Photophysics and Triplet−Triplet Annihilation Analysis for Axially

Fax: 86-10-82617315. ... A novel solid matrix doped with tetra-α-(4-tert-butylphenoxy) gallium phthalocyanine (TBP-GaPc) was prepared. Photophysical ...
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J. Phys. Chem. C 2009, 113, 11943–11951

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Photophysics and Triplet-Triplet Annihilation Analysis for Axially Substituted Gallium Phthalocyanine Doped in Solid Matrix† Jun Chen,‡ Shayu Li,‡ Fangbin Gong,‡ Zhipei Yang,‡ Shuangqing Wang,*,‡ Huijun Xu,‡ Yi Li,§ Jin Shi Ma,‡ and Guoqiang Yang*,‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, and Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: April 29, 2009

A novel solid matrix doped with tetra-R-(4-tert-butylphenoxy) gallium phthalocyanine (TBP-GaPc) was prepared. Photophysical properties were investigated in order to analyze the effect of triplet-triplet annihilation (TTA) of the phthalocyanine (Pc) both in a solid matrix and in a tetrahydrofuran (THF) solution at different concentrations. The rigid surroundings of the Pc molecules in the solid matrix greatly reduce the aggregation, resulting in much lower probability of the TTA process. With the deductive kinetic models, the rate constant and the percentage of TTA were obtained. In a THF solution, the summation of the first-order exponential decay rate constants k1 and the apparent rate constants of triplet-triplet annihilation k′2 is much larger than that in the solid matrix, and the percentages of TTA in THF solutions are much higher (7.74-50.9%) than that in solid matrixes (1.05-6.51%) from 5.0 × 10-6 to 1.0 × 10-3 M. The results of the TTA process and photophysical properties strongly suggest that the Pc doped in the solid matrix will give better optical limiting behaviors than that in liquid solution. 1. Introduction In the past decades, phthalocyanines (Pcs), as one kind of classical dyes, were widely used in advanced materials including optical limiting materials, semiconductor devices, electrochromic display devices, and photosensitizers in photodynamic therapy (PDF).1-6 However, Pcs have strong intermolecular interactions because of their extensive delocalization of the 18-π electron conjugation system of the planar macrocycle, which results in strong influences on their photophysical properties especially for those involving excitons such as fluorescence quenching, singlet-singlet annihilation, triplet-triplet annihilation (TTA), etc.7-11 Optical limiting (OL) performance can be improved by reverse saturable absorption (RSA) from excited triplet state absorption.12-16 A five-level model can be used to discuss the RSA process of Pcs (Figure 1). Each molecule at the S0 state absorbs the first photon to the S1 state; then, part of the S1 state molecules undergo fast intersystem crossing to the T1 state. When the irradiation is strong enough, the molecules at the excited states continue to absorb another photon to reach higher excited states Sn or Tn. When the excited state absorption cross section σex is greater than that of the ground state σg, the process is a so-called reverse saturable absorption, which is the origin of the optical limiting performance. However, for the nanosecond scale optical limiting, the S1 to Sn absorption is too weak to be ignored, compared to the absorption of the T1 to Tn state.17 Thus, for the Pc compound, the optical limiting behaviors greatly depend on the †

Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. Fax: 86-10-82617315. E-mail: [email protected] (G.Y.); [email protected] (S.W.). ‡ Institute of Chemistry, Chinese Academy of Sciences. § Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.

Figure 1. Five-level models of the RSA process for an optical limiting effect.

triplet state absorption from T1 to Tn. Molecules with a higher intersystem crossing rate from S1 to T1 will give a higher quantum yield (ΦT) of the triplet state,18 and a higher triplet quantum yield (ΦT) accompanied with a longer triplet lifetime (τT) will produce higher probability of T1 to Tn absorption (RSA). Thus, the ΦT and τT are important parameters to affect the optical limiting performance. Because the TTA is an important process leading to a quench in the triplet state,19,20 it would greatly weaken the effect of optical limiting materials. The TTA process corresponds to the short-range energy transfer process21 as

T1 + T1 f S1 + S0 in which two triplet molecules yield a singlet excited state and another ground state molecule.22 In addition, some other processes are produced such as delayed fluorescence and internal conversion (S1 f S0) as well as intersystem crossing (S1 f T1). As it is known, the triplet state is deactivated in two major photophysical processes.23,24 One is the first-order exponential decay through phosphorescence and internal conversion directly

10.1021/jp902723h CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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Figure 2. Structure of tetra-R-(4-tert-butylphenoxy) gallium phthalocyanine (TBP-GaPc) and the solid matrix sample doped with TBPGaPc.

