Study on Nonlinear Spectroscopy of Tetraphenylporphyrin and

Dec 1, 2007 - Zhi-Bo Liu, Yan Zhu, Yi-Zhou Zhu, Jian-Guo Tian*, and Jian-Yu Zheng*. The Key Laboratory of Weak Light Nonlinear Photonics, Ministry of ...
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J. Phys. Chem. B 2007, 111, 14136-14142

Study on Nonlinear Spectroscopy of Tetraphenylporphyrin and Dithiaporphyrin Diacids Zhi-Bo Liu,† Yan Zhu,‡ Yi-Zhou Zhu,‡ Jian-Guo Tian,*,† and Jian-Yu Zheng*,‡ The Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School, Nankai UniVersity, Tianjin 300457, People’s Republic of China, and State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed: July 12, 2007; In Final Form: September 19, 2007

The nonlinear absorption of two porphyrin diacids (H4TPP2+ and H2DSP2+), the diprotonated forms of free base tetraphenylporphyrin (H2TPP) and dithiaporphyrin (DSP), were studied in the wavelength range of 500650 nm. The two porphyrin diacids exhibited perturbed static and dynamic characteristics and enhanced nonlinear absorption properties relative to their parent neutral complexes in solution. Furthermore, for the dithiaporphyrin diacid, the introduction of S-atoms into the porphyrin core makes it a better candidate for optical limiting relative to the simple porphyrin. Their photophysical parameters such as ground and excited states absorption cross-sections, together with fluorescence lifetime and intersystem crossing time, were determined.

Introduction Porphyrins have a wonderful repertoire of optical and electronic properties and play a variety of roles in nature and in technical applications.1,2 There has been a great deal of interest in nonlinear optical (NLO) responses of porphyrins because of their potential applications in optical switching, optical memory, optical limiting, and 3-D microfabrication.3-6 The nonlinear optical responses of porphyrins depend strongly on different patterns of peripheral substitutions, π-conjugations, or central metal incorporations.3,4,6 However, outer rim modification could only affect the central electronic structure of the porphyrin slightly. Core alteration of porphyrins will change the electronic structure of the macroring more directly and remarkably.7-10 An understanding of ground and excited state properties of coremodified porphyrins is an important prerequisite for their possible diverse applications. Recently, the photophysical properties of dithiaporphyrins were reported in comparison to tetraphenylporphyrins.11-13 The nonlinear optical characteristics of porphyrins depend on the porphyrin excited (singlet and triplet) state lifetimes and quantum yields, which in turn depend on environmental characteristics, such as pH, ionic strength, etc. Acid-base behavior of porphyrins has also been well-studied. The tetrapyrrole skeleton allows the loss of two N-H protons to form a dianion in a strong base or the gain of two protons to form a dication in acid. The addition of two protons to the tertiary nitrogens of a porphyrin core can lead to nonplanar forms of porphyrins. It is evident that upon protonation, the absorption maxima are both chronically shifted relative to the neutral species for normal porphyrins and thiaporphyrins.11,13 The protonation changes the characteristics of the ground and excited states through modifying the electronic structure of chromophores and thus is expected to affect the properties of nonlinear optical absorption of porphyrins. To apply these materials in photonics and medicine with higher efficiency, a * Corresponding authors. E-mail: (J.-G.T.) [email protected] and (J.-Y.Z.) [email protected]. † The Key Laboratory of Weak Light Nonlinear Photonics. ‡ State Key Laboratory and Institute of Elemento-Organic Chemistry.

