Mechanism of Chain Polymerization in Self-Assembled Monolayers of

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Mechanism of Chain Polymerization in Self-Assembled Monolayers of Diacetylene on Graphite Surface Daisuke Takajo, and Koichi Sudoh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03737 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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Mechanism of Chain Polymerization in Self-Assembled Monolayers of Diacetylene on Graphite Surface

Daisuke Takajo*,1 and Koichi Sudoh2

1Research

Center for Structural Thermodynamics, Graduate School of Science, Osaka

University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan 2The

Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

1 ACS Paragon Plus Environment

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ABSTRACT: We have studied the mechanism of the photo-induced chain polymerization in the self-assembled monolayer of a diacetylene compound, 10,12-pentacosadiyn-1-ol, on the graphite surface. We statistically analyze the polymerization degree of the polydiacetylene chains formed at different temperatures, using scanning tunneling microscopy. The distributions of the polymerization degree agree well with the prediction from a simple probabilistic model allowing for the addition reaction and deactivation at both the ends of the chain as stochastic events. The estimated activation energies of the addition reaction and deactivation are noticeably different from those for the conventional solid-state polymerization in the bulk crystals of diacetylene.

Keywords: photoinitiated reaction, surface reaction, STM, polymerization degree, activation energy, radical, oxidation


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INTRODUCTION With the advances in material chemistry, surface reaction (or on-surface synthesis) is a promising way of creating highly controlled functional materials.1–6 Linear chain polymerization of diacetylene (DA) monomers in self-assembled monolayers on substrate surfaces has attracted great interest since the polydiacetylene (PDA) chains are expected to work as conductive nanowires for molecular scale electronics.7–16 It is known that chain polymerization reactions occur in the self-assembled monolayers of DA compounds adsorbed on solid substrates by ultraviolet (UV) irradiation or stimulation using a scanning tunneling microscope (STM) tip.7–18 Although the reaction mechanism of the solid-state polymerization in bulk crystals of DA molecules has been studied in detail,19,20 our understanding on the chain polymerization in the self-assembled monolayers adsorbed on solid substrates is still very poor. Recently, we have shown that the reactivity of the chain polymerization of 10,12-pentacosadiyn-1-ol (PCDYol, Fig. 1) depends on the molecular arrangement of the self-assembled monolayer, using STM.17 In this work, we use STM to study the reaction mechanism of the chain polymerization in the self-assembled monolayers of PCDYol on graphite surfaces by statistical analysis of the polymerization degree.21,22


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We show that the distributions of the polymerization degree obtained from the STM observations are well reproduced by a simple probabilistic model, where the addition reaction and deactivation of radicals are introduced as stochastic events. The activation energies of the addition reaction and deactivation are evaluated from the temperature dependence of the PDA number density and polymerization degree distribution, revealing the difference of the polymerization mechanism in the self-assembled monolayer from that in the bulk crystals.23,24

Figure 1. Molecular structure of PCDYol.


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EXPERIMENTAL SECTION PCDYol was purchased from Tokyo Chemical Industry, Co., Ltd., Tokyo, Japan and used without further purification. The self-assembled PCDYol monolayer was formed on a freshly cleaved (0001) surface of HOPG (grade ZYH, Momentive Performance Materials Inc., Strongsville, OH) by means of horizontal dipping.17,25,26 The PCDYol solution in chloroform 50 L at 0.1 mg/mL was spread onto a purified water surface in a Petri dish with an inner diameter of 27 mm at 20 °C. After the chloroform was evaporated, a part of the thin film was detached from the water interface by bringing the horizontally oriented HOPG substrate in contact with the film from the air atmosphere side. To initiate the solid-state polymerization of the PCDYol layer adsorbed on the HOPG substrate, we irradiated the surface with a hand-held UV lamp (UVG-11 Compact, 4 W, wavelength 254 nm, UVP Inc., Upland, CA) under air atmosphere, while controlling the temperature in the range of 8~28 °C. The power density at the sample position was maintained at 1.1 mW/cm2. After UV-irradiation, the STM images were acquired with a Nanoscope IIIa scanning probe microscope (Digital Instruments Inc., Santa Barbara, CA) using mechanically cut Pt/Ir tips (90/10) under air atmosphere at room temperature. The sample bias voltage (tip grounded) −800 mV and tunneling current 6~10 pA were applied in constant current mode.