from the T1 to S0 state. Another is TTA which results in delayed fluorescence and/or the nonradiative decay process. The two processes are competitive and depend on the concentration and the diffusion constant of the triplet state. In recent years, researchers have focused on the optical limiting properties of solidified samples for practical application as OL materials and some solid OL materials displayed better OL behavior than that in liquid solution.18,25-27 Because the exciton mobility is diffusion controlled,28-30 the effect of TTA for samples in liquid solutions and in a rigid surrounding should be quite different. In order to get a better insight into the difference of OL behaviors between liquid and solidified samples, the TTA process needs to be carefully investigated. Up to now, some researchers have investigated the influence of relaxation on the phosphorescence decay kinetics by the TTA process, 31-34 but rare is the research concerning the TTA process of phthalocyanine-based materials that give weak or nonemission of phosphorescence and delayed fluorescence. Thus, the rate coefficient for the TTA should be an important parameter to be studied for understanding the whole photophysical process and the intrinsic factor for optical limiting performance. In our previous work,35,36 investigations had focused on the optical limiting properties of a series of R-substituted Al, Ga, and In phthalocyanines in liquid solutions and found that Pc molecules displayed excellent OL behaviors. In this paper, novel solid matrixes doped with tetra-R-(4-tert-butylphenoxy) gallium phthalocyanine (TBP-GaPc) in different concentrations were prepared. Photophysical properties were investigated, both in solid matrixes and in tetrahydrofuran (THF) solutions, to observe the effect of concentration and the dispersive medium on the TTA process and to evaluate the effect of TTA on OL performance of phthalocyanine-based materials. The rate constants and the percentages of TTA were obtained and analyzed with the kinetic models of the triplet state. The results are discussed and provide useful information for the application of phthalocyanine-based materials. 2. Experimental Section 2.1. Materials. Tetra-R-(4-tert-butylphenoxy) gallium phthalocyanine (TBP-GaPc) was synthesized with similar methods as described in the literature (Figure 2).35-37 All the samples for TBP-GaPc both in solid matrixes and in THF solutions were at eight different concentrations. EP3302UCL, one kind of commercially available epoxy resin, was used as received. All other organic solvents were dried and distilled by appropriate methods before use. 2.2. Methods. UV-vis absorption spectra were recorded on a Hitachi U-3010 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. The time-resolved fluorescence spectra and fluorescence lifetime

Chen et al. were investigated with an Edinburgh FL900 spectrophotometer. Nanosecond flash photolysis experiments were investigated both in solid matrixes and in argon-saturated THF solutions. The excitation light was the harmonic of the Nd:YAG laser (Continuum Surelite II, 355 nm and 7 ns fwhm). A pulsed xenon arc lamp was used to provide the analyzing light. The laser intensity used in our experiment is 2.69 mJ/pulse (with 10% floating). The configuration of the monitoring light with respect to the excitation laser pulse is right-angle geometry. The liquid samples (1.0 cm quartz cell) and solid matrix (0.2 cm thickness) were settled on the platform at the intersection of the monitoring light and the excitation pulse. All the samples are optically dilute at the laser excitation wavelength. For all the studies related to the flash photolysis in solution and in the solid state, the laser intensity and the other parameters are fixed. The signals were detected by the Edinburgh LP900 and recorded on the Tektronix TDS 3012B oscilloscope and computer. The triplet-minusground state extinction coefficients (∆εT) were calculated by the method of total depletion or saturation.35 The quantum yields of the triplet state were determined by the comparative method,36 using unsubstituted ZnPc in 1-chloronaphthalene as a reference (ΦT ) 0.65). The triplet lifetimes were obtained by kinetic analysis of the transient absorption. 2.3. Preparation of Samples. A mixture of bisphenol A epoxy resin (EP3302UCL-A) (2 g) and polyamine (EP3302UCLB) (1 g) were added to a weighing bottle and stirred at room temperature until the two components were fully mixed; then a solution of TBP-GaPc in CH2Cl2 was added and completely stirred until the mixture was homogeneous and transparent. The mixture was degassed under vacuum at room temperature for about 2 h; then it was carefully poured into a flat-bottomed quartz utensil and desiccated at 80 °C for 8 h to obtain the homogeneous, transparent, and firm solid matrixes doped with TBP-GaPc in eight different concentrations of 1 (5.0 × 10-6 M), 2 (7.5 × 10-6 M), 3 (1.0 × 10-5 M), 4 (5.0 × 10-5 M), 5 (1.0 × 10-4 M), 6 (2.5 × 10-4 M), 7 (5.0 × 10-4 M), and 8 (1.0 × 10-3 M). The corresponding liquid samples of TBPGaPc in THF solutions with the same concentrations of 1-8 were prepared as a reference, as listed in Table 2. 3. Results and Discussion 3.1. Ground State Absorption. Figure 3 shows the ground state absorption spectra of TBP-GaPc in solid matrixes and in THF solutions. The ground state absorption spectra of TBPGaPc both in the solid matrixes and in THF solutions are similar, each showing an intense S0 f S1 transition with a small shoulder in the red region and a broad Soret band around 350 nm. The absorption maxima of TBP-GaPc in the solid matrixes and in THF solutions, λmax, are at 723 and 714 nm, respectively. One distinct difference observed in the THF solution is an additional small shoulder appearing at about 680 nm, which results from the absorption of aggregated Pc molecules, and the intensity is slightly increased with an increasing concentration of TBPGaPc. Great deviation from Beer’s law for the Pcs in THF, particularly in the spectral region around this shoulder, could be considered as the support of aggregation formation. However, this small shoulder is not observed for the TBP-GaPc doped in solid matrixes. Although the length of the light that passes through the solid matrixes (0.2 cm) is shorter than that in THF (1.0 cm), the OD values of all absorptions are higher for the TBP-GaPc doped in solid matrixes than in THF solutions at the same concentration. These results suggest that Pc molecules are more easily aggregated in a THF solution than in a solid matrix, especially at higher concentrations.

