Charge and Structural Dynamics in Photoinduced Phase Transition of

Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan...
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Charge and Structural Dynamics in Photoinduced Phase Transition of (EDO-TTF)2PF6 Examined by Picosecond Time-Resolved Vibrational Spectroscopy Naoto Fukazawa,† Minoru Shimizu,† Tadahiko Ishikawa,† Yoichi Okimoto,† Shin-ya Koshihara,†,‡ Takaaki Hiramatsu,§ Yoshiaki Nakano,§ Hideki Yamochi,§ Gunzi Saito,∥ and Ken Onda*,⊥,# †

Department of Chemistry and Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan CREST, Japan Science and Technology Agency, Japan § Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ∥ Research Institute, Meijo University, Tempaku-ku, Nagoya 468-8502, Japan ⊥ Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan # PRESTO, Japan Science and Technology Agency, Japan ‡

S Supporting Information *

ABSTRACT: Using time-resolved near-infrared reflectance spectroscopy and time-resolved mid-infrared vibrational spectroscopy, we studied photoinduced phase transition of the charge-ordered insulating phase in a charge-transfer complex (EDO-TTF)2PF6 (EDO-TTF: ethylenedioxy-tetrathiafulvalene) in the hundred picosecond range after photoexcitation. The temporal profiles at 0.83−1.03 eV, which are a characteristic of the photoinduced charge-disproportionate phase immediately after photoexcitation, suggested the formation of a new metastable phase in the hundred picosecond range. Time-resolved vibrational spectra at 1300−1700 cm−1, where charge- and structure-sensitive CC stretching vibrational modes are located, elucidated that the nature of the new phase is very close to that of the high-temperature metallic phase and it takes about 100 ps for the new phase to emerge accompanied with charge and structure fluctuation. pulse.12,13 PIPT is macroscopic phase transition triggered by photoinjected local excitation by virtue of cooperative interaction, and its application to ultrafast photoswitching devices is expected. Thus, the processes and microscopic mechanisms of PIPT have been intensively studied; however, they have not been revealed yet because of the nonequilibrium, ultrafast, and complex dynamics of PIPT. (EDO-TTF)2PF6 (EDO-TTF: ethylenedioxy-tetrathiafulvalene) was the first quarter-filled organic CT complex to show ultrafast and gigantic reflectivity change stimulated by a light pulse. This phenomenon was attributed to PIPT from the CO insulating phase to the metallic phase.14,15 Later, the photoinduced metallic phase was identified as a unique photoinduced charge-disproportionate phase due to strong electron−phonon interactions and Coulomb interactions between electrons.16 In the (EDO-TTF)2PF6 complex, the EDO-TTF molecules act as electron donors and form a quarter-filled, quasi-one-

1. INTRODUCTION Charge-transfer (CT) complexes consisting of π-electron organic molecules have been attracting considerable attention because many CT complexes exhibit a wide variety of phases, such as Mott insulators, charge-ordered (CO) insulators, charge density wave insulators, and superconductors, by weak external stimuli.1−4 The main cause of the variety of phases is the fact that the Coulomb interactions between on-site and/or off-site electrons are comparable to the transfer energy between molecules; that is, these complexes are strongly correlated electron systems.5−7 In addition, the complexes often have a strong electron−lattice interaction because of their softness. The competition among these interactions makes them sensitive to external stimuli such as temperature, pressure, and electromagnetic fields including light. Because of the variety of responses to external stimuli, many studies, from fundamentals to applications, have been conducted.1−11 Among these studies, photoinduced phase transition (PIPT) is one of the most extensively studied topics because there is a potential to control various physical properties extremely quickly and efficiently and without contact using a weak light © 2012 American Chemical Society

Received: November 8, 2011 Revised: February 5, 2012 Published: February 7, 2012 5892