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RESULTS AND DISCUSSION There are two polymorphic forms for the self-assembled PCDYol monolayer on the HOPG substrate as shown in Fig. 2. One is the herringbone (H) arrangement, where the extended PCDYol chains present a feather-like pattern as shown in Fig. 2(b). The other is the parallel (P) arrangement, where the all PCDYol molecules align in the same direction as shown in Fig. 2(c). The periods of the lamellar structures in the H- and P-arrangements are estimated to be 6.2 ± 0.2 nm and 6.8 ± 0.2 nm, respectively. The two polymorphic forms are easily identified by the difference in the period in the STM images. Previously, we have shown that the P-arrangement is thermodynamically stable while the H-arrangement is metastable.26

Figure 2. STM images of a self-assembled PCDYol monolayer on a HOPG surface obtained before UV-irradiation. Domains of the H- and P-arrangements show the stripe intervals of 6.2 nm and 6.8 nm, respectively, as indicated by pairs of white lines. (a) A large-scan area image of the polycrystal composed of the H- and P-domains (Iset = 6 pA, Vbias = –800 mV, 1.94 Hz). The interfaces between the H- and P-arrangement domains are suggested by a black line in (a). High-resolution images of (b) the H-arrangement (Iset = 6 pA, Vbias = –800 mV, 3.16 Hz) and (c) P-arrangement (Iset = 6 pA, Vbias = –800 mV, 2.37 Hz). The adsorption models are superimposed at the bottom in (b) and (c).


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Figure 3 shows typical STM images of the PCDYol self-assembled monolayers after UV-irradiation, demonstrating that the UV-irradiation initiates the chain polymerization. The images in Figs. 3(a) and 3(b) were acquired after UV-irradiation at 23 °C for 20 min and 50 min, respectively. We find that PDA chains with π-conjugated system, which appear as the bright lines, are formed along the lamella direction in both the P- and H-domains.17 We observed no further initiation of the chain polymerization during the STM measurements without UV-irradiation. In Fig. 4, the numbers of the observed bright lines attributed to the polymer backbone per area are plotted as a function of UV-irradiation time. We can see that the number densities of PDAs formed in the P- and H-arrangement domains are proportional to the UV-irradiation time with the strong dependence of the reaction efficiency on the monomer arrangement in the self-assembled monolayer.17


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Figure 3. STM images of the PCDYol monolayer on HOPG after UV-irradiation at 23 °C for (a) 20 min (Iset = 6 pA, Vbias = –800 mV, 3.62 Hz) and (b) 50 min (Iset = 10 pA, Vbias = –800 mV, 4.07 Hz). The PDA chains are observed to form in both the P- and H-arrangements as indicated by green and yellow arrowheads, respectively. The separations of the neighboring molecular rows are indicated by pairs of white lines.

Figure 4. The plots of the number density D for the PDAs produced in the P- and H-arrangements against the irradiation time, as examined by STM. By UV-irradiation at 23 °C, D increases in proportion to the irradiation time as suggested by black lines.


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It is considered that both the ends of the PDA chains observed in the STM images are deactivated because we found no further growth of the chain during STM observation. Figure 5 shows the frequency distributions of the polymerization degree in the P-arrangement domains for the samples after UV-irradiation at 23 °C for 20 min and 50 min. For the statistical analysis of the polymerization degree, the PDA chains terminated at the domain boundaries are eliminated. The values of the polymerization degree are estimated by dividing the PDA length measured from the STM images by the DA monomeric unit lengths of 0.48 nm and 0.53 nm for the P- and H-arrangements, respectively.17,25 There is no distinct difference between the two distributions. This strongly suggests that after the initiation of the chain growth both the ends are deactivated rather quickly at least much shorter than the time scale of the UV-irradiation.