Figure 1. Molecular structures of porphyrin diacids.

complete determination of their photophysical parameters and their dependence on external conditions are required. Very few studies on nonlinear absorption of the diacid form of porphyrins, especially for dithiaporphyrins, have been conducted. For mesotetra(sulfonatophyenyl)porphyrins (TPPS4), the change of nonlinear absorption and photophysical properties due to protonation has been investigated.14-16 Furthermore, the dication form of TPPS4 is characterized by a high ratio (∼10) of the cross-section of the excited state to that of the ground state at 532 nm, and it should be useful for optical limiting.16 In this study, we compared the nonlinear absorption spectra of neutral forms with those of diacid derivatives of free base tetraphenylporphyrin and dithiaporphyrins (H2TPP and DSP, respectively) in the range of 500-650 nm. The Z-scan technique was utilized for the characterization of their nonlinear absorption properties. For all four compounds, UV-vis absorption spectroscopy and time-resolved fluorescence spectroscopy were measured to obtain complementary data that are necessary for the fitting procedures. The five-level model is applied to analyze and explain the experimental results. Experimental Procedures Tetraphenylporphyrin (H2TPP) and 5,10-diphenyl-15,20-bis4-tolyl-21,23- dithiaporphyrin (DSP) were synthesized following literature methods.10,17 Diacid species [H4TPP] (CF3COO)2 and [H2DSP] (CF3COO)2 (H4TPP2+ and H2DSP2+, see Figure 1)

10.1021/jp075472f CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2007

Tetraphenylporphyrin and Dithiaporphyrin Diacids

J. Phys. Chem. B, Vol. 111, No. 51, 2007 14137

Figure 2. UV-vis absorption spectra of H2TPP and H4TPP2+ (a) and DSP and H2DSP2+ (b).

TABLE 1: Absorption Data of H2TPP, H4TPP2+, DSP, and H2DSP2+ in Chloroform porphyrin

Soret-band λ (nm)

H2TPP H4TPP2+ DSP H2DSP2+

417 449 435 466

IV

Q-band λ (nm) III II

515

548

514

548

592 625 637 712

I 647 674 699 755

were prepared by adding trifluoroacetic acid into solutions of the corresponding neutral porphyrins. All measurements were carried out on chloroform solutions of these samples. Absorption spectra were recorded using a Cary 300 Scan spectrophotometer. Corrected steady state fluorescence spectra under a 90° angle between the excitation and the detection direction were obtained by a VARIAN Cary Eclips spectrometer. The time-resolved fluorescence measurements were carried out on an Acton Research SpectraPro-300i spectrograph coupled with an ultrafast gated ICCD camera (LaVision, PicoStar) with a time resolution of 20 ps. A frequency-doubled Ti:sapphire laser system (Spectra Physics) performing at 1 kHz was used as an excitation source, and its typical pulse width was less than 130 fs (fwhm) at 440 nm. In measurements of nonlinear absorption using the Z-scan technique,18 a Q-switched Nd:YAG laser (Continuum SureliteII) and a mode-locked Nd:YAG laser (Continuum Model PY61) were used to generate 5 ns pulses and 30 ps pulses at 532 nm. The lasers in the wavelength region of 510-650 nm were produced from a commercial optical parametric oscillator (Continuum Panther Ex OPO) pumped by Continuum SureliteII with a repetition rate of 10 Hz and a pulse duration of 4-5 ns. The beam waist was 18-24 µm for different wavelengths and different pulse durations. The incident and transmitted pulse energies were measured simultaneously by two energy detectors (Molectron J3S-10). All solutions of porphyrins used in the Z-scan measurements had a concentration of 2.0 × 10-4 M and were poured into a 1 mm quartz cuvette. Results and Discussion Linear Absorption and Fluorescence Spectra. The absorption spectra of H2TPP and DSP used in our experiments are shown in Figure 2, together with those of H4TPP2+ and H2DSP2+, and the absorption maxima data of the four porphyrins are listed in Table 1. The addition of two more protons to the central nitrogens of the porphyrin macrocycle is expected to

Figure 3. Steady fluorescence spectra of H2TPP and H4TPP2+ (a) and DSP and H2DSP2+ (b).

