Titanyl Phthalocyanine as a Water Photooxidation Agent - The Journal

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Titanyl Phthalocyanine as Water Photooxidation Agent Olaf Wojciech Morawski, Katarzyna Izdebska, Elena Karpiuk, Andrzej Suchocki, Yaroslav Zhydachevskii, and Andrzej L. Sobolewski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05083 • Publication Date (Web): 31 May 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Titanyl Phthalocyanine as Water Photooxidation Agent Olaf Morawskia*‡, Katarzyna Izdebskaa, Elena Karpiuka, Andrzej Suchockia, Yaroslav Zhydachevskyya, and Andrzej L. Sobolewskia‡, AUTHOR ADDRESS a

Institute of Physics Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland

KEYWORDS Photoinduced partial water splitting, hydroxyl radicals, hydrogen bonded complex, the ligand-tometal charge-transfer state.

ABSTRACT

Photooxidation of water by a titanyl phthalocyanine (TiOPc) layer deposited on fused silica plates was investigated with excitation to the Soret and Q bands of the absorption spectrum. Partial water photolysis, evidenced by hydroxyl radical dosimeters, occurs upon excitation within the Soret absorption band, but is absent upon excitation to the Q band. These results confirm the first step of the mechanism of photoinduced partial water splitting proposed theoretically by Sobolewski and Domcke (PCCP, 2012, 14, 12807-12817). The photocatalytic

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water splitting occurs without external voltage or pH bias. TiOPc is found to be a more efficient photocatalyst

for

partial

water

oxidation

than

the

previously

reported

titanyl

tetraphenylporphyrin.

INTRODUCTION The production of clean and renewable fuels is of top interest and the photocatalytic water splitting reaction is studied intensively with the aim to generate H2 and O2 directly with solar light. The vast efforts towards water splitting can be classified into two groups. In the photoelectrochemical approach to water splitting, a semiconductor photoanode is used to generate holes which are able to oxidize water molecules1-7. In an alternative concept, mimicking natural photosynthesis, a supramolecular structure consisting of electron donor, acceptor and chromophore absorbs photons and triggers charge separation. The separated charges may neutralize hydroxide anions and protons8–14. Recently a new approach to water splitting - the direct photo-induced homolytic cleavage of H2O molecules into H• and OH• radicals with oxotitantium porphyrin (TiOP) as photo-catalytically active chromophore - has been proposed15. The mechanism comprises of two steps: first the splitting of the photo-excited TiOP-H2O hydrogen bonded complex into biradical pair of TiOPH• and OH•, and the subsequent cleavage of photo-excited TiOPH• into TiOP and H•. This cycle of reactions requires absorption of two photons for excitation of the TiOP-H2O complex and later - the TiOPH• radical and results in two products: H• and OH• radicals. Thus the theoretically predicted mechanism15 leads to partial water oxidation what differentiates it from the photoelectrochemical approach1-7 which leads to total water oxidation and results in generation of molecular hydrogen and oxygen. The first step of the theoretically predicted concept of the partial water splitting was experimentally confirmed


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with oxotitanium tetraphenylporphyrin (TiOPP) as photo-catalyst. It was demonstrated that photoexcitation of TiOPP immersed in water leads to the generation of OH• radicals16.

The investigation of partial water splitting with titanyl phthalocyanine (TiOPc) may provide further verification of the mechanism proposed in15 due to larger energy gap between optically allowed B (Soret) and Q states in TiOPc than in TiOPP (the gap is important in water oxidation process15). The study of water splitting with TiOPc is also interesting since this is a robust compound and it is widely used because of its favorable photoelectrical properties. TiOPc is a highly photosensitive xerographic pigment used in digital electrophotography17-18. The photocarrier generation process depends on the shape of TiOPc microcrystallites19, with the Ypolymorph (Y-TiOPc) having the best photoconductivity among the polymorphs of TiOPc, other metallo-phthalocyanines (CuPc, VOPc, AlClPc) and bare H2Pc20. TiOPc absorbs in the visible and near infrared offering a good match with the solar spectrum. It was used in photovoltaic devices showing a 2.4% power conversion efficiency21-24. Its derivatives used in dye-sensitized solar cells (DSSC) were reported to reach a power conversion efficiency of 3.6%25. Photoelectrical properties of TiOPc are influenced by humidity indicating that water plays an important role in the carrier photogeneration process26-28. In Y-TiOPc an external electric field quenches the fluorescence, which indicates some charge-transfer character of the lowest excited singlet state27. The quenching of fluorescence by adsorbed water molecules suggests that H2O plays a role in the charge generation process27. Another investigation of Y-TiOPc absorption and fluorescence spectra as well as fluorescence decays has confirmed that an external electric field and adsorption of water lead to the same effects: quenching of fluorescence, faster fluorescence


