ARTICLE pubs.acs.org/JPCA
Photochromic Properties of [2.2]Paracyclophane-Bridged Imidazole Dimer with Increased Photosensitivity by Introducing Pyrenyl Moiety Hiroaki Yamashita† 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 ‡ Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan
bS Supporting Information ABSTRACT: The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. A new [2.2] paracyclophane-bridged imidazole dimer, pseudogem-PPI-DPI[2.2]PC, with high photosensitivity to UVA radiation was developed. To enhance the photosensitivity, we introduced pyrenyl moieties to the [2.2]paracyclophane-bridged imidazole dimer. The localized π π* transition of pyrenyl moieties appears in the UVA radiation region by introducing a pyrenyl moietiy on the 4-position of the imidazole rings. The expansion of the π-electron system also affects the absorption spectrum of the colored species. The broad absorption band of the colored species covers the whole range of visible light region and its absorbance is approximately equal throughout the visible light region. Thus, pseudogem-PPI-DPI[2.2]PC shows the photochromic reaction coloring black upon light irradiation and successive fast thermal bleaching following the monoexponential kinetics with a time constant of 12 ms at room temperature.
1. INTRODUCTION Considerable attention has been paid to the organic photochromic materials that change their color upon light irradiation. The photogenerated colored-species can be reversed to the initial colorless-species either by thermal means or by subsequent irradiation with a specific wavelength of light. The thermally reversible photochromic molecules are allowed to change and reset the molecular properties by simply turning a light source on and off.1 The development of fast photochromic materials is a major challenge for the photochemistry and material science because of their potential applications including smart windows, solar protection lenses and decorative objects. We have recently developed a unique series of photochromic [2.2]PC-bridged imidazole dimer, with a [2.2]paracyclophane ([2.2]PC) moiety that couples two diphenylimidazole groups, that shows instantaneous coloration upon exposure to UV light and rapid fading in the dark.2 Upon UV light irradiation, the C N bond between two imidazole rings of the [2.2]PC-bridged imidazole dimer is homolytically cleaved to give a pair of imidazolyl radicals and the color of the solution changes from colorless to blue. The blue color fades very rapidly following monoexponential thermal bleaching kinetics after ceasing the light irradiation. Thus, the molecular design based on the photochromism of hexaarylbiimidazole (HABI)3 would be a very promising candidate for the development of a new family of photochromic compounds with unprecedented switching speeds, which could eventually evolve into solid state photonic materials with unique photoresponsive characters. r 2011 American Chemical Society
The increase in the photosensitivity to solar ultraviolet A (UVA) radiation region is essential for the applications in smart windows and ophthalmic glasses. HABIs can be sensitized by a large number of sensitizing dyes. HABI-based initiator systems, which are sensitive to visible light, have been developed for laser imaging and other radiation sources with high visible emission. The sensitization reaches out into the near-infrared region. For example, aryl ketones and p-(dialkylamino)aryl aldehydes are known as efficient HABI sensitizers.4 On the other hand, we have successfully achieved the enhancement of the photosensitivity to UVA light with the help of the charge-transfer (CT) transition.2d,g The [2.2]PC-bridged imidazole dimer has two types of imidazole rings (Im1 and Im2) as shown in Scheme 1. It should be noted that the two imidazole rings are not equivalent in their electronic environment. Im1 is a resonant planar structure that has a typical bond distance for a 6π electron system with an electron-donating characteristic, whereas Im2 has two localized CdN double bonds and one sp3 carbon connecting Im1, consistent with a 4π-electron system with an electron-withdrawing characteristic. By introducing electron-donating dimethoxy groups on the phenyl rings attached to the electron-withdrawing Im2, we confirmed the appearance of the CT transition band from the electron-donating dimethoxysubstituted phenyl rings to Im2 by the TDDFT calculation.2g This strategy of the molecular design for the enhancement of the Received: May 12, 2011 Revised: September 19, 2011 Published: October 08, 2011 13332
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The Journal of Physical Chemistry A Scheme 1. Photochromic Reaction of [2.2]Paracyclophane-Bridged Imidazole Dimers
photosensitivity was proved to be quite effective even for the polymer systems carrying [2.2]PC-bridged imidazole dimer.2c In this study, we propose another approach to enhance the photosensitivity to UVA light based on the introduction of larger aromatic groups as a sensitizing unit. As described in our previous paper, the introduction of a pyrenyl group on the 2-position of the imidazole ring of hexaarylbiimidazole was found to induce the CT-like transition from the frontier molecular orbitals delocalized over the pyrenyl moieties and Im1 to the LUMO delocalized mainly in Im2 on the basis of the TDDFT calculations.5 Moreover, the intense absorption band attributable to the localized π π* transition band of the pyrenyl moieties was revealed to enhance the photosensitivity. Here we developed a new type of [2.2]PC-bridged imidazole dimer introduced a pyrenyl group on the 4-position of imidazole ring and investigated the photochromic properties of the molecule.
