Poly(quinoxaline-2,3-diyl) as a Multifunctional Chiral Scaffold for

Apr 3, 2017 - Random poly(quinoxaline-2,3-diyl) copolymers, containing achiral 5,8-diarylquinoxaline units and chiral units bearing (S)-2-butoxymethyl...
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Poly(quinoxaline-2,3-diyl) as a Multifunctional Chiral Scaffold for Circularly Polarized Luminescent Materials: Color Tuning, Energy Transfer, and Switching of the CPL Handedness Tsuyoshi Nishikawa, Yuuya Nagata,* and Michinori Suginome* Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan S Supporting Information *

ABSTRACT: Random poly(quinoxaline-2,3-diyl) copolymers, containing achiral 5,8-diarylquinoxaline units and chiral units bearing (S)-2-butoxymethyl groups, exhibited circularly polarized luminescence (CPL). The emission color was fully tunable by changing the aryl substituents on the 5,8-diarylquinoxaline units. An energy transfer from the quinoxaline main chain to the 5,8diarylquinoxaline units was observed. The handedness of the CPL was dependent on the helical chirality of the polymer main chain, which could be switched by changing the solvent from CHCl3 to 1,1,2-trichloroethane or n-octane to cyclooctane, depending on the chiral side chain.

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color-tunable selective reflection of circularly polarized light on the basis of cholesteric superstructure whose chirality as well as pitch depends on the solvent vapor used in the annealing process.14 Our attention was then focused on the luminescent material. We found that the main chains of PQXs exhibits blue CPL, whose handedness can be switched by a solventdependent helix inversion of the polymer main chain.15 However, the PQX main chain exhibited only blue emission with a low photoluminescent quantum yield (ΦPL < 0.7%). To achieve a more efficient CPL using the PQX scaffolds, we envisioned that the introduction of simple π-groups on the quinoxaline rings could enhance the photoluminescence quantum yield by forming efficient luminophores. Furthermore, the formation of new luminophores may also lead to a switch of the handedness of the CPL through induction of local chirality at the luminophores. So far, only a few examples of lowmolecular-weight CPL materials based on the chirality-transfer from chiral substituents to a luminophore have been reported,16 and the switch of CPL handedness still remains to be accomplished. In this paper, we describe the incorporation of achiral luminescent units to PQXs that bear chiral side chains. The emission color of the PQXs was tuned from blue to red by changing the structure of the π-group. These PQXs exhibited CPL due to the chirality transfer from the single-handed backbone to the luminophores. Furthermore, the CPL handedness could be switched by the solvent effect similar to the solvent-induced switch of chirality of the PQX main chain.

hiral materials exhibiting circularly polarized luminescence (CPL) have lately received increased attention,1 on account of their potential applications in chemical sensors,2 biological probes,3 and three-dimensional displays.4 Recent research interests have focused on the switch of the CPL handedness,5 for example, by using achiral external stimuli such as solvent,6 nonpolarized light,7 or an achiral matrix.8 However, in these examples, full emission color tuning9 could not be accomplished, as the mechanism of the switch of the CPL handedness therein is closely related to the structure of the luminophores, which govern the emission color. Therefore, new chiral scaffolds, including helical polymers and foldamers,10 are in high demand, as they should allow an independent design of the luminophore and the chirality-switching moiety. Recently, we have reported that poly(quinoxaline-2,3-diyl)s (PQXs) can serve as a new helical macromolecular scaffold that exhibits a solvent-dependent switch of its helical chirality.11 We demonstrated that PQXphos, that is, single-handed PQXs containing diarylphosphino groups,12 can serve as effective chiral ligands in various asymmetric reactions.13 In the system, the helical chirality of the polymer backbone induced by the chiral units should be efficiently transferred to the local conformation of the achiral coordination units in close spatial proximity to the reaction sites, where highly enantioselective reactions take place. The PQX-based macromolecular ligand system features its modular molecular design, in which units having different functions can be incorporated into the scaffold of PQX, which serves as a platform for integrated molecular function. We have explored PQX-based materials that generate handedness-switchable circularly polarized light. We reported dry thin films of PQXs exhibiting handedness-switchable, full© XXXX American Chemical Society

