Efficient Cycloreversion Reaction of a Diarylethene Derivative in

Letter pubs.acs.org/JPCL

Efficient Cycloreversion Reaction of a Diarylethene Derivative in Higher Excited States Attained by Off-Resonant Simultaneous TwoPhoton Absorption Hikaru Sotome,† Tatsuhiro Nagasaka,† Kanako Une,† Chiaki Okui,† Yukihide Ishibashi,†,‡ Kenji Kamada,§ Seiya Kobatake,# Masahiro Irie,*,¶ and Hiroshi Miyasaka*,† †

Division of Frontier Materials Science and Center for Advanced Interdisciplinary Research, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ‡ Department of Applied Chemistry, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan § National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-5877, Japan # Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sumiyoshi, Osaka 558-8585, Japan ¶ Department of Chemistry and Research Center for Smart Molecules, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan S Supporting Information *

ABSTRACT: Off-resonant excitation of the closed-ring isomer of a photochromic diarylethene derivative at 730 nm induced the efficient cycloreversion reaction with a yield of ∼20%, while the reaction yield was only 2% under onephoton excitation at 365 nm. Excitation wavelength dependence of the onephoton cycloreversion reaction yield under steady-state irradiation in a wide wavelength range showed that the specific electronic state leading to the large cycloreversion reaction yield, which is originally forbidden in the optical transition but partially allowed owing to the low symmetry of the molecule, is spectrally overlapped with the electronic state accessible by the allowed onephoton optical transition in the UV region. Femtosecond transient absorption spectroscopy also revealed that the off-resonant two-photon excitation preferentially pumped the molecule into the specific state, leading to the 10fold enhancement of the cycloreversion reaction.

E

isomers and has been attracting much attention from various viewpoints of photofunctional materials.12 In some derivatives, the stepwise two-photon excitation at 532 nm enhances the cycloreversion reaction yield up to ∼50%, while the yield is ≤1% under visible one-photon excitation. Importantly, this marked enhancement of the reaction is not obtained under ultraviolet (UV) one-photon excitation corresponding to the same energy as the two-photon process. These results indicate that the two-photon allowed state plays a key role in this efficient reaction, leading to high-order photofunctions.13 In the present work, we have studied of f-resonant simultaneous two-photon excitation leading to the efficient cycloreversion reaction of a typical diarylethene derivative, PT (Figure 1a), in the higher excited state. Off-resonant twophoton excitation produces specific one-photon forbidden excited states more directly than the stepwise two-photon process. Moreover, simultaneous two-photon excitation has an

lectronic excited states produced by light absorption play important roles in various processes, such as photonenergy conversion, natural and artificial photosynthesis, photofunctional materials, and so forth. Most primary photochemical reactions, in principle, proceed in the lowest singlet excited state (S1 state) in the condensed phase because higher excited states are rapidly relaxed into the S1 state via internal conversion within a few picoseconds, leading to the loss of excitation energy originally obtained by the photoabsorption, particularly in large molecules consisting of more than 10−20 atoms (Kasha’s rule).1 This fundamental restriction, which lies behind conventional photochemistry, also inhibits the utilization of a variety of electronic states inherently existing in molecules in the wide energy range. Several photochemical responses specific to the higher excited states, such as photoisomerization,2−7 electron/energy transfer,8,9 and proton transfer,10,11 have been recently reported as exceptions of Kasha’s rule. For the photoisomerization reaction, we have so far reported the stepwise two-photon cycloreversion (ring-opening) reaction of diarylethene derivatives.2,3 A series of these compounds shows a reversible photoisomerization reaction between the closed- and open-ring © XXXX American Chemical Society