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Figure 5. The frequency distribution of the polymerization degree N for PDA chains formed in the P-arrangement extracted from STM images. The values of the frequency are normalized so that its sum is equal to unity. The plots of UV-irradiation at 23 °C for 20 min and 50 min approximately coincide with each other. The solid line represents the fitted curve with the formula (𝑁 ― 1)𝑝𝑁 ― 2(1 ― 𝑝)2 (p; the probability that the addition reaction will occur).


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Figure 6 shows the STM images of PDA chains formed by 50 min UV-irradiation at different temperatures. We find that the number density and average polymerization degree increase with temperature. Figure 7 shows the Arrhenius plots of the number density of PDAs, suggesting an activation energy of 8 ± 2 kcal/mol for the P-arrangement. Figure 8 clearly shows the average length of PDA chains increases with temperature. These temperature effects suggest that thermally activated processes play crucial roles in the chain propagation.

Figure 6. STM images of PDA chains obtained after UV-irradiation of the PCDYol layers (a) at 20 °C for 50 min (Iset = 10 pA, Vbias = –800 mV, 2.00 Hz) and (b) at 26 °C for 50 min (Iset = 6 pA, Vbias = –800 mV, 1.00 Hz). The number density and average length of the PDA increase with temperature in both the P- and H-arrangements.


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Figure 7. Arrhenius plot of the number density D of the PDA formed in the P- and H-arrangements per minute. The slope of the approximate straight line shows that the activation energy Ea of the addition reaction is estimated to be 8 ± 2 kcal/mol for the P-arrangement.

Figure 8. The temperature dependence of the average polymerization degree of the PDA chains. The length of the chain tends to be longer with temperature in both arrangements.


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The mechanism of the chain polymerization in DA bulk crystals has been extensively studied.19,20 The reaction mechanism known for the solid-state polymerization is illustrated in Fig. 9. First, the radical structure is optically generated by UV-irradiation of monomer molecules. The subsequent chain polymerization reaction involves thermal vibration of the adjacent DA monomers. The addition reactions occur at both the ends of the chain, where reactive chemical species are located. The chain growth is eventually terminated by the deactivation of the reactive species. Our observations of the chain polymerization in the self-assembled monolayers are qualitatively consistent with this mechanism for the conventional solid-state polymerization of DA crystals. However, there is significant difference between the polymerization degrees of the PDA chains obtained in the self-assembled monolayers and bulk crystals. As shown in Fig. 8, for the PDA chains formed in the self-assembled monolayers the polymerization degree is normally less than 100, which is much lower than the polymerization degrees of 1000~2000 reported for the solid-state polymerization from bulk crystals at room temperature.27,28


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Figure 9. The mechanism for the conventional solid-state polymerization of DA crystals. (a) An array of DA molecules. (b) The DA moiety of one of the monomers (M) is excited by UV-stimulation. Both ends of the DA group are activated as a diradical monomer (R1) as indicated by red circles. (c) An addition reaction of the neighboring M can occur through the thermal process as suggested by red arrows. (d) The diradical dimer (R2) is produced as the source of the PDA chain. (e) The extended PDA chain grows through a chain propagation reaction towards both the sides until reactive species are deactivated as suggested by blue circles.


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In order to understand the reaction mechanism of the chain polymerization in the self-assembled monolayers in more detail, we compare the experimentally obtained distributions of polymerization degree with the predictions from a simple probabilistic model for the chain polymerization as schematically illustrated in Fig. 9. Here, we consider the chain growth from one dimer towards both directions using discrete probability distributions. The chain propagation towards one direction is modeled by a sequence of Bernoulli trials, where the addition of a monomer molecule and deactivation are two possible outcomes.29 The number of trials, n, until occurrence of the first deactivation event follows the geometric distribution, 𝑃𝑔(𝑛) = 𝑝𝑛 ― 1(1 ― 𝑝), where p is the addition reaction probability. Since the chain grows towards both the sides of the PDA molecule, the polymerization degree N of the resultant PDA chain is given by 𝑛 + + 𝑛 ― , where 𝑛 + and 𝑛 ― are the numbers of trials until the occurrence of the deactivation event for the two opposite directions. Of course, both 𝑛 + and 𝑛 ― take positive integer values, and thus dimer (N = 2) is the minimum product in the chain polymerization. ∑𝑛

+

The

probability

𝑃 (𝑛 + )𝑃𝑔(𝑛 ― ), + 𝑛― = 𝑁 𝑔

density

function

P

(N)

is

given

by

and thus we obtain 𝑃(𝑁) = (𝑁 ― 1)𝑝𝑁 ― 2(1 ― 𝑝)2.

