Photosensitization and Photocurrent Switching in Carminic Acid

J. Phys. Chem. C 2008, 112, 19131–19141

19131

Photosensitization and Photocurrent Switching in Carminic Acid/Titanium Dioxide Hybrid Material Sylwia Gawe¸da, Grazˆyna Stochel, and Konrad Szaciłowski* Uniwersytet Jagiellon´ski, Wydział Chemii, ul. Romana Ingardena 3, 30-060 Krako´w, Poland ReceiVed: May 28, 2008; ReVised Manuscript ReceiVed: September 10, 2008

New photoactive material is obtained by chemisorption of carminic acid onto nanocrystalline titanium dioxide. Organic chromophore is anchored at the semiconductor surface via the neighboring carboxylate and phenolate groups; the anchorage is strengthened by formation of the six-membered chelate ring. Photoelectrochemical studies reveal strong photosensitization of the new material toward visible light: The photoelectrodes comprised of carminic acid-titanium dioxide hybrid material generate photocurrent within 300-650 nm window. Moreover, the direction of the photocurrent can be changed from anodic to cathodic and vice versa by application of appropriate photoelectrode potential, changes in electrolyte composition, and incident light wavelength. Geometry, electronic structure, and the photophysics of the neat chromophore and its model complex with TiIV oxo-species are studied in detail using the density functional theory method. Photoelectrochemical and spectroscopic studies supplemented with quantum-chemical modeling allow elucidation of photocurrent switching mechanism. The carminic acid-titanium dioxide material constitutes an efficient platform for light harvesting antennae, optoelectronic switches, and other optoelectronic devices. 1. Introduction Constantly growing demand for clean energy sources has turned the attention of the chemical community toward light harvesting and solar energy conversion. There are numerous approaches toward utilization of light in the production of energy and/or useful chemicals, but the most successful seems to be the photosensitized solar cells. These devices can be regarded as “biomimetic” or “bioinspired” because their principle of operation is closely related to photosynthetic reactions. Light absorbed by semiconducting electrodes generates couples of charge carriers: electrons and holes. They in turn are transferred to the conducting substrate (photovoltaics)1 or involved in secondary chemical reactions with the electrolyte components (photocatalysis).2 Various semiconducting materials have been used for construction of the photocells, but the most successful appears to be nanocrystalline titanium dioxide. It is cheap, nontoxic, chemically inert, and resistant toward photocorrosion, but because of a quite large bang gap (∼3.2 eV), it absorbs only UV light. Therefore, unmodified titanium dioxide-based devices cannot harvest more than 10% of total solar light flux.3 The efficiency of the photovoltaic device can be greatly enhanced by photosensitization toward visible light. One strategy consists of doping TiO2 with transition metal ions (V,4 Cr,5,6 Ni,7 Pt,7-11 etc.). This results in formation of intrabandgap electronic states close to the conduction or valence band edges, respectively, which in turn induces visible light absorption upon sub-band gap irradiation. This material usually suffers from photocorrosion and enhanced recombination of photogenerated charges. Recently, doping of TiO2 with main group elements like boron,12,13 carbon,14-16 nitrogen,17-21 sulfur,16,22 and others (Cl, Br,23 I,24,25 Pb26) has attracted significant interest and yields stable photocatalysts and photovoltaic materials.2 The most commonly used technique of TiO2 photosesitization involves surface modification. Photosensitization of UV absorbing semi* Corresponding author. E-mail: [email protected]. Phone: +48 12 663 2208. Fax: +48 12 634 0515.

conductors to the longer wavelengths of the electromagnetic spectrum can be achieved by electron injection from visible light absorbing molecules which are adsorbed (via formation of covalent or ionic bonds between semiconductor surface and chromophoric molecules) onto the surface of semiconducting material. This photosensitization protocol usually involves organic dyes (porphyrins,27-32 phtalocyanines,33 thiacarbocyanine dyes,34,35 natural dyes36-43), transition metal complexes,36,37,44-46 or low bandgap semiconductor quantum dots.47 This approach has been very successful in the case of the regenerative dye-sensitized solar cells (“Gra¨tzel cells”48). These systems show no electron delocalization between surface chromophore and semiconductor bands. Photoexcitation of these systems results in indirect electron injection from photoexcited surface chromophores into the conduction band according to the Sakata-Hashimoto-Hiramoto model (SHH).49,50 Conversely, some dyes and other surface species strongly interact with the TiO2 surface, and in these cases, strong electronic coupling is observed. In these cases, optical properties of surface chromophores are strongly perturbed by interaction with semiconductor, and new allowed optical transitions are observed. Cyanoferrates51-55 and catechols56-59 are standard examples of such systems. Photoexcitation results in direct electron injection from the surface species into the semiconductor conduction band.52,60,61 An inspiration of light harvesting molecules and antenna systems capable of titania photosensitization comes from various biosystems.62 There are numerous trials of solar cell construction which are based on biomolecules and supramolecular systems, for instance, chlorophylls,63,64 porphyrins,27-31,33 phtalocyanines,33 and other natural or bioinspired dyes.36-40,42,65 Hybrid materials incorporating biomolecules immobilized on conducting or semiconducting surfaces are unique systems combining collective properties of solids with structural diversity of molecules,66-68 which besides photosensitization show other unique electrochemical and catalytical properties.69-74 Strong and stable chromophores derived from anthraquinone framework

