Article pubs.acs.org/JPCA
Two-Photon-Induced Isomerization of Spiropyran/Merocyanine at the Air/Water Interface Probed by Second Harmonic Generation Lu Lin,†,‡ Zhen Zhang,‡ Zhou Lu,‡ Yuan Guo,*,‡,§ and Minghua Liu*,‡,∥ †
National Center for Nanoscience and Technology, Beijing 100190, P. R. China Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ∥ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, P. R. China ‡
S Supporting Information *
ABSTRACT: Photochromic molecules often exhibit switchable hyperpolarizabilities upon photoisomerization between two molecular states and can be widely applied in nonlinear optical materials. Photoisomerization can occur through either one-photon or two-photon processes. Two-photon-induced isomerization has several advantages over one-photon process but has not been fully explored. In the present study, we have used second harmonic generation to investigate the two-photon-induced isomerization between spiropyran and merocyanine at the air/water interface. We show that spiropyran and merocyanine can be converted into each other reversibly with 780-nm laser-beam irradiation through two-photon processes. We also investigated the isomerization rates under various incident laser powers. Quantitative analysis revealed that the isomerization rates of spiropyran and merocyanine depend differently on the laser power. We attribute the difference to the distinct molecular structures of spiropyran and merocyanine. At the interface, nonplanar spiropyran molecules exist mainly as monomers, whereas planar merocyanine molecules form aggregates. Upon aggregation, steric hindrance effects and excitonic coupling efficiently arrest the photoisomerization of merocyanine. This work provides an in-depth understanding of two-photon-induced isomerization at the interface, which is beneficial for designing and controlling optical thin-film materials.
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than in the bulk.8 For instance, Kajikawa et al. investigated the J-aggregation of MC at interfaces and demonstrated the longterm stability of the MC aggregates.21,22 As a result, the photoisomerization kinetics at the interface should differ from that in the bulk phase. This difference was partially confirmed by Siebenhofer et al., who observed slower isomerization in a densely packed MC monolayer than in solution.19 However, a comprehensive description of SP/MC isomerization kinetics at the interface is still lacking in the literature. Photoisomerization between SP and MC can occur through either one-photon or two-photon processes. It is worth noting that previous studies concerning photoinduced SP/MC isomerization focused mainly on the one-photon excitation process, whereas the two-photon-induced isomerization between SP and MC has not yet been thoroughly investigated. Although it was previously reported that SP can be photoswitched by two-photon-induced isomerization using 780-nm near-IR radiation,23,24 detailed knowledge of the isomerization kinetics of the two-photon-induced processes, in particular, the two-photon-induced ring closure of MC, remains lacking. In
INTRODUCTION Photochromic molecules, such as spiropyrans, fulgimides, and azobenzenes, are widely applicable in nonlinear optical (NLO) materials because their hyperpolarizabilities are switchable upon photoisomerization between two molecular states.1−5 To make the most of their NLO properties, such molecules are often fabricated into thin films because even-order NLO processes are allowed only at interfaces with broken centrosymmetry.6,7 It has been reported that the isomerization rate, the reversibility of photoisomerization, and the NLO contrast are critical factors determining the efficiency of thin-film materials.8 A deep understanding of photoisomerization kinetics at the interface is hence needed to construct better NLO films. In studies of photochromic molecules, the photoisomerization between spiropyran (SP) and merocyanine (MC) has gained considerable attention in the past two decades9−20 because these molecules have entirely different properties, which make them more than simple photoswitches.8 Most previous studies, including those of photoisomerization kinetics, were conducted in the gas phase or in solution rather than at interfaces. Unlike in the bulk phase, where the ringopen MC form is thermally unstable and can easily change into the more stable ring-closed SP form,18 MC can form aggregates at the interface and become more thermodynamically stable © XXXX American Chemical Society
Received: August 9, 2016 Revised: September 20, 2016
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DOI: 10.1021/acs.jpca.6b08053 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Figure 1. (a) Molecular structures of SP and MC used in this work and illustration of photoisomerization. (b) SHG intensities as a function of time from an SP monolayer spread on a pure water subphase: (top) as spread and (bottom) after irradiation with a UV lamp for 20 min. The surface pressures of the SP and MC monolayers were 0 and 6 mN/m, respectively. The incident laser power was 800 mW. The incident light was ppolarized without an analyzer before the detector. Significant fluctuations in the SHG intensity were observed for the as-spread SP monolayer, whereas a strong enhancement of the SHG intensity followed by a gradual decrease was observed for the UV-irradiated monolayer, implying isomerization from SP to MC upon UV irradiation and isomerization from MC to SP upon laser-beam irradiation.
