Relationship between Activation Volume and Polymer Matrix Effects

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Relationship between Activation Volume and Polymer Matrix Effects on Photochromic Performance: Bridging Molecular Parameter to Macroscale Effect Kentaro Shima,† Katsuya Mutoh,† Yoichi Kobayashi,† and Jiro Abe*,†,‡ †

Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan ‡ CREST, Japan Science and Technology Agency (JST), K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan S Supporting Information *

ABSTRACT: Photochromic compounds have attracted attention as ophthalmic lenses because of their reversible color modulation upon irradiation with light. However, the efficiency of the photochromism is strongly affected by their surrounding because of the structural changes concomitant with the photochromism, which causes the decrease in the photochromic performance in the polymer matrix. Therefore, the clarification of the degree of the structural changes is necessary to apply to the ophthalmic lenses. Bridged imidazole dimers are one of the fast photoswitch molecules possessing high photochromic quantum yield and durability. Although the enhancement of the photochromic properties of bridged imidazole dimers has been vigorously studied, the quantitative information about the structural changes has not been revealed in detail. In this study, we investigated the pressure effects on the photochromic properties of bridged imidazole dimers. The activation volume for the thermal back-reaction of the photogenerated biradical species becomes an effective measure to predict the degree of the structural change during the photochromic reaction. We revealed that the smaller activation volume is suitable for keeping the efficient photochromic reaction in the polymer matrix because the photochromic reaction is not affected by the surroundings. These fundamental insights into the molecular dynamics provide valuable information to develop fast photochromic compounds that are suitable for the use in the polymer matrix and pressure sensitive photochromic materials. ophthalmic lenses and real-time holographic materials.35 However, the switching speed of the photochromic compounds is generally affected by the viscosity of the surrounding medium because photochromic compounds change their structure accompanied by the photochromism.36−40 The molecular structures of bridged imidazole dimers also instantaneously change upon UV light irradiation: the alignment of the two imidazole rings is drastically changed, associated with the photochromic reaction. Although the two imidazole rings of bridged imidazole dimers are perpendicularly arranged, those of the colored biradical species are arranged in face-to-face alignment. The thermal decoloration reaction rate and the photoconversion efficiency of bridged imidazole dimers decrease in the polymer matrix due to the smaller free volume.41 One of the answers to overcome the issue is using the plasticizer to reduce the grass transition temperature (Tg) of the polymer matrix;42 however, it also induces the lowmechanical strength of the films.43 Therefore, fast photochromic materials that are not affected by the surrounding are required. However, quantitative information about the

1. INTRODUCTION Photochromic compounds have potential applications to ophthalmic lenses, magnetic and electric switches, fluorescence modulations, and so on.1−10 The reversible color modulation and the switch of electronic properties between the two structural isomers are effectively utilized for those applications. On the contrary, not only the color modulation but also the structural changes associated with photochromic reactions have received much attention to develop light-driven molecular machines and to control the biological functions upon irradiation with light.11−20 As a result of amplifying the microscopic structural changes to the macroscopic volume changes, the photoinduced bending of the photochromic crystal and liquid crystalline films have been also reported, to develop photoactuators.21−23 Bridged imidazole dimers are one of the attractive T-type photochromic compounds whose photogenerated colored species exhibit the fast thermal backreaction.24 We have designed and synthesized several bridged imidazole dimers to enhance their photochromic properties.25−34 Bridged imidazole dimers show the C−N bond cleavage reaction upon UV light irradiation, resulting in the generation of the colored biradical species. The fast color modulation within a several tens of milliseconds and desirable high photochromic quantum yield lead to the application to the © XXXX American Chemical Society

