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Kinetic Analysis by Laser Flash Photolysis of Porphyrin Molecules’ Orientation Change at the Surface of Silicate Nanosheet Miharu Eguchi,*,† Tetsuya Shimada,‡ Haruo Inoue,‡ and Shinsuke Takagi*,‡ †

National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan

‡

S Supporting Information *

ABSTRACT: In a mixed solvent of water and dimethylformamide (DMF), porphyrin molecules have two types of orientation, tilted and parallel, toward a surface of silicate nanosheet. In the solvent, tilted species have lower energy. The Tn ← T1 absorption of porphyrin molecules adsorbed at the surface of the nanosheet in the mixed solvent was observed at five different temperatures. The decay curve was analyzed with an equation for transient absorption difference, describing the behavior of parallel and tilted adsorbed species in the ground state and excited state to determine the rate constants for the orientation change and the radiationless deactivation. The rate constants of the orientation change increased with the temperature. The activation energy and energy gap between parallel and tilted species were estimated by analyzing the temperature dependence of the rate constants. The energy gap obtained in this kinetic study was consistent with our thermodynamically obtained value previously reported.

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INTRODUCTION Living organisms that obtain energy by photosynthesis have regular assemblies of dyes that utilize sunlight energy effectively. For instance, purple photosynthetic bacteria have a circular chlorophyll assembly that allows selective, effective energy transfer and electron transfer among ring structures with different diameters.1,2 For artificial photosynthetic systems to use sunlight energy effectively, a regularly arranged dye assembly must be built from the bottom up. Many methods for constructing dye assemblies have been developed. The following are examples in which porphyrin molecules are used as dyes: (i) self-assembly using guest−guest interactions such as hydrogen bonding,3−7 (ii) supermolecules using covalent bonds,8−13 and (iii) self-assembly on substrates such as a Au surface or HOPG surface.14−21 In these methods, the interactions between the porphyrin molecules determine the structure of the porphyrin assembly. We have obtained porphyrin molecule assemblies by using layered silicate, fully exfoliated, to form a nanosheet. The silicate nanosheet (also known as a clay nanosheet) has been used extensively as a host material for guest molecules.22−43 In the host−guest hybrid complex, cationic porphyrins are used as guests to exploit electrostatic attractions with the nanosheet. The advantage of the hybrid complex in dye assembly formation is that the electrostatic attraction between the dye and nanosheet becomes much larger than the interaction between dyes that induces self-aggregation and shortens the excitation lifetime. For our system containing tetracationic porphyrins and layered silicates dispersed in water, a distance © XXXX American Chemical Society

that varies within 0.21 nm between average negative charges and the distance between intramolecular cationic sites is required to obtain a nonaggregated monomeric porphyrin arrangement.44−46 Under these conditions, an excitation lifetime that was long enough for a photoreaction was obtained.44,45,47 In fact, energy transfer between porphyrin molecules on the nanosheet was confirmed.44,45,48,49 In addition, we determined the orientation angle between the porphyrin molecule axis and the surface of the layered silicate by dichroic measurements with a waveguide.45,50 In this system, the orientation angle was estimated to be less than 5°; thus, the molecule plane is almost parallel to the surface (parallel species). We changed the orientation angles by tuning the physical environment of the system.45,51 For example, when the hybrid complex was dispersed in an organic solvent such as DMF with 10% (v/v) water, the angle was 68° (tilted species). The ratio of the tilted to parallel species increased with the ratio of DMF to water. For instance, for 60% (v/v) DMF, the ratio of parallel and tilted species was estimated to be 31:69 by separation of the absorbance spectra. Further experiments revealed that the hydrogen bonding strength of the solvent controlled the change in orientation angle in the hybrid dispersion.52 A thermodynamic study revealed that the tilted species are more stable. Received: February 3, 2016 Revised: March 8, 2016

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DOI: 10.1021/acs.jpcc.6b01211 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. Structure of cis-DPyP.