2. EXPERIMENTAL SECTION 2.1. Synthesis. All reactions were monitored by thin-layer chromatography using 0.2 mm E. Merck silica gel plates (60F254). Column chromatography was performed on silica gel (Wakogel C-300). All reagents were purchased from TCI, Wako Co. Ltd., Aldrich Chemical Co., Inc., and Acros Oraganics, and were used without further purification. 1H NMR spectra were recorded at 500 MHz on a JEOL JMN-ECP500A spectrometer. Chemical shifts are reported in parts per million (ppm) relative to tetramethylsilane, and the coupling constants (J) are reported in hertz (Hz). FAB mass spectra were measured with an MStation MS-700 (JEOL) spectrometer using 3-nitrobenzyl alcohol as a matrix. 1-Phenylethynylpyrene (1) and 1-(1-pyrenyl)-2-phenylethane1,2-dione (2) were synthesized according to Scheme 2. pseudogemPPI-DPI[2.2] (5) was synthesized according to Scheme 2 using pseudogem-[4-formyl-13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]paracyclophane (3) as a starting material. pseudogem-[4-Formyl13-(4,5-diphenyl-1H-imidazol-2-yl)][2.2]paracyclophane2b (3) was prepared according to a literature procedure.
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1-Phenylethynylpyrene (1). 1-Bromopyrene (3.67 g, 13.1 mmol), ethynylbenzene (1.43 mL, 13.0 mmol), Pd(PPh3)4 (0.274 mg, 0.237 mmol), and copper iodide (0.306 mg, 1.61 mmol) were refluxed in Et3N (60 mL) for 48 h. After cooling to room temperature, the reaction mixture was concentrated in vacuo. The crude mixture was purified with silica gel column chromatography using hexane/CHCl3 = 5/1 as eluent to give a yellow powder (1), 3.85 g (12.7 mmol, 98%). 1H NMR (500 MHz, DMSO-d6), δ: 8.68 (d, J = 9.0 Hz, 1H), 8.25 8.19 (m, 4H), 8.15 (d, J = 6.0 Hz, 1H), 8.11 (d, J = 8.5 Hz, 1H), 8.07 (d, J = 10.0 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.74 7.73 (m, 2H), 7.54 7.52 (m, 1H), 7.46 7.40 (m, 3H). FAB MS: m/z 302 [M + H]+. 1-(1-Pyrenyl)-2-phenylethane-1,2-dione (2). To a solution of 1 (4.03 g, 13.3 mmol), sodium hydrogen carbonate (1.30 g, 15.4 mmol), and tetraethylammonium bromide (2.80 g, 13.3 mmol) in CH2Cl2 (100 mL) was added the potassium permanganate (6.40 g, 40.5 mmol) in water, and the reaction mixture was vigorously stirred for 40 h. Saturated solution of sodium hydrogen sulfite in water was added to quench excess oxidant. The reaction mixture was filtered, and the residue was washed with CH2Cl2 and water. The organic layer was separated, dried over Na2SO4, and concentrated in vacuo. The residue was purified with silica gel column chromatography hexane/CHCl3 = 5/1 as eluent to give a orange powder (2), 2.48 g (7.43 mmol, 56%). 1H NMR (500 MHz, DMSO-d6), δ: 9.49 (d, J = 9.8 Hz, 1H), 8.61 8.24 (m, 8H), 8.07 (d, J = 7.9 Hz, 2H), 7.82 (t, J = 7.0 Hz, 1H), 7.66 (t, J = 7.0 Hz, 2H). FAB MS: m/z 335 [M + H]+. pseudogem-(4,5-Diphenyl-1H-imidazol-2-yl-4-pyrenyl-5-phe nyl-1H-imidazol-2-yl)[2.2]paracyclophane (4). 