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dimensional electronic structure.17 At room temperature, this complex shows metallic behavior and the charge is distributed equivalently (+0.5) at each site with an almost planar shape of the donor molecule. With decreasing temperature, this complex undergoes metal to insulator (M−I) phase transition at 279 K18 accompanied with a large structural change based on the molecular deformation of EDO-TTF and a doubling of unit cells compared with the metallic phase. This M−I transition is considered to originate from the cooperation of Peierls, charge ordering, and order−disorder transitions of the counteranion.17,19,20 In the low-temperature phase (CO insulating phase), two boat-shaped and two flat EDO-TTF molecules with neutral charge and +1 charge, respectively, form a tetramer of (0110), where the parentheses represent the order of neutral (0) and cationic (1) EDO-TTF molecules. The charge patterns in the low- and high-temperature phases are revealed by electronic and vibrational spectroscopy,21,22 and the structures are determined by X-ray diffraction.17,23 The schematic diagram of these electronic and structural phase transitions is summarized in Figure 1. A numerical calculation based on

studied intensively. Despite the importance of basic knowledge about this process in solid-state physics under nonequilibrium conditions, this is true not only for this material but also for the other material showing PIPT with strong electron−electron and electron−phonon interactions. In addition, elucidation of the relaxation process is essential in order to actively control PIPT in applications. In this study, we measured not only temporal profiles of reflectivity change in the near-infrared region but also time-dependent vibrational spectra in the midinfrared region in the hundred picosecond range. In particular, time-resolved vibrational spectroscopy is expected to become a powerful method to investigate dynamics in the later processes in PIPT because some of the vibrational bands of the constituent molecules are very sensitive to molecular charge and structure. IR spectroscopy has been utilized to investigate the phase transitions of various CT complexes in thermal equilibrium.7,22,25−32

2. EXPERIMENTAL METHODS A single crystal (typically 0.5 mm ×2 mm ×0.1 mm) of (EDOTTF)2PF6 was obtained by an electrocrystallization technique.17 The EDO-TTF molecule was synthesized by the previously reported procedure.33 All optical measurements were performed on a (001) crystal face, which is the most developed crystal face elongated along the b axis. Highresolution linear reflectivity spectra of the crystal in the midinfrared region were measured using a Fourier transform infrared (FT-IR) spectrometer equipped with a reflective objective lens. Near-infrared reflectivity change spectra were measured by the pump−probe method using a 120 fs Ti:sapphire regenerative amplifier (fs-RGA, photon energy = 1.55 eV, spectral width = 150 cm−1, repetition rate = 1 kHz) as the light source. A part of the output of the fs-RGA was used for the pump pulse. The near-infrared pulse (photon energy = 0.83−1.03 eV) for the probe pulse was generated by optical parametric amplification (OPA) from the remaining output of the fs-RGA. The photon energy of the pump light was fixed at 1.55 eV, which is nearly resonant to the charge-transfer band from (0110) to {0200} (CT2);21,22 { } represents a localized photoexcited state as distinguished from a long-range chargeorder state. The polarizations of the pump and probe pulses were parallel to the donor stacking axis, which corresponds to a quasi-one-dimensional conducting direction. Time-resolved mid-infrared vibrational spectra were also measured by the pump−probe method. The experimental setup is shown in Figure 2. To resolve narrow and dense vibrational

Figure 1. Schematic of charge patterns and molecular structures of (EDO-TTF)2PF6 for the low-temperature CO insulating phase, hightemperature metallic phase, and photoinduced (1010) chargedisproportionate phase.

density functional theory suggests that this strong charge ordering originates from large electric potential bias within a tetramer of EDO-TTF molecules owing to long-range Coulomb interactions.24 The photoexcitation of CT transition in the CO insulating phase creates the photoinduced (1010) charge-disproportionate phase at around 100 fs after photoirradiation, as shown in Figure 1.16 This was revealed by measurement of an ultrafast reflectivity change spectrum ranging from the farinfrared (69 meV) to visible (2.1 eV) range and theoretical calculation using the extended Hubbard model, including both the Peierls- and Holstein-type electron−phonon interactions. The photoinduced phase has a (1010) charge-order pattern; however, theory predicts that, unlike the thermal equilibrium phases, the lattice potential in this phase fluctuates, and that, under the influence of such a lattice potential, some carriers exist. This is the first CT complex that does not show any of the thermal equilibrium phases but displays a hidden and original phase by photoexcitation.16 The next question is how this unique photoinduced phase relaxes to the thermal equilibrium phase. Until this date, the relaxation process of the photoinduced phase has never been

