Molecular Power Spring: Circular Dichroism Inversion of

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Molecular Power Spring: Circular Dichroism Inversion of Polythiophene Aggregates from Right-handed Helix to Left-Handed Helix Shingo Hattori, Stefaan Vandendriessche, Toshiyuki Hirano, Fumitoshi Sato, Guy Koeckelberghs, Thierry Verbiest, and Kazuyuki Ishii J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b11832 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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The Journal of Physical Chemistry

Molecular Power Spring: Circular Dichroism Inversion of Polythiophene Aggregates from Right-Handed Helix to Left-Handed Helix Shingo Hattori1, Stefaan Vandendriessche2, Toshiyuki Hirano1, Fumitoshi Sato1, Guy Koeckelberghs3, Thierry Verbiest2 and Kazuyuki Ishii1,*

1Institute

of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku,

Tokyo 153-8505, Japan 2Molecular

Imaging and Photonics, KU Leuven, Celestijnenlaan 200D, Box 2425, 3001

Heverlee, Belgium 3Polymer

Chemistry and Materials, KU Leuven, Celestijnenlaan 200F, Box 2404, 3001

Heverlee, Belgium

(K. I.) [email protected] Telephone: +81-3-5452-6306 Fax: +81-3-5452-6306

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ABSTRACT In molecular rotors, a type of molecular machine, spontaneous rotors without the need for an external stimulus are promising because conventional molecular rotors require a continuous energy supply. In this study, we demonstrate spontaneous transformation from kinetically favored metastable right-handed helical aggregates to thermodynamically stable left-handed helical aggregates after an evaporation procedure. In addition, we propose the conditions for preparation of metastable right-handed helical aggregates, whose chirality can be spontaneously inverted, based on kinetic analysis. This molecular power spring will be useful for designing new types of molecular machines.


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Introduction Molecular machines are nanorobots that perform intelligent functions via programmed motions.1 Recently, a nanocar race in which competitors aim to move single molecules a distance of 100 nm has attracted attention.2 Controlling molecular rotation by an external stimulus is a useful method for driving molecules. Feringa and co-workers have developed various types of unidirectional motors since they first reported a unidirectionally rotating molecule driven by photoirradiation and heating, where the helicity of the molecule can be reversed by cis–trans photoisomerization followed by heating.3–5 Kelly and co-workers reported chiral molecular motors driven by chemical fuels.6–8 In biological systems, adenosine triphosphate (ATP) synthases driven by protons and ATP, which are called F0 and F1 motors, respectively, unidirectionally rotate.9–12 Thus, a continuous energy supply, such as photoirradiation, heating, or chemicals, is needed to rotate molecules in these artificial and biological systems. Development of systems in which molecules spontaneously rotate after an initial energy supply is one of the main targets for molecular rotors. Several examples of chiral inversion of supramolecular chiral aggregate formation after addition of poor solvents,13,14 chiral compounds,15–17 and cooling of the solution18,19 have been reported. Chiral inversion corresponds to dynamic transformation of chiral supramolecules from a 3 ACS Paragon Plus Environment


kinetically favored metastable state to the thermodynamically stable state. Thus, we focus on dynamic chiral inversion from a kinetically favored metastable state as a possible candidate for spontaneous unidirectional molecular rotors. Recently, using a rotary evaporator, we demonstrated evaporation-rate-based supramolecular chiral selection of poly(3-((S)-3,7-dimethyloctyl)thiophene) (PT-1) for the first time, and we proposed the chiral selection mechanism.20–22 In this system, the supramolecular chirality of the PT-1 aggregate is determined by the retention time of initial aggregation. The right-handed helical (P-type) aggregate, which is the product of the kinetically favored state, is maintained in the fast evaporation process, while it transforms to the left-handed helical (M-type) aggregate, which is the product of the thermodynamically stable state, in the slow evaporation process. If the metastable P-type aggregate can be selectively extracted by an evaporation process, spontaneous chiral inversion could be achieved without addition of chemicals or modulation of the temperature. This motivated us to develop spontaneous unidirectional molecular rotors using metastable states (i.e., molecular power springs). Herein, we report a novel molecular machine, namely, a molecular power spring, that can spontaneously rotate after evaporation (Fig. 1). We succeeded in direct observation of the dynamic transformation from the P-type aggregates to the M-type aggregates by 4 ACS Paragon Plus Environment

