Photodimerization Kinetics of a Styrylquinoline Derivative in Langmuir

Oct 5, 2017 - It has been shown that the number density of molecules within a functionalized monolayer can largely affect its surface morphology, mole...
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Article Cite This: J. Phys. Chem. C 2017, 121, 23541-23550

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Photodimerization Kinetics of a Styrylquinoline Derivative in Langmuir−Blodgett Monolayers Monitored by Second Harmonic Generation Yingxue Ma,†,‡ Yu Xie,§ Lu Lin,∥ Li Zhang,† Minghua Liu,† Yuan Guo,†,‡ Zhenggang Lan,*,§ and Zhou Lu*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy Sciences, Beijing 100049, China § Key Laboratory of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 Shandong, China ∥ National Center for Nanoscience and Technology, Beijing 100190, China S Supporting Information *

ABSTRACT: To explore the influence of surface packing densities on the interfacial photochemical kinetics, the surface-selective second harmonic generation (SHG) technique was used to investigate the kinetics of two-photon induced [2 + 2] photocycloadditions of a styrylquinoline alkoxy derivative within the Langmuir−Blodgett (LB) monolayers. The laser power dependence experiment revealed that this interfacial photodimerization is a first-order reaction, which implies that the photoexcitation is the rate-limiting step. Interestingly, a comparison of photodimerization kinetics at different surface packing densities shows a nonmonotonic distribution of reaction rate constants, which can be attributed to a result of combined effects of the topochemical mechanism and steric hindrance. The atomic force microscopy measurements and theoretical calculations were also employed to help understand the [2 + 2] photocycloaddition mechanisms. The results presented in this work demonstrate that the surface packing density plays an important role in regulating the interfacial photoreactions within the LB monolayers composed of the conjugated aromatic molecular systems.

I. INTRODUCTION

situ SHG measurements have been successfully used to study the photopolymerization at the air/water interface and the reversible photoswitching reactions of conjugated organic molecules within the self-assembled monolayers on solid substrates.24−26 We recently employed the SHG technique to investigate the two-photon induced photoisomerization between spiropyran and merocyanine at the air/water interface.27 The isomerization rate constants of the forward and backward reactions were discovered to depend differently on the incident laser power, which was possibly due to the different aggregation structures of spiropyran and merocyanine in the monolayer. However, the packing structures within the monolayers were not experimentally characterized and purposely controlled in that study. The adjustment of the packing densities, in either a Langmuir monolayer at the air/aqueous solution interface or Langmuir−

The photoresponsive behaviors of chromophores in twodimensional confinements often considerably differ from those in bulk materials.1−7 For example, the cis-to-trans photoisomerization dynamics of amphiphilic merocyanine at the air/ water interface was reported to be several orders of magnitude slower than that in bulk solution, which possibly is a result of restrained molecular geometries and motions in densely packed monolayers.7 Nevertheless, the photoreactions in monolayers and ultrathin films are still poorly understood, partly due to the lack of the nonintrusive spectroscopic method that is sensitive enough to the short optical path length of ultrathin films with only one or a few monolayers. The fast developed second harmonic generation (SHG) and sum frequency generation (SFG) spectroscopies have become promising techniques to solve this problem. With the intrinsic surface selectivity and submonolayer sensitivity, SHG and SFG have proved to be versatile and powerful optical probes for molecular structures and motions at various interfaces.1−4,8−23 In respect of monitoring the photochemistry in monolayers, in © 2017 American Chemical Society

Received: August 23, 2017 Revised: October 3, 2017 Published: October 5, 2017 23541

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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The Journal of Physical Chemistry C

present in this work clearly demonstrate the important roles played by film packing density in regulating the photoreactions at surfaces.