Figure 2. Experimental setup for the time-resolved mid-infrared vibrational spectroscopy. 5893

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sharp vibrational bands are clearly observed in the wavenumber region less than 2000 cm−1.22,35 As previously reported,16 the photoexcitation at the CT2 band does not create a charge-melted (0.5, 0.5, 0.5, 0.5) phase but creates the nonequilibrium photoinduced (1010) chargedisproportionate phase immediately after photoexcitation. This photoinduced phase was assigned to the photoinduced reflectivity spectrum over a wide photon energy range from 69 meV to 2.1 eV. To reveal the destiny of the photoinduced phase, we first measured temporal profiles of the reflectivity change at several probe photon energies up to 1000 ps. As a result, it was found that the reflectivity changes persist for more than 1000 ps after photoexcitation and the temporal profiles of the reflectivity change, especially in the near-infrared region from 0.8 to 1.0 eV, show characteristic behavior. Figure 4 shows

bands of organic molecules, we used a narrow-band 3 ps Ti:sapphire regenerative amplifier (ps-RGA, photon energy = 1.55 eV, spectral width = 10 cm−1, repetition rate = 1 kHz) for the light source.34 In the same manner as the near-infrared measurement, a part of the output of the ps-RGA was used for the pump pulse, and the mid-infrared pulse (tunable range: 1000−3700 cm−1) was generated by OPA and difference frequency generation (DFG) from the remaining output of the ps-RGA. The polarization of the pump pulse was also parallel to the donor stacking axis, whereas the polarization of the probe pulse was perpendicular to the donor stacking axis so as to avoid strong reflectivity from conduction carriers along the donor stacking axis. To measure transient vibrational spectra with high signal-to-noise (S/N) ratios, the signals were accumulated at a negative delay and a positive delay (t) alternatively at each wavenumber of the probe pulse. The reflectivity change at the delay time ΔR/R(t) was estimated using the equation ΔR/R(t) = R(t)/R(negative delay) − 1. The sample crystal was held inside a cryostat and was maintained at 180 K for all pump−probe measurements.

3. RESULTS AND DISCUSSION 3.1. Time-Resolved Near-Infrared Reflectance Spectroscopy. Figure 3 shows the linear reflectivity spectra of

Figure 4. Temporal profiles of reflectivity change (ΔR/R) in the nearinfrared region at 0.83, 0.89, 0.95, and 1.03 eV by photoexcitation at 1.55 eV in the CO insulating phase of (EDO-TTF)2PF6 at 180 K. Polarizations of pump and probe pulses are parallel to the donor stacking axis (E ∥ stack). The horizontal and vertical scales change at 3 ps.

Figure 3. Polarized linear reflectivity spectra of (EDO-TTF)2PF6 in the CO insulating phase (blue and back) and in the metallic phase (red and orange). The horizontal axis is drawn in logarithmic scale. Thick and thin lines represent the polarized spectra in the direction parallel and perpendicular to the donor stacking axis, respectively. The vertical dashed lines represent the region measured by time-resolved mid-infrared vibrational spectroscopy. The arrows indicate the corresponding energies of the probe and pump light when timeresolved spectroscopy was performed in the near-infrared region.

the temporal profiles of the reflectivity change at 0.83, 0.89, 0.95, and 1.03 eV (arrows in Figure 3 indicate the corresponding energies). The excitation density was 2.0 mJ/ cm2 (8.0 × 1015 photons/cm2). In a previous report,36 the direction of the reflectivity change in this probe photon energy region was a key to assign the photoinduced phase. That is, a positive reflectivity change indicates the appearance of a phase similar to the thermally induced metallic phase, whereas a negative reflectivity change indicates the generation of the photoinduced (1010) charge-disproportionate phase. Note that the photoinduced reflectivity change at 0.95 eV is exceptionally positive immediately after photoexcitation because of a large coherent phonon but becomes negative after the coherent phonon is dumped at around 1 ps. For all probe photon energies shown in Figure 4, the reflectivity changes at 1 ps are negative, indicating the appearance of the photoinduced (1010) phase. At delay times longer than 3 ps, weak and irregular reflectivity oscillations are observed up to 500 ps, and over 500 ps, the reflectivity changes become stable. The directions of reflectivity changes over 500 ps are different from those at 1 ps,

