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Ultrafast excitation relaxation in titanylphthalocyanine thin film
Optical Materials 27 (2004) 377–382 www.elsevier.com/locate/optmat
Ultrafast excitation relaxation in titanylphthalocyanine thin film Jiong Zhou, Jun Mi, Rongyi Zhu, Bo Li, Shixiong Qian
*
Department of Physics, Fudan University, Shanghai 200433, PR China Received 9 January 2004
Abstract Ultrafast excitation relaxation in the whole Q band of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond time-resolved pump–probe technique. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the laser beam and consists of three quite different processes: an ultrafast process with a lifetime of 0.5–5 ps, a fast and a long-lived processes with lifetimes of about 5–10 ps and longer than 100 ps, respectively. The initial ultrafast decay appearing to be excitation intensity dependent is suggested to represent a bimolecular exciton–exciton annihilation process with a t1/2 time dependence of the excited-state population, assigned to a one-dimensional exciton diffusion. The exciton– exciton annihilation is observed in the pump intensity as low as 0.27 GW/cm2. Ó 2004 Elsevier B.V. All rights reserved. PACS: 71.35.Cc; 78.66.w; 42.65.k; 82.53.k Keywords: Phthalocyanine; Excited state; Ultrafast dynamics
1. Introduction Phthalocyanines (Pc) are promising materials to be applied in photonic devices, for example optical switching and optical limiting filters [1,2], for their unique properties such as large nonlinear optical susceptibility and ultrafast optical response due to their macrocycle delocalized 18 p-electrons systems [3]. These systems are characterized by ultrafast response, chemical and thermal stability, relative ease of thin-film fabrication, and a high laser damage threshold. The relaxation mechanism of the light-induced excitations is one of the most important factors determining the optical properties and potential applications of such materials. Several studies in femtosecond domain have been reported on the ultrafast excited-state dynamics of phthalocyanine film [4–9]. Ho and Peyghambarian [4] found that the polycrystalline AlClPc film possesses two extre*
Corresponding author. Tel.: +86 21 656 42084; fax: +86 21 656 41344/651 04949. E-mail address: [email protected] (S. Qian). 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.09.003
mely fast routes of ground state recovery, i.e., a fast exciton–exciton annihilation which determines the subpicosecond kinetics at high excitation intensities and the linear relaxation on the picosecond time scale due to exciton–phonon coupling. From the ultrafast evolution of the transient absorption spectra of phthalocyanines, Williams et al. suggested that a subgap state is present, which is occupied during the first picosecond after the film excitation by a 100 fs pulse [5]. Gulbinas et al. [6,7] ascribed the ultrafast blue shift of the red edge of the induced absorption band to the exciton relaxation via excitation transfer between different molecular species. The excited-state dynamics of phthalocyanines was found to be strongly dependent on the molecular arrangement (phase form) and to be sensitive to the host environment [8,9]. In thin Pc film, where the molecules are spaced near Van der Waals distances, electronic coupling and energy transfer between the Pc molecules can become efficient, and exciton decay rates can become larger than that for the monomer. This can significantly affect the relaxation dynamics of the photoexcited states. When there is a high exciton densities, nonlinear
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J. Zhou et al. / Optical Materials 27 (2004) 377–382
bimolecular exciton–exciton annihilation becomes dominating. All these previous studies have observed an ultrafast nonlinear annihilation of exciton which determines the main relaxation channel on a ultrashort time scale. Intensity-dependent bimolecular decay processes are thus expected to play an important role in the dynamics of optically excited Pc films. For the nonplanar conformation of the titanylphthalocyanine (TiOPc) molecule, the central Ti ion (TiO2+) locates significantly out of the convex molecular plane and forms a square pyramid together with four nitrogen atoms. TiOPc possesses a permanent dipole perpendicular to the Pc ring plane, which results in a relative larger and faster nonlinear optical response compared with other planar conformation Pcs, such as magnesium phthalocyanine and zinc phthalocyanine. TiOPc is one of the most sensitive organic photoconductors in the near-IR region and therefore, it is a subject of numerous experimental and theoretical investigations. Oka et al. [10] analyzed crystalline structure of various crystal modifications. Absorption spectra of molecular-beamdeposited films were investigated by Yamashita et al. [11]. Electroabsorption measurements performed by Saito et al. [12] revealed the charge transfer character of the near infrared absorption band of phase II and Y-form TiOPc. Dynamics of the charge carrier photogeneration in Y-form TiOPc was investigated by Popovic et al. [13]. Gulbinas et al. [14] and Mizuguchi et al. [15] performed careful analysis of the absorption spectra of TiOPc solids and explained their peculiarities in terms of molecular distortion. Recently, Gulbinas [16] systematically studied the transient absorption of photoexcited titanylphthalocyanine in various molecular arrangements, especially in m-form. However, there is not much knowledge about the transient behavior of amorphous form TiOPc in these investigations. From the above viewpoints, the present study was focused on the study of the relaxation mechanisms of the excited-state in amorphous TiOPc thin films fabricated by physical jet deposition, by using femtosecond timeresolved pump–probe technique. Intense wavelength dependent dynamics in the whole Q band of TiOPc was observed, and an ultrafast intensity dependant excitation relaxation was followed by a fast decaying process and a long-lived process. After the description of the experimental, a detailed discussion was performed.