Received: February 20, 2017 Accepted: March 30, 2017

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DOI: 10.1021/acsmacrolett.7b00131 ACS Macro Lett. 2017, 6, 431−435

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ACS Macro Letters We prepared random co-100mers (1a−g, Mw/Mn = 1.11− 1.26) by living polymerization, which bear common chiral units containing (S)-2-butoxymethyl side chains in combination with various 5,8-diarylqunoxalaine units (Figure 1a). These polymers

being red-shifted with increasing electron-donating ability of the aryl groups at the 5,8-positions on the quinoxaline ring. 1a and 1b with 4-trifluoromethylphenyl and p-tolyl groups exhibited blue emission, while 1c and 1d, containing electron-rich 4-methoxyphenyl and 3,4,5-trimethoxyphenyl groups, showed green emission. 1e, 1f, and 1g, bearing more electron-rich 3- or 5-methylthienyl, and 4-dimethylaminophenyl groups, exhibited orange to red emission. This trend may be explained by the mechanism of the fluorescent emission operative, which involves a donor−acceptor interaction between the aryl substituents and the electron-deficient quinoxaline ring. 1g showed solvatochromic fluorescence in n-hexane, CHCl3, and tetrahydrofuran (THF), whose peak was bathochromically shifted with increasing solvent polarity (573, 600, and 617 nm, respectively), suggesting that the photoluminescence emission occurred from an intramolecular chargetransfer state (see SI). It should be noted that an increased content of luminophore units caused a red shift of the emission wavelength. For example, a copolymer with a ratio of 60/40 containing chiral units bearing (S)-2-butoxymethyl groups and 5-methylthienyl-substituted luminophore units, 1f(60/40), showed an emission peak at 627 nm due to an excitonic interaction of neighboring luminophore units (see SI). We also prepared polymer PQ, which does not contain 5,8-diarylquinoxaline units, and low-molecular-weight model compound 2, which corresponds to a substructure of 1c (Figure 1a,e). While PQ exhibited a completely different PL spectra from 1c, 2 showed an identical PL spectrum to 1c, clearly indicating that the 5,8-bis(4-methoxyphenyl)quinoxaline unit is the luminophore in the helical backbone of 1c. In contrast, 1a and PQ showed similar PL spectra, suggesting the major luminophore in 1a is not 5,8-bis(4-trifluoromethylphenyl)quinoxaline unit but the chiral unit in the main chain. To gain a better understanding of the photophysical properties of these polymers, the ΦPL and the photoluminescent life times (τ1) of 1a−g, 2, and PQ were measured. Although PQ showed blue emission with low ΦPL (0.3%), the ΦPL increased up to 49.7% by random incorporation of five 5,8diarylquinoxaline units per 100 units (1f, Table 1). Photoluminescence was also observed at higher excitation wavelength (400−500 nm), which preferentially excite the luminophore. Although ΦPL is higher (55.3% for 1e at 420 nm) than excitation at 350 nm (41.7% for 1e, Table 1), the apparent brightness of the luminescence excited at 420 nm was much weaker than that at 350 nm because of the small absorption coefficient at 420 nm.

Figure 1. (a) Structures of copolymers 1a−g, model compound 2, and homopolymer PQ; (b, c) UV−vis absorption spectra; (d) PL spectra of 1a−g; (e) PL spectra of 1c, 2, and PQ in CHCl3 (1.83−3.17 × 10−2 g/L for 1a−g and PQ, and 4.95 × 10−3 g/L for 2) .

showed almost identical UV−vis absorption spectra (Figure 1b), although 1f and 1g exhibited broad, weak peaks around 400−500 nm (Figure 1c), which are assignable to 5methylthienyl or 4-dimethylaminophenyl substituted quinoxaline units. The photoluminescence (PL) spectra of 1a−g were measured with excitation wavelength of 350−351 nm, which were the absorption maxima of these polymers. The mean emission wavelength covered the entire visible light region, Table 1. Optical Properties of 1a−g, PQ, and 2 in CHCl3 PQ 1a 1b 1c 1d 1e 1f 1g 2

λabsa (nm)

λPLb (nm)

ΦPLc,d (%)

τ1e,d (ns)

kff (ns−1)