Received: November 5, 2014 Revised: January 13, 2015

A

DOI: 10.1021/jp511074y J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Scheme 1. Photochromic Reactions of Bridged Imidazole Dimer Derivatives

were dissolved in spectroscopic grade toluene for all measurements (HH, 6.7 × 10−5 M; HT, 5.4 × 10−5 M; TT, 6.2 × 10−5 M; BINOL, 6.7 × 10−5 M). The photochromic film doped with HH and TT (5 wt %) were prepared on a glass substrate by a cast method. The host polymers were prepared by doping the tricresyl phosphate (TCP), as a plasticizer, with different ratios (80, 70, 60, 50, 40, and 36 wt %) into poly(methyl methacrylate) (PMMA), according to our previous report.42 The polymer was dissolved in chloroform for the cast method. 2.2. Laser Flash Photolysis Measurements. We conducted laser flash photolysis measurements of the bridged imidazole dimer derivatives by using the same setup as described in our previous report.42 In detail, the laser flash photolysis experiments were carried out with a TSP-1000 timeresolved spectrophotometer (Unisoku). A 10 Hz Q-switched Nd:YAG (Continuum Minilite II) laser with the third harmonic at 355 nm (ca. 4 mJ per 5 ns pulse) was employed for the excitation light. The probe beam from a halogen lamp (OSRAM HLX64623) was guided with an optical fiber scope to be arranged in an orientation perpendicular to the exciting laser beam. The probe beam was monitored with a photomultiplier tube (Hamamatsu R2949) through a spectrometer (Unisoku MD200) for the decay profile and the transient absorption spectroscopy of HT and BINOL, and monitored with a multichannel spectrophotometer (Unisoku MSP-1000V1) for the transient absorption spectroscopy of HH, HT, and TT. An inner-cell type high pressure optical measurement cell (light pass length, 4 mm; Syn-Corporation Ltd. PCI-500) and a high pressure hand pump (Syn-Corporation Ltd. HP-500) were used for all measurements.

structural changes associated with photochromic reactions of bridged imidazole dimers has not been revealed in detail. Generally, the investigation of pressure effects on chemical reaction rates provides important insight into the conformational changes during chemical reactions.44−49 The activation volume ΔV‡ estimated from the pressure dependence of reaction rates brings out the effective information to elucidate the structural changes because (ΔV‡) is generally correlated with the difference in the partial molar volumes between the reactant and transition states. Recently, we demonstrated the pressure dependence of the thermal-back-reaction rates of the biradical species photogenerated from the bridged imidazole dimer derivatives to understand the relationship between the bridging-structure and the applied pressure.50 The thermal back-reaction of the biradical species photogenerated from pseudogem-bisDPI[2.2]PC ([2.2]PC-bridged imidazole dimer) is not affected by pressure because the structural change during the thermal back-reaction is small. On the contrary, the 1,1′-bi2-naphthol (BINOL) bridged imidazole dimer (BINOL) shows the pressure dependent photochromism because of the flexibility of the BINOL moiety. The activation entropy (ΔS‡) shows good agreement with the structural flexibility of the bridging moiety and the ΔV‡ for the thermal back-reaction. We revealed that the pressure dependence of the photochromic properties would be predicted from the ΔS‡ value. In this study, we investigate the pressure dependence of the photochromic properties of [2.2]PC-bridged imidazole dimer derivatives (head-to-head form, HH, head-to-tail form, HT, and tail-totail form, TT, Scheme 1). The photochromic properties of the structural isomers are drastically changed by flipping their imidazole rings.32 Moreover, we demonstrated that the compound that possesses small ΔV‡ for the thermal backreaction is suitable for use in the polymer matrix. The investigation of pressure effects on the photochromism of HH, HT, and TT gives deep understanding about the conformational change concomitant with the photochromic reactions.