have electronic effects on the photophysical properties of the diacids H4TPP2+ and H2DSP2+ relative to H2TPP and DSP. H2TPP and DSP exhibit characteristic I-IV Q-bands and one strong Soret-band. Upon protonation of H2TPP, the absorption spectrum in the visible region is characterized by two strong maxima at 625 and 674 nm, as seen from the dashed line in Figure 2a, and the Soret-band shifts to 449 nm. A similar characteristic of the absorption spectrum to that of H4TPP2+ can be found in H2DSP2+. Once the ground state absorption R0 and the molecular concentration N are measured, the ground state absorption cross-section σ0 can be obtained by the relation R0 ) Nσ0. The values of σ0 obtained at 532 nm were 2.63 × 10-17 and 3.82 × 10-17 cm2 for H2TPP and DSP, respectively, and 1.69 × 10-17 and 0.86 × 10-17 cm2 for H4TPP2+ and H2DSP2+, respectively. It can seen that the values of σ0 decrease after protonation for both H2TPP and DSP. Figure 3 shows a comparison of fluorescence spectra of H4TPP2+ and H2DSP2+ with those of the neutral parent complexes H2TPP and DSP, and the emission maxima data are also marked in Figure 3. The features of fluorescence spectra are as follows: (1) the fluorescence intensity is quenched considerably, and the emission maxima are red-shifted after protonation for both H2TPP and DSP and (2) the H4TPP2+ fluorescence spectrum is unusual in that it consists of a single broad, structureless band with a maximum at 689 nm, which is substantially displaced from the absorption maximum. The emission behavior is also different from that of neutral H2TPP; it has a wellresolved vibrational structure and a Q(0,0)-band that is modestly displaced from the absorption maximum. All of these effects can be understood if diacid formation is accompanied by both purely electronic effects and consequences of nonplanar distortions.19 Fluorescence Lifetimes. The time-resolved fluorescence measurements were carried out on H2TPP, DSP, H4TPP2+, and H2DSP2+ (Figure 4), and the fitting data of fluorescence lifetime τF are shown in Table 2. The fluorescence decay curves for both H2TPP and DSP are successfully simulated by a biexponential decay model. Results obtained by this fitting procedure show that the singlet excited state lifetimes of H4TPP2+ and H2DSP2+ decrease greatly as compared to H2TPP and DSP. The heavier sulfur has empty d-orbitals, which have an appropriate symmetry for better coupling with π-systems of porphyrins, resulting in the decrease of S1 state lifetimes. The S1 state lifetime mainly depends on the increase of the S1-T1 intersys-

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Liu et al. Nonlinear Absorption in Picosecond and Nanosecond Regime. Excited state absorption (ESA) and two-photon absorption (TPA) are important processes leading to nonlinear absorption behavior in porphyrins. ESA is a sequential χ(1):χ(1) process effectively giving rise to third-order nonlinearity, and TPA itself is a χ(3) process involving four fields. An electronic nonlinear optical process is resonant when the excitation wavelength is closer to any absorbing electronic transition. Under resonant and near resonant excitation, ESA is the dominant mechanism in porphyrins.21 The nonlinear absorption behaviors observed in these porphyrins exhibit ESA on both nanosecond and picosecond time scales. We applied a five-level model depicting molecule energy levels to explain their nonlinear absorption processes and to fit the Z-scan experimental data.22,23 For a sample satisfying L < z0, the equation that governs the irradiance I is

dI ) -RI ) -(σ0N0 + σ1NS1 + σ2NT1)I dz

Figure 4. Time-resolved fluorescence spectra of H2TPP and H4TPP2+ (a) and DSP and H2DSP2+ (b).

tem crossing and the S1-S0 internal conversion rates. The increase in the internal conversion rate can be attributed to the enhancement of the Franck-Condon factor that is associated with structural reorganization in the excited state.20 The increase of the intersystem crossing rate is attributed to spin-orbit coupling, which arises partly owing to the participation of appropriate empty d-orbitals (dxy and dyz) of sulfur atoms with the porphyrin π-orbital.12 The fluorescence decay profiles of H4TPP2+ and H2DSP2+ are not biexponential. Much better fits are obtained with a monoexponential decay model. Relative to the neutral parent complexes, both H4TPP2+ and H2DSP2+ exhibit reduced S1 state lifetimes. Vibrations involving the central protons do not appear to materially affect S1-S0 internal conversion in free-base porphyrin. The substantially increased internal conversion and the reduced S1 state lifetimes found in the diacids are not easily understood simply on electronic or vibrational grounds. It has been suggested that the enhanced S1-S0 internal conversion in H4TPP2+ may be derived from the participation of a low-energy charge-transfer excited state involving the macrocycle and peripheral rings.

