Large and ultrafast photoinduced reflectivity change in the charge separated phase of ${\text{Et}}_2{\text{Me}}_2\text{Sb}{\left[\text{Pd}{\left(1,3-\text{dithiol}-2-\text{thione}-4,5-\text{dithiolate}\right)}_2\right]}_2$

A large photoinduced change in reflectivity has been observed in the low-temperature charge separated (CS) phase of a dimeric radical anion salt, ${\text{Et}}_2{\text{Me}}_2\text{Sb}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ ($\text{dmit}=1,3$-dithiol-2-thione-4,5-dithiolate). Just after the photoexcitation, the reflectivity abruptly changed reflecting the appearance of a photoinduced metastable state, indicating occurrence of recrystallizing of the CS phase by intradimer photoexcitation within a picosecond. Quantitative analysis considering the linear combination of the dielectric functions of the CS and the dimer-Mott state suggests a rather high efficiency of the photoinduced phase transition. One photon can change the valence of about five dimers. This photoinduced metastable state relaxed to the initial CS state via two successive types of relaxation processes, a fast and a slow one. The relaxation time $\left(\tau \right)$ and the reflectivity of the fast process showed a clear excitation intensity and temperature dependence. In particular, $\tau$ and the estimated domain size were enhanced up to the transition temperature $\left(T_{\text{c}}\right)$ with increasing temperature. This phenomenon, a sort of critical slowing down around $T_{\text{c}}$, suggests that the density of the photoinduced state as well as the external temperature plays an important role in determining the relaxation dynamics of the photoinduced state. The results obtained indicate that this photoinduced phenomenon can be classified as a tuning of the charge in crystals via cooperative interaction between the degrees of freedom of “charge” and “molecular orbital” of the constituents, i.e., as a type of photoinduced phase transition.

Figure 1

Schematic structure of the anion layer of ${\text{Et}}_2{\text{Me}}_2\text{Sb}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ viewed along the long direction of the molecules. (a) End-on projection of the DM insulating phase, (b) that for the charge separated (CS) phase. $X$ and $Y$ denote two crystallographically independent dimers. $X$ is an expanded divalent dimer and $Y$ is a constricted neutral dimer. [15] Solid lines represent the end-on projection of the $\mathit{ab}$ plane in the unit cell.

Figure 2

Temperature dependence of (a) resistivity $\left(\rho \right)$, (b) reflectivity observed at 1.31 eV $\left(R\right)$, (c) the relaxation time $\left(\tau \right)$ estimated from the time profile of the photoinduced reflectivity change, and (d) the estimated relative photoinduced domain size at 0.5 ps (see context in Sec. 7), respectively. The pump photon energy for pump-probe measurements was 1.55 eV with an intensity of $1.18\times {10}^{20}/{\text{cm}}^3$. For the probe light, the photon energy was 1.31 eV. The transition temperature is about 70 K judging from the kink of resistivity and reflectivity (indicated by dashed vertical line). The relative size of the photoinduced domain [in (d)] is normalized with that observed at 16.6 K.

Figure 3

(a) Schematic energy diagram of $\text{Pd}{\left(\text{dmit}\right)}_2$ monomer and dimer. Schematic shape of the $\text{Pd}{\left(\text{dmit}\right)}_2$ monomer and dimer are also drawn under the energy diagram. (b) Schematic structure of the monovalent $\left(-1\right)$ $\text{Pd}{\left(\text{dmit}\right)}_2$ dimer and its energy level. (c) Those for the neutral (0) and divalent $\left(-2\right)$ $\text{Pd}{\left(\text{dmit}\right)}_2$ dimers and their energy levels. Arrows drawn in (b) and (c) represent the intradimer transition observed in the optical reflectivity in the near-IR region (see Fig. 4 and Ref. 13). $A$, $B$, and $C$ correspond to the peak structures in Fig. 4. The values of the energy between the bonding and antibonding state are quoted from Ref. 12.

Figure 4

(Color online) Closed and open circles represent the photoinduced reflectivity spectra ($\mathit{E}\parallel \mathit{a}$ axis) with excitation photon energy of 1.55 and 1.18 eV, respectively, at (a) 0.5, (b) 5, and (c) 19 ps after photoirradiation. The excitation intensity was $1.4\times {10}^{20}/{\text{cm}}^3$ at 1.55 eV pump and $7.6\times {10}^{19}/{\text{cm}}^3$ at 1.18 eV pump. Solid lines show the spectra without photoexcitation at 50 (CS phase) and 200 K (DM phase) in each figure. The colored solid lines and dashed lines show the simulated reflectivity spectra using the multilayer model with several photoconversion efficiency (see text). A, B, and C indicate the peak structures corresponding to Figs. 3b, 3c.

Figure 5

(Color online) (a) Typical time profile of the pump-probe signal $\left(\Delta R/R\right)$ with excitation of 1.55 eV light (closed circles). The probe photon energy was 1.31 eV. The dashed blue (gray), dashed green (light gray), and solid red (gray) curves represent fast relaxation, remnant part, and their summation, respectively (see context), estimated based on the fitting procedure. (b) Schematic energy diagram of the photoinduced relaxation processes.

Figure 6

(a), (b), and (c) Excitation photon-energy dependence of time profiles of the pump-probe signal $\left(\Delta R/R\right)$ observed at three probe photon energies (1.31, 1.36, and 1.40 eV). The closed circles represent the signal for the pump photon energy of 1.55 eV with intensity of $1.4\times {10}^{20}/{\text{cm}}^3$. The open circles represent the pump photon energy of 1.18 eV with intensity of $7.6\times {10}^{19}/{\text{cm}}^3$. (d), (e) Time profiles of the pump-probe signal with different pump photon energies. (f), (g) Oscillatory components extracted by subtracting the electronic component (see text). (h), (i) Contour maps of the results of wavelet analysis of the oscillatory components. The inset in (i) shows the scale of these plots.

Figure 7

(a) Excitation intensity dependence of the pump-probe signal $\left(\Delta R/R\right)$ of ${\text{Et}}_2{\text{Me}}_2\text{Sb}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ at 50 K. The polarization of the 1.55 eV pump and 1.31 eV probe lights was parallel to the $\mathit{a}$ axis of the crystal. The photon densities of the pump light were $2.3\times {10}^{20}$, $1.1\times {10}^{20}$, $5.6\times {10}^{19}$, $2.8\times {10}^{19}$, and $1.4\times {10}^{19}/{\text{cm}}^3$.

Figure 8

(a), (b), (c) Excitation intensity dependence of the reflectivity change $\left(\Delta R/R\right)$ at 50, 60, and 65 K. Closed triangles, open circles, and closed circles show the data at 0.5, 5.0, and 15.0 ps after photoexcitation, respectively. (d), (e), (f) Excitation intensity dependence of the relaxation time of the fast relaxation $\left(\tau \right)$ with error bars at 50, 60, and 65 K, respectively.