Origin of Near-Infrared Absorption for Azulene-Containing Conjugated

Article pubs.acs.org/JPCB

Origin of Near-Infrared Absorption for Azulene-Containing Conjugated Polymers upon Protonation or Oxidation Tao Tang,† Tingting Lin,§̧ FuKe Wang,*,‡,§̧ and Chaobin He*,†,§̧ †

Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, Singapore 117576 ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, Singapore 117543 §̧ Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, Singapore, 117602 S Supporting Information *

ABSTRACT: A series of azulene-containing conjugated polymers were studied to elucidate their tunable absorption properties in near-infrared (NIR) regions (i.e., 1.2−2.5 μm) upon protonation/oxidation. Density function theory (DFT) revealed that protonation-induced intramolecular charge transfer (ICT) in the polymer backbone lead to strong NIR absorption. Distinct spectral change was observed when tiny amount of peroxide was added to the protonated polymer in trifluoroacetic acid/chloroform solution. Electron paramagnetic resonance (EPR) study confirmed the presence of radical cation, which results in the occurrence of newly formed absorption band after the addition of peroxide. The spectro-electrochemical results and DFT study indicate that polarons and polaron pairs were formed during p-doping process, and both the chemical oxidation and electrochemical oxidation could be facilitated by TFA protonation. This represents the first reported mechanisms of NIR absorption under various protonation/oxidation conditions in a single polymer system.

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INTRODUCTION Azulene and its derivatives have received a plethora of interests during the past a few decades due to its unique electronic and structural characters such as a dipolar structure with the dipole moment around 1.0 D, high electron affinity, low ionization energy and unusual S2−S0 fluorescence.1−8 The large dipole moment arises from the electron drift from the sevenmembered ring to five-membered ring, which could be considered as an aromatic 6π electron tropylium cation fused by a 6π electron cyclopentadienyl anion.9,10 This remarkable electronic structure of azulene allows for cation stabilization through aromatization of the seven-membered ring, which has promising applications such as electrochromic or switching materials,11 NIR sensor12 and conducting polymers.13 Our involvement of azulene and its derivatives stems from our interest in preparing solution-processable polymers with tunable NIR absorption via a simple way, which is widely used in the areas of telecommunication, defense, medical therapy, heat absorbers, and optical recording.14−17 Common strategy to achieve low band gap conjugated chromophores is to adopt donor−acceptor (D−A) configuration, and this has become a widely used method for synthesizing conjugated polymers with narrow band gap through tailoring the donor and acceptor units.18−21 Furthermore, methods such as protonation,10,22 chemical oxidation,23 Lewis acid adducts,24,25 and electrochemistry26 are also used to achieve desired low band gap chromophores. © 2015 American Chemical Society

It has also been demonstrated that anodic potentials or strong oxidants can lead to the decrease of band gap and occurrence of NIR absorption.13,27 A commonly accepted model for this phenomenon is the formation of polarons, bipolarons and polaron pairs with a nondegenerate ground state.28,29 A polaron is simply a radical cation or anion which is associated with a geometrical deformation delocalized over a limited number of monomeric units.26 When doping level is increased, an extra polaron is formed. These two polarons can coalesce into a bipolaron. However, polaron pairs are generated if the distance between the two polarons is large enough to inhibit the formation of bipolaron. Polarons are also the essential charge carriers in sensors, solar cells and electrochromic devices (ECD) with great electronic effect. 30 Conjugated polymers such as polythiophenes and polyfluorenes with eletrochromic behavior are extensively studied due to their good conductivity, stability, and high contrast. Under anodic or cathodic potential, these polymers all exhibit reduced bandgap with extending the maximum absorption into NIR range. Recently, we have investigated a series of azulene containing chromophores and polymers with tunable NIR absorption ranging from 1.2 to 2.5 μm after protonation,22,31,32 which also exhibit outstanding spectroscopic properties and thermal stability. Unlike the occurrence of NIR absorption in Received: February 17, 2015 Revised: May 20, 2015 Published: May 20, 2015 8176