Blodgett (LB) monolayer on solid substrates, is one of the easiest ways to regulate the intermolecular interactions and assembly structures at surfaces. It has been shown that the number density of molecules within a functionalized monolayer can largely affect its surface morphology, molecular orientations, supramolecular chiral structures, and surface reactions with gaseous molecules/radicals.28−36 However, the influence of molecular packing densities and aggregation structures on the photochemical reaction rates in LB monolayers has not been systematically explored yet. In this study, the two-photon induced dimerization of amphiphilic styrylquinoline alkoxy derivatives in a LB monolayer was selected as a model reaction to investigate the dependence of surface photochemical reaction kinetics on interfacial molecular densities. Previous studies have shown interesting photochemical responses of styrylquinoline (SQ) molecules in the LB films.32,37 Opposite to the reversible trans-to-cis photoisomerization in methanol solutions when exposed to the UV radiation, the [2 + 2] photocycloaddition was observed to dominate in the LB films of styrylquinoline alkoxy derivatives with long alkyl chains. The supramolecular chirality and acidichromism were also demonstrated for the SQ derivative LB films, revealing their potential applications in fabricating new photosensing and photoswitching devices.32,37 Most recently, we have successfully employed the SHG and atomic force microscopy (AFM) techniques to characterize acidic effects on the morphologies and two-photon induced photodimerization kinetics of amphiphilic SQ derivatives within LB monolayers, showing the key roles played by the intermolecular interactions and aggregation structures in the surface photochemical reactions.38 The aim of the current study is to explore the interrelations between the monolayer packing densities and photodimerization kinetics of the SQ derivatives in LB monolayers. To do so, the single-layered LB films of styrylquinoline alkoxy derivatives 3-(4-(octadecyloxy)styryl) quinoxalin-2(1H)-one (SQC18, as shown in Scheme 1) were prepared at three distinct surface

II. EXPERIMENTAL AND COMPUTATIONAL METHODS The synthesis, purification, and structure analysis of styrylquinoxaline derivative 3-(4-(octadecyloxy)styryl)quinoxalin2(1H)-one (SQC18) were all carried out according to the procedures in the literature.32,37 Chloroform (analytical reagent grade) was purchased from Xilong Chemical Company Limited in China and used without further purification. To avoid the trans-to-cis isomerization of styrylquinoxaline derivatives, the solution preparation and film deposition were both carried out in the dark. The LB film depositions were carried out using a KSV NIMA minitrough with a surface area of 270 cm2 (360 mm × 75 mm). A volume 30 μL of SQC18 solution in chloroform (0.9 mM) was spread onto the surface of ultrapure water (double distilled water further purified by a Millipore Simplicity 185 system, 18 MΩ cm). After the evaporation of chloroform for 20 min, the insoluble amphiphilic film at the air/water interface was compressed at a speed of 10 mm/min. The surface pressure− area (π−A) isotherm was measured with a Wihelmy plate during the film compression. At the desired surface pressures, the monolayers were transferred to cleaned fused quartz substrates for SHG measurements or freshly cleaved mica substrates for AFM measurements, both by the vertical lifting method at a speed of ∼11 mm/min. The real-time photodimerization kinetics of the SQC18 monolayer was monitored by a SHG setup that is based on a mode-locked Ti:sapphire laser (Tsunami 3960C, SpectraPhysics) with a repetition rate of 82 MHz and a pulse width of 80 fs.17,27,38,39 The center wavelength of the laser output was tuned to 790 nm. The laser beam was focused onto the top surface of the fused silica substrate by a convex focal lens (focal length = 10 cm) at an incident angle of 70° with respect to the surface normal. The incident laser beam not only served as the fundamental light source of the SHG measurements but also triggered the two-photon photochemical processes. The incident laser power was adjusted by a variable neutral density filter and the polarization was controlled by a half-wave plate. SHG signals were detected in a reflective geometry by a highgain photomultiplier (R585, Hamamatsu) and a gated photon counter (SR400, Stanford Research Systems). A short-pass filter and a monochromator were used to remove the fundamental beam from SHG signals. In this study, the SHG measurements were carried out with a P-in/all-out polarization mode, i.e., the incident laser was P-polarized while the SHG signals at all polarizations were collected. Because of the low SHG photon counts per laser shot, all the signals were averaged over ∼8.2 × 107 pulses. In addition, all of the SHG measurements were carried out under a controlled room temperature of about 22 °C and humidity below 40%. The AFM images were recorded using the tapping mode and silicon cantilever probes by a Digital Instrument Nanoscope IIIa Multimode system (Santa Barbara, CA). To assist the understanding of the experimental results, the geometries and excited states of SQ momomers, π-stacking complexes, and cycloadduct dimers were examined theoretically. Here the π-stacking complex refers to the two SQ monomers that were paired together by the noncovalent π−π interactions. All calculations were carried out with the Gaussian 09 program (Gaussian, Inc., Wallingford, CT, 2009). The

Scheme 1. Photodimeration Process of SQC18: (A) SQC18 and (B) the Photocycloadduct Dimer

pressures, followed by time-dependent SHG studies on the photodimerization kinetics. The laser power dependence measurements were carried out in SHG experiments to help establish the photodimerization rate law for SQC18 LB monolayers, based on which we were able to further compare the photodimerization rate constants for different SQC18 surface densities. In addition, AFM measurements and density function theory (DFT) calculations were also used to reveal the reaction mechanisms. The experimental and theoretical results 23542

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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The Journal of Physical Chemistry C ground-state and excited-state calculations were performed using density functional theory (DFT) and time-dependent DFT (TDDFT), respectively. Because both the inclusion of the dispersion interaction and proper treatment of the chargetransfer excited state need to be considered, the ωB97XD functional was used with the 6-31G* basis set.

