Near-Ultraviolet Circular Dichroism of Achiral Phenolic Termini

Aug 11, 2016 - Density functional theory (DFT) and time-dependent DFT calculations suggested the existence of multiple through-space intramolecular CH...
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Near-Ultraviolet Circular Dichroism of Achiral Phenolic Termini Induced by Nonchromophoric Poly(L,L‑lactide) and Poly(D,D‑lactide) Kai Kan,†,‡ Michiya Fujiki,*,† Mitsuru Akashi,*,§ and Hiroharu Ajiro*,†,‡,⊥ †

Graduate School of Materials Science and ‡Institute for Research Initiatives, Division for Research Strategy, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan § Graduate School of Frontier Biosciences, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan ⊥ JST PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Herein, we present the first induced chirality of vanillin and its phenolic analogs attached to the chain ends of poly(L,L-lactide) and poly(D,D-lactide). Vanillin analogs were used as chromophoric and luminophoric, but achiral, ringopening initiators of corresponding chiral cyclic lactides. Induced chirality was evident from clear circular dichroism bands at 270−320 nm due to π−π* and n−π* transitions at the vanillin moiety. However, no circularly polarized luminescence band was detected. Density functional theory (DFT) and time-dependent DFT calculations suggested the existence of multiple through-space intramolecular CH/O interactions between the ortho-methoxy moiety of vanillin and nearestneighbor lactic acid units. The terminus sensitively indicated whether the main-chain chirality was L or D.

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of which are as follows. (1) What is/are the most critical interaction(s) between chiral catalysts and prochiral monomers in the initiation process of stereoregular polymerization? (2) Can the end termini or terminus interact with the main chain in stabilizing chiral/helical conformations during the propagation phase of chiral polymerization? Nonchromophoric, but chiral and helical, poly(L,L-lactide) (PLLA) and poly(D,D-lactide) (PDLA) ranging from the nearultraviolet (UV) and visible region are possible candidates for elucidating interactions between the main chain and chain end(s). Thus, far, several block copolymers involving L,L- and 12 D,D-lactide sequences have revealed induced chirality/helicity. Side-chain PLLA and PDLA moieties have also been observed to induce helicity in poly(phenylacetylene)s.10 To address the above-mentioned questions, we designed PLLA and PDLA bearing a chromophoric achiral terminus (Figure 1). Vanillin and its phenolic analogs were selected as achiral, chromophoric, and luminophoric ring-opening initiators of cyclic (L,L)- and (D,D)-lactides, followed by termination with benzyl alcohol (Figure 1).13,14 Note that vanillin shows intramolecular charge transfer (ICT) characteristics in the near-UV region, as proven by infrared and Raman spectroscopies,15 UV spectroscopy, and fluorimetry.16 This property has enabled us to use vanillin as a powerful molecular circular dichroism (CD)/UV probe when it is noncovalently surrounded by helical amyloses.17 These new

nduced chirality and helicity by intramolecular and intermolecular chiral biases have been widely investigated in the realms of artificial and biological polymers and supramolecules for decades. These phenomena arise from the restricted rotational freedom of stereogenic single, double, and triple bonds. The main motivation for this research arises from several elaborate functions of biological polymers consisting of multiple stereogenic centers and bonds.1,2 These chiral/helical motifs allow an efficient transfer of structural information, enabling them to perform various functions. Accordingly, many artificial chiral/helical polymers have been designed to mimic these biological polymers’ chirality and helicity.3 Induced mainchain chirality and helicity play crucial roles in controlling highordered structures, functions of nanostructures,4 and stereoregularity.5 However, the role of two termini in most cases is not yet completely understood due to a lack of proper polymers in solution. This is because of the minute proportion of terminal units relative to main-chain units, on the order of 1/ 103−1/106. A few examples are that the N-terminal domain of a prion plays a key role in inducing self-coordination with a Cu(II) ion6a,b along with conformational transitions by polypeptides.6c,d Main-chain chirality and helicity associated with chain ends or side groups can be categorized into the following groups: chromophoric chain ends with chiral alkyl substituents,7 nonchromophoric chiral units at growing chain ends,8 chromophoric chiral side groups,9 nonchromophoric chiral side groups,10 and others.11 However, unresolved issues in chiral/helical vinyl and ring-opening polymerizations by organocatalysts leave numerous unanswered questions, some © XXXX American Chemical Society

Received: July 4, 2016 Accepted: August 10, 2016

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Figure 1. Schematic of induced chirality from helical poly(L,L-lactide) and poly(D,D-lactide) to an achiral terminus including vanillin and its phenolic analogs (see Table 1 and Table S1).

