. Take the synthesis of ITT1 as example. To a mixture of ITBr (807 mg, 1.08 mmol), sodium 4-hexyl-2-thienylboronate (1.09 g, 4.32 mmol), 2.00 M Na2CO3 (5.00 mL), and THF (50.0 mL) in a dried 100 mL roundbottle flask was added Pd(PPh3)4 (60 mg, 0.05 mmol) as a catalyst. The solution mixture was heated at 100 °C under N2 for overnight. After cooling to room temperature, the reaction mixture was poured into water. The resulting mixture was extracted with ethyl acetate (3 × 30 mL), and the organic layer was dried over anhydrous MgSO4. The filtrate was concentrated under reduced pressure to afford the crude product. Further purification was performed by silica gel column chromatography using CH2Cl2/hexane (1:15) as eluent to provide 851 mg of ITT1. The loadings of Pd catalyst for the synthesis of ITT2− ITT4 are all in 5−10 mol % of the substrates. The eluents for ITT2, ITT3, and ITT4 are CH2Cl2/hexane (1:15), CH2Cl2/hexane (1:13), and CH2Cl2/hexane (1:10), respectively. Typical Procedures for the α-Bromination of Terminal Thiophenes. Take the synthesis of ITT1Br as example. A mixture of ITT1 (634 mg, 0.630 mmol), N-bromosuccinimide (334 mg, 1.88 mmol), AcOH (2.00 mL), and chloroform (30.0 mL) was stirred at 0 °C for overnight and then warmed up to room temperature. The solution was extracted with CH2Cl2 and water. The organic layer was
holes are present in poly(ITT2), and cylindrical holes grow in poly(ITT3) and poly(ITT4). The holes and cylindrical features are not uniform, typically ranging from 400 nm to 2 microns in diameter. A depth and a width as large as 478 nm and 3 microns, respectively, have also been observed. (Figure S4). For comparison, the AFM images of spin-cast films of the starshaped monomers in hexane were also recorded (Figure S5). Unlike the electrodeposited polymer films, the spin-cast films are microscopically smooth with a vertical roughness less than 100 nm. These results show that the film morphology of poly(ITTn) is strongly affected by the OT arm length. The higher surface roughness of poly(ITT4) as compared to that of electrodeposited films of rod-shaped OT systems39 also reflect in part the effect of molecular shape on the dynamics of polymer chain growth. However, a direct correlation between the monomer structure and the polymer morphology is not possible at current stage. Further studies on ITTn and related systems might shed light on this issue.
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CONCLUSION We have integrated the branched OP core of isotruxene and linear OT arms to form four star-shaped OP−OT hybrids ITTn (n = 1−4). The ortho−para connectivity and the rigid planar scaffold of the isotruxene core allow strong electronic couplings among the OT arms. As a result of the two-dimensional conjugation, ITTn possess high fluorescence quantum yields, low oxidation potentials, and good electropolymerization activity, resembling long-chain OT systems. Electropolymerization of ITTn leads to the first class of isotruxene-containing πconjugated polymers poly(ITTn). The length of the OT segments in these polymer films determines the nature of the oxidized form (p-type doping) of the film. In addition to radical cations (polarons), formation of dications (bipolarons) becomes more favorable through the series (n = 1 → 4). The electrodeposited films display a larger degree of surface roughness as the n value is larger. This work highlights that isotruxene is an ideal building block for constructing twodimensionally conjugated organic electronic materials of not only OP but also OT π-backbones.
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EXPERIMENTAL SECTION
Spectroscopy. All of the spectral and electrochemical data were collected at room temperature (23 ± 1 °C). The UV−vis spectra of ITTn were measured on a Cary300 double beam spectrophotometer. Fluorescence spectra were recorded on a PTI QuantaMaster C-60 spectrometer and corrected for the response of the detector. The fluorescence quantum yield was determined with N2-outgassed solutions or solid powders using an integrating sphere (150 mm diameter, BaSO4 coating) of Edinburgh Instruments by the Edinburgh FLS920 spectrometer. Fluorescence decays were determined by the Edinburgh FLS920 spectrometer with a gated hydrogen arc lamp using a scatter solution to profile the instrument response function. The goodness of the nonlinear least-squares fit was judged by the reduced χ2 value (