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decay and enhancement of carrier generation28. Two mechanisms have been postulated as origin of the water effect: (i) a charge transfer or a hydrogen bonded complex between TiOPc and H2O in which TiOPc acts as proton acceptor, or (ii) a water-induced local electric field that influences the photophysics of TiOPc in similar way as an external electric field does. The differences in the photophysics of the various forms of TiOPc have been observed28 and related to distortion of the molecular structure of TiOPc19,29. Extensive theoretical explorations of the photophysics of the hydrogen-bonded complex of TiOP with a single water molecule (TiOP–H2O)15 revealed that a remarkable electronic charge rearrangement in the ligand-to-metal charge-transfer (LMCT) states occurs through an excitedstate proton-coupled electron-transfer (PCET) reaction: the hole in the π-orbital of porphyrin is filled by electronic charge from the water molecule, resulting in formation of a neutral TiPOH•– OH• biradical. It was also postulated that the water-splitting process in the TiOP–H2O complex can be completed by the absorption of a second photon by the TiPOH• radical via its B band and radiationless population of the lower-lying dark πσ* doublet state, which results in a barrierless photodetachment of the H• radical from TiPOH•. Thus, in principle, after absorption of two visible photons, a water molecule is cleaved into free OH• and H• radicals and the photocatalyser TiOP is recovered. In the present work, we investigated the first step of the theoretically predicted partial water splitting process, the formation of OH• radicals with visible light in microcrystalline TiOPc in water. The interpretation of the experimental data is supported by electronic structure calculations.


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EXPERIMENTAL METHODS TiOPc (Fig. 1) was purchased from Aldrich and used as received.

Figure 1. The structure of titanyl phthalocyanine. Like other unsubstituted metal phthalocyanines which are barely soluble in common solvents30, TiOPc does not dissolve in water. For the photochemical experiments we deposited TiOPc either on bare fused silica plates or on fused silica plates covered by a thin gold layer (in order to attain electrical contact for current measurements). The deposition was performed in a table-top thermal evaporation system, model EVD22B-TT, of Plasmionique Inc. The evaporation was performed under high vacuum condition with a pressure of the order of 10-6 hPa on quartz plates of temperature 18oC. TiOPc evaporates effectively at 330-3800C - a temperatures which are distinctly different from 1850C, at which the impurity present in the purchased material evaporates. Keeping the shutter over the crucible of Knudsen cell closed for temperatures up to 3000C was enough to discriminate the impurity from the target molecular layer. The evaporated molecular layers had a thickness of 8-10 nm. The fused silica plates had the same size as the gold electrodes’ plates (45 x 8 x 1.5 mm). Standard fused silica cuvettes were used in all experiments.

Absorption spectra have been measured with a PerkinElmer UV/VIS spectrometer model Lambda 35. Fluorescence spectra have been recorded with the Fluorolog-3 spectrometer. The


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sensitive Andor DU420A-BR-DD CCD camera, electronically cooled to - 90 degree Celsius was used in measurement of weak, near-IR fluorescence of TiOPc. Photovoltaic measurements were performed in a quartz cuvette filled with neat deionized water and two immersed gold electrodes (one with deposited molecular layer). Two identical gold electrodes were used to avoid any bias arising from difference in standard electrode potentials. Photo-response excitation spectra were recorded using one gold electrode (with deposited molecular layer) and a platinum wire separated by 4mm as the second electrode, to allow the spatially elongated beam of the spectrofluorimeter illuminate the molecular layer on the gold electrode. Voltage and current intensity were measured with a Keithly Model 197 microvolt voltmeter. In all measurements, distilled and deionized water of conductivity less than 0.1 µS/cm was used. The spectrally selective irradiation of molecular layers of TiOPc was performed with a Mira pumped by a Verdi laser of Coherent Inc., solid-state lasers and with Mercury and Xenon lamps. In radicals generation measurements several sources of light impinging on the sample were used: Mercury lamp (2.6 mW at 365 nm line), Xenon lamp (4.51, 8.61 and 10.23 mW at 315 nm) and the 660 nm solid-state laser of the 90 mW power. In measurements of photo-electrochemical effect the solid state lasers (75 mW at 405 nm and 66 mW at 660 nm) and the Mira laser (75 mW at 730 nm and 264 mW at 800 nm) were applied for the sample irradiation.