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Figure 3. Spectra of ground state absorption for TBP-GaPc in solid matrixes and THF solutions. The concentration for the samples is listed in Table 2. The length of the light passing through the samples is 0.2 and 1.0 cm for solid matrixes and THF solutions, respectively.

Figure 4. Fluorescence emission spectra of TBP-GaPc in solid matrixes and THF solutions (excited at 610 nm). The concentrations for the samples are listed in Table 2.

Figure 5. Time-resolved fluorescence spectra of TBP-GaPc both in THF (a) and in a solid matrix (b) at concentrations of 1.0 × 10-5 M (3), 1.0 × 10-4 M (5), and 1.0 × 10-3 M (8), respectively. λex ) 370 nm.

3.2. Fluorescence Emission. Fluorescence spectra of TBPGaPc in liquid and solidified solutions are shown in Figure 4. At lower concentrations of 1, 2, and 3, TBP-GaPc doped in a solid matrix displays smaller Stokes shifts, because of the rigid surrounding which decreases the energy relaxation of Pc molecules, while in a THF solution, a larger Stokes shift is observed. However, at higher concentrations of 4, 5, 6, 7, and 8, significant red shifts were observed. For the solidified samples, the interaction between Pc molecules might be slightly changed with an increase in the concentration of TBP-GaPc because of the isolation of the molecules by the polymer. Thus, the red shifts could be mainly ascribed to the stronger selfabsorption of the Pc molecules. In the liquid THF solutions, both molecular interaction and self-absorption should be greatly increased with increasing the concentrations of the Pc molecules, resulting in larger red shifts of the observed emission and

changing the spectral shapes as shown in Figure 4. Especially at higher concentrations in THF, the observed emission shifts to much longer wavelengths and becomes much broader. It is the sum results of comprehensive effects of energy relaxation, self-absorption, and molecular aggregation.38 Time-resolved fluorescence and fluorescence lifetime measurements of TBP-GaPc both in a solid matrix and in THF were investigated. The time-resolved emission spectra are shown in Figure 5. In THF, the fluorescence maximum is located at 728 nm with lifetimes of τ1 ) 3.8 ns (94.7%) and τ2 ) 17.3 ns (5.3%) at a lower concentration of 1.0 × 10-5 M. A small shoulder around 785 nm is observed with lifetimes of τ1 ) 4.2 ns (90.5%) and τ2 ) 16.7 ns (9.5%). There is no distinct selfabsorption around the Q band absorption at this lower concentration. With the concentration increasing to 1.0 × 10-4 M, a red shift is observed for the fluorescence maximum, located at

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TABLE 1: Lifetimes of Time-Resolved Fluorescence Emission after Being Fitted by a Sum of Two Exponential Decays samples THF

c/M 1.0 × 10

-5

1.0 × 10-4 1.0 × 10-3 matrix

a

1.0 × 10-5 1.0 × 10-4 1.0 × 10-3

λ (em)/nm

τ1/ns

A1a/%

τ2/ns

A2a/%

730 785 745 785 745 785 745 746 746

3.8 4.2 4.3 4.4 4.3 4.5 0.60 0.44 0.34

94.7 90.5 93.2 88.3 93.0 78.2 96.3 96.1 92.9

17.3 16.7 19.0 21.0 17.7 15.1 2.7 2.4 2.2

5.3 9.5 6.8 11.7 7.0 21.8 3.7 3.9 7.1

Note: An is the percentage for the emissive species with different decay lifetimes.

Figure 6. Transient absorptions of TBP-GaPc in solid matrix and in THF at the concentration of 1.0 × 10-5 M (excited at 355 nm).