Corp., PF filter, X2+G-10 column). Organic solvents for spectroscopy were obtained from Kanto Chemical Co., Inc. (spectral grade) and were used as received without any further purification. Sample Preparation. Samples for absorption spectra were prepared as follows. Nanosheet−porphyrin hybrid complexes were prepared by mixing the aqueous nanosheet dispersion with the aqueous porphyrin solutions. A less-polar organic solvent that was miscible with water was added to the dispersion containing the hybrid complexes. The porphyrin loading was 10% with respect to the CEC of the nanosheet. The concentration of anionic sites of the nanosheet in the dispersion was calculated to be 2.0 × 10−5 equiv L−1 from the CEC. Equipment. The amounts of water in the nanosheet and porphyrin reagent were determined by thermogravimetric/ differential thermal analysis (DTG-60H, Shimadzu). Their molecular weights were corrected according to the water content. Atomic force microscopy (SPI-4000, Seiko Instruments Inc.) was performed in contact mode. The transient absorptions were observed by laser flash photolysis, which was also utilized for our study on the clarification of the mechanism of the photo-oxygenation reaction of porphyrin molecules via the excited triplet states.53 An Nd:YAG laser (LS2127, LotisTII) and an optical parametric oscillator (COPO2200, SOLAR TII) laser were used for sample excitation. The excitation wavelength was 460 nm, and the laser pulse had a full width at half-maximum of 20 ns. A Xe flash lamp (XF-80, Tokyo Instruments) and a continuous wave Xe lamp (UXL-500D-0, Ushio) were used as reference light sources for absorbance measurements. The unnecessary wavelength component of the reference light was eliminated with optical sharp cut filters and interference filters. A polychromator detector (82-499, Thermo Jarrell-Ash) and a multichannel photodiode detector (IRY-512, Princeton Instruments) were used for the transient absorption spectral measurements. For the time-variation profile of the transient absorption at arbitrary wavelengths, a monochromator (MC-30, Ritsu) and a photomultiplier (Hamamatsu Photonics, R636) were used. Signals from the photomultiplier were recorded with an oscilloscope (TDS680B, Tektronix). The temperature of the quartz cuvette (optical length, 1.0 cm), which was placed between the Xe lamp and the detector, was adjustable from 5 to 55 °C with an accuracy of ±1 °C. The sample in the cuvette was irradiated with a laser pulse perpendicular to the optical path of the reference light following the pulse elongation to 1 cm in the optical path of the uniform intensity spot with a microlens array. The laser intensity at the surface of the cuvette was less than 5 mJ/cm2. Measurements were executed with the sample dispersion bubbled with dry nitrogen. The time profiles of the transient absorption were analyzed by means of a nonlinear least-squares method (Levenberg−Marquardt method) to evaluate the lifetimes. This analysis was performed with IGOR Pro6 (WaveMetrics, Inc.).

checked by thin-layer chromatography (TLC). A synthetic saponite-layered silicate, Sumecton SA, was obtained from Kunimine Industries Co. Ltd. Atomic force microscopy showed that the diameter of the clay nanosheet particles was 20−50 nm. The nanosheet was purified by repeated decanting with water and washing with ethanol. The cation exchange capacity (CEC) was 99.7 mequiv/100 g. Water was deionized just before use with a water purification system (BB-5A, Organo

RESULTS AND DISCUSSION Tn ← T1 Transient Absorption Spectra. Tn ← T1 transient absorption spectra of the porphyrin−nanosheet hybrid complex dispersed in 60/40 (v/v) DMF/water were observed from just after excitation (t = 0) to 800 μs (Figure 3, left). The spectral shape change was observed within 10 μs. Additionally, transient absorption spectra of the complex dispersed in 0/100 (v/v) (black line) and 90/10 (v/v)

We examined the kinetics of the change in the porphyrin orientation by laser flash photolysis. In the hybrid complex dispersed in 60/40 (v/v) DMF/water, parallel (G1 in Figure 1)