Figure 2 The structure of terephthalic acid. The dosimetry of the OH• radicals was described previously16. Here we briefly summarize the method. Hydroxyl radicals were detected with a well-known OH• scavenger, terephthalic acid


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(TA, Fig. 2) which is OH• specific and does not react with other radicals potentially present in the sample (e.g. HO2 or H2O2)31-33. The reaction with OH• radicals converts the non-fluorescent TA into strongly luminescent hydroxyl-terephthalic acid (HTA). The solution of TA was prepared in neat water by dissolving first NaOH (35*10-4M) and then TA (5*10-4M). The TiOPc plates were immersed in this solution and irradiated for a given time at a wavelength corresponding to either the Q or Soret bands. The fluorescence spectrum of the solution (the HTA emission) was recorded with excitation at 315 nm. The increase of the HTA fluorescence after irradiation of the plates with deposited TiOPc molecules is the proof of photo-production of hydroxyl radicals from water. With this method only a lower limit of the efficiency of the watersplitting reaction can be estimated, since HTA is produced in a chain of reactions. The first step is the actual splitting of water into biradicals TiOPc + H2O + hν -> TiOPcH• + OH•

(1)

The hydroxyl radicals then diffuse to encounter TA and react with the latter to give HTA and water TA + 2OH• -> HTA + H2O

(2)

In the above scheme, the OH• + OH• → H2O2 reaction and the possible reaction of OH• with TiOPc, competing with (2), have been neglected. Therefore, the experimentally determined efficiency of TA → HTA conversion represents a lower limit for the water-splitting efficiency.

In order to exclude a possible pH effect on the reaction, a different, but less effective, OH• scavenger - benzoic acid (BA) - has also been used. BA dissolves in pure water and attaches free


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OH• radicals at three positions: meta, ortho and para36. The presence of hydroxyl radicals can be detected by recording the fluorescence of 2-hydroxybenzoic acid (salicylic acid, SA) which absorbs at lower energies than BA37. OH• radicals are detected by the buildup of SA fluorescence formed by the reaction BA + 2OH• → SA + H2O.

(3)

The concentration of BA in water in the irradiation experiments was 4.54*10-4M. The efficiency of hydroxyl radicals generation was determined by analysis of the buildup curves of the HTA fluorescence in cuvette of small (750 microliters) volume. The Fluorolog 3 spectrophotometer was set in the “kinetics” mode allowing for continuous recording of HTA fluorescence over the time of irradiation. Depending on the Xenon lamp power (adjusted with the excitation slits width) the build-up characteristic time ranges from 3 to 11 minutes. The exponential grow A*(1-e-t/τ) formula with two free parameters “A” and “τ“ was fit to the experimental points, with τ being the build-up time and “A” representing intensity of HTA fluorescence at the infinity time. In calculation of the TA->HTA conversion efficiency the “A” parameter was replaced with the initial concentration of TA molecules, as it was assumed that all TA molecules have been converted into HTA by irradiation of TiOPc immersed in water.

THEORETICAL METHODS The equilibrium geometry of the hydrogen-bonded complex of TiOPc with a water molecule (TiOPc-H2O) in the electronic ground state was determined with density functional theory (DFT), employing the B3LYP functional. The vertical excitation energies of the lowest excited


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singlet states were calculated with the TDDFT method, employing the same functional. The equilibrium geometry of the photoproduct of the reaction, the biradical TiOPcH•-OH•, has been obtained by geometry optimization of the lowest open-shell singlet (or triplet) state at the spinunrestricted DFT (UDFT) level. The correlation-consistent double-zeta basis set (cc-pVDZ) was employed for the first-row atoms, while the cc-pVTZ basis was used for the Ti atom34. The TURBOMOLE program package was used for all electronic-structure calculations38,39.

Earlier extensive explorations of spectroscopic and photophysical properties of TiOP and the TiOP-H2O complex performed with several electronic-structure methods provided evidence that the TDDFT/UDFT protocol is a reliable method for systems of this kind15. The well-known deficiency of the TDDFT method with standard exchange-correlation functionals for excited electronic states of charge-transfer character35 was not a problem for the systems considered here.