elsewhere [17]. TiOPc powder was heated to around 390 °C in vacuum chamber at pressure of 102 Torr, the vapor of TiOPc was carried by the high speed flowing Ar-gas and deposited on the fused silica substrate. The thickness of the film was about 1 lm. The absorption spectrum was measured using UV3101-PC spectrophotometer. The excited-states dynamics of titanylphthalocyanine film was studied by using femtosecond time-resolved pump–probe technique. To investigate the dynamics in whole Q band of the TiOPc, we used two laser systems to generate the pump and probe beams: First, we used the laser beam with pulses of 120 fs duration, 5 nJ pulse energy and 82 MHz repetition rate, which was generated by a mode-locked Ti-Sapphire oscillator (Spectra Physics, Tsunami). In the experiment, we set the wavelength of the output pulses at 800 and 840 nm. For the measurement at shorter wavelength, we used the second laser source. The pulses at 800 nm from the oscillation stage were amplified by a regenerated amplifier pumped by a Q-switched Nd:YLF laser. The amplified pulses were used as the pump source of an optical parametric amplifier to produce tunable infrared laser pulses at wavelength from 1.1 to 1.4 lm and frequency was doubled using a BBO crystal to yield pulses at 550–700 nm. In the experiment, the output beam was split into two beams, the intense beam was chopped and was used as a pump after passing through a variable optical delay line, while the second weak beam was used as a probe beam. The intensity of the pump beam was controlled by a quarter-wave plate-polarizer combination. Two parallel polarized beams were overlapped spatially on the sample, and were focused to a 50-lm-diameter spots by a lens with focal length of 5 cm. After transmitting through the sample, the pump beam was blocked by an aperture and the differential transmission spectrum (DTS), which is the difference between the probe transmission in the presence and absence of the pump pulse, i.e. DTS = T T0, where T and T0 are the perturbed and unperturbed transmissions detected by a photodiode, was amplified by a lock-in amplifier and monitored as a function of time delay between the pump and the probe beam.
3. Results and discussion 3.1. Steady-state absorption spectrum
2. Experimental Titanylphthalocyanine powder was supplied by Jiaotong University, Shanghai. The synthesis procedure was similar to other metallophthalocyanines (MPc) and the powder was purified by sublimation process. The titanylphthalocyanine thin films were fabricated by physical jet deposition method described in detail
The Q band absorption spectrum of the TiOPc thin film under consideration is shown in Fig. 1. The inset is the TiOPc molecular structure. The TiOPc thin film has a broad Q absorption band extending from 550 to 900 nm, with a peak at 720 nm and a shoulder around 650 nm. Similar spectral characteristics have been observed in other papers [12,15,16,18], and were assigned
J. Zhou et al. / Optical Materials 27 (2004) 377–382
379
0.8 0.8
N
0.6
N
Ti
N
800 nm 670 nm 665 nm 650 nm 635 nm 620 nm 610 nm 600 nm
N
0.4
N N
∆ T(a.u.)
Absorbance
840 nm
N O N
0.4
0.0
590 nm
-0.4
0.2
568 nm
0.0 400
-0.8
500
600
700
800
900
1000
Wavelength (nm) Fig. 1. The absorption spectrum of titanylphthalocyanine film in the visible region. The molecular structure of TiOPc is given in the inset.
to the amorphous phase. The TiOPc molecule consists of a planar phthalocyanine ring with a titanium ion at its centre and an out-of-plane oxygen ion bonded to the titanium. The ring contains a two-dimensional conjugated p-electron system where 18 p-electrons are distributed over the 16 carbon–nitrogen sites of the inner ring. This delocalized system has two highly allowed p–p* electronic transitions resulting in two intense optical absorption bands: the Q-band in the visible region and the B or Soret band in the ultraviolet region [19]. In Pc thin films, the Q-band becomes splitting or much broader as compared to the monomer due to exciton coupling effects (include the Davydov splitting) of the allowed transitions [20,21] or inter-molecular interactions resulting in molecular distortion [15] leading to the reduction of the double degeneration of the two LOMO orbits. As it was shown that molecular distortion is weak in the amorphous phase [15], therefore, exciton coupling effects might be the origin for the broadening of the Q band. The resulting broadened Q-band indicates that the constituent molecules of the thin film experience many local site environments, with the spectral shifts as predicted by the molecular exciton model for closely spaced chromophores [21]. 3.2. Excited-state dynamics The excited state relaxation dynamics of the TiOPc film measured by femtosecond pump–probe technique under the excitation at different wavelength is shown in Fig. 2. We can see that at initial time after excitation, the relaxation dynamics in the whole Q band shows a strong dependence on the wavelength. There is an ultrafast bleaching when the wavelength of pulses is longer than 610 nm, and it changes into photoabsorption when the wavelength is shorter than 610 nm which locates at
-2
0
2
4
6
8
10
12
14
Time delay (ps) Fig. 2. The differential transmission spectrum of TiOPc at different wavelength in the whole Q band.