Figure 1. Equilibrium between the parallel and tilted adsorbed porphyrin molecules in the ground and excited states.

and tilted (G2 in Figure 1) species in the ground state were in equilibrium. Equilibrium constant K was determined by reproducing the ground state absorption spectra for the hybrid complex in 60/40 (v/v) DMF/water with those in water and 90/10 (v/v) DMF/water as K = kG/kG′ = 2.23 at 40 °C, where kG is the rate constant for the orientation change from parallel to tilted species and kG′ is the rate constant for the backward orientation change. When a 460 nm laser pulse irradiated the sample dispersion, a part of the parallel species (G1) was selectively excited to the excited state (E1). As a result, the equilibrium condition in the ground state was disturbed. The excited state was also not in equilibrium. Following the recovery process between the parallel and tilted species in the ground and excited states, the deactivation from the excited to ground states was examined by transient absorption. Furthermore, the rate constants were determined at five temperatures to estimate the activation energy of the orientation change and the energy gap between the parallel and tilted species.

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EXPERIMENTAL SECTION Chemicals. The dicationic porphyrin cis-bis(N-methylpyridinium-4-yl)diphenylporphyrin (cis-DPyP) was purchased from Mid-Century Chemicals (Figure 2). The purity was

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Figure 3. (Left) Transition spectra of the porphyrin−nanosheet hybrid complex dispersed in 60/40 (v/v) DMF/water at t = 0, 10, 100, 400, and 800 μs. (Right) Transition spectra of the porphyrin−nanosheet hybrid complex dispersed in 0/100 (v/v) (black line) and 90/10 (v/v) DMF/water (gray line) at t = 0.

Figure 4. (Left) Normalized transient absorption spectra of the porphyrin−nanosheet hybrid complex dispersion at t = 0 in water (blue line), 90/10 (v/v) DMF/water (red line), and 60/40 DMF/water (green line). The spectra were normalized to be the maximum and minimum 1 and 0, respectively (Normalized ΔOD), to facilitate a comparison of the spectra. (Right) Absorption spectra of the porphyrin−nanosheet hybrid complex dispersion in water (blue line) and 90/10 (v/v) DMF/water (red line).

constant for the tilted to parallel or parallel to tilted orientation changes in species in the ground states. Time-Course Profile of Transient Absorption. The time-course profile of the transient absorption (ΔOD(t)) of the hybrid complex dispersed in 40/60 (v/v) DMF/water at 40 °C was observed at 430 and 450 nm to analyze the time variation of each of the components G1, G2, E1, and E2 in Figure 1 (Figure 5). At these wavelengths, the transient rise and decay of the tilted and parallel species could be observed, respectively. The time-course profiles obtained were well-fit by three exponential components, as expected theoretically (theoretical details are explained in the following section). Specifically, the three rate constants at 430 nm were determined as 2.7 × 103, 2.0 × 104, and 5.0 × 105 s−1. The smallest rate constant was assigned to nonradiative deactivation of the excited states (E1 and E2) because it is consistent with the experimentally obtained radiationless deactivation rate constants of the parallel and tilted species (3000−4000 s−1, cf. Figure 3). Here, the inset of Figure 5 shows an increase in depletion of the transient absorption at 430 nm and a decrease in depletion at 450 nm in 5 × 10−6 s after the excitation at 460 nm. The depletion increase indicates a decrease in tilted species in the ground state, and the depletion decrease indicates the recovery (increase) of parallel species in the ground state. This suggests that some tilted species change to a parallel orientation in the ground state. The orientation change is thought to be the recovery process from disturbed equilibrium in the ground state