3. RESULTS AND DISCUSSION 3.1. Pressure Effects on the Transient Absorption Spectra. Figure 1 shows the transient absorption spectra of HH, HT, TT, and BINOL in toluene at 298 K under the pressure range from 0.1 to 400 MPa. Regarding the broad absorption band at around 800 nm of the transient absorption spectrum of HH, it is found to be the characteristic absorption band of the biradical species HHR photogenerated from HH. This absorption band is attributable to the radical−radical interaction due to the face-to-face alignment of the two imidazole rings in the biradical state.51 The π-orbirals of the radical species are perpendicular to the plane of the imidazole rings. The interaction between the π-orbirals of the two

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. pseudogem-BisDPI[2.2]PC (HH) from Kanto Chemical Co., Inc. was used as received. The synthetic procedures for the derivatives of [2.2]PC-bridged imidazole dimers (HT, TT, and BINOL) were followed by same procedures to our previous papers.32,33 These compounds B

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The Journal of Physical Chemistry A

Figure 1. Transient absorption spectra of HH, HT, TT, and BINOL in toluene at 298 K (4 mm light path length; excitation wavelength, 355 nm; pulse width, 5 ns; power, 4 mJ/pulse). Each spectrum was recorded at 0.1 (red), 200 (green), and 400 (purple) MPa.

imidazolyl radicals can be controlled by changing the overlap integral between the π-orbirals. By flipping the imidazole rings of the bridged imidazole dimer, the absorption band derived from the radical−radical interaction decreases and shifts to longer wavelength in the order corresponding to HTR and TTR (the biradical species photogenerated from HT and TT, respectively) owing to the increment in the distance and the displacement between two imidazole rings. As a result, TTR has almost no interaction between the two imidazole rings, whereby TTR exhibits an absorption band similar to that of the typical triphenylimidazolyl radical (TPIR). The biradical species generated from the BINOL bridged imidazole dimer also has a small absorption band at 800 nm at 0.1 MPa because the distance between the two imidazolyl radicals is large in solution due to the flexibility of the BINOL moiety. Under high pressures, the two imidazolyl radicals bridged by the BINOL moiety deduced to be closely spaced and the interaction between the two imidazolyl radicals becomes large, resulting in the increment of the absorption band at around 800 nm, as previously reported (Figure 1d).50 On the contrary, almost no changes were observed in the absorption bands of HHR, HTR, and TTR ascribed to the radical−radical interaction under high pressures, in contrast to that of the BINOL-bridged imidazolyl radicals (Figure 1a−c). It suggests that the imidazole rings bridged by the [2.2]PC moiety has little flexibility whereas the conformation of imidazole rings bridged by the flexible BINOL moiety easily changes depending on the applying pressure. Although both the absorption spectrum and thermal-backreaction rate of HHR are scarcely affected by pressure, only the thermal-back-reaction rates of HTR and TTR distinctly depend on pressure (as described later). Therefore, the geometries of HTR and TTR would change under high pressures, but these results presumably indicate that the structural flexibility of the [2.2]PC moiety is not enough to increase the overlap integral between the two imidazole rings by responding to pressure, or

merely the wave functions between the two imidazole rings would not be in phase due to the flipping of imidazole rings. 3.2. Pressure Effects on the Thermal-Back-Reaction Rate. The pressure dependence of the thermal-back-reaction rates at 298 K are investigated in detail. Because the decay profiles of the absorbance of the photogenerated radical species monitored at 400 nm follow the first-order kinetics under high pressures as demonstrated by the first-order plots, the intramolecular recombination reactions to form the C−N bond would only proceed under high pressures. The transient absorption dynamics of HH shows almost no pressure dependence in the pressure range from 0.1 to 400 MPa (Figure 2a). On the contrary, panels b and c of Figure 2 show the transient absorption dynamics of HT and TT, respectively, in the pressure range from 0.1 to 400 MPa. The thermal backreactions of HTR and TTR are accelerated with increasing pressure in contrast to that of HHR. The activation parameters estimated from Eyring analysis (Supporting Information) are summarized in Table 1. The ΔH‡ value of BINOL is much smaller than that of HH. The transition state of HH would have the steric strain because the two imidazole rings are closely spaced to make the C−N bond. On the contrary, because BINOL is the flexible bridging moiety, the transition state of BINOL can relax the steric strain as compared to HH. Thus, the difference in the enthalpy between the radical and transition states of BINOL is small. In addition, it has been experimentally revealed that the incorporation of the chlorine atom into the 2-phenyl ring of the imidazole ring reduces the ΔH‡.33 We previously demonstrated that the ΔS‡ value for the thermal back-reaction becomes an effective measure to predict the pressure effects.50 Because high pressures increase the viscosity of solvent, the large conformational changes concomitant with the thermal back-reaction of photogenerated biradical species are suppressed under high pressures. Because the thermal back-reaction of HHR possesses a small ΔS‡ value (ΔS‡ = −21.4 J K−1 mol−1 in toluene), the thermal-backC

