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Luminescent Main-Chain Organoborane Polymers: Highly Robust, Electron-Deficient Poly(oligothiophene borane)s via Stille Coupling Polymerization Xiaodong Yin, Fang Guo, Roger A. Lalancette, and Frieder Jak̈ le* Department of Chemistry, Rutgers University Newark, 73 Warren Street, Newark, New Jersey 07102, United States S Supporting Information *

ABSTRACT: A series of polymers (PBnT, n = 2−5) with boron atoms incorporated into the conjugated polythiophene main chain have been prepared via Pd-catalyzed coupling of stannylated thienylborane monomers. The polymers exhibit excellent long-term chemical stability to air and moisture and remarkable thermal stability with decomposition temperatures reaching over 300 °C. The high stability is achieved by placing very bulky pendant groups, 2,4,6-tri-tert-butylphenyl (Mes*) and 2,4,6-tris(trifluoromethyl)phenyl (FMes), on boron that prevent attack by nucleophiles. All these polymers display strong absorptions in the visible region and intense fluorescence in both solution and the solid state with quantum yields of up to 38% and fast radiative decay constants (kr) of up to 3.3 × 108 s−1. Density functional theory (DFT) studies on diborylated oligothiophene model compounds suggest that the strong absorption of the polymers results from π−π* transitions on the oligothiophene borane main chain with significant charge transfer to boron. The unusually intense luminescence in the solid state is favored by the rigid planar skeleton and steric shielding of the bulky pendent groups. The emission color can be tuned from blue to deep orange by varying the length of the π-conjugated oligothiophene spacer between the boron atoms. Spectroelectrochemical studies on a dimeric model compound in THF solution reveal reversible two-step reductions to give highly colored species, while the corresponding polymeric material precipitates at higher potentials after undergoing an initial reversible reduction. The LUMO energy levels of the polymers can be effectively lowered by introduction of electron-withdrawing pendent groups on boron, affording a versatile approach for development of electron-deficient boroncontaining polymers with controllable electronic structures and photophysical properties. The facile modular synthetic approach combined with the exceptional stability opens the door to broad adoption of electron-deficient organoboranes in conjugated materials design and development.

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INTRODUCTION Over the past several decades, conjugated polymers have become one of the most rapidly growing fields in material science because of potential applications, for example, in display technology,1 sensory materials,2 solar cells,3 and transistors.4 In recent years, much effort has been directed toward the functionalization of organic conjugated polymers with main group elements.5 In particular, the incorporation of tricoordinate organoborane groups into conjugated polymers has attracted considerable attention as advantageous optical and electronic properties result from the interaction between the empty p-orbital of boron and organic π-conjugated substituents, enabling applications as optoelectronic and sensory materials.6 However, the presence of the empty p-orbital tends to make boranes prone to attack by Lewis bases, which can ultimately lead to hydrolytic or oxidative degradation. Therefore, electrondeficient yet robust conjugated polymers with tricoordinated boron in the main chain remain rare. Among the most successful routes for the incorporation of tricoordinate organoborane moieties into conjugated polymers are hydroboration6c,g,7 and Sn/B exchange protocols.8 However, they © XXXX American Chemical Society

rely on polymerization processes that involve reactive borane groups, and due to the high sensitivity of the precursors, these methods typically require inert atmosphere techniques and relatively complex synthetic procedures. To make functional conjugated borane polymers readily accessible to researchers across the disciplines, it is therefore desirable to develop new robust borane monomers that not only exhibit high stability to air, water and even strong acid/base but also can be easily further modified with functional groups such as halides, boronic acid, or stannyl groups for transition-metal-catalyzed polymerization reactions.9 The stability of organoboranes can be very effectively enhanced by bulky substituents on the boron atom.10 The presence of two mesityl or fluoromesityl groups generally results in high stability of molecular compounds,11 and dimesitylboryl groups have also been used successfully used for the side chain modification of conjugated polymers.12 Received: November 10, 2015 Revised: December 23, 2015

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DOI: 10.1021/acs.macromol.5b02446 Macromolecules XXXX, XXX, XXX−XXX

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insights into the influence of (a) different sterically demanding pendent groups on boron and (b) variations in the length of the conjugated linkers on the electronic structure and photophysical properties.

However, when incorporated into the main chain of conjugated oligomers or polymers, only one position at boron remains available for introduction of a bulky substituent. The triisopropylphenyl (Tip) group has generally proved to be suitable,13 but degradation tends to still occur after prolonged exposure to ambient atmosphere. Recently, we introduced a series of exceptionally stable, yet highly electron-deficient triarylborane compounds (A), which are obtained in a few simple steps and can be readily further functionalized at the thiophene α-positions.11g The coplanar structure of the dithienylborane moiety is reinforced by the sterically demanding Mes* (R = t-Bu)14 or FMes (R = CF3) groups,11g,k,l while the fluorinated FMes groups also further enhance the electron-deficient character of boron. Herein, we report the facile incorporation of the organoborane building block A into conjugated polymers via Stille-type (co)polymerization,15 providing access to a series of well-defined main-chain organoborane polymers that are perfectly stable to air and moisture for periods of well over a year (Figure 1). In sharp

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RESULTS AND DISCUSSION To incorporate building blocks A into polymers using Stilletype methods, we first prepared the distannylated compounds BDTSn2 and FBDTSn2 by dilithiation of BDT and FBDT11g and subsequent treatment with trimethylstannyl chloride (Scheme 1). Recrystallization from methanol furnished BDTSn2 as a white microcrystalline solid, whereas purification by flash column chromatography on alumina gel with hexanes/ triethylamine = 100/1 as the eluent gave pure FBDTSn2 as a light yellow oil. Both compounds were obtained in high yield and fully characterized by multinuclear NMR and highresolution mass spectrometry. Polymerization of the borane monomers was accomplished by Pd-catalyzed Stille-type cross-coupling with the assistance of microwave irradiation. Microwave irradiation has proved to be a highly efficient method for the synthesis of conjugated polymers with reaction times that are commonly reduced by more than a factor of 10, higher yields, and fewer side products.16 Using this method, the boron-containing conjugated polymers PB2T (35% isolated yield) and PFB2T (43% isolated yield) were easily obtained within only 40 min. MALDI-TOF mass spectra (Supporting Information) further confirmed the formation of the desired macromolecules; multiple peak series with the expected peak separation based on the molecular weight of the repeating are observed in all cases. Dimeric model compounds, OB2T (60% yield) and FOB2T (55% yield), that mimic the repeating units in the polymer chain were also synthesized according to the procedures shown in Scheme 1. The polymers and model compounds are stable to air and moisture and can be easily purified without any special precautions. The polymers were isolated as yellow (PB2T) and orange (PFB2T) powdery solids after precipitation into

