Remarkable Acceleration for Back-Reaction of a Fast Photochromic

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Remarkable Acceleration for Back-Reaction of a Fast Photochromic Molecule Yuka Harada, Sayaka Hatano, Atsushi Kimoto, and Jiro Abe* Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan

ABSTRACT We demonstrate that photochromism based on light-driven reversible C-N single bond cleavage can enable rapid coloration upon UV light irradiation and successive fast thermal back-reaction within tens of microseconds at room temperature. According to Marcus theory, the thermal back-reaction would be accelerated with increasing ΔG0, which is closely linked to the decrease in ΔG‡. We have considered that the ΔG0 of the thermal back-reaction could be enlarged by destabilizing the colored species and designed pseudogem-DPI-PI[2.2]PC, with a [2.2]paracyclophane moiety that couples diphenylimidazole and phenanthroimidazole groups. The present study demonstrates that controlling the stability of the biradical state is effective in accelerating the thermal back-reaction for the photochromic [2.2]paracyclophane-bridged imidazole dimer. SECTION Kinetics, Spectroscopy

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onsiderable interest has been focused on organic photochromic materials that change their color upon irradiation with light; the photogenerated species can be reversed to the initial species either thermally or by subsequent irradiation with a specific wavelength of light.1-3 In particular, thermally reversible photochromic molecules offer the opportunity to change and reset the molecular properties by simply turning a light source on and off. However, the thermal back-reaction of colored species toward their colorless form is generally on the time scale of tens of seconds to minutes, which precludes their practical use in certain applications, such as optical data processing and light modulators.4,5 Increasing the thermal bleaching rate for thermally reversible photochromic molecules is essential for the development of revolutionary optical switching devices. In this study, we demonstrate a novel photochromic molecule, the colored species of which have a half-life in solution of 35 μs at room temperature. We recently developed two novel fast photochromic molecules, 1-NDPI-8-TPI-naphthalene6 and pseudogem-bisDPI[2.2]PC7 (Scheme 1a and b), which show instantaneous coloration upon exposure to UV light and rapid fading in the dark. Both of the photochromic molecules show photoinduced homolytic bond cleavage of the C-N bond between the imidazole rings and successive fast C-N bond formation. The back-reaction is not accelerated by irradiation with visible light. The half-lives of the colored species at 25 °C of 1-NDPI-8-TPI-naphthalene and pseudogem-bisDPI[2.2]PC in benzene are 170 and 33 ms, respectively. Fast thermal bleaching kinetics enable a solution color change only where it is irradiated with UV light because the thermal bleaching rate is much faster than the diffusion rate of the colored species at room temperature. It should be noted that the afterimage seen in the photochromic reaction of 1-NDPI-8-

r 2010 American Chemical Society

TPI-naphthalene is invisible to the human eye in the case of pseudogem-bisDPI[2.2]PC. In view of the thermal bleaching rate, the fast photochromism of pseudogem-bisDPI[2.2]PC is applicable to real-time image processing at video frame rates. Thus, molecular design based on the photochromism of hexaarylbiimidazole (HABI)8-14 can lead to the development of a new family of photochromic compounds with unprecedented switching speeds and remarkable stability, which could eventually evolve into solid-state photonic materials with unique photoresponsive characters. Photochromic materials showing such intense photocoloration and fast thermal bleaching performance are promising materials for prospective fast light modulator applications. The remarkable stability of the colored species generated from the photochromic reaction of pseudogem-bisDPI[2.2]PC can be attributed to the inhibition of the diffusion of reactive radicals and rapid geminate recombination in the nascent radical pair. However, a molecular design that significantly accelerates the thermal back-reaction is necessary for practical use in fast light modulators. A significant feature of the synthetic character of [2.2]paracyclophane-bridged bisimidazole is a stepwise formation of two imidazole rings by the condensation reaction between aromatic 1,2-diketones and two aldehyde groups of pseudogem-bisformyl[2.2]paracyclophane as a starting material. (Scheme S1, Supporting Information) This type of stepwise reaction permits rational design for achieving desired performances such as thermal bleaching rate, coloring, and photosensitivity.

