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A pH-responsive, TiO2-attached sensitizer was prepared based on the adsorption of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) onto TiO2 nanoparticles. This colloidally dispersed TiO2-attached TCPP behaves as a single-phase colloidal sensitizer at pH 1.0–3.3 with quantum yields of singlet oxygen production (ΦΔ) between 0.20 and 0.25, as a heterogeneous particle sensitizer at pH 3.5–6.0 with ΦΔ between 0.25 and 0.50, and as homogeneous free TCPP molecules in alkaline solutions with ΦΔ = 0.53. The changes in ΦΔ are fully consistent with pH dependent adsorption of TCPP on TiO2 surface. Recovery yields of 99.8% for TCPP and 98.8% for TiO2 were obtained from 1.4 mM TiO2-attached TCPP. We attribute its photosensitization ability to retain TCPP solubility on TiO2 surface and hence activity. This novel system shows a potential to bridge the gap between easily recoverable and highly efficient sensitizers.
The use of heterogeneous sensitizers for singlet oxygen (1O2) photooxidation in a solution facilitates product separation and analysis. The first heterogeneous sensitizer was a polymer-attached rose Bengal synthesized by Neckers, Schaap, and their co-workers,1,2 which was followed by a series of immobilized dyes on polymers and metal oxides.3–9 The adsorption of dye molecules on metal oxides has been well characterized, e.g., in the field of dye-sensitized nanocrystalline TiO2 solar cells.10–13 It is known that a homogeneous sensitizer is more efficient than its heterogeneous counterpart,2 and far less affected by limitations due to short lifetime of 1O2, and slow transport of reactants and products.14 Similar awareness existed for the use of catalysts in chemical reactions, in which strategies were developed to facilitate the recovery and recycling of homogeneous catalysts.15–22 Understanding the dependence of 1O2 production on dye adsorption on support materials may bridge the gap between easily recoverable and highly efficient sensitizers, and benefit areas of 1O2-based organic synthesis and water purification. However, such information has been limited.
In present work, 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) adsorbed on TiO2 surface was examined for its ability of 1O2 photosensitization. The selection of TiO2-attached TCPP was based on its pH-responsive stability and photoactivity, as indicated in the following. (1) The substantial interaction between TiO2 and deprotonated carboxyl groups in TCPP has been evidenced by femtosecond electron injection,23 variation in TCPP fluorescence intensity with pH in a colloid TiO2 solution,24 X-ray photoelectron spectroscopy, resonance Raman spectroscopy and the red-shift in TCPP Q-band.25 Due to the strong anchoring of TCPP on TiO2 surface, the electrostatic attraction between - COO− groups in TCPP and positively charged TiO2 nanoparticles prevails at pH < 5. In alkaline solution where the surface charge of TiO2 is negative, the adsorption of anionic TCPP on TiO2 surface is inhibited. (2) The solubility of TCPP in acidic solution is very low but can be greatly enhanced by adsorbing on colloidal TiO2 surface. Due to pH-responsive stability and irreversible formation of TiO2-attached TCPP, the recovery of both TCPP and TiO2 can be simply achieved by centrifugation at certain pHs. (3) Similar to other porphyrin sensitizers, TCPP possesses the ability to produce 1O2 in organic solvents (toluene, methanol, acetone),26 in neutral water (from TCPP-cyclodextrin supramolecular complexes),27 and in pH 13 aqueous solution.28 Therefore, in present work we characterize in detail the preparation and recovery of TiO2-attached TCPP sensitizer, and examine their photosensitization ability in a wide pH range.