DOI: 10.1021/acs.jpcb.5b01613 J. Phys. Chem. B 2015, 119, 8176−8183

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The Journal of Physical Chemistry B

on a Shimadzu UV-3600. Cyclic voltammetry (CV) measurements were conducted on Autolab PGSTAT 30 electrochemical workstation. In situ spectro-electrochemical properties of the thin films were also characterized using an Autolab PGSTAT30 electrochemical workstation and Shimadzu UV3600. Electrochemistry was performed by using a threeelectrode quartz cell with a platinum wire as the counter electrode, a silver wire pseudoreference electrode, and an ITO/ PET as the working electrode. 0.1 M LiClO4/acetonitrile and 0.1 M LiClO4/acetonitrile with 15 vol % TFA were used as electrolytes. Polymer films were prepared by spin-casting the appropriate solution (15 mg/mL in toluene) onto an ITO/PET substrate at 2000 rpm under an atmosphere of N2. mChloroperoxybenzoic acid (m-CPBA) solution was obtained by dissolving 0.1 g of m-CPBA in 2 mL of chloroform. Room temperature X-band EPR was performed on a commercial JEOL FA200 ESR spectrometer. Theoretical Calculations. All calculations were performed with the Gaussian 09 program employing the Becke three parameter hybrid functionals Lee−Yang−Parr (B3LYP) in conjunction with the 6-31G(d) basis set.38 Full geometry optimizations without symmetry constraints were carried out in the gas phase for the singlet ground states (S0). The energies of highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and other frontier orbitals were calculated at the optimized structure. The first 20 singlet− singlet transition energies and oscillation strengths for model monomers were computed at the optimized S0 geometries by using the time-dependent DFT (TD-DFT) methodology.39 The excited states were calculated at the optimized ground state geometry using TD-DFT and at the same level of theory as ground state geometry optimization.

electrochromic materials, the newly appearing long-wavelength absorption upon protonation is attributed to the intramolecular charge transfer (ICT) interaction. ICT interaction between the electron donor (D) and electron acceptor (A) units can facilitate ready manipulation of their electronic structures and optoelectronic properties.33,34 C-1 and C-3 positions in the 5membered ring of azulene are basic enough to get facile hydrogen exchange (monoprotonation) to form stable azulenylium ion.10,35,36 Therefore, upon the treatment of trifluoroacetic acid (TFA), π-electron drift from the sevenmembered ring of azulene to the five-membered ring generates cycloheptatrienyl (tropylium) cation and makes azulene become a strong electron-acceptor.2,37 Significant redistribution of charge is formed and generates distinct charge separation. As a result, substantial charge transfer from HOMO to LUMO after excitation leads to strong ICT which could lower the bandgap and result in long wavelength absorption. Though many researches have been done on the optical property of chromophores under protonation or oxidation, the mechanism and the possible synergetic effect between protonation and oxidation induced long-wavelength absorption are not fully studied. In this study, we systematically study the protonation, chemical oxidation and electrochemical oxidation of an azulene based system. We also investigate the synergetic effect when combining the protonation with oxidation, with an aim to elucidate the underlining mechanism. Strikingly, addition of peroxide, which induces chemical oxidation, into protonated polymer in TFA/CHCl3 solution leads to the appearance of new absorption band and decrease of previously protonation/ICT induced band in NIR region, i.e., small amount of peroxide could alter the overall optical and electronic properties of protonated polymer. Such spectral change is attributed to the formation of the radical cation and confirmed by optical spectroscopy, EPR spectroscopy, and DFT calculation. Electrochemical oxidation with/without the TFA was also examined. Similar to the previous reports, polarons and polaron pairs are the origin of NIR absorption under the electrochemical oxidation. In addition, electrochemical oxidation potential could be lowered with addition of TFA. As a result, desired NIR absorption could be obtained at a lower applied voltage with the aid of TFA. Our study demonstrates a simple approach toward tunable NIR absorption materials through varying the TFA concentration, TFA/peroxide ratio, or applied voltage.

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RESULTS AND DISCUSSION Four azulene containing polymers (PTAzs) listed in Scheme 1 were synthesized according to our previous researches through Suzuki and Stille coupling.22,31 Scheme 1. Chemical Structures of the Azulene-Containing Conjugated Polymers22,31

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EXPERIMENTAL SECTION Chemicals. Unless stated otherwise, starting materials were purchased and used as received. Azulene (99%), 3-bromothiophene (97%), biphenyl-4,4′-diboronic acid bis(pinacol) ester (95%), methyltin trichloride (97%), and tri(o-tolyl)phosphine and magnesium turnings were purchased from Alfa Aesar. NBromosuccinimide(97%), 2,1,3-Benzothiadiazole-4,7-bis(boronic acid pinacol ester) (95%), trifluoroacetic acid (99%) and copper bromide (98%) were purchased form Aldrich. Ni(dppp)Cl2 and n-BuLi were purchased from Acros Organics. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl solution. Other commercially available solvents and reagents were used as received. Four conjugated polymers based on the poly(thenyl-azulene)s (PTAzs) were synthesized as depicted in our previous researches.10,22,31 Equipment. UV−vis−NIR spectra for protonation and chemical oxidation were conducted on Agilent Cary 5000. UV− vis−NIR spectra for electrochemical oxidation was conducted