Received: June 2, 2017 Accepted: July 5, 2017 Published: July 5, 2017 3272

DOI: 10.1021/acs.jpclett.7b01388 J. Phys. Chem. Lett. 2017, 8, 3272−3276

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is ≤1% in the 600−650 nm wavelength range, it shows a subtle increase as the excitation wavelength becomes shorter toward the UV region. This excitation wavelength dependence can be interpreted as follows. The photoexcitation at shorter wavelengths in the visible region produces the 2A state with excess thermal energy after ultrafast 1B → 2A internal conversion, and the excess energy assists the 2A molecules in overcoming the activation barrier, leading to an increase of the reaction yield.21 This interpretation is also rationalized by the temperature dependence of the reaction yield (Figure S1 in the Supporting Information). Although the reaction yield increases with shortening of the excitation wavelength also in the UV region, it shows decreases at around 310 and 370 nm corresponding to the two absorption peaks of PT(c) in the UV region. This negative correlation between the reaction yield and the absorption maxima strongly suggests that the two excited states leading to the different reactivity are spectrally overlapped with each other in the UV region. That is, the strong allowed transition into the electronic state resulting in lower reactivity behaves as an intrinsic filter, inhibiting the weak transition into the specific electronic state, leading to the large reaction yield. Although optical transitions are generally regulated by the selection rule and could be classified such as into one- and two-photon transitions, it is partially broken for molecules with lower symmetry. The excitation wavelength dependence in Figure 1b suggests that the weak transition originally forbidden in the selection rule was partially allowed. In general, it is expected that the electronic state originally inaccessible by the one-photon absorption is more easily accessible by the two-photon absorption. Accordingly, to directly elucidate the mechanism underlying in the excitation wavelength dependence in Figure 1b, we examined the cycloreversion reaction of PT(c) by off-resonant two-photon excitation at 730 nm, whose two-photon energy is almost the same as that in the one-photon energy of the 370 nm absorption band. Prior to a detailed discussion on the reaction dynamics, we show the excitation intensity dependence of the transient absorbance of PT(c) in n-hexane in Figure 2a, where the transient absorbance at the monitoring wavelength of 600 nm at 2 ps following irradiation is plotted as a function of excitation intensity of the femtosecond laser pulse at 730 nm. The transient signal at 600 nm corresponds to ground-state bleaching. The slope of this dependence is ∼1.8. This quadratic dependence clearly shows that the ground-state molecules are pumped up into the excited state upon two-photon excitation.

Figure 1. (a) Chemical structure of a diarylethene derivative, PT, and its photochromic reactions. (b) Excitation wavelength dependence of the cycloreversion reaction yields (red circle) of PT(c) in n-hexane solution under the one-photon excitation condition. Steady-state absorption spectra of PT(c) (blue solid curve) and PT(o) (black dotted curve). Spectra of excitation pulses at 730 and 365 nm in the present study are also shown.

additional advantage for application to three-dimensional microfabrication14,15 and multilayered optical storage memory.16−18 In the following, we discuss the initial dynamics and chemical reactivity in the higher excited states by comparing time-resolved data obtained by two-photon excitation with that by the one-photon excitation with an equivalent photon energy. Figure 1b shows steady-state absorption spectra of the closed- and open-ring isomers in n-hexane solution. While the open-ring isomer, PT(o), has absorption only in the UV region below 360 nm, the closed-ring isomer, PT(c), shows three absorption peaks in the UV−visible regions, at around 310, 370, and 575 nm. The 575 nm band in the visible region mainly corresponds to the transition from the 1A (ground state) to the 1B state. The excitation wavelength dependence of the one-photon cycloreversion reaction yield is shown in Figure 1b. The detailed method for the derivation of the reaction quantum yield was described previously.19,20 Although the reaction yield

Figure 2. (a) Excitation power dependence of transient absorbance changes of PT(c) in n-hexane solution at 2 ps after excitation with a femtosecond laser pulse at 730 nm. The monitoring wavelength was 600 nm. (b) Correlation trace of the permanent bleaching induced by off-resonant simultaneous two-photon absorption as a function of time intervals between two 730 nm excitation pulses. A signal from neat solvent is also shown for comparison. (c) Temporal sequence of excitation and probe pulses used in the correlation measurements. 3273

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Figure 3. Transient absorption spectra of PT(c) in n-hexane solution excited at 365 nm (a) and those at 730 nm (b). The spectra at around 730 nm are not shown due to strong scattering of the excitation pulse at 730 nm. The spectral features denoted by asterisks are due to stimulated Raman scattering of the solvent. The excitation powers were 170 and 740 nJ for excitation at 365 and 730 nm, respectively.

Figure 4. Time profiles of transient absorbance changes of PT(c) in n-hexane solution excited at 730 and 365 nm in the early state after excitation (a) and those in a wide temporal window (b). Monitoring wavelengths were 590 and 540 nm for (a) and (b), respectively.