III. RESULTS AND DISCUSSION Monolayer Formations. To regulate the molecular densities in monolayers, Langmuir monolayers were prepared at the air/water interface with controlled surface pressures and then transferred onto solid substrates for either the SHG or AFM measurements. Figure 1 displayed the surface pressure−

Figure 2. Typical time-dependence of normalized SHG intensities for the SQC18 LB monolayer with an MMA of 32 Å2 at various incident laser powers. The solid lines were the data fitting results using eqs 2 and 3.

nm.32,37 Therefore, the SHG activities of SQC18 molecules were resonantly enhanced, causing the large initial SHG amplitudes at time zero in Figure 2. The decay of the SHG intensities as shown in Figure 2 had to be induced by the fundamental laser beam in the SHG measurements since it was the only external field applied on the sample during our experiments. For the further verification, we blocked the laser beam for a period of ∼2 min at a randomly selected time delay during the SHG measurement. The SHG signal level was observed to be the same before and after the laser blockage period (as shown in Figure S1), confirming that the same laser beam used for the SHG measurement was responsible for the decrease of SHG amplitudes.27 Several reasons can possibly cause the laser-induced decay of second order nonlinear optical signals: (1) the ablation of SQC18 molecules by strong laser irradiations, (2) the change in net orientations of π-electrons as a consequence of the laser excitations,4,5 and (3) the alternation of hyperpolarizabilities during photoreactions.24−27,40,41 The fact that the SHG signals always reached equilibria and never completely vanished after a long period of laser exposures indicated that a significant portion of SQC18 molecules remained on the surfaces of fused silica substrates, therefore ruling out the possibility of laser ablations. On the other hand, if the depletion of SHG signals was purely caused by the laser-induced reorientation or disordering of interfacial SQC18 molecules, one would expect the restoration of SHG intensities when the laser beam stopped illuminating on the sample, just as what was observed in the previous ultrafast SFG and SHG investigations on the orientational dynamics of photoexcited organic dye molecules at the air/water interface.4,5 However, in the current study, we did not observe any recovery of SHG intensities after blocking the laser beam for a relatively long period, showing that the photoinitiated decay of SHG signals from SQC18 monolayers was permanent and the pure orientational change caused by the laser excitation was not the main reason. Previous SHG studies have shown that both the trans-to-cis and ring-opening/closure reactions of the conjugated organic dye molecules could result in significant changes of molecular hyperpolarizabilities and cause the decline of SHG intensities.25−27,40 Therefore, the interfacial photochemical reactions can greatly affect the second order nonlinear optical signals. In the case of SQC18 or other similar long chain SQ derivatives in multilayered LB films, irreversible [2 + 2] cycloadditions dominated upon the UV radiations as revealed by the UV−vis, FTIR, and TOF-MS measurements.32,37 For example, the FTIR

Figure 1. Surface pressure−area (π−A) isotherms of SQC18 on pure water subphase at room temperature. The surface pressures at which the Langmuir−Blodgett monolayers were prepared are marked by the arrows.