3 (513 mg, 1.13 mmol), 2 (584 mg, 1.75 mmol), and ammonium acetate (957 mg, 12.4 mmol) were refluxed in acetic acid (5 mL) for 24 h. After cooling to room temperature, the reaction mixture was neutralized with aqueous NH3. The resulting precipitate was dissolved CH2Cl2 and washed with water. The organic layer was dried over Na2SO4, filtered, and concentrated in vacuo. The crude mixture was purified with silica gel column chromatography using CH2Cl2 as eluent to give a yellow powder 780 mg (the mixture of structural isomers). This was used in the next step without further purification. FAB MS: m/z 769 [M+H]+. pseudogem-PPI-DPI[2.2]PC. All manipulations were carried out with the exclusion of light. Under nitrogen, to a solution of 4 (765 mg, 0.995 mmol) in benzene (40 mL) was added the solution of potassium ferricyanide (8.24 g, 25.0 mmol) and KOH (4.85 mg, 86.4 mmol) in water (50 mL), and the reaction mixture was vigorously stirred for 30 min at room temperature. The organic layer was separated, exhaustively washed with water, and concentrated in vacuo. Then the crude mixture of the structural isomers was purified with silica gel column chromatography hexane/AcOEt = 3/2 as eluent. The separated pseudogem-PPIDPI[2.2]PC was recrystallized from CH2Cl2/hexane to give yellow crystal, 90 mg (0.117 mmol, 12%). 1H NMR (500 MHz, CDCl3), δ: 8.63 (d, J = 9.2 Hz, 1H) 8.12 8.08 (m, 2H), 8.02 (d, J = 9.2 Hz, 1H), 8.00 7.90 (m, 3H), 7.86 (d, J = 7.9 Hz, 1H), 7.70 (d, J = 7.9 Hz, 1H), 7.47 7.40 (m, 3H), 7.36 7.31 (m, 3H), 7.27 7.26 (m, 2H), 7.20 7.17 (m, 3H), 7.11 7.07 (m, 3H), 6.91 6.82 (m, 3H), 6.71 6.82 (m, 3H), 6.71 6.64 (m, 2H), 6.56 (d, J = 7.3, 1H), 6.50 6.48 (m, 1H) 4.68 4.63 (m, 1H), 3.58 3.53 (m, 1H), 3.35 3.19 (m, 4H), 3.11 3.06 (m, 2H). FAB MS: m/z 767 [M + H]+. 13333
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Scheme 2. Synthetic Procedure of pseudogem-PPI-DPI[2.2]PC
2.2. X-ray Crystallographic Analysis. The diffraction data of the single crystal were collected on the Bruker APEX II CCD area detector (Mo Kα, λ = 0.710 73 nm). During the data collection, the lead glass doors of the diffractometer were covered to exclude the room light. The data refinement was carried out by the Bruker APEXII software package with SHELXT program.6 All non-hydrogen atoms were anisotropically refined. 2.3. DFT Calculation. All calculations were carried out using the Gaussian 03 program (Revision E.01).7 The molecular structures were fully optimized at the MPW1PW91/6-31G(d) level of the theory, and analytical second derivatives were computed using vibrational analysis to confirm each stationary point to be a minimum. The TDDFT calculations were performed at the MPW1PW91/6-31 +G(d) level of the theory for the optimized geometries. 2.4. Measurement. Time-resolved vis NIR absorption spectra were recorded on a Unisoku TSP-1000 time-resolved spectrophotometer. A Continuum Minilite II Nd:YAG (Q-switched) laser with the third harmonic at 355 nm (ca. 8 mJ per 5 ns pulse) was employed for the excitation light. The probe beam from an Osram HLX64623 halogen lamp 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 Hamamatsu R2949 photomultiplier tube through a spectrometer (Unisoku MD2000). The steady state UV vis absorption spectra were carried out with a Shimadzu UV3150 spectrometer.