(1)

According to this probability distribution, we easily find that the expected value of the


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polymerization degree is given by 𝐸(𝑁) =

2 . 1―𝑝

(2)

Figure 10 shows the frequency distributions of the polymerization degrees obtained from the STM observations for the samples after UV-irradiation at different temperatures for the P- and H-arrangements. The solid curves indicate the best fits by Eq. (1), showing that the probabilistic model reproduces well the experimental results on the distributions of the polymerization degree. Using the experimentally obtained values of the average polymerization degree in Fig. 8, we can estimate the addition reaction probability p by Eq. (2). In Fig. 11, the obtained values of p are plotted as a function of temperature, showing that the addition reaction probability p increases with temperature.


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Figure 10. The frequency distributions of the polymerization degree N for the PDA chains formed in (a) the P- and (b) H-arrangements extracted from STM images. The values of the frequency are normalized so that its sum is equal to unity. The solid lines represent the fitted curves with the formula (𝑁 ― 1)𝑝𝑁 ― 2 (1 ― 𝑝)2 (p; the probability that the addition reaction will occur). The specific temperatures and duration times applied for UV-irradiation are given in the legends.

Figure 11. The temperature dependence of the addition reaction probability p. The solid lines represent 𝜈𝑑

the fitted curves with the formula (1 + 𝜈𝑎exp

𝐸𝑎 ― 𝐸𝑑

(

𝑅𝑇

))

―1

. The difference Ea  Ed of the activation energy

between the addition reaction and deactivation is estimated to be 7 ± 1 kcal/mol in both the P- and H-arrangements.


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The temperature dependence of the addition reaction probability provides us significant information on the polymerization mechanism. The addition reaction probability p is determined by the relative reaction rate of the addition reaction to the deactivation. Thus, p is written as 𝑝=

𝑘𝑎 𝑘𝑎 + 𝑘𝑑

(3)

,

where ka and kd are the reaction rate constants of the addition reaction and deactivation, respectively. Using Arrhenius’ equation for the temperature dependence of the rate constants,30 we have the following temperature dependence of the addition reaction probability: 𝑝=

1 𝜈𝑑 𝐸𝑎 ― 𝐸𝑑 1 + exp 𝜈𝑎 𝑅𝑇

(

)

,

(4)

where a and d are the frequency factors, Ea and Ed are the activation energies for the addition reaction and deactivation, respectively. We find that the behavior of the temperature dependence changes by the magnitude relationship between Ea and Ed. Our observation where the addition reaction probability increases with increasing temperature implies that Ea > Ed. The solid curves in Fig. 11 indicate the best fits by Eq. (4). From the curve fittings, we find that for both the P- and H-arrangements Ea – Ed = 7 ± 1 kcal/mol. Since the addition reaction is irreversible, the PDA chain generation obeys the rate 18 ACS Paragon Plus Environment

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equation: 𝑑𝐷 = 𝑘𝑎[R1] , 𝑑𝑡

(5)

where D is the number density of the PDA chain, and [R1] is the number density of monomer radical generated by UV-irradiation. If we assume a steady state, we have [R1] =

𝛼 𝑘𝑎 + 𝛼′

(6)

[M] ,

where  and ' are the rate constants of the excitation and relaxation of radicals, respectively, and [M] is the number density of DA monomers. At low temperatures, where ka is much smaller than ', the rate limiting process for the PDA chain generation is the addition reaction involving vibrational excitation of the DA molecules. In this temperature range, 𝑘𝑎[R1] ≈ (𝛼/𝛼′)𝑘𝑎[M], and thus we find that the PDA chain generation rate is given by 𝑑𝐷 𝛼𝑘𝑎 = [M] . 𝑑𝑡 𝛼′

(7)

