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.
2.3.2 Cycloreversion Reaction
In contrast to the cyclization reaction described in Section 2.3.1, it is difficult to directly measure the time constant of the ring‐opening (cycloreversion) reaction, because the absorption spectrum of the photogenerated open‐ring isomer and that of the closed‐ring isomer overlap in the UV region. The time constant was estimated from the decay of the transient absorption spectra as well as the fluorescence signal of the closed‐ring isomer.
Figure 2.12 shows time‐resolved transient absorption spectra of the closed‐ring isomer of 1,2‐bis(2‐methyl‐5‐phenyl‐3‐thienyl)perfluorocyclopentene (20c) in a n‐hexane solution excited with a femtosecond laser pulse at 600 nm [12]. Time evolution of the spectra following the excitation can be divided into three stages. In the time region immediately after the excitation (0–0.5 ps), the absorption at 860 nm appears together with the negative signal due to the bleaching of the steady‐state absorption at around 600 nm. In the initial 0.5‐ps time range, the intensity of the absorption at 860 nm slightly decreases and that at 890 nm relatively increases, together with the appearance of the absorption around 700 nm. The absorption at 860 nm is due to the initially formed 1B state, while the subsequently growing bands at 890 and 700 nm are attributable to the 2A state. In the following second stage at the time region of 0.5–5 ps, the relative ratio of the transient absorption at 890 and 700 nm to that at 860 nm gradually increases. Finally, after 5 ps following the excitation, the decay of the positive and negative absorption intensities to the baseline was observed with no remarkable evolution of the spectral band shape.
Figure 2.12 Time‐resolved transient absorption spectra of 20c in n‐hexane excited with a 600 nm femtosecond laser pulse.
Source: Reprinted with permission from Ref. [12]. Copyright 2017 American Chemical Society.
The time profiles of the transient absorbance monitored at 840 and 700 nm are reproduced by a triple exponential function with time constants of 200 fs, 3.0 ps, and 12 ps. As shown in Figures 2.7 and 2.8, the 1BFC(c) state produced by photoexcitation of the closed‐ring isomer 20c undergoes the internal conversion to the local minimum of 2A state (2Ac) via 1B/2A CI(c). The 2Ac state goes over the energy barrier, 2ATS1, in competition with the direct non‐radiative as well as radiative decay processes to the ground state. According to this mechanism, the shortest time constant of 200 fs is attributed to the internal conversion from the 1B state to the 2Ac state. The bands at 890 and 700 nm are ascribable to the 2Ac state, because the rise of these bands is accompanied by the decay of the band at 860 nm assigned to 1B state. To elucidate the origin of the second 3‐ps component, the time profile of the transient absorbance at 700 nm divided by that at 840 nm was plotted. The relative intensity rapidly increased within 1 ps and was followed by the gradual increase in several picosecond time region. This time profile indicates that the rapid internal conversion yields the 2Ac state with the time constant of 200 fs and the slow 3‐ps process leads to the spectral evolution in the 2Ac state. The time constant of 3.0 ps is attributed to the geometrical rearrangement in the excited state, because 3 ps is in the typical temporal scale of vibrational cooling in solution. Finally, the longest time constant of 12 ps is ascribable to the lifetime of the 2Ac state, because the recovery of the transient bleaching of the ground state and the decay of the positive transient absorption have the same time constant of 12 ps. Temperature dependence of the transient absorption decay and the fluorescence decay at 700 nm was measured and it was found that both transient absorption