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Cyclization Reaction Dynamics of a Photochromic Diarylethene Derivative as Revealed by Femtosecond to Microsecond Time-Resolved Spectroscopy Yukihide Ishibashi,† Mika Fujiwara,† Toshiyuki Umesato,† Hisayuki Saito,† Seiya Kobatake,‡ Masahiro Irie,§,* and Hiroshi Miyasaka†,* †
Division of Frontier Materials Science, Graduate School of Engineering Science, Center for Quantum Science and Technology under Extreme Conditions, Osaka University and CREST, JST, Toyonaka, Osaka 560-8531, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sumiyoshi, Osaka 558-8585, Japan § Department of Chemistry, School of Science, Rikkyo University, Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan ABSTRACT: Cyclization reaction of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), in nonpolar alkane solutions with different viscosity was investigated by means of femtosecond-microsecond transient absorption spectroscopy and a time-correlated single-photon counting method. Transient absorption measurements revealed that a ring closure rapidly occurred with a time constant of 450 fs. In addition to this rapid cyclization, transient species with longer lifetimes (ca. 150 ps and ca. 1 μs) were observed. The faster time constant of 150 ps was independent of the solvent viscosity and was assigned to the fluorescence lifetime of a conformer with molecular geometry unfavorable for the ring closure. The longer component was strongly quenched in the solution purged with O2 and was attributed to the triplet state of the open-ring form. Steady-state measurement and nanosecond transient absorption spectroscopy revealed that the cyclization process did not occur via the triplet state of BT. These results indicate that only the rapid reaction taking place in subpicosecond time region was responsible for the cyclization process. The key factors regulating the cyclization reaction of diarylethene derivatives were discussed on the basis of the solvent viscosity dependence, by comparing the present results with those obtained for other diarylethene derivatives.
’ INTRODUCTION Photochromism is a photoinduced reversible transformation in a chemical species between two isomers. The quick change of molecular properties via the photoinduced chemical bond reconstruction has been attracting much attention.1-14 Diarylethene derivatives3,4 are one of the representative organic photochromic molecular systems, where cyclization and cycloreversion reactions between open- and closed-ring isomers take place. Near-coplanar geometry of two heteroaryl groups in the closed-ring isomer leads to the visible absorption owing to the πconjugation throughout the molecule, whereas the open-ring isomer usually has absorption bands only in the UV region. As the correlation diagram of the 6π-electrocyclic reaction3a,15 suggests, both isomers in diarylethene derivatives are generally thermally stable and, in actual, it is estimated for several derivatives that the closed-ring isomer could keep its molecular structure without thermal reaction as long as several thousands years at room temperature.3a For the change of physical and chemical properties accompanied with photochromic reactions, a large number of investigations have been reported for fluorescence spectra,9 refractive indices,10 oxidation/reduction potentials,11 and chiral properties.12 In addition, it was revealed for several derivatives that the photochromic reaction can take r 2011 American Chemical Society
place even in the crystalline phase,13 of which properties have been attracting attention also from the viewpoint of mesoscopic photodriven actuators. Also, for the reaction dynamics of diarylethene derivatives, a number of researches have been accumulated by means of timeresolved detections.16-32 From these results, it was revealed for many diarylethene derivatives that cyclization reactions take place with time constants shorter than several picoseconds. It should be noted that the open-ring isomer of diarylethene derivatives has two conformations; the geometry with two-aryl groups oriented in parallel (Cs symmetry, P form) and that with the two aryl groups in antiparallel orientation (C2 symmetry, AP form).14 These two conformers can be separately detected by steady state NMR spectroscopy14 and, hence, the interconversion time between the two conformers in the ground state is estimated to be much longer than μs. From the fast cyclization time constant and the slow interconversion time, it is inferred that only the C2 conformer (AP form) is responsible for the cyclization in the excited state. Received: December 29, 2010 Revised: February 1, 2011 Published: February 21, 2011 4265