employed this strategy in the present work to investigate the kinetics of two-photon-induced SP/MC isomerization at the air/water interface. We show that SP and MC can be converted reversibly using 780-nm laser-beam irradiation. The dependencies of the isomerization rates on the incident laser power are entirely different for SP and MC, which is attributed to the molecular aggregation of MC at the interface.
fact, two-photon-induced isomerization has several advantages over one-photon excitation process in the fabrication of NLO materials. For instance, two-photon excitation can be achieved with a near-IR light source, which provides deeper penetration into materials.25 Moreover, because of the relatively small twophoton absorption cross section, isomerization induced by twophoton excitation can occur only in the focal volume of a laser beam.26 These characteristics give rise to potential threedimensional control of NLO materials.27 As a result, it is necessary to investigate the kinetics of the two-photon-induced isomerization between SP and MC at interfaces. So far, interfacial structures and dynamics have been probed by a variety of techniques, including scanning probe microscopy, surface-enhanced Raman spectroscopy, neutron reflectometry, X-ray photoelectron spectroscopy, ellipsometry, second harmonic generation (SHG), and sum frequency generation (SFG).28,29 SHG is interface-selective and sensitive to the components, structures, and molecular orientations of interfacial monolayers, and thus, it has been successfully employed for in situ characterization of the dynamic processes occurring at various surfaces and/or interfaces.30−34 SHG intensity is proportional to the square of the mode of the second-order nonlinear susceptibility of the interface, χ, which is related to the hyperpolarizabilities, β, of the molecules at the interface. Both SP and MC have large hyperpolarizabilities, with that of the latter being greater.35 Consequently, these two molecules can result in different SHG responses. The photoisomerization between SP and MC at interfaces can hence be monitored in real time by SHG detection. Most SHG measurements at present are conducted using Ti:sapphire femtosecond lasers, which are also suitable for twophoton photochemistry.36 It is thus possible to trigger SP/MC isomerization at the interface and monitor it simultaneously using only one femtosecond laser beam. Accordingly, we
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EXPERIMENTAL SECTION
SP was synthesized according to the procedure in the literature.37 Chloroform (purity ≥ 99.9%) was purchased from Sigma-Aldrich and used as received. Ultrapure water (Millipore, 18.2 MΩ·cm−1) was used as the subphase. Forty microliters of SP/chloroform solution with a concentration of 0.5 mM was spread onto the subphase contained in a Teflon beaker with a microsyringe. The surface area was 80 cm2, and the average area occupied by one molecule was 0.66 nm2. Before detection, the solvent was allowed to evaporate for 20 min. A mode-locked femtosecond Ti:sapphire laser (Tsunami 3960C, Spectra-Physics) with a pulse width of 80 fs and a repetition rate of 82 MHz was used for two-photon-induced isomerization and SHG measurements. The incident angle of the laser beam was set at 70° with respect to the surface normal. SHG signals in the reflected direction were detected with a high-gain photomultiplier tube (R585, Hamamatsu) and a gated photon counter (SR400, Stanford Research Systems). The incident light was p-polarized, and no analyzer was placed before the detector. The incident laser power was adjusted with an attenuator. A UV lamp that could provide 355-nm light was placed above the sample. All experiments were carried out under controlled room temperature of about 22 °C and humidity below 40%. B