The stability of colloidal TiO2 nanoparticles is pH dependent and can be measured by zeta potential. A higher zeta potential favors the formation of a stable suspension resulted from electrostatic repulsion among nanoparticles. It is well recognized that the surface of TiO2 in an aqueous environment takes electric charges that are dependent on pH. The positive charge occurred at acidic conditions (pH < 5) due to the presence of TiOH2+ groups where the zeta potentials were sufficiently large to form stable colloids, near neutral charge at zero zeta potential around pH 5–7,29,30 and negative charge at basic conditions (pH > 7) owing to TiO− groups31 where TiO2 precipitation from the solution was observed. Carboxylic acid functionality was the most commonly used anchoring group for dye-sensitized solar cells. These carboxylic acid groups, while ensuring efficient adsorption of dyes on TiO2 surface also promoted electronic coupling between the donor levels of the excited chromophore and the acceptor levels of the TiO2 semiconductor. Some of the possible modes of chelation/derivatization ranged from chemical bonding (chelating or bridging mode) to H-bonding.12 Our results showed that TiO2-attached TCPP colloids were extremely stable in a pH range of 1.0–3.3 (d in scheme 1) but aggregate at pH > 3.5 (e in scheme 1). The desorption of TCPP from TiO2 surface was largely enhanced in basic solutions due to fair solubility of TCPP and electrostatic repulsion between negatively charged TiO2 and TCPP anions (f in scheme 1).
The quantitative adsorption of TCPP onto TiO2 surface was examined by two experiments. First, an experiment was carried out in order to determine whether a decrease in pH in colloidal solution coincides with the adsorption of TCPP onto TiO2 surface. A concurrent decrease in pH of the aqueous solution was found, which related to the deprotonation of carboxylic acid groups in TCPP upon its binding on TiO2 surface. The adsorption of TC over 7 days corresponded to a pH reduction from 2.06 to 1.99. Due to the similar sizes of TiO2 nanoparticles (1 nm) and TCPP molecules (1.5 nm), it was reasonable to assume that a TCPP molecule could bind vertically on the surface of one TiO2 nanoparticle through one of the four carboxylic acid anchoring groups. The molar ratio of [H+]dissociated/TiO2 particles was found to be increased from initial zero to 1.1, which revealed that an average of ca. one TCPP molecule per colloidal TiO2 nanoparticle. Second, the extinction coefficient of TCPP molecules chemisorbed on colloidal TiO2 surface in pH 2.3 aqueous solution, ε = 6.8×104 M−1 cm−1 at 420 nm was determined from the slope of a calibration curve relating TCPP concentration to absorption (figure 4). The amount of TCPP chemisorbed on TiO2 surface was measured at 420 nm according to Lambert-Beer’s law. Typically an average of 0.7 TCPP molecules loaded on per TiO2 particle was obtained. The loading amounts of TCPP on TiO2 surface from both experiments were comparable with each other. A little bit higher value (compare 1.1 to 0.7) obtained based on pH measurements for the ratio of TCPP molecules over TiO2 particles might result from partial presence of two carboxylic acid anchor groups on one TiO2 particle and/or the inconsequential decomposition of TCPP molecules during preparation.
As shown in figure 1, the absorption spectrum of TCPP in pH 2.3 HCl-NaOH solution was characterized by a quite broad Soret band between 405 and 436 nm, and Q band at 524 nm, 560 nm, 597 nm and 650 nm, respectively. The protonation of the nitrogen atoms of the central macrocycle of porphyrins has been well studied. The longer wavelength Q band becomes the most intense upon protonation with the band diminishing in intensity in the order of decreasing wavelength.32–34 The longest-wavelength band is the typical for dictations and metal compexes of porphyrins Q band, which is reflected in the spectra of colloidal TiO2-attached TCPP (black line in figure 1). TCPP was sparingly soluble in acidic solution (blue line in figure 1). Its solubility however could be improved at a low limit of ca. 6×10−6 M in the presence of sodium salt (red line in figure 1). The lowest intensity Q band at the longest wavelength of 650 nm for TCPP in pH 2.3 HCl-NaOH solution indicates its deprotonated spectral feature, which is probably due to the competition of sodium cations with protons for the central nitrogen atoms. In neutral and alkaline solutions, the presence of TiO2 had hardly any effect on Q band (table 1). While in acidic solution, a small but reproducible red shift at 650 