Article pubs.acs.org/JPCC
Cooperative Conformational Change and Excitation Migration of Biphenyl-PMO Amorphous Film, As Revealed by Femtosecond TimeResolved Spectroscopy Yukihide Ishibashi,†,‡,⊥ Tetsuro Katayama,† Hisayuki Saito,† Ken-ichi Yamanaka,§,⊥ Yasutomo Goto,§,⊥ Takao Tani,§,⊥ Tadashi Okada,∥ Shinji Inagaki,*,§,⊥ and Hiroshi Miyasaka*,†,⊥ †
Division of Frontier Materials Science, Graduate School of Engineering Science, Center for Quantum Science and Technology under Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531, Japan ‡ Department of Applied Chemistry, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan § Toyota Central R&D Laboratories., Inc., Nagakute, Aichi 480-1192, Japan ∥ Toyota Physical & Chemical Research Institute, Nagakute, Aichi 480-1192, Japan ⊥ Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST), Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Excited state dynamics of biphenyl-bridged mesoporous organosilica (Bp-PMO) film was investigated by femtosecond transient absorption and dichroism measurements at various excitation intensities. Under the excitation condition with low intensity (ca. 0.1 μJ/pulse), the relaxation from the excited Franck−Condon state with skewed structure of the two phenyl rings to the preplanar state occurred with a time constant of 550 fs, followed by the excimer formation with two time constants of 9.0 and 140 ps. Under higher excitation condition with 1.0 μJ/pulse, very rapid excimer formation within 500 fs was observed. From the analysis of the transient absorption spectra, it was revealed that the cooperative geometrical relaxation from skewed to planar structures, in addition to the energy migration, led to the rapid excimer formation under the high excitation condition. By integrating these results with the fluorescence dynamics, the photoprimary processes in Bp-PMO, such as energy migration, annihilation, and excimer formation, were discussed.
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INTRODUCTION Periodic mesoporous organosilicas (PMOs)1−5 are a new class of functional materials with organic−inorganic hybrid frameworks and well-ordered mesopores. Various functionalities can be designed by proper selection of organic groups incorporated in the framework.6−10 So far, a wide variety of PMOs have been synthesized and found to exhibit excellent chemical, physical, electrical, and optical properties.11−19 Especially, biphenyl (Bp)-bridged PMO was reported to show large fluorescence yield, unique light harvesting functionalities and photoinduced electron donating properties, which opened the potential of PMOs for construction of novel light-emitting and photoreaction systems.17−19 For example, it was found for a Bp-PMO doped with a coumarin dye that efficient excitation energy transfer occurs from Bp units in the framework to a small amount of the dye in the meso-channels.17 Bp-PMO can be obtained as both powder and transparent film using the different preparation methods. To advance these functionalities related to electronically excited states, it is of great importance to acquire the detailed © 2014 American Chemical Society
information on the dynamic behaviors in the excited state through the direct elucidation of the bridging Bp groups in the framework of Bp-PMO. Along this line, Yamanaka et al. investigated excited-state dynamics of Bp-PMO in powders by femtosecond transient diffuse reflectance (TDR) and picosecond time-resolved fluorescence spectroscopies.20 From these investigations, it was revealed that Bp-PMO in the excited state underwent the relaxation in the following manner: (i) the excited Franck−Condon (FC) state with the molecular structure of the two phenyl rings being skewed to each other to the lowest single excited (S1) state (two phenyl rings are still skewed to some extent) with a time constant of 730 fs, (ii) formation of three types of excimers (E1, E2, and E3) from the S1 state with two time constants of 7.0 ps (E1: ca. 64%) and 170 ps (E2 and E3: ca. 36%), and (iii) decays of these excimers with three different time constants of 1.3 ns (E1), 8.2 ns (E2), and 27 Received: March 19, 2014 Revised: April 15, 2014 Published: April 15, 2014 9419
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Scheme 1. Conformation of Bp-PMO and Bp-Et-PMO Films
