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Photo-Switchable and Wavelength Selective Axial Ligation of Thiol Appended Molecules to Zinc Tetraphenylporphyrin: Spectral and Charge Transfer Kinetics Studies Samrat Devaramani, Xiaofang Ma, Shouting Zhang, Mahgoub Ibrahim Shinger, Dong-Dong Qin, Duoliang Shan, and Xiao-Quan Lu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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The Journal of Physical Chemistry

Photo-Switchable and Wavelength Selective Axial Ligation of Thiol Appended Molecules to Zinc Tetraphenylporphyrin: Spectral and Charge Transfer Kinetics Studies Samrat Devaramani[a], Xiaofang Ma[a], Shouting Zhang[ab], Mahgoub Ibrahim Shinger[a], Dongdong Qin[a], Duoliang Shan[a], Xiaoquan Lu*[ab]

a

Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College

of Chemistry & Chemical Engineering Northwest Normal University, Lanzhou, 730070, P. R. China. E-mail: [email protected], [email protected] Phone: +86 13993146276. b

Department of Chemistry, Tianjin University, Tianjin, 300072, P. R. China

1 ACS Paragon Plus Environment

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ABSTRACT: Axial ligation property of metalloporphyrins has got vital importance in the architecture of donor-acceptor dyads, 3D cages and modular assemblies made up of porphyrins. Nitrogenous ligands have been extensively used in this regard. In this paper, we made a unique attempt to axially ligate thiol appended molecule to tetraphenylporphyrinato zinc (II), (ZnTPP). Taking the advantage of photoexcitation of metalloporphyrins, axial ligation of 1-Dodecanethiol (DDT) to ZnTPP in presence of visible light was invented. This has been ascertained by recording the charge transfer between ZnTPP and DDT. Photoinduced axial ligation of DDT to ZnTPP under visible light was studied by varying the wavelength and intensity of the light source. The axial ligation reaction was found to be photoswitching and wavelength selective. The binding constant of ZnTPP-DDT complex was found to be comparable with that of ligands bind to ZnTPP though nitrogen. The consequence of the axial ligation on the HOMO and LUMO and the photoinduced charge transfer kinetics of the planar chromophore, ZnTPP were studied. Scanning electrochemical microscope was used to extract the heterogeneous charge transfer constant values in the case of ZnTPP and ZnTPP-DDT at the film/electrolyte solution.


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INTRODUCTION Multifunctional properties exhibited by the porphyrin derivatives and their metallated forms excited the scientific community to investigate them in their natural and artificial systems. Their properties can be modulated by functionalizing the meso or β positions of the aromatic ring and by placing the suitable metal ion at the tetradentate core.1 An axial ligation phenomenon of metalloporphyrins has been used in multifaceted applications. Axial ligation of porphyrins was exploited to construct the 3D multiporphyrinic cages.2 Catalytic and enzymatic applications of porphyrins can be achieved through the photoinduced ligation process.3,4 Numbers of donoracceptor dyads, rotors comprised of porphyrins have been constructed based on the axial ligation.5-7 In most of the cases, ligands bearing nitrogen as a donor atom have been utilized for this purpose so far.2,8 9 Though there are no serious drawbacks related to the axial ligation of nitrogenous ligands, search for new possible ligands as an alternative is a natural research interest. There is an another class of thiol appended molecules such as alkanes, oligophenylenes (OPs) oligo(phenyleneethynylenes) (OPEs), and oligo(phenylenevinylenes) (OPVs) have been thoroughly investigated in molecular electronics by forming the monolayers.10 Based on the nature of these molecules placed between the donor and acceptor, tunneling and hopping mechanism prevails. But these molecules have not been tried as axial ligands in combination with porphyrins in the above mentioned applications.

