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Excited-State Absorption and Sign Tuning of Nonlinear Refraction in Porphyrin Derivatives Li Jiang,† Tonggang Jiu,† Yuliang Li,*,† Yunbo Li,‡ Junyi Yang,§ Junbo Li,† Cuihong Li,† Huibiao Liu,† and Yinglin Song*,‡,§ Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R. China, School of Physical Science and Technology, Suzhou UniVersity, Suzhou 215006, P.R. China, and Department of Physics, Harbin Institute of Technology, Harbin 150001, P.R. China ReceiVed: August 13, 2007; In Final Form: October 29, 2007
The nonlinear absorptions and nonlinear refractions of free-base porphyrin (P2) and Zn-porphyrin (ZnP2) were studied using the Z-scan technique at 532 nm with different pulse durations. Both P2 and ZnP2 exhibit reverse saturated absorption attributed to excited-state absorption. The coordination of P2 by Zn ion can alter the nonlinear refraction sign from negative to positive at 4 ns pulse. The results indicate that the sign of self-lensing can be tuned by the coordination of Zn in the porphyrin derivatives. In the case of longer pulse duration, the thermal effect was enhanced to dominate the nonlinear refraction sign, leading to the negative nonlinear refraction repeated appearance.
Introduction Recent progress in the field of high-speed optoelectronics and information technology has encouraged the search for materials with excellent nonlinear optical properties.1 Organic materials with delocalized π electrons have the advantages of architectural flexibility, easy fabrication,2 and electron delocalization in conjugated systems which contributes to the ultrafast response capability and large third-order nonlinearity.3 Investigation of well-defined π conjugated molecules is of considerable interest, since it can provide insight into the structural and electronic properties. Over the past few years, a wide range of functionalized π conjugated molecules have been designed and synthesized to tune the desirable nonlinear optical properties of the functional materials. Among these molecules, porphyrins undoubtedly are one of the most studied classes of compounds because of their abundance in nature and their potential applications in all-optical signal processing.4 Their macrocycles with a large polarizable π conjugated system constitute a twodimensional framework for electronic communication. By changing the metal center5 and the nature of the substituents at the peripheral sites of the macrocycle,6 their optical properties can be remodeled properly. There have been a number of reports on the third-order nonlinear optical properties of porphyrin derivatives.7 These materials present either a reverse saturable absorption (RSA) or a saturable absorption (SA) effect, depending on the molecular structure and the pump wavelength.8 RSA in porphyrins was first reported by Blau. et al. for laser pulses at 532 nm with 80 ps duration.9 The RSA, in which the excited-state absorption cross-section (σex) is larger than the ground-state absorption cross-section (σg), is the basis for optical-limiting action in resonant nonlinear absorbers. A series of porphyrin * Authors to whom correspondence should be addressed. E-mail:
[email protected] (Y.L.);
[email protected] (Y.S.). Fax: (+86)10-82616576. Phone: (+86)10-82616576. † Chinese Academy of Sciences. ‡ Harbin Institute of Technology. § Suzhou University.