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Figure 2. Pressure dependence of the transient absorption dynamics of (a) HH, (b) HT, (c) TT, and (d) BINOL monitored at 400 nm in toluene at 298 K. The insets show the plot of ln(kP/k0.1) against pressure in toluene.

contrary, the absolute value of ΔV‡ for the thermal backreaction of BINOL is relatively small despite that the ΔS‡ value is large and negative, compared with that of TT, indicative of the difference in the solvation effects between the photochromism of the [2.2]PC-bridged imidazole dimer and the BINOL-bridged imidazole dimer. As expected from the ΔV‡ values, the photochromism of TT and BINOL would be affected by their surroundings because they show large structural changes during the photochromic reaction. 3.3. Effects of the Viscosity of Polymer Matrix on the Thermal-Back-Reaction Rate. We investigated the thermalback-reaction rates of HHR and TTR in the PMMA/TCP films possessing several Tg to demonstrate that the effect of the free volume around the molecules on the thermal back-reaction is predicted from the ΔV‡. The previously estimated Tg’s of the polymer films are summarized in Table 2. Figure 3 shows the thermal-back-reaction rates of HHR and TTR in the polymer films. The effect of the viscosity of the polymer matrix is small on the thermal-back-reaction rate of HHR, which possesses small ΔV‡. On the contrary, that of TTR possessing large ΔV‡ is strongly affected by the polymer matrix. The thermal-backreaction rates of HHR and TTR do not obey the first-order reaction kinetics in the films with high Tg, and the slow component is observed by the biexponential fitting. The rate constants (k) and the ratio of the fast and slow components for the thermal back-reactions of HHR and TTR in PMMA/TCP

reaction rate of HHR is almost independent of pressure. On the contrary, the difference in the entropy between the initial biradical and the transition state is relatively large in HT and TT (ΔS‡ = −27.1 and −34.5 J K−1 mol−1 in toluene, respectively). The large difference in the entropy suggests large structural changes between the disordered biradical and the ordered transition states of the thermal back-reactions. Thus, the thermal-back-reaction rate of TTR shows distinct pressure dependence, compared with that of HTR. The effects of pressure were observed most clearly in the thermal backreaction of the biradical species bridged by the flexible BINOL moiety as predicted from the large difference in the entropy between the initial biradical state and the transition state (ΔS‡ = −107.8 J K−1 mol−1, Figure 2d).50 ΔV‡ is one of the effective measures to elucidate the structural change associated with chemical reactions because ΔV‡ originates from the difference in the partial molar volumes between the initial state and the transition state. In this study, the ΔV‡ values for the thermal back-reactions were determined from eqs 1−3,46,51 ⎛ ∂ ln kP ⎞ ⎟ ΔV ‡ = −RT ⎜ ⎝ ∂P ⎠T

(1)

ln(kP /k 0.1) = aP + b ln(1 + cP)

(2)

ΔV ‡ = −(a + bc)RT

(3)

where kP is the rate constant at pressure P. The insets of Figure 2 show the plot of ln(kP/k0.1) against pressure for HT, TT, and BINOL. The ΔV‡ values for the thermal back-reactions are summarized in Table 1. Both ΔS‡ and ΔV‡ decrease in the order corresponding to HH, HT, TT, and BINOL. In particular, the ΔS‡ values of HH, HT, and TT exhibit reasonable correlation with the ΔV‡ values. This result indicates that we can quantitatively presume the degree of conformational changes during the photochromic reactions from the activation parameters for the structural isomers. On the

Table 1. Activation Parameters for the Thermal BackReactions of Photogenerated Biradical Species in Toluene