Perhaps due to less reactivity with

metalloporphyrins and binding constant issues compared to their nitrogenous counterparts. Iron and nickel porphyrins have been extensively investigated for their photoexcited ligation/deligation dynamics.11,12 We believe, zinc porphyrin is a simpler system among the other metalloporphyrins for the following reasons. There is no ambiguity about the oxidation state, 2+,



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of the metal; it prefers to form five coordinated complex by accepting only one axial ligand. And the d orbitals of the Zn2+ are completely filled; hence, no participation of empty d orbital in axial ligation. Hence, tetraphenylporphyrinato zinc (II) (ZnTPP) was chosen as the representative metalloporphyrin. Whereas 1-dodecanethiol (DDT) was used as the ligand represents the thiol appended molecules. In this paper, we propose an innovative, simple yet effective method to react thiol appended molecules as axial ligands to metalloporphyrins. The reaction between thiol ligands and ZnTPP do not take place under ambient conditions. But just by using light as a driving force, thiol appended molecule can be successfully axially ligated to the ZnTPP. And this photo-driven reaction is very fast, axial ligation was observed to be complete within 45 minutes. Photoinduced axial ligation was confirmed by spectral studies. Various parameters which can influence the photoinduced reaction such as the wavelength of light source, solvent, the time required for the completion of reaction were studied in detail. Important characteristics of the ZnTPP-DDT, ZnTPP-TPhol, ZnTPP-TPh complexes such as stoichiometry and binding constant were investigated by constructing the Hill’s plot. Consequences of the axial ligation were investigated after detailed characterization of the photoinduced axial ligation, fundamental properties and various parameters which influence. Porphyrin molecules are well known for their light harvesting property and their chemistry at the solid-liquid interface is of prime importance. Also, axial ligands with different nature have been used as linker molecules between donor and acceptor systems.5-7 Depending on the nature of the axial ligand hopping and tunneling mechanism prevails. Understanding the effect of the axial ligand on the charge transfer is very important. Hence, going one step further, the consequence of axial ligation on the photoinduced charge transfer kinetics at the solid-liquid interface was


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systematically studied using scanning electrochemical microscope (SECM). Effect of binding of DDT to ZnTPP on the HOMO and LUMO energy levels of the latter was understood through cyclic voltammetric experiments. SECM is a versatile tool to study the charge transfer kinetics across the different kinds of interfaces, surface films etc.13-15 Working principle of the SECM and the theoretical equations used in such studies has been thoroughly discussed elsewhere.16,17 In this study SECM was employed to compare the heterogeneous electron transfer kinetics of the ZnTPP and ZnTPPDDT coated electrodes (substrates) via feedback mode. Our objective was to study the influence of DDT molecules present across the film/electrolyte solution interface on the extent of photoinduced charge transfer. Detailed studies were reported on the charge transfer kinetics of the partially blocked conducting substrates.18 Finite heterogeneous kinetics at such substrates can be determined with the help of theoretical simulations. Encouraged by such reports, we undertook the investigation of a conducting substrate believed to be partially blocked due to the presence of insulating molecules i.e DDTs. EXPERIMENTAL SECTION

Reagents. Potassium ferricyanide and potassium chloride were purchased from SigmaAldrich.

1-Dodecanethiol,

1-Butyl-3-methylimidazolium

hexafluorophosphate,

K2HPO4,

KH2PO4, CH2Cl2were purchased from Aladdin Chemistry Co. and Fuchen Chemicals (TianJin, China). All the chemicals used are analytical grade. ZnTPP was synthesized and recrystallized in our lab by following the reported procedure.19

Measurements. Electronic spectra were recorded using a UV-1102 spectrophotometer (Shanghai, China).