(1)

where z is the longitudinal coordinate and z0 is the diffraction length of the beam. σ0, σ1, and σ2 and N0, NS1, and NT1 are the absorption cross-sections and population of ground state, first singlet excited state, and first triplet excited state, respectively. In general, it is difficult to obtain analytical solutions of timeand space-dependent differential equations. We used the standard Runge-Kutte fourth-order method to solve eq 1 and the rate equations numerically, and the intensity distribution of the beam at the exit face of the sample was obtained. By applying Huygens’s principle and a zero-order Hankel transformation, we could also obtain the far-field electric field at the aperture plane and the normalized transmittance of the Z-scan. Relaxation rates of higher lying singlet and triplet states are generally much faster than the durations of picosecond and nanosecond pulses. It is normally assumed that these rates are so fast that the population densities of these states, designated by S2 and T2, are very small and can be ignored.24 Meanwhile, since the relaxation time τT1 from T1 to S0 is in the microsecond time scale, the transition from T1 to S0 also can be ignored for nanosecond and picosecond excited pulsed durations.22 When the input laser pulse duration is on the order of picoseconds, populations in the triplet state can be neglected, and the nonlinear absorption arises mostly from singlet-singlet transitions since the intersystem crossing time (τISC) of porphyrins is in the nanosecond time scale. For nanosecond pulsed laser incidence, it is necessary to simultaneously consider the contribution of both singlet-singlet and triplet-triplet state absorptions. If the intersystem crossing time is faster and the S1 lifetime is much smaller than nanosecond pulse duration, triplet-triplet absorption will dominate.22 Thus, it is suitable to measure the value of σ1 by using a picosecond pulsed laser and the value of σ2 by using a nanosecond pulsed laser. Solving numerically eq 1 and rate equations with the assumptions and initial conditions state previously, the values

TABLE 2: Photophysical Parameters of H2TPP, H4TPP2+, DSP, and H2DSP2+ at 532 nma

a

σ0 (10-17 cm2)

σ1 (10-17 cm2)

σ2 (10-17 cm2)

τF (ns)

τISC (ns)

τ0 (ns)

H2TPP

2.63

6.9

7.8

25.0

8.9

H4TPP2+ DSP

1.69 3.82

7.6 7.5

12.5 6.9

7.0 3.2

3.6 1.7

H2DSP2+

0.86

5.1

9.8

1.84/6.61 (0.32/0.68) 2.4 0.19/1.21 (0.10/0.90) 0.32

1.9

0.38

σ0, σ1, and σ2 are absorption cross-sections of ground state, first singlet excited state, and first triplet excited state, respectively, τF is fluorescence lifetime, τISC is intersystem crossing time, and τ0 is lifetime of the first singlet excited state.

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Figure 5. Open-aperture Z-scan results of H2TPP and H4TPP2+ (a and b) and DSP and H2DSP2+ (c and d) at 532 nm with picosecond and nanosecond pulses.