dx.doi.org/10.1021/jp112370a | J. Phys. Chem. C 2011, 115, 4265–4272
The Journal of Physical Chemistry C Scheme 1. Photochromic reactions of BT
For several systems, however, it has been reported that a slow reaction channel also contributes to the cyclization. The ring closing reaction of 1,2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA) was found21 to proceed in several hundred picoseconds time region in addition to the fast reaction taking place within a few picoseconds. The fluorescence lifetime of TMTMA in several hundred picoseconds time region increases together with a decrease in the cyclization reaction yield with increasing solvent viscosity, whereas the cyclization yield in a rapid channel is independent of the solvent viscosity. In addition, it has been reported that some diarylethene derivatives undergo a cyclization reaction in the triplet state. For a diaryletheneruthenium(II) polypyridine dyad, the cyclization reaction takes place via the triplet state of the diarylethene via the energy transfer from the Ru(II)polypyridine chromophore.32 For the diarylethene-perylenebisimide dyads,33 it was also found that the cyclization reaction takes place in the triplet state of the diarylethene unit. Although these results indicate that the cyclization reaction could take place in various channels, factors enabling these reactions, especially slow channels of cyclization, have not yet been elucidated. To unveil these factors, in the present article, we have investigated the cyclization dynamics of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene, BT, by using femtosecond to microsecond transient absorption spectroscopic methods and fluorescence measurements. In addition, BT is a representative diarylethene derivative. Hence, the experimental clarification of the whole picture of cyclization process is important also for the general investigation covering various diarylethene derivatives. In the following, we will show the reaction dynamics and discuss the mechanism of the cyclization reactions in various channels on the basis of the solvent viscosity dependence.
’ EXPERIMENTAL SECTION A photochromic diarylethene derivative, 1,2-bis(2-methyl-3benzothienyl)perfluorocyclopentene (BT) was synthesized and purified. This diarylethene derivative undergoes photochromic reactions between the open- and closed-ring isomers as shown in Scheme 1. Five alkanes with different viscosity were used as solvents. n-Hexane (Wako, infinity pure grade), cyclohexane (Wako, infinity pure grade), and isooctane (Wako, infinity pure grade) were used without further purification. However, transdecalin and cis-decalin (Wako, Special Guarantee) were purified by passing through a column of silica gel (Wako, Wakogel 200). Femtosecond dual NOPA/OPA laser system was used for transient absorption spectroscopy.24 The output of a femtosecond Ti:sapphire laser (Tsunami, Spectra-Physics) pumped by the SHG of a cw Nd3þ:YVO4 laser (Millennia Pro, Spectra-Physics) was amplified with 1 kHz repetition rate by
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using a regenerative amplifier (Spitfire, Spectra-Physics). The amplified pulse (802 nm, 0.9 mJ/pulse energy, 85 fs fwhm, 1 kHz) was divided into two pulses with the same energy (50%). One of the two pulses was guided into a NOPA system (TOPASwhite, Light-Conversion), which covers the wavelength region between 500 and 780 nm with 1-40 mW output energy with ca. 20-40 fs fwhm. The wavelength of the NOPA was tuned at 620 nm in the present study and was frequency-doubled by a 100 μm BBO crystal. After the compression by a prism pair, the SHG in the UV region (310 nm) was used as a pump pulse with the intensity of 0.5 μJ/pulse. Pulse duration at the sample position was estimated to be ca. 80 fs fwhm by FROG signals. The other pulse at 802 nm was guided into an OPA system (OPA-800, Spectra-Physics) and converted to 1200 nm pulse, which was focused into 3 mm CaF2 plate to generate a white-light continuum covering the wavelength region from 350 to 1000 nm. This white light was used as a probe pulse. Polarization angle between the pump and probe pulses was set at the magic angle for all of the measurements. The probe pulse was divided into signal and reference pulses and detected with multichannel photodiode array systems (PMA-10, Hamamatsu) and sent to a personal computer for further