area (π−A) isotherm of SQC18 on the water subphase at room temperature. With the increasing surface pressure, two distinct regions could be identified on the isotherm: a coexistence of the gas (G) phase and liquid condensed (LC) phase and the pure LC phase. Furthermore, the limiting area obtained by extrapolating the linear part of the isotherms to zero surface pressure was 44 Å2. On the basis of the Corey−Pauling− Koltum (CPK) molecular model, a vertically and flatly oriented aromatic ring of SQC18 will at least occupy an area of 25 and 78 Å2 per molecule, respectively.32,37 Therefore, we can infer that the styrylquinoline ring of SQC18 took an inclined orientation at the air/water interface. In the present work, LB monolayers were deposited at three different surface pressures, 11 mN/m, 22 mN/m, and 28 mN/ m (marked by arrows in Figure 1). The corresponding mean molecular areas (MMA) are 48 Å2, 32 Å2, and 19 Å2, respectively. Two-Photon Induced Photodimerizatin of SQC18. The real-time SHG measurements were carried out to characterize the kinetics of SQC18 photodimerization at interfaces. Figure 2 shows the typical time-dependence of normalized SHG intensities for the SQC18 LB monolayer with an MMA of 32 Å2 at various incident laser powers. The SHG intensities were observed to decay gradually toward equilibria with the increasing laser exposure time. It is known that the SHG intensity, which relies on the second order nonlinear optical susceptibility, is highly sensitive to the degree of conjugations in molecules under investigation.25,27,40 In SQC18 molecules, the conjugation between the quinoxaline and benzene rings results in a broad UV−vis band that is centered at ∼386 nm and coincides with the second harmonic (SH) wavelength at 395 23543

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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The Journal of Physical Chemistry C ISHG(t ) ∝ |χM |2 ∝ θM(t )2

measurements showed the disappearance of vibrational features for CC bonds of a 30-layer film of SQC18 upon UV radiation at 365 nm, therefore, supporting the photodimerization process within the LB films instead of the trans-to-cis photoisomerization.37 Although the near-infrared laser wavelength centered at 790 nm was not in resonance with any electronic transitions of SQC18 and the photodimerization could not be triggered by single-photon absorptions, a twophoton induced [2 + 2] cycloaddition of SQC18 could occur considering the large peak laser power density of 1.5−3 × 107 W/cm2 that was used in our investigations and the SH wavelength of 395 nm near the absorption maximum of SQC18. The breaking of the CC double bonds and the formation of new σ bonds during such photodimerizations would disrupt the conjugated structures and lead to smaller absorption cross sections as shown in the UV−vis spectra of multilayered LB films of SQC18.32,37 The DFT calculations also confirmed a negligible transition probability at the SH wavelength of 395 nm when the dimerization occurs (see Figure 7 and the detailed discussions below). Therefore, a significantly weaker resonant enhancement of SHG process, which was a result of the formation of SQC18 cycloadduct dimers, was responsible for the observed decay of the SHG intensities as shown in Figure 2.38 The 790 nm laser beam not only served as the fundamental probe pulses during the timedependent SHG measurements but also triggered the twophoton dimerization processes. This is very similar to what happened in our recent SHG studies on the two-photon induced spiropyran-merocyanine isomerizations at the air/ water interface.27 The two-photon induced photochemical reaction occurred within the very limited area of laser spot on the LB monolayer. To directly measure such a small quantity of the two-photon dimerization products and other possible side products, the second-order nonlinear optical tools with the spectroscopic information, such as the vibrational sum frequency generation spectroscopy (VSFG), electronic sum frequency generation (ESFG), and SHG with the tunable fundamental wavelengths, shall be employed. Such a kind of research will further verify the two-photon induced dimerization products, however, is beyond the scope of the current work and will be carried out in the future. Reaction Rate Law. The time-dependence of the normalized SHG intensities can be quantitatively analyzed to obtain the kinetics of the two-photon induced photodimerization of SQC18 in the LB monolayers. The SHG intensity ISHG(t) at the given time t is determined by24,42 ISHG(t ) ∝ |χFS + χM + χD |2

(2)

in which θM(t) is the percentages of the unreacted monomers within the LB monolayer at the given time t. Up to date, both the first and second order reaction rate laws have been proposed for various photochemical reactions, including the photodimerizations in thin films and photopoloymerization at the air/aqueous interface.24,43−46 For the first order reaction model, θM(t) exponentially decays; while for the second order rate law, θM(t) is inversely proportional to the time. Since the photodimerization of SQC18 in the LB monolayer was never observed to be 100% complete, we further include a new parameter θM(∞) to describe the percentage of unreacted monomers at the reaction equilibria. The kinetics of θM(t) for the first order reaction can then be described as θM(t ) = [1 − θM(∞)] e−kt + θM(∞)

(3)

and that for the second order reaction can be represented by θM(t ) = [1 − θM(∞)]/kt + θM(∞)