Femtosecond dual NOPA/OPA laser system was used for transient absorption spectroscopy.21,22 The output of a femtosecond Ti:Sapphire laser (Tsunami, Spectra-Physics) pumped by SHG of a cw Nd3+:YVO4 laser (Millennia Pro, Spectra-Physics) was amplified with 1 kHz repetition rate by 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 (TOPAS-white, Light-Conversion), which covers the wavelength region between 500 and 780 nm with 1−40 mW output energy and ca. 20−40 fs fwhm. In the present study, the output of the NOPA was tuned at 540 nm and was frequency-doubled by a 100 μm BBO crystal. After the compression by a prism pair, the SHG centered at 270 nm was used as a pump pulse with the intensity of 0.1−1.2 μJ/pulse. The pulse duration (fwhm) at the sample position was estimated to be ca. 30 fs by FROG signals. The focusing radius of the pump laser was ca. 170 μm, which was estimated from the colored area of a photochromic thin film by the excitation light. The other pulse at 802 nm was focused into 3 mm CaF2 plate to generate white light continuum, which was collimated with a parabolic mirror and used as a monitoring pulse in the wavelength range of 370−730 nm. Polarization between the pump and probe pulses was set at the magic angle for transient absorption spectroscopy. The probe pulse was divided into signal and reference pulses and detected with multichannel photodiode array systems (PMA-10, Hamamatsu), of which signals were sent to a PC 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 600 nm. At and after 3 ps following the excitation, the broad absorption signal > 600 nm diminishes and the spectrum with the maximum at 390 nm remains. This 390 nm band is in good agreement with that of the excimer, indicating that large amount of the excimer is produced in the early stage after the excitation under the high excitation condition. In the time window up to 200 ps, the 390 nm band due to the excimer gradually decreases. These characteristic behaviors under the high excitation condition can be summarized in the following; the broad absorption band appeared in the wavelength region >600 nm within 1 ps after the excitation, the rapid formation of the excimer was observed within a few ps, and the excimer deactivated in several tens of ps time region. Figure 4 shows the excitation intensity dependence of time profiles for the transient absorbance at 480 nm. The time profiles in the short time region (0−10 ps) are strongly dependent on the excitation intensity. That is, the rapid decay component is pronounced with an increase in the excitation intensity. In general, the rapid decay of the excited states in the system with large density of chromophores is ascribable to the S1−S1 (or exciton−exciton) annihilation owing to the dense population of the excited species under the intense excitation. In the present case, however, this rapid decay is not simply attributable to the S1−S1 annihilation, because the spectral evolution under the higher excitation condition in Figure 3b
Et-PMO (Figure 2a), this absorption is safely assigned to the FC state with the skewed structure. With an increase in the delay time (0.2 ps to ca. 3 ps), the absorption band around 440 nm gradually increases, together with a decrease of the band at 480 nm. Although the 440 nm band is similar to that observed in Bp-Et-PMO (Figure 2a), the broad absorption band around 680 nm was not clearly observed. Similar absorption spectrum was also observed for the Bp-PMO powder system.20 On the basis of the investigation of the geometrical relaxation of biphenyl and its derivatives in solution,25 this spectral feature was attributed to the geometry where the two phenyl rings of Bp in Bp-PMO are still distorted to some extent (preplanar structure). That is, the Bp moiety in the Bp-PMO powder system does not take a full planar geometry even after relaxation to the S1 state. Also for Bp-PMO film, similar spectral feature was observed in the transient absorption at 0.2−3 ps. In contrast, the geometrical relaxation to planar structure was detected in the Bp-Et-PMO film as discussed above (Figure 2). These results indicate that the dense population of Bp moieties in Bp-PMO inhibits the full relaxation in the molecular geometry. At and after several picoseconds following the excitation, the band at 440 nm slightly decreases and a new peak at 390 nm appears. This new band around 390 nm was in good agreement with the transient absorption spectrum at 40 ns, which was assigned to the excimer of Bp moieties. The present result indicates that geometrical relaxation to preplanar structure in the excited state is followed by the excimer formation. Initial structural relaxation and subsequent excimer formation processes of Bp-PMO film were very similar to those for BpPMO powder.20 In addition, similar lifetimes of the excimers (1.6, 8.6, and 30 ns) were also obtained by the time-resolved fluorescence study of Bp-PMO film (see the Supporting Information). These results suggest that intermolecular interaction between Bp moieties in the framework of BpPMO film is almost the same with that in the Bp-PMO powder. From the XRD measurements,17 however, the periodicity of Bp moieties in the framework of Bp-PMO was not detected, while 9422
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Figure 4. Dependence of time profile of transient absorbance of BpPMO film at 480 nm on the excitation light intensity in the range of 0.1−1.2 μJ/pulse.