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time-dependent circular dichroism (CD) measurements after evaporation. That is, spontaneous rotational transformation from the kinetically favored metastable P-type aggregates to the thermodynamically stable M-type aggregates, which corresponds to a molecular power spring. Kinetic analysis reveals the preparation conditions of metastable aggregates whose CD signal can be inverted over time, which will provide guidelines for designing novel molecular machines.

P-type

S

S

S

S

M-type

S

=

S n

PT-1 Figure 1. Evaporation-triggered molecular power spring. 5 ACS Paragon Plus Environment


Experimental section Materials

PT-1 with a number-average molar mass of 12.7 kg/mol was

synthesized by a previously described method.23-26 The PT-1 aggregates were prepared as follows. First, 3 mL of MeOH was added to 7 mL of a CHCl3 solution of PT-1. This solution containing non-aggregated PT-1 (5 × 10−5 M) was evaporated by a rotary evaporator, and PT-1 aggregates formed. Evaporation was performed using a rotary evaporator at 80 hPa with a water bath at 20 °C, which was similar to room temperature. For 2 and 8 min evaporation, the final concentrations of PT-1 were 6 × 10−5 and 8 × 10−5 M, respectively.

Instrumental Techniques Electronic absorption and CD spectra were measured with a JASCO V-570 spectrometer and a JASCO J-725 spectrodichrometer, respectively, using an optical quartz cuvette with a 1 cm path length.


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Results and Discussion Spectroscopic features of PT-1 and its chiral aggregates

Figure 2a and 2b shows the

electronic absorption and CD spectra of non-aggregated randomly coiled PT-1 in CHCl3. The absorption peak at about 445 nm is attributed to a π–π* transition, while there is no distinguishable CD signal. For a highly concentrated solution of PT-1 in a mixed MeOH/CHCl3 solvent, the absorption peak red shifts and two new peaks appear. The red shift originates from the high planarity of the main thiophene chain, which extends the effective π-conjugation length, while the more intense band at shorter wavelength indicates formation of face-to-face type aggregates of PT-1 (i.e., H-aggregates).20 In the CD spectra, the kinetically favored P-type aggregates show a negative/positive spectral pattern, while the thermodynamically stable M-type aggregates show the opposite positive/negative spectral pattern. As previously reported, the supramolecular chirality of PT-1 aggregates can be selectively prepared based on the evaporation rate.20



Figure 2. a) Electronic absorption and b) CD spectra of non-aggregated PT-1 (black dashed line) and aggregated PT-1 (red and blue solid lines for aggregates prepared by fast and slow evaporation, respectively).20 c) Electronic absorption and d) CD spectra after stopping evaporation at the initial stage of aggregation (red and blue solid lines are the spectra 1 and 90 min after stopping evaporation, respectively: [PT-1] = 5 × 10−5 M, MeOH/CHCl3 = 3/7 in the initial state). e) A time profile (red dotted line, [PT-1] = 5 × 10−5 M, MeOH/CHCl3 = 3/7 in the initial state) of the CD signal at 490 nm with its fitted curve (black solid line). The insert shows the expanded fast time region.

Circular dichroism inversion

To investigate whether chiral inversion from the

kinetically favored P-type aggregates to the thermodynamically stable M-type aggregates is possible, we examined the time-dependence of the CD spectra. Figure 2c and 2d shows the electronic absorption and CD spectra of the PT-1 aggregates at the initial stage of aggregation after 2 min evaporation. The red and blue solid lines are the spectra 1 and 90 8 ACS Paragon Plus Environment

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min after stopping evaporation, respectively. The electronic absorption spectra indicate coexistence of non-aggregated PT-1 and aggregates, which are almost completely maintained. The CD spectral pattern is dynamically inverted from negative/positive to positive/negative.27-30 That is, we succeeded in spontaneous transformation of the supramolecular chirality of PT-1 aggregates from P-type to M-type at the initial stage of aggregation.