Electrochemical and photoelectrochemical experiments were performed

using a CHI900 electrochemical workstation (CH Instruments, Austin, TX). Three electrode cell



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was used: platinum wire as the counter electrode, Ag/AgCl in 3M KCl as the reference electrode and FTO based photoanodes as a working electrode (substrate). Teflon electrochemical cell with a 0.5 cm opening at the bottom with an O-ring was used. An electrochemical cell was mounted after correcting tilt if present. A 150 W Xenon lamp was used as a light source and the radiation was illuminated to the bottom of the substrate. The SECM approach curves were recorded using 25 µm diameter Pt tip. This tip was polished on the emery sheets to bring down its RG value to about 4 and this was confirmed using a microscope. Before using this tip to record probe approach curve, it was polished with 0.05 µm alumina slurry on the polishing pad. Then cyclic voltammogram was recorded to observe the sigmoidal shaped voltammogram, which confirms the surface of the tip is clean (Figure S1, Supporting Information). Photoinduced axial ligation experiments were carried out in the 5 ml glass vials with the rubber septum. Reactants taken in the vial were degassed by purging pure nitrogen for about 10 minutes. Then the vial was exposed to the light for the desired time. The light source was placed 10 cm away from the vial. Unless otherwise mentioned highly focused triple LED array (Autolab Optical bench) attached to the potentiostat (AUT83071) was used as the light source to carry out the axial ligation. 0.3 V was applied throughout the experiment to get the light source output. The inert atmosphere inside the vial was not maintained after the completion of the reaction. Samples were closed tightly with cap and stored under dark. Photocurrent measurements were carried out in 0.1 M phosphate buffer of pH 6.8 without applying the bias potential to the substrate.

Results and Discussion There exist a good number of classical investigations which shed light on the dependence of the electronic spectra of metalloporphyrins on the nature of their axial ligands.20,21 Detailed studies


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have been reported on the characteristic changes in the electronic spectra of metalloporphyrins relating to the solvents, axial ligands, and type of atom which is participating in the ligation.22-24 Generally, axial ligation between the metalloporphyrin and ligand will happen through the charge transfer between the two. Hence, electronic spectroscopy will give a perfect evidence of axial ligation of metalloporphyrin.

Photoinduced axial ligation study of 1-dodecanethiol. As shown in Figure 1 following changes were observed in the UV-vis absorption spectrum of ZnTPP after reacting it with DDT in the presence of cyan LED light. i) The red shift of the Soret band. ii) Two new peaks appeared. One peak located at the high energy side of the Soret band, 321 nm. Another peak appeared at the longer wavelength, 651 nm. And iii) Change in the relative intensities of Q1 and Q2 bands. Above mentioned changes i and iii are generally noticed even in the case of ligand bearing N or O as donor atom.9, 25 But when S is the donor atom in the form of thiol, only then can change ii be expected in addition to i and iii.26 Possible reason for the changes i and iii is the transfer of negative charges to the porphyrin ring through the zinc ion from S and its polarizable nature.26 The appearance of the peak at higher energy side is believed to be the charge transfer from sulfur P to porphyrin eg (π*).27 Control experiments were carried out by keeping the reaction mixtures in the dark at room temperature. Even after 72 h, ZnTPP remains unreacted with DDT. This was confirmed by recording the absorption spectra (Figure S2, Supporting Information). All the above observations undoubtedly proved the successful axial ligation of DDT to ZnTPP only in the presence of light.



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Figure 1. UV-vis absorption spectral changes observed after the addition of DDT to ZnTPP solution and exposed to cyan LED for 30 minutes under inert atmosphere. Since the spectral properties of ZnTPP are strongly dependent on the solvent, its light induced reaction with DDT was studied in various solvents such as dimethyl formamide (DMF), dimethylsulfoxide (DMSO), ethyl alcohol, trichloromethane. Axial ligation was found to be facile in di- and trichloromethane only. Less solubility of ZnTPP in DMSO and ethyl alcohol may be the reason. The nitrogen present in DMF may compete with the sulfur of DDT lead to the less facile reaction. Hence, in all further studies, photoinduced ligation was carried out in dichloromethane.


