Article pubs.acs.org/Macromolecules
Torsionally Responsive Tropone-Fused Conjugated Polymers Kyung-su Kim,† Inhwan Cha,† Daeheum Cho,† Jongho Ahn,† Chinnadurai Satheeshkumar,† Ki Seok Yang,† Jin Yong Lee,† Yunmi Lee,‡ and Changsik Song*,† †
Department of Chemistry, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Republic of Korea Department of Chemistry, Kwangwoon University, Seoul 139-701, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Torsionally responsive molecular systems can change their electronic properties according to the dihedral angles and can be utilized as sensory materials. We have designed and synthesized novel tropone-fused conjugated polymers PBTr, PBTr-T, and PBTr-Tz that showed interesting dihedral-angledependent variations in UV−vis absorptions. Tropone-fused thiophene derivatives were prepared from one-step condensation of thiophene-3,4-dialdehyde and aliphatic ketones via a modular, facile, and high-yielding method. Subsequent halogenation and Stille cross-coupling polymerization with a bis(stannyl)benzodithiophene resulted in a tropone-fused conjugated polymer PBTr. We were also able to prepare thiophene- and thiazolebridged polymers, PBTr-T and PBTr-Tz, respectively, using similar synthetic methods. Electronic absorptions of the newly synthesized PBTrs were measured in solutions and in films states. Substantial red-shifts occurred in the case of thiophene-bridged PBTr-T, whereas almost no shift was observed for thiazolebridged PBTr-Tz. We attributed this to the substantial change in the torsional angle between the tropone-fused thiophene moiety and thiophene, which was further supported by density functional theory (DFT) calculations. Similar spectral changes of UV−vis absorptions were observed when a poor solvent (methanol) was introduced to a chloroform solution of PBTr-T. Reverse torsional angle variations were realized with initially planar PBTr-Tz by introducing steric hindrance through protonation on the thiazole rings. We believe that torsionally responsive tropone-fused conjugated polymers are promising as novel platforms for sensory applications.
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INTRODUCTION Conjugated polymers have “semiconducting” properties derived from their extended π-bonds in the polymer backbone, and their interesting electric and optoelectronic properties support their applications in organic light-emitting diodes, photovoltaics, conductors, and sensors.1 Control of the conjugation of π-bonds and their elaboration enable switchings of electrical and optoelectronic properties of conjugated polymers. For example, it is well-known that polythiophene derivatives have thermochromic or solvatochromic properties; they have a rather flexible backbone with a rotational energy barrier of 90%). We strategically used a long-chain ketone (nonadecan-10-one) to ensure the solubility of the resulting compounds and their polymers, but we found the same efficiency with short-chain ketones as well (e.g., pentan-3-one, data not shown). This onestep method is quite simple and high yielding when compared to other methods (such as selenium dioxide oxidation of cycloheptatriene17 and Hofmann degradation of tropinone18) and enables the synthesis of many derivatives of different side chains. Next, in order for cross-coupling polymerization, we attempted halogenation at the thiophene moiety of 2, but bromination either with N-bromosuccinimide or bromine was
Table 1. Physical and Optical Properties of Tropone-Fused Conjugated Polymers λmax (nm) Mna PBTr PBTr-T PBTr-Tz
(kDa)
33.2 91.3b 19.9
PDI
solutionc
film
40
1.2
23.2
1.16
479 480 546
527 564 555
Mwa
(kDa)
a
a
Determined by gel permeation chromatography (GPC) with polystyrene standard. bDetermined by dynamic light scattering (DLS). cChloroform solution. C
DOI: 10.1021/acs.macromol.5b01421 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) UV−vis absorption spectra of 2 (black), 4 (red), and 6 (blue) in CHCl3 solutions (2.0 × 10−5 M). (b) HOMO and LUMO orbitals and the NICS(1) values of tropone ring of 2 (top), 4 (middle), and 6 (bottom).
Table 2. NICS(1) Values of Geometrical Ring Centers (Tropone and Thiophene Moieties) in Tropone-Fused Compounds 2, 4, and 6, and Bond Lengths of the Carbonyl Groups in Optimized Structures tropone NICS(1) on tropone NICS(1) on thiophene bond length (CO), Å
thiophene
2
4
6
−10.479
−1.453 −10.807 1.236
−1.430 −9.102 1.237
−1.655 −7.957 1.237
−4.178 1.240
annelation was also reported in previous research when benzene or naphthalene was fused at the 4,5-position of tropone.21 When heteroarenes were appended to tropone-fused thiophene, NICS(1) values of tropone moiety were little changed (Table 2). We found that the NICS(1) values of thiopene moiety became slightly positive: −9.102 for thiophene-appended and −7.957 for thiazole-appended derivatives. We attributed this slight decrease in aromaticity of thiophene moiety to delocalization of π-electrons to neighbor heteroarenes, and the greater effect of thiazole appendage seems due to enhanced planarization. UV−vis spectroscopy of CHCl3 solutions of tropone-fused thiophene 2, bithiophene derivative 4, and bithiazole derivative 6 shed light on their electronic structures (Figure 1a). The π−π* transition of tropone-fused thiophene 2 was observed at λmax = 329 nm, and the molar absorptivity appears much smaller than the other derivatives. For bithiophene derivative 4, the similar transition appeared to be blue-shifted to λmax = 308 nm, while the shoulder transition was observed at around 396 nm. The lower energy transition is no surprise due to the increased conjugation. The bithiazole derivative 6 showed a similar trend in electronic absorption as 4. However, there are slight but meaningful differences; the main π−π* transition was red-shifted to λmax = 318 nm, and the shoulder transition at 401 nm became stronger when compared to those of 4. We attributed this difference to enhanced conjugation by thiazole heteroarenes, which was evidenced by DFT calculations. Figure 1b shows the frontier orbitals (highest-occupied and lowestunoccupied molecular orbital, or HOMO and LUMO) of 2, 4, and 6. The shapes of HOMOs and LUMOs are largely similar, but increased conjugation was observed for thiazole derivative 6 in both orbitals when compared to thiophene derivative 4, which appears responsible for the increased shoulder transitions in the UV−vis spectrum. Investigations of the optical properties of the tropone-fused conjugated polymers in solutions and films revealed torsional angle-dependent alteration of π-electron conjugations in the polymers, which is supported by DFT calculations (Figure 2). We measured the electronic absorptions of tropone-fused