Kinetic analyses of circular dichroism inversion

The time course of the CD signal

corresponding to the second peak of the Cotton effect at 490 nm was measured (dotted red line, Fig. 2e). The negative CD signal instantly inverts to a positive CD signal, followed by a gradual increase in the positive signal. The fast and slow components coexist in this transformation, which differs from a simple transformation from P-type to M-type, such as that without intermediates. Thus, transformation via a non-helical intermediate (NH) is considered using the formation (kI+, I = M and P) and deformation (kI−, I = M and P) rate constants for M-type and P-type aggregates (Scheme 1).



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Scheme 1. Transformation from the P-type aggregate to the M-type aggregate via the NH intermediate.

Here, the model is simplified to only three species (i.e., NH, the P-type aggregate, and the M-type aggregate) whose degrees of aggregation are similar. This approximation is supported by the fact that the electronic absorption spectrum is almost entirely maintained during the transformation; that is, the nucleation and elongation processes are negligible (Fig. 2c).13,14 By solving differential equations, the difference between [M] and [P] is represented by a double-exponential function (see the Supporting Information):31 (1) where Ai (i = 1, 2, and 3) and γj (j = 1 and 2) consist of kI+ and kI− (I = M and P), and [P]0 denotes [P] just after stopping evaporation. The time profile of the CD signal is well reproduced by equation (1) (black line in Fig. 2e), and the fitting parameters are summarized in Table 1. As shown in Scheme 1, this result indicates that chiral inversion from the P-type aggregate to the M-type aggregate occurs via the NH intermediate, which can be attributed to a PT-1 aggregate with long intermolecular distances by considering previous studies (Supporting Information).31 Thus, the dynamic chiral transformation from the P-type aggregates to the M-type aggregates can be proposed to be a molecular 10 ACS Paragon Plus Environment

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power spring, which is the first example of a molecular power spring spontaneously driven without addition of chemicals or modulation of the temperature.

Table 1. Kinetic parameters of Scheme 1 for reproducing the CD time profiles. Evaporation time

2 min

8 min

γ1 / s−1

−6.4 × 10−5

−1.8 × 10−4

γ2 / s−1

−6.5 × 10−3

−3.6 × 10−3

A1

−3.4 × 10−1

−4.0 × 10−1

A2

−1.0

−2.6 × 10−1

A3

4.3 × 10−1

−3.1 × 10−1

kP− / s−1

6.9 × 10−3

9.9 × 10−4

Dependence of the degree of aggregation To clarify the conditions where chiral inversion occurs, the evaporation time was extended to 8 min. The CD signal at 490 nm decreases and is not inverted even after 6 h (dotted blue line, Fig. S1). The electronic absorption spectra (Fig. S2) indicate that the degree of aggregation after 8 min evaporation is higher than after 2 min evaporation. Thus, CD inversion does not occur when the evaporation time is long enough to lead to the excessive degree of aggregation due to the increase in the final concentrations of PT-1 and the MeOH/CHCl3 ratio.27



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Next, transformation of the P-type aggregate to the NH intermediate was kinetically clarified. The activation Gibbs energy in deformation from the P-type aggregate to the NH intermediate (∆G‡P→NH) was evaluated by the deformation rate constant of the P-type aggregate kP–. Using Ai (i = 1, 2, and 3) and γj (j = 1 and 2), kP– can be expressed as (see the Supporting Information)

(2) Using the parameters that reproduce the CD time profiles, the kP– value for 2 min evaporation is 6.9 × 10−3 s−1, which is seven times greater than that for 8 min evaporation (9.9 × 10−4 s−1). That is, deformation of the P-type aggregate is dependent on the evaporation time. Using the ratio of the kP– values for 2 and 8 min evaporation, the energy difference in ∆G‡P→NH between 2 and 8 min evaporation is calculated to be 4.8 kJ mol−1 (see the Supporting Information). Thus, the ∆G‡P→NH value increases by extending the evaporation time, which accelerates aggregation (Fig. 3). This can be explained by two factors. (1) The increase in the MeOH/CHCl3 ratio stabilizes the P-type aggregate compared with the NH intermediate. (2) The excessive degree of aggregation inhibits the deformation pathway from the P-type aggregate to the NH intermediate because of the difficulty in structural changes.