Received Date: February 18, 2010 Accepted Date: March 11, 2010 Published on Web Date: March 16, 2010

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DOI: 10.1021/jz100228w |J. Phys. Chem. Lett. 2010, 1, 1112–1115

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Scheme 1. Photochromism of (a) 1-NDPI-8-TPI-naphthalene, (b) Pseudogem-bisDPI[2.2]PC, and (c) Pseudogem-DPI-PI[2.2]PC

Figure 1. Crystal structure of pseudogem-DPI-PI[2.2]PC with thermal ellipsoids (50% probability). The hydrogen atoms and the solvent molecules are omitted, and nitrogen atoms are highlighted in blue.

did not form a colorless PC-bridged phenanthroimidazole dimer. The molecular structure of pseudogem-DPI-PI[2.2]PC revealed by X-ray crystallographic analysis is shown in Figure 1. Two enantiomeric forms of the molecule are found to make up the racemic crystal, and the C-N bond length connecting two imidazole rings (1.484(2) Å) is approximately equal to that of pseudogem-bisDPI[2.2]PC (1.4876(15) Å). Upon irradiation with UV light in solution, no color change is discernible at room temperature, but the solution changes from colorless to blue at liquid nitrogen temperature. That is, the thermal back-reaction is too rapid for detection by a human eye at room temperature. Laser flash photolysis measurements confirm the photochromic properties of pseudogem-DPI-PI[2.2]PC with the formation of a transient colored species. Figure 2a shows the transient vis-NIR absorption spectra of pseudogem-DPI-PI[2.2]PC in benzene at 25 °C, excited with a nanosecond laser pulse at 355 nm. A sharp absorption band at 400 nm and a broad absorption band ranging from 450 to 1000 nm can be ascribed to the colored biradical, pseudogem-DPIR-PIR[2.2]PC. Indeed, the transient absorption spectra are almost identical to those for pseudogembisDPI[2.2]PC. All of the absorption bands decay with the same time constant, indicating the presence of a single conformation for the colored species. The half-life of the colored species is 35 μs at 25 °C. Figure 2b shows the time profiles of the transient absorbance at 400 nm, measured at temperatures ranging from 5 to 40 °C. The thermal bleaching process obeys first-order kinetics, and the half-life of the colored species varies from 110 μs at 5 °C to 17 μs at 40 °C. It should be emphasized that the original state is fully restored within 200 μs in benzene at 25 °C. This fast colorationdecoloration cycle would make it possible to operate light modulator applications at a repetition frequency of 5 kHz. The enthalpies and entropies of activation (ΔH‡ and ΔS‡, respectively) for the thermal back-reaction were estimated from Eyring plots over temperatures ranging from 5 to 40 °C. The Eyring plots produce an excellent straight line (Figure S7, Supporting Information), and the ΔH‡ and ΔS‡ values estimated from standard least-squares analysis of the Eyring plots are 35.4 kJ mol-1 and -44.1 J K-1 mol-1, respectively. The free-energy barrier (ΔG‡ = ΔH‡ - TΔS‡) is 48.6 kJ mol-1 at 25 °C.

Although Marcus theory was originally developed for electron-transfer reactions,15-17 it has been successfully applied to a wide variety of organic reactions.18-21 Following standard Marcus theory, an increase in the change in Gibbs energy, ΔG0, between the reactant and the product would lead to a decrease in the free energy of activation, ΔG‡, and consequently, the rate constant for the reaction would be accelerated. Thus, Marcus theory gave us crucial insight into controlling the rate of the radical recombination reaction. We have considered that the ΔG0 of the thermal-back reaction could be enlarged by destabilizing the colored species and designed pseudogem-DPI-PI[2.2]PC (Scheme 1c), with a [2.2]paracyclophane (PC) moiety that couples diphenylimidazole and phenanthroimidazole groups. The steric repulsion between the rigid phenanthroimidazole group and the phenyl rings facing each other should destabilize the biradical state (pseudogem-DPIR-PIR[2.2]PC), whereas the rotational motion along the C-C single bond between the imidazole group and the phenyl ring in pseudogem-bisDPIR[2.2]PC should relax the steric hindrance between the phenyl rings facing each other. Since it was expected that the steric repulsion between the phenanthroimidazole groups introduced on both sides of the geminal position of [2.2]paracyclophane should enhance the destabilization of the biradical state, we attempted to synthesize a PC-bridged phenanthroimidazole dimer. We successfully synthesized PC-bridged bisphenanthroimidazole via the reaction of pseudogem-bisformyl[2.2]paracyclophane and 9,10-phenanthrenequinone. However, the green-colored biradical species obtained by the chemical oxidation of PC-bridged bisphenanthroimidazole


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Table 1. Kinetic Parameters Associated with the Thermal Back-Reactions at 298 K k s-1

ΔH‡ kJ mol-1

pseudogem-DPI-PI[2.2]PC

1.96 104a

35.4 (37.8)b

-44.1 (-16.8)

48.6 (42.8)

not measured (83.7)

pseudogem-bisDPI[2.2]PC

2.06 101

59.8 (70.3)

-19.1 (-10.5)

65.5 (73.4)

not measured (63.3)

compound

a

ΔS‡ J mol-1 K-1

ΔG‡ kJ mol-1

ΔG0 kJ mol-1

First-order rate constants measured in benzene solution. b The calculated values obtained by the DFT M05-2X/6-31G(d) are given in parentheses.