(EDO-TTF)2PF6 using polarized light. The thick blue and red lines represent the spectra at 180 K (CO insulating phase) and 290 K (metallic phase), respectively, with the light polarized parallel to the donor stacking axis. The broad bands located at 0.6 eV (CT1) and 1.4 eV (CT2) at 180 K are assigned to the charge-transfer transition between donor molecules.21,22 In the spectrum at 290 K, these charge-transfer bands disappear and a large reflectivity increase due to carriers along the donor stacking axis is observed in the photon energy region less than 1.0 eV. These features are consistent with previous reports.21,22 The thin black and orange lines represent the spectra at 180 and 300 K, respectively, with the light polarized in the direction perpendicular to the donor stacking axis. In this direction, the reflectivity increase due to carriers is suppressed and several 5894

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is difficult to observe the absorption bands of νβ and νγ with neutral charge due to their small intensities. Thus, we assign only να with charge +1, +0.5, and 0 in the spectra. Figure 5c shows the transient vibrational spectra (ΔR/R) observed at 1, 20, and 300 ps after photoexcitation. The excitation intensity of the pump pulse was 1.4 mJ/cm2 (5.7 × 1015 photons/cm2). At 1 ps, the reflectivity increase in the wide spectral region covering 1300−1700 cm−1 and three sharp bleach bands at 1380, 1550, and 1643 cm−1 are observed. Since the wavenumbers of these bleach bands are in good agreement with the vibrational band characteristics of the low-temperature (0110) CO insulating phase, the bleach bands indicate that a part of the (0110) CO insulating phase in the ground state disappears. The temporal profiles at these bleach bands (typical example observed at 1380 cm−1 is plotted in Figure 7) indicate that the (0110) CO insulating phase does not revert within 300 ps. In addition to the bleach bands, the broad reflectivity band appears immediately after photoexcitation and its intensity decreases considerably within 20 ps, although a portion remains even at 300 ps after photoexcitation. The temporal profile at 1424 cm−1, where no vibrational band is observed, is plotted in Figure 7 as a typical example. According to the previous theoretical calculation,16 the dynamical fluctuation of the ionicity of every constituent EDO-TTF molecule, i.e., hole density, in the subpicosecond time scale may reach up to ±0.1−0.2 accompanied with the appearance of the photoinduced (1010) phase. Thus, it is reasonable that the observed short-lived broad band can be attributed to the broadening of the charge-sensitive band due to the large fluctuation of the degree of charge on each EDO-TTF donor site accompanied with the photoinduced phase conversion from (0110) to (1010), as schematically shown in Figure 8b. At 300 ps, in addition to the small remnant reflectivity increase, an obvious narrow band appears at around 1572 cm−1. Compared with the linear reflectivity spectrum, the να band of the metallic (0.5, 0.5, 0.5, 0.5) phase is located at this wavenumber. Thus, the narrow band at 1572 cm−1 is most plausibly attributed to the formation of the metallic phase similar to the thermally induced phase. To support this assignment, we applied the normal multilayer model, which is often utilized for studying photoinduced phase transition,37−40 to analyze the ΔR/R spectra in detail. The details of this model are described in the Supporting Information. The spectrum calculated by this model with the yields of the photoinduced phase (γ) = 0.2 is indicated in Figure 5c as the dotted red line. This spectrum is in good agreement with the reflectivity change spectrum at 300 ps, indicating that approximately 20% of the photoexcited state turns into the state having the same optical constants as the high-temperature metallic phase. To evaluate the domain size of the photoinduced metallic phase, we also performed the calculation using an effective-medium approximation (EMA)-based multilayer model, as shown in Figure 6. However, the spectrum calculated by the EMA-based multilayer model is quite similar to that calculated by the normal multilayer model. This is because the complex dielectric function difference between 180 and 300 K is quite small in this region. Thus, it is difficult to determine the domain size of the photoinduced metallic phase from these calculations. The details of the EMA-based multilayer model are also described in the Supporting Information. The temporal behavior of the photoinduced metallic phase is derived from the temporal profiles of the reflectivity change