derivatives such as tetrabenzoporphyrin (TBP) and tetraphenylporphyrin (TPP) have been found to exhibit intensitydependent excited-state absorption.8,10 In general, many RSA materials also possess nonlinear refractive properties, particularly under high irradiances of incident light. The presence of refractive nonlinearities may be used to enhance the opticallimiting performance if a focusing geometry is employed and an on-axis aperture is positioned in front of the optical sensor. However, the occurrence of the nonlinear refraction can also distort the transmitted light beam. The beam distortion imposes serious problems in the device design for optical-limiting and pulse-shape application. Therefore, it is necessary to comprehensively investigate the nonlinear refraction in porphyrins aside from the nonlinear absorption. In previous works, the sign alteration of nonlinear refraction was observed only in metal clusters.11 By varying the ratio of two clusters, the nonlinear refraction of the mixture can experience an alteration from self-focusing to self-defocusing.11a More attention was focused on the transferable nonlinear refraction of the metal clusters depending on the laser pulses.11b,c Until now, the studies on nonlinear refraction of porphyrin molecules are short of sharing. It is recently reported that the metallization in the self-assembled porphyrins can result in a disappearance of self-lensing properties.12 In this paper, we synthesized porphyrin derivatives P2 and ZnP2 and studied both nonlinear refraction and nonlinear absorption of by using the Z-scan technique at 532 nm in different laser pulse conditions. The alteration from selfdefocusing to self-focusing of nonlinear refraction in the porphyrin molecules was observed through the coordination of zinc atom into the macrocycle. Experimental Section General. Most of the chemical reagents were purchased from Alfa Aesar, Acros Ltd., or Aldrich Chemicals and were utilized as received unless indicated otherwise. All solvents were purified using standard procedures. Columm chromatography was performed on silica gel (size 200-300 mesh). 1H NMR spectra
10.1021/jp076515v CCC: $40.75 © 2008 American Chemical Society Published on Web 12/28/2007
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SCHEME 1: The Molecular Structures of P2 and ZnP2
were recorded on either a Bruker ARX400 or DMX300 spectrometer. Matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra (TOF) were measured on a Bruker Biflex III MALDI-TOF instrument. The molecular structures of P2 and ZnP2 are shown in Scheme 1. Synthesis of 5,15-Bimesitylene-10,20-bi(1,2-diphenylethene)porphyrin (P2). A mixture of (Z)-2-(mesityl(2H-pyrrol-2ylidene)methyl)-1H-pyrrole (0.41 g, 8.5 mmol) and (E)-1-(4styrylphenyl)ethanone13 (0.33 g, 8.5 mmol) in 280 mL of distilled dichloromethane was degassed by nitrogen for 30 min under ice bath. After CF3COOH (0.093 mmol, 7 mL) was added, the solution was stirred overnight. Following the addition of tetrachlorobenzoquinone (0.65 g, 2.6 mmol), the resulting solution was stirred for a further 3 h. After being concentrated on a rotary evaporator, the residue was separated on a silica gel column to obtain the pure product as a purple solid (yield 18%). 1H NMR (400 MHz, CDCl3): δ (ppm) -2.55 (s, 2H), 1.85 (s, 12H), 2.62 (s, 6H), 7.28 (s, 4H), 7.33 (m, 2H), 7.44 (m, 8H), 7.67 (d, 4H), 7.89 (d, 4H), 8.22 (d, 4H, 8.0 Hz), 8.70 (d, 4H), 8.85 (d, 4H). MALDI-TOF MS (m/z): 903.5 [M + H]+. Synthesis of ZnP2. To a solution of P2 (45 mg, 0.05 mmol) in 50 mL of THF solution was added Zn(CH3COO)2‚2H2O (165 mg, 0.5 mmol), and this was stirred for 2 h. This solution was located on a silica gel column and eluted with dichloromethane. The products as aubergine solid were obtained. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.84 (s, 12H), 2.63 (s, 6H), 7.28 (s, 4H), 7.33 (m, 2H), 7.44 (m, 8H), 7.66 (d, 4H), 7.89 (d, 4H), 8.22 (d, 4H), 8.79 (d, 4H), 8.94 (d, 4H). MALDI-TOF MS (m/ z): 964.4 [M + H]+. Measurements. UV-vis spectra were taken on a Hitachi U-3010 spectrometer, and steady-state fluorescence spectra were measured on a Hitachi F-4500 spectrometer. Nonlinear refraction and reverse saturable absorption were measured by using the closed- and open-apterture Z-scan.14 A Q-switched Nd:YAG laser (Continuum model PY61) were used to a generate 4 and 10 ns pulses with a repetition rate of 1 Hz at 532 nm. The spatial profiles of optical pulses were nearly Gaussian obtained by spatial filtering. The sample solution was placed in quartz cells of 2 mm thickness. The quartz cell with the sample was placed on a translation state controlled by a computer that moved the sample along the z-axis with respect to the focal point of a 308 mm focal lens. The laser pulses adjusted by an attenuator were separated into two beams by using a splitter. The two beams were simultaneously measured by using two energy detectors (818J-09B energy probe, Newport Corporation) linked to the energy meter (model 2835-C, Newport). A personal computer was used to collect data coming from the energy meter through
Figure 1. Normalized absorption spectra of P2 (solid line) and ZnP2 (dashed line) in dichloromethane at a concentration of 1 × 10-5 mol/ L.