indicated that the rapid decay of the absorbance at 480 nm was accompanied by the appearance of the excimer at 390 nm. In addition, the broad absorption band was observed in the longer wavelength region (>600 nm) in Figure 3b. These results suggest that new relaxation process is opened under the strong excitation condition. Analysis of the Spectra and Time Profiles. To elucidate the complicated relaxation process of Bp-PMO film, we analyzed transient absorption spectra into four components: (1) the excited state with the skewed structure, (2) the excited state with the preplanar structure, (3) the excited state with the planar structure, and (4) the excimer state. Reference spectra of (1) and (3) can be obtained from the transient absorption spectra of Bp-Et-PMO system in Figure 2. The spectrum in longer delay time for the transient absorption of Bp-PMO system with low excitation intensity is safely attributed to the excimer states and used as the reference for (4). For the spectrum of the preplanar structure, (3), we employed the transient absorption spectra of Bp-PMO observed at the delay time of ca. 5 ps under the lower excitation intensity. These four spectra are shown in Figure 5a. The transient spectrum at t, A(λ, t), in Figure 3 was analyzed into the four components by the linear least-squares method, as represented by eq 1.
Figure 5. (a) Transient absorption spectra of four components. Red, black, blue and orange lines are skewed, planar, excimers, and preplanar states, respectively. (b) Representative fitting results by a least-squares method on the basis of eq 1, together with the residual error.
the skewed structure. At and after 1.5 ps following the excitation, the preplanar state gradually decreases to zero with two time constants of 9.0 and 140 ps. Along with these decay processes, the gradual rise of the excimer was observed with the same time constants (Figure 6b). The slower time constant of the excimer formation was in good agreement with that observed in the time-resolved fluorescence spectra in Figure S1 in the Supporting Information. The excited state with the planar structure was not clearly observed in the time profiles for the excitation intensity 0.1 μJ/pulse. By integrating the transient absorption and the time-resolved fluorescence spectra, we can summarize the excited state dynamics with the low excitation intensity in Scheme 2. Figure 6c and d shows time profiles of the four components under the higher excitation intensity (1.0 μJ/pulse). As clearly shown in these figures, all the components appear within 1 ps and decrease rapidly, of which results are quite different from those under the lower excitation condition. Time evolution under the higher excitation condition is summarized as follows: (i) the skewed state appearing within the response function decreased with a time constant of 0.35 ps, (ii) the preplanar state increased with a time constant of 0.35 ps and was followed by the decay with two distinct time constants (3.0 and 20 ps), (iii) the planar state appeared within time resolution ( k2. In the case where the decay of the planar state is faster than 0.25 ps, the decay component observed in the time profile is due to the time constant of the formation of the planar state. The small contribution of the planar state also supports the short lifetime of this state. The time constant of the quick appearance of the excimer was 0.20 ps, which is similar to the time constant of the appearance of the planar state and that of the decay of the skewed state. This result suggests that the initial excimer formation mainly arises from the planar state. In the time region > 1 ps, ca. 20% of the decay was observed for the excimer within a few picoseconds. This decay is attributed to the annihilation process of the excimers as will be discussed later. The time constants of the excimer formation within several tens of picoseconds time regions correspond to the decay of the preplanar state. From the above results and discussion, the dynamics in the early stage after the excitation with the high intensity could be summarized in Scheme 3. Before the discussion on the annihilation processes of excimers immediately after the excitation, we first concentrate on the mechanism of this rapid geometrical relaxation. On the geometrical change of the excited molecule in the solid state with dense population of chromophores, several studies have been reported for cooperative reaction processes occurring with