3. RESULTS AND DISCUSSION 3.1. X-ray Crystallographic Analysis. The molecular structure of pseudogem-PPI-DPI[2.2]PC was determined by single
crystal X-ray crystallographic analysis (Figure 1). The C N bond length connecting two imidazole rings (1.487(3) Å) is approximately equal to that of pseudogem-bisDPI[2.2]PC (1.4876(15) Å). The imidazole ring attached with the pyrenyl moiety acts as an electron-donating unit. The pyrenyl moiety is linked to the 4-position of the electron-donating Im1. The bond distances of the pyrenyl moiety are almost equal to those of unsubstituted pyrene. In addition, we could not observe a significant difference in the bond lengths in the [2.2]PC moiety, Im1, Im2, and the phenyl rings of pseudogem-PPI-DPI[2.2]PC compared with those of pseudogem-bisDPI[2.2]PC. Thus, it is expected that the pyrenyl moiety is relatively weakly coupled with other parts in the molecule. 3.2. UV Vis Absorption Spectra. Figure 2 shows the UV vis absorption spectra of pseudogem-PPI-DPI[2.2]PC and pseudogem-bisDPI[2.2]PC in benzene at room temperature, along with the calculated oscillator strength indicated by the red perpendicular lines for pseudogem-PPI-DPI[2.2]PC. The experimental absorption spectrum is in good agreement with the theoretical one. Pseudogem-bisDPI[2.2]PC does not show a distinct absorption band in the UVA light region, whereas pseudogem-PPI-DPI[2.2]PC shows an intense absorption band in the UVA light region. The S0 f S1 transition at 501 nm of pseudogem-PPI-DPI[2.2]PC demonstrated by the TDDFT calculation is attributable to the HOMO f LUMO transition. The oscillator strength for this transition is almost zero (f = 0.0002) due to the small orbital overlap between HOMO localized on Im1 and LUMO localized on Im2. The S0 f S5 transition at 356 nm of pseudogem-PPI-DPI[2.2]PC can be described by the HOMO f LUMO+1 transition, which is characterized by the 13334
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Figure 2. UV vis absorption spectra for pseudogem-PPI-DPI[2.2]PC and pseudogem-bisDPI[2.2]PC and TDDFT calculations for pseudogemPPI-DPI[2.2]PC. The calculated spectra (MPW1PW91/6-31+G(d)// MPW1PW91/6-31G(d)) are shown by the red perpendicular lines. Figure 1. ORTEP representation of the molecular structure of pseudogem-PPI-DPI[2.2]PC with thermal ellipsoids (50% probability), where nitrogen atoms are highlighted in blue. The hydrogen atoms are omitted for clarity.
localized π π* transition of the pyrenyl moiety (Figure 3). The large orbital overlap between HOMO and LUMO+1 results in the large oscillator strength (f = 0.6019). Indeed, the photosensitivity to UVA radiation can be improved by displacing the phenyl ring by the pyrenyl moiety comparing with that of the parent compound, lacking the sensitizing group. Our previous work demonstrated that the introduction of larger aromatic groups, instead of phenyl rings, is an effective method to improve the photochromic properties of HABI derivatives that have higher sensitivity and an absorption band in the longer wavelength region.5 The absorption tail in the UV vis absorption spectrum of the HABI derivative with a pyrenyl unit on the 2-position of the imidazole rings (Py-HABI) is fairly long and it extends even beyond 450 nm. This long absorption tail can be assigned to the CT-like transition from the frontier molecular orbital delocalized over the pyrenyl moiety and Im1 to the LUMO delocalized mainly in Im2 on the basis of the TDDFT calculations. In fact, Py-HABI shows coloration by the visible light irradiation. The time-resolved transient absorption spectroscopy and fluorescence measurements showed that the pyrenyl unit influences the dissociation reaction rate by modifying the surface crossing between the attractive potential surface of the locally excited state of pyrenyl moiety and the repulsive potential surface along the C N bond elongation axis. Though the pyrenyl moiety is introduced on the 4-position of the imidazole ring for pseudogem-PPI-DPI[2.2]PC, we consider that the difference in the location of the pyrenyl group would not be an essential matter. Thus, we considered that the photosensitivity of pseudogem-PPI-DPI[2.2]PC would also be enhanced in a similar manner as Py-HABI. Consequently, we concluded that the introduction of a pyrenyl group is an effective method for enhancing the photosensitivity to the UVA radiation region for the photochromic [2.2]PC-bridged imidazole dimer. 3.3. Laser Flash Photolysis Measurement. The photochromic property of pseudogem-PPI-DPI[2.2]PC was investigated by laser flash photolysis measurement. The solution of pseudogemPPI-DPI[2.2]PC rapidly reaches the photostationary state by the
Figure 3. Relevant molecular orbitals of pseudogem-PPI-DPI[2.2]PC obtained at the MPW1PW91/6-31+G(d) level.