and the largest positive reflectivity change occurs at 0.95 eV. This feature is similar to the thermally induced reflectivity change from the CO insulating phase to the metallic phase, as shown in Figure 1 in ref 16. These results suggest the formation of a new metastable phase in the relaxation process of the photoinduced (1010) phase. The high-temperature metallic phase is a strong candidate for the new phase. To identify and clarify the nature of this new metastable phase and its formation process, it is essential to know the charge- and lattice-coupled dynamics of this system in the wide delay time region ranging from one to hundreds of picoseconds. Time-resolved vibrational spectroscopy is a suitable method for this because it enables us to probe the changes in charge and structure in the pico- and nanosecond regions simultaneously. 3.2. Time-Resolved Mid-Infrared Vibrational Spectroscopy. For comparison with the transient vibrational spectra, Figure 5b shows the linear reflectivity spectra at 1300−1700

Figure 5. (a) Molecular structure of the EDO-TTF molecule and the schematic view of its CC stretching vibrational mode expected to be observed from 1300 to 1700 cm−1. The arrows on the molecular structures represent major vibrations for each vibrational mode. (b) Polarized linear reflectivity spectra of (EDO-TTF)2PF6 are in the direction perpendicular to the stacking axis (E ⊥ stack) in the CO insulating phase at 180 K (thick blue line) and in the metallic phase at 300 K (thin red line). (c) Photoinduced vibrational reflectivity change (ΔR/R) spectra at 1, 20, and 300 ps by excitation at 1.55 eV in the CO insulating phase at 180 K. Polarizations of pump and probe pulses are parallel and perpendicular to the donor stacking axis, respectively. The red dotted line on the spectrum at 300 ps is the simulated spectrum using the normal multilayer model (for details, see the text).

cm−1 measured with a linearly polarized light, perpendicular to the donor stacking axis at 180 K (CO insulating phase) and 300 K (metallic phase). According to refs 22 and 35, the molecular vibrational modes of να, νβ, and νγ are expected to be observed in this region. As shown in Figure 5a, these three modes mainly consist of CC stretching vibrations. These modes are sensitive to molecular charge and structure.22,35 However, it 5895

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Figure 6. Calculated spectra using the normal multilayer model and the effective-medium approximation (EMA)-based multilayer model together with the photoinduced vibrational reflectivity change spectrum at 300 ps. See the text for the details of these calculations.

observed at 1572 cm−1. As shown in Figure 7, the reflectivity at 1572 cm−1 quickly increases within the pulse duration (∼3 ps) and then decreases within a few picoseconds. Thereafter, reflectivity begins to increase again gradually over 200 ps. To analyze this temporal profile together with the other profiles, we carried out nonlinear least-squares fitting using the following function: ⎛ t ⎞ A(t ) = A exp⎜ − ⎟ ⎝ τ1 ⎠ ⎧ ⎛ t ⎞ τ1 B(t ) = B⎨1 − exp⎜ − ⎟ τ1 − τ2 ⎝ τ1 ⎠ ⎩ ⎛ t ⎞⎫ τ2 exp⎜ − ⎟⎬ + τ1 − τ2 ⎝ τ2 ⎠⎭ ⎪

⎪

⎪

⎪

Figure 7. Temporal profiles of reflectivity change at 1380, 1424, and 1572 cm−1. The solid red lines represent fitting curves using eq 1. The short-dashed (blue), long-dashed and short-dashed (orange), and dashed (green) lines are the temporal profiles of the components A(t), B(t), and C(t), respectively. The inset in the top panel shows the diagram of the three-level model used for derivation of the fitting function (1).