D

compd

ΔH‡/ kJ mol−1

ΔS‡/ J K−1 mol−1

ΔG‡/ kJ mol−1

ΔV‡/ cm3 mol−1

HH HT TT BINOL

60.1 53.6 49.4 19.1

−21.4 −27.1 −34.5 −107.8

66.5 61.7 59.6 51.3

−3.8 −11.1 −14.4

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The Journal of Physical Chemistry A Table 2. Rate Constants for the Thermal Back-Reaction of HHR and TTR in PMMA/TCP Films ratio of TCP/wt %

Tg/°C

80 70 60 50 40 36

−54 −49 −48 −37 −28 −8

k1,HHR/s−1 21.85 22.34 21.38 19.05 26.57 20.81

(100%) (100%) (100%) (100%) (64%) (87%)

k2,HHR/s−1

k1,TTR/s−1

5.69 (36%) 3.74 (13%)

196.5 (100%) 108.5 (100%) 94.28 (67%) 91.47 (62%) 47.39 (28%) 1053.4 (49%)

k2,TTR/s−1

23.92 12.76 3.62 6.07

(33%) (38%) (72%) (51%)

the BINOL moiety changes in response to pressure because the distance between the radicals are closely spaced under high pressures. The ΔV‡ estimated from the pressure dependence of the thermal-back-reaction rate of the biradical species shows good correlation to the ΔS‡ value, which indicates that we can predict the degree of structural changes during the photochromic reaction from the ΔS‡ value. Moreover, the ΔV‡ value for the thermal back-reaction is one of the effective measures to evaluate the efficiency of the photochromic reaction in the polymer matrix to apply the fast switchable photochromic compound to the ophthalmic lenses. These results open up the effective development of the pressure-sensitive photochromic compounds and will offer the fundamental information about the molecular dynamics of the bridged imidazole dimers, to develop attractive photochromic materials.

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ASSOCIATED CONTENT

* Supporting Information S

Kinetics for the thermal back-reaction, including first-order kinetic plots, first-order rate constants, and Eyring plots. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Figure 3. Thermal back-reaction kinetics of (a) HHR and (b) TTR in the PMMA films doped with tricresyl phosphate (TCP), as a plasticizer, with different ratios (80, 70, 60, 50, 40, and 36 wt %).

AUTHOR INFORMATION

Corresponding Author

*J. Abe. E-mail: [email protected]. Tel: +81-42-7596225. Author Contributions

films with several Tg are summarized in Table 2. The slow component of HHR is observed above Tg = −28 °C with increasing Tg even though that of TTR is observed above Tg = −48 °C. In addition, the ratio of the fast component for the thermal back-reaction of TTR distinctly decreases with increasing the Tg except the component in the film with Tg = −8 °C. It is obvious that the large structural change of TTR expected from the ΔV‡ is difficult to proceed in the rigid polymer film. On the contrary, because the conformational relaxation of TTR to the face-to-face alignment after the C−N bond cleavage reaction would be suppressed and the conformation is destabilized in the higher Tg due to the smaller free-volume, the very fast component (k = 1053 s−1) is observed in the film with Tg = −8 °C. Therefore, the photochromic compounds possessing small ΔV‡ is suitable for the use in the solid films.

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by the Core Research for Evolutionary Science and Technology (CREST) program of the Japan Science and Technology Agency (JST) and a Grantin-Aid for Scientific Research on Innovative Areas “Photosynergetics” (No. 26107010) from MEXT, Japan.

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4. CONCLUSION We investigated the pressure dependence of the thermal radical recombination reactions of bridged imidazole dimer derivatives. The absorption spectra of the photogenerated biradical species bridged by the [2.2]PC moiety are not affected by pressure because two imidazole rings are strongly constrained by the [2.2]PC moiety. On the contrary, the radical−radical interaction between the two imidazolyl radicals bridged by E

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DOI: 10.1021/jp511074y J. Phys. Chem. A XXXX, XXX, XXX−XXX