DOI: 10.1021/acs.jpca.6b08053 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Time-dependent SHG signals from the monolayer are shown in Figure 1. First, fluctuations of the SHG signals from the SP monolayer were observed (Figure 1b, top) in this experiment, indicating that either disordered structures emerged or the laser heat induced lateral diffusion of the SP molecules.38 Then, the laser beam was blocked, and the SP monolayer was irradiated with a UV lamp for 20 min. After that, the UV lamp was turned off, and the block was removed. A strong enhancement of the SHG intensity compared with that from the initial SP monolayer was observed, followed by a gradual decrease (Figure 1b, bottom). Upon UV illumination, SP molecules at the interface can be converted into MC molecules.18,22,23 The strong enhancement of SHG is due to the facts that the MC molecule has a larger hyperpolarizability than the SP molecule and that MC molecules form well-ordered structures, as evidenced by an increased surface pressure after UV illumination [see Figure S1 in the Supporting Information (SI)]. The decrease of the SHG signal can be attributed to the laser-induced isomerization of MC. To confirm this assumption, we blocked the laser beam during the SHG decrease, kept the sample in the dark for 15 min, and then removed the block. We found that the SHG intensity remained unchanged during this time interval (see Figure S2 in SI). Hence, the decrease of SHG is indeed caused by laser illumination. Upon photoexcitation, MC can switch to its cis isomer and then to SP. The lifetime of the cis isomer is on the time scale of microseconds to milliseconds.13 Moreover, SP generates much weaker SHG than MC does. Therefore, it is reasonable to propose that MC can be converted into SP by laser illumination. Additionally, no fluctuations were observed from the monolayer after UV irradiation because, in this case, only the MC molecules in the laser spot (50 μm in diameter) were converted back into SP and the lateral diffusion was confined by the surrounding MC molecules out of the laser spot. To verify the reversibility of the photoisomerization between SP and MC, the monolayer was again irradiated with the UV lamp after the decreasing SHG signal had reached equilibrium. The procedures were as follows: First, the laser beam was blocked, and the UV lamp was turned on, and then the monolayer was irradiated for a certain time. After that, the lamp was turned off, the laser block was removed, SHG signals were collected for 1 s, and then the laser was blocked again. These procedures were repeated, and the SHG intensities as the function of the UV irradiation time were recorded, as shown in Figure 2. It was found that the SHG intensity gradually increased with the UV irradiation. After about 20 min, the intensity reached a maximum, which was the same as that of the initial MC monolayer. These results support our suggestions that MC can be converted into SP by laser-beam induction and that the photoisomerization is reversible. We also assessed the fatigue of the isomerization by alternating UV and laser illumination over several cycles. The SHG intensities and photoisomerization kinetics were highly reproducible (see Figure S2 in SI). In addition, we noticed that the fundamental wavelength (780 nm) is not in resonance with MC, whereas the second harmonic wavelength (390 nm) is covered by the absorbance band of MC (see Figures S3 and S4 in SI).15,39 On the basis of these facts, we propose that this laser-induced isomerization is a two-photon process.
Figure 2. SHG intensity of an SP/MC monolayer as a function of UV irradiation time. The incident power was 800 mW, and the incident light was p-polarized without an analyzer before the detector. The vertical axis was normalized to the SHG intensity from the initial MC monolayer. The red circles are experimental results, and the solid line is a guide to the eye. The SHG gradually recovered upon UV irradiation, indicating the reversibility of the photoisomerization between SP and MC.
We then implemented time-dependent SHG experiments under various incident laser powers to investigate the kinetics of the two-photon-induced isomerization between SP and MC. These results are shown in Figure 3 with the SHG intensities
Figure 3. Time dependence of normalized SHG intensities under different incident powers. The solid lines are fits to eq 2. The final SHG intensities are different, indicating the coexistence of MC and SP in the laser spot at a ratio that depends on the incident laser power.