n-hexane solution excited with a femtosecond laser pulse at 365 and 730 nm, respectively. The 365 nm excitation is rigorously resonant with the 370 nm absorption band. For the 365 nm excitation, an absorption peak appeared at 590 nm with broad absorption in the range of 400−1000 nm at the time origin. This 590 nm peak is ascribed to absorption from the higher excited state (Sn state) produced by the 365 nm excitation because it was not observed under one-photon excitation in the visible region.2−5 This peak disappeared in the subpicosecond time region, and the corresponding rise was observed in the whole spectral region. The resultant spectra are characterized by a negative signal due to ground-state bleaching in the 510− 640 nm range and positive bands in other regions. This spectral shape is similar to that obtained by the visible one-photon excitation at 600 nm, although it is slightly broader probably due to larger excess vibrational energy in the 2A state (S1 state). These bands decrease on the time scale of a few tens of ps. At 100 ps, following the excitation, little bleaching signal remained, which is consistent with the small one-photon cycloreversion reaction yield of ∼2%. On the other hand, band shapes of transient spectra by twophoton excitation at 730 nm are distinctly different from those by 365 nm one-photon excitation in the whole time window. Although the spectra around the time origin are largely contaminated by the coherent artifact due to solvent response, the spectrum at 200 fs shows absorption bands in the 650−800 nm range, which is largely different from that by 365 nm

In addition, we measured the permanent bleaching signal of the closed-ring isomer excited with two sequential excitation pulses at 730 nm. In this measurement, the time interval between the two pulses was scanned, as shown in Figure 2c, and the permanent bleaching was monitored at 400 ps after irradiation of the first excitation pulse, at which the cycloreversion reaction is perfectly completed. The correlation trace in Figure 2b shows that the reaction amount, that is, the permanent bleaching signal, shows the maximum at Δt = 0 with a correlation width of ∼130 fs fwhm. Because this value is in good agreement with the autocorrelation width (140 fs fwhm) of the excitation pulse, the cycloreversion reaction is induced by simultaneous two-photon absorption. It was also confirmed that this signal does not arise from coherent artifacts due to the solvent by comparing with the signal of neat solvent under the same experimental setup. Efficient excitation of PT(c) is due to the rather large two-photon cross section at 730 nm. The Zscan measurement revealed that the two-photon absorption cross section of PT(c) was 380 GM at 730 nm (Figure S2 in the SI), of which the value is larger than that of typical organic dyes due to the wavelength at 730 nm being close to the onephoton absorption band.22,23 In order to directly investigate the dynamics and reactivity by two-photon excitation at 730 nm, we applied transient absorption spectroscopy and compared the result with that by the UV one-photon excitation at 365 nm. Figure 3a,b, respectively, presents transient absorption spectra of PT(c) in 3274

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∼20%. This result shows that the yield is significantly enhanced by a factor of ∼10 compared to that by one-photon excitation at 365 nm, ∼2%. Reaction pathways leading to enhancement in the cycloreversion quantum yield are worth mentioning. As shown in Figures 3 and 4, the relaxed state from the highly excited state (Sn′ state) produced by two-photon absorption at 730 nm is different from the 2A state (S1 state) produced by one-photon absorption. In a previous study on stepwise two-photon excitation using a picosecond 532 nm pulse, it was revealed for PT(c) that the relaxed state around the S1 state from the highly excited state by stepwise two-photon absorption did not significantly contribute to the cycloreversion reaction.3 This result indicates that at least one excited state exists around the 2A state of PT(c), and this state has low cycloreversion reactivity. The presence of the excited state with low reactivity has been found also in the stepwise two-photon cycloreversion reaction of several diarylethene derivatives.3,24,25 These results imply that the lower excited state around the S1 state after relaxation from the higher excited state by two-photon 730 nm excitation does not contribute to enhancement of the cycloreversion reaction. It is worth noting the branching ratio at the conical intersection of PT. The cyclization (ring-closing) reaction yield of PT(o) is more than 0.6 in solutions and almost unity in the crystalline phase. By taking into account the presence of the conformer (parallel conformer, P conformer) undergoing no cyclization reaction in the solution phase,12 the actual cyclization yield could be estimated to be much larger. The large cyclization yield indicates that branching into the closed-ring isomer is close to almost unity at the conical intersection accessible from the low excited states. These reaction behaviors in lower excited states strongly suggest that the present enhancement in the cycloreversion takes place via some specific channels leading to the open-ring isomer without passing through the S1/S0 conical intersection of the low excited state produced by one-photon absorption. Figure 5 summarizes the cycloreversion reaction dynamics of PT(c) in higher excited states by UV one-photon and off-