Here, , ' and [M] are temperature independent parameters. Thus, the PDA chain generation rate follows the Arrhenius type temperature dependence with the activation energy Ea of the addition reaction. The experimentally observed temperature dependence of the PDA number density shown in Fig. 7 is consistent with the above discussion. From the Arrhenius plot in Fig. 7, we evaluate the activation energy Ea to be 8 ± 2 kcal/mol for the P-arrangement. Using the estimation of Ea – Ed mentioned above,


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the activation energy Ed is found to be 1 ± 1 kcal/mol for the P-arrangement. The values of the activation energies obtained for the polymerization in the self-assembled monolayer on HOPG in this work are rather different from those reported for the conventional solid-state polymerization in DA bulk crystals. It has been reported that the activation energy of the addition reaction for the conventional solid-state polymerization is 2~4 kcal/mol,23,24 which is smaller than that obtained in this work. We attribute the increase in the activation energy for the polymerization in the monolayer on HOPG surfaces to the suppression of thermal vibration of the DA monomer by the molecule-substrate interaction.31 On the deactivation process, the activation energy estimated in this work is much smaller than that for the process involving intramolecular hydrogen transfer, which is predicted as the deactivation mechanism for the conventional solid-state polymerization.32,33 The activation energy for the isomerization of chain molecules due to the intramolecular hydrogen transfer has been estimated to be 10~30 kcal/mol.34–36 The large difference in the activation energy strongly suggests that the deactivation mechanism in the self-assembled monolayer is completely different from that in the bulk crystals. A conceivable deactivation process is oxidation, since the self-assembled monolayer is exposed to atmospheric air in our experiment. It is known that the spin-allowed reaction between the radical and O2 gas is


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rapid.37–39 We consider that the much smaller polymerization degree for the chain polymerization in the self-assembled monolayers than that for the conventional solid-state polymerization is due to the fast deactivation by oxidation.


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CONCLUSIONS We studied the mechanism of the photo-induced polymerization of the PCDYol in the self-assembled monolayer on the HOPG surface. The distributions of the polymerization degree derived from the STM images agree well with the prediction from a simple probabilistic model for the chain formation through the addition reaction and deactivation event at both the ends. Based on the observed temperature dependence of the PDA number density and polymerization degree distribution, the activation energies of the addition reaction and deactivation in the P-arrangement domain are estimated to be 8 ± 2 kcal/mol and 1 ± 1 kcal/mol, respectively. These values are considerably different from those known for the solid-state polymerization in bulk crystals. We consider that in the self-assembled monolayer, which is exposed to air, the radicals are deactivated by oxidation instead of by the process involving intramolecular hydrogen transfer, which is predicted as the deactivation mechanism for the conventional solid-state polymerization in bulk crystals. The relatively low degree of polymerization for the chain polymerization in the self-assembled monolayer is considered to be due to the fast deactivation of the radicals by oxidation.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone: +81-6-6850-5524.

ACKNOWLEDGMENTS This work is Contribution No. 61 from the Research Center for Structural Thermodynamics.

REFERENCES (1) Davis, E. M.; Davis, J. R. Fundamentals of Chemical Reaction Engineering. McGraw-Hill, New York, 2003.

(2) Bond, C. G. Heterogeneous Catalysis: Principles and Applications. Clarendon Press, Oxford, 1974.

(3) Han, P.; Akagi, K.; Canova, F. F.; Mutoh, H.; Shiraki, S.; Iwaya, K.; Weiss, P. S.; Asao, N.; Hitosugi, T. Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity. ACS Nano 2014, 8, 9181–9187.