normalized for clarity. One can immediately observe in Figure 3 that the isomerization rates are strongly dependent on the incident laser power. Unexpectedly, one can also see that the final SHG intensities vary dramatically with the incident power. This variation indicates that the equilibrium of the photoisomerization between MC and SP also depends on the incident power, because if the MC molecules in the laser spot were completely converted into SP, then the final SHG intensities or, more precisely, the ratios of the final and initial SHG intensities would be the same, independent of the incident laser power. The power dependence of the equilibrium can be ascribed to the fact that 780-nm laser irradiation can also photoswitch SP into MC through a two-photon process as has been reported.23,24 The photoinduced equilibrium between MC and SP can be expressed as 2hν , k1
MC XooooooY SP 2hν , k 2
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(1) DOI: 10.1021/acs.jpca.6b08053 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 1. Fitting Parameters and Corresponding Logarithms P (mW) 500 600 700 800 900 945 1055
k1 (s−1) 0.00023 0.00129 0.00260 0.00680 0.01316 0.02104 0.03421
± ± ± ± ± ± ±
0.00001 0.00001 0.00001 0.00002 0.00003 0.00006 0.00011
k2 (s−1) 0.00165 0.00255 0.00281 0.00430 0.00541 0.00744 0.01127
± ± ± ± ± ± ±
0.00001 0.00001 0.00001 0.00002 0.00003 0.00004 0.00007
b
log P
log k1
log k2
−5.62 ± 0.07
2.70 2.78 2.84 2.90 2.95 2.98 3.02
−3.64 −2.89 −2.58 −2.17 −1.88 −1.68 −1.46
−2.78 −2.59 −2.55 −2.37 −2.27 −2.13 −1.95
where k1 and k2 are rate constants of the forward and reverse reactions, respectively. It is obvious that both of the rate constants depend on the incident power and their reliances on the power are different, resulting in an equilibrium between MC and SP that is dependent on the incident power. The rate constants can be obtained by fitting the time dependence curves of the SHG intensity in Figure 3. Denoting the initial amounts of MC and SP as c0 and 0, respectively, we derived the following relationship between SHG intensity and time ⎧ ⎡ ⎤ k 2t I ∝ ⎨c0b⎢e−(k1+ k 2)t + ⎥ 1 + (k 1 + k 2 )t ⎦ ⎩ ⎣ ⎪
⎪
2 ⎤⎫ ⎡ k 2t −(k1+ k 2)t ⎬ + c 0 ⎢1 − e − ⎥ 1 + (k 1 + k 2 )t ⎦⎭ ⎣
Figure 4. Logarithms of the reaction rate constants plotted against the logarithm of the incident power. The circles are experimental results, and the solid lines are fits to a linear function. log k1 and log k2 are linearly dependent on log P, whereas their slopes are significantly distinct because of the different molecular ordering and aggregation behaviors of MC and SP at the air/water interface.
⎪
⎪
(2)
where b = βMC/βSP is the ratio of the first hyperpolarizabilities of MC and SP. The detailed derivation and further description of eq 2 can be found in the SI. Because b is related to the molecular properties of MC and SP, which are independent of the incident power, we employed global fitting to fit the curves in Figure 3 simultaneously using eq 2 with parameter b linked. The fitting results, as well as the logarithms of k1, k2, and the incident powers, are listed in Table 1. Note that the value of c0 is arbitrary and is not included in the table. The curves in Figure 3 can be well fitted with eq 2. The fit value for parameter b is equal to −5.62, indicating that βMC and βSP have opposite signs and that βMC is about 5 times βSP. The negative value of b originates from the negative value of βMC, which has been attributed to the zwitterionic characteristic of merocyanine’s ground state.40 It is necessary to compare the hyperpolarizability contrast obtained by the above fitting with that from theoretical calculations. Plaquet et al. calculated the first hyperpolarizabilities of various SP/MC couples with different substituents.35 One of these couples (no. 21 in ref 35) exhibited molecular structures similar to those of the couple studied in the present work. The calculated hyperpolarizability contrast for this couple is 5.34, which is close to the absolute value of the fit result for b.41 This consistency in the hyperpolarizability contrast, including both the absolute value and the sign, between our experimental results and the previous theoretical work further supports the validity of the reaction model proposed in this work. As a result of the two-photon-induced processes suggested above, it is expected that k1 and k2 in the isomerization between MC and SP must be proportional to the square of the incident power and that log k1 and log k2 should be linearly dependent on log P with a slope of 2. However, when log k1 and log k2 are plotted against log P and fitted to a linear function, as shown in Figure 4, the slopes are 6.6 ± 0.3 for log k1 and 2.4 ± 0.2 for log k2. This discrepancy between the fitting results and the expected value, especially for k1, which denotes the rate