RESULTS

TiOPc deposited in vacuum on fused silica plates crystalizes in the form of micro-cristallites. An SEM image of TiOPc microcrystalites is shown in Fig. 3a. The stability of the evaporated molecular layer depends on the experimental conditions and the concentration of the OH• scavenger. In experiments performed in the presence of hydroxyl radical scavengers (TA or BA), the TiOPc layer exhibits constant absorbance for 4 hours. In pure water, its absorbance starts to decrease after 30 minutes of excitation to the Soret absorption band. This is accompanied by changes in the SEM image of the layer (Fig. 3b). The size of the micro-cristallites is considerably


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reduced, which indicates that the photoproducts attack the TiOPc surface in the absence or low concentration of OH• radical scavengers.

(a)

(b)

Figure 3. SEM images of a TiOPc layer deposited on a quartz plate: (a) fresh after evaporation, (b) after 4.5 h of irradiation in an aqueous solution of TA.

Absorption and fluorescence spectra of a TiOPc layer evaporated on a fused silica plate and measured in air are shown in Figure 4. The most intense band in the absorption spectrum is the Q-band with a maximum at 720 nm (1.72 eV). The Soret (B) band at 352 nm (3.52 eV) has a slightly weaker intensity. As mentioned in the introduction, several polymorphic phases of TiOPc layers deposited on glass plates have been identified40. The absorption spectrum of the TiOPc layer evaporated by us corresponds to the amorphous, “as evaporated”, phase reported earlier in literature40,41. The fluorescence of the TiOPc layer is very weak and its maximum at 900 nm is shifted with respect to the Q-band at 720 nm.


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Figure 4. Absorption (black line) and fluorescence (red line) spectra of an 8 nm thick TiOPc layer on a fused silica plate. Absorption and fluorescence spectra were recorded with 1 nm resolution. The fluorescence spectrum was normalized to fit the maximum of the Q-band.

The vertical excitation energies and oscillator strengths of the lowest singlet states of TiOPc, computed with the TD-DFT/B3-LYP/cc-pVDZ(cc-pVTZ at Ti) method, are collected in Table 1. The frontier Kohn-Sham orbitals involved in these excitations; occupied (πP) and unoccupied (π*P) π-orbitals of phthalocyanine ring and the lowest unoccupied d-orbital of titanium (dTi), are displayed in Fig. S1 of the Electronic Supporting Information (ESI). Analogously to the titanyl porphyrin15, the lowest dipole-allowed singlet transitions involve πPπ*P excitations within the phthalocyanine ring of TiOPc. They fall into two bands observed at 700 nm and 350 nm


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being analogs of the Q band and the B band of porphyrin, respectively. These bands are interloped by several “dark” states most of which are also of the πPπ*P orbital nature.

Table 1. Vertical excitation energy (∆E), vertical transition wavelength (λ) and oscillator strength (f) of the lowest excited singlet states of TiOPc computed with the TDDFT/B3-LYP/ccpVDZ (cc-TZVP at Ti) method at the DFT-optimized geometry of the ground state. state

∆E [eV]

λ [nm]

f

πPπ*P(Q)

2.02

613,8

0.75

πPdTi(CT)

2.36

525,4

0.0

πPπ*P

3.27

379,2

0.0

πPπ*P

3.27

379,2

0.0

1 1

1

1

1

πPπ*P

3.31

374,6

0.015

1

πPπ*P

3.48

356,3

0.002

1

πPπ*P

3.50

354,2

0.005

1

πPπ*P(B)

3.58

346,3

0.20

1

πPπ*P(B)

3.66

338,8

0.90

1

πPπ*P(B)

3.72

333,3

0.22

Like in TiOPP, the lowest “dark” state is of the ligand-to-metal charge-transfer (LMCT) character, in which π electron of the phthalocyanine ring is excited to a d orbital of the titanium atom. This state is dark in absorption from the ground state due to the small overlap of πP and dTi orbitals.


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Hydroxyl radicals were detected with chemical dosimetry by terephthalic acid as described in the experimental section. Shortly, the plates with a layer of TiOPc were immersed in aqueous solution of NaOH and TA and irradiated with light at 365 nm (excitation of the Soret band) or 660 nm (excitation of the Q-band). If OH• radicals are produced, they react with TA molecules converting them into HTA which are strongly fluorescent when excited at 315 nm.