the blue side of the Q band. From the absorption spectrum, we can see that there is a intense ground state absorption in the 600–800 nm region, while the excited state absorption is small at this region for the TiOPc in the amorphous phase [16]. Thus, a bleaching signal is reasonable due to the saturated absorption. On the contrary, at the wavelength shorter than 610 nm, the absorption of the ground state is weak while the excited state absorption could be more intense than the absorption of the ground state, leading to the observed transient absorption. From this measurement, we can see that TiOPc thin film could be a very useful optical limiting device due to its intense transient absorption at the blue side of the Q band. The differential transmission spectrum was fitted by combination of three exponential decay processes for the whole Q band: DT =T ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ A3 expðt=t3 Þ þ C 0;
ð1Þ
where A1, A2, A3 and t1, t2, t3 are the contributions and lifetimes of three corresponding components, respectively, C0 is a constant. The fitting results are given in Table 1. We can see that the bleaching decay is composed of three processes, an ultrafast component with a lifetime 0.5–5 ps, a fast component lasts about 10– 20 ps, and a long-lived component persists more than 100 ps. Because the ultrafast decay rate appears to be excitation intensity dependent as being shown in Fig. 3, it is considered to represent a bimolecular process, such as exciton–exciton annihilation. It was reported that the exciton–phonon coupling tends to take place in the relatively ordered crystal film [4,8], while in the amorphous TiOPc thin film exciton–phonon coupling is
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Table 1 Parameters of the decay processes with excitation at different wavelengths k (nm)
A1
t1 (ps)
A2
t2 (ps)
A3
t3 (ps)
840 800 650 635 610 600 568
0.83 0.64 0.40 0.33 0.74 1.08 0.76
0.38 0.41 1.448 5.20 0.53 0.82 0.44
0.11 0.29 0.30 0.08 0.04 0.13 0.31
2.53 2.82 10.15 21.58 46.49 6.48 16.66
0.2 0.27 0.36 0.62 0.28 0.17 0.10
>100 >100 >100 >100 >100 >100 >100
comparatively weak. Therefore, we suggest that the ultrafast component can be assigned to exciton–exciton annihilation. From our measurement, we did not observed fluorescence emission from TiOPc film, which is in agreement with other reports [22]. This result demonstrates that the nonradiative relaxation is a dominating process for the depopulation of the excited state in TiOPc film. It was also reported that the inter-system crossing (ISC) process is greatly enhanced in solid phase material and in MPc materials, ISC process could be ranged in ps domain [23], even in fs scale [24], which would dominate the lifetime of the singlet excited state. Therefore, the fast component with recovery time about several ps can be assigned to the decay of singlet excited state via ISC process, while the long-lived component is due to the nonradiative relaxation from triplet state to the ground state [25]. From the data in Table 1, we can find that the relaxation at longer wavelength is faster than that at shorter wavelength. The values of T1 and T2 are becoming bigger when the wavelength is becoming shorter from 840 to 635 nm. We think that this phenomenon is due to the inter-band decay in the excited state. In the solid state MPc, the excited state is always broadened due
3.3. Exciton–exciton annihilation Exciton–exciton annihilation is a bimolecular process. Therefore, its dynamics cannot be described by a single exponential decay process. But the excitation dynamics shown in Fig. 2 was performed under relative low excitation intensity. Under this condition, exciton– exciton annihilation (bimolecular process) is relative weak and the ultrafast process can be phenomenologically approximated as intensity-independent exponential decay process as formula (1) and the decay lifetime for this process was obtained. The intensity dependent of the initial ultrafast decay of the bleaching via exciton–exciton annihilation can be described phenomenologically by SunaÕs theory [26] o nðtÞ ¼ a bðtÞnðtÞ cn2 ðtÞ; ot
ð2Þ
where n(t) is the uniform exciton density, b is the monomolecular decay rate, a = N n(t) is the rate of producing excitons randomly, and c is the macroscopic annihilation rate of the exciton. In the case with intense excitation, (2) can be reduced to o 2 nðtÞ ¼ cnðtÞ : ot
0.6
ð3Þ
2
0.27 GW/cm 2 0.12 GW/cm 2 0.03 GW/cm
0.5
∆ T(a.u.)
to the inter-molecular interactions and thus forms a continuous excited band. The laser beam at shorter wavelength can excite the electron to the higher energy levels in the excited state, while the beam at longer wavelength can only excite the electron to the lower levels in the excited state. After excitation, electron decays from the higher levels to lower levels in the excited state via vibrational decay and then decay to the triplet state by ISC progress. Therefore, if the electron is excited to the higher levels in the excited state, the vibrational decay will be slower, leading to the slower relaxation at shorter wavelength.
0.4 0.3 0.2 0.1 0.0 0
2
4
6
8
10
12
14
Time Delay (ps) Fig. 3. Excitation intensity dependence of DTS decay dynamics under excitation by 120 fs, 800 nm pulses.