351, 290 351, 290 351, 291 350, 291 351, 291 350, 290 (459), 351, 291 (455), 350, 291 361, 285

(650), 416 (646), 426 473 512 537 569 577 600 512

0.3 0.5 3.1 33.0 12.7 40.7h 49.7 24.6 84.0

1.7 × 10−1 4.5 × 10−1 8.0 × 10−1 7.6 8.4 10.8 10.4 7.8 11.8

1.8 1.1 3.9 4.3 1.5 3.9 4.6 3.2 7.4

× × × × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

knrg (ns−1) 5.9 2.2 1.2 8.8 1.0 5.7 4.7 9.7 1.4

× × × × × ×

10−2 10−1 10−2 10−2 10−2 10−2

Maximum absorption wavelength. bMaximum PL wavelength. cAbsolute photoluminescent quantum yield. dExcitation wavelength, λex = 351 nm for PQ, 1a, 1b, 1d, and 1f. λex = 350 nm for 1c, 1e and 1g. λex = 361 nm for 2. eFluorescence decay time constant (λex = 378 nm). fFluorescence rate constant. gNonradiative decay rate constant. hΦPL was 55.3% excited at 420 nm. a

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ACS Macro Letters It is assumed that most of the excitation light (∼350 nm) was initially absorbed by the chiral units, which potentially cause a luminescence quenching. However, because the PQX showed the increased ΦPL, the photoexcited energy should be transferred from the chiral units to the 5,8-diarylquinoxaline units to exhibit photoluminescence. To assume the efficiency of the energy transfer, a PL spectrum of a 95:5 mixture of PQ and 2 was measured. It showed ΦPL of 5.1%, which is much lower than that of 1c (33.0%), suggesting the energy transfer efficiency of 34.0% from the main chain to the luminophore (see SI). The energy transfer efficiency can also be assumed by comparing the absorption spectrum and the corresponding excitation spectrum.17 1e and 1g exhibited the energy transfer efficiency of 48.1% and 27.6% at the excitation wavelength of 350 nm (see SI). Although the fluorescence rate constants (kf) of all polymers and 2 were comparable, the nonradiative rate constants (knr) were dependent on the molecular structure. While PQ, 1a, and 1b exhibited large knr values (5.9−1.2 ns−1), 1c−g and 2 showed smaller values (1.0 × 10−1−1.4 × 10−2 ns−1). This apparent decrease of knr can be explained by the increase in the ΦPL caused by the energy transfer from the lowemissive chiral units to the luminescent 5,8-diarylquinoxaline units in the polymer main chain of 1c−g. We also prepared PQXs with 5-(4-methoxyphenyl)-8methylquinoxaline (3) or 5-(2-(diphenylphosphanyl)phenyl)8-methylquinoxaline units (4),13 which contain only one aryl group on the quinoxaline ring. However, these monoarylsubstituted PQXs exhibited low fluorescence quantum yields (3: ΦPL = 8.6%, λPL = 485 nm; 4: ΦPL = 0.7%, λPL = 634 nm; for further details, see SI), suggesting that the diaryl-substituted quinoxaline unit plays an important role for the fluorescent emission.

Figure 2. CD spectra of 1a−g in (a) CHCl3 and (b) 1,1,2-TCE (1.83−3.17 × 10−2 g/L); a mixed 1,1,2-TCE/THF (8/2, v/v) solvent was used for 1a, 1b, and 1g on account of their low solubility in neat 1,1,2-TCE.