10.1021/jp804700d CCC: $40.75  2008 American Chemical Society Published on Web 11/11/2008

19132 J. Phys. Chem. C, Vol. 112, No. 48, 2008 CHART 1: Molecular Structure of Carminic Acid

(e.g., carminic acid) have not been applied as photosensitizers of wide band gap semiconductors. Furthermore, spectral and photophysical properties of carminic acid have been only briefly described.75,76 Carminic acid (Chart 1) is produced by cochineal insect (Dactylopius coccus) living on cacti from the genus Opuntia. This intensely red dye is widely used in the cosmetic industry and as a food colorant.77 Its identification code as food additive is E-120. Furthermore, it has found some applications in analytical chemistry for photometric determination of boron,78 beryllium,79 uranium,80 thorium,80 and osmium.81 It was also used recently for preparation of red silica glasses using sol-gel method.82 Because of its specific interactions with proteins,83 nucleic acids,84 and lipids,85 carminic acid also found numerous applications in biochemical studies. Structure of the carminic acid molecule suggests strong covalent binding via carboxylic and/or hydroxyl functionalities. Furthermore, this interaction may provide an efficient platform for electron delocalization and formation of surface charge-transfer complexes, which in turn may result in strong photosensitization and photocurrent switching effects. The latter phenomena may be in turn utilized in molecular information processing, which is the ultimate mission of chemistry.86,87 This paper reports the synthesis and properties of carminic acid-modified titanium dioxide and its photophysical and photoelectrochemical properties. 2. Experimental Section 2.1. Materials. TiO2 (Degussa P25, ca. 70% anatase, 30% rutile; 50 m2 g-1) was used to prepare porous electrodes. All other chemicals were supplied by Fluka and used as received. Twenty-five milligrams of carminic acid was dissolved in 5 mL of dimethylformamide (DMF). A small sample of TiO2 powder (150 mg) was suspended in the resulting solution and stirred for 5 min in ultrasonic bath. Modified TiO2 was removed by centrifugation and washed 3 times with DMF and then 3 times with distilled water. Upon drying in air, carminic acid-modified TiO2 was obtained as purple powder. This material was used for diffuse reflectance measurements and for all photoelectrochemical studies. The ITO-coated glass slides (∼2 cm2, Aldrich) were etched for at least 12 h in 15% NaOH solution, washed with distilled water and acetone, and air-dried. Modified semiconductor nanopowder (50 mg) was suspended in distilled water (1 mL) and sonicated for 60 s. An aliquot of resulting suspension (50 µL) was cast onto cleaned surface of ITO-coated glass slides (1.5 cm2). Upon drying in hot air, copper wire was attached to the ITO surface using copper adhesive tape (Elfa, Sweden), and the whole junction area was protected with water resistant self-adhesive insulation tape. Titanium dioxide nanoparticles were synthesized from hydrolyzed isopropyl orthotitanate. A mixture prepared from 2.5 cm3 of titanate ester and 45 cm3 of isopropyl alcohol was added dropwise over 1 h to 450 cm3 of diluted perchloric acid solution of pH ) 1 at 1 °C. Whitish colloidal solution was stirred overnight in an ice bath