742 nm (τ1 ) 4.3 ns, 93.2%; τ2 ) 19.0 ns, 6.8%) with a broad shoulder of 785 nm (τ1 ) 4.4 ns, 88.3%; τ2 ) 21.0 ns, 11.7%). At the higher concentration of 1.0 × 10-3 M, the fluorescence maximum shifts to much longer wavelength and becomes much broader, and the λmax is located at 752 nm (τ1 ) 4.3 ns, 93.0%; τ2 ) 17.7 ns, 7.0%) with a broad shoulder of 785 nm (τ1 ) 4.5 ns, 78.2%; τ2 ) 15.1 ns, 21.8%). A notable self-absorption is observed at the Q band absorption center (714 nm), the fluorescence has almost disappeared even at the beginning of excitation. At lower concentration, the fluorescence maximum located at 728 nm is assigned to the monomer with a lifetime of ∼4 ns. With an increase in the concentration, the A1 and A2 percentages change a little at the wavelength of λmax, while at 785 nm, the A1 percentage decreases from 90.5 to 78.2% and the A2 percentage increases from 9.5 to 21.8%. This indicates that there are two distinct species around 785 nm, one is the vibrational emission of the same luminescent state of the monomer with a lifetime of ∼4 ns and another is the excimer or aggregate emission formed in THF,39 with a longer lifetime of ∼20 ns. At the higher concentration, fluorescence from the monomer quickly disappears because of strong self-absorption and the increasing scale of aggregation or excimer formation. However, in the solid matrix, almost no shoulder is observed at 785 nm in the time-resolved fluorescence spectrum. It indicates that the rigid surrounding of the matrix effectively restricts the molecular vibration and interaction, leading to the disappearance of the shoulder caused by both vibronic progression and aggregation. The fluorescence maximum is 745 nm with lifetimes of τ1 ) 0.6 ns, 96.3% and τ2 ) 2.7 ns, 3.7% at 1.0 × 10-5 M and 746 nm with lifetimes of τ1 ) 0.44 ns, 96.1% and τ2 ) 2.4 ns, 3.9% at 1.0 × 10-4 M (Table 1). Even at the higher concentration of 1.0 × 10-3 M, there is no distinct shoulder observed at the longer wavelength. The decay at 746 nm shows a little change with lifetimes of τ1 ) 0.34 ns, 92.9% and τ2 ) 2.4 ns, 7.1%. The results suggest that the aggregation or excimer formation in a solid matrix is much smaller than that in a liquid solution. The τ1, about 0.4 ns, is the fluorescence

lifetime of the monomer, and the τ2, about 3 ns, should be the lifetime of the excimer or aggregate at a longer wavelength, as that in liquid solution, although the signal at 785 nm is weak in the time-resolved emission measurement. It is interesting that the decay process of the fluorescence both for the monomer and for the aggregate is greatly quickened in the solid matrix. The lifetimes are about 10 times longer in THF than that in the solid matrix. The rigid surroundings in the solid matrix greatly restrict the molecular migration, and the nonradiative transition is accordingly reduced. Thus, the deactivation process by nonradiative transition is greatly restricted, and the intersystem crossing will be a dominant process for the decay of the S1 state. Generally, for the samples of the solid state, a longer excited lifetime will be observed. However, the much shorter lifetimes of the S1 state observed in this solid matrix could be only ascribed to a faster deactivation process caused by the intersystem crossing. The faster decay of the S1 state could be the result of a higher probability of intersystem crossing from the S1 to the T1 state with a faster intersystem crossing rate. The faster intersystem crossing can be approved by evaluation of kisc values from flash photolysis experiments. 3.3. Laser Flash Photolysis. Nanosecond flash photolysis experiments were investigated both in solid matrixes and in argon-saturated THF solutions. Figure 6 shows the transient absorption of TBP-GaPc in the two media. The positive ∆A signals are attributed to the absorption from the transient species, such as the T1 state, while the negative ∆A signals are the results of bleaching due to the transition from the ground state at the wavelength in the sample. The broad and intense positive absorption of the T1 to Tn transition in the visible region with a peak at 600 nm in the solid matrix and 580 nm in THF are observed, respectively. It is interesting to point out that the bleaching signals at the Q (320-400 nm) and B (670-740 nm) bands of TBP-GaPc were observed in THF solution, but in the solid matrix, even positive ∆A signals were obtained around 710 and 340 nm. For the TBP-GaPc both in the solid matrix

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and in THF, the bleaching process by ground state transition would occur synchronously at the Q and B bands after excitation. In the solid matrix, the positive ∆A indicated that there was strong transient absorption in these regions and the significant positive values were the sum result of the transient absorption and the ground state bleaching. It indicated that the transient absorption region in the solid matrix was much broader than that in liquid solution. The triplet-minus-ground state extinction coefficients (∆εT) were calculated by the method of total depletion or saturation.35 They were 6.55 × 104 M-1 cm-1 in the solid matrix and 2.88 × 104 M-1 cm-1 in THF, respectively. The quantum yields of the triplet state, ΦT, determined by the comparative method with ZnPc as the reference (ΦT ) 0.65)36 were 0.85 and 0.61 in the solid matrix and in THF, respectively. It is obvious that in the solid matrix the ∆εT and ΦT are higher than that in the THF solution. Furthermore, the intersystem crossing rate constant, kisc, was evaluated,36 and they were 1.93 × 109 and 1.61 × 108 s-1 in the solid matrix and in THF, respectively. The higher value of ΦT in the solid matrix could be attributed to the faster intersystem crossing process from the S1 to the T1 state. As discussed in the ground state absorption section, the λmax was 723 and 714 nm for the TBP-GaPc doped in solids and in THF, respectively. It indicates that the TBP-GaPc possess a lower energy level of the S1 state in the solid than that in THF, which results in a smaller energy gap between the S1 and the T1 state. It is reasonable to consider that the smaller energy gap between the S1 and the T1 state would induce a faster intersystem crossing process. The results are well consistent with the photophysical processes discussed above. Moreover, the delayed fluorescence is attempted to be detected with a 355 nm laser pulse as the excitation wavelength. The emission was detected during the period of 2000 ns, 200 µs, and 10 ms after excitation, respectively. There are not any delayed fluorescence signals both in THF and in the solid matrix at the concentrations we used. 3.4. Decay of Triplet State. The molecules at the S0 state absorb the first photon to the S1 state; then part of the S1 state molecules are relaxed to the T1 state through fast intersystem crossing (2). The triplet state is an important intermediate40,41 which would undergo many pathways in relaxation and results in several processes as described in (3)-(7).