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Light is the sole driving force of the reaction between ZnTPP and DDT and ZnTPP strongly absorbs in the Soret region (about 420 nm) and weekly in the Q region (about 550 nm). This motivated us to examine the feasibility of axial ligation reaction under the varied wavelength light source. Hence, six different wavelengths (such as 420, 505, 530, 590, 610 and 627 nm) LEDs were tested to explore the effect on the reaction. Based on the intensity of peak appeared at the 442 nm, following conclusions were drawn. In the case of LEDs with 420, 530 and 590 nm wavelength, the reaction occurred but to less extent as compared to that of the cyan LED. The axial interaction was not at all observed when 610 and 627 nm LEDs were used as a light source. Among all the LEDs tested, cyan LED with 505 nm was found to be more effective to carry out the axial ligation, which resulted in the highest intense peak at 442 nm. This was not unexpected, because purple colored ZnTPP solution should strongly absorb its complementary colored, green (cyan), LED light and hence the facile reaction. Hence, it can be concluded that feasibility of the photoinduced axial ligation reaction is in accordance with the extent to which light can be absorbed by the ZnTPP. The active role of the ligand in photoinduced ligation process based on different wavelengths can be ruled out as DDT is UV-vis inactive. Though Zn2+ in the ZnTPP is expected to form a five coordinated complex, a number of DDT molecules bind to ZnTPP and the binding constant were determined by the following studies. 3.4 µM ZnTPP in dichloromethane was titrated against a varied concentration of DDT and the absorption spectrum of each sample was recorded. The Hill plot28 was constructed from the titration curves by considering the change in the absorbance value at 418 nm. A straight line with coefficient value, 1.3, near to one was obtained. That indicates the 1:1 stoichiometry between ZnTPP and DDT (Figure 2). An intercept of the straight line, 8.208 gave the logk value. From that, the binding constant was found to be 1.61 x 108 M-1. This was found to be comparable 9 ACS Paragon Plus Environment


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with that of ligands bind to ZnTPP though nitrogen.5,9,26,29 A non-sigmoidal curve was obtained from the plot of the change in the absorption intensity at 418 versus the various DDT concentrations added for the titration (Figure 3). This ascertains the non-cooperative binding between the ZnTPP and DDT. The photo-switchability of the axial ligation reaction between DDT and ZnTPP was tested using cyan LED as the light source. UV-vis absorption spectra were recorded for the reaction mixture before exposing it to the light source and at different intervals of time by keeping the LED on and off. Results presented in Figure 4 indicate that the proposed photoinduced axial ligation can be stopped by turning the LED off and can be steadily restarted by turning the LED on. Hence, the proposed photo-driven method for reacting DDT with ZnTPP is photo-switchable.


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Figure 2. The Hill plot fitting of ZnTPP titrated against DDT, change in the absorption intensities were measured at 418 nm.

Figure 3. The plot of [DDT] added to 3.4 µM ZnTPP versus change in the absorption intensity measured at 418 nm.



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Figure 4. The plot of UV-vis absorption values measured for DDT – ZnTPP mixture at 418 nm during the ON/OFF of cyan LED vs time. Experiments were conducted to understand the relationship between the concentration of the reaction mixture and the exposure time of the reaction mixture to the LED light. Three different sets of an equimolar mixture of ZnTPP and DDT of 5.5, 4.0 and 2.5 µM were taken. One sample from each set was exposed to the cyan LED for a period of particular time and then its absorption spectrum was recorded. Peak intensity at 418 nm was measured for the samples exposed to light for a different duration of time. Then the measured peak intensity was plotted against the exposure time as shown in Figure 5. From the plot it is clear that samples of all the three sets took almost same time, 45 min to complete the photoinduced axial ligation of


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the mentioned concentration. Another important thing to notice is that there is linearity between the exposure time and axial ligation from the initial stage to the completion of the reaction. The intensity of the light source used is the other important parameter which may influence the completion of photoinduced axial ligation reaction with respect to exposed time. This has been systematically studied by exposing the three samples, an equimolar mixture of ZnTPP and DDT, of same concentration i.e 5.5 µM to the Xe lamp set at different intensity, 30, 60 and 100 %. Obtained results are systematically presented as 3D bars as shown in Figure 6. Even though the concentration of reactants is equal in all the three cases, a significant difference in the absorption measured at 418 nm can be seen due to the different intensity of light source. Though the axial ligation reaction happened in the presence of light with 30 % intensity, reaction was not complete even after 45 minutes. But the progress of the reaction was continuous in all the three cases. Since the experiment was not conducted beyond 45 minutes, the fate of the reaction after that time period is not clear.