The available maximum pump intensity in the present study was about 0.3 GW/cm2, which is below the saturation intensity of several GW/cm2 [27]. It is shown in Fig. 3 that the available pump intensity in the present study is higher than the threshold intensity for TiOPc thin film to occur exciton–exciton annihilation. Therefore Eq. (3) is applicable in the analysis of the nonlinear exciton decay dynamics. Exciton–exciton annihilation is conceived as a composite process, separable into two steps; the migration of the two excitons toward one another, and the annihilation of the two excitons once they are sufficiently close to interact. Previous studies have measured timedependent annihilation rate which are comprised of, and determined by, the individual microscopic rates of these two steps. In principle, however, time-dependent
J. Zhou et al. / Optical Materials 27 (2004) 377–382
energy-transfer rates are expected in measurements of exciton–exciton annihilation. This arise from the fact that subsequent to the initial creation of a spatial homogeneous exciton population, proximate pairs of excitons, will interact first. Progressively greater inter-particle distance result in decreasing characteristic interaction rates. Both the motion-limited diffusion theory and annihilation via Forster long range dipole–dipole interactions lead to a time-dependent annihilation constant, which can approximately be expressed as c = c0th [28], where h may take a value between 1/6 and 1/2, the exact value depending on the dimensionality of the exciton motion. Then, the solution of equation of (2) gives the temporal dependence of the exciton density as 1
n ¼ ð1=nt¼0 þ 2c0 t1h Þ :
ð4Þ
Kinetics obeying the rate law (Eq. (3)) will yield a straight line with slope 2c. We calculated the decay curve from Eq. (4) to fit to the experiment data to find the parameter h. In Fig. 4, the time dependence of the excitation density is plotted as N/n N/n0 versus t1/2. We can see that the best value for h is 1/2, which corresponds to the case of one-dimensional diffusion-limited annihilation. A low-dimensional diffusion is expected because phthalocyanine crystals have a columnar-like stacked structure with different closest-neighbor distances within a plane of molecules and between such planes, the inter-planar distance is the smallest. Therefore, the excitons may migrate along the columnar direction. One-dimensional diffusion-limited exciton–exciton annihilation has been observed by many previous studies [6,7,28].
381
4. Conclusions In this study, ultrafast excitation relaxation of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond pump–probe technique. The main results can be summarized as following: TiOPc thin film shows a broad Q band from 550 to 900 nm, which is characterized by a peak at 720 nm and a shoulder locates at the high energy side around 650 nm. This absorption spectrum is assigned to the amorphous phase. It is exciton coupling effects rather than molecular distortion that results in the broad Q band. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the excitation. In general, the delay processes consist of three quite different processes: an ultrafast process with a lifetime of 0.5–5 fs, a fast and a long-lived processes with lifetimes of 5–10 ps and longer than 100 ps, respectively, which are attributed to ultrafast exciton–exciton annihilation, decay of singlet excited state via ISC process and nonradiative relaxation. The observed exciton–exciton annihilation with a t1/2 time dependence of the excited state population, is explained by using a one-dimensional exciton diffusion model.
Acknowledgment We are grateful to the financial support by National Science Foundation of China under Grant No. 10274013 and 10374020.
2.5 2.0
N/n-N/n
0
References 1.5 1.0 0.5 0.0 0.0
0.5
1.0
t
1/2
1.5
2.0
1/2
(ps )
Fig. 4. Plot of (N/n N/n0) versus t1/2, where N is the molecule density, n0 is the exciton density at initial time after excitation, n is the exciton density at time t after the excitation. Exciton density n and n0 were calculated from the bleaching decay dynamics at 800 nm created by a pump with the intensity of 0.27 GW/cm2.
[1] F.Z. Henari, J. Opt. A: Pure Appl. Opt. 3 (2001) 188. [2] P. Wang, S. Zhang, P. Wu, C. Ye, H. Liu, F. Xi, Chem. Phys. Lett. 340 (2001) 261. [3] Leznoff C.C., Lever A.B.P. (Eds.), Phthalocyanine. Principles and Properties, vol. III, VCH Pulications, New York, 1993. [4] Z.Z. Ho, N. Peyghambarian, Chem. Phys. Lett. 148 (1988) 107. [5] V.S. Williams, S. Mazumdar, N.R. Armstrong, Z.Z. Ho, N. Peyghambarian, J. Phys. Chem. 96 (1992) 4500. [6] V. Gulbinas, M. Chachiilis, A. Persson, S. Svanberg, V. Sundstrom, J. Phys. Chem. 98 (1994) 8118. [7] V. Gulbinas, M. Chachiilis, L. Valkunas, V. Sundstrom, J. Phys. Chem. 100 (1996) 2213. [8] A. Terasaki, M. Hosoda, T. Wada, H. Tada, A. Koma, A. Yamada, H. Sasabe, A.F. Garito, T. Kobayashi, J. Phys. Chem. 96 (1992) 10534. [9] L. Howe, J.Z. Zhang, J. Phys. Chem. A 101 (1997) 3207. [10] K. Oka, O. Okada, K. Nukada, Jpn. J. Appl. Phys. 31 (1992) 2181.