constant of isomerization from MC to SP, is probably due to steric hindrance effects and excitonic coupling, as have been studied in self-assembled monolayers of azobenzene derivatives.42−46 For instance, Tamada et al. observed a marked deviation from the first-order plot in the reaction kinetics for photoinduced trans to cis isomerization of azobenzene disulfide SAMs.42 Similar effects are also likely to play key roles in the photoisomerization between MC and SP, as discussed below in detail. SP molecules have a nonplanar configuration and cannot form aggregates. As a result, they exist mainly as monomers at the interface. Therefore, the isomerization from SP to MC is simply a two-photon process, and the slope for log k2 is approximately equal to 2. On the contrary, interactions between MC molecules with planar configuration are much stronger so that they can readily form supramolecular assemblies through π−π stacking. Actually, self-aggregation of merocyanine dyes in solution was reported in previous studies.47,48 When at the air/ water interface where the capacity of molecular surfactants is limited and the molecular orientation is restricted by hydrophilic and hydrophobic forces, amphiphilic MC molecules are more likely to form aggregates than when in solution. In molecular aggregates, photoisomerization can be inhibited by steric hindrance from adjacent molecules. Moreover, photoinduced excitons can also be transferred to neighboring molecules, leading to quenching of the photoexcited molecules. In other words, the MC aggregates can be treated as an ensemble, in which photoisomerization can occur only when all of the MC molecules are excited simultaneously by absorbing multiple photons. This gives rise to a slope that is much larger than 2 for log k1. D
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(3) Schulze, M.; Utecht, M.; Hebert, A.; Rück-Braun, K.; Saalfrank, P.; Tegeder, P. Reversible Photoswitching of the Interfacial Nonlinear Optical Response. J. Phys. Chem. Lett. 2015, 6, 505−509. (4) Champagne, B.; Plaquet, A.; Pozzo, J.-L.; Rodriguez, V.; Castet, F. Nonlinear Optical Molecular Switches as Selective Cation Sensors. J. Am. Chem. Soc. 2012, 134, 8101−8103. (5) Fihey, A.; Perrier, A.; Browne, W. R.; Jacquemin, D. Multiphotochromic Molecular Systems. Chem. Soc. Rev. 2015, 44, 3719−3759. (6) Boyd, R. W. Nonlinear Optics, 3rd ed.; Academic Press: Burlington, MA, 2008. (7) Shen, Y. R. The Principles of Nonlinear Optics; Wiley-Interscience: Hoboken, N.J., 2003. (8) Klajn, R. Spiropyran-Based Dynamic Materials. Chem. Soc. Rev. 2014, 43, 148−184. (9) Ernsting, N. P.; Arthen-Engeland, T. Photochemical RingOpening Reaction of Indolinespiropyrans Studied by Subpicosecond Transient Absorption. J. Phys. Chem. 1991, 95, 5502−5509. (10) Aramaki, S.; Atkinson, G. H. Spironaphthopyran Photochromism: Picosecond Time-Resolved Spectroscopy. J. Am. Chem. Soc. 1992, 114, 438−444. (11) Zhang, J. Z.; Schwartz, B. J.; King, J. C.; Harris, C. B. Ultrafast Studies of Photochromic Spiropyrans in Solution. J. Am. Chem. Soc. 1992, 114, 10921−10927. (12) Görner, H.; Atabekyan, L. S.; Chibisov, A. K. Photoprocesses in Spiropyran-Derived Merocyanines: Singlet versus Triplet Pathway. Chem. Phys. Lett. 1996, 260, 59−64. (13) Chibisov, A. K.; Görner, H. Photoprocesses in SpiropyranDerived Merocyanines. J. Phys. Chem. A 1997, 101, 4305−4312. (14) Chibisov, A. K.; Görner, H. Photochromism of Spirobenzopyranindolines and Spironaphthopyranindolines. Phys. Chem. Chem. Phys. 2001, 3, 424−431. (15) Hobley, J.; Pfeifer-Fukumura, U.; Bletz, M.; Asahi, T.; Masuhara, H.; Fukumura, H. Ultrafast Photo-Dynamics of a Reversible Photochromic Spiropyran. J. Phys. Chem. A 2002, 106, 2265−2270. (16) Holm, A.-K.; Rini, M.; Nibbering, E. T. J.; Fidder, H. Femtosecond UV/Mid-IR Study of Photochromism of the Spiropyran 1′, 3′-Dihydro-1′, 3′, 3′-Trimethyl-6-Nitrospiro [2H-1-Benzopyran-2, 2′-(2H)-Indole] in Solution. Chem. Phys. Lett. 2003, 376, 214−219. (17) Holm, A.