(4)

where k stands for the photodimerization rate constant. Equations 2−4 were then used to fit the normalized SHG decay curves in Figure 2. It was found that both the first and second order kinetics models can reasonably fit the observed decay of SHG signals from the SQC18 LB monolayers. This is not surprising because the determination of the correct kinetics model by the curve fitting of time-dependent spectroscopic signals sometimes can be quite a challenge. In fact, similar problems have been encountered in a previous SHG study on the UV-radiation triggered polymerization of vinyl sterate monolayers at the air/water interface.24 To discern which the correct kinetics model is, additional measurements are needed. As discussed above, the SQC18 photocycloaddition observed in the current study occurred through a two-photon process. It is therefore expected that the rate constant k is proportional to the square of incident laser power P. In order to unambiguously distinguish whether the SQC18 photodimerization has the first or second order dependence on the surface densities of monomers, we examined the effect of laser powers on the SQC18 photodimerization rates. As shown in Figure 2, the decay kinetics of SHG signals strongly depended on the laser powers. These SHG curves taken at different laser powers were then fitted by both the first and second order reaction models, yielding two different sets of the photocycloaddition rate constants k. Figure 3 summarized the obtained rate constants k as a function of the incident laser power P in a logarithmic scale. It can be seen that there were linear relationships between log k and log P for both kinetic models, yielding the slopes of 1.8 ± 0.1 and 1.3 ± 0.1 for the first and second order reaction models, respectively. Obviously, the k values obtained from the firstorder kinetics model are approximately proportional to the square of the laser power. Therefore, the first-order reaction model agrees with the mechanisms of two-photon induced photodimerizations, implying that the photoexcitation is the rate-limiting step.45 The first-order kinetics also suggests that the dimerizaton might occur between a monomer in the excited state and a second monomer in the ground state according to previous studies on the photodimerization of solid C60 films.45 However, we will show later on in this article that the two SQC18 monomers involved in photodimerization are possibly not separated from each other but already form aggregations by

(1)

where χFS, χM, and χD are the second order nonlinear susceptibilities of the fused silica substrate, SQC18 monomer, and cycloadduct dimer, respectively. The photodimerization products only have negligible cross sections at the SH wavelength of 395 nm according to the DFT calculations (see detailed discussions below), therefore do not significantly contribute to the SHG intensities due to the weak resonant enhancement. Similarly, the SHG amplitude from the fused silica substrate was also insignificant. As a result, the observed strong SHG signals were dominated only by the unreacted SQC18 monomers and their aggregations. Equation 1 can then be simplified as 23544

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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Figure 3. SQC18 photodimerization reaction rate constants k as a function of incident laser powers. Both the k value data sets obtained by the first- (■) or second-order (○) kinetics models were included. The data were plotted in logarithmic scales. Each k value was an averaged result from three measurements. The solid and dashed lines are the results of linear fitting of log k vs log P.

Figure 4. Typical SHG decay curves during photodimerization of SQC18 LB monolayers at surface pressures of 11, 22, and 28 mN (corresponding to the MMA of 48, 32, and 19 Å2, respectively). The incident laser intensity was 500 mW. The solid lines were the data fitting results using eqs 2 and 3.

π−π stacking before absorbing any photons. Therefore, the dimerization shall follow the photoexcitation of the whole πstacking SQC18 complexes instead of exciting individual monomers. Nevertheless, here we show that the laser power dependence experiments can be quite effective in determining the kinetics models of the photoreactions which are otherwise difficult to be differentiated. In previous studies on photodimerizations of conjugated organic molecules in the solid state crystals, the reaction kinetics was also often described by the Johnson−Mehl− Avrami−Kolmogorov (JMAK) model for nucleation and growth:47−49 y = 1 − e−kt

distribution of photocycloaddition rate constants k that were summarized in Figure 5. It was seen that the photocycloaddition rate constant k does not monotonically depend on the surface packing densities of SQC18. As shown in Figure 5, the photocycloaddition rate constants at the low surface densities with the MMA of 48 Å2

n

(5)

where y is the yield of the product and n is a parameter describing the dimensionality of the growth of the product phase. The values of n = 2, 3, and 4 represent the one, two, and three-dimensional growth, respectively. In the current work, the first order photodimerization kinetics means n = 1, implying the homogeneous product growth and thorough light penetration.49 This is understandable since our sample has a monolayer structure that can be easily accessed by light. Effect of the Molecular Packing Densities on Photodimerization Kinetics. To investigate the influence of molecular packing densities on the photodimerization kinetics, the monolayers were transferred from the air/water interface to the fused silica substrates at the surface pressures of 11 mN/m, 22 mN/m, and 28 mN/m, corresponding to the MMA of 48 Å2, 32 Å2, and 19 Å2, respectively. We then carried out the timedependent SHG measurements on these LB monolayers at constant laser power of 500 mW. Figure 4 displays the typical time-evolutions of SHG signals for the three different MMAs. As discussed above, these SHG decay curves resulted from the two-photon induced [2 + 2] photocycloadditions of SQC18 molecules, therefore providing the direct measurement of the photodimerization kinetics within the LB monolayers. Because SQC18 molecules tended to form microdomains and were not evenly distributed at surfaces, we repeated the SHG measurements at more than 20 different positions for each LB monolayer to obtain the statistical results of the photodimerization kinetics. Each of these time-dependent SHG curves was then fitted by eqs 2 and 3, yielding the statistic