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Figure 5. The plot of exposure time vs. absorbance measured at the 418 nm for the equimolar reaction mixtures of ZnTPP and DDT of three different sets of different concentration.

Figure 6. Effect of intensity on the completion of photoinduced axial ligation reaction between the three sets of an equimolar mixture of ZnTPP and DDT of 5.5 µM concentration. HOMO-LUMO. Energy levels, HOMO and LUMO play a key role in the photoinduced charge transfer of the ZnTPP and ZnTPP-DDT. Also, it was expected that axial ligation of the DDT will alter the energy levels of the ZnTPP.

Generally, electrochemical oxidation and

reduction potential onset values will be used to calculate HOMO and LUMO of the molecule. Cyclic voltammograms for both molecules were recorded in dichloromethane using 1-butyl-3methylimidazolium hexafluorophosphate as an electrolyte. In the case of ZnTPP, two set of quasi


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reversible one electron reduction processes were observed at -1.53 and -1.88 V due to the monoand dianion formation respectively. Two set of peaks due to reduction processes were observed even in the case of ZnTPPDDT at -1.14 and -1.58 V (Figure S3, Supporting Information). This positive shift in reduction peak potentials can be ascertained to the charge transfer between ZnTPP and DDT. The shift in the formal potentials is directly proportional to the magnitude of charge transfer between the two.30 A similar positive shift in the reduction peak potentials was also observed on axial ligation of nitrogen and sulfur donating atoms different metalloporphyrins.31,32 It is evident that strong charge transfer exists between them, an observation that parallels the results of absorption spectral studies. Due to charge transfer from the ligand to porphyrin ring via zinc resulted in the two new bands at wavelengths 321 and 651 nm. With the help of reduction potential onset values (Eonsetred), LUMO of the molecules was calculated. Then energy gap (Eg) between the HOMO and LUMO of the molecule was found out with the help of wavelength onset (λonset) value of each molecule (Table S1, Supporting Information). HOMO energy levels were back-calculated using the experimentally obtained LUMO and Eg values. Obtained results and the data used for the calculation are summarized in Table 1. It can be inferred that energy levels of HOMO and LUMO of the ZnTPP become slightly deeper after axial ligation (Figure 7).



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Figure 7. Cartoon represents the energy level diagram of ZnTPP and ZnTPP-DDT. Data derived from the electrochemical experiments and absorption spectra. Also, depicts the direction of charge carriers in which they move during the photoinduced charge transfer in FTO-TiO2-ZnTPP and FTO-TiO2-ZnTPP-DDT. Table 1. Reduction potential onset values, HOMO and LUMO energy levels calculated for ZnTPP and ZnTPP-DDT Eredonset (V)

ELUMO (eV)

Eg (eV)

EHOMO (eV)