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[11] A. Yamashita, T. Moruno, T. Hayashi, J. Phys. Chem. 97 (1993) 4567. [12] T. Saito, W. Sisk, T. Kobayashi, S. Suzuki, T. Iwayanagi, J. Phys. Chem. 97 (1993) 8026. [13] Z.D. Popovic, M. Iltaf Khan, S.J. Atherton, A.-M. Hor, J.L. Goodman, J. Phys. Chem. B 102 (1998) 657. [14] V. Gulbinas, R. Jakubenas, S. Pakalnis, A. Undzenas, J. Chem. Phys. 107 (1997) 4927. [15] J. Mizuguchi, G. Ribs, H.R. Karfunkel, J. Phys. Chem. 99 (1995) 16217. [16] V. Gulbinas, Chem. Phys. 261 (2000) 469. [17] J. Qian, C. Xu, S. Qian, W. Peng, Chem. Phys. Lett. 257 (1996) 563. [18] A. Yamashita, T. Maruno, T. Hayashi, J. Phys. Chem. 97 (1993) 4567. [19] K. Toyota, J. Hasegawa, H. Nakatsuji, J. Phys. Chem. A 101 (1997) 446.
[20] L.K. Chau, C.D. England, S. Chen, N.R. Armstrong, J. Phys. Chem. 97 (1993) 2699. [21] M. Kasha, in: B. DiBartolo (Ed.), Spectroscopy of the Excited State, Plenum Press, New York, 1976, p. 337. [22] Y. Sakakiara, R.N. Bera, T. Mizutani, K. Ishida, M. Tokumoto, T. Tani, J. Phys. Chem. B 105 (2001) 1547. [23] A.V. Nikolaitchik, O. Korth, M.A.J. Roders, J. Phys. Chem. A 103 (1999) 7587. [24] J. Rodrigues, C. Kirmaicer, D. Holten, J. Am. Chem. Soc. 111 (1989) 6500. [25] M.K. Casstevens, M. Samoc, J. Pfeger, P.N. Prasad, J. Chem. Phys. 92 (1990) 2019. [26] A. Suna, Phys. Rev. B 1 (1970) 1716. [27] B.I. Greene, R.R. Millard, Phys. Rev. Lett. 55 (1985) 1331. [28] R. Kopelmna, S. Parus, J. Prasad, Chem. Phys. 128 (1988) 209.
Ultrafast excitation relaxation in titanylphthalocyanine thin film Jiong Zhou, Jun Mi, Rongyi Zhu, Bo Li, Shixiong Qian
*
Department of Physics, Fudan University, Shanghai 200433, PR China Received 9 January 2004
Abstract Ultrafast excitation relaxation in the whole Q band of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond time-resolved pump–probe technique. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the laser beam and consists of three quite different processes: an ultrafast process with a lifetime of 0.5–5 ps, a fast and a long-lived processes with lifetimes of about 5–10 ps and longer than 100 ps, respectively. The initial ultrafast decay appearing to be excitation intensity dependent is suggested to represent a bimolecular exciton–exciton annihilation process with a t1/2 time dependence of the excited-state population, assigned to a one-dimensional exciton diffusion. The exciton– exciton annihilation is observed in the pump intensity as low as 0.27 GW/cm2. Ó 2004 Elsevier B.V. All rights reserved. PACS: 71.35.Cc; 78.66.w; 42.65.k; 82.53.k Keywords: Phthalocyanine; Excited state; Ultrafast dynamics
1. Introduction Phthalocyanines (Pc) are promising materials to be applied in photonic devices, for example optical switching and optical limiting filters [1,2], for their unique properties such as large nonlinear optical susceptibility and ultrafast optical response due to their macrocycle delocalized 18 p-electrons systems [3]. These systems are characterized by ultrafast response, chemical and thermal stability, relative ease of thin-film fabrication, and a high laser damage threshold. The relaxation mechanism of the light-induced excitations is one of the most important factors determining the optical properties and potential applications of such materials. Several studies in femtosecond domain have been reported on the ultrafast excited-state dynamics of phthalocyanine film [4–9]. Ho and Peyghambarian [4] found that the polycrystalline AlClPc film possesses two extre*
Corresponding author. Tel.: +86 21 656 42084; fax: +86 21 656 41344/651 04949. E-mail address: [email protected] (S. Qian). 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.09.003
mely fast routes of ground state recovery, i.e., a fast exciton–exciton annihilation which determines the subpicosecond kinetics at high excitation intensities and the linear relaxation on the picosecond time scale due to exciton–phonon coupling. From the ultrafast evolution of the transient absorption spectra of phthalocyanines, Williams et al. suggested that a subgap state is present, which is occupied during the first picosecond after the film excitation by a 100 fs pulse [5]. Gulbinas et al. [6,7] ascribed the ultrafast blue shift of the red edge of the induced absorption band to the exciton relaxation via excitation transfer between different molecular species. The excited-state dynamics of phthalocyanines was found to be strongly dependent on the molecular arrangement (phase form) and to be sensitive to the host environment [8,9]. In thin Pc film, where the molecules are spaced near Van der Waals distances, electronic coupling and energy transfer between the Pc molecules can become efficient, and exciton decay rates can become larger than that for the monomer. This can significantly affect the relaxation dynamics of the photoexcited states. When there is a high exciton densities, nonlinear
378
J. Zhou et al. / Optical Materials 27 (2004) 377–382
