Читать книгу Diarylethene Molecular Photoswitches - Masahiro Irie - Страница 17

As discussed in the theoretical study in Section 2.2, diarylethenes are anticipated to undergo very fast ring‐closing (cyclization) reactions. The rapid cyclization dynamics was confirmed using femtosecond absorption spectroscopies in solution as well as in the single crystalline phase [9].

The cyclization reaction of 1,2‐bis(2,4‐dimethyl‐5‐phenyl‐3‐thienyl)perfluoro‐cyclopentene (5) was investigated in the single crystalline phase. In solution, the open‐ring isomer has two conformations, photoactive antiparallel and photoinactive parallel ones, as shown in Scheme 2.4. Coexistence of the two conformations in solution prevents clear resolution of spectral features associated with the photocyclization reaction. On the other hand, in the single crystalline phase of 5o, all molecules are fixed in the photoactive antiparallel conformation and this simplifies the analysis of reaction dynamics.

Scheme 2.4 A diarylethene open‐ring isomer has two conformations with two aryl groups in mirror symmetry and C₂ symmetry, which are called parallel and antiparallel conformations, respectively.

Figure 2.9 shows the transient absorption spectra associated with the cyclization reaction in the single crystalline phase upon irradiation with a 343 nm femtosecond laser pulse. The formation of the spectrum ascribed to the closed‐ring isomer is clearly observed. After approximately 20 ps, the transient spectrum is essentially fully converged to that of the final product. The subpicosecond dynamics following excitation with a 343 nm pulse indicates that the initially broad photoinduced absorption seen at 75 fs progressively shifts from the red (635 nm) to the blue (480 nm) end of the visible spectrum in the first 500 fs following the UV excitation. The progressive spectral shift occurs prior to the ring‐closure itself and is attributed to the evolution on the excited state potential of the open‐ring isomer due to the difference in the equilibrium geometry between ground and excited states. The absorption band at 635 nm is assigned to the excited state absorption of the open‐ring isomer initially in the equilibrium geometry associated with ground state charge distribution. The absorption in the blue end is associated with the relaxed structure of the open‐ring isomer in the excited state. The formation of this intermediate occurs with a time constant of 200 fs.

Figure 2.9 Transient absorption spectra of 5 during the ring‐closing reaction in the single crystal excited with a 343 nm femtosecond laser pulse. Selected spectra for various time delays between −200 fs and +50 ps (black curves), as compared to their associated τ_∞ measurements (blue curves). The 100‐fs spectrum was extracted from a separated data set taken with higher resolution time steps.

Source: Adapted with permission from Ref. [9]. Copyright 2011 American Chemical Society.

Figure 2.10a shows the convergence on the picosecond time scale of the transient absorption toward that of the closed‐ring photoproduct. Initially, at 0.5 ps after the pulse the absorption associated with the relaxed open‐ring isomer in the excited state dominates the signal. In the time delay between 0.5 and 20 ps, the spectral feature associated with the intermediate decays in concert with the growth of the ground state absorption associated to the closed‐ring isomer centered at 635 nm. Figure 2.10b shows the transient time traces at 490 and 635 nm. The latter is the central wavelength of the absorption associated with ground state absorption of the closed‐ring isomer and follows the growth of the photoproduct. The former follows the decay of the excited state absorption associated with the relaxed open‐ring isomer. The time constants for the rise at 635 nm and the decay at 490 nm are 7.3 ± 0.8 and 5.3 ± 0.3 ps, respectively. It was anticipated that the rise and decay of the signals would be equally matched, as the closed‐ring photoproduct is thought to be directly formed from the relaxed excited state of the open‐ring isomer through the conical intersection. The discrepancy in the time constants is attributed to the vibrational relaxation of the nascent closed‐ring isomer. The growth of the signal at 635 nm is the convolution of the cyclization reaction, vibrational cooling and lattice relaxation. Therefore, the decay of the excited absorption of the open‐ring isomer is assigned to the time constant of the ring‐closing (cyclization) reaction, that is 5.3 ± 0.3 ps. The cyclization time constant in the single crystalline phase is slightly slower than the values in solution, ∼ subpicoseconds. This is presumably attributed due to the impeding influence of the close‐packed environment in crystal.