Figure 5. Statistical results of SQC18 photodimerization reaction rate constants k for different MMAs: (a) 48 Å2 (surface pressure ∼11 mN/ m), (b) 32 Å2 (surface pressure ∼22 mN/m), and (c) 19 Å2 (surface pressure ∼28 mN/m). 23545

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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Figure 6. AFM images of SQC18 LB monolayer deposited from the water subphase onto the mica for different MMAs: (a) 48 Å2 (surface pressure ∼11 mN/m), (b) 32 Å2 (surface pressure ∼22 mN/m), and (c) 19 Å2 (28 mN/m). The scan scale is 5 × 5 μm2. The area of the inserted image in part c is 1.2 × 1.2 μm2.

were mainly in the range of 1.2−2.4 × 10−3 s−1. When the surface densities increased, the rate constants also increased, ranging between 2.7 × 10−3 s−1 and 3.9 × 10−3 s−1 for the MMA of 32 Å2. However, when the surface densities further increased, the photocycloaddition rate constants decreased again, dropping into the range of 2.1−3.0 × 10−3 s−1 for the MMA of 19 Å2. This nonmonotonic dependence of photocycloaddition rate constant on the surface packing densities is remarkably different from the previous surface potential or surface pressure measurements on the photopolymerization of unsaturated amphiphilic esters at the air/aqueous interfaces in which the polymerization rate constants constantly decreased when MMA decreased.43 In order to understand why the photodimerization kinetics of interfacial SQC18 molecules does not correlate with the surface densities monotonically, we carried out the AFM measurements on the surface topographies of SQC18 LB monolayers. Figure 6a displays the AFM image of the SQC18 monolayer deposited at 11 mN/m (MMA ∼48 Å2). Apart from the flat regions of the mica substrate, short stick-like nanostructures were observed, implying the strong π−π interactions between the conjugated aromatic rings. The heights of these nanostructures were found to be distributed between 0.75 and 1.75 nm, much smaller compared with ∼3.5 nm that was estimated for the length of SQC18 molecules using the CPK model. This indicates that at large MMA such ∼48 Å2, the alkyl chains in SQC18 molecules were tilted away from the surface normal. Noticeable changes could be observed when surface densities increased. As shown in Figure 6b, much more densely packed nanostick domains were seen at the higher surface pressure of 22 mN/m (MMA ∼32 Å2). The depth analysis revealed that the heights of these nanostructures increased to 1.0−2.75 nm, indicating an orientational change of SQC18 molecules when the surface became more crowded. This is also in agreement with the picture of SQC18 aromatic rings oriented more vertically at the reducing MMA as mentioned above in the π−A isotherm measurements. The more vertically standing SQC18 molecules at MMA ∼32 Å2 can help the formation of face-toface aggregations since conjugated aromatic rings were parallel to each other in this arrangement. In addition, the more densely packed nanostructures and more upright SQC18 surface