bimolecular exciton–exciton annihilation becomes dominating. All these previous studies have observed an ultrafast nonlinear annihilation of exciton which determines the main relaxation channel on a ultrashort time scale. Intensity-dependent bimolecular decay processes are thus expected to play an important role in the dynamics of optically excited Pc films. For the nonplanar conformation of the titanylphthalocyanine (TiOPc) molecule, the central Ti ion (TiO2+) locates significantly out of the convex molecular plane and forms a square pyramid together with four nitrogen atoms. TiOPc possesses a permanent dipole perpendicular to the Pc ring plane, which results in a relative larger and faster nonlinear optical response compared with other planar conformation Pcs, such as magnesium phthalocyanine and zinc phthalocyanine. TiOPc is one of the most sensitive organic photoconductors in the near-IR region and therefore, it is a subject of numerous experimental and theoretical investigations. Oka et al. [10] analyzed crystalline structure of various crystal modifications. Absorption spectra of molecular-beamdeposited films were investigated by Yamashita et al. [11]. Electroabsorption measurements performed by Saito et al. [12] revealed the charge transfer character of the near infrared absorption band of phase II and Y-form TiOPc. Dynamics of the charge carrier photogeneration in Y-form TiOPc was investigated by Popovic et al. [13]. Gulbinas et al. [14] and Mizuguchi et al. [15] performed careful analysis of the absorption spectra of TiOPc solids and explained their peculiarities in terms of molecular distortion. Recently, Gulbinas [16] systematically studied the transient absorption of photoexcited titanylphthalocyanine in various molecular arrangements, especially in m-form. However, there is not much knowledge about the transient behavior of amorphous form TiOPc in these investigations. From the above viewpoints, the present study was focused on the study of the relaxation mechanisms of the excited-state in amorphous TiOPc thin films fabricated by physical jet deposition, by using femtosecond timeresolved pump–probe technique. Intense wavelength dependent dynamics in the whole Q band of TiOPc was observed, and an ultrafast intensity dependant excitation relaxation was followed by a fast decaying process and a long-lived process. After the description of the experimental, a detailed discussion was performed.
elsewhere [17]. TiOPc powder was heated to around 390 °C in vacuum chamber at pressure of 102 Torr, the vapor of TiOPc was carried by the high speed flowing Ar-gas and deposited on the fused silica substrate. The thickness of the film was about 1 lm. The absorption spectrum was measured using UV3101-PC spectrophotometer. The excited-states dynamics of titanylphthalocyanine film was studied by using femtosecond time-resolved pump–probe technique. To investigate the dynamics in whole Q band of the TiOPc, we used two laser systems to generate the pump and probe beams: First, we used the laser beam with pulses of 120 fs duration, 5 nJ pulse energy and 82 MHz repetition rate, which was generated by a mode-locked Ti-Sapphire oscillator (Spectra Physics, Tsunami). In the experiment, we set the wavelength of the output pulses at 800 and 840 nm. For the measurement at shorter wavelength, we used the second laser source. The pulses at 800 nm from the oscillation stage were amplified by a regenerated amplifier pumped by a Q-switched Nd:YLF laser. The amplified pulses were used as the pump source of an optical parametric amplifier to produce tunable infrared laser pulses at wavelength from 1.1 to 1.4 lm and frequency was doubled using a BBO crystal to yield pulses at 550–700 nm. In the experiment, the output beam was split into two beams, the intense beam was chopped and was used as a pump after passing through a variable optical delay line, while the second weak beam was used as a probe beam. The intensity of the pump beam was controlled by a quarter-wave plate-polarizer combination. Two parallel polarized beams were overlapped spatially on the sample, and were focused to a 50-lm-diameter spots by a lens with focal length of 5 cm. After transmitting through the sample, the pump beam was blocked by an aperture and the differential transmission spectrum (DTS), which is the difference between the probe transmission in the presence and absence of the pump pulse, i.e. DTS = T T0, where T and T0 are the perturbed and unperturbed transmissions detected by a photodiode, was amplified by a lock-in amplifier and monitored as a function of time delay between the pump and the probe beam.
3. Results and discussion 3.1. Steady-state absorption spectrum
2. Experimental Titanylphthalocyanine powder was supplied by Jiaotong University, Shanghai. The synthesis procedure was similar to other metallophthalocyanines (MPc) and the powder was purified by sublimation process. The titanylphthalocyanine thin films were fabricated by physical jet deposition method described in detail
The Q band absorption spectrum of the TiOPc thin film under consideration is shown in Fig. 1. The inset is the TiOPc molecular structure. The TiOPc thin film has a broad Q absorption band extending from 550 to 900 nm, with a peak at 720 nm and a shoulder around 650 nm. Similar spectral characteristics have been observed in other papers [12,15,16,18], and were assigned
J. Zhou et al. / Optical Materials 27 (2004) 377–382
379
0.8 0.8
N
0.6
N
Ti
N
800 nm 670 nm 665 nm 650 nm 635 nm 620 nm 610 nm 600 nm
N
0.4
N N
∆ T(a.u.)