Solution Properties. Optical Spectroscopy. As shown in Figure 1, all PTAzs showed a strong absorption band in the visible range due to the π−π* transition. PTAz-3 showed two absorption maxima at 325 and 528 nm. The low-wavelength absorption peak is attributed to the π−π* transition whereas the high-wavelength absorption peak is resulted from the intramolecular π−π* transition due to D−A interaction. Compared with PTAz-1, bathochromic shift of 20 and 52 nm was obtained by PTAz-2 and PTAz-4 through the extension of 8177


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The Journal of Physical Chemistry B Scheme 2. Simplified Picture of Di-PTAz-2 under Protonation and Chemical Oxidation

Figure 1. UV−vis spectra of PTAzs chloroform solution.

conjugation length and substitution of strong electron-donor (alkoxy-thiophene), respectively. More strikingly, significant optical change happened when PTAzs underwent treatments: protonation, chemical oxidation and electrochemical oxidation. (Figure 2 and Supporting Information). Further addition of m-CPBA led to the strong and blue-shifted absorption at 1105 nm. Moreover, distinct and broad NIR absorption bands were generated under the electrochemical oxidation. The chemical oxidation and electrochemical oxidation will be discussed in details later. Chemical oxidation of PTAz-2 in chloroform/TFA/metachloroperoxybenzoic acid (m-CPBA) (or benzoyl peroxide (BPO)) solution was also studied (Figure 3 and Supporting Information). In the chloroform/TFA/m-CPBA (v/v/v, 88.3/ 10/1.7) solution, absorbance intensities at 680 nm of PTAz-2 changed rapidly as a function of time up to 400 s. Then, the absorbance intensities decreased a bit and remained relatively stable with small variations (Figure S2a, Supporting Information). Concerned about this phenomenon, UV−vis−NIR spectra of PTAz-2 was conducted after the solution was kept for 15 min, which resulted in stable spectrum. Likewise, absorbance intensities increased rapidly as a linear function of time at 1100 nm. After 500 s, absorbance intensities, which still maintained a linear relation with time, continued increasing slowly (Figure S2b, Supporting Information). The different trend of absorption intensities for PTAz-2 as a function of time or as a function of m-CPBA concentration, which is discussed below, indicates distinct origin of absorption band at 680 and 1100 nm. Compared with protonation, an extra peak centered at 1105 nm appeared when 3% m-CPBA was added to the chloroform/ TFA solution. Further increase of m-CPBA concentration from 5% to 15% leaded to continuing increase of absorption intensities at 1105 nm, but significantly decreased absorption intensities at 1860 nm. As the concentration reached 20%, the absorption bands at 1105 and 1860 nm remained stable with little variations. However, the absorption intensity (680 nm) resulted from the formation of azulenylium ions in the mCPBA concentration range (3%−30%) remained same, indicating the saturation of azulenylium ions. The newly appearing peak in the NIR region (1105 nm) should be attributed to the radical cation resulted from chemical oxidation of peroxide. Aromatic peracids (m-CPBA) represent a class of strong oxidants, characterized by high selectivity in the oxidation of nonactivated C−H bands.40 mCPBA could exhibit more prominent radical process for its less

Figure 2. UV−vis−NIR spectra of PTAz-2 under various conditions: in chloroform (black line), in TFA/chloroform (red line, v/v: 1/9), in TFA/m-CPBA/chloroform (blue line, v/v/v: 1/3/6) and electrochemical oxidation (green line).