nm of the Q band (650 + 8 = 658 nm) was observed. One possible mechanism for this shift was a charge transfer band. Coupling between the wave functions of excited electronic state of TCPP and the charge separated state could give mixed states.35,36 Such strong interaction was accomplished through a chemical bonding between TCPP central macrocycle and TiO2. The adsorption was also evident by the pronounced effect of pH on ΦΔ (see discussion below.). A sharp change in ΦΔ was observed when pH closed to the point of zero charge (pH ~ 5.0 for TiO2),37 where the transition from free to surface-bound TCPP occurred. These spectral features clearly indicated the adsorption of TCPP onto TiO2 surface at acidic pHs and were consistent with the literature reports using various techniques including femtosecond electron injection,23 fluorescence,24 X-ray photoelectron spectroscopy, resonance Raman spectroscopy and UV/Vis.25 The change in driving force for the electron transfer might also result in an inefficient interaction between TiO2 and TCPP molecules at higher pH. It was reasonable to suppose that the oxidation potential of porphyrin ring based dyes adsorbed on TiO2 surface was weakly pH-dependent or pH-independent, especially in the neutral and alkaline solutions. While in TiO2 with increasing pH, the valence and conduction band edges move to more cathodic potentials. This would reduce considerably the driving force for charge separation.38 Therefore, we concluded that TCPP was strongly adsorbed on TiO2 surface through chemical bonding at acidic pH but released in neutral and alkaline pH region. The spectroscopic data is summarized in table 1.
TiO2-attached TCPP remains as a homogeneous colloid at pH 1.0–3.3, a heterogeneous aggregate at pH 3.5–6.0 and free soluble TCPP molecules at pH > 7. Formation and conversions among these forms are shown in scheme 1. Due to pH-responsive stability and irreversible formation of TiO2-attached TCPP, the recovery of both TCPP and TiO2 were simply achieved by centrifugation at certain pHs. To test the recovery yields, a homogeneous solution of TiO2-attached TCPP at pH between 1.0 and 3.3 was first alkalinized with NaOH to destroy colloids, then followed by centrifugation. The supernatant was analyzed at pH 2.3 to quantify remaining TCPP and TiO2 concentrations by monitoring TiO2 at 215 nm using ε = 6050 M−1 cm−1 and TCPP at 408 nm using ε = 9.5×104 M−1 cm−1, respectively (see Experimental Section). The best recovery yields of 99.8% for TCPP and 98.8% for TiO2 were obtained from 1.4 mM TiO2-attached TCPP by centrifugation at pH between 4 and 5.
1O2 luminescence was detected at 1270 nm upon irradiation of TiO2-attached TCPP at 532 nm. The data shown in figure 2 was assigned to 1O2 phosphorescence because both kinetics and intensity of the signals were sensitive to the concentrations of oxygen and azide ions that quenches 1O2 at a rate constant of 5.0×108 M−1 s−1 in H2O.39 Figure 2 also indicated that azide ions quenches not only the lifetime of 1O2 but also the initial intensity of 1O2 luminescence, which can be explained by its reactions with both 1O2 and excited states of a sensitizer.40 The kinetic decay was exponential with rate constants (kd) of 1O2 in 80% D2O between 1.2×105 and 4.4×104 s−1 for a pH range of 1 to 10, respectively. For all calculations in this work, kinetic traces have been corrected for the interference from other rapid events synchronized with laser pulses (i.e., scattered light and fluorescence), using a same but N2-saturated sample as a control. Recently we reported 1O2 production upon two-photon excitation of TiO2 in CHCl3 at 532 nm.41 Compared to TCPP (ΦΔ = 0.53 for free TCPP molecules, measured in the work), 1O2 production from TiO2 was much less efficient, and could be disregarded. The generation of 1O2 is based on a bimolecular sensitization reaction, represented by predominantly a triplet quenching process of a sensitizer in the presence of 3O2 (equation 1 and 2).
1O2 production upon visible irradiation of TiO2-attached TCPP was also evidenced by steady-state experiments using [2-dicyclohexylphosphino)ethyl]trimethylammonium chloride as a chemical trap of 1O2. The loss of phosphine was monitored by 31P NMR. Our results indicated that 100% conversion yields of phosphonate were obtained upon 20 minutes visible irradiation of 0.6 μmol phosphine in the presence of 1.0×10−5 M TiO2-attached TCPP at pH 2.3, wherein a homogeneous TiO2-attached TCPP colloids were formed. We attribute photosensitizing ability to retain TCPP solubility on TiO2 surface and hence activity.
