We present our experimental results on the measurements of excited state dynamics in 2, 9, 16, 23-phenoxy-phthalocyanine (Pc1) and 2, 9, 16, 23-phenoxy-phthalocyanine-zinc (Pc2) using the pump-probe experiment. The results show that the lifetime of the first triplet excited state of the Pc2 longer than Pc1. The lifetimes of the triplet excited state for Pc2 and Pc1 are 12.8 μs and 10.1 μs at the same intensity, respectively. Moreover, analysis of modulation characteristics of all-optical switching (A-OS) shows that the stronger the light intensity of the pump light is, the smaller the normalized transmittance is, and the lower the A-OS response time is. The consequences of such short lifetimes are also discussed in view of the strong A-OS properties of these molecules.
©2013 Optical Society of America
The phthalocyanines (Pc) and metallophthalocyanines (MPc) have been studied in great deal for many years. Recently, much attention has been attracted to the dynamics of the triplet excited states in Pc because of their aromatic 18-π electron system and cability of containing more than 70 kinds of metallic and non-metallic ions in the ring cavity [1, 2]. Furthermore, the large nonlinear optical response of MPc, arising from its two-dimensional conjugated π-electrons system, lends it as a candidate in various photonics devices, such as power limiting, all optical switching, optical bi-stability [3–5] etc. All-optical modulation has attracted much interest especially in the ðelds of optical communication. All-optical switching (A-OS) is a fundamental building block of information processing for the future.
A-OS has been reported in fullerene (C60) [6–8], polydiacetylene [9, 10], bacteriorhodopsin [11–14], metalloporphyrins , PVK  and azobenzene dyes  using pump probe method. For Pc and MPc materials, the dynamic and steady-state behaviors of reverse saturable absorption have been researched by C. f. Li et al. . M. C. Larciprete et al. reported nonlinear optical properties of zinc-phthalocyanines using Z-Scan . Up to now, the influence factor on optical switching effect of the phenoxy-phthalocyanines, combined experiment result with theoretical analysis is seldom reported.
In this paper, based on our team’s previous work about synthesis and experiment of the nonlinear optical properties of the Pc and MPc material , we further investigate the relation between the A-OS response time of these material and the lifetimes of the triplet state by using pump-probe method. Our theoretical analysis and experimental measurements show that the A-OS response time is determined by the lifetime of the first triplet excited state of molecule. The smaller the lifetime of the first triplet excited state of molecule is, the lower the A-OS response time is. The experimental results provide reliable reference for the application of A-OS using the Pc and MPc material.
2. Synthesis and characterization
The 2, 9, 16, 23-phenoxy-phthalocyanine (Pc1) and 2, 9, 16, 23-phenoxy-phthalocyanine-zinc (Pc2) are a hydrogen or zinc connected with four ligands by nitrogen bridges. Especially Pc1 and Pc2 are two-dimensional (2D) large molecules as shown in Fig. 1(a) , and their molecular weights are 883u and 946u, respectively. In our experiment, Pc1 and Pc2 are synthesized using mild reaction coordination method. The proton nuclear magnetic resonance of the two synthesized phthalocyanines compounds have been reported by Ref . The results show that the number and relative intensities of the spectrum peaks, and splittings are identical with the target product (Pc1, Pc2) structure.
All the experiments were performed with samples dissolved in N, N-dimethylformamide (DMF). The thickness of the sample cell containing the solution is 1 mm. The solution is of concentration 4.53 × 10−4 mol/L. The absorption spectra of Pc1 and Pc2 in ground state (S0) have been reported by Ref . For both phthalocyanines in DMF there are two strong broad bands: the B-band (π→π*) in the near-UV (λmax = 314 and 298 nm for Pc1 and Pc2, respectively), and the Q-band (n→π*) in the red (λmax = 712 nm for Pc1 and 704 nm for Pc2). It is shown that linear absorption does not appear at pump light 532 nm and for the selected signal the nonlinear absorption appears. The excited-state linear absorption spectra of MPc with different center metals have similar, which have been reported by Ref . The results show that the molecule of first triplet excited state (T1) does not absorb the pump light at 532 nm, but absorbs the probe light at 632.8 nm.
3. Theory and experiment
As shown in Fig. 1(b), the molecules of S0 absorb photon of the pump light and transit to vibrational level of the first excited singlet state (S1), and then the molecules of S1 relax back to S0 or make intersystem-crossing transition to T1. The molecules of T1 absorb photon of the probe light and transit to a higher excited level (Tn). Because the lifetime of Tn is very short, and the molecules returns to T1. Molecules of T1 have a longer lifetime, and can return back to S0 in the form of non-radiative transition. Then the molecular population in T1 disappears. Therefore, the light intensity of the probe light restores to its original strength, and under the control of the pulse light, the switch operation of probe light is realized.
The characteristics of A-OS and modulation of the samples are investigated by pump-probe method. The experimental arrangements have been reported by Ref , as show in Fig. 2 . In our experiment, a frequency-doubled Nd: YAG 532 nm laser with repetition rate of 10 Hz is employed to provide exciting pulse, and its pulse width is 10 ns. A He-Ne laser of continuous wave with 632.8 nm is used as the probe light.
Because the lifetime of Tn is very short (less than ps), the population of these levels can be neglected. The population changes can be described by the rate equations of different levels as21]. γl, σ0, and IL are pump frequency, absorption cross-section of S0 state and pump intensity.
