Femtosecond dynamics of the ring closing process of diarylethene: a case study of electrocyclic reactions in photochromic single crystals

The cyclization reaction of the photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene was studied in its single crystal phase with femtosecond transient absorption spectroscopy. The transient absorption measurements were performed with a robust acquisition scheme that explicitly exploits the photoreversibility of the molecular system and monitors the reversibility conditions. The crystalline system demonstrated 3 × 10(4) repeatable cycles before significant degradation was observed. Immediately following photoexcitation, the excited state absorption associated with the open-ring conformation undergoes a large spectral shift with a time constant of approximately 200 fs. Following this evolution on the excited state potential energy surface, the ring closure occurs with a time constant of 5.3 ps, which is significantly slower than previously reported measurements for similar derivatives in the solution phase. Time resolved electron diffraction studies were used to further demonstrate the assignment of the transient absorption dynamics to the ring closing reaction. The mechanistic details of the ring closing are discussed in the context of prior computational work along with a vibrational mode analysis using density functional theory to give some insight into the primary motions involved in the ring closing reaction.

Photochromism of diarylethene single molecules and single crystals

Photochromism is characterized as a reversible colour change of a chemical species upon photoirradiation. Although vast numbers of photochromic molecules have been so far synthesized, molecules which exhibit thermally irreversible photochromic reactivity are limited within a few families of compounds. Among the thermally irreversible photochromic molecules, diarylethene derivatives are the most promising candidates for applications to opto-electronic devices because of their fatigue resistance, high sensitivity and rapid response. In this review, characteristic photochromic performances of the derivatives, detection and analysis of the photochromic reactions at a single-molecule level and application of the single-crystalline photochromism to molecular machinery will be described.

Photochromism of diarylethene molecules and crystals

Photochromism is defined as a reversible transformation of a chemical species between two isomers upon photoirradiation. Although vast numbers of photochromic molecules have been so far reported, photochromic molecules which exhibit thermally irreversible photochromic reactivity are limited to a few examples. The thermal irreversibility is an indispensable property for the application of photochromic molecules to optical memories and switches. We have developed a new class of photochromic molecules named "diarylethenes", which show the thermally irreversible photochromic reactivity. The well designed diarylethene derivatives provide outstanding photochromic performance: both isomers are thermally stable for more than 470,000 years, photoinduced coloration/decoloration can be repeated more than 10(5) cycles, the quantum yield of cyclization reaction is close to 1 (100%), and the response times of both coloration and decoloration are less than 10 ps. This review describes theoretical background of the photochromic reactions, color changes of the derivatives in solution as well as in the single crystalline phase, and application of the crystals to light-driven actuators.

Photochromic diarylethene molecules and crystals

Some of the diarylethene derivatives were found to undergo photoisomerization reactions between two isomers even in the single-crystalline phase. The two isomers have different electronic as well as geometrical structures. The electronic structure change induces the reversible color change of the crystal. Single crystals composed of three different kinds of diarylethenes having similar geometrical structures, bisoxazolylethene, bisthiazolylethene, and bisthienylethene have been prepared. The crystals exhibited multicolor photochromism. The colorless crystals turned yellow, red, and blue upon irradiation with the appropriate wavelength of light. The geometrical structure change of component diarylethene molecules in crystals induces reversible surface morphology as well as shape and size changes of the crystals. The photoinduced geometrical structure changes of the molecules in the densely packed crystals were successfully linked to the macro-scale mechanical motion of the solid crystals and resulted in the net task of launching a tiny silica particle.

One- and multi-photon cycloreversion reaction dynamics of diarylethene derivative with asymmetrical structure, as revealed by ultrafast laser spectroscopy

A cycloreversion reaction of a photochromic diarylethene derivative, 1-(2-methyl-3-benzothienyl)-2-(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene, with asymmetrical structure, in n-hexane solution, was investigated by means of picosecond and femtosecond laser spectroscopic methods. Femtosecond laser spectroscopy revealed that the excited state (S(1)) of the closed form with a lifetime of 1.3 ps undergoes the cycloreversion reaction in competition with the internal conversion and the apparent reaction yield was independent of the excitation intensity. On the other hand, picosecond laser excitation at 532 nm led to the drastic enhancement of the cycloreversion reaction yield by a successive two-photon absorption process via the S(1) state, leading to higher excited states with a large cycloreversion yield of 0.60 +/- 0.1. For the multiphoton-enhanced cycloreversion reaction, we reported that the efficient cycloreversion reaction of a diarylethene derivative with C(2v) symmetry could occur by stepwise, visible two-photon excitation but did not take place by UV one-photon absorption to a higher excited state. In the present asymmetrical system, both UV one-photon and visible two-photon absorption opened the efficient cycloreversion reaction. Similarities and differences of the reaction profiles in higher excited states between diarylethene derivatives with symmetrical and asymmetrical structures are discussed from the viewpoint of the selection rule of optical transition and its dependence on molecular structures.

5-n-Butyl-4-[2-(2-ethyl-1-benzothiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopent-1-en-1-yl]thiophene-2-carbaldehyde

The title compound, C₂₄H₂₀F₆OS₂, exhibiting photochromic behaviour, has thienyl and benzothienyl substituents attached to the double-bond C atoms of the envelope-shaped cyclo­pentene ring. The mean planes of aromatic systems form dihedral angles of 43.0 (1) (thien­yl) and 73.8 (1)° (benzothien­yl) with the mean plane of the C—C=C—C portion of the cyclo­pentene ring. This conformation avoids steric hindrance between the n-butyl and ethyl substituents. The formyl substituent of the thienyl group, as well as the ethyl substituent of the benzothienyl group, are disordered [occupancies of 0.788 (17):0.212 (17) and 0.64 (5):0.36 (5), respectively].

Synthesis, crystal structure and optoelectronic properties of a new unsymmetrical photochromic diarylethene

A new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (1a), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its optical and electrochemical properties, including photochromic reactivity both in solution and in the solid state (PMMA film and the single-crystalline phase), fluorescence and electrochemical properties were investigated in detail. The compound showed excellent photochromism even in the single-crystalline phase by photo-irradiation. In acetonitrile, the open-ring isomer of diarylethene 1 exhibited relatively strong fluorescence at 470 nm when excited at 300 nm, and its emission intensity decreased along with the photochromism upon irradiation with 313 nm light. Its closed-ring isomer showed almost no fluorescence. The electrochemical properties of diarylethene were investigated by performing cyclic voltammetry experiment and its HOMO and LUMO energy level were calculated.

1-(2-Chloro-phen-yl)-2-(2-methyl-5-phenyl-3-thien-yl)-3,3,4,4,5,5-hexa-fluoro-cyclo-pent-1-ene: a new photochromic diaryl-ethene

The title compound, C(22)H(13)ClF(6)S, is a hybrid diaryl-ethene derivative with one 3-thienyl substituent, and a Cl-substituted six-membered aryl unit bonded to the double bond of a hexa-fluoro-cyclo-pentene ring. In the crystal structure, the mol-ecule adopts a photo-active anti-parallel conformation that can undergo effective photocyclization reactions. The distance between the two reactive C atoms is 3.848 (3) Å. The dihedral angles between the least-squares cyclo-pentene plane and those of the adjacent thio-phene and chloro-phenyl rings are 49.39 (8) and 59.88 (8)°, respectively. The F atoms are disordered over two positions, with site occupancy factors of 0.6 and 0.4.