Figure 1. Modular strategy for the synthesis of perfectly air-stable boron-containing π-conjugated polymers.

contrast to the nonemissive building blocks A, the resulting polymers are strongly luminescent in solution and the solid state. They are more easily reduced, and their UV−vis absorption bands are significantly red-shifted, indicating highly effective π-conjugation throughout the polymer main chain. In addition, a detailed structure−property correlation offers

Scheme 1. Synthesis of Borane Polymers PB2T and PFB2T and the Corresponding Dimeric Species OB2T and FOB2T

B


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Macromolecules methanol. GPC analyses of the isolated products reveal Mw,GPC = 7.8 kDa, Đ = 1.34 (DPGPC = 14) for PB2T and Mw,GPC = 6.2 kDa, Đ = 1.24 (DPGPC = 10) for PFB2T. The model dimers were purified by column chromatography using hexanes/ dichloromethane (DCM) = 5:1 as the eluent and obtained as white (OB2T) and yellow (FOB2T) microcrystalline solids. Recrystallization from hexanes gave single crystals of OB2T suitable for X-ray analysis. As shown in Figure 2, the thiophene

transitions, respectively. A similar vibronic splitting is also seen in the emission, but the 0 → 1 transition at 481 nm for PB2T and 515 nm for PFB2T dominates the fluorescence spectra. The strong fluorescence of the polymers and dimers in THF with quantum yields >20% stands in stark contrast to the almost completely nonemissive nature11g of the monomeric building blocks. Thin films of the polymers exhibit almost identical absorptions as in solution without any evidence of intermolecular association in the ground state (Figure S4). Interestingly, the fluorescence remained quite strong with quantum yields of 5.3 ± 3% for PB2T and 8.9 ± 3% for PFB2T and even larger quantum yields for the dimers (Table 1). A clear red-shift in comparison to the emission in THF solution suggests that intermolecular interactions do affect the excited state photophysical properties. The strong fluorescence of the dimers and polymers not only in solution but also in the solid state is remarkable and is likely related to the presence of the bulky Mes* and FMes groups, which rigidify the conjugated backbone and also hinder intermolecular association to some extent, thereby limiting nonradiative decay. DFT calculations on the dimer model compounds were carried out at the B3LYP/6-31+G* level of theory to gain further insight into the origin of the photophysical characteristics, including the red-shifts in the absorptions and dramatic increases in the fluorescence quantum yields relative to the monomeric species. The HOMOs and LUMOs for the dimers are fully delocalized over the bithienyldiborane main chain (Figure 4) with strong contributions of the boron p-orbitals to the LUMOs. When oligomers with five repeating units were used to better approximate the corresponding polymers, all the thiophenes still adopted a coplanar conformation, with the frontier orbitals extending over almost the entire conjugated main chain (Figures S10 and S11). The delocalization of both the HOMO and LUMO over the polythiophene main chain contrasts our observation that the HOMO is localized on the pendent Mes* group for the monomeric building block BDT.11g The different behavior for the dimer OB2T and the corresponding pentamer is likely related to the more electronrich character and larger conjugation length of the bithiophene bridge. According to TD-DFT calculations, the dominant absorption band of OB2T and FOB2T at 418 nm is due to a high intensity S0 → S1 (HOMO → LUMO) transition that is mainly π−π* in character, with some degree of charge transfer to boron. It is this intense transition that results in the remarkably strong emission of the dimers and a similar transition presumably also occurs upon further chain extension, leading to the observed strong luminescence for the polymers. The rigid planar skeleton of the dimers and polymers further enhances the emission, whereas for the monomers rapid rotation of the thiophene moieties may facilitate nonradiative decay, leading to effective quenching of the emission. The electronic structure of the polymers was further studied by cyclic voltammetry in THF solution (Figure 5). The voltammograms reveal two broad overlapping redox waves with remarkably low onsets of reduction, especially for the fluorinated polymer (EPB2T = −1.82 V and EPFB2T = −1.43 red red + V relative to the Fc/Fc couple). A comparison with the monomers and dimer models again provides important insights. For the monomers, BDT and FBDT, only a single redox wave is observed, but two distinct reversible reductions are evident for the dimers, OB2T and FOB2T, indicating stepwise

Figure 2. ORTEP plot of the single crystal X-ray structure of OB2T (50% thermal ellipsoids).

rings of OB2T adopt an almost coplanar conformation with a small dihedral angle ∠Th1−Th2 of 14.6°. The Mes* groups are positioned orthogonal with dihedral angles of 85.2° and 84.4° relative to the thiophene rings in the main chain. No π−π stacking was observed in the extended structure due to the high steric demand of the bulky Mes* groups, but C−H···S and C− H···π contacts between neighboring molecules are evident as illustrated in Figure S3. These weak interactions may also affect the relative orientation of the terminal and internal thiophenes. Thus, a trans-arrangement is favored, and almost no disorder of the terminal thiophene rings (position of the S) is observed in contrast to the structure of the corresponding monomer. The UV−vis absorption maxima of the polymers in THF solution are clearly red-shifted in comparison to those of the corresponding monomers (BDT, FBDT) and dimers (OB2T, FOB2T) (Figure 3 and Table 1). Two vibronic bands of similar intensity are observed for PB2T (433, 458 nm) and PFB2T (456, 476 nm), which can be assigned to 0 → 1 and 0 → 2

Figure 3. UV−vis absorption and fluorescence spectra of dimers and polymers in THF solution. C


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Macromolecules Table 1. Photophysical Properties of Dimers and Polymers λabsa (solid)/nm e

BDT (A) FBDT (A)e OB2T FOB2T PB2T PFB2T

324 326 418 418 458 476

(420) (420) (457) (480)