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(4) Xiao, B.; Niu, Z.; Wang, Y.; Jia, W.; Shang, J.; Zhang, L.; Wang, D.; Fu, Y.; Zeng, J.; He, W.; Wu, K.; Li, J.; Yang, J.; Liu, L.; Li, Y. Copper Nanocrystal Plane Effect on Stereoselectivity of Catalytic Deoxygenation of Aromatic Epoxides. J. Am. Chem. Soc. 2015, 137, 3791–3794. (5) Tafreshi, S. S.; Roldan, A.; De Leeuw, H. N. Density Functional Theory Calculations of the Hydrazine Decomposition Mechanism on the Planar and Stepped Cu (111) Surfaces. Phys. Chem. Chem. Phys. 2015, 17, 21533–21546. (6) Klappenberger, F.; Zhang, Y.; Björk, J.; Klyatskaya, S.; Ruben, M.; Barth, J. V. On-Surface Synthesis of Carbon-Based Scaffolds and Nanomaterials Using Terminal Alkynes. Acc. Chem. Res. 2015, 48, 2140–2150. (7) Okawa, Y.; Aono, M. Nanoscale Control of Chain Polymerization. Nature 2001, 409, 683–684.

(8) Okawa, Y.; Aono, M. Linear Chain Polymerization Initiated by a Scanning Tunneling Microscope Tip at Designated Positions. J. Chem. Phys. 2001, 115, 2317– 2322.

(9) Okawa, Y.; Aono, M. Nanoscale Wiring by Controlled Chain Polymerization. Surf. Sci. 2002, 514, 41–47.


Page 24 of 30

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(10) Qiao, Y.; Zeng, Q.; Tan, Z.; Xu, S.; Wang, D.; Wang, C.; Wan, L.; Bai, C. Photoinduced Organic Nanowires from Self-Assembled Monolayers. J. Vac. Sci. Technol. B 2002, 20, 2466–2469.

(11) Akai-Kasaya, M.; Shimizu, K.; Watanabe, Y.; Saito, A.; Aono, M.; Kuwahara, Y. Electronic Structure of a Polydiacetylene Nanowire Fabricated on Highly Ordered Pyrolytic Graphite. Phys. Rev. Lett. 2003, 91, 255501.

(12) Takajo, D.; Okawa, Y.; Hasegawa, T.; Aono, M. Chain Polymerization of Diacetylene Compound Multilayer Films on the Topmost Surface Initiated by a Scanning Tunneling Microscope Tip. Langmuir 2007, 23, 5247–5250.

(13) Giridharagopal, R.; Kelly, K. F. Substrate-Dependent Properties of Polydiacetylene Nanowires on Graphite and MoS2. ACS Nano 2008, 2, 1571–1580.

(14) Wang, D.; Chen, Q.; Wan, L. Structural Transition of Molecular Assembly under Photo-Irradiation: an STM Study. Phys. Chem. Chem. Phys. 2008, 10, 6467–6478.

(15) Deshpande, A.; Sham, C.; Alaboson, J. M. P.; Mullin, J. M.; Schatz, G. C.; Hersam, M. C. Self-Assembly and Photopolymerization of Sub-2 nm One-Dimensional Organic Nanostructures on Graphene J. Am. Chem. Soc. 2012, 134, 16759–16764.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16) Alaboson, J. M. P.; Sham, C.; Kewalramani, S.; Emery, J. D.; Johns, J. E.; Deshpande, A.; Chien, T.; Bedzyk, M. J.; Elam, J. W.; Pellin, M. J.; Hersam, M. C. Templating Sub-10 nm Atomic Layer Deposited Oxide Nanostructures on Graphene via One-Dimensional Organic Self-Assembled Monolayers. Nano Lett. 2013, 13, 5763– 5770. (17) Takajo, D.; Inaba, A.; Sudoh, K. Two-Dimensional Solid-State Topochemical Reactions of 10,12-Pentacosadiyn-1-ol Adsorbed on Graphite. Langmuir 2014, 30, 2738–2744. (18) Guo, C.; Xue, J. D.; Cheng, L. X.; Liu, R. C.; Kang, S. Z.; Zeng, Q. D.; Li, M. Phys. Chem. Chem. Phys. 2017, 19, 16213–16218. (19) Neumann, W.; Sixl, H. The Mechanism of the Low Temperature Polymerization Reaction in Diacetylene Crystals. Chem. Phys. 1981, 58, 303–312. (20) Sixl, H. Spectroscopy of the Intermediate States of the Solid State Polymerization Reaction in Diacetylene Crystals. Adv. Polym. Sci. 1984, 63, 51−88.