Figure 5a contains a series of HTA fluorescence spectra recorded after 0 to 4.5 hours of irradiation of TiOPc in aqueous TA solution. Before irradiation of the photocatalyst, the emission spectrum of the solution (excited at 315 nm) is dominated by the Raman band of water at 352 nm, which can be assigned to the strong and well-known 3400 cm-1 OH stretching vibration of the H2O molecule. Irradiation of the TiOPc layer results in the buildup of HTA fluorescence centered around 425 nm. The intensity of HTA fluorescence increases linearly with time of excitation of TiOPc to the Soret absorption band (see inset in Fig.5a). In contrast to that, only very weak HTA fluorescence is observed for Q-band excitation of TiOPc (Fig.5b), although the irradiation power was 35 times higher (90 mW for the Q-band excitation vs. 2.6 mW for the Soret band). Thus we conclude that excitation of TiOPc within the Soret band leads to the generation of hydroxyl radicals in the water. The residual buildup of HTA fluorescence observed for Q-band excitation of the TiOPc reveals only the inherent, systematic error of the method used for the OH• radical dosimetry. Although the 315 nm beam exciting HTA was lead parallel to the plate with TiOPc molecular layer and did not illuminate it directly, some scattered light may reach the molecular layer exciting TiOPc which has strong absorption at that wavelength as seen in Fig.4. In result the production of a small amount of OH• radicals can be expected at every recording of the HTA fluorescence spectrum. Thus the OH• dosimetry method has the systematic error which, in case of the TiOPc absorbing at 315 nm, influences the HTA spectra.


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Taking into account that error and the very weak HTA fluorescence observed for the red light irradiation to the Q-band, one may conclude that the excitation to the Q-band of the TiOPc molecular layer immersed in water does not lead to the generation of the hydroxyl radicals.


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Figure 5. The buildup of fluorescence spectra of HTA in result of (a) irradiation of TiOPc with 365nm line of Mercury lamp (excitation within Soret band), power 2,6 mW, (b) irradiation of TiOPc with 660nm solid state laser (Q band excitation), power 90 mW. The inset shows the HTA fluorescence integral intensity versus time of irradiation. Spectra have been recorded after irradiation of a TiOPc layer evaporated on fused silica plate and immersed in water solution of TA. Legends specify the total time of TiOPc layer irradiation.

To check whether generation of OH• occurs also in pure water, the plate with the TiOPc layer was immersed in water solution of BA and irradiated with the 365 nm line of a mercury lamp (excitation within the Soret band). Fig. 6 presents series of SA fluorescence spectra recorded after irradiation of the TiOPc layer. The spectra are dominated by the Raman band of water, but the buildup of the salicylic acid fluorescence in response to TiOPc layer irradiation is clearly


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seen. This result reveals that BA is converted to SA upon TiOPc irradiation and thus provides experimental evidence that the OH• radical generation process occurs also in pure water.

Figure 6. The buildup of SA fluorescence in water. Spectra have been recorded after irradiation of a TiOPc layer in a solution of BA. The legend specifies the total TiOPc layer irradiation time for each spectrum. The inset shows the BA fluorescence integral intensity versus total time of TiOPc layer irradiation. Note that the position of Raman band maximum in Fig.6 (334 nm) differs from that of Fig.5 (352 nm) because SA was excited at 300 nm whereas HTA at 315 nm.


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Efficiency of hydroxyl radicals generation In the photoelectrochemical approach to water splitting the efficiency of PEC cells is defined in terms of the free enthalpy of liquid water, the rate of hydrogen generation, the bias voltage applied to the cell and solar irradiance2. Here we study only the first step of the theoretically proposed partial water oxidation process which results in generation of hydroxyl radicals. To evaluate the performance of that step we define its efficiency as the ratio of quantity of hydroxyl radicals generated to number of absorbed photons. As hydroxyl radicals are detected with TA molecules, the actual measure of efficiency must be related to HTA fluorescence. Such measure has the meaning of the quantum yield of the process studied and gives information of the overall effectiveness of the (1) and (2) reactions chain. If one limits the number of TA molecules in water, it should be possible to extinct the TA population and thus observe saturation of the intensity of HTA fluorescence. In a volume of several mililiters with a concentration of TA of the order 5*10-4 M, the fluorescence of HTA is found to grow linearly with time for several hours of irradiation, showing that the initial quantity of TA molecules is sufficient for these time periods to react with the OH• radicals generated. By reducing the volume of the aqueous solution of TA one may limit the initial amount of TA molecules and observe stabilization of HTA fluorescence intensity. This was demonstrated in the study of photocatalytic partial water splitting by titanyl tetraphenylporphyrin16. TiOPc has strong absorption at wavelengths used for the excitation of the fluorescence of HTA (300–320 nm - figure 4). Thus the scattered light of the beam exciting HTA excites TiOPc and causes also generation of OH• radicals, as discussed above. In small volumes of TA aqueous solutions the light scattering is especially effective and does not allow to separate the quantity of radicals generated with TiOPc Soret band excitation (beam perpendicular to the molecular layer)


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from the quantity of OH• produced by scattered light (beam parallel to the layer). Therefore we decided to excite the HTA and TiOPc concurrently with beam perpendicular to the TiOPc molecular layer. The beam of light passes first the thin (2.5mm) layer of TA aqueous solution and falls onto TiOPc layer of absorbance at 315nm equal to 0.13. As the of absorbance of the HTA solution at 315nm is less than 0.01 (at the end of the experiment when all TA molecules are converted info HTA), one can state that most of photons are absorbed by TiOPc layer and the negligible absorbance of HTA does not affect the kinetics of the HTA fluorescence build-up.