kf + kic

S1 98 S0 + hν/∆

(1)

kisc

S1 98 T1 RSA

T1 + hν 98 Tn

(n ) 2, 3, ...)

(2)

(3)

SEN

T1 + A 98 S0 + 3A

(4)

T1 + O2 f S0 + 1O2

(5)

k1

T1 98 S0 + hν/∆ k2

T1 + T1 98 S* + S0

(6)

(7)

Equation 3 is the so-called RSA process, resulting in an optical limiting effect. It directly depends on the quantum yield and lifetime of the T1 state. Equations 4 and 5 are the processes of sensitization and singlet oxygen formation, respectively. These two processes could be easily avoided by removing sensitizer A and O2 from the system. Equation 6 is the first-order decay of the triplet state by internal conversion (∆) and phosphorescence (hν), and (7) is the TTA process resulting in the delayed fluorescence and the loss of the triplet state. Thus, the RSA process (3) is mainly influenced by the TTA process (7), the first-order decay process (6), and the intersystem crossing process (2). Decay of the triplet state was investigated for TBP-GaPc both in solid matrixes and in THF solutions with flash photolysis experiments, as shown in Figure 7 and Table 2. It is obvious that TBP-GaPc possesses longer triplet lifetimes in solid matrixes, about 3 times longer than that in THF solutions. Moreover, with increasing concentration of TBP-GaPc, the triplet state lifetimes become shorter both in the solid matrixes and in the THF solutions. At lower concentrations, the values of ∆A are linearly enhanced with increasing TBP-GaPc concentration. However, at higher concentrations, the ∆A values increase nonlinearly with increasing concentration for TBP-GaPc in the solid matrixes and even decrease in the THF solution. In the solid matrixes, the triplet decays much more slowly, which basically obeys the first-order exponential decay. While in the THF solutions, the triplet decays much faster, especially at the high concentrations, which do not obey the first-order expo-

Figure 7. Triplet decay profiles of TBP-GaPc in solid matrixes (detected at 600 nm) and in THF solutions (detected at 580 nm).

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nential decay very well. The results suggest that the rigid surroundings of the solid matrix greatly restrict the rate of molecular diffusion and then reduce the intermolecular interaction of Pc molecules; thus, different decay modes would be observed between the solid samples and the THF solutions due to the diversity of triplet diffusion. Triplet diffusion is an important physical process that could induce triplet state decay via the TTA process.42-44 TTA occurs when two triplets meet each other to form a single excited state (S*, it is then mainly relaxed with nonradiative transition in our case because of no observation of delayed fluorescence), which greatly causes the loss of the triplet. This process depends significantly on the concentration of the T1 states.28,45-47 At higher concentrations, triplet molecule collision should be more frequent, resulting in faster decay of the triplet state. This is the reason why the triplet lifetimes are shortened greatly with an increase in the concentration of the Pc molecules. TTA also depends on the diffusion constant of the triplet state molecules.19,30,48-50 Faster diffusion increases the probability of collision, resulting in faster decay. Obviously, in the THF solution, the diffusion constant of the triplet molecules is much larger than that in the solid matrix, leading to a higher probability of the TTA process and faster decay of the triplet state. In the solid matrix, the rigid surroundings greatly restrict the migration of Pc molecules. The triplet state molecules can only move or vibrate in a small interspace and collide with their adjacent Pc molecules. Even so, the scale of this collision is very small, especially at a lower concentration. Therefore, much longer triplet state lifetimes were observed in the solid matrix. 3.5. Evaluation of Triplet-Triplet Annihilation. The triplet state is formed through intersystem crossing from the S1 to the T1 state (2). Then the decay of the triplet state undergoes two main pathways: One is the first-order exponential decay through phosphorescence and internal conversion directly from the T1 to the S0 state, while another is the TTA process, resulting in delayed fluorescence or internal conversion as described in (6) and (7), where k1 is the summation of rate constants of the firstorder exponential decays, k2 is the overall rate constant of TTA, and kisc is the rate constant of intersystem crossing. So two differential equations that need to be solved are eqs 8 and 9:

-

d[T1] ) k1[T1] + k2[T1]2 - kisc[S1] dt

(8)

-

d[S1] ) -k2[T1]2 + (kisc + kf + kic)[S1] dt

(9)

For the intersystem crossing (nanosecond scale) of the S1 state formed from the S0 state, the process is quite short-lived in contrast to the microsecond scale of the triplet state of the phthalocyanine-based materials. Furthermore, this intersystem crossing is an earlier process to form the T1 state; then comes forth the TTA process. In fact, the S1 state may only consist in the very beginning of the TTA process. As discussed above, the values of kisc both in the solid matrix and in THF are on the scale of 108-109 s-1, and the lifetimes of the S1 state are about 1-4 ns. The decay of the S1 state is much faster than the decay of the T1 state in the microsecond scale. Hundreds of nanoseconds (i.e., 100 ns) after excitation, the concentration of this S1 state turns out to be very small. For the S1 molecules from the TTA process, it is much smaller than that from the S0 state with excitation. The concentration of the T1 state formed by the TTAmediated S1 state is also too small to have an effect on the TTA again, because this kind of T1 state is formed after several physical steps, and the final yield of the T1 state formed by this TTA-mediated S1 state should be very small. So, the effects of the S1 state molecule are small enough to be ignored in eq 8 and the following simplified equations are obtained:

dc ) k1c + k2c2 dt

(10)

∫ dt ) k c +dc k c2

(11)

-

-

1

c)

(

2

1 k k2 1 2 + ek1t c0 k1 k1

)

(12)

in which c is the time-dependent concentration of the triplet state [T1] and c0 is the initial excited triplet state concentration at the beginning time. Here, triplet-triplet annihilation is assumed to be a process with two molecules, and the probability of multimolecule TTA processes are small enough to be neglected.

TABLE 2: Parameters of Triplet Decay Processes for TBP-GaPc Doped in Solid Matrixes and in THF Solutions state solid matrix

THF solution

a

compounds 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

c/M -6

5.0 × 10 7.5 × 10-6 1.0 × 10-5 5.0 × 10-5 1.0 × 10-4 2.5 × 10-4 5.0 × 10-4 1.0 × 10-3 5.0 × 10-6 7.5 × 10-6 1.0 × 10-5 5.0 × 10-5 1.0 × 10-4 2.5 × 10-4 5.0 × 10-4 1.0 × 10-3

τTa/µs

k1/×103 s-1

k2′/×103 s-1

∆A0/×10-2

c2/%

285.7 281.7 277.8 277.8 274.7 273.2 275.5 273.2 77.5 76.3 75.7 74.6 71.4 63.2 62.1 61.3

3.50 ( 0.01 3.55 ( 0.02 3.60 ( 0.05 3.60 ( 0.01 3.64 ( 0.01 3.66 ( 0.01 3.63 ( 0.01 3.66 ( 0.02 12.9 ( 0.22 13.1 ( 0.14 13.2 ( 0.13 13.4 ( 0.16 14.0 ( 0.06 15.8 ( 0.09 16.1 ( 0.12 16.3 ( 0.21

2.40 ( 0.14 2.30 ( 0.13 2.10 ( 0.10 2.00 ( 0.33 2.20 ( 0.15 1.90 ( 0.13 1.80 ( 0.11 2.10 ( 0.21 241 ( 1.82 253 ( 2.61 249 ( 1.38 254 ( 1.57 265 ( 3.32 280 ( 4.47 395 ( 2.77 421 ( 2.68

3.09 ( 0.007 4.25 ( 0.011 5.20 ( 0.017 12.2 ( 0.027 14.6 ( 0.024 15.8 ( 0.024 19.0 ( 0.032 24.8 ( 0.10 0.92 ( 0.008 0.99 ( 0.008 1.68 ( 0.008 6.06 ( 0.001 7.36 ( 0.001 8.45 ( 0.002 9.14 ( 0.002 10.1 ( 0.018

1.05 1.34 1.51 3.24 3.80 4.28 4.84 6.51 7.74 8.52 13.1 33.4 37.3 40.2 46.8 50.9

τT ) 1/k1, and it is the lifetime of triplet state with simplex monomer decay.

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Figure 8. Fitting of decay profiles for TBP-GaPc in solid matrixes (a) and in THF solutions (b) at concentrations of 1 × 10-5 M (3) and 1 × 10-3 M (8) using the kinetics of eq 15.

According to Beer’s law,51,52 the value of c is correlative with the value of ∆A, which could be detected by laser flash photolysis, as described by the following kinetics equations.