DMF/water (gray line) just after the excitation (t = 0) are also shown in Figure 3 (right). Those spectra decayed monotonically with a rate constant of 3000−4000 s−1, owing to the radiationless deactivation of the excited triplet state. Normalized Tn ← T1 transient absorption spectra of the porphyrin−nanosheet hybrid complex dispersed in water, 60/ 40 (v/v) DMF/water, and 90/10 (v/v) DMF/water observed immediately after excitation (t = 0) are shown in Figure 4 (left) with the ground state absorption spectra (Figure 4, right) for the complex in water (blue line) and in 90/10 (v/v) DMF/ water (red line). The tilted and parallel species showed absorption maxima at 423 and 455 nm, respectively, in the ground state, which corresponds to the dips immediately after excitation. In the transient absorption spectra of the sample in 60/40 (v/v) DMF/water (left figure, green line), the dips were at 423 and 450 nm, respectively, indicating that both species coexisted in the dispersion (indicated by arrows). An apparent spectral change at 10 μs of the Tn ← T1 transient absorption spectra for the complex in 60/40 DMF/ water (Figure 3, left) indicated that tilted- and parallel-adsorbed species did not decay monotonically, in contrast to when the two species were separate (Figure 3, right). The spectral change suggests that the ratio of tilted and parallel species shifted on a time scale of several microseconds to several hundred microseconds. Therefore, decay curves for transient absorption at 430 and 450 nm were analyzed to determine the rate C


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constants were determined from the time-course profile with this equation. Temperature Dependence of Rate Constants and the Derivation of Activation Energy. The transient absorption time courses were analyzed at 10, 20, 30, and 50 °C in the same manner. The rate constants determined at each temperature are summarized in Table 1. Although the rate constants for Table 1. Rate Constants of the Orientation Change at Each Temperature

Figure 5. Time-course profiles of transient absorptions at 430 and 450 nm for the porphyrin−nanosheet hybrid complex dispersion in 40/60 (v/v) DMF/water at 40 °C on a time scale of 5 × 10−5 and 5 × 10−6 s/div (inset). The solid black lines indicate fitting curves obtained by lifetime analysis.

temp (°C)

kG + kG′ (s−1)

kE + kE′ (s−1)

10 20 30 40 50

× × × × ×

× × × × ×

6.4 1.5 2.7 5.0 1.1

4

2.8 1.5 1.6 2.0 2.4

10 105 105 105 106

3

10 104 104 104 104

kd (s−1) 2.5 2.5 2.5 2.7 2.7

× × × × ×

103 103 103 103 103

radiationless deactivation (kd) showed no temperature dependency, kG + kG′ and kE + kE′ increased with the temperature rise. The values of kG and kG′ were determined from kG + kG′ and the equilibrium constant, K, which was obtained from the absorption spectral division. The calculated values of ln(k/T) around kG and kG′ were plotted against 1/T (Eyring plot, Figure 6). The line of best fit was used to derive the activation

through the selective excitation of parallel species. Therefore, the largest rate constant of 5.0 × 106 s−1 was assigned to kG + kG′, which corresponds to the recovery process of the equilibrium between G1 and G2. The remaining rate constant, 2.0 × 104 s−1, was assigned to kE + kE′, where kE is the rate constant for the orientation change from parallel to tilted species in the excited state and kE′ is the rate constant for the backward orientation change, which corresponds to the recovery process of the equilibrium between E1 and E2. Analysis Model and Theoretical Equations. Based on the model in Figure 1, the time-variable differential equations that G1, G2, E1, and E2 satisfy are shown as follows. dG1(t ) = −k GG1(t ) + k G′G2(t ) + kd1E1(t ) dt dG2(t ) = k GG1(t ) − k G′G2(t ) + kd2 E 2(t ) dt

Figure 6. Eyring plot for kG and kG′.

dE1(t ) = −[kE + kd1]E1(t ) + kE′E 2(t ) dt

energy for the orientation change, which was obtained from the slope and intercept according to eq 2. The derived activation energies for the orientation change are summarized in Table 2.

dE 2(t ) = kE E1(t ) − [kE′ + kd2]E 2(t ) dt

Table 2. Activation Energies of the Orientation Change in Ground States at 300 K

Time-variable transient absorption is expressed by the sum of each intermediate (G1, G2, E1, and E2) and its molar extinction coefficient. For the rate constants obtained from the timecourse profile (Figure 5), the following approximations are allowed.