Information) Here, PTAz-2 is used as an example. Upon the addition of 10% TFA, protonation of polymers afforded a new transition with peak at 677 nm as well as a broad transition in the infrared region (1500−2200 nm) while the π−π* transition bleached. Scheme 2 indicates the process of converting neutral PTAz-2 to ion PTAz-2 upon protonation via attaching one proton to five-membered ring of azulene. (Di-PTAz-2 is used as a surrogate for representative dimer of PTAz-2 and DFT calculation.) Under this circumstance, azulenylium ion became a very powerful electron acceptor and resulted in the efficient charge separation. On the basis of previous researches, protonation of azulene lead to change of the sp2 carbon to sp3 carbon. Newly appeared peak in the infrared region was attributed to the charge separation and ICT. Meanwhile, the spectra change of PTAzs as a function of TFA concentration was studied (see the Supporting Information). The NIR absorption intensity gradually increased with the increase of TFA concentration. Finally, the spectrum did not vary much when TFA concentration reached 30%. Interestingly, the protonation−deprotonation process in solution or solid state was reversible (Figure S1, Supporting 8178


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Figure 3. (a) UV−vis−NIR spectra change of PTAz-2 as a function of m-CPBA concentration in chloroform/TFA solution. (b) UV−vis−NIR spectra change of PTAz-2 as a function of TFA concentration in chloroform/m-CPBA solution. (c) Absorbance study of PTAz-2 as a function of mCPBA concentration in chloroform/10% TFA solution at 1105 and 1860 nm.

Figure 4. Spatial distributions of the calculated HOMOs and LUMOs of model compounds Di-PTAz-2 at protonation and chemical oxidation states.

results in the formation of radical cation, leading to the formation of new absorption band. Similar to the protonation, this chemical oxidation is determined by the pH value. For example, the spectrum did not show any change upon the addition of acetic acid and m-CPBA. However, similar spectrum could be obtained through adding methanesulfonic acid and m-CPBA. It is supposed only strong acid such as TFA (pKa = 0.23) and methanesulfonic acid (pKa = −1.9) rather than weak acid like acetic acid (pKa = 4.75) could lead to the

activated property. However, the spectrum remained unchanged upon the addition of m-CPBA only (Figure 3b). Other strong oxidants such as benzoyl peroxide (BPO) and hydrogen peroxide (H2O2) were also studied and showed similar phenomenon as m-CPBA. Newly formed absorption band can appear only when both TFA and aromatic peroxide were added into the solution. It has been shown that oxidation process could be initiated and promoted by acid.41 In detail, PTAz-2 could be protonated by the acid first. The protonated species can then undergo nucleophilic attack by peroxide which 8179


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The Journal of Physical Chemistry B protonation or chemical oxidation with the presence of peroxide. UV−vis−NIR spectra change of PTAz-2 as a function of TFA concentration in chloroform/m-CPBA solution was also studied. Similar to protonation process, a rise in TFA concentration afforded enhancement of protonation and increased absorption intensities in both visible and NIR region (680 and 1900 nm) while the π−π* transition of the neutral state gradually disappeared. Additionally, it is assumed that the electrophilic nature of the oxidant could be further enhanced by protonation.42 Hence, absorbance intensity at 1105 nm was also promoted upon further addition of TFA. Solution oxidation of PTAz-2 as a function of BPO concentration gives results very similar to what was described for m-CPBA above. The spectrum changed dramatically even at the lower concentration of BPO, which may indicate the oxidation potential of BPO is strong than that of m-CPBA. Though chemical oxidation showed outstanding spectra change upon adding TFA and small quantity of BPO, the system is quite stable in a seal cuvette. The UV−vis−NIR study showed spectra with small variations in a sealed system for hours (Figure S 2d, Supporting Information). DFT Study. DFT calculations reproduced well the spectral behavior of protonated states for polymers (Figure 4 and Figure S4). The calculated dihedral angles of protonated Di-PTAzs are all much larger than those of neutral compounds, indicating the transition from sp2 carbon to sp3 carbon. Di-PTAz-2 was here used as an example to study the charge separation. The HOMO in the protonated Di-PTAz-2 is delocalized on monomeric unit and azulenes, while the LUMO is mainly localized on one azulene unit at one side. A large charge separation and effective ICT resulted by protonation then lead to a lower HOMO− LUMO band gap and absorbance band at long wavelength. Upon the addition of m-CPBA, the HOMO now is delocalized the whole backbone except one azulene−bithiophene unit, while LUMO is merely delocalized on one azulene unit. Though ICT could also be generated by the chemical oxidation, the charge separation is not as effective as that of protonation. Time dependent (TD)-DFT calculation of excitation energies and oscillation strength were done (See the Supporting Information). The first 20 singlet−singlet transition energies were calculated for Di-PTAz-2 under protonation and chemical oxidation. Moreover, the calculated maxima absorption for protonation and chemical oxidation is 2109.19 (oscillation strength f = 0.1291) and 1685.55 (0.6346) nm, respectively, which match well with the experimental trend. Therefore, a combinational effect of protonation and radical cation could lead to the blue shift of absorbance band. Electron Paramagnetic Resonance Spectroscopy. Electron paramagnetic resonance (EPR) spectroscopy of four polymers in solution all showed strong signal and confirmed the presence of paramagnetic radical cation upon oxidation. Here, PTAz-2 is used as the example. As shown in Figure 5, PTAz-2 in chloroform/TFA and solution without PTAz-2 did not show any signal. PTAz-2 in chloroform/TFA/m-CPBA exhibited the strong EPR signal. The radical cation of PTAz-2 has an EPR signal centered at a g value of 2.00248 with a peak-to-peak width of 4 G. No hyperfine coupling was observed, which could be attributed to the delocalization of the radical cation wave function. Solid State Properties. In-situ UV−Vis−NIR Spectroelectrochemistry. Spectroelectrochemistry acts as an indispensable role in studying the optical changes that occur upon doping or