λema (solid)/nm

445 450 481 515

ΦFb(THF)/%

ΦFb(film)/%

± ± ± ±

18 ± 5 12 ± 5 5.3 ± 3 8.9 ± 3

(495) (500) (484) (574)

38 21 38 23

9 5 9 6

τa,c/ns

0.7 0.6 1.2 0.8

± ± ± ±

0.02 0.02 0.03 0.02

krd/s−1

5.4 3.5 3.2 3.1

× × × ×

108 108 108 108

a Measured in THF solution. bAbsolute quantum yield determined using an integrating sphere. cFluorescence liftetime. dRadiative rate constant (kr) determined using the equation kr = Φ/τ. eTaken from ref 11g.

reduction of the borane moieties. The first reduction potential of OB2T (−2.06 V) and FOB2T (−1.72 V) experiences a large anodic shift relative to that of the monomers as a result of chain extension and electronic communication between the boron atoms. This is consistent with a highly conjugated main chain structure. However, the redox splitting for OB2T and FOB2T (ΔE = 288 mV for OB2T; ΔE = 256 mV for FOB2T) is smaller than for related monothiophene-bridged diboron compounds,11g indicative of weaker electronic communication between the boron centers through the longer bithiophene bridge. To elucidate the nature of the reduced species, spectroelectrochemical studies were carried out on the dimer OB2T and polymer PB2T in THF solution with 0.1 M Bu4NPF6 as supporting electrolyte, using a gold honeycomb and a Ag reference electrode. As seen in Figure 6a, absorption features that are distinctly different from those of OB2T are observed when the applied voltage is gradually decreased to −1.9 V and then to −2.2 V. At −1.9 V, a new absorption band develops with a maximum at 640 nm and a high-energy shoulder at ca. 580 nm. In addition, a very broad absorption is observed at ca. 1460 nm. Collectively, these absorptions are attributed to formation of the radical anion [OB2T]•−. These bands give way to lower energy absorptions at ca. 760 and 840 nm when the potential is further decreased to −2.2 V, due to formation of the dianion [OB2T]2−. Importantly, these color changes are fully reversible as evidenced by recovery of the initial absorption bands for neutral OB2T at 397 and 418 nm when the potential is set to −0.2 V. It is interesting to compare the absorption features for the radical anion [OB2T]•− and dianion [OB2T]2− to literature data for the boron radical [Mes2BPh]•− (Mes = 2,4,6-trimethylphenyl) and the corresponding singly

Figure 4. Plots of Kohn−Sham orbital and electronic transitions for OB2T and FOB2T (geometry optimized at the B3LYP/6-31+G* level and single point energy at the B3PW91/6-211+G* level).

Figure 5. Cyclic voltammetry plots of monomeric, dimeric, and polymeric compounds in THF solution (ca. 1 × 10−3 mol L−1 using 0.1 M Bu4NPF6 as supporting electrolyte, Fc (ferrocene) as internal standard, scan rate 100 mV s−1).

Figure 6. Spectroelectrochemical changes for the reduction of (a) OB2T and (b) PB2T (potential vs Ag/AgCl) in THF solutions containing 0.1 M Bu4NPF6 as supporting electrolyte. D


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as shown in Scheme 2. Similar to the synthesis of PB2T, polymers with longer thiophene linkers, PB3T, PB4T, and

and doubly reduced phenylenediborane species derived from Mes2B−C6H4−BMes2 and Mes2B−C6H4−C6H4−BMes2.17 The absorption maximum for [Mes2BPh]•− in THF occurs at 692 nm, whereas the longest wavelength absorptions of [Mes2B− C6H4−BMes2]•− (872, 424 nm) and [Mes2B−C6H4−C6H4− BMes2]•− (1305, 522 nm) are found at considerably lower energy. Kaim and co-workers17a attributed the lowest energy bands for the radical anions to SOMO-to-LUMO transitions with intervalence charge transfer character and the higher energy bands to HOMO-to-SOMO transitions. Relative to these data, both the long wavelength and short wavelength absorptions for [OB2T]•− occur at significantly lower energy. The absorption maxima for the dianion [OB2T]2− (840, 760 nm) are also found at lower energy than for [Mes2B−C6H4− BMes2]2− (665 nm) and [Mes2B−C6H4−C6H4−BMes2]2− (736 nm), consistent with more effective electronic delocalization across the bithiophene bridge. For the polymer, PB2T, only the first reduction process could be clearly observed, and the absorption maximum at 623 nm is in a similar range as that of OB2T (Figure 6b). Further reduction led to strongly diminished signal intensity, presumably due to precipitation of the highly charged polymeric species. Using the data derived from electrochemical and UV−vis absorption measurements, the HOMO and LUMO energy levels of the molecular and polymeric materials can be determined as shown in Figure 7. For both fluorinated and

Scheme 2. Synthesis and Molecular Weight Data for Isolated Boron-Containing Polymers with Different Lengths of the Oligothiophene Bridges

PB5T, were prepared via microwave-assisted Stille coupling using distannylated oligothiophenes and diiodinated BDT. A hexyl group was introduced on the central thiophene ring of PB5T to increase the solubility of the resulting polymer. For the synthesis of PB1T with a single thiophene bridge between the boron atoms a tin−boron exchange protocol was utilized since it cannot be synthesized by C−C coupling methods. GPC analyses indicated that the polymers are obtained with relatively high molecular weights and reasonably low dispersities (Đ). The molecular weight of PB1T is somewhat lower than that of the other polymers, possibly because the tin−boron exchange reaction is more prone to side reactions such as Sn−Me bond cleavage that can result in chain termination. Again, for all these polymers, no obvious decomposition was observed after exposure to ambient atmosphere for more than a year, suggesting excellent stability to oxygen and moisture. Besides, thermogravimetric analysis (TGA) results indicate that all polymers exhibit excellent thermal stability with decomposition temperatures that are over 300 °C in most cases (Figure S7). This is in contrast to polymers prepared via hydroboration polymerization which are reported to undergo depolymerization at temperatures above 100 °C.6c A comparison of the absorption and emission characteristics of the polymers in THF solution is provided in Figure 8. A strong red-shift with increasing length of the (oligo)thiophene bridge indicates that the band gap becomes more narrow as the