of the relaxation times and excited state absorption cross-sections can be obtained by the best fitting of the Z-scan data. Figure 5 gives the open-aperture Z-scan results of the four porphyrins at 532 nm. The solid lines are the theoretical fittings obtained by the five-level model, and the fitting parameters σ0, σ1, σ2, τ0, and τISC of porphyrins at 532 nm are shown in Table 2. The Z-scan curves of H4TPP2+ and H2DSP2+ exhibit obviously different profiles from neutral porphyrins. Because the experimental conditions of the Z-scan are identical for all porphyrins and there is no change of beam waist, the different Z-scan profiles shown in Figure 5 should be caused by different parameters of excited states, such as σ1, σ2, τ0, and τISC. Relative to neutral parent porphyrins, both H4TPP2+ and H2DSP2+ have larger ratios of excited state absorption cross-section (σ1 and σ2) to ground state absorption cross-section σ0 since they exhibit obviously smaller σ0 and larger σ2 values. For example, the addition of two more protons to the central nitrogen of DSP caused a marked enhancement of σ2 from 6.9 × 10-17 to 9.8 × 10-17 cm2, and σ0 decreased from 3.8 × 10-17 to 0.86 × 10-17 cm2. The values of σ1/σ0 and σ2/σ0 are 5.9 and 11.4 for H2DSP2+ and 1.9 and 1.8 for DSP, respectively. Generally, the ratio of the excited state cross-section to the ground state absorption cross-section is considered as the figure of merit that is an important parameter in the application of optical limiting, optical switching, and so on. The large enhancement of σ1/σ0 and σ2/σ0 implies that H4TPP2+ and H2DSP2+ have a greater potential utility as materials of optical limiting and optical switching than their neutral parent complexes H2TPP and DSP. Nonlinear Absorption Spectroscopy. For many applications that depend on nonlinear absorption, the nonlinear absorption spectrum is required to be measured in a wide range of

wavelengths. In porphyrins, the useful reverse saturable absorption (RSA) region lies between the Q and Soret absorption bands, where the linear absorption is low. From Figure 2, we can see that for H2TPP and DSP, the useful RSA region typically extends over the range of about 440-500 nm. For the diacid forms of the two porphyrins, the broadening and red-shift of this region at about 500-650 nm can be observed, which is useful to extend the application of RSA in optical limiting, optical switching, and so on. Although RSA was observed at 532 nm for H2TPP and DSP, there are large ground state absorptions at this wavelength due to the fact that 532 nm is located in the Q-bands, as given in the results of the previous section. We applied the Z-scan technique with a nanosecond pulsed laser to measure the nonlinear absorption spectrum of the four porphyrins in the wavelength range of 500-650 nm. In openaperture Z-scan measurements, the Z-scan curves are expected to be symmetric with respect to the focus (z ) 0) where they have a minimum transmittance (e.g., RSA) or maximum transmittance (e.g., saturable absorption (SA)). Figure 6 shows the change of transmittance at the focus (T0) at different wavelengths for the four porphyrins. As we see in the Figure 6, there is large difference of nonlinear absorption between the neutral porphyrins and their diacids. At most of the wavelengths used in our experiments, the neutral porphyrins H2TPP and DSP have a small RSA as compared to their diacids. Furthermore, H2TPP and DSP present a large SA at 520 nm since this wavelength is located at about the strongest absorption peak of the Q-band and both H2TPP and DSP have a large ground state absorption at 520 nm. The addition of two protons to H2TPP and DSP causes RSA enhancement at all wavelengths, except

14140 J. Phys. Chem. B, Vol. 111, No. 51, 2007

Figure 6. Change of transmittance at the focus (T0) with different wavelengths for H2TPP and H4TPP2+ (a) and DSP and H2DSP2+ (b).

Figure 7. Open-aperture Z-scan curves of H2TPP and H4TPP2+ (a) and DSP and H2DSP2+ (b) at 620 nm.

650 nm for H4TPP2+. However, the enhancement of DSP’s RSA due to protonation is larger than that of the simple porphyrin H2TPP. Figure 7 shows the open-aperture Z-scan curves of H2TPP, DSP, H4TPP2+, and H2DSP2+ at 620 nm. It shows that the nonlinear absorption is obviously enhanced due to the addition of two protons. The values of T0 are 0.92 for H2TPP, 0.58 for H4TPP2+, 0.94 for DSP, and 0.41 for H2DSP2+, respectively. Similar to the fittings of Z-scan curves for a picosecond and nanosecond pulsed laser at 532 nm, a five-level model was used to theoretically fit the Z-scan results shown in Figure 6. The parameters σ0, σ1, and σ2 for H2TPP, DSP, H4TPP2+, and H2DSP2+ at different wavelengths are shown in Figure 8. In accordance with Figure 6, large excited state absorption cross-