ZnTPP

-1.47

-3.33

2.03

-5.36

ZnTPP-DDT

-1.08

-3.72

1.76

-5.48


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Photoinduced charge transfer at a solid-liquid interface. SECM was used to study the consequences of the presence of axially ligated DDT on the photoinduced charge transfer kinetics of the ZnTPP at the solid-liquid interface. It is important to understand the photocurrent response of the material prior to the recording of probe approach curves. Hence, to gather the photoresponse of the ZnTPP-DDT in comparison with that of ZnTPP, photocurrent measurements were done by recording the I-t curves. Observed photocurrent for both materials was in the magnitude of 10-8 A. It was difficult to achieve the satisfactory SECM results when the same materials were used as such. This can be suspected, primarily, due to the lesser magnitude of the photocurrents exhibited by the ZnTPP and ZnTPP-DDT. Secondly, DDTs are bad conductors which may be acting as an obstacle for the charge transfer between the microelectrode probe of the SECM and the ZnTPP-DDT substrate. This problem was overcome as explained below. It is well demonstrated in the literature that, stabilization of the photoinduced charge separated states will result in long lived radical ion-pair species.25 This will further lead to the more facile charge transfer across the interface by outperforming the recombination of the photogenerated charges. Encouraged by the seminal work on the combination of TiO2 and metalloporphyrins as an efficient light harvesting photoanodes,33 TiO2 was introduced into our system. Each material was coated onto the FTO-TiO2 electrode to improve the photocurrent response. Again, photocurrent measurements were performed systematically to observe the change. As shown in Figure S4 (Supporting Information) improved photocurrent was observed for ZnTPP and ZnTPP-DDT coated on the FTO-TiO2 compared to pristine ZnTPP, ZnTPP-DDT, and TiO2. This can be attributed to the following well established mechanism. Photoexcitaion of ZnTPP due to illumination of visible light leads to the generation of charge carriers, electrons and holes in the LUMO and HOMO respectively. Excited electrons



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of ZnTPP injected into the conduction band of the TiO2 surface. Then these electrons will enter the FTO to reach the counter electrode through the external circuit. Holes left behind in the ZnTPP will cause the oxidation reaction thereby result the improved anodic current.

SECM study. A family of approach curves, SECM tip current-distance relationships, was recorded for ZnTPP and ZnTPP-DDT coated substrates. All approach curves were recorded under the same conditions explained below. 1 mM ferricyanide solution as a redox mediator was used with 0.1 M KCl as a supporting electrolyte. The tip was poised at a sufficiently negative potential vs. Ag/AgCl to cause the reduction of the mediator [Fe(CN)6]3- to [Fe(CN)6]4-. To make the opposite reaction happen i.e oxidation of [Fe(CN)6]4- to [Fe(CN)6]3- at the substrate, the required positive over potential was not applied to the substrate. Instead of potential, the light was illuminated to the substrate from the bottom. Thereby charge carriers were generated in the material (ZnTPP or ZnTPP-DDT) present on the substrate. Therefore the oxidation of the reduced mediator was expected to happen at the photoexcited material on the substrate under light illumination. Hence, positive and negative feedback approach curves were anticipated in the presence and absence of light respectively. Approach curves recorded for ZnTPP and ZnTPP-DDT are shown in Figure 8. It can be seen that negative feedback resulted in the dark and nature of the approach curve was transitioned from negative to positive after illuminating light for both the materials. This indicates the formation of diffusion cone of the mediator between the SECM tip and substrate. The shape of the curves more or less remained same for both the substrates. But the tip distance (L) at which current started rising was altered. In the case of ZnTPP-DDT substrate, shorter distance (L≤0.3) was required to result in the rise of current. Whereas, ZnTPP substrate exhibit the same phenomenon at a distance L≤0.45. Seems that the DDT molecules present at 18 ACS Paragon Plus Environment

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film/electrolyte interface are posing an obstacle for the facile diffusion of the mediator. Therefore tip should be present at the closer proximity of the substrate to overcome this and to result in the regeneration of the tip reduced mediator species. Important and expected result is the considerable decrease in the magnitude of the tip current in case of ZnTPP-DDT compared to ZnTPP substrate. A Number of reports appeared on the detailed investigation of the charge transfer kinetics across the electroinactive alkanethiol monolayer. And the blocking property of the alkanethiol molecules is a well established result [18]. Though the DDT molecules are perpendicularly bound to ZnTPP, we are not claiming the formation of a film composed of the monolayer of DDT on the electrode in the case of