excitation. This difference indicates that the excited state (Sn′ state) pumped by two-photon excitation at 730 nm is different from the Sn state attained by one-photon absorption at 365 nm. The absorption band in the 800−900 nm range increases within 1 ps, and the absorption bands in the 650−900 nm range show a rather flat shape in the picosecond time region, which is still different from that observed in the case of the 365 nm excitation. This result indicates that the Sn′ state is relaxed, at least in part, into a lower excited state different from the 2A state. These positive absorption bands decrease into the baseline within several tens of picoseconds, and the permanent bleaching markedly remains at and after 100 ps following the excitation. To quantitatively evaluate the reaction dynamics, we analyzed temporal evolutions of the transient absorption signals. Figure 4a shows the time profile of transient absorbance of PT(c) monitored at 590 nm. The time profile after excitation with a femtosecond 365 nm laser pulse shows ultrafast decay at 590 nm within 1 ps and recovery of the negative signal in several tens of picoseconds. This time profile was reproduced with a triple exponential function. The first time constant was ∼130 fs, which is ascribable to the lifetime of the Sn state. The short lifetime of the high-lying excited state is consistent with those reported for similar diarylethene derivatives.4,5 The second time constant, 3 ps, is attributable to vibrational cooling in the 2A state relaxed from the Sn state because spectral evolution, such as sharpening of the absorption bands in Figure 3a, is a typical feature of vibrational cooling and the time constant is in the range of the cooling process of typical organic compounds in solutions. The final time constant, 12 ps, is almost identical to that of the S1 state by visible one-photon excitation.2,3 Also, for the time profile at 590 nm by two-photon excitation at 730 nm, we could reproduce the time profile in the early stage after excitation with ∼100 fs and 3 ps time constants following the coherent spike at around the time zero. It should be noted that the transient spectra in the few to a few tens of picoseconds time range (Figure 3b) involved some species other than the S1 state (2A), and the spectral shape of the positive absorption bands temporarily evolved in a few tens of picoseconds. Accordingly, we analyzed time profiles with a quadruple exponential function. The decay time constants in a few tens of picoseconds thus obtained were 5 and 12 ps. These time constants were also observed at other wavelengths (Figure S3). The time constant of 12 ps is attributable to the S1 (2A) state, and that with 5 ps could be ascribed to the lifetime of another excited state (S1′ state). After all of the transient species diminished, the permanent bleaching remained, as shown in Figure 3b. Actually, as Figure 4b shows, the normalized profiles with a large constant negative signal in the longer time region indicates that the 730 nm excitation gives rise to a larger reaction amount than that by the 365 nm excitation. To quantitatively evaluate the reactivity under simultaneous two-photon excitation at 730 nm, we calculated the reaction yield defined below (the details are shown in the Supporting Information). ΦHES =

# Newly formed PT(o) # Sn ′ states produced by excitation

Figure 5. Reaction scheme of the cycloreversion reaction of PT(c) in the higher excited states by one-photon excitation at 365 nm and offresonant simultaneous two-photon excitation at 730 nm.

resonant simultaneous two-photon absorption. Photoexcitation at 365 nm first produces the higher excited state (Sn state), showing characteristic absorption at 590 nm. This Sn state is rapidly relaxed into the 2A state with excess vibrational energy within 200 fs and finally leads to the formation of the open-ring isomer with a reaction yield of 2%, which is slightly larger than that obtained by visible excitation at 600 nm (ΦCO ≈ 1%). On the other hand, simultaneous two-photon excitation at 730 nm generates the higher excited state (Sn′ state) distinct from the Sn state. The relaxation from this Sn′ state results in more efficient formation of the open-ring isomer than that with the 365 nm excitation. It is noteworthy that in both excitation

(1)

That is, ΦHES corresponds to the reaction yield for two-photon excitation at 730 nm, and this reaction yield was estimated to be 3275

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The Journal of Physical Chemistry Letters conditions the Sn and Sn′ states are generated with different intensities owing to rather low symmetry of PT(c). As shown in Figure 3, the transient absorption band at 590 nm corresponding to the Sn state produced by the 365 nm onephoton excitation is slightly observed also by the two-photon 730 nm excitation (typically at 200 fs). The mode of the excitation, one- and two-photon excitation, can lead to higher contrast of the reaction leading to efficient control of photochromic reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01388. Detailed description of sample materials, experimental methods, and supplementary steady-state and timeresolved data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.I.). *E-mail: [email protected] (H.M.). ORCID

Seiya Kobatake: 0000-0002-1526-4629 Hiroshi Miyasaka: 0000-0002-6020-6591 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI Grant Numbers JP26107002, JP26107004, and JP26107013 in Scientific Research on Innovative Areas “Photosynergetics”.

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REFERENCES

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