Figure 7. Comparison of the experimentally determined energy levels of small molecules and polymers. LUMO levels are determined as E = −(4.8 + Ered) eV and HOMO levels as EHOMO = ELUMO − Eg using energy gaps (Eg) from UV−vis spectroscopy according to the equation Eg = 1240/λabs.

non-fluorinated compounds, the HOMOs increase and the LUMOs decrease in energy as the chain is extended, resulting in a much narrower band gap for the polymers than for the small molecules. This phenomenon is typical of conjugated polymers and indicates that the boron p-orbital effectively participates in the extension of conjugation throughout the polymer main chain. A comparison of the fluorinated and nonfluorinated derivatives reveals that the pendent group on boron can be used very effectively to lower the LUMO energy level, thereby enhancing the electron-deficient character of the boron-containing polymers. However, both the HOMO and LUMO levels decrease in energy due to the electronwithdrawing effect of the trifluoromethyl groups, resulting in only subtle changes to the band gap upon fluorination. Further tuning of the optical absorption properties is therefore best accomplished by variations of the electronic structure of the conjugated linker between the borane moieties in the polymer. To further study the influence of the linker group on the band gap, we synthesized a series of (oligo)thiophene borane polymers by Stille coupling and tin−boron exchange reactions, E


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Figure 8. (a) UV−vis (left) and fluorescence (right) spectra of boron-containing polymers in THF. (b) CIE (1931) chromaticity diagram indicating the polymer fluorescence in THF solution.

Table 2. Comparison of Photophysical and Electrochemical Data of Polymers PBnT (n = 1−5) polymers

λAbs,THFa/nm

Egoptb/eV

PB1T PB2T PB3T PB4T PB5T

465 494 534 583 590

2.66 2.55 2.35 2.29 2.25

λEm,THF(solid)c/nm 450 481 537 571 590

(495) (484) (595) (615) (657)

ΦTHF(solid)d/% 12 38 18 17 15

± ± ± ± ±

3 9 3 3 5

(1.2 ± 1) (5.3 ± 3) (12 ± 3) (1.2 ± 1) (0.9 ± 0.5)

τe/ns

kr × 108/s−1

EredCVf/V

EHOMOg/eV

ELUMOg/eV

± ± ± ± ±

1.3 3.3 2.4 2.4 1.6

−1.775 −1.820 −1.880 −1.890 −1.950

−5.68 −5.53 −5.26 −5.19 −5.10

−3.02 −2.98 −2.91 −2.90 −2.85

0.95 1.16 0.75 0.71 0.81

0.01 0.02 0.01 0.01 0.01

a

Onset of absorption. bEgopt = 1240/λAbsonset. cEmission maximum. dAbsolute quantum yield measured using a precalibrated Quanta-φ integrating sphere. eLifetime measured using a 350 nm Nano-LED for excitation of PB1T and a 388 nm Nano-LED for all other polymers. fCV data are reported vs Fc0/+ in THF solution; Ered was estimated from the reduction peak onsets. gELUMO = −(4.8 + Ered), EHOMO = ELUMO − Egopt.

ratio of thiophene to borane moieties increases. The bandgaps were estimated based on the UV−vis spectra, according to the equation Eg = 1240/λabs (Table 2). The observed decrease in band gap with elongation of the oligothiophene linker is consistent with DFT calculations on diborylated model compounds with different oligothiophene linker length (B3T, B4T, and B5T; see Figures S12 and S13) and in good agreement with results on molecular diborylated oligothiophenes recently reported by Marder et al.11e The HOMO and LUMO energies were assessed using electrochemical studies. Cyclic voltammetry experiments show that the reduction potentials fall in a relatively narrow range from Eonset = −1.77 to −1.95 V (Figure S6) but show a clear trend in that they gradually become more negative as the thiophene linker gets longer. Thus, the LUMO energy increases by 0.17 eV from PB1T to PB5T. This effect is attributed to a decrease in the boron “density” that leads to less pronounced electronic coupling between neighboring boron centers. Using the reduction potentials and absorption data, the HOMO energy levels can also be estimated (Figure 9). The results indicate a much more dramatic effect of the thiophene linker length on the HOMO levels, which increase by 0.58 eV from PB1T to PB5T. These changes result in an overall decrease of the band gap from 2.66 eV for PB1T to 2.25 eV for PB5T. The polymers are strongly fluorescent with quantum yields in the range of 12−38% (Table 2). A clear correlation between linker length and fluorescence quantum yield is not apparent. The higher quantum yield for PB2T is likely related to the relatively larger number of pendent Mes* groups that make the polymer more rigid and limit intermolecular interactions. PB2T also shows the longest lifetime at about τ = 1.2 ns, whereas that of the other polymers is less than 1 ns, which is similar to data reported for oligothiophenes.18,11e Using the equation Φ = krτ, radiative rate constants in the range from kr = 1.3 × 108 to 3.3 × 108 s−1 are derived, which are comparable to previously reported boryl-substituted oligothiophene derivatives.19,11e The

Figure 9. Experimentally determined HOMO and LUMO energy levels of polymers with different thiophene linker length. LUMO levels are determined as E = −(4.8 + Ered) eV and HOMO levels as EHOMO = ELUMO − Eg using energy gaps (Eg) from UV−vis spectroscopy according to the equation Eg = 1240/λabs.

polymers also show remarkably strong fluorescence in the thin film state with quantum yields up to 12%. Again, the high quantum yields are likely related to the steric effect of the bulky Mes* groups, which can block π−π interactions between neighboring macromolecules, thereby inhibiting nonradiative decay. The emission color of the five polymers ranges from blue to deep orange (Figure 8c), which further substantiates that the photophysical properties can be effectively tuned by changing the structure of the π-conjugated bridge between the boron atoms.