PLLA and PDLA thus enabled us to directly detect the terminus by CD/UV spectroscopy. Herein, we report the first induced CD (ICD) signal at 270−320 nm, which was attributed to the π−π* and n−π* ICT bands of vanillin analogs16 covalently attached at the chain ends of PLLA and PDLA. The terminus was found to be an excellent CD/UV probe in indicating the chirality of the main chain. All polymers used in this study are listed in Table 1. These polymers in dilute chloroform solution clearly showed single positive- and negative-sign CD bands at CD extrema (λext = 270−306 nm). Single-sign CD band profiles are coincident with the corresponding ICT UV absorptions because of the terminus of vanillin and its analogs (Figures 2a and 3). To quantitatively assess the degree of induced chirality, the magnitude of the dimensionless dissymmetry ratio, defined as gCD = Δε/ε, was on the order of 10−4 (Tables 1 and S1, Figure 2b). These results indicate that the ICD signal originated from a single chiral chromophore at ICT π−π* and n−π* transitions rather than from an exciton couplet oscillator.18 We compared the values of gCD and λext for five pairs of phenolic initiators that commonly have electron-donating ortho-alkoxy group(s) on the benzene ring. Table 1 summarizes all chiroptical results of PDLA and PLLA obtained with vanillin (entries 1 and 2), guiacol (entries 3 and 4), ethyl vanilate (entries 5 and 6), ethyl vanillin (entries 7 and 8), and syringaldehyde (entries 9 and 10). Four initiators with electronaccepting para-aldehyde or para-ester groups revealed CD signals ranging from 276 to 306 nm, while guiacol showed a single CD signal at 270 nm. This very small λext is possibly because of the absence of ICT owing to the lack of an electronwithdrawing para-aldehyde or ester group. Figure 2a shows the CD and UV spectra of PLLA-vanillin (1) and PDLA-vanillin (2). Polymers 1 and 2 had two negativesign and two positive-sign CD bands, respectively, both at 255

Figure 2. (a) CD and UV−vis spectra of PLLA-vanillin (1) and PDLA-vanillin (2) in dilute chloroform at 20 °C. Concentration of the solution is 0.05 mol L−1. (b) The gCD value at 306 nm as a function of Mn−1 in chloroform at 20 °C.

Figure 3. CD and UV−vis absorption spectra of the PLLA-guiacol (3) and PDLA-guiacol (4) in chloroform solution at 20 °C. Concentration of the solution is 0.025 mol L−1.

Table 1. Values of gCD and λext of PLLA and PDLA Derivatives with Terminus of Vanillin and Its Analogs substituents of phenolic initiators (see Figure 1) entry

polymers

R1

R2

R3

gCD (× 10−4)

λext (nm)

1 2 3 4 5 6 7 8 9 10

PLLA-vanillin (1) PDLA-vanillin (2) PLLA-guiacol (3) PDLA-guiacol (4) PLLA-ethyl vanilate (5) PDLA- ethyl vanilate (6) PLLA-ethyl vanillin (7) PDLA-ethyl vanillin (8) PLLA-syringaldehyde(9) PDLA-syringaldehyde (10)