To record the build-up of the HTA fluorescence we used small cuvette and 750 microliters of H2O with 32.13*10-4 M NaOH and 4.66*10-4 M TA. Applying the irradiation of the TiOPc plate at 315 nm we were able to reach the saturation of the HTA fluorescence intensity within few minutes (Fig. 7) and fit the exponential grow curve to experimental points of HTA fluorescence intensity.


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Figure 7. The increase of HTA fluorescence intensity observed at 425nm in result of TiOPc layer irradiation at 315nm. The red solid curve represents fit of A*(1-e-t/τ) formula to the experimental points, where τ is the build-up time determined by the fit. The inset presents the linear fit to TA->HTA rate constants, defined as reciprocal of τ, as a function of the power of the irradiation light.

The rate constant for HTA fluorescence build-up (the reciprocal of characteristic time τ) grows linearly with the power of the irradiation beam (inset to Fig. 7). This proves that the extinction of TA molecules is governed by the process (2) and that other second – order reactions like the reaction with H2O2 (formed in OH• + OH• → H2O2 process) may be neglected in the conversion of TA into HTA. As the absorbance of the TiOPc molecular layer before and after irradiation is the same, the destruction of the molecular layer is negligible, and one may calculate the molar rate constant and the efficiency for the chain reactions (1) and (2) knowing the power


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of irradiation beam, optical density of the TiOPc molecular layer and the initial concentration of TA. The results are presented in Table 2.

Table 2. The quantum efficiency and rate constant for the build-up of HTA concentration. Efficiency is defined as the number of HTA molecules generated per absorbed photon.

Light power [mW] 4.51 8.61 10.23

Efficiency 0.16 0.14 0.13

HTA build-up rate [M/s] 1.8*10-6 2.0*10-6 2.1*10-6

The values of the TA → HTA conversion efficiency obtained for TiOPc are higher than those previously reported for titanyl tetraphenylporphyrin16.

Water splitting via the ionic channel The earlier study of partial water photolysis with TiPPO revealed the existence of an ionic channel of water splitting16. The ionic channel in TiPPO was found to be much less efficient than the radical channel, but occurs even in pure water and without external voltage bias. In the present work, we tested if the generation of electrically charged products is possible also with TiOPc. Measurements of voltage and electric current were performed for TiOPc deposited on a gold electrode and immersed in distilled and deionized water. Under these non-biased conditions we have detected a photocurrent of very low intensity (20 – 40 nA) and a photo-induced voltage of the order of 0.1 V. The photocurrent excitation spectrum has bands in spectral regions of the absorption spectrum of TiOPc (Fig. 8). The photocurrent excitation spectrum is not corrected for the spectral distribution of the xenon lamp intensity, which results in a continuous decrease of the photocurrent at


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wavelengths longer than 600 nm. Nevertheless, the correspondence between regions of strong absorption and intense photocurrent is clear, see Fig. 8.

Figure 8. Photocurrent excitation spectrum (solid green line) and absorption spectrum (dashed black line) of TiOPc. The (spectrally uncorrected)

photocurrent excitation spectrum was

recorded with 15 nm spectral resolution in short circuit with two different electrodes, a platinum wire and a gold electrode with deposited TiOPc layer, both immersed in pure water. The absorption spectrum was recorded with 1 nm spectral resolution. The maximal value of the photocurrent intensity is 40 nA.

In pure water in the dark the gold electrode with a TiOPc layer has small negative potential of 10 mV against the second (bare gold) electrode. Upon illumination with a laser of 45mW power


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and 660 nm wavelength, the potential of the TiOPc layer jumps to +100 mV and then grows within a few seconds to 110-120 mV. After switching the laser off, the potential of the TiOPc layer decreases slowly, within several minutes, to the initial negative value. Fig. 9 presents jumps and drops of the photo-induced voltage of the TiOPc layer in pure water. The “light on” and “light off” periods (0.95 and 2.83 seconds respectively) are too short to observe the potential stabilization and therefore the voltage does not drop to the initial value observed in the dark.