ZnTPP-DDT coated substrate. Rather

ZnTPP-DDT molecules are randomly casted on the electrode surface. Ill defined arrangement and the possible scenario of charge transfer between the tip and substrates are shown in Figure 9. Hence, the decrease in heterogeneous charge transfer rate constant, keff, value in case of ZnTPPDDT compared to ZnTPP is obvious. This has been studied more systematically in the next sections. Previous studies undertaken by our group ascertained the effect of wavelength and intensity of the light used on the finite kinetics of the [Fe(CN)6]3-/4-. Hence, importance has not been given on these parameters. Moreover, ZnTPP-DDT was believed to be a case of hindered charge transfer, the maximum intensity of the light was used throughout the experiment. For the same reason cut-off filters were also not used.



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Figure 8. Experimental SECM probe approach curves (dotted lines) at ZnTPP and ZnTPP-DDT substrates recorded in 1 mM [Fe(CN)6]3-with 0.1 M KCl in the presence and absence of light. The tip was held at sufficiently negative potential to reduce the mediator.


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Figure 9. Diagrammatic representation of the photoinduced charge transfer reaction at the interface of FTO-TiO2-ZnTPP, FTO-TiO2-ZnTPP-DDT electrode, and ferricyanide electrolyte solution. SECM microelectrode tip was held at 0.35 V.

Since the ZnTPP and ZnTPP-DDT molecules are present on the electrode in a disordered manner, it is logical to extract the keff value by placing the tip at the different locations. Hence, the average keff value can be obtained and our hypothesis can be strengthened by comparing this average value obtained for the two substrates. Throughout these studies, initial tip-substrate distance was kept same for both the substrates. Firstly, approach curve was recorded by positioning the tip at the initial position on the substrate. Then moved in the x or y direction of the substrate to spot the new location and the approach curve was recorded for that particular spot. Four approach curves to the ZnTPP substrate (Figure 10 (two not shown)) are obtained by following the above procedure. It can be seen that there is no observable change in the shape and 21 ACS Paragon Plus Environment


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magnitude of normalized current. The heterogeneous charge transfer rate constant value was extracted by fitting these experimental curves with the theoretical simulations. This was done using the suitable expressions deduced elsewhere [16]. Extracted keff values for all the four curves ranged between 9.6 and 8.9 10-2cm s-1 without significant change. This indicated the all the four spots studied are equally reactive, which means the distribution of ZnTPP on the electrode is uniform.

Figure 10. Experimental SECM probe approach curves at the ZnTPP substrate in the presence of light fitted to the theoretical ones’. The probe approach curves were recorded at the initial position and spot located 100 µm away from the initial position of the substrate. In a similar way, approach curves were recorded for the ZnTPP-DDT substrate by positioning the tip 5, 10, 100 µm away from the initial position in the x direction (Figure 11). A


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significant decrease in the magnitude of the normalized tip currents was observed. Extracted keff values, in this case, ranged between 5.2 and 1.6 10-2cm s-1. When the tip situated very near to its prior position, change in the keff was also less (5.2 and 3.7 10-2 cm s-1). But the keff value was relatively considerably changed as the tip located far away from the initial position indicating the significant change in the environment of the film/electrolyte solution interface. Change in keff values was observed when the tip was located at different spots by moving 20, 40 µm away from the initial position in the y direction (Figure 12). Tip situated at 20 and 40 µm away from the initial position yielded keff values equal to 3.3 and 2.2 10-2cm s-1 respectively. Extracted keff values in all the above mentioned conditions are summarized in Table 2.



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Figure 11. Experimental SECM probe approach curves at the ZnTPP-DDT substrate in the presence of light fitted to the theoretical ones’. The probe approach curves were recorded at the initial position and the spots located 5, 10 and 100 µm away in the x direction from the initial position of the substrate


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Figure 12. Experimental SECM probe approach curves at the ZnTPP-DDT substrate in the presence of light fitted to the theoretical ones’. The probe approach curves were recorded at the initial position and the spots located 20 and 40 µm away in the y direction from the initial position of the substrate.