Scheme 2. Excited-State Relaxation Process of Bp-PMO Film under Lower Excitation Intensity
and decayed with a time constant of 0.25 ps, and (iv) the excimer appeared with a time constant of 0.2 ps and 20% of signal intensity decreased within 2 ps, followed by the rise with the 3- and 20 ps time constants and the gradual decay in subns time region. The same time constant (0.35 ps) for the decay of the skewed state and the increase of the preplanar state indicate that the structural relaxation occurs from the skewed to the preplanar states. This time constant was slightly faster than that obtained under the lower excitation intensity. The decay time constants of the preplanar state (3.0 and 20 ps) were much shorter than those obtained under lower excitation conditions (9.0 and 140 ps). As will be discussed in later, these rapid decay processes are related to the excimer formation. Interestingly, the rapid appearance ( k1, the coefficient of exp(−k1t) is negative. That is, the time constant is observed as the decay component. (27) Suzuki, M.; Asahi, T.; Masuhara, H. Photochromic Reactions of Crystalline Spiropyrans and Spirooxazines Induced by Intense 9427
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Femtosecond Laser Excitation. Phys. Chem. Chem. Phys. 2002, 4, 185− 192. (28) Asahi, T.; Suzuki, M.; Masuhara, H. Cooperative Photochemical Reaction in Molecular Crystal Induced by Intense Femtosecond Laser Excitation: Photochromism of Spironaphthooxazin. J. Phys. Chem. A 2002, 106, 2335−2340. (29) Suzuki, M.; Asahi, T.; Takahashi, K.; Masuhara, H. Ultrafast Dynamics of Photoinduced Ring-opening and the Subsequent Ringclosure Reactions of Spirooxazines in Crystalline State. Chem. Phys. Lett. 2003, 368, 384−392. (30) Uchida, K.; Yamaguchi, S.; Yamada, H.; Akazawa, M.; Katayama, T.; Ishibashi, Y.; Miyasaka, H. Photoisomerization of an Azobenzene Gel by Pulsed Laser Irradiation. Chem. Commun. 2009, 4420−4422. (31) Yamanaka, K.; Okada, T.; Goto, Y.; Ikai, M.; Tani, T.; Inagaki, S. Dynamics of Excitation Energy Transfer from Biphenylylene Excimers in Pore Walls of Periodic Mesoporous Organosilica to Coumarin 1 in the Mesochannels. J. Phys. Chem. C 2013, 117, 14865−14871. (32) Berlman, I. B. Energy transfer parameters of aromatic compounds; Academic Press: New York, 1973. (33) Fö rster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 437, 55−75. (34) Förster, T. Experimentelle und Theoretische Untersuchung des Zwischenmolekularen Ü bergangs von Elektronenanregungsenergie. Z. Naturforsch., A: Phys. Sci. 1949, 4, 321−327. (35) Förster, T. 10th Spiers Memorial Lecture. Transfer mechanisms of electronic excitation. Discuss. Faraday Soc. 1959, 27, 7−17. (36) We estimated the molar coefficient for the excimer-state absorption in Bp-PMO from the comparison of the transient absorption of the organic dye Nile Blue (NB) in ethanol as a reference. The molar coefficient of NB in the ground state was wellknown, and the excited molecules can be easily estimated. Under the same excitation condition, the maximum transient signal of Bp-PMO was less than 1/10 times as small as that of NB, of which the molar coefficient has at least more than 80 000 M−1 cm−1. Therefore, the molar coefficient of excimer state at the maximum was estimated to be ca. 10 000 M−1 cm−1.
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