the RS-232C interface. The on-axis peak energy at focus is 7.73 µJ for 4 ns pulse and 14.9 µJ for 10 ns pulse, respectively. Results and Discussion Synthesis. The general strategy employed for the synthesis of P2 and ZnP2 was summarized in the Experimental Section. The molecular structures of P2 and ZnP2 are shown in Scheme 1. Their structures have been verified by spectroscopic analyses including 1H NMR and MALDI-TOF mass spectra. Absorption and Fluorescence Spectra Measurements. The absorption spectra of P2 and ZnP2 shown in Figure 1 present the characteristic bands of porphyrins.15 It is well-known that porphyrin molecules present two absorption bands in the visible spectra, B (Soret) and Q bands, which are attributed to transitions of the molecular π and π* orbitals of the porphyrin rings. As a closed-shell ion, a slight influence of d levels of Zn2+ on π-π* transitions of the porphyrin ring is expected. The Soret bands locate around 424 nm for P2 and 426 nm for ZnP2, respectively, while the Q bands cover the region from 500 to 700 nm. ZnP2 showed a red-shift of Q band compared with P2 because of the more delocalized π electrons on the macrocycle; the absorption peak of Q(1,0) is shifted from 550 to 610 nm. For both P2 and ZnP2, the absorption around 532 nm used in the nonlinear optical experiments is weak. That means only Q bands are excited at 532 nm. Figure 2 presents the ZnP2 fluorescence spectrum with two peaks at 603 and 650 nm and the P2 fluorescence spectrum with two peaks at 656 and 720 nm. The two peaks are related to the absorption bands Q(0,0) and Q(0,1), respectively.16 Compared with P2, ZnP2 showed a 50 nm blue-shift emission when excited at 424 nm. While excited at 532 nm, the wavelength used in Z-scan measurements, the emission spectrum of ZnP2 stays changeless. The emission peaks of P2, however, obviously differ from the emission peaks obtained at 424 nm. The peak at 650 nm decreases slightly accompanied with a remarkable increase of the peak at 725 nm. Nonlinear Optical Measurements. The Z-scan technique is a single-beam method for measuring both nonlinear absorption and nonlinear refraction. Its operation involves measurements of the far-field sample transmittance (transmitted energy detected by D2 divided by input energy monitored by D1) of a focused Gaussian beam as a function of the sample position (z) relative to the beam waist. An aperture is optionally placed in front of
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Figure 2. Normalized fluorescence spectra of P2 (solid line) and ZnP2 (dashed line) with excitation at 424 and 532 nm, respectively.
Figure 4. Normalized closed-aperture Z-scan curves of P2 (squares) and ZnP2 (circles) in CHCl2 for the cases of (a) 4 ns and (b) 10 ns. The solid lines are the fitting curves.
Figure 3. Normalized open-aperture Z-scan curves of P2 (squares) and ZnP2 (circles) with (a) 4 ns and (b) 10 ns pulse duration at 532 nm. The solid lines are the fitting curves.