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CONCLUSIONS We have demonstrated that microwave-assisted Stille crosscoupling is a very effective and versatile method for the synthesis of conjugated polymers with tricoordinate boranes embedded in the main chain. A range of new polymers with different linkers and pendent groups on boron have been prepared from simple and highly robust triarylborane building F


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Arg-Bradykinin (904.4681), Angiotensin I (1296.6853), Glu-Fibrinopeptide B (1570.6774), ACTH (clip 1−17) (2093.0867), ACTH (clip 18−39) (2465.1989), and ACTH (clip 7−38) (3657.9294) with αhydroxy-4-cyanocinnamic acid as the matrix). UV−vis absorption data were acquired on a Varian Cary 5000 UV− vis/NIR spectrophotometer. The fluorescence data and lifetimes were measured using an Horiba Fluorolog-3 spectrofluorometer equipped with a 350 or 388 nm nanoLED for excitation and a FluoroHub R-928 detector. Absolute quantum yields (ΦF) were measured on the HORIBA Fluorolog-3 using a precalibrated Quanta-φ integrating sphere. Light from the sample compartment is directed into the sphere via a fiber-optic cable and the F-3000 fiber-optic adapter and then returned to the sample compartment (and to the emission monochromator) via a second fiber-optic cable and the F-3000. Cyclic voltammetry (CV) and square wave voltammetry (SWV) experiments were carried out on a CV-50W analyzer from BASi. The three-electrode system consisted of an Au disk as working electrode, a Pt wire as counter electrode, and an Ag wire as the reference electrode. The voltammograms were recorded with ca. 10−3−10−4 M solutions in THF containing Bu4N[PF6] (0.1 M) as the supporting electrolyte. The scans were referenced after the addition of a small amount of ferrocene as internal standard. The potentials are reported relative to the ferrocene/ferrocenium couple. Spectroelectrochemistry experiments were performed with a gold honeycomb electrode (Pine Research Instrumentation) and a Ag wire as reference in THF containing Bu4N[PF6] (0.1 M) as the supporting electrolyte. The potentiostat was set to a certain potential, and the current was monitored. UV− vis−NIR data were acquired after 100 s and then again after the current flow subsided (typically ca. 300 s). GPC analyses were performed using a Viscotek GPCmax equipped with a VE 2001 GPC solvent/sample module, a 2600 PDA detector, a TDA 305 triple detector array, and a columns set consisting of a PLgel 5 mm mixed-D and two PLgel 5 mm mixed-C columns. The system was calibrated against narrow polystyrene standards (10) in the molecular weight range of 580−371 100 Da. X-ray diffraction intensities were collected on a Bruker SMART APEX CCD diffractometer using Cu Kα (1.541 78 Å) radiation at 100 K. The structure of OB2T was refined by full-matrix least-squares based on F2 with all reflections.22 Non-hydrogen atoms were refined with anisotropic displacement coefficients, and hydrogen atoms were treated as idealized contribution. SADABS (Sheldrick, 12 G.M. SADABS (2.01), Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998) absorption correction was applied. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication CCDC 1426034. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: [email protected]). DFT calculations were performed with the Gaussian09 suite of programs.23 The input files were generated from single crystal structures when available or otherwise generated in Chem3D and then preoptimized in Spartan ’08 V 1.2.0. Geometries were then optimized in Gaussian09 using the hybrid density functional B3LYP with a 6-31+G* basis set. Frequency calculations were performed to confirm the presence of local minima (positive frequencies). The orbital energy levels were calculated by single point calculations using the B3PW91 functional with a 6-311+G* basis set (the larger basis set gave more reasonable results for the dimeric species in comparison to the monomers). Vertical excitations were then calculated using TDDFT methods (B3LYP/6-31+G*). Orbital representations were plotted with Gaussview 5.08 (scaling radii of 75%, isovalue of 0.02). Synthesis of BDTSn2. Bis(thien-2-yl)(2,4,6-tri-tert-butylphenyl)borane (1.19 g, 2.80 mmol) was charged into a 100 mL Schlenk flask under nitrogen. Then 60 mL of degassed ether were added by syringe, and the solution was cooled to −78 °C. n-BuLi (3.9 mL, 1.6 M in hexane) was added dropwise over a period of 30 min. The mixture was stirred at −78 °C for 0.5 h and then warmed to rt and kept stirring for another 1 h. Then the flask was cooled to −78 °C again, and a solution of chlorotrimethylstannane (1.23 g, 6.17 mmol) in 10 mL of ether was added dropwise. The mixture was stirred at rt overnight and then

blocks with reasonable molecular weights, good solubility in common organic solvents, and exceptional thermal and oxidative stability. We emphasize that the reported highly robust polymers cannot be prepared by the previously reported hydroboration polymerization or Sn/B exchange methods. The reduction potentials and band gaps of the polymers are much lower than those of the corresponding monomeric building blocks, indicative of a highly conjugated structure. All the polymers show strong fluorescence both in solution and the solid state with quantum yields up to 38 ± 9% for PB2T in THF solution. The origin of these advantageous optical properties was further examined using the model dimers OB2T and FOB2T. Similar to the polymers, they display strong fluorescence in solution and the solid state, which, according to theoretical calculations, can be attributed to strong π−n/π* electronic transitions from S0 to S1 that are localized on the thienylborane main chain. Electrochemical and DFT results reveal that electron-withdrawing pendent groups (FMes vs Mes*) significantly lower both the HOMO and LUMO levels. When the same pendent group is used but the length of the thiophene linker is increased, then the LUMO level changes only slightly, but the energy of the HOMO level increases dramatically. As a result, the band gaps of the polymers can be easily tuned in a predictable manner by varying the linker length, ultimately resulting in changes in the fluorescence of the polymers from blue to deep orange. Given the modular synthesis, facile tunability, and exceptionally high stability, the approach to boron-containing conjugated polymers described in here lends itself to broad adoption in the development of novel optoelectronic materials.