Figure 5. EPR spectrum of PTAz-2 in chloroform/TFA/m-CPBA (v/ v/v, 75/10/15) solution (blue line), PTAz-2 in chloroform/TFA/ (v/ v, 90/10) solution (red line) and chloroform/TFA/m-CPBA (v/v/v, 75/10/15) without PTAz-2 (black line).

dedoping of an electrochromic film.43 The in situ UV−vis−NIR spectra obtained during the electrochemical doping of polymers are presented in Figure 6 and Figure S4. Since high positive potentials decreased the stability of the polymer films, such potentials were avoided. Neutral films displayed only the π−π* transition at 385, 545, 413, and 451 nm for PTAz-1, PTAz-2, PTAz-3, and PTAz-4, respectively. Redshifts in positions of long wavelength maxima were observed in thin film spectra compared with the spectra in solutions indicate the expected more coplanar structure in the solid state compared with solution state for all four systems. These polymers exhibited similar trend when the potential was increased stepwise. PTAz-2 was here used an example to demonstrate UV−vis−NIR spectra under doping process. The spectrum remained nearly unchanged when the potential was increased stepwise from −1.6 to +0.6 V. As the potential was increased to 0.8 V, the absorption band (413 nm) from neutral polymer diminished, while two broad bands emerged at 648 and 1400 nm. Both newly formed absorption peaks increased in intensity with increased doping level to 1.6 V and gradually shifted to lower wavelengths, accompanied by the decreased absorbance intensities at 413 nm. As shown in Scheme 3, at lower positive potentials, the newly formed peaks in both visible and NIR regions are ascribed to the radical cation (polaron), which is confined to a limited number of monomeric units. As a result of the characteristic strong coupling of the electronic structure with the geometric structure for pconjugated materials, this deformation causes new electronic states to appear within the π−π* band gap of the polymer.44 With increasing doping levels, more than one charge is removed from a single chain. Absorption band in the NIR region gradually blue-shifted as it increased intensity. On the basis of previous researches, two polarons on a single chain rather than bipolaron is formed.26,30 At a high doping level, PTAz-2 does not display classic polaron spectra: transition from polarons to bipolarons leads to a suppression of the transition between the two in-gap levels and that the remaining absorptions shift to higher energies. For polymers, the distance between the polarons is large enough to inhibit the recombination of the two polarons into a bipolaron, because the energy gained from the decrease in the Coulomb repulsion is supposed to be greater than the energy lost by creating two geometrical deformations instead of one. Meanwhile, the presence of two equally intense peaks (620 and 1099 nm) at 8180


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Figure 6. UV−vis−NIR spectra of PTAz-2 under different potentials measured in (a) 0.1 M LiClO4/acetonitrile solution and (b) 0.1 M LiClO4/ acetonitrile/TFA (ACN/TFA 85:15) solution.