Figure 3. Energy diagrams for the thermal back-reactions of pseudogem-bisDPI[2.2]PC and pseudogem-DPI-PI[2.2]PC calculated by the DFT M05-2X/6-31G(d). The optimized structures and energies relating to pseudogem-DPI-PI[2.2]PC are emphasized in blue.

recombination reaction. The molecular geometries of biradicals, imidazole dimers, and transition states (TS) were fully optimized by using the highly parametrized empirical M052X function with the 6-31G(d) basis set.23 The M05-2X function is known to give improved performance for energies of reactions involving radicals for barrier heights of radical reactions.24 The stationary nature of the structures is confirmed by harmonic vibrational frequency calculations, that is, equilibrium species possess all real frequencies, whereas transition states possess only one imaginary frequency. The zero-point energy (ZPE) corrections were obtained at the same level of theory. The calculated kinetic parameters are listed in Table 1 along with those obtained by the kinetic analysis of the absorbance decay of the biradicals. The calculated values are in fair agreement with the experimental values, except for the entropies of activation, ΔS‡. To clarify the energy levels of the thermal back-reaction, the relative free energies, ΔGrel, are depicted in Figure 3. The absolute energies for pseudogem-DPI-PI[2.2]PC and pseudogembisDPI[2.2]PC could not be compared, owing to the differences in the numbers of electrons and nuclei. Since we are focusing on the relative free energy between the biradical and the imidazole dimer, both of the free energies of the imidazole dimers are set to zero in Figure 3. According to Marcus theory, the thermal back-reaction would be accelerated with increasing ΔG0, which is closely linked to decreasing ΔG‡. Indeed, the relative free-energy level of the biradical (83.7 kJ mol-1) of pseudogem-DPI-PI[2.2]PC is higher than that (63.3 kJ mol-1) of pseudogem-bisDPI[2.2]PC, whereas the relative free-energy level of the transition state (126.5 kJ mol-1) of the former is lower than that (136.7 kJ mol-1) of the latter. Thus, it can be seen that the difference in the rate of the back-reaction is

Figure 2. (a) Transient vis-NIR absorption spectra of pseudogemDPI-PI[2.2]PC in degassed benzene at 25 °C (3.4 10-5 M; light path length: 10 mm). Each of the spectra was recorded at 20 μs intervals after excitation with a nanosecond laser pulse (excitation wavelength, 355 nm; pulse width, 5 ns; power, 8 mJ/pulse). (b) Time profiles of transient absorbance of the colored species generated from pseudogem-DPI-PI[2.2]PC, monitored at 400 nm in degassed benzene (3.6 10-4 M). The measurements were performed at temperatures ranging from 5 to 40 °C.

It is worth noting that the thermal back-reaction of pseudogem-DPI-PI[2.2]PC is accelerated about 1000 times compared with that of pseudogem-bisDPI[2.2]PC, while maintaining its optical density in the colored state. Moreover, ΔOD values immediately after laser excitation are not influenced by a temperature change, as can be seen in Figure 2b. These photochromic behaviors demonstrate the distinguishing feature of the PC-bridged imidazole dimer. The dissociation of the imidazole dimer into the geminate radical pair occurs via the singlet excited state, and the repulsive nature of the potential energy surface along the C-N bond elongation axis is considered to lead to a high quantum yield (close to unity) from analogy with the conventional imidazole dimer (HABI derivative).22 Theoretical investigations in the gas phase were conducted to confirm our theory regarding controlling the rate of radical


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related to the difference in ΔG0, as expected from Marcus theory. Thus, the present study demonstrated that controlling the stability of the biradical state is effective in controlling the thermal bleaching rate for the photochromic PC-bridged imidazole dimer. The method for controlling the stability of the biradical state is not restricted to utilizing the steric effect since the electronic effect induced by substituent groups would also be effective. As compared with another photochromic system, PC-bridged imidazole dimers are characterized by their diverse molecular design. Moreover, the thermal bleaching rates are found to be predictable from the DFT M052X calculations. We can prepare a wide variety of PC-bridged imidazole dimer derivatives by selecting appropriate aromatic 1,2-diketones to meet requirements. We believe that this work will lead to an exciting new avenue in the future development of photochromic molecules.

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SUPPORTING INFORMATION AVAILABLE Synthesis of

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pseudogem-DPI-PI[2.2]PC, experimental details of spectroscopic measurements, Eyring plots, crystallographic data in CIF format, and details of DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: (18)

*To whom correspondence should be addressed. E-mail: jiro_abe@ chem.aoyama.ac.jp.

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ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Science Research in a Priority Area “New Frontiers in Photochromism (No. 471)” 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.

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Remarkable Acceleration for Back-Reaction of a Fast Photochromic

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