Liu et al. sections σ1 and σ2 were obtained for the diacids H4TPP2+ and H2DSP2+ as compared to H2TPP and DSP. The calculated parameters σ1/σ0 and σ2/σ0 are listed in Table 3. At most of the wavelengths, the values of σ1/σ0 and σ2/σ0 of H4TPP2+ and H2DSP2+ are larger than those of H2TPP and DSP. The largest σ1/σ0 and σ2/σ0 values found in H2DSP2+ at 580 nm were 11.61 and 26.79, respectively. A high ratio of the cross-section of the excited state to that of the ground state should be useful for applications in optical limiting. Under resonant conditions, nonlinear absorption depends on the excited state population and the co-effect of the ground and excited state absorption. In applications of nonlinear absorption, first a large ratio of excited state cross-section to ground state cross-section should be emphasized. Second, materials should have a sufficiently large ground state cross-section to completely populate the excited state.25 Although there are comparable or smaller σ1/σ0 and σ2/σ0 values for DSP and H2DSP2+ at 610-650 nm as listed in Table 3, the normalized transmittance T0 at the focus (z ) 0) for H2DSP2+ is much larger than that for DSP. A similar case can be found in H2TPP. The protonation changes the characteristics of the ground and excited states of H2TPP and DSP. It can be seen in Figure 8 that the protonation increases not only the cross-section of excited states but also the cross-section of the ground state for both H2TPP and DSP at 610-650 nm. Large ground state cross-sections can make a more sufficient population of excited state, which is useful for the enhancement of nonlinear absorption. On the other hand, in the wavelength range of 540-570 nm, the normalized transmittance T0 of two diacids H4TPP2+ and H2DSP2+ was close to that of their neutral parent complexes, but there are larger σ1/σ0 and σ2/σ0 values for H4TPP2+ and H2DSP2+. The reason should be that there is a reduction of ground state crosssection when σ1/σ0 and σ2/σ0 are enhanced due to protonation, and thus, the population of the excited state decreases. It is obvious that nonlinear absorption depends not only on the ratio of cross-section of excited state to that of ground state but also on the magnitude of ground state cross-section. For applications of nonlinear absorption such as optical limiting and optical switching, large σ1/σ0 (or σ2/σ0) values and appropriate ground state absorption should be required. Among the two neutral porphyrins and their diacids, H2DSP2+ has better nonlinear absorption properties and larger σ1/σ0 and σ2/σ0 values. Thus, it appears that for the porphyrin diacid, the introduction of S-atoms into the porphyrin core makes it a better candidate for optical limiting relative to the simple porphyrin. The introduction of S-atoms into the porphyrin core modifies the electronic structure of chromophores and thereby affects nonlinear optical absorption of porphyrins due to changes of photophysical parameters. However, the comparison of the spectroscopic changes upon protonation of DSP and H2TPP also reveals many similarities, which presumably arise from similarities in structures of the porphyrin core. Although the nonlinear absorption enhancement of DSP and H2TPP was obtained by protonation, the changes of photophysical parameters such as the obvious increase of internal conversion rates and the resulting decrease of S1 lifetime found in the diacids cannot be understood as purely electronic effects. It has been suggested that the enhanced S1-S0 internal conversion in H4TPP2+ may be derived from the participation of a low-energy charge-transfer excited state involving the macrocycle and peripheral phenyl ring.26 The diacid formation is accompanied by both purely electronic effects and the consequences of nonplanar distortions.19 These two sources may complement or counterbalance each other in giving rise to the individual photophysical

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J. Phys. Chem. B, Vol. 111, No. 51, 2007 14141

Figure 8. Parameters σ0, σ1, and σ2 for H2TPP (a), H4TPP2+ (b), DSP (c), and H2DSP2+ (d) at different wavelengths.