D2. When the aperture is absent, total transmitted energy is detected, and the resultant Z-scan curve reveals nonlinear absorption alone. When the aperture is present, beam distortion (broadening or narrowing) induced by nonlinear phase shift, in addition to nonlinear absorption, affects the amount of energy through the aperture. The result thus contains both nonlinear absorption and nonlinear refraction. Division of the Z-scan curve obtained with an aperture by that without an aperture yields a
curve with nonlinear absorption effectively eliminated. This curve, referred to as the divided Z-scan curve, reveals the effect of nonlinear refraction alone. The Z-scan measurements were performed in dichloromethane solution at the concentration of 2.2 × 10-4 mol/L for P2 and 1.3 × 10-4 mol/L for ZnP2. Figure 3 gives the nonlinear absorptive curves of P2 and ZnP2 with a 4 and 10 ns pulse duration under an open-aperture Z-scan configuration. The fitting curves were obtained on the basis of a five-level energy model which took into account the dynamic thermal effect resulting from the transient excited-state absorption. The valleys of the normalized transmittances indicate the laser pulse experience reverse saturable absorption in both P2 and ZnP2. For P2 and ZnP2, the nonlinear absorption under 4 ns pulse is slightly weaker than the one under 10 ns pulse. This phenomenon may be attributed to the more obvious thermal effect under 10 ns pulse which usually leads to the saturated absorption response. It is noticeable that the coordination of the zinc atom results in an increase of RSA which is independent of the laser pulse width. Since the reverse saturable absorption of porphyrin derivatives is well-known to result from excited-state absorption,4g,17 the coordination interaction of zinc atom makes the excited states in ZnP2 differ from those in P2. Figure 4 gives the nonlinear refraction Z-scan curves of P2 and ZnP2 obtained by dividing closed-aperture Z-scan data by corresponding open-aperture Z-scan data. The distortion of
Excited-State Absorption and Sign Tuning closed-aperture Z-scan curves, including the shifting base lines and the asymmetry with a narrow peak and a broad valley, should result from the competition between the thermal nonlinearities for the existing excited-state absorption and the excited-state refraction. In the case of 4 ns pulse duration, P2 has a negative sign for the nonlinear refraction and exhibits a self-defocusing behavior while ZnP2 exhibits an opposite selffocusing response. It is the coordination interaction that alters the sign of nonlinear refraction in porphyrins. It has been reported that positive nonlinear refraction is attributed to population transitions among singlet states; the refraction volume of the singlet excited state is larger than that of the ground state.11b On the basis of this theory, it can be deduced that the Zn atom increases the refraction volume of the singlet excitedstate so that ZnP2 exhibits positive nonlinear refraction at short pulses, which is less than the lifetime of the triplet excited states. The results indicate that the sign of self-lensing can be tuned by the coordination of Zn in the porphyrin derivatives at short pulses. In the case of the 10 ns pulse duration, however, both P2 and ZnP2 exhibit strong self-defocusing behaviors and the difference of nonlinear refraction sign between P2 and ZnP2 could not be observed anymore. It is believed that the obviously thermal effect at longer pulse duration, which is stronger than the coordination effect, plays a crucial role of dominating the sign of nonlinear refraction in porphyrins. For the same reason, it can be seen that an opposite sign of nonlinear refraction exists between 4 and 10 ns laser pulses for ZnP2. The pulse-dependent sign alteration of nonlinear refraction in this paper is consistent with the previous work.11 In ZnP2, the thermal effect counteracts the coordination effect to result in the negative nonlinear refraction being weaker than P2, as shown in Figure 4. More detailed relative works on mechanism are underway. Conclusion In summary, the porphyrin derivatives of P2 and ZnP2 were synthesized and characterized and their nonlinear absorptions and nonlinear refractions were studied using the Z-scan technique at 532 nm with different pulse durations. Both P2 and ZnP2 exhibit reverse saturated absorption rooting in excitedstate absorption. Importantly, the coordination of P2 by Zn ion is able to tune the nonlinear refraction sign from negative to positive at 4 ns pulse. The results indicate that the sign of selflensing can be controlled by the coordination interaction of Zn and porphyrins. In the case of longer pulse duration, the thermal effect was enhanced to dominate the nonlinear refraction sign, leading to the negative nonlinear refraction repeated appearance. This may provide a possible route to solve the problems of the beam distortion in the device design in the future for opticallimiting and pulse-shape application. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20531060, 10474101, 20571078, 20473102, 20418001, and 20421101) and the National Basic Research 973 Program of China (Grant No. 2007CB936401 and 2006CB806200). References and Notes (1) (a) Zyss, J., Ed. Molecular Nonlinear Optics: Materials, Physics and DeVices; Academic Press: Boston, MA, 1994. (b) Bosshard, C.; Sutter, K.; Preˆtre, P.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter. P. Organic Nonlinear Optical Materials; Gordon and Breach Science Publishers: Amsterdam, The Netherlands, 1995; Vol. 1. (c) Nalwa, H. S.; Myata, S. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: Boca Raton, FL, 1997. (2) (a) Bredas, J. L.; Adant, C.; Tackx, P.; Petersoons, A.; Pierce, B. M. Chem. ReV. 1994, 94, 243. (b) Nalwa, H. S. AdV. Mater. 1993, 5, 341.
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