Figure 9. Voltage induced in photo-electrochemical effect of a TiOPc layer deposited on a gold plate electrode in pure water. The 660 nm laser of 45 mW power was used for the irradiation of the TiOPc layer, and its beam was chopped into 0.95 sec “light on” and 2.83 sec “light off” periods.


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The increase of the potential of the TiOPc electrode to positive values means that upon TiOPc excitation the molecular layer releases anions into water or absorbs cations from the solution. One may point at two reactions leading to that. First, it can be hydrogen ion abstraction from neutral water by excited TiOPc molecule: TiOPc* + H2O -> TiOPcH+ + OH-, an ionic analogue to the radical channel of partial water-spliting process (1). Absorbance of cations could be linked with association of excited TiOPc with hydrogen ions: TiOPc* + H+ + OH- -> TiOPcH+ + OH-. Both reactions result in same products - reduced TiOPcH+ and hydroxyl anions. The second process requires reaction with hydronium H3O+ always present in water at low concentration what can explain very low photocurrent intensity. Although concentration of H3O+ can be changed with pH but that requires adding base or acid and introduces new charged species to the solution what makes the photocurrent behavior complex, limiting the possibility to discriminate experimentally one of the ionic reactions. The detailed description of the ionic channel mechanism is beyond the scope of this work, as it shall take into account adsorption and desorption at the electrodes, the mass transport and non-faradaic processes important at that low currents as observed here.

Table 3 presents the values of short circuit current intensity and open circuit voltage. The photon to electric charge conversion quantum efficiency is very low, of the order of 10-6, and decreases at longer wavelengths of excitation. The values for TiOPc are slightly smaller than those reported for TiOPP16. In contrast to the hydroxyl radical generation, which can be produced only with excitation of the Soret band, the charge separation occurs for excitation of the Soret band as


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well as of the Q band. This indicates that mechanism of charge generation in ionic channel is different from the mechanism of OH• production in radical channel.

Table 3. Photo-electrochemical data obtained for TiOPc in deionized water. Voc – open circuit voltage, Isc – short circuit current intensity, efficiency = number of generated elementary charges per absorbed photon. wavelength power [nm] [mW] 405 75 660 66 730 75 800 264

Voc [V] 0.12 0.12 0.11 0.11

Isc [µA] 0.06 0.05 0.02 0.02

efficiency 3*10-6 1.5*10-6 4.6*10-7 1.5*10-7

DISCUSSION The theoretical results of Ref. 15 provided evidence that the dark LMCT state of the TiOP-H2O hydrogen-bonded complex plays a decisive role for the photoinduced generation of the TiPOH• – OH• biradical pair. Like in TiOP-H2O, in TiOPc-H2O the reactive LMCT state is theoretically predicted in between the Q and B bands. The present experimental results show that for TiOPc the photooxidation of water occurs only upon excitation of the B band. This means that the reactive state for H-atom abstraction from water can be populated only from B excited electronic state. The efficiency of the reaction (generation of hydroxyl radical or charged moieties) is higher in TiOPc than in TiOPP16 where Q and B bands are closer in energy. This indicates that a higher energy gap between the absorbing state and the dark LMCT state is favorable for an efficient population of the reactive LMCT state.


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Fig. 10 shows a potential-energy diagram of the electronic states involved in the photoinduced reaction of TiOPc with H2O. Computed energy of the states and the most relevant molecular orbitals are displayed in Table S1 and Figure S2 of ESI. The left part of the figure shows the vertical excitation energies of the lowest singlet states (dashed lines) computed with the TDDFT/B3LYP/cc-pVDZ (cc-pVTZ at Ti) method at the DFT-optimized ground-state geometry of the complex. On the right side of the figure, the UDFT energy of the biradical pair computed at its equilibrium geometry (red solid line) is shown. The black dashed line denotes vertical energy of the TiOPcH+-OH- ion pair (which correlates to the closed-shell ground state) computed at the biradicalic equilibrium geometry.