ΦΔ is an important measure of the efficacy of 1O2 photosensitization. The determination of ΦΔ from a colloidal solution was not very precise because of light scattering by suspended nanoparticles. However, ΦΔ could be approximated according to equation (4) by comparing the initial 1O2 intensities with those from a well developed reference sensitizer meso-tetrasulphonatophenyl porphyrin (TSPP) using ΦΔ 0.63 in D2O.42 TiO2-attached TCPP remained as a homogeneous colloidal sensitizer at pH between 1 and 3.3 but aggregated in a pH range of 3.5 to 6. In neutral and basic solutions, 1O2 was mainly resulted from free TCPP molecules. ΦΔ exhibited in the sigmoidal curve was pH-dependent and fully consistent with pH dependent TCPP adsorption on TiO2 surface (figure 3). Observed sharp change in ΦΔ took place when the pH was close to the point of zero charge (pH ~ 5.0 for TiO2),37 where the transition from free to surface-bound TCPP occurred. The higher ΦΔ obtained in neutral and alkaline solutions (0.54 from figure 3) were perfectly in agreements with both of our value 0.53 for free TCPP measured in pH 10 NaOH solution and literature value 0.47 determined at pH 13.28 Similar observations were reported for weak fluorescence quenching at higher pH performed by Hartland for anthracene dyes bound to TiO2,43 and for weak electron transfer in alkaline solutions studied by Gratzel for Zn porphyrin in colloidal TiO2 solution.44 We therefore concluded that efficient energy transfer from triplet TCPP to 3O2 in neutral and alkaline conditions was due to the lack of the adsorption of anionic TCPP on negatively charged TiO2 surface. The electron transfer between TiO2 and TCPP could compete favorably with the energy transfer process in acidic conditions, subsequently resulting in lower ΦΔ.
To quantify apparent quenching of 1O2, total rate constants of 1O2 removal (kT) by free TCPP and TiO2-attached TCPP were determined by Stern-Volmer analysis using TSPP as a sensitizer. kT (1.1±0.1)×108 M−1 s−1 measured from TiO2-attached TCPP in pH 2.3 HCl solution was comparable to those from other porphyrin sensitizers, e.g., 6×107 M−1 s−1 for TPP in C6D6,45 and < 108 M−1 s−1 for free- and metallo-TSPP in D2O.46 A higher value of (1.1±0.1)×108 M−1 s−1 for TiO2-attached TCPP than that of (5.0±0.1)×107 M−1 s−1 for free TCPP measured in pH 10 NaOH solution might result from fast quenching of 1O2 by TiO2 nanoparticles.41
TiO2-attached TCPP retained photosensitization ability in a wide pH range, e.g., as a homogeneous colloid at pH between 1.0 and 3.3, a heterogeneous aggregate in a pH range of 3.5 to 6.0 and free soluble TCPP molecules at pH > 7. Both TCPP and TiO2 could be easily separated from solutions by centrifugation at pH between 3.5 and 5 with satisfied recovery yields. This pH-responsive, TiO2-attached TCPP has advantages of high dispersion and subsequent efficient 1O2 production. The novel system shows a potential to bridge the gap between easily recoverable and highly efficient sensitizers. Also, it provides a general methodology for the separation of dyes based on their interaction with inorganic metal oxides. The recoverable sensitizers can be of great significance to aqueous phase organic synthesis, water detoxification and disinfection. Due to the photocatalytic activity of TiO2 under UV illumination, the application of TiO2-attached TCPP should be restricted to visible light. A challenge for future development is to tailor recoverable sensitizers with the aim of using a wide range of light energy for photosenstization.