UV light irradiation and the photogenerated species immediately turns back to the initial colorless state according to monoexponential kinetics when UV irradiation ceases. The transient vis NIR absorption spectra of the colored species, pseudogem-PPIRDPIR[2.2]PC, of the parent imidazole dimer, pseudogem-PPIDPI[2.2]PC, are shown in Figure 4. A sharp absorption band at 400 nm and a broad absorption band ranging from 500 to 13335
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Figure 4. Transient vis NIR absorption spectra of pseudogem-PPIRDPIR[2.2]PC in degassed benzene at 25 °C (2.1 10 5 M, light path length 10 mm). Each of the spectra was recorded at 2 ms intervals after excitation with a nanosecond laser pulse (excitation wavelength, 355 nm; pulse width, 5 ns; power, 8 mJ/pulse).
1000 nm can be ascribed to the colored species. The broad absorption band covers whole range of visible light region and its absorbance is approximately equal throughout the visible region. Thus, pseudogem-PPI-DPI[2.2]PC shows instantaneous coloration from colorless to black upon UV light irradiation. Previously reported [2.2]PC-bridged imidazole dimers show the color change from colorless to blue or green due to the presence of a valley around 500 nm in the transient vis NIR absorption spectra, whereas pseudogem-PPIR-DPIR[2.2]PC colors black in benzene. It is worth noting that pseudogem-PPI-DPI[2.2]PC colors black in a single component system. In our previous work, we reported that the absorption band of pseudogem-bisDPIR[2.2]PC at longer wavelength regions is attributable to the intramolecular radical radical interaction.2f The introduction of the pyrenyl moiety would affect the spin density distribution and intramolecular radical radical interaction. Indeed, pseudogem-PPIR-DPIR[2.2]PC absorbs the longer-wavelength light than pseudogem-bisDPIR[2.2]PC. The TDDFT calculation for pseudogem-PPIR-DPIR[2.2]PC was carried out to explain this red-shift in absorption spectrum as shown in Figure 5. The blue and red curves indicate the experimental vis NIR absorption spectrum of pseudogem-bisDPIR[2.2]PC and pseudogem-PPIRDPIR[2.2]PC, respectively. The red perpendicular lines are the theoretical oscillator strength for pseudogem-PPIR-DPIR[2.2]PC. The S0 f S1 transition at 1047 nm (f = 0.0071) and the S0 f S2 transition at 936 nm (f = 0.0834) are described by the αHOMO f αLUMO and βHOMO f βLUMO transitions of pseudogem-PPIR-DPIR[2.2]PC. These transitions are characterized by intramolecular CT transition from the electron-donating pyrenyl moiety to the 5π-electron imidazole ring. Figure 6 shows decay profiles of the colored species pseudogemPPI-DPI[2.2]PC. The thermal bleaching process obeys the firstorder kinetics, and the half-life of the colored species is 12 ms in benzene at 25 °C. The thermal bleaching rate of pseudogem-PPIRDPIR[2.2]PC is larger than that of pseudogem-bisTPIR[2.2]PC (33 ms in benzene at 298 K), indicating that the introduction of pyrenyl moiety can preserve the rapid thermal bleaching characteristic to the [2.2]PC-bridged imidazole dimer. The decay profiles were measured over the temperature range from 5 to 40 °C. The activation parameters for the thermal back-reaction were estimated from the Eyring plots. The Eyring plots gave an excellent straight line, and ΔH‡ and ΔS‡ values were estimated from the standard least-squares analysis of the Eyring plots. The
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Figure 5. Transient vis NIR spectra for pseudogem-PPIR-DPIR[2.2]PC and pseudogem-bisDPIR[2.2]PC and the theoretical spectrum by the TDDFT calculations for pseudogem-PPIR-DPIR[2.2]PC. The calculated spectra (UMPW1PW91/6-31+G(d)//UMPW1PW91/6-31G(d)) are shown by the red perpendicular lines.