orientations indicate the shorter intermolecular distances within the aggregations. According to the “topochemical postulate”,50−53 the occurrence of photocycloaddition in solid crystals requires that the reacting double bonds are parallel to each other and their distance is less than 4.2 Å. Although the SQC18 molecules under investigation here were not in the crystalline form, the molecules in the LB monolayer were restrained within the well-structured aggregations and partially anchored on solid substrates. Therefore, the SQC18 photodimerization in the LB monolayer is still a topochemically controlled reaction. The increasing SQC18 photocycloaddition rate constant at MMA ∼32 Å2 can be explained by the topochemical mechanism that the photodimerizations occur more easily with the shorter distances and more parallel orientations between the two reacting molecules. The SQC18 photodimerization slowed down again at the even smaller MMA of ∼19 Å2 (corresponding to the surface pressure of 28 mN/m) as shown in Figure 5c. The AFM images in Figure 6c revealed a uniform and highly dense SQC18 film under this condition. The stick-like nanostructures were no longer observed and the mica surface was almost completely covered by SQC18 molecules. Small undulations with height variations less than 1.2 nm could be seen in the enlarged AFM images (the inserted image in Figure 6c). In addition, nanosized protrusions that were 2−5 nm above the SQC18 monolayer were also occasionally observed, indicating that some SQC18 aggregations stuck out from the monolayer. In fact, the MMA of ∼19 Å2 is smaller than the area of 25 Å2 that a vertically oriented SQ ring was supposed to occupy as estimated by the CPK model. With such high surface density, it is not surprising that some SQC18 molecules were squeezed away from the surface. The decreasing photodimerizaition rate constants at MMA as low as ∼19 Å2 could therefore be attributed the increasing steric hindrance as the interface became too crowded. The bimodal distributions of photocycloaddition rate constants as a function of the SQC18 surface densities reflect the complicated nature of the LB monolayers. For the previously studied photopolymerization at the air/aqueous interface, the molecules are not anchored at any particular site and can diffuse around and find its reactive partners. Therefore, 23546

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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The Journal of Physical Chemistry C the photopolymerization rate constants were discovered to constantly decrease when MMA decreased, which purely resulted from the steric effects during the formation of photochemical products.24,43 However, here the SQC18 molecules are within the LB monolayer. They only had the very limited fluidity and could not freely diffuse. As a result, the photodimerization was slow at low surface densities due to the large intermolecular distances and possibly unfavorable molecular orientations. The photocycloaddition of SQC18 was then topochemically controlled, which is a characteristic of solid state photochemistry.50−53 On the other hand, the LB monolayers were fabricated by transferring the films from air/ water interface at desired surface pressure to a solid substrate. The packing densities and aggregation structures of LB monolayers could be more easily adjusted compared with the solid crystalline structures. At the significantly high surface densities where the film was almost fully compressed, the steric hindrance began to play a dominant role. Therefore, the observed nonmonotonic dependence of SQC18 photocycloaddtion kinetics on surface densities within LB monolayers was in fact a result of combined effects of the topochemical mechanism and steric restrictions. Computational Modeling on SQ Photodimerization. To further verify the photocycloaddition mechanisms as proposed above, we carried out the DFT and TDDFT calculations on ground and excited states of both the reactants and products. For the simplicity of computations, the calculations were only performed on the SQ rings and did not include the alkyl chains within the derivatives. The role of solid substrates was also neglected. During our computational modeling, both the trans- and cis-isomers of SQ molecules were considered. For each isomer, several possible conformations for photocycloadduct dimers and their corresponding π-stacking complexes were compared. It was discovered that the trans-SQ monomer is about 0.2 eV more stable than the cis-SQ monomer. Other properties, such as the energy gaps between the ground and excited states, oscillation strengths, and major geometrical changes during the photocycloadditions were all qualitatively similar between the trans- and cis- isomers. Thus, only the computational results about the trans-isomers will be discussed in detail. The energy diagrams of the SQ monomer, π-stacking complexes, and photocycloadduct dimers are shown in Figure S4. It is seen that the π-stacking complexes can be around 1 eV more stable than monomers, which well explains the formation of strong aggregated structures that were observed in the AFM images. On the basis of the calculated energy gaps and oscillation strengths, the UV−vis absorption spectra for the SQ monomer, π-stacking complex, and cycloadduct dimer with the most typical conformations were also simulated and displayed in Figure 7. It can be observed that both the monomer and πstacking complex have absorption maxima around 400 nm, while the peak absorption is blue-shifted and the cross section is significantly reduced for the cycloadduct dimer. These results fully support our previous assumptions that the resonant enhancement of SHG signals at 395 nm were significantly reduced when [2 + 2] photocycloaddition occurred. Moreover, the computational spectra as shown in Figure 6 are in good agreement with previously reported experimental UV−vis absorption measurements for multilayer LB films of SQC16 and SQC18 molecules before and after the UV radiation at 365 nm,32,37 showing that our DFT calculations are reliable.

Figure 7. Simulated UV−vis spectra of SQ monmer, π-stacking complex, and photocycloadduct dimers.