Reagents and solvents were obtained commercially and used without further purification. meso-Tetra(4-carboxylphenyl) porphine (TCPP) and meso-Tetra(4-sulfonatophenyl)porphine dihydrochloride (TSPP) were purchased from Frontier Scientific, Inc., [2-(dicyclohexyl phosphino) ethyl]trimethyl ammonium chloride (> 98%) from Strem Chemicals, Inc., 1.005-0.995 M standard NaOH solution from Fisher Scientific, D2O (99% atom) from Sigma-Aldrich and sodium azide (99%) from Acros Organics. Deionized water was obtained from a Nanopure Water (Barnsted System, USA). A Q-switched Nd:YAG laser with pulse duration of 3–4 ns and a maximum energy of 30 mJ at 532 nm (Polaris II-20, New Wave Research Merchantek Products), and a liquid N2-cooled germanium photodetector (Applied Detector Corporation) were used for time-resolved 1O2 luminescence measurements. Steady-state photooxidation was conducted in oxygen-saturated solution using a 300 W mercury lamp (Newport Oriel Instruments, 68911 ARC Lamp Power Supply, 200–500 watts Xe/HgXe) equipped with a water filter and a 500 nm cutoff filter. Other instruments employed in this research include a BioMate 3 UV-Vis spectrophotometer (Thermo Scientific) or a Cary 300 UV-Vis spectrophotometer (Varian, Inc.) for taking absorbance and spectra, and a centrifuge (Fisher Scientific, Inc, Centrific Model 228) for heterogeneous separation. The determinations of photooxidation products were done with a 300 MHz Bruker Spectrospin FT-NMR and/or a 400 MHz JEOL Eclipse FT-NMR. All measurements were carried out at ambient temperature.
Colloidal TiO2 solutions were prepared by hydrolysis of TiCl4 as previously described.47,48 Briefly 2.5 mL TiCl4 at 0°C was introduced under a stream of nitrogen and vigorous stirring into 70 mL water/ice solution of 0.1 M HCl. After 30 min stirring at 0°C, the solution was dialyzed against aqueous HCl of pH 2.3 using Spectrapor membrane tubing (Spectrum Medical Industries) with MW 6000–8000 cutoff pores. A transparent solution containing ca. 20 g/L TiO2 at pH 2.3 was obtained. Molecular TiO2 concentration was determined by spectrophotometric measurements at 215 nm using molar extinction coefficient 6050 M−1 cm−1.47 The solution was kept in refrigerator at 2–5°C and used within 3 months. Colloidal TiO2 nanoparticles such prepared had an average diameter 1.0 nm with 90% of the diameters in the range 0.8–1.3 nm.48 TiO2 concentrations were reported in terms of TiO2 particles (CTiO2 particles, molarity) to better reflect experimental conditions. The particle concentration was calculated according to equation 1 using a TiO2 density (dTiO2, g/cm3) of 4 g/cm3 and an average diameter (TiO2, nm) of 1.0 nm.
TiO2 nanoparticles modified with TCPP molecules were prepared by mixing 0.0063 g TCPP with 20.00 mL pH 2.3 1.7 g/L colloidal TiO2 solution for 7 days under stirring. The mixture was kept in dark during preparation to avoid photocatalytic decomposition of TCPP. The excessive TiO2 particles and TCPP molecules were then separated from aqueous phase by centrifugation. TiO2-attached TCPP colloids such prepared were extremely stable in the pH range of 1.0–3.3 with an extinction coefficient ε = 6.8×104 M−1 cm−1 at 420 nm measured at pH 2.3. For extinction coefficient measurement, 100% adsorption of TCPP on TiO2 surface was required. We therefore used a minimum 10 fold excess of TiO2 particles relative to the TCPP molecular concentrations. The remaining TiO2 particles were removed from the solution by centrifugation. The absorption spectra and calibration curve are shown in figure 4. All spectra were measured against pH 2.3 aqueous HCl solution. The amount of TCPP chemisorbed on TiO2 surface was therefore determined at 420 nm according to Lambert-Beer’s law using ε420 nm = 6.8×104 M−1 s−1. Typically, an average load of one TCPP molecules per TiO2 particle was obtained.