∆A ) ∆εTcl -

k2 d∆A ) k1∆A + ∆A2 dt ∆εTl

∆A )

(

1 k′2 k1t k′2 1 + e ∆A0 k1 k1

)

(13)

(14)

process of the triplet state decay and the longer lifetime of the triplet state are displayed for the TBP-GaPc doped in the solid matrix. The τT is the lifetime of the triplet state with simplex monomer decay. It is obvious that the τT in the solid matrix is much longer than that in THF. Furthermore, in order to investigate the effect of both concentration and dispersive medium on the TTA process, the consumed concentrations (c2) for TTA were evaluated by the following models. The concentration for the first-order exponential decay (c1) obeys the kinetics in eqs 16-18:

(15)

where k2′ ) k2/∆εTl is defined as the apparent rate constant of TTA. The ∆εT is the triplet-minus-ground state extinction coefficient, and l is the length of the laser passed through the sample. The decay kinetics of TBP-GaPc both in solid matrixes and in THF solutions were analyzed by fitting the decay curves using eq 15 as shown in Figure 8. All the decay curves at different concentrations fit the proposed kinetics very well with high relativity, and the values of k1, k2′, and ∆A0 were obtained and shown in Table 2. The initial values of ∆A0 obey Beer’s law at low concentrations but are nonlinearly increasing at higher concentrations of TBP-GaPc. Values of k1 and k2′ stay unaltered at lower concentrations, especially in the solid matrixes. They just display little changes with the concentration increases. It is interesting to point out that the values of k1 and k2′ in the solid matrixes are much lower than that in the THF solutions. In addition, in the solid matrix, the value of k′2 is smaller than the value of k1, suggesting that the first-order decay is the main pathway for the decay of the triplet state. In THF, the value of k2′ is ∼20 times higher than that of k1. It indicates that the TTA process is an important pathway for the triplet state deactivation. As a result, the slower

-

dc dc ) k1c + k2c2 ⇒ -dt ) dt k1c + k2c2

dc1 ) -k1c dt )

k1c k1c + k2c2

∫0c dc1 ) ∫0t k1c dt ) ∫0c

(17)

dc

k1c

0

1

k1c + k2c2

(16)

dc

(18)

where c1 is the concentration consumed by the first-order exponential decay. After standard kinetic analysis, the following equations were obtained.

c1 )

(

k1 k2c0 ln 1 + k2 k1

)

(19)

11950

J. Phys. Chem. C, Vol. 113, No. 27, 2009

(

) (

Chen et al.

k′2 ∆A1 ) ′ ln 1 + ∆A0 k1 k2 k1

∆A2 ) ∆A0 - ∆A1 ) ∆A0 -

k1 k′2

ln 1 +

(20)

)

k′2 A k1 0

(21)

Here, ∆A1 and ∆A2 are the consumed values of ∆A for the firstorder exponential decay and for the TTA, respectively. Using the final equation of 21, the consumed values of ∆A for the first-order exponential decay (∆A1) and for the TTA (∆A2) were obtained, as shown in Figure 9 and Table 2. In Figure 9, the initial ∆A0 values of the triplet state linearly increase at lower concentrations both in solid matrixes and in THF solutions. When c > 5 × 10-5 M, the ∆A0 values do not obey Beer’s law and slowly reach a balance. In the solid matrixes, the values of ∆A1 almost obey the same trend with values ∆A0, while the values of ∆A2 are much smaller compared to values of ∆A1 and ∆A0 and change a little with an increase in the concentration of TBP-GaPc. This fact indicates that in the solid matrix the major decay process of the triplet state is the first-order exponential decay, and the effect of TTA is small enough to be neglected. In THF solutions, however, the values of ∆A2 greatly increase, especially at higher concentrations as shown in Figure 9. At lower concentrations of THF solutions, the first-order exponential decay dominates for the decay of the triplet, but with an increase in concentration to higher scales, consumed values of ∆A2 for the TTA are greatly increased and even become larger than the consumed values of ∆A1 for the first-order exponential decay. The percentages of concentration for TTA were easily obtained, as shown in Figure 10 and Table 2. At lower concentrations of TBP-GaPc, the percentages of c2 for TTA increase quickly with an increase in the TBP-GaPc concentra-

tion. When c > 5 × 10-5 M, c2 follows the curvilinear growth. Moreover, the percentages of c2 in the solid matrixes increased from 1.05 to 6.51%, which are much smaller than those in the THF solutions that ranged from 7.74 to 53.7%. At the same concentration of TBP-GaPc, the c2 for triplet-triplet annihilation in the THF solution is about 10 times higher than that in the solid matrix. Even at very high concentration, the effects of TTA for the TBP-GaPc doped in the solid matrix are much smaller than those in the THF solution. The results are consistent with the facts that the triplet state of the TBP-GaPc molecule possesses a much longer lifetime in the solid matrix than that in the THF solution, and the lifetime of the triplet is shortened with an increase in the concentration of TBP-GaPc molecules. Generally, rigid surroundings greatly restrict the molecular diffusion and aggregation, resulting in lower probability of molecular collision and nonradiative transition and leading to a lower proportion of the TTA process for the TBP-GaPc doped in the solid matrix than that in THF solution. Thus, higher values of ΦT, kisc, τT, and ∆εT are obtained. For the phthalocyaninebased materials, higher values of kisc, ∆εT, and ΦT, longer lifetime of the triplet state, and lower probability of the TTA process would make many more triplet molecules to achieve the reverse saturable absorption (RSA) process; thus, the optical limiting effect which directly depends on RSA would be accordingly enhanced, just as reported by the published papers.25-27,53-55 Therefore, the optical limiting properties are greatly influenced by the TTA process which depends on the concentration and the dispersive medium of phthalocyaninebased compounds. 4. Conclusions Novel solid matrixes doped with tetra-R-(4-tert-butylphenoxy) gallium phthalocyanine in epoxy resin were prepared. The photophysical properties were investigated both in THF solutions and in solid matrixes with transient spectroscopies. A detailed analysis is introduced to evaluate the TTA process. In THF