scattering due to its solubility issue, and the results were 91.3 kDa. The relatively narrow molecular-weight distributions were attributed to slight fractionation through multiple purifications. The product yields were as high as 70% even after fractional precipitation. In order to gain insights into structural and electronic properties of tropone-fused thiophenes, density functional theory (DFT) calculations were carried out at the B3LYP level with a 6-31G* basis set for tropone-fused thiopene 2, bithiophene derivative 4, and bithiazole derivative 6 (Figure 1b and Figure S3). The nucleus-independent chemical shift (NICS) values for 2, 4, and 6 shown in Table 2 reveal that the aromaticity of tropone ring was quite decreased upon annealing, while the aromaticity of thiophene ring was not significantly changed. First, we investigated the polarization of the CO bonds since it indicates the resonance contribution of the tropylium-like structure, and we found that the CO bond length was decreased from 1.240 Å (tropone) to 1.236 Å (tropone-fused thiophene). Reduced polarization may reveal that the tropylium-like character is reduced in tropone-fused thiophene, which results in its reduced aromatic character. The reduced aromaticity of tropone moiety upon annelation was also reflected in the augmented bond alternation; the lengths of single bonds were increased and those of double bonds were decreased (Figure S3). Nucleus-independent chemical shift (NICS) calculations clearly show the decreased aromaticity of tropone moiety upon annelation. NICS values have been widely used to probe aromaticity since Schleyer and co-workers proposed19 and the large negative values represent aromaticity while the large positive values indicate antiaromaticity. The GIAO method at the B3LYP/6-31+G* level revealed that NICS(1) value of tropone moiety in tropone-fused thiophene became positive to −1.453 from −4.178 of parent tropone, which means reduced aromaticity. NICS values were calculated at 1 Å above the center of rings in order to minimize the local magnetic interference.20 However, NICS(1) values of thiophene moiety was almost maintained (−10.807 and −10.479 of parent thiophene). The reduced aromaticity of tropones upon D
DOI: 10.1021/acs.macromol.5b01421 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. (a−c) UV−vis absorption spectra of tropone-fused conjugated polymers in CHCl3 solutions (1.0 × 10−5 M) and film states of PBTr (a), PBTr-T (b), and PBTr-Tz (c) at room temperature. For the solution of PBTr-T (b), the UV−vis spectrum was also measured at the elevated temperature (55 °C). (d−f) Calculated structures and dihedral angles of part of tropone-fused polymers PBTr (d), PBTr-T (e), and PBTr-Tz (f).
conjugated polymers in solutions and films via UV−vis spectroscopy. The least conjugated polymer PBTr (i.e., the shortest conjugation length) colored orange (λmax = 479 nm) in a chloroform solution (1.0 × 10−5 M), while the thiazolebridged polymer PBTr-Tz showed a wine-red color (λmax = 546 nm) in a similar solution. We attributed this shift of absorption maximum (∼67 nm) to the increase of conjugation length through the thiazole bridge. However, we found that the color of a chloroform solution of thiophene-bridged PBTr-T (1.0 × 10−5 M) was slightly dark orange with an absorption maximum of 480 nm, which is very close to that of PBTr. Considering the conjugation length through the thiophene bridge, the absorption maximum of PBTr-T was substantially blue-shifted. We observed the shoulder at ∼630 nm in the case of PBTr-T, which may be the result of the enhanced conjugation. However, in solutions at least, most of the π-electron conjugation of PBTr-T appears “broken”, while in contrast that of PBTr-Tz seems well connected. In order to investigate the shoulder peak at ∼630 nm in the PBTr-T solution, we measured concentration-dependent and temperature-dependent absorption spectra (Supporting Information, Figures S1 and S2).
Interestingly, the shoulder at 630 nm in the UV−vis spectrum of the PBTr-T solution (Figure 2b) was also present in the very dilute concentration (∼0.2 × 10−5 M). When we measured temperature-dependent UV−vis spectra, we observed that the shoulder at 630 nm for the PBTr-T solution was decreased at above 45 °C, which suggests that the shoulder peak is due to the polymer’s aggregation. It is interesting that the peak from aggregation in thiophene-bridged PBTr-T still exist in dilute conditions. It should be noted here that the shapes of UV−vis spectra for thiazole-bridged PBTr-Tz were not changed even in dilute concentrations and elevated temperatures (up to 55 °C in CHCl3). These results strongly suggest that thiazole-bridged polymer PBTr-Tz is “molecularly dissolved” and not aggregated in the solution state. Thus, the red-shifted feature of PBTr-Tz (Figure 2c) is not from polymer aggregation but due to its conformation.. Interestingly, substantial red-shift (∼84 nm) of electronic absorptions of thiophene-bridged PBTr-T was observed when cast in films. The film of thiazole-bridged PBTr-Tz showed very little shift (