known to yield reasonable results for charged conjugated polymers. The neutral Di-PTAz-2 mainly shows absorption band at 459.4 nm (oscillation strength f = 1.8409), which is quite similar to experimental data. After removal of one electron, absorbance intensity at 459 nm decreases with the occurrence of two peaks at 4847 (0.5731) and 1705 nm (0.1697). The latter absorption should be the same as observed in the UV− vis−NIR spectra in Figure 6a, whereas the absorption with a maximum at 4847 nm is outside the wavelength range used in our UV−vis−NIR measurements. Recently, Nöll and Ö sterholm all proposed that a polaron absorption should be found at wavelengths above 2500 nm.26,27 Upon further oxidation, absorption band at 1758 nm (1.0277) is formed, which shows higher oscillation strength than other band. This is in good agreement with Ö sterholm’s study on polyazulene at high doping level. For the double oxidized form, both singlet and triplet states were calculated. Triplet state, has lower total energy, is then more stable than the singlet state. On the basis of calculation, polaron of Di-PTAz-2 has one spin and double oxidized Di-PTAz-2 has two spins (See the Supporting Information). Hence, double-oxidized, spinless species (polaron pairs/triplet states) are formed at high doping level. Meanwhile, the dihedral angles (particularly those between azulene and thiophene) decrease as the degree of oxidization increases.

Scheme 3. Simplified Picture of Di-PTAz-2 under Oxidation

the doubly oxidized state is not compatible with a bipolaron structure, for which only one strong absorption band is expected. Instead, this phenomenon is resulted by two separated polarons on a single chain. Therefore, polaron pairs are formed at high doping level for polymers. In addition, TFA influence on spectroelectrochemistry behavior was also examined. At the same applied voltage, PTAz-2 in acid exhibits higher intensity of NIR absorption than that conducted in neutral electrolyte, which corresponds well with the fact that acid can lower the oxidation potential of materials. Protonation could also improve the electron transport in polymer film with less voltage loss, leading to the evenly applied potential. Finally, absorption band at 1099 nm generated by polaron pairs can also be observed under the electrochemical oxidation when the potential reached 1.4 V, indicating oxidation process played a major part at a high voltage level. The influence of TFA on chemical oxidation and electrochemical oxidation is quite similar based on the study. In other words, TFA mainly has two roles: (1) protonation and (2) lowering the oxidation potential as acid-catalyzed process. DFT Study. In order to gain insight into the electronic nature of positively charged PTAzs in neutral electrolyte, we performed density functional calculations on dimer model compounds using the Gaussian 09 program. In our study we used the B3LYP/6-31G(d) hybrid functional because this is

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CONCLUSIONS In this work, NIR absorption mechanism for azulene-containing polymers upon protonation and oxidation are studied. On the basis of experimental results and density function calculation (DFT), it could be concluded that intramolecular charge transfer is the key to the remarkable lowering of band gap after protonation. Electron paramagnetic resonance confirmed the presence of radical cation by chemical oxidation of peroxide. The oxidation can be accelerated by strong organic acid. In addition, the optical absorption bands arising from polarons and polaron pairs were found at similar wavelengths and closely overlapped in films. Both experimental results and DFT suggest the formation of polaron pairs in polymers at higher doping levels instead of bipolarons. Moreover, it is shown that TFA could have a profound effect in promoting the chemical oxidation and electrochemical oxidation. Distinct NIR absorption could be obtained in a single system via adopting different protonation/oxidation methods. A better understanding of the phenomenon and related mechanisms could 8181


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be further used as a guide for design new NIR absorption materials.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1, UV−vis−NIR spectra change of PTAzs as a function of TFA concentration in chloroform solution; Figure S2, Dynamic absorbance study of PTAz-2 as a function of time in chloroform/TFA/m-CPBA and UV−vis−NIR spectra change of PTAz-2 under different peroxide concentration; Figure S3, EPR spectrum of PTAz-2 in chloroform/TFA/m-CPBA/TEA solution; Figure S4, UV−vis−NIR spectra of PTAzs under different potentials measured in a 0.1 M LiClO4/acetonitrile solution; Figure S5, CV curves of polymer PTAzs without/with 15% TFA measured in a 0.1 M LiClO4/acetonitrile solution; Table S1, Milliken atomic spin densities for polaron and polaron pairs; Tables S2 and S3, TD-DFT calculations of the absorption energies and oscillator strengths for the Di-PTAz-2 under protonation, chemical oxidation, and electrical oxidation. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b01613.

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AUTHOR INFORMATION

Corresponding Authors

*(C.H.) E-mail: [email protected]; [email protected]. *(F.W.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support of this work from the Science and Engineering Research Council (Grant 1123004024) of Agency for Science, Technology and Research of Singapore. T.T. also acknowledges scholarship support from National University of Singapore. The DFT calculation was supported by the A*STAR Computational Resource Centre through the use of its high performance computing facilities.

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Origin of Near-Infrared Absorption for Azulene-Containing Conjugated

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