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EXPERIMENTAL SECTION

Materials and General Methods. All reactions were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inert-atmosphere glovebox. Ether solvents were distilled from Na/benzophenone prior to use. Hydrocarbon and chlorinated solvents were purified using a solvent purification system (alumina/copper columns for hydrocarbon solvents), and the chlorinated solvents were subsequently distilled from CaH2 and degassed via several freeze−pump−thaw cycles. All commercially available chemicals were directly used without further purification. Bis(thien-2-yl)(2,4,6-tri-tert-butylphenyl)borane (BDT),11g bis(thien2-yl)(2,4,6-tris(trifluoromethyl)phenyl) borane (FBDT),11g dibromo(thien-2-yl)borane,11g 2,2′-bis(trimethylstannyl)-5,5′-bithiophene,20 and 3′-hexyl-2,2′:5′,2″-terthiophene21 were synthesized according to literature procedures. 499.9 MHz 1H NMR, 125.7 MHz 13C NMR, 160.4 MHz 11B NMR, and 470.4 MHz 19F NMR spectra were recorded at ambient temperature on a 500 MHz Varian INOVA spectrometer, and all 599.7 MHz 1H, 150.8 MHz 13C, and 192.4 MHz 11B NMR spectra were recorded on a Varian INOVA 600 spectrometer equipped with a boron-free 5 mm dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA). 11B NMR spectra were acquired with boron-free quartz NMR tubes. MALDI-MS measurements for small molecules were performed on an Apex-ultra 7T Hybrid FT-MS (Bruker Daltonics) in reflectron (+) or (−) mode. Anthracene (10 mg/mL) was used as the matrix, mixed with the samples (10 mg/mL in chloroform) in a 10:1 ratio, and then spotted on the wells of a target plate. MALDI-TOF measurements for polymers were performed on an Applied Biosystems 4700 Proteomics Analyzer or a Bruker Ultraflextreme as specified in reflectron (+) or (−) mode with delayed extraction. Anthracene or trans-2[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile was used as the matrix (10 mg/mL chloroform). Samples were prepared in chloroform (10 mg/mL), mixed with the matrix in a 1:10 ratio, and then spotted on the wells of a sample plate. Peptides were used for calibration (DesG


Article

Macromolecules

CDCl3): δ = 54.2 (w1/2 = 3200 Hz). HRMS: m/z = 842.4362 ([M]+, calcd for C52H68B2S4 842.4401). Synthesis of 5,5′-Bis((thien-2-yl)(2,4,6-tris(trifluoromethyl)phenyl)boryl)-2,2′-bithiophene (FOB2T). 1,3,5-Tris(trifluoromethyl)benzene (2.52 g, 8.93 mmol) was charged into a 100 mL Schlenk flask. Degassed ether (60 mL) was added by syringe, and the mixture was cooled to −78 °C. n-BuLi (6.2 mL, 1.6 M in hexane, 1.1 equiv) was added dropwise by syringe over a period of 30 min. The mixture was stirred at −78 °C for 0.5 h and then stirred in an ice bath for another 5 h. After solvent removal at 0 °C under high vacuum, 20 mL of dry toluene was added to the residue by syringe. A solution of 5,5′-bis(bromo(thien-2-yl)boryl)-2,2′-bithiophene (2.00 g, 3.90 mmol) in 10 mL of toluene was added into the suspension dropwise at −78 °C. Then, the mixture was stirred at rt overnight. The reaction was quenched by addition of 50 mL of water, followed by 20 mL of ether. The aqueous phase was discarded, and the organic phase was washed with 50 mL of water and then dried over anhydrous sodium sulfate. All volatile components were removed on a rotary evaporator. Purification by column chromatography (silica gel, hexanes:DCM = 5:1) gave the product as a light yellow solid (1.96 g, 55%). 1H NMR (599.7 MHz, CDCl3): δ = 8.18 (s, 4H, FMes), 8.00 (d, J = 4.6 Hz, 2H, Th), 7.60 (d, J = 3.6 Hz, 2H, Th), 7.51 (d, J = 3.6 Hz, 2H, Th), 7.49 (d, J = 3.6 Hz, 2H, Th), 7.31 (t, J = 4.0 Hz, 2H, Th). 13C NMR (150.8 MHz, CDCl3): δ = 148.63, 144.58 (br, B−C), 143.67, 142.96, 142.35 (br, B−C), 141.54 (br, B−C), 138.56, 134.69 (q, JC−F = 32 Hz), 132.10 (q, JC−F = 34 Hz), 129.44, 127.41, 126.28, 123.68 (q, JC−F = 276 Hz), 122.99 (q, JC−F = 273 Hz). 11B NMR (192.4 MHz, CDCl3): δ = 51.1 (w1/2 = 1900 Hz). 19F NMR (470.4 MHz, CDCl3): δ = −56.2 (s), −63.1 (s). HRMS: m/z = 913.9839 ([M]+, calcd for C34H14B2F18S4 913.9883). Synthesis of (3′-Hexyl-[2,2′:5′,2″-terthiophene]-5,5″-diyl)bis(trimethylstannane) (3TH-Sn2). 3′-Hexyl-2,2′:5′,2″-terthiophene (3.32 g, 10 mmol) was charged into a 100 mL Schlenk flask under nitrogen, followed by adding of 60 mL of degassed ether and cooled to −78 °C. n-BuLi (13.8 mL, 1.6 M in hexane, 22 mmol) was added dropwise over a period of 30 min. The mixture was then stirred at −78 °C for 0.5 h, warmed to rt, and kept stirring for another 1 h. Then the flask was cooled to −78 °C again, and a solution of chlorotrimethylstannane (5.0 g, 25 mmol) in 10 mL of ether was added dropwise. The mixture was stirred at rt overnight and then quenched by addition of 50 mL of water. The aqueous phase was discarded, and the organic phase was washed with water and brine and dried over anhydrous sodium sulfate. The solvent was removed in vacuo, and the residue was recrystallized from methanol to give the product as light-yellow crystals (5.1 g, 78%). 1H NMR (599.7 MHz, CDCl3): δ = 7.26 (d, J = 3.3 Hz, 1H, Th), 7.24 (d, J = 3.4 Hz, 1H, Th), 7.14 (d, J = 3.4 Hz, 1H, Th), 7.09 (d, J = 3.3 Hz, 1H, Th), 7.00 (s, 1H, Th), 2.76−2.72 (m, 2H, α-CH2), 1.71−1.62 (m, 2H, β-CH2), 1.44−1.36 (m, 2H, γ-CH2), 1.36−1.26 (m, 4H, δ/ε-CH2), 0.89 (t, J = 7.0 Hz, 3H, CH3), 0.40 (s/d, J(117/119Sn,H) = 55/58 Hz, 9H, SnMe3), 0.39 (s/d, J(117/119Sn,H) = 55/58 Hz, 9H, SnMe3). 13C NMR (150.8 MHz, CDCl3): δ = 143.06, 141.99, 140.02, 138.21, 137.51, 136.10, 135.67, 135.11, 129.91, 126.97, 126.65, 124.88, 77.43, 77.22, 77.01, 31.88, 30.67, 29.62, 29.42, 22.82, 14.32, −8.00 (s/d, J(117/119Sn, 13C) = 365 Hz). HRMS: m/z = 658.0007 ([M]+, calcd for C24H36S3Sn2 658.0016). Synthesis of PB1T. Under nitrogen atmosphere, a solution of BDT-2Sn (740 mg, 0.99 mmol) in 10 mL of toluene was added dropwise to a precooled (−78 °C) solution of BBr3 (250 mg, 1.00 mmol) in 10 mL of toluene. Then the mixture was stirred for 2 days. All solvents were removed in vacuo, and the solid residue was redissolved in toluene. The solution was slowly added to a suspension of Mes*Li (300 mg, 1.20 mmol) in 30 mL of toluene at −78 °C, which was prepared as described previously. The mixture was allowed to warm to rt and stirred overnight. The solvents were removed in vacuo, and the solid residue was dissolved in dichloromethane. The solution was washed with water and brine and dried over anhydrous sodium sulfate, and the solvent was removed on a rotary evaporator. The resulting solid was redissolved in 2 mL of THF and then precipitated in 50 mL of methanol twice to obtain polymer PB1T (200 mg, 30%)