-K.; Mohammed, O. F.; Rini, M.; Mukhtar, E.; Nibbering, E. T. J.; Fidder, H. Sequential Merocyanine Product Isomerization Following Femtosecond UV Excitation of a Spiropyran. J. Phys. Chem. A 2005, 109, 8962−8968. (18) Darwish, T. A.; Tong, Y.-j.; James, M.; Hanley, T. L.; Peng, Q.-l.; Ye, S. Characterizing the Photoinduced Switching Process of a Nitrospiropyran Self-Assembled Monolayer Using in Situ Sum Frequency Generation Spectroscopy. Langmuir 2012, 28, 13852− 13860. (19) Siebenhofer, B.; Gorelik, S.; Lear, M. J.; Song, H. Y.; Nowak, C.; Hobley, J. Transient Absorption Spectroscopy on Spiropyran Monolayers Using Nanosecond Pump-Probe Brewster Angle Reflectometry. Photoch. Photobio. Sci. 2013, 12, 848−853. (20) Markworth, P. B.; Adamson, B. D.; Coughlan, N. J. A.; Goerigk, L.; Bieske, E. J. Photoisomerization Action Spectroscopy: Flicking the Protonated Merocyanine-Spiropyran Switch in the Gas Phase. Phys. Chem. Chem. Phys. 2015, 17, 25676−25688. (21) Kajikawa, K.; Takezoe, H.; Fukuda, A. Orientational Structure of Noncentrosymmetric Domain of Merocyanine J-Aggregates Studied by Surface Second-Harmonic Generation. Chem. Phys. Lett. 1993, 205, 225−228. (22) Kajikawa, K.; Anzai, T.; Takezoe, H.; Fukuda, A. Noncentrosymmetric and Centrosymmetric J-Aggregation in Photomerocyanine Monolayer Studied by Second-Harmonic Generation and Absorption Spectroscopy. Thin Solid Films 1994, 243, 587−591. (23) Ivashenko, O.; van Herpt, J. T.; Feringa, B. L.; Rudolf, P.; Browne, W. R. UV/Vis and NIR Light-Responsive Spiropyran SelfAssembled Monolayers. Langmuir 2013, 29, 4290−4297. (24) Zhu, M.-Q.; Zhang, G.-F.; Li, C.; Aldred, M. P.; Chang, E.; Drezek, R. A.; Li, A. D. Q. Reversible Two-Photon Photoswitching and
CONCLUSIONS In summary, we studied the two-photon-induced isomerization between SP and MC at the air/water interface. We found that SP and MC molecules can be converted into each other reversibly by 780-nm laser irradiation through two-photon processes. The photoisomerization rates of SP and MC show different dependences on the incident laser power. We propose that the difference originates from the molecular ordering and aggregation at the interface. Nonplanar SP molecules exist mainly as monomers at the interfaces; hence, the isomerization rate is approximately proportional to the square of the laser power, as expected for a two-photon-induced process. In contrast, MC molecules form aggregates at the interface. As a consequence, the photoisomerization of MC is strongly affected by steric hindrance effects and excitonic coupling. These findings provide new insights into two-photoninduced reaction kinetics at the air/water interface and are beneficial for the preparation of NLO materials. Unlike in the gas phase or solution, molecules at interfaces are highly oriented and have strong interactions between them, giving rise to particular dynamic characteristics upon light irradiation that can lead to some interesting results. For instance, the ratio between MC and SP molecules can be controlled by adjusting the incident power, as shown in Figure 3. Consequently, if MC/SP molecules were used to fabricate optical switches or other molecular devices, the NLO contrast between the switching states would be readily manipulable. In addition, photoisomerization between MC and SP is highly reversible, and the NLO contrast is nearly 100%. These properties make MC/SP appropriate for NLO materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b08053. Surface pressure−area isotherms of SP and MC, SHG experimental results upon UV/laser illumination cycles, UV/vis spectra of SP and MC, spectrum of the laser output, and derivation of the relationship between SHG intensity and time (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Basic Research Development Program (2013CB834504) and the National Natural Science Foundation of China (21227802 and 21073199) for financial support.
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REFERENCES
(1) Delaire, J. A.; Nakatani, K. Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials. Chem. Rev. 2000, 100, 1817−1846. (2) Castet, F.; Rodriguez, V.; Pozzo, J.-L.; Ducasse, L.; Plaquet, A.; Champagne, B. Design and Characterization of Molecular Nonlinear Optical Switches. Acc. Chem. Res. 2013, 46, 2656−2665. E
DOI: 10.1021/acs.jpca.6b08053 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpca.6b08053 J. Phys. Chem. A XXXX, XXX, XXX−XXX