kG kG′

ΔH⧧ (kJ mol−1)

ΔS⧧ (J K−1 mol−1)

ΔG⧧ (kJ mol−1)

44 68

1.8 72

43 46

kd1 + kd2 ≪ kE + kE′

The positive values of the activation enthalpy and entropy indicate the extended distance between cationic sites of porphyrin and anionic sites on the nanosheet (ΔH > 0), and the resulting increase in the degree of freedom of porphyrin molecules on the nanosheet (ΔS > 0) in the transition state. The activation free energy of the forward reaction (parallel to tilted) was determined as 43 kJ mol−1, and that of the backward reaction was determined as 46 kJ mol−1. The energy diagrams for the parallel and tilted species are shown in Figure 7. The activation energy obtained from the Eyring plot is

kd1 ≅ kd2 ≡ kd

Differential equations for each species were solved to derive an equation for the transient absorption difference (ΔOD(t)), which is trinomial. ΔOD(t ) = C1e−(kG+ kG′)t + C2e−(kE+ kE′)t + C3e−kdt

(1)

Here, Cn are constants that include the molar extinction coefficient of each intermediate. Details of the elicitation process are described in the Supporting Information. Rate D


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Figure 7. Energy diagram of the transition between the parallel and tilted adsorbed species in the ground state at 300 K, revealed by static and kinetic studies. Enthalpy diagram (top left), entropy diagram (top right), and free energy diagram (bottom). Energy values in fine print were obtained statically, and the values in boldface were obtained dynamically.

⎛ k ⎞ ΔS ⧧ ⎛k⎞ ΔH ⧧ ln⎜ ⎟ = ln⎜ B ⎟ + − ⎝T ⎠ ⎝h⎠ R RT

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(2)

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

where kB is the Boltzmann constant and h is the Planck constant.

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Notes

The authors declare no competing financial interest.

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CONCLUSIONS In our previous report, we revealed the values of ΔH, ΔS, and ΔG in the reorientation from parallel to tilted species (in 40/60 (v/v) DMF/water) at 300 K) using thermodynamic experiments.52 These values are −26 kJ mol−1, −78 J K−1 mol−1, and −2.5 kJ mol−1, respectively. Because the surface of the hybrid is hydrophobic, we concluded that the parallel species are stabilized on the surface by the ΔS term, because the smaller surface area of the hybrid (compared to that of hybrid with tilted species) results in a decrease in the rigidity of the hydrogen bonds among solvent molecules around the hybrid. On the other hand, the tilted species are stabilized by the ΔH term because of the electrostatic interactions between porphyrin and nanosheet and interactions between the solvent molecules and hybrid surfaces. In the present paper, the activation energy (ΔG⧧ = 46 kJ mol−1 at 300 K) of the orientation change of the porphyrin molecules at the surface of the layered silicate was obtained by measuring the Tn ← T1 transient absorption and determining the orientation change rate constants at different temperatures. It is noteworthy that the energy gap between the parallel and tilted species that was determined in our previous study is consistent with that obtained in the present study (ΔΔH⧧ = 44 − 68 = −24 kJ mol−1, ΔΔS⧧ = 1.8 − 72 = −70 J K−1 mol−1, ΔΔG⧧ = 43 − 46 = −3.0 kJ mol−1).

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AUTHOR INFORMATION

Corresponding Authors

ACKNOWLEDGMENTS This work was partly supported by a PRESTO/JST program, Innovative Use of Light and Materials/Life, and a Grant-in-Aid for Scientific Research on Innovative Areas, All Nippon Artificial Photosynthesis Project for Living Earth (AnApple) (Grant 25107521), from the JSPS.

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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.6b01211. The elicitation process of eq 1 for the transient absorption difference (ΔOD(t)) (PDF) E


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