Absorbance
840 nm
N O N
0.4
0.0
590 nm
-0.4
0.2
568 nm
0.0 400
-0.8
500
600
700
800
900
1000
Wavelength (nm) Fig. 1. The absorption spectrum of titanylphthalocyanine film in the visible region. The molecular structure of TiOPc is given in the inset.
to the amorphous phase. The TiOPc molecule consists of a planar phthalocyanine ring with a titanium ion at its centre and an out-of-plane oxygen ion bonded to the titanium. The ring contains a two-dimensional conjugated p-electron system where 18 p-electrons are distributed over the 16 carbon–nitrogen sites of the inner ring. This delocalized system has two highly allowed p–p* electronic transitions resulting in two intense optical absorption bands: the Q-band in the visible region and the B or Soret band in the ultraviolet region [19]. In Pc thin films, the Q-band becomes splitting or much broader as compared to the monomer due to exciton coupling effects (include the Davydov splitting) of the allowed transitions [20,21] or inter-molecular interactions resulting in molecular distortion [15] leading to the reduction of the double degeneration of the two LOMO orbits. As it was shown that molecular distortion is weak in the amorphous phase [15], therefore, exciton coupling effects might be the origin for the broadening of the Q band. The resulting broadened Q-band indicates that the constituent molecules of the thin film experience many local site environments, with the spectral shifts as predicted by the molecular exciton model for closely spaced chromophores [21]. 3.2. Excited-state dynamics The excited state relaxation dynamics of the TiOPc film measured by femtosecond pump–probe technique under the excitation at different wavelength is shown in Fig. 2. We can see that at initial time after excitation, the relaxation dynamics in the whole Q band shows a strong dependence on the wavelength. There is an ultrafast bleaching when the wavelength of pulses is longer than 610 nm, and it changes into photoabsorption when the wavelength is shorter than 610 nm which locates at
-2
0
2
4
6
8
10
12
14
Time delay (ps) Fig. 2. The differential transmission spectrum of TiOPc at different wavelength in the whole Q band.
the blue side of the Q band. From the absorption spectrum, we can see that there is a intense ground state absorption in the 600–800 nm region, while the excited state absorption is small at this region for the TiOPc in the amorphous phase [16]. Thus, a bleaching signal is reasonable due to the saturated absorption. On the contrary, at the wavelength shorter than 610 nm, the absorption of the ground state is weak while the excited state absorption could be more intense than the absorption of the ground state, leading to the observed transient absorption. From this measurement, we can see that TiOPc thin film could be a very useful optical limiting device due to its intense transient absorption at the blue side of the Q band. The differential transmission spectrum was fitted by combination of three exponential decay processes for the whole Q band: DT =T ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ A3 expðt=t3 Þ þ C 0;
ð1Þ
where A1, A2, A3 and t1, t2, t3 are the contributions and lifetimes of three corresponding components, respectively, C0 is a constant. The fitting results are given in Table 1. We can see that the bleaching decay is composed of three processes, an ultrafast component with a lifetime 0.5–5 ps, a fast component lasts about 10– 20 ps, and a long-lived component persists more than 100 ps. Because the ultrafast decay rate appears to be excitation intensity dependent as being shown in Fig. 3, it is considered to represent a bimolecular process, such as exciton–exciton annihilation. It was reported that the exciton–phonon coupling tends to take place in the relatively ordered crystal film [4,8], while in the amorphous TiOPc thin film exciton–phonon coupling is
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Table 1 Parameters of the decay processes with excitation at different wavelengths k (nm)
A1
t1 (ps)
A2
t2 (ps)
A3
t3 (ps)
840 800 650 635 610 600 568
0.83 0.64 0.40 0.33 0.74 1.08 0.76
0.38 0.41 1.448 5.20 0.53 0.82 0.44
0.11 0.29 0.30 0.08 0.04 0.13 0.31
2.53 2.82 10.15 21.58 46.49 6.48 16.66
0.2 0.27 0.36 0.62 0.28 0.17 0.10
>100 >100 >100 >100 >100 >100 >100
comparatively weak. Therefore, we suggest that the ultrafast component can be assigned to exciton–exciton annihilation. From our measurement, we did not observed fluorescence emission from TiOPc film, which is in agreement with other reports [22]. This result demonstrates that the nonradiative relaxation is a dominating process for the depopulation of the excited state in TiOPc film. It was also reported that the inter-system crossing (ISC) process is greatly enhanced in solid phase material and in MPc materials, ISC process could be ranged in ps domain [23], even in fs scale [24], which would dominate the lifetime of the singlet excited state. Therefore, the fast component with recovery time about several ps can be assigned to the decay of singlet excited state via ISC process, while the long-lived component is due to the nonradiative relaxation from triplet state to the ground state [25]. From the data in Table 1, we can find that the relaxation at longer wavelength is faster than that at shorter wavelength. The values of T1 and T2 are becoming bigger when the wavelength is becoming shorter from 840 to 635 nm. We think that this phenomenon is due to the inter-band decay in the excited state. In the solid state MPc, the excited state is always broadened due
3.3. Exciton–exciton annihilation Exciton–exciton annihilation is a bimolecular process. Therefore, its dynamics cannot be described by a single exponential decay process. But the excitation dynamics shown in Fig. 2 was performed under relative low excitation intensity. Under this condition, exciton– exciton annihilation (bimolecular process) is relative weak and the ultrafast process can be phenomenologically approximated as intensity-independent exponential decay process as formula (1) and the decay lifetime for this process was obtained. The intensity dependent of the initial ultrafast decay of the bleaching via exciton–exciton annihilation can be described phenomenologically by SunaÕs theory [26] o nðtÞ ¼ a bðtÞnðtÞ cn2 ðtÞ; ot
ð2Þ
where n(t) is the uniform exciton density, b is the monomolecular decay rate, a = N n(t) is the rate of producing excitons randomly, and c is the macroscopic annihilation rate of the exciton. In the case with intense excitation, (2) can be reduced to o 2 nðtÞ ¼ cnðtÞ : ot
0.6
ð3Þ
2
0.27 GW/cm 2 0.12 GW/cm 2 0.03 GW/cm
0.5
∆ T(a.u.)