TCPP was nearly insoluble in acidic solution (blue line in figure 1). Its solubility however could be improved at a low limit of ca. 6×10−6 M in the presence of sodium salt. A series of standard TCPP solutions at pH 2.3 were therefore prepared by first dissolving TCPP in pH 10 NaOH water, then followed by acidifying the solution to pH 2.3 with HCl. The recovery of TiO2-attached TCPP aggregates was achieved by centrifugation at pH between 4 and 5. The remaining TCPP concentrations were therefore calculated according to Lambert-Beer’s law using e = 9.5×104 M−1 cm−1 measured at 408 nm in pH 2.3 acidic solution as shown in figure 5.
ΦΔ were determined in aerated solutions by comparing the intensity of 1O2 phosphorescence at 1270 nm from TiO2-attached TCPP with that from a reference sensitizer TSPP in D2O using a known ΦΔ, TSPP 0.63.42 1O2 phosphorescence at 1270 nm was monitored as previously described.49 The initial 1O2 intensity was extrapolated to time zero and corrected by using a same but N2-saturated sample as a control. The data points of the initial 3–5 ns were not used due to electronic interference signals from the detector. The absorbances of TiO2-attached TCPP and TSPP were matched to be the same at excitation wavelength of 532 nm. ΦΔ were calculated according to equation (2).
Here ΦΔ, TiO2-attached TCPP and ΦΔ, TSPP are the ΦΔ from TiO2-attached TCPP and TSPP, respectively; and IΔ, TiO2-attached TCPP and IΔ, TSPP the 1O2 intensities from TiO2-attached TCPP and TSPP, respectively.
Many sensitizers also quench 1O2 via chemical and/or physical reactions. In order to quantify the apparent strong quenching of 1O2 by TiO2-attached TCPP, we determined the total quenching rate constant of 1O2 removal (kT) for both of colloidal TiO2 particles and TiO2-attached TCPP by Stern-Volmer analysis. The quenching rate constants were calculated in terms of TiO2 particle concentrations. Measurements were carried out at 532 nm excitation using TSPP in pH 2.3 D2O as a sensitizer. Considering relatively low concentrations of TiO2-attached TCPP used in quenching experiments and high ΦΔ from TSPP,42 1O2 signals resulted from TCPP could be disregarded if comparing to these from TSPP. Our data indicated that the kinetics of 1O2 luminescence decay at 1270 nm followed Stern-Volmer equation.
Where k is the observed 1st-order rate constant of 1O2 decay after laser pulse, kd the 1st-order rate constant of 1O2 decay in the absence of quencher, kT the 2nd-order rate constant for bimolecular quenching of 1O2, or total quenching rate constant of 1O2 removal. Changes in 1O2 lifetimes were observed by the addition of TiO2 particles or TiO2-attached TCPP into the solutions. Stern-Volmer plots gave a good linear correlation between k and quencher concentrations [Q]. kT values then can be derived from the slopes of the straight lines.
1O2 photooxidation of phosphines led to the formation of either phosphonate or a mixture of phosphonate and phosphinate.50,51 A water soluble phosphine, [2-(dicyclohexylphosphino)ethyl]trimethylammonium chloride was employed as a 1O2 trap in steady-state photolysis experiments. Photooxidation products were determined by 31P NMR as previously reported.50,51 The reactions were carried out in the presence of 1.00×10−5 M TiO2-attached TCPP in an O2-saturated 90% D2O solution at pH 2.3. 1O2 photooxygenation led to a formation of a sole product, phosphonate (figure 6). The percent yields were calculated by comparison of the integrated 31P peaks of phosphine with those of phosphonate in 31P NMR spectra. Control experiments were carried out in order to correct phosphine oxidation by ground state oxygen molecules (line 2 in figure 6). Examples of 31P NMR spectra shown in figure 6 indicated peaks of phosphine δ −6.75 (s, 1P) and phosphonate δ 60.47 (s, 1P).
Supports from programs of NSF-PREM (DMR-0611539) and NIH-RCMI (2G12RR013459) are gratefully acknowledged. We thank Professor Edward Valente from Mississippi College for his support on 31P NMR measurements.