Figure 2.10 Transient absorption spectra of 5 during the ring‐closing reaction in the single crystal excited with a 343 nm femtosecond laser pulse. (a) Transient absorption spectra for time delay between 0.5 and 50 ps. (b) Transient traces for the probe wavelengths at 635 and 490 nm. Monoexponential fits are shown, with time constants of 7.3 and 5.3 ps for 635 and 490 nm, respectively. The inset demonstrates the sub‐picosecond behavior.

Source: Reprinted with permission from Ref. [9]. Copyright 2011 American Chemical Society.

Although in solution both types of conformers, antiparallel and parallel ones, are equally excited by the pump pulse and the coexistence of the two conformers prevents clear resolution of the transient absorption spectra, it is still possible to appropriately analyze the spectra, because of the distinct dynamics of the two conformers. Femtosecond laser spectroscopy study was carried out in cyclohexane solutions containing 1,2‐bis(2‐methyl‐5‐phenyl‐3‐thienyl)perfluorocyclopentene (20) and nonfluorinated (perhydro) analogue 1,2‐bis(2‐methyl‐5‐phenyl‐3‐theinyl)cyclopentene (21), and the effect of fluorination in the photcyclization reaction was examined [10]. The dynamics after photoexcitation of 20o and 21o can be expressed in three stages: (i) pre‐switching due to the excited‐state mixing and relaxation, (ii) the ring‐closure, and (iii) post‐switching related to the vibrational cooling. In all stages, the fluorinated diarylethene 20o was found to switch faster than its nonfluorinated analogue 21o. The mixing and relaxation time constants of 21, 70 ± 15 and 150 ± 30 fs, respectively, were accelerated to the time constants of 50 ± 10 and 120 ± 30 fs in 20. The ring‐closure reaction rate also increased from 4.2 to 0.9 ps. The results indicate that fluorinated switch is faster and more efficient than the nonfluorinated switch.

The cyclization dynamics of 1,2‐bis(2‐methyl‐1‐benzothiophen‐3‐yl)perfluorocyclopentene (13) was also studied in detail in a n‐hexane solution [11]. Figure 2.11 shows the time profile of the transient absorbance in n‐hexane excited with a 310 nm femtosecond laser pulse. The time profile monitored at 520 nm in the initial 5 ps after the excitation shows that the positive signal appears within the response of the apparatus and gradually rises in a few picosecond time regions. The spectral shape and absorption maximum around 520 nm are the same as those of the ground state absorption of 13c. The solid line is the result calculated with a time constant of 450 fs. On the other hand, the time profiles monitored at 420 and 620 nm decay with the time constants of 480 and 420 fs, respectively. These time constants agree with the rise constant at 520 nm. The decreasing signals observed at 420 and 620 nm are assigned to the decay of the excited state of 13o undergoing the cyclization. The rise and decay profiles indicate that the open‐ring isomer having a broad absorption from 420 to 620 nm converts to the closed‐ring isomer having the absorption maximum at 520 nm with the time constant of 450 fs.

Figure 2.11 Time profiles of transient absorbance of 13 in n‐hexane excited with a 310 nm femtosecond laser pulse. The detection wavelength is 520 nm for (a), 420 nm for (b), and 620 nm for (c), respectively. Solid lines in each of the frames are calculated curves by taking into account the pulse durations and the time constants.

Source: Adapted with permission from Ref. [11]. Copyright 2011 American Chemical Society.

These transient spectroscopic studies in solution indicate that the cyclization reactions take place in less than 1 ps. Although in the crystalline phase the cyclization time constant is slowed to several picosecond region, it is safe to say that the central carbon–carbon bond is made within 20 ps. The very fast cyclization dynamics in solution and in the single crystalline phase is consistent with the prediction of theoretical study.