analysis. The chirping of the monitoring white-light continuum was corrected for transient absorption spectra. The fwhm of the cross correlation between the pump and probe pulses was ca. 100 fs at the sample position. Picosecond laser photolysis system with a repetitive modelocked Nd3þ:YAG laser was used for transient absorption spectral measurements in picosecond-nanosecond time region.35 Third harmonic laser pulse (355 nm) with 15 ps fwhm was focused into a spot with a diameter of ca. 1.5 mm. Picosecond white-light continuum generated by focusing a fundamental pulse into a 10 cm quartz cell containing D2O and H2O mixture (3:1) was employed as a monitoring light. The sample solution was circulated during the measurement under the repetition rate 600 nm where BT(c) in the ground state has no absorption, indicating that a long-living transient species such as a triplet state is produced. 4267
dx.doi.org/10.1021/jp112370a |J. Phys. Chem. C 2011, 115, 4265–4272
The Journal of Physical Chemistry C
Figure 4. Time profiles of transient absorbance of BT(o) in n-hexane solution, excited with a femtosecond 310 nm laser pulse. The detection wavelength is 520 nm for (a) and (d), 420 nm for (b) and (e), and 620 nm for (c) and (f), respectively. Solid lines in each of the frames are calculated curves by taking into account the pulse durations and the time constants.
Figure 5. Fluorescence time profiles of BT(o) in n-hexane and cisdecalin solutions. Excitation and monitoring wavelengths were 355 and 420 nm, respectively. Solid lines are results obtained by the analysis.
Figure 4 shows the time profiles of the transient absorbance of BT(o) in n-hexane, excited with a femtosecond 310-nm laser pulse. The time profile monitored at 520 nm in the initial 5 ps after the excitation (part a of Figure 4) shows that the positive signal appearing within the response of the apparatus is followed by the gradual rise in a few picoseconds time region. The solid line is the result calculated with the instrumental response and the monophasic rise with a time constant of 450 fs, indicating that the curve thus calculated well reproduces the experimental result. On the other hand, the time profiles monitored at 420 nm (part b of Figure 4) and 620 nm (part c of Figure 4) show rapid decays following the appearance of the positive transient absorbance within the response of the apparatus. Solid lines are results calculated with the instrumental response and the monophasic decay with a time constant of 480 fs for part b of Figure 4 and that, with 420 fs time constant for part c of Figure 4, indicating that these solid lines well reproduce the experimental result. The time constant of ca. 450 fs is in agreement with the rise constant at 520 nm, which corresponds to the production of BT(c). Therefore, the decreasing signals observed in time profiles at 420 and 620 nm are assigned to the decay of the excited state of BT(o) undergoing the cyclization.
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Figure 6. Solvent viscosity dependencies of the cyclization reaction yield (closed circles) and the fluorescence lifetime of BT(o) (open circles) in alkane solutions. A closed circlea is the cyclization reaction yield in n-hexane obtained by other methods.34
Time profiles in the initial 1 ns region are exhibited in parts df of Figure 4. Positive signals at 520 and 620 nm gradually decay in subns time region, whereas a gradual rise was observed at 420 nm. At and after ca. 500 ps, the signals at three monitoring wavelengths remain constant values. Time profiles at and after a few picoseconds following the excitation were reproduced by a single exponential function with the time constant of ca. 150 ps (151 ps at 520 nm, 141 ps at 420 nm, and 165 ps at 620 nm). To precisely elucidate the 150 ps component in the cyclization process, fluorescence dynamics was measured by using the timecorrelated single-photon counting system. Figure 5 shows the fluorescence time profiles of BT(o) in n-hexane and cis-decalin solutions, together with instrumental response function (IRF) curve. Excitation and monitoring wavelengths are respectively 360 and 420 nm. The time profile in n-hexane solution is well reproduced by a single-exponential function with a time constant of (150 ( 10) ps. For the time profile in cis-decalin solution, biphasic decay function with two time constants of (150 ( 10) ps and (1.2 ( 0.1) ns reproduces the experimental result. Preexponential factors for the faster and the slower components are respectively 99% and