quenched by addition of 50 mL of water. The aqueous phase was discarded, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed under vacuum, and the residue was recrystallized from methanol to give the product as light-yellow crystals (1.49 g, 68%). 1H NMR (499.9 MHz, CDCl3): δ = 7.75 (s, 2H, Th), 7.44 (s, 2H, Mes*), 7.26 (d, J = 3.0 Hz, 2H, Th), 1.41 (s, 9H, p-tBu), 1.16 (s, 18H, o-tBu), 0.42 (s/d, J(117/119Sn, H) = 57 Hz, 18H, SnMe3). 13C NMR (125.7 MHz, CDCl3): δ = 154.39 (br, B−C), 151.52, 149.99, 148.06, 142.05 (br, B−C), 136.13 (s/d, J(117/119Sn, 13 C) = 31 Hz), 135.8 (br, B−C) 122.46, 77.25, 77.02, 76.87, 38.60, 34.89, 34.67, 31.46, −8.05 (s/d, J(117/119Sn, 13C) = 354/371 Hz). 11B NMR (160.3 MHz, CDCl3): δ = 52.8 (w1/2 = 2000 Hz). HRMS: m/z = 747.1414 ([M−H]+, calcd for C32H50BS2Sn2 747.1495). Synthesis of FBDTSn2. Bis(thien-2-yl)(2,4,6-tris(trifluoromethyl)phenyl)borane (FBDT) (1.37 g, 3.0 mmol) was charged into a 100 mL Schlenk flask under nitrogen. Then 60 mL of degassed ether were added by syringe and cooled to −78 °C. n-BuLi (4.1 mL, 1.6 M in hexane, 2.2 equiv) was added dropwise over a period of 30 min. The mixture was stirred at −78 °C for 0.5 h and then warmed to rt and kept stirring for another 1 h. Then the flask was cooled to −78 °C again, and a solution of chlorotrimethylstannane (1.49 g, 7.5 mmol, 2.5 equiv) in 10 mL of ether was added dropwise. The mixture was stirred at rt overnight and then quenched by addition of 50 mL of water. The aqueous phase was discarded, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed under vacuum, and the residue was purified by column chromatography (alumina gel, hexanes:triethylamine = 100:1) to give the product as a yellow oil (1.7 g, 72%). 1H NMR (499.9 MHz, CDCl3): δ = 8.15 (s, 2H, FMes), 7.63 (d, J = 3.4 Hz, 2H, Th), 7.35 (d, J = 3.4 Hz, 2H, Th), 0.43 (s/d, J(117/119Sn,H) = 57 Hz, 18H, −SnMe3). 13C NMR (125.7 MHz, CDCl3): δ = 153.89, 147.67, 145.68, 142.79 (s/d, J(117/119Sn, 13C) = 40 Hz), 136.65, 134.30 (q, JC−F = 32 Hz), 131.32 (q, JC−F = 34 Hz), 125.92, 123.6 (q, JC−F = 275 Hz), 122.91 (q, JC−F = 273 Hz), −8.04 (s/d, J(117/119Sn, 13C) = 357/373 Hz). 11B NMR (160.3 MHz, CDCl3): δ = 49.8 (w1/2 = 1600 Hz). HRMS: m/z = 782.9250 ([M− H]−, calcd for C23H23BF9S2Sn2 782.9235). Synthesis of 5,5′-Bis(bromo(thien-2-yl)boryl)-2,2′-bithiophene. Under protection of nitrogen, to a solution of dibromo(thien-2-yl)borane (8.00 g, 31.6 mmol) in 5 mL of dichloromethane was added a solution of 2,2′-bis(trimethylstannyl)-5,5′-bithiophene (7.74 g, 15.8 mmol, 0.5 equiv) in 5 mL of dichloromethane at 0 °C. The mixture was stirred for 5 h at rt. The solvent was then removed under high vacuum to leave the product as a yellowish powder (7.28 g, 14.2 mmol, crude yield: 90%). The product was used in the next step without further purification. Synthesis of 5,5′-Bis((thien-2-yl)(2,4,6-tri-tert-butylphenyl)boryl)-2,2′-bithiophene (OB2T). 2-Bromo-1,3,5-tri-tert-butylbenzene (2.90 g, 8.91 mmol) was charged into a 100 mL Schlenk flask. Degassed ether (60 mL) was added by syringe, and the mixture was cooled to −78 °C. n-BuLi (6.5 mL, 1.6 M in hexane, 1.1 equiv) was then added dropwise by syringe over a period of 30 min. The mixture was stirred at −78 °C for 0.5 h and subsequently stirred in an ice bath for another 2 h. After solvent removal at 0 °C under vacuum, 20 mL of dry toluene was added to the residue by syringe. A solution of 5,5′-bis(bromo(thien-2-yl)boryl)-2,2′-bithiophene (2.00 g, 3.90 mmol) in 10 mL of toluene was added to the suspension dropwise at −78 °C. Then, the mixture was stirred at rt overnight. The reaction was quenched by addition of 50 mL of water, followed by 20 mL of ether. The aqueous phase was discarded, and the organic phase was washed with 50 mL of water and then dried over anhydrous sodium sulfate. All volatile components were removed on a rotary evaporator. Purification by column chromatography (silica gel, hexanes:DCM = 5:1) gave the product as a white solid (1.97 g, 60%). 1H NMR (499.9 MHz, CDCl3): δ = 7.84 (d, J = 4.4 Hz, 2H, Th), 7.74 (br, 2H, Th), 7.61 (br, 2H, Th), 7.45 (s, 4H, Mes*), 7.42 (d, J = 3.2 Hz, 2H, Th), 7.22 (t, J = 3.5 Hz, 2H, Th), 1.40 (s, 18H, tBu), 1.19 (s, 36H, tBu). 13 C NMR (125.7 MHz, CDCl3): δ = 151.76, 148.61, 148.26 (br), 147.88, 146.84, 142.41, 141.21, 135.57, 128.53, 126.17, 122.64, 77.25, 77.00, 76.75, 38.64, 34.92, 34.71, 31.42. 11B NMR (160.3 MHz, H