Figure 6. Decay profiles of the colored species pseudogem-PPIR-DPIR[2.2]PC monitored at 400 nm in degassed benzene (2.1 10 5 M, light path length 10 mm). The measurements were performed in the temperature range from 5 to 40 °C.
enthalpy of activation ΔH‡ is 55.0 kJ mol 1, and the entropy of activation ΔS‡ is 26.6 J mol 1 K 1, respectively. The free energy barrier (ΔG‡ = ΔH‡ T ΔS‡) at 25 °C is 63.0 kJ mol 1. The acceleration of the radical recombination reaction rate was explained by the Marcus theory in our previous paper.2b,d According to the standard Marcus theory, an increase in the change in Gibbs free energy, ΔG0, between the reactant and the product leads to a decrease in the free energy of activation, ΔG‡. We have considered that the radical recombination reaction is accelerated by the destabilization of the colored species. Indeed, the radical recombination reaction rate of the colored species of pseudogem-DPI-PI[2.2]PC with a [2.2]PC moiety that couples dipheylimidazole and phenanthroimidazole groups, is accelerated about 1000 times compared with that of pseudogem-bisDPIR[2.2]PC. The steric repulsion between the phenanthroimidazole and the phenyl rigns facing each other should destabilize the colored species. Consequently, the radical recombination reaction rate is accelerated as explained by the Marcus theory. Because the pyrenyl moiety is also sterically bulky, we considered that the above discussion for the acceleration for the backreaction of the colored species of pseudogem-DPI-PI[2.2]PC can be applied to that of pseudogem-PPI-DPI[2.2]PC. To explain the acceleration of the radical recombination reaction of pseudogem-PPIR-DPIR[2.2]PC, the Gibbs free energy of pseudogemPPIR-DPIR[2.2]PC and pseudogem-bisDPIR[2.2]PC were theoretically investigated by the DFT calculation. Because the 13336
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The Journal of Physical Chemistry A absolute free energy could not be compared due to the difference in the number of electrons and nuclei between them, the relative free energies calculated from the difference between the free energy of the colored species and that of the imidazole dimer were compared. As mentioned in the Experimental Section, the molecular structures were fully optimized at the MPW1PW91/6-31G(d) level of the theory, and the zero-point energy (ZPE) correction was obtained at the same level of theory. The relative free energy of pseudogem-PPIR-DPIR[2.2]PC was 51.3 kJ mol 1, whereas that of pseudogembisDPIR[2.2]PC was 23.2 kJ mol 1. Therefore, the relative free energy of pseudogem-PPIR-DPIR[2.2]PC is higher than that of pseudogem-bisDPIR[2.2]PC. Thus, the radical recombination reaction rate of pseudogem-PPI-DPI[2.2]PC can be expected larger than that of pseudogem-bisDPI[2.2]PC by following the Marcus theory.
4. CONCLUSIONS We have demonstrated that the introduction of a pyrenyl moiety is effective method for enhancing the photosensitivity to UVA light of the photochromic [2.2]PC-bridged imidazole dimer. The localized excitation of the pyrenyl moiety with large oscillator strength (f = 0.6019) leads to an increase in the photosensitivity to UVA light. In addition, the introduction of a pyrenyl moiety would affect the absorption spectrum of the colored species. The broad absorption band in the UVA radiation region results in black colored species fading in a time constant of 12 ms at room temperature. Moreover, it is revealed that the absorption band at longer wavelength region reaches more than 1000 nm due to the intramolecular CT from the pyrenyl moiety to 5π-electron imidazole ring. Thus, the present study demonstrated the advantage of the introduction of pyrenyl moiety on the 4-position of imidazole ring of [2.2]PC-bridged imidazole dimer. We believe that the diversity of the molecular design makes this class of photochromic molecules highly attractive for the applications including smart windows, solar protection lenses and decorative objects. ’ ASSOCIATED CONTENT
bS
1
H-NMR spectra of pseudogem-PPI-DPI[2.2]PC, HPLC chromatograms, X-ray crystallographic analysis data, kinetics for the thermal back-reaction, details of the DFT calculations, optimized structures, and atomic coordinates. CIF file. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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’ ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (A) (No. 22245025) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and by a High-Tech Research Center project for private universities with the matching fund subsidy from MEXT, and by the NAIST Advanced Research Partnership Project. 13337
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