OCH3 OCH3 OCH3 OCH3 OCH3 OCH3 OCH2CH3 OCH2CH3 OCH3 OCH3

H H H H H H H H OCH3 OCH3

CHO CHO H H COOCH2CH3 COOCH2CH3 CHO CHO CHO CHO

−1.7 +1.8 −3.0 +2.3 −2.6 +2.2 −2.4 +2.1 −1.2 +0.5

306 306 270 270 293 293 276 276 278 278

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250−450 nm are thus very promising as CD and UV probes to investigate the behavior of the terminus. To ascertain the origin of the ICD band at ca. 300 nm, we employed density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, which allowed us to assume multiple through-space intramolecular CH/O interactions between the ortho-alkoxy moiety of the terminus and nearestneighbor lactide residue. The potential energy surface of L-lactide-vanillin as a function of the dihedral angle in C1−C2−O−C4 was obtained using Gaussian 09 (TD-DFT, B3LYP, 6-31G(d) basis set), suggesting that optimized L-lactide-vanillin has two conformers, A (local minimum) and B (global minimum) (Figure S30). However, the energy difference between these two conformers is only 0.1 kcal mol−1 with a barrier height of 1.5−2.0 kcal mol−1. These similar energies should lead to rapid interconversion between the two conformers with a subtle preference of A or B. Both A and B appear to be stabilized by multiple through-space CH/O interactions. This idea was evidenced by the existence of several very short CH/O distances beyond the expectation from van der Waals contact by Bondi radii21 such that: (1) H atoms of benzene rings and O atom of the carbonyl, (2) H atom of benzene rings and O atom of aldehyde, and (3) O atom of benzene ring and H atom of the methoxy group and H atom of L-lactide (Figure S30), respectively. All these O−H distances were shorter than the 0.272 nm given by the Bondi radii, suggesting the existence of intramolecular chiral CH/O interactions. Chiral CH/O interactions are responsible for the observed ICD signals at 306 nm. Even at the achiral vanillin terminus, a geometrically distorted chiral motif is inducible by the stereogenic centers (L or D) of PLLA and PDLA (Figure 2a). Surprisingly, simulated CD spectra (TD-DFT, B3LYP, 6311G+ basis set) of conformers A and B had a similar shape with opposite signs (Figure 4), suggesting that A and B are a pair of diastereomeric conformers.

and 306 nm. The CD signal at 306 nm was ascribed to ICT π−π* and n−π* bands of the vanillin moiety.16 This was further reconfirmed by PLLA and PDLA obtained using 1dodecanol as a nonaromatic initiator.14 PLLA and PDLA with a 1-dodecanol terminus had no CD bands in the range of 250− 306 nm, showing solely intense n−π* transitions at 240−250 nm due to the lactide ester (Figure S29). The observed CD signal at 306 nm for polymers 1 and 2 was therefore assigned as arising from a certain chiral conformation of the vanillin terminus induced by the main-chain chirality of PLLA and PDLA. The ICD sign of the terminus was thus used to detect the main-chain chirality (L or D) of PLA. Next, we evaluated the relation between the gCD value at 306 nm and the number-average molecular weight (Mn) of 1 and 2 (Figure 2b). We found that as Mn increased the ε value decreased, and the Δε value was nearly constant (Figure S21a). The nearly constant Δε value and the decrease in ε led to an expectation that the gCD values would converge to certain constant values when the Mn increased (Figure S21b). This meant that the Mn of PLLA and PDLA with a very small proportion of vanillin termini becomes sufficiently high. By extrapolating the gCD values at M∞ as a function of reciprocal Mn and DPn, 1 and 2 were ±2.2 × 10−4 at 306 nm, respectively (Figures 2b and S21c). A plausible reason for this is that sufficiently high molecular weight PLLA and PDLA suppress the thermally induced molecular motion of the terminus, which contributes to a thermally stabilized chiral conformation associated with the terminus.19 This Mn−gCD relation will catalyze an investigation of invisible interactions between the achiral terminus and resulting main chain of chiral/helical polymers. To confirm our observations, we synthesized PDLA and PLLA with several phenolic initiators based on the hypothesis that there exist intramolecular CH/O interactions20 between the ortho-alkoxy moiety of the terminus and nearest-neighbor lactic residue. This hypothesis was supported by CD bands at 250−270 nm and 300−320 nm. The proton signal of phenyl ring at the ortho-position broadened in 1H NMR spectra (Figures S1 and S2), suggesting the possible intramolecular interaction. We tested 12 phenolic initiators without orthomethoxy groups and with electron-donating groups (Table S1, entries 1−4) as well as those without ortho-methoxy and with electron-withdrawing groups (Table S1, entries 5−8) in the para-position, in addition to four phenol derivatives with orthomethoxy or ortho-ethoxy groups (Table 1, entries 3−10). PLLA-guiacol (3) and PDLA-guiacol (4) clearly showed two weak CD signals whose spectral profiles matched well with the corresponding UV absorptions at 270 and 285 nm, while no detectable CD or UV band was observed in the range of 300− 320 nm (Figure 3). The spectral profiles of 3 and 4 clearly differed from those of 1 and 2. These spectral profile results also agree with phenolic initiators without ortho-methoxy groups and with electron-donating groups (Table S1, entries 1−4) as well as those without ortho-methoxy and with electronwithdrawing groups (Table S1, entries 5−8) in the paraposition (Figures S25−S28). This means that the CD-active ICT transition requires two substituents, such as aldehyde and alkoxy groups, on the benzene ring. We therefore assumed that the presence of the ortho-alkoxy group (Table 1, entries 5−10) was responsible for the two ICD bands due to the π−π* transitions at 250−270 nm and ICT π−π* and n−π* transitions at 300−320 nm (Figures S22−S24). Chromophoric initiators revealing π−π* and n−π* transitions in the range of