We then measured the CPL spectra of 1a−g in dilute CHCl3 and 1,1,2-TCE solutions and observed that 1a afforded weak CPL signals (Figure 3a), which mainly originated from chiral units in the main chain as previously reported.15 It should be noted that 1b−g showed clear CPL peaks, suggesting that 5,8diaryl units, in which the axial chirality at the aryl-quinoxaline axes is induced by the helical polymer backbone, are responsible for the emission process (Figure 3b−g; see also SI for theoretical calculations for the luminophore moieties). All polymers exhibited positive CPL signals in CHCl3, due to their M-helical main chains. The gCPL values at maximum PL wavelength in CHCl3 (−3.2 to −11.5 × 10−4; Table 2) are comparable to previously reported nonswitchable CPL materials without rare earth elements (typical |gCPL| = 10−5− 10−3),16 except for exceptionally efficient helical molecules (| gCPL| = 10−2−10−1).18 In 1,1,2-TCE or mixed 1,1,2-TCE/THF (8/2, v/v) solvent, these polymers showed positive peaks (gCPL = +1.2 to +10.5 × 10−4), except for 1f, which showed no CPL emission. At present, the reason for this exception remains to be determined. The achiral model compound 2 exhibited no CPL signal. This result suggests that the chiral side chains of 1b−g induce helical chirality on the backbones, which provides a chiral environment for the diaryl-substituted quinoxalines to exhibit the CPL signals. This assumption is also supported by a result that 1c to which low se (39%) was induced by using a mixture of 1,1,2-TCE and CHCl3 (65/35, v/v) showed |gCPL| (gCPL = −2.7 × 10−4, see SI) as low as 29% of the |gCPL| in pure CHCl3 (gCPL = −9.2 × 10−4). Subsequently, 1c−f were dissolved in CHCl3 or 1,1,2-TCE, while 1a, 1b, and 1g were dissolved in a mixed 1,1,2-TCE/THF solvent (8/2, v/v) and examined under 365 nm UV light (Figure 3i). Although the emission of 1a was too weak to be visualized photographically, 1b−g clearly exhibited photoluminescence in the visible light region. Again, the emission color and the ΦPL values for each copolymer were comparable for P- and M-helical conformations, which were readily recognizable by the naked eye. We recently demonstrated a solvent-dependent switch of the helical sense of PQXs bearing (S)-3-octyloxymethyl side chains in alkane solvents.11d As the functions of the chiral and luminescent units should be orthogonal, the replacement of the

Subsequently, circular dichroism (CD) spectra of 1a−g were measured in CHCl3 or 1,1,2-trichloroethane (1,1,2-TCE). As discussed in our previous paper, PQ adopts pure M- or Phelical conformations in CHCl3 or 1,1,2-TCE, respectively.11a The gabs values of 1a−g around the peak top (−2.55 to −2.31 × 10−3 at 366.0 nm) indicate that these polymers adopt pure Mhelical structures in CHCl3 (Figure 2a). The CD spectra of 1a− g in 1,1,2-TCE (1a, 1b, and 1g were dissolved in the mixed solvent 1,1,2-TCE/THF = 8/2 due to their low solubility) were almost mirror images of the spectra in CHCl3, suggesting that these polymers invert their screw sense to P-helical conformations (Figure 2b). The gabs values of 1a−g at 366.0 nm in 1,1,2-TCE (2.46 to 2.92 × 10−3) suggest that these polymers adopt almost pure P-helical structures. It should be noted that the 5% incorporation of 5,8-diarylquinoxaline units into the backbone did not affect the screw-sense induction and the solvent-dependent helix inversion. On the other hand, the solvent-dependent helix inversion was significantly retarded at room temperature for 1f(60/40), of which the incorporation rate of 5,8-diarylquinoxaline units was 40% (see SI). The halflife values for the helix inversion of 1c at 20 °C were also determined: 7.8 min for the inversion from the P-helix in 1,1,2TCE to the M-helix in CHCl3, and 73.0 min for the inversion from the M-helix in CHCl3 to the P-helix in 1,1,2-TCE (see SI). 433

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ACS Macro Letters

Table 2. PL and CPL Properties of 1a−g, PQ, and 2 in CHCl3 or 1,1,2-TCE 1,1,2-TCEa

CHCl3 1a 1b 1c 1d 1e 1f 1g 2

λPLb (nm)

gCPLc (×10−4)

λPLb (nm)

gCPLc (×10−4)

ΦPLd (%)

427 473 513 535 561 579 599 515

−3.2 −11.0 −9.2 −3.3 −5.1 −6.4 −6.8 (−1.6)e

424 473 512 536 588 577 615 513

6.7 7.6 4.2 6.2 1.2 −0.9 5.1 (−1.6)e

0.4 4.0 38.8 15.4 42.9 53.6 25.7 86.0

a

A mixed 1,1,2-TCE/THF (8/2, v/v) solvent was used for 1a, 1b, and 1g on account of their low solubility in neat 1,1,2-TCE. bMaximum PL wavelength. cgCPL was determined at maximum PL wavelength. d Absolute photoluminescent quantum yield. eThese signals are artifact peaks originating from the CPL instrument.