Last but not least, the theoretical calculations also provide us further evidence for the competitive roles of the topochemical mechanism and steric effect played in the SQC18 photodimerization kinetics. As revealed by the optimized molecular geometries in Figure 8, the SQ rings are almost parallel with

Figure 8. Molecular structures of SQ π-stacking complex and photocycloadduct dimer with the typical comformations.

each other within π-stacking complexes, which are the favorable orientations according to the topochemical postulate.50−52 The large stabilization energies of these π-stacking complexes also suggest strong π−π interactions and short intermolecular distances between the SQ rings before dimerizations. The redistribution of valence electrons during the photoexcitation step is then likely not to be localized on the individual SQC18 monomer but occur within the range of whole π-stacking complexes. On the other hand, a detailed analysis of the changes of molecular geometries also revealed the origin of steric hindrances that SQC18 experienced during the photodimerization reactions. When the [2 + 2] cycloaddition occurs, the reactive double bonds break and new σ bonds form to yield the cyclobutane structures. As a result, the originally sp2 hybridized carbon atoms become sp3 hybridized, which leads to an obvious bending angle between the quinoxaline and benzene rings although these two rings initially were almost in the same plane (Figure 8). Because of the bent molecular structures of the cycloadduct dimer, more space is needed to accommodate the photocydimerization products. At high packing densities such as the one with MMA ∼19 Å2, the intermolecular space is extremely limited. Therefore, some of molecules have to be squeezed out of the monolayer so that the photocycloaddition can proceed. The action of squeezing the molecules out of the monolayer costs extra energy, which 23547

DOI: 10.1021/acs.jpcc.7b08409 J. Phys. Chem. C 2017, 121, 23541−23550

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The Journal of Physical Chemistry C ORCID

would result in the slower kinetics of [2 + 2] photocycloaddition for SQC18 at MMA ∼19 Å2.

Yu Xie: 0000-0001-8925-6958 Lu Lin: 0000-0003-1308-9158 Minghua Liu: 0000-0002-6603-1251 Zhou Lu: 0000-0001-8527-0381

IV. CONCLUSIONS In this work, the [2 + 2] photocycloaddition kinetics of SQC18 molecules in the LB monolayer was systematically investigated by the real-time SHG technique. It was discovered that the photocycloaddition was induced by a two-photon process. The 790 nm femtosecond laser beam that was used as the fundamental pulse in the SHG measurement also triggered the photochemical reaction. The laser power dependence experiments further revealed that SQC18 photocycloaddition is a first order reaction, showing that the photoexcitation is the rate-limiting step. Moreover, the interfacial photocycloaddition rate constants of SQC18 molecules were found to first increase then decrease with the increasing surface packing densities. With the assistance of AFM measurements and theoretical calculations, we infer that the topochemical mechanism dominates at lower surface packing densities in a way that the photodimerization is favored with the shorter distances and more parallel orientations between the two reactive SQ rings. On the other hand, at the significantly higher surface packing densities, the steric hindrance became too large and the photodimerization slowed down again. Previously, the kinetics of the photochemical reactions within the bulk materials (including multilayer films) can be monitored by various conventional linear spectroscopic methods45−49,52,53 while the monolayer at the air/liquid interface can be recorded by the surface pressure, surface potential, and viscosity measurements.43,44,54 However, these methods are not applicable for the LB monolayers. In addition, the surface plasmon resonance method only works for a limited number of metal surfaces and is not suitable for the dielectric surfaces.55 The current work shows that the second order nonlinear optical tool such as SHG can be very successful in detecting the kinetics of monolayer-thin materials supported on any kind of solid substrates. Additionally, the results obtained in this article indicate that the LB monolayer has a rather complicated nature with both characteristics of the solid state materials and the air/aqueous interface. Yet this work provides an excellent example that the kinetics of the interfacial photochemical reaction of a LB monolayer is possible to be purposely regulated due to the easiness of adjusting the surface packing densities and aggregation structures, which is one of the advantages of the Langmuir−Blodgett method as a promising technique to fabricate future optoelectronic and photosensing devices.

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21473217, 21227802, 21503248, 21673266, and 21303216), the Chinese Ministry of Science and Technology (Grant No. 2013CB834504), and the Natural Science Foundation of Shandong Province for Distinguished Young Scholars (Grant JQ201504). The authors also thank the Supercomputing Centre, Computer Network Information Center, CAS, and the Super Computational Centre of CAS-QIBEBT for providing computational resources.

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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.jpcc.7b08409. Additional evidence of laser-induced decay of SHG signals, detailed calculation results including the optimized molecular geometries, the energy diagrams, and simulated UV−vis spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 23548

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