Figure 9. Kinetic processes of consumed ∆A values for the first-order exponential decay (∆A1) and for the TTA (∆A2) at different concentrations.

Figure 10. Overall percentage of triplet state concentration consumed by the TTA process (c2) in solid matrixes and in THF solutions of 1-8.

Axially Substituted Gallium Phthalocyanine solutions, the rate constants of the first-order exponential decay (k1) and the apparent rate constants of TTA (k′) 2 are much larger than those in the solid matrixes. It gives a longer triplet excited state lifetime in the solid matrix. With an increase in the concentration of TBP-GaPc from 5.0 × 10-6 to 1.0 × 10-3 M, the percentages of TTA in the solid matrixes ranged from 1.05 to 6.51%. It is much lower than that in THF solutions, which ranged from 7.74 to 50.9%. A lower probability of Pc molecular aggregation and a much lower proportion of the TTA process are generated for the TBP-GaPc doped in the solid matrix than that in THF solution. Accordingly, higher values of ΦT, kisc, τT, and ∆εT are obtained in the solid matrix which will produce higher probability of the reverse saturable absorption (RSA) process to result in better optical limiting behaviors for phthalocyanine-based materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 50773085, 20733007, and 20703049) and the National Basic Research Program (2007CB808004). References and Notes (1) Leznoff, C. C.; Lever, A. B. P., Eds. Phthalocyanines: Properties and Applications; VCH Publishers Inc.: New York, NY, 1989, 1993, 1996; Vols. 1-4. (2) De la Torre, G.; Va´zquez, P.; Agullo´-Lo´pez, F.; Torres, T. Chem. ReV. 2004, 104, 3723–3750. (3) Ino, D.; Watanabe, K.; Takagi, N.; Matsumoto, Y. J. Phys. Chem. B 2005, 109, 18018–18024. (4) Hasobe, T.; Kamat, P. V. J. Phys. Chem. C 2007, 111, 16626– 16634. (5) Kobayashi, N.; Lam, H.; Andrew Nevin, W.; Janda, P.; Leznoff, C. C.; Koyama, T.; Monden, A.; Shirai, H. J. Am. Chem. Soc. 1994, 116, 879–890. (6) Detty, M. R.; Gibson, S. L.; Wagner, S. J. J. Med. Chem. 2004, 47, 3897–3915. (7) Denes, G. J. Am. Chem. Soc. 1998, 120, 241–242. (8) Mizuguchi, J. J. Phys. Chem. A 2001, 105, 10719–10722. (9) O’Flaherty, S. M.; Wiegart, L.; Struth, B. J. Phys. Chem. B 2006, 110, 19375–19379. (10) Sheng, Z.; Ye, X.; Zheng, Z.; Yu, S.; Ng, D. K. P.; Ngai, T.; Wu, C. Macromolecules 2002, 35, 3681–3685. (11) Li, X. Y.; He, X.; Ng, A. C. H.; Wu, C.; Ng, D. K. P. Macromolecules 2000, 33, 2119–2123. (12) Tutt, L. W.; Boggess, T. F. Prog. Quantum Electron. 1993, 17, 299–338. (13) Sun, Y. P.; Riggs, J. E. Int. ReV. Phys. Chem. 1999, 18, 43–90. (14) Sun, W.; Wang, G.; Li, Y.; Calvete, M. J. F.; Dini, D.; Hanack, M. J. Phys. Chem. A 2007, 111, 3263–3270. (15) Kim, K. Y.; Farley, R. T.; Schanze, K. S. J. Phys. Chem. B 2006, 110, 17302–17304. (16) Jiang, L.; Jiu, T.; Li, Y.; Li, Y.; Yang, J.; Li, J.; Li, C.; Liu, H.; Song, Y. J. Phys. Chem. B 2008, 112, 756–759. (17) Perry, J. W.; Mansour, K.; Marder, S. R.; Perry, K. J.; Alvarez, D.; Choong, I. Opt. Lett. 1994, 19, 625–627. (18) Slodek, A.; Wo¨hrle, D.; Doyle, J. J.; Blau, W. Macromol. Symp. 2006, 235, 9–18. (19) Shaw, G. B.; Papanikolas, J. M. J. Phys. Chem. B 2002, 106, 6156– 6162.

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