Figure 4. Simulated CD and UV spectra of L-lactide-vanillin using Gaussian 09 (TD-DFT, B3LYP, 6-31G(d) basis sets).

To understand this ICD effect, we calculated the gCD value at λext = 306 nm of L-lactide-vanillin, i.e., the smallest model of polymer 1, and compared it with the extrapolated gCD at M∞ of PLLA with vanillin (Figure 2b). We estimated that the extrapolated gCD value of PLLA with vanillin is −2.2 × 10−4 at 305.5 nm (Figure 2b), whereas those of its model conformers A and B are +7.9 × 10−4 at 309.9 nm and −8.7 × 10−4 at 307.9 nm, respectively (Figure 4). Note that the thermally more stable conformer B with a negative CD exists in a 22% excess over the thermally less stable conformer A with a 1016

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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (S) from the Ministry of Education, Culture, Sports, Science and Technology (23225004). H.A. acknowledges support by JST PRESTO “Molecular Technology” with Dr. Takashi Kato.

positive CD. This suggests that the observed negative CD agrees well with the simulated value. Thus, the vanillin terminus in PLLA-vanillin (1) in the solution at 20 °C preferentially adopts chiral conformer B rather than chiral conformer A with opposite chirality. Based on the ICD effect of these aromatic termini, we surveyed the possibility of using our findings in (chir)optically amplified photonic materials. The transfer of chirality/helicity to other constituents followed by chiroptical amplification has been of increasing interest for practical applications such as chiroptical switches and generation of circularly polarized luminescence (CPL).22 The photophysical properties of 1 and 2 in dilute CHCl3 are shown in Figure S31. As shown in Figures S31a and S31b, the UV−vis absorption and photoluminescence (PL) spectra of 1 (λabs = 288 nm, λem = 321 nm) and 2 (λabs = 291 nm, λem = 324 nm) originate from those of vanillin because these absorption and PL bands are almost identical to those of vanillin. Unexpectedly, 1 and 2 did not provide any detectable CPL signal (Figure S32), possibly owing to a low-energy barrier in the photoexcited state as well as ground state (1.5−2.0 kcal mol−1) (Figure S30). A more sophisticated molecular design of the terminus structures of PLLA and PDLA in the photoexcited and ground states will be required to realize CPL-functionalized materials using gels,23 films,24 aggregations,25 and complexation with metals.26 This work is currently in progress. In summary, we demonstrated the first example of induced chirality of vanillin and its phenolic analogs that detect the main-chain chirality of poly(L,L-lactide) and poly(D,D-lactide) when used as termini. Vanillin analogs were chromophoric and luminophoric probes and acted as achiral ring-opening initiators of cyclic (L,L)- and (D,D)-lactides. We observed invariably clear CD bands at 270−320 nm due to π−π* and n−π* transitions at the phenolic terminus moiety but could not detect its CPL band. DFT and TD-DFT calculations suggested the existence of multiple through-space intramolecular CH/O interactions between the ortho-methoxy moiety of vanillin and nearestneighbor lactic acid units. Our findings shed light on the elucidation of invisible weak interactions between the achiral terminus and chiral polymers and suggest applications of materials with amplified chiroptical CPL and CD in the future.

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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/acsmacrolett.6b00513. Experimental details (including GPC data), additional structures and spectroscopic measurements (1NMR, CD/UV−vis, CPL/PL), and Gaussian 09 calculations are presented (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest. 1017

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