to the inter-molecular interactions and thus forms a continuous excited band. The laser beam at shorter wavelength can excite the electron to the higher energy levels in the excited state, while the beam at longer wavelength can only excite the electron to the lower levels in the excited state. After excitation, electron decays from the higher levels to lower levels in the excited state via vibrational decay and then decay to the triplet state by ISC progress. Therefore, if the electron is excited to the higher levels in the excited state, the vibrational decay will be slower, leading to the slower relaxation at shorter wavelength.
0.4 0.3 0.2 0.1 0.0 0
2
4
6
8
10
12
14
Time Delay (ps) Fig. 3. Excitation intensity dependence of DTS decay dynamics under excitation by 120 fs, 800 nm pulses.
The available maximum pump intensity in the present study was about 0.3 GW/cm2, which is below the saturation intensity of several GW/cm2 [27]. It is shown in Fig. 3 that the available pump intensity in the present study is higher than the threshold intensity for TiOPc thin film to occur exciton–exciton annihilation. Therefore Eq. (3) is applicable in the analysis of the nonlinear exciton decay dynamics. Exciton–exciton annihilation is conceived as a composite process, separable into two steps; the migration of the two excitons toward one another, and the annihilation of the two excitons once they are sufficiently close to interact. Previous studies have measured timedependent annihilation rate which are comprised of, and determined by, the individual microscopic rates of these two steps. In principle, however, time-dependent
J. Zhou et al. / Optical Materials 27 (2004) 377–382
energy-transfer rates are expected in measurements of exciton–exciton annihilation. This arise from the fact that subsequent to the initial creation of a spatial homogeneous exciton population, proximate pairs of excitons, will interact first. Progressively greater inter-particle distance result in decreasing characteristic interaction rates. Both the motion-limited diffusion theory and annihilation via Forster long range dipole–dipole interactions lead to a time-dependent annihilation constant, which can approximately be expressed as c = c0th [28], where h may take a value between 1/6 and 1/2, the exact value depending on the dimensionality of the exciton motion. Then, the solution of equation of (2) gives the temporal dependence of the exciton density as 1
n ¼ ð1=nt¼0 þ 2c0 t1h Þ :
ð4Þ
Kinetics obeying the rate law (Eq. (3)) will yield a straight line with slope 2c. We calculated the decay curve from Eq. (4) to fit to the experiment data to find the parameter h. In Fig. 4, the time dependence of the excitation density is plotted as N/n N/n0 versus t1/2. We can see that the best value for h is 1/2, which corresponds to the case of one-dimensional diffusion-limited annihilation. A low-dimensional diffusion is expected because phthalocyanine crystals have a columnar-like stacked structure with different closest-neighbor distances within a plane of molecules and between such planes, the inter-planar distance is the smallest. Therefore, the excitons may migrate along the columnar direction. One-dimensional diffusion-limited exciton–exciton annihilation has been observed by many previous studies [6,7,28].
381
4. Conclusions In this study, ultrafast excitation relaxation of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond pump–probe technique. The main results can be summarized as following: TiOPc thin film shows a broad Q band from 550 to 900 nm, which is characterized by a peak at 720 nm and a shoulder locates at the high energy side around 650 nm. This absorption spectrum is assigned to the amorphous phase. It is exciton coupling effects rather than molecular distortion that results in the broad Q band. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the excitation. In general, the delay processes consist of three quite different processes: an ultrafast process with a lifetime of 0.5–5 fs, a fast and a long-lived processes with lifetimes of 5–10 ps and longer than 100 ps, respectively, which are attributed to ultrafast exciton–exciton annihilation, decay of singlet excited state via ISC process and nonradiative relaxation. The observed exciton–exciton annihilation with a t1/2 time dependence of the excited state population, is explained by using a one-dimensional exciton diffusion model.
Acknowledgment We are grateful to the financial support by National Science Foundation of China under Grant No. 10274013 and 10374020.
2.5 2.0
N/n-N/n
0
References 1.5 1.0 0.5 0.0 0.0
0.5
1.0
t
1/2
1.5
2.0
1/2
(ps )
Fig. 4. Plot of (N/n N/n0) versus t1/2, where N is the molecule density, n0 is the exciton density at initial time after excitation, n is the exciton density at time t after the excitation. Exciton density n and n0 were calculated from the bleaching decay dynamics at 800 nm created by a pump with the intensity of 0.27 GW/cm2.
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