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Table 2. Heterogeneous charge transfer rate constant values obtained for ZnTPP and ZnTPPDDT substrates by fitting the experimental approach curves to theoretical ones.

Location of the tip away from the initial position

ZnTPP

Initial

5 µm

10 µm

100 µm

20 µm

40 µm

position

along the

along the

along the

along the

along the

x-axis

x-axis

x-axis

y-axis

y-axis

k

13

12

12.5

12.7

keff / 10-2 cms-1

9.6

8.9

9.3

9.4

ZnTPP-

k

7

5

3

2.2

4.5

3

DDT

keff / 10-2 cms-1

5.2

3.7

2.2

1.6

3.3

2.2

*Diffusion coefficient of the mediator [Fe(CN)6]3- was found to be 9.27x10-6 cm2s-1. And the same was used in the calculation of keff. Average keff value for ZnTPP and ZnTPP-DDT were found to be 9.3 and 3 respectively. Comparison of the average keff values obtained for the ZnTPP and ZnTPP-DDT evidenced the hindrance posed by the DDT molecules for the photoinduced charge transfer. But, none of the studied spots on the ZnTPP-DDT substrate resulted in the complete negative feedback. Overall, it can be acceptable that the presence of DDT molecules at the film/electrolyte solution interface diminished the extent of charge transfer; hence the decrease in keff value was observed. It can also be surmised that this effect is not uniform throughout the substrate as the molecules are not systematically arranged.


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CONCLUSIONS We have explored the photoinduced axial ligation of thiol appended molecule, DDT to ZnTPP in presence of visible light. Binding of thiol appended molecule to zinc using light as a driving force is first of its kind. Axial ligation via charge transfer was confirmed by spectral measurements. Stoichiometric ratio between ZnTPP and DDT was found to be 1:1 with the binding constant comparable to those of nitrogen bearing ligands. Proposed photoreaction is wavelength selective and photo switchable. Also, the intensity of the light source will play an important role on the completion of reaction over a specific time. HOMO and LUMO energy levels of the ZnTPP deepened after the axial ligation and it was deduced with the help of cyclic voltammograms and absorption spectra. Further, SECM was used to study the heterogeneous charge transfer rate constant, keff of ZnTPP and ZnTPP-DDT films coated on the transparent electrode. This was systematically studied by positioning the SECM tip at a different location of the substrates. Experimentally recorded approach curves were well fitted to analytical approximations to extract the keff value. Hindrance for the free diffusion of the mediator species was posed by the DDT molecules and hence the diminishing values of keff were observed in the case of ZnTPP-DDT film.

ASSOCIATED CONTENT Supporting Information Available: Steady state voltammogram recorded for the 1 mM [Fe(CN)6]3- with 0.1 M KCl at a platinum microelectrode (Figure S1), UV-vis absorption spectra of ZnTPP and the mixture of ZnTPP and DDT solutions measured after keeping in the dark for 72 hours (Figure S2), cyclic voltammograms recorded for 0.05 M ZnTPP and ZnTPP-DDT (Figure S3), Eg values of ZnTPP



Page 28 of 34

and ZnTPP-DDT (Table S1), photocurrent response of FTO-TiO2-ZnTPP, FTO-TiO2-ZnTPPDDT, FTO-ZnTPP, FTO-TiO2-ZnTPP-DDT (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Phone: +86 13993146276.

ACKNOWLEDGEMENTS This work was supported by Natural Science Foundation of China (21575115, 21327005), program for China Chang jiang Scholars and Innovative Research Team, Ministry of Education, China (IRT-16R61). The Program of Innovation and Entrepreneurial for Talent, Lan Zhou, Gansu Province, China (2014-RC-39). Thanks to Prof. Yao Meng, The school of science, Northwestern Polytechnical University for useful discussions.


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