⎛ t ⎞⎫ τ2 ⎧ ⎛ t ⎞ ⎨exp⎜ − ⎟ − exp⎜ − ⎟⎬ C(t ) = C τ1 − τ2 ⎩ ⎝ τ1 ⎠ ⎝ τ2 ⎠⎭ ⎪

⎪

⎪

⎪

⎧ (t < 0) ⎪0 f (t ) = ⎨ ⎪ ⎩ A(t ) + B(t ) + C(t ) (t ≥ 0)

Table 1. Parameters Determined by the Fitting Analysis Using Eq 1 1424 and 1572 cm−1

⎡ 4 ln 2 ⎤ g (t ) = exp⎢ − 2 t 2 ⎥ ⎣ α ⎦ ΔR(t )/R =

∞

∫−∞ f (t − t′)·g(t′) dt′

A B C τ1 (ps) τ2 (ps) α (ps)

(1)

where A, B, and C are the magnitude of components and τ1 and τ2 are the decay constants. We derived the function from the three-level rate equation as shown as the inset of Figure 7 so as to reproduce the slow increase after the quick decrease. A(t), B(t), and C(t) represent the intensities proportional to population of the highest, lowest, and middle levels, respectively. g(t) is the Gaussian function with full width at half-maximum α = 4 ps, which is determined by crosscorrelation between the pump and probe pulses. To take the instrumental function of the measurement system into account, the fitting function was convoluted with the Gaussian function as the instrumental function. The fitting curves are shown in Figure 7 as red solid lines, and the obtained fitting parameters are summarized in Table 1. The other temporal profile

1572 cm−1

1424 cm−1

0.172 ± 0.020 0.141 ± 0.003 0.031 ± 0.004 4.4 ± 0.9 94 ± 9 4 (fixed)

0.081 ± 0.005 0.021 ± 0.002 0.023 ± 0.002 4.4 ± 0.9 (fixed) 94 ± 9 (fixed) 4 (fixed)

observed at 1424 cm−1 is also well-explained using the same fitting function with the fixed value of τ1 and τ2 derived from the temporal profile at 1572 cm−1, and estimated parameters are also summarized in Table 1. We did not apply this fitting analysis to the temporal profile at 1380 cm−1 because we need more unknown parameters representing bleaching in addition to these parameters. According to a previous report, the dynamical change in the spectral shape in the vis−IR region indicated that the lifetime of 5896

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the (1010) charge-disproportionate phase is within 5 ps.36 The present results shown in Figure 4 are closely consistent with this result. Therefore, we conclude that the fast decay process commonly observed at 1424 and 1572 cm−1 is due to electronic relaxation of the photoinduced (1010) phase. In other words, component A(t) in Figure 7 is attributed to the contribution of the (1010) phase. This assignment is closely consistent to the previously discussed idea that the broad component in the vibrational spectra is enhanced because of the fluctuation of the molecular iconicity accompanied with the appearance of the (1010) phase. The gradual increase of reflectivity following the fast decay due to the disappearance of the (1010) phase is clearly observed at 1572 cm−1, indicating growth of the new absorption band with rather narrow bandwidth, which is a characteristic of the thermally induced metallic phase (component B(t) in Figure 7). This growing time constant is estimated to be 94 ps and is long enough to deform the lattice or molecular structure; thus, the lattice and/or molecular structure must change during the relaxation process of the photoinduced state. The observed spectral changes at 300 ps in the molecular vibrational region, which strongly correlate with both charge and lattice structures, are well-explained by the formation of the thermally induced metallic phase on the crystal surface as discussed above. Therefore, not only the iconicity of the donor site but also the structure probably become almost equivalent to those in the thermally induced metallic phase through the growing time constant, as schematically shown in Figure 8c. This state should correspond to the intermediate state we assumed in the temporal profile analysis. In conclusion, immediately after photoexcitation, the structure remains the same as that in the (0110) CO insulating phase (see Figure 8, parts a and b), and for the metallic phase, in addition to CO melting, a large structural change occurs (see Figure 8d). The cause of this large structural change is probably that this system takes about 100 ps after photoexcitation to form the metallic phase. In the process of the formation of the metallic phase, the appearance of a largely broadened and dispersed spectral shape in the molecular vibrational region is expected, as schematically shown in Figure 8c, reflecting the melting of CO with the (1010) pattern. Indeed, the dispersed reflection band has been observed immediately after the disappearance of (1010) CO (at 20 ps after excitation), as shown in Figure 5c, and it seems to be consistent with this model. Of course, the observed spectral region in this report is limited. For a more detailed and quantitative analysis of the total vibrational spectrum, development in the IR detector will be essential. With regard to the intermediate state [component C(t)], the microscopic origin is not clear at the present stage. One possible explanation is that induced by photoexcitation, the carrier moves comparatively freely. Coexistence of a small number of photoinjected carriers and the (1010) photoinduced phase has been predicted by a theoretical study.16 Also the broadening of the vibrational bands due to charge randomization as shown in Figure 8c contributes to the component C(t). Thus, component C(t) consists of at least two different origins; however, it is difficult to distinguish their contributions. We compare our results to later processes in PIPT in other CT complexes. In general, the photoexcitation initially creates many nuclei in the high-temperature phase within 100 fs, followed by the nuclei condensing over tens of picoseconds. The relaxation period of this condensed metallic phase is over 1