1π

PπP(B) 1p d (CT) W Ti

IC 1p W

πP(CT) 1BP

1π d (CT) P Ti 1π

PπP(Q)

fl

abs

S0

Figure 10. Potential-energy diagram for the photoinduced water oxidation

reaction in the

TiOPc-H2O complex. The vertical excitation energies of the lowest dipole-allowed and charge-


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transfer singlet states of the complex (blue and red dashed lines on the left hand side, respectively) were computed at the equilibrium geometry of the ground state. The solid red line on the right hand side denotes the optimized energy of the TiOPcH•-OH• biradical, while the dashed black line denotes the energy of the TiOPcH+-OH- ion computed at this geometry. Blue arrows indicate the vertical excitation of the B and Q states and fluorescence from the Q state. The black dotted arrow indicates schematically the radiationless transition from the optically bright B states to the dark and reactive CT states, while red arrows indicate reactive channels for water oxidation. Only dipole-allowed Q and B bands and charge-transfer states are indicated.

Inspection of the figure reveals that the formation of the biradical pair is energetically feasible for optical excitation of the TiOPc-H2O complex within the Soret band. The charge-transfer state (1pWdTi ) where an electron from the p orbital of water oxygen atom is transferred to the d orbital of titanium atom is located within the band and can be non-adiabatically populated from optically excited 1πPπP(B) state. This state is directly (diabatically) correlated to the ground state of the biradicalic product (cf. Table S1, Fig S1 and Fig. S3 of ESI). However, the non-adiabatic coupling between the B and CT states is expected to be rather weak since this involves twoelectron process. Thus the non-adiabatic one-electron transition from B band to the lower lying CT state (1pWπP) may be preferred. Formation of the biradical pair from this state involves oneelectron process and is still energetically feasible. The reaction from the lowest CT state (1πPdTi) is endoenergetic and would rather result in population of the fluorescing Q state. The calculated data qualitatively explain experimental observations. One has to kept in mind, however, the rather approximate nature of the electronic-structure calculations. This concerns,


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for example, the comparison of TDDFT excited-state energies (generally underestimated) with the UDFT energy (usually overestimated) of the radical pair. Moreover, the computational results were obtained for the isolated TiOPc-H2O complex, while the experimental measurements were performed for molecular micro-crystals immersed in aqueous environment. Fig. 10 therefore provides only an explanation of the mechanisms of the photooxidation of water with the TiOPc chromophore. The photophysical scheme of water oxidation with TiOPc which emerges from this study differs from that determined previously for the TiOTPP chromophore where the reaction was observed for excitation within both (B and Q) absorption bands of the oxidation dye16.

CONCLUSIONS

TiOPc is an efficient water photooxidation agent - the photoinduced partial splitting of water occurs in a heterogeneous system consisting of micro-crystallites of TiOPc immersed in water and excited within UV range of the absorption spectra. The experimental results confirm the theoretically predicted photoreactivity of TiOPc in liquid water leading to the generation of radicals. The mechanism of the generation of OH• radicals proposed by Sobolewski and Domcke15 for the TiPO-H2O complex seems to apply also in the TiOPc-H2O complex. The observation of small photocurrents reveals another, ionic, channel of water splitting, albeit with extremely low efficiency. In both cases, the reaction occurs without pH-bias (electrolyte is not required) and without external potential bias. The efficiency of hydroxyl generation is found to be higher for TiOPc than for TiOPP16. We relate this observation to different energy gaps between the absorbing B state and the reactive LMCT state of the TiOPP-H2O and TiOPc-H2O


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complexes. The larger energetic gap in TiOPc provides more excess energy and therefore a higher probability of the H-atom transfer reaction in the LMCT state. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: (+48) 22 116 32 34 Author Contributions ‡

O.M. and A.L.S. contributed to this work equally.

Funding Sources The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the NSC (the National Science Center of Poland) for funding this project, the grant no. 2012/04/A/ST2/00100. This work was partly supported by the European Union within European Regional Development Fund through the Innovative Economy Grant (POIG. 01.01.0200-108/09). We thank Wolfgang Domcke for stimulating discussions during the course of this study.

SUPPORTING INFORMATION


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Vertical excitation energy, vertical transition wavelength and oscillator strength of the lowest excited singlet states of TiOPc-H2O complex. Frontier Kohn-Sham molecular orbitals involved into lowest electronic excitations of TiOPc. Kohn-Sham natural orbitals singly occupied in the ground state of the TiOPcH•-OH• biradical pair. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

ABBREVIATIONS TiOPc, titanyl phthalocyanine; TiOP, oxotitantium porphyrin; TiOPP, oxotitanium tetraphenylporphyrin; TA, terephthalic acid; HTA, hydroxyl-terephthalic acid; BA, benzoic acid; SA, salicylic acid (2-hydroxybenzoic acid); LMCT, ligand-to-metal charge-transfer; SEM, Scanning Electron Microscope.

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Titanyl Phthalocyanine as a Water Photooxidation Agent - The Journal

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