(21) Flory, J. P. Molecular Size Distribution in Linear Condensation Polymers. J. Am. Chem. Soc. 1936, 58, 1877–1885.


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(22) Taylor, B. G. The Relation of the Viscosity of Nylon Solution in Formic Acid to Molecular Weight as Determined by End-Group Measurements. J. Am. Chem. Soc. 1947, 69, 638–644. (23) Wegner, G. Topochemical Reactions of Monomers with Conjugated Triple-Bonds IV. Polymerization of Bis-(p-Toluene Sulfonate) of 2,4-Hesadiin-1,6-diol. Makromol. Chem. 1971, 145, 85–94. (24) Chance, R. R.; Patel, G. N. Solid-State Polymerization of a Diacetylene Crystal: Thermal,

Ultraviolet,

and

-Ray

Polymerization

of

2,4-Hexadiyne-1,6-diol

Bis-(p-Toluene Sulfonate). J. Polym. Sci. 1978, 16, 859–881. (25) Takajo, D.; Inaba, A.; Sudoh, K. Two Polymorphic Forms of 10,12-Pentacosadiyn-1-ol Molecular Layers on Graphite Transferred from an Aqueous Surface. Surf. Sci. 2014, 620, 38–44. (26) Takajo, D.; Sudoh, K. Stability of Two-Dimensional Polymorphs for 10,12-Pentacosadyn-1-ol on Graphite Investigated by SPM. Appl. Sci. 2018, 8, 503. (27) Patel, G. N.; Walsh, E. K. Polydiacetylenes: Solution Conformation Characteristics. J. Polym. Sci. 1979, 17, 203–208. (28) Wenz, G.; Wegner, G. Kinetics of the Radiation Induced Polymerization of a Diacetylene Studied by Analysis of the Molecular Distribution. Mol. Cryst. Liq. Cryst.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1983, 96, 99–108. (29) Bertsekas, P. D.; Tsitsiklis, N. J. Introduction to Probability. Athena Scientific, Nashua, 2008. (30) Laidler, K. J. J. Chem. Educ. 1984, 61, 494–498. (31) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. Imaging Alkane Layers at the Liquid/Graphite Interface with the Scanning Tunneling Microscope. J. Appl. Phys. Lett. 1990, 57, 28–30. (32) Wright, J. D. Molecular Crystals. Cambridge University Press, Cambridge, 1987. (33) Niederwald, H.; Schwoerer, M. Kinetics of the UV-Photopolymerization in TS Diacetylene Single Crystals as Studied by Time Resolved Optical Spectroscopy. Z. Naturforsch. 1983, 38 a, 749–761. (34) Dormans, G. J. M.; Buck, H. M. Mechanism of the Thermal [1,5]-H Shift in cis-1,3-Pentadiene. Kinetic Isotope Effect and Vibrationally Assisted Tunneling. J. Am. Chem. Soc.1986, 108, 3253–3258. (35) Heiss, A.; Sahetchian, K. Isomerization Reactions of the n-C4H9O and n-OOC4H8OH Radicals in Oxygen. Int. J. Chem. Kinet. 1996, 28, 531–544. (36) Zheng, J.; Truhlar, D. G. Kinetics of Hydrogen-Transfer Isomerizations of Butoxyl Radicals. Phys. Chem. Chem. Phys. 2010, 12, 7782–7793.


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(37) Sander, W. W. Reaction of Diphenylmethylene and Phenylmethylene with Oxygen. A Matrix Isolation Study. J. Org. Chem. 1989, 54, 333–339. (38) Barclay, L. R. C.; Ingold, K. U. Autoxidation of Biological Molecules. 2. The Autoxidation of a Model Membrane. A Comparison of the Autoxidation of Egg Lecithin Phosphatidylcholine in Water and in Chlorobenzene. J. Am. Chem. Soc. 1981, 103, 6478–6485. (39) Adams, G. E.; Willson, R. L. Pulse Radiolysis Studies on the Oxidation of Organic Radicals in Aqueous Solution. Trans. Faraday Soc. 1969, 65, 2981– 2987.


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The graphic for the table of contents.


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Mechanism of Chain Polymerization in Self-Assembled Monolayers of

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