TABLE 3: Calculated Parameters σ1/σ0 and σ2/σ0 of H2TPP, H4TPP2+, DSP, and H2DSP2+ at Different Wavelengths λ H2TPP DSP H4TPP2+ wavelength (nm) σ1/σ0 σ2/σ0 σ1/σ0 σ2/σ0 σ1/σ0 σ2/σ0 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650

0.78 0.23 0.15 0.63 2.00 1.22 1.38 2.83 2.38 1.32 1.49 2.04 3.11 2.86 2.38 0.86

5.14 2.76 1.64 2.4 7.14 2.71 2.62 5.19 4.76 2.29 2.9 4.01 4.92 4.76 3.38 1.88

3.39 1.77 1.29 0.81 1.35 1.66 1.88 2.94 3.46 1.79 1.68 1.60 1.32 1.28 0.94 1.07

3.69 3.23 3.59 5.11 6.08 5.25 4.84 5.59 7.23 3.21 2.77 2.64 2.18 1.81 1.22 0.64

H2DSP2+ σ1/σ0 σ2/σ0

1.03 3.15 9.30 10.47 0.58 1.02 6.51 10.95 0.46 0.67 5.37 8.68 0.83 0.96 7.38 13.93 1.86 1.53 2.78 6.78 1.09 1.21 3.95 7.24 1.10 1.52 4.93 8.45 1.38 1.88 6.16 12.33 2.35 2.94 11.61 26.79 2.96 3.80 6.76 14.86 7.14 8.93 6.93 12.77 9.57 10.43 5.67 9.11 5.00 6.56 4.49 6.48 2.54 3.24 3.27 4.79 1.80 2.40 2.15 3.14 3.57 4.29 1.37 2.05

properties of a given diacid, including a magnitude change relative to its neutral complex. Conclusion A detailed study on nonlinear absorption parameters of two porphyrin diacids as well as their neutral parent complexes was performed. Results showed that the states of nonprotonated and protonated DSP and H2TPP possess a reverse saturable absorption caused by the singlet excited state population in the picosecond regime but mainly by the triplet excited state population in the nanosecond regime. The nonlinear absorption

spectroscopy of DSP, H2TPP, and their two diacids was also obtained; results show that the protonation can change the properties of ground and excited states and lead to a large enhancement of nonlinear absorption of porphyrins, especially DSP. The protonated forms of DSP and H2TPP exhibit large RSA and a high ratio of excited state cross-section to ground state cross-section in the wavelength range of 500-640 nm. Therefore, they should be useful for applications in optical limiting. Acknowledgment. This work was supported by the Natural Science Foundation of China (10574075, 60708020, 20572048, and 20421202), Chinese National Key Basic Research Special Fund (2006CB921703), and Preparatory Project of the National Key Fundamental Research Program (2004CCA04400). References and Notes (1) Nalwa, H. R.; Miyata, S. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: Boca Raton, FL, 1997. (2) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (3) Krivokapic, A.; Anderson, H. L.; Bourhill, G.; Ives, R.; Clark, S.; McEwan, K. J. AdV. Mater. 2001, 13, 652. (4) de la Torre, G.; Vaquez, P.; Agullo-Lopez, F.; Torres, T. Chem. ReV. 2004, 104, 3723. (5) Chen, Y.; Hanack, M.; Araki, Y.; Ito, O. Chem. Soc. ReV. 2005, 34, 517. (6) Calvete, M.; Yang, G. Y.; Hanack, M. Synth. Met. 2004, 141, 231. (7) Gupta, I.; Hung, C. H.; Ravikanth, M. Eur. J. Org. Chem. 2003, 19, 4392. (8) Gupta, I.; Ravikanth, M. Coord. Chem. ReV. 2006, 250, 468. (9) Sridevi, B.; Narayanan, S. J.; Srinivasan, A.; Reddy, M. V.; Chandrashekar, T. K. J. Porphyrins Phthalocyanines 1998, 2, 69. (10) Ulman, A.; Manassen, J. J. Am. Chem. Soc. 1975, 97, 6540.

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