The modulating pump laser pulse is given by
The pump peak power density IL0 is 1.2 MW∕cm2. IP is incident power density on the surface of the sample. Boundary conditions take the following form22]. Optical switching characteristics, namely the changes in the normalized transmitted intensity of the probe beam with time, can be obtained by simulations using Eqs. (1)-(4).
4. Results and discussion
The curves of N0, NS and NT of Pc1 and Pc2 excited by a single pump pulse are shown in Fig. 3 , obtained using Eqs. (1)-(4). The results show that most of the particle population in T1, and molecules of T1 have a longer lifetime. Molecules of excited states will return to the ground state in a single pulse after a sufficiently long time.
Figure 4 presents the temporal profiles recorded at a few selected wavelengths following photoexcitation of Pc1 and Pc2 in DMF along with the best-ðt functions. The temporal profile is associated with an initial rise of excited-state absorption (ESA) with the instrument response time limit followed by further biexponential rise of ESA with the lifetimes of about 7.8 and 4.3 ps (internal conversion time τIC) for Pc1 and Pc2, respectively. Long decay time of ESA indicates the long lifetime of the S1 state. This is supported by a long ñuorescence (FL) lifetime of the S1 state, which has been determined to be about 18.5 and 7.5 ns for Pc1 and Pc2, respectively, using time correlated single photon counting technique. The excited state dynamics of samples in DMF can be schematically represented by the potential energy surface (PES) diagram as shown in Fig. 1(c). The FL lifetime is similar to pulse-width of the pump light; the lifetimes of Tn and Sn are very short (less than ps). Therefore, the analysis shows that the response time of probe light is not determined by the lifetime of S1 of the molecule.
Depending on the energy levels of these molecules, one would expect multi-exponential decay following different excitation mechanisms. As shown in Fig. 5 , the transmittance signal of the experimental measurement of the sample changes with the time, based on pumped light. In the pumping processing, the pumping field excites molecules from the ground state to the excited states. The absorption of excited state leads to the decrease of probe field energy. When the molecules return back to the ground state, the absorption at the excited state disappears. The probing power resumes the initial value. The corresponding time of this process is considered as the lifetime of the excited state, which is equal to the A-OS response time. For Pc2 switch-off and -on time are 1.2 and 11.6 μs, and for Pc1 the corresponding time are 2.3 and 7.8 μs, under the same pump peak power density. The results show that the A-OS response time of Pc2 is longer than that of Pc1. Because the metal-zinc is introduced into phthalocyanine, and the d-orbital of Zn2+ neither participates in composition of the frontier orbitals, nor interacts with of the macrocyclic ligand orbital mixture, making that the particles number in excited states has longer duration.
Pc1 and Pc2 are closely related compounds. Both of them exhibit strong absorption in the 300–400 nm and 600–780 nm range and very low absorption at 532 nm, as show in Ref . It would also require larger intensity of the pump beam to populate triplet state. When the incident light intensity I0 increases, the majority of particles are excited to T1, which plays a dominant role for the nonlinear absorption. The nonlinear absorption of organic molecules for long-pulsed nanosecond light is mainly caused by the first triplet excited-state.
Figure 6 shows the dependence of the normalized transmittance of the probe light on the energy of pump pulse. The results show that the normalized transmittance decreases with increasing the energy of pump pulse. As energy of pump pulse increases, population of the first triplet state increases accordingly, resulting in the decreases of probe light normalized transmittance. Therefore, we can adjust the energy of pump pulse to modulate the probe light transmittance. However, the modulation of probe light transmittance saturates after a certain value due to saturation of the population of triplet state.
Moreover, we further studied the eﬀect of variation of pump energy on transmitted probe beam intensity for diﬀerent concentrations of Pc2. As the concentration of the sample increases, percentage modulation of the probe beam increases due to larger population of the T1, as show in Fig. 7 . The inset in Fig. 7 shows normalized modulation sensitivity (q = ΔM/ΔE) of Pc2 at the diﬀerent concentrations. For Pc2 samples of 23%, 48% and 68% linear transmission (at 532 nm), the probe beam gets modulated by 11%, 30% and 20%, respectively, with 1.2 mJ pump energy. After a certain value of concentration (48%) further increase in concentration results in decrease in the modulation of the probe beam due to increase in the linear absorption of the probe beam. Therefore, modulation characteristics of A-OS for concentration 68% is decrease. There is an optimum value of sample concentration which maximum modulation of the probe beam can be achieved. Percentage modulation of the probe beam of Pc1 is similar to that of Pc2, because they are closely related compounds.
We have investigated the triplet state lifetime of Pc1 and Pc2 in DMF. The experimental results show that the lifetime of the first triplet excited state of the Pc1 faster than that of Pc2 at the same intensitiy. Switching results have been explained by carrying out a theoretical analysis based on the rate equation model. The theoretical fits are in good agreement with the experimental results. The results shows that the A-OS response time is determined by the lifetime of the first triplet excited state of molecule. In addition, under the modulation excitation of the pump, both Pc1 and Pc2 have good modulation characteristic of A-OS. We hope the Pc and MPc based switches would be potentially useful in optical signal processing.
The research is supported by the National Natural Science Foundation of China under Grant No. 61078006 and 61275066, and the National Youth Natural Science Foundation of China under Grant No. 51002041.
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