Figure 8. Schematic of proposed time evolution of the charge pattern and molecular structure after photoexcitation of the CT transition in the CO insulating phase of (EDO-TTF)2PF6. The illustration on the right-hand side is the expected spectra in the molecular vibrational region from these charge patterns and molecular structures.

ns. For example, α-(BEDT-TTF)2I3 [BEDT-TTF = bis(ethylenedithiolo)tetrathafulvalene] also shows PIPT from the CO insulating phase to the metallic phase,41−43 and PIPT follows this process. Although TTF-CA (tetrathiafulvalene-pchloranil) shows photoinduced ionic to neutral phase transition,37,44 the neutral phase is created and relaxes via a similar process. The microscopic one-dimensional (1D) neutral domains are created initially, and then the domains are multiplied through a cooperative interstack interaction. This neutral phase lasts over 1 ns. In contrast, (EDO-TTF)2PF6 shows two different phases on a different time scale. First, the photoinduced charge-disproportionate phase emerges within 100 fs, and then the charge-melted metallic phase is created over 100 ps. The origin of this phenomenon is probably multistability resulting from competing interactions such as Coulomb interactions, electron−phonon interactions, and Peierls instability.

4. CONCLUSION Using near-infrared reflectivity change spectroscopy and timeresolved infrared vibrational spectroscopy, we revealed the later processes in photoinduced phase transition (PIPT) after photoexcitation of the charge-ordered (CO) insulating phase in (EDO-TTF)2PF6. We found that the spectrum estimated from remnant components over 500 ps in the temporal profiles in the near-infrared region obviously differs from that at 1 ps, 5897

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when the photoinduced (1010) charge-disproportionate phase exists, suggesting that a phase similar to the high-temperature metallic phase emerges up to 500 ps. To assign this slow and weak spectral change, we applied time-resolved infrared vibrational spectroscopy and succeeded in observing the transient vibrational spectra reflecting the transient molecular charges and structures in the PIPT process. From the transient vibrational spectra, we discovered the gradual growth of the photoinduced metallic phase with about 100 ps time constant after the relaxation of the photoinduced charge-disproportionate phase. This result shows that time-resolved vibrational spectroscopy is a powerful tool to investigate photoinduced dynamics in not only CT complexes but also many π-electron organic crystals, such as in organic electronic devices. Moreover, the importance of later processes in the hundred picosecond range in photoinduced dynamics is clarified. Development of dynamical vibrational spectroscopic methods or other methods that are sensitive to not only molecular charge but also molecular structure, for example, time-resolved X-ray or electron diffraction measurements, are expected to increase in importance in the materials science field.

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ASSOCIATED CONTENT

S Supporting Information *

The details of the normal and EMA-based multilayer model. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone and Fax: +81-45-924-5891. E-mail: konda@chemenv. titech.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Assistant Professor K. Iwano (Institute of Materials Structure Sciences, KEK in Japan) and Associate Professor K. Yonemitsu (Institute for Molecular Science, in Japan) for beneficial discussions. This work was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 20340074) and on Innovative Areas (No. 21110512) and the G-COE program of the Tokyo Institute of Technology.

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dx.doi.org/10.1021/jp210708q | J. Phys. Chem. C 2012, 116, 5892−5899

The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp210708q | J. Phys. Chem. C 2012, 116, 5892−5899