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Figure 3. Schematic representations of the potential energy surfaces after 2 min (left) and 8 min evaporation (right).

Molecular dynamics simulations To investigate the nature of the energy difference between the P-type and M-type aggregates, the computational structures of models (octamers of tetra(3-((S)-3,7-dimethyloctyl)thiophene)) were obtained by molecular dynamics simulations. The energies of the optimized octamers were then calculated by density functional theory (B3LYP/6-31G) (see the Supporting Information).32 The energy of the P-type octamer is higher than that of the M-type octamer (Fig. S3), which is consistent with the experimental results. This energy difference can be explained by steric hindrance, which is larger in the P-type octamer than in the M-type octamer. In the Ptype octamer, the chiral origin (i.e., the 3-methyl group) in one unit is much closer to the 13 ACS Paragon Plus Environment


octyl chain of the neighboring unit compared with the M-type octamer. This slight structural difference is the origin of the metastable state from which unidirectional rotation spontaneously occurs.


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Conclusions In conclusion, we have discovered a spontaneous molecular machine that unidirectionally rotates from the kinetically favored metastable state to the thermodynamically stable state. This originates from the inherent chirality of the alkyl side chain of PT-1, which causes a slight structural difference based on steric hindrance. Therefore, design of chiral alkyl side chains should be a useful approach to construct spontaneously rotating molecular machines. Because the present molecular machine, which is called a molecular power spring, does not require a continuous external stimulus after evaporation, in contrast to conventional molecular machines that require a continuous external stimulus such as photoirradiation, heating, or a supply of chemicals, this study will aid in the design of new types of molecular machines.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI. Kinetic analysis of transformation from P-type aggregate to M-type aggregate via the NH intermediate, interpretation of the NH intermediate, time profile of the circular dichroism signal at 490 nm after 8 min evaporation, calculation of the energy difference in the activation Gibbs energy, comparison of the electronic absorption spectra, and 15 ACS Paragon Plus Environment


calculations of the difference between P-type and M-type aggregates.

Acknowledgements This work was supported by JSPS KAKENHI (grant numbers JP17H06375 and JP16H04128). T.V. and G.K. acknowledge financial support from the KU Leuven.


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26. Willot, P.; Steverlynck, J.; Moerman, D.; Leclere, P.; Lazzaroni, R.; Koeckelberghs, G. Poly(3-alkylthiophene) with Tuneable Regioregularity: Synthesis and Self-Assembling Properties. Polym. Chem. 2013, 4, 2662–2671. 27. By 1H-NMR measurements, the ratios of MeOH/CHCl3 were evaluated to be 3.7:6.3 and 4.6:5.4 for 2 and 8 min evaporation, respectively. 28. At higher concentration (2 × 10-4 M), a CD spectrum of the P-type aggregates was not changed depending on the time. 29. The repeatability based on the cycle of evaporation and solvent-addition was confirmed. 30. Although the electronic absorption and CD spectra of PT-1 in the mixed solvent of MeOH/CHCl3 were investigated with different volume ratios (Figure S1 in ref. 20), a mixed solvent of MeOH/CHCl3 (= 3:7) was appropriate for observing the CD inversion. 31. Martin, J.; Nogales, A.; Martín-González, M. The Smectic−Isotropic Transition of P3HT Determines the Formation of Nanowires or Nanotubes into Porous Templates. Macromolecules 2013, 46, 1477–1483. 32. Dag S.; Wang, L.-M. Packing Structure of Poly(3-hexylthiophene) Crystal: Ab Initio and Molecular Dynamics Studies. J. Phys. Chem. B 2010, 114, 5997–6000. 21 ACS Paragon Plus Environment


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