Macromolecules

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as a light yellow solid. 1H NMR (499.9 MHz, CDCl3): δ = 7.80 (Th), 7.39 (Mes*), 1.37 (p-tBu), 1.15 (o-tBu). 11B NMR (160.3 MHz, CDCl3): δ = 56.4 (w1/2 = 4800 Hz). GPC: Mn = 2650, Mw = 4060, Đ = 1.53. General Procedure for Stille Coupling Polymerization. In a glovebox, the organotin and iodo monomers were place in a 10 mL reaction tube, followed by addition of Pd2(dba)3 (6%), t-Bu3P (30%), and 3 mL of dry xylene. Then the tube was heated in a microwave reactor for 40 min (200 °C, 300 W). The mixture was allowed to cool to room temperature, diluted with 10 mL of THF, and then filtered through a fritted funnel with a small layer of silica gel to remove insoluble compounds and the catalyst. The polymer was precipitated twice from THF solution into 100 mL of methanol. Synthesis of PB2T. BDT-2I (180 mg, 0.27 mmol), BDT-2Sn (200 mg, 0.27 mmol), Pd2(dba)3 (15 mg, 0.016 mmol), and tBu3P (30 mg, 0.15 mmol) were used for this reaction. The product was obtained as a yellow solid. Yield: 80 mg, 35%. 1H NMR (499.9 MHz, CDCl3): δ = 7.62 (Th), 7.46 (Mes*, Th), 1.41 (p-tBu), 1.20 (o-tBu). 11B NMR (160.3 MHz, CDCl3): δ = 48.4 (w1/2 = 4500 Hz). GPC: Mn = 5840, Mw = 7830K, Đ = 1.34. Synthesis of PFB2T. FBDT-2I (270 mg, 0.38 mmol), FBDT-2Sn (300 mg, 0.38 mmol), Pd2(dba)3 (20 mg, 0.022 mmol), and t-Bu3P (40 mg, 0.20 mmol) were used for this reaction. The product was obtained as a yellow solid. Yield: 149 mg, 43%. 1H NMR (499.9 MHz, CDCl3): δ = 8.20 (FMes), 7.51 (Th). 11B NMR (160.3 MHz, CDCl3): δ = 45.3 (w1/2 = 3000 Hz). GPC: Mn = 5020, Mw = 6220, Đ = 1.24. Synthesis of PB3T. Bis(5-iodothien-2-yl)(2,4,6-tri-tert-butylphenyl)borane (296 mg, 0.44 mmol), 2,5-bis(trimethylstannyl)thiophene (180 mg, 0.44 mmol), Pd2(dba)3 (35 mg, 0.038 mmol), and t-Bu3P (60 mg, 0.30 mmol) were used for this reaction. Yield: 100 mg, 45%. 1H NMR (499.9 MHz, CDCl3): δ = 7.60 (Th), 7.47 (Mes*, Th), 7.30 (Th), 1.42 (p-tBu), 1.24 (o-tBu). 11B NMR (160.3 MHz, CDCl3): δ = 52.6 (w1/2 = 4800 Hz). GPC: Mn = 11 200, Mw = 17 900, Đ = 1.49. Synthesis of PB4T. Bis(5-iodothien-2-yl)(2,4,6-tri-tert-butylphenyl)borane (377 mg, 0.56 mmol), 5,5′-bis(trimethylstannyl)-2,2′bithiophene (275 mg, 0.56 mmol), Pd2(dba)3 (51 mg, 0.056 mmol), and t-Bu3P (91 mg, 0.45 mmol) were used for this reaction. Yield: 171 mg, 52%. 1H NMR (499.9 MHz, CDCl3): δ = 7.59 (Th), 7.46−7.44 (Mes*, Th), 7.15 (Th), 1.41 (p-tBu), 1.24 (o-tBu). 11B NMR (160.3 MHz, CDCl3): δ = 48.5 (w1/2 = 4800 Hz). GPC: Mn = 10 000, Mw = 15 100, Đ = 1.51. Synthesis of PB5T. Bis(5-iodothien-2-yl)(2,4,6-tri-tert-butylphenyl)borane (417 mg, 0.62 mmol), (3′-hexyl-[2,2′:5′,2″-terthiophene]-5,5″-diyl)bis(trimethylstannane) (408 mg, 0.62 mmol), Pd2(dba)3 (55 mg, 0.060 mmol), and t-Bu3P (98 mg, 0.48 mmol) were used for this reaction to obtain product as red solid. Yield: 251 mg, 54%. 1H NMR (499.9 MHz, CDCl3): δ = 7.60 (Th), 7.47 (Mes*, Th), 7.32 (Th), 7.09−7.03 (Th), 2.79 (α-CH2), 1.69 (β-CH2), 1.42 (p-tBu), 1.35 (γ,δ−CH2CH2) 1.24 (o-tBu), 0.91 (CH3). 11B NMR (160.3 MHz, CDCl3): δ = 47.6 (w1/2 = 4800 Hz). GPC: Mn = 12 000, Mw = 23 300, Đ = 1.95.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (F.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grants CHE-1362460 and CHE1112195.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02446. Characterization details including NMR and mass spectra, GPC traces, X-ray diffraction analysis parameters, cyclic voltammograms, absorption and emission spectra in solution and thin films, thermogravimetric analysis data, computed orbital plot, and results from TD-DFT calculations (PDF) Structure of OB2T (CIF) I


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