Figure 4. (a) Chemical structure of polymer 5, and (b) CPL and PL spectra of 5 in n-octane and cyclooctane (2.70 × 10−2 g/L).

Figure 3. (a−h) CPL and PL spectra of 1a−g and 2 in CHCl3 or 1,1,2-TCE (1.83−3.17 × 10−2 g/L for 1a−g and PQ, and 4.95 × 10−3 g/L for 2); a mixed 1,1,2-TCE/THF (8/2, v/v) solvent was used for 1a, 1b, and 1g on account of their low solubility in neat 1,1,2-TCE. (i) Photograph of 1a−g dissolved in CHCl3 or 1,1,2-TCE under UV light (365 nm). A mixed 1,1,2-TCE/THF solvent (8/2, v/v) was used for 1a, 1b, and 1g on account of their low solubility in neat 1,1,2-TCE.

polymer backbone. Furthermore, the use of a PQX-based scaffold enabled an independent design of a luminophore and chiral units for the full tuning of the emission color and solventdependent switch of the CPL handedness. In summary, we have demonstrated that PQXs bearing 5,8diarylquinoxaline units as luminophores exhibited multicolored emission, the color of which depends on the substituents on the aryl groups on the luminophore units. An energy transfer from the backbone of the PQXs to the luminophore units accounts for the avoidance of the luminescence quenching, even though the PQXs contain only a small number of luminophore units compared to the nonluminescent chiral units. The CPL spectra of PQXs bearing (S)-2-butoxymethyl side chains in 1,1,2-TCE were almost perfect mirror images of the spectra measured in CHCl3 except for a PQX bearing 5-methylthienyl-substituted luminophore units. Taking advantage of modular molecular design, we replaced the (S)-2-butoxymethyl side chains with (S)-3-octyloxymethyl side chains to obtain a PQX exhibiting switch of CPL handedness between n-octane and cyclooctane. These results clearly demonstrates that in such chiral PQXs, the

2-butoxy groups with 3-octyloxy groups is expected to change the solvent dependency of the CPL handedness dramatically, without changing the wavelength of the CPL. To proof this concept, polymer 5, bearing (S)-3-octyloxymethyl groups and 5,7-bis(4-methyloxyphenyl)quinoxaline units, was prepared and its CPL spectra were measured (Figure 4a). We also carried out the UV−vis absorption and CD measurements to confirm that 5 adopted pure M- or P-helical conformations in n-octane or cyclooctane (see SI). In n-octane, 5 exhibited a negative CPL peak around 500 nm, owing to the M-helical conformation of the backbone (Figure 4b). On the other hand, 5 exhibited a positive CPL peak in cyclooctane. These results clearly indicate that the sign of the CPL from the achiral luminophore is determined by the helix sense of the 434

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ACS Macro Letters

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luminophore and the chirality-determining units can be designed independently. To the best of our knowledge, this represents the first example of multicolored and chiralityswitchable CPL materials. Further applications of such chiralityswitchable luminescent PQXs as novel chiral materials are currently under investigation in our laboratory, together with mechanistic studies of the solvent-dependent chirality transfer from the helical main chain to the incorporated achiral units.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00131. Experimental procedures and spectral data for the new compounds (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuuya Nagata: 0000-0001-5926-5845 Michinori Suginome: 0000-0003-3023-2219 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Prof. Yoshiki Chujo (Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University) for CPL measurements and to Prof. Kenji Matsuda and Dr. Takashi Hirose (Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University) for fluorescence lifetime measurements and fruitful discussion. Financial support for this research was provided by the Japan Science and Technology Corporation (CREST, “Establishment of Molecular Technology towards the Creation of New Function” Area) and MEXT Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions” (No. JA 15H00994). Computation time was provided by the SuperComputer System, Institute for Chemical Research, Kyoto University.

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REFERENCES

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DOI: 10.1021/acsmacrolett.7b00131 ACS Macro Lett. 2017, 6, 431−435