Gaweˆda et al. yielding a transparent solution of TiO2 nanoparticles. Thus obtained titanium dioxide quantum dots were used for luminescence measurements and laser flash photolysis experiments. 2.2. Instrumentation. Absorption spectra were recorded on HP 8453 (Hewlett-Packard, U.S.A.) diode array spectrophotometer. Fluorescence spectra were recorded on LS45 (PerkinElmer, U.S.A.) instrument operating with 5 nm bandpass. Quantum yields were measured against erythrosin B reference (Φ ) 0.02).88 Spectroelectrochemical measurements were performed in a 2 mm quartz cell equipped with Pt-Ir minigrid working electrode (5% Ir), auxiliary electrode (Pt wire) and reference Ag/AgCl FLEXREF electrode (World Precision Instruments, U.S.A.); 0.1 M n-Bu4NBF4 in DMF was used as supporting electrolyte. Laser photolysis was performed using laser spectrometer LKS 50 (Applied Photophysics, U.K.) equipped with Nd:YAG laser Surelite Sl I-10 (Continuum, U.S.A.). Infrared spectra were recorded in KBr pellets on Fourier transform IR spectrometer IFS 48 (Bruker, Germany). The typical three-electrode setup was employed for electrochemical and photoelectrochemical measurements. Electrochemical measurements were performed in DMF solutions of 0.1 M tetrabutylammonium tetrafluoroborate as a supporting electrolyte. All of the photoelectrochemical experiments were performed in 0.1 M aqueous solution of KNO3 which was purged with oxygen or argon for at least 15 min prior to the measurement. Platinum and Ag/AgCl (3 M NaCl, E1/2 ) +209 mV vs NHE) were used as auxiliary and reference electrodes, respectively. A 150 W XBO lamp (Osram, Germany) equipped with water cooled housing and LPS 200 power supply (Photon Technology International, U.K.) was used as a light source. The working electrodes were irradiated from the backside (through the ITOglass) in order to minimize the influence of thickness of the semiconductor layer on the photocurrent. An automatically controlled monochromator and a shutter were applied to choose the appropriate energy of radiation. Dark electrochemical measurements were performed using platinum disk electrodes. The electrochemical measurements (CV, CV + chopped light, photocurrent action spectra) were controlled by a BAS 50W (Bioanalytical Instruments, U.S.A.) or M161 (MTM, Poland) electrochemical analyzers. Square wave voltammograms were recorded with the following parameters: potential step 4 mV, square wave amplitude 25 mV, square wave frequency 15 Hz. Photocurrent action spectra were not corrected for incident light intensity. Diffuse reflectance spectra were recorded on Lambda 15 (Perkin-Elmer, U.S.A.) spectrophotometer equipped with an integrating sphere of 5 cm diameter. Barium sulfate of spectral purity was used as a reference material. Conduction band edge potential was determined using modified Roy′s procedure.89 A 40 mg sample of semiconductor powder was suspended in 70 cm3 of 0.1 M aqueous potassium nitrate and sonicated for 5 min. Thirty milligrams of methylviologen bis(hexafluorophosphate) was added, and the resulting mixture was acidified with 1 cm3 of concentrated perchloric acid. Suspension was placed in rectangular glass vessel equipped with combined pH electrode, platinum foil electrode (area 2.5 cm2) and reference Ag/ AgCl electrode (FLEXREF, World Precision Instruments, U.S.A.). The vessel was vigorously purged with argon and irradiated with full light of an HBO 200 mercury high pressure lamp. The solution was titrated with 0.1 M solution of Na2CO3 using computer controlled infusion pump Medipan 610 B.S (Medipan, Poland) equipped with calibrated Hamilton syringes and a custom-built interface. Potential of the platinum electrode was measured using BM-811 digital multimeter (Brymen, Taiwan).

Photosensitization and Photocurrent Switching

J. Phys. Chem. C, Vol. 112, No. 48, 2008 19133

Figure 1. Molecular structure and electrostatic potential distribution of carminic acid in its ground state (a) and the change in electrostatic potential upon S1 excitation (b) together with the contours of the frontier molecular orbitals (c) as calculated on the B3PW91/6-311+G(d,p) level of theory.

The adsorption isotherm in the carminic acid-titanium dioxide system was determined using the procedure of Regazzoni and co-workers.90 Samples of 100 mg of TiO2 powder were suspended in DMF solutions of carminic acid (10 mL, 20-140 µmol dm-3). Samples were sonicated for 15 min, then stirred for additional 40 min at 298 K. Upon centrifugation (5 min, 5000 rpm) equilibrium concentration of carminic acid was determined by absorption measurement at 510 nm. The surface coverage Γ was calculated according to the following equation:

Γ)

V · (ci - ceq) S · mTiO2

(1)

where V is the volume of the suspension, S is the specific surface area (here 51 m2 g-1), mTiO2 is the mass of the semiconductor sample, ci is the initial concentration of carminic acid, and ceq is the equilibrium concentration of carminic acid. 2.3. Calculations. Theoretical modeling was performed with Gaussian 03 Rev. D.01 (Gaussian, Inc., U.S.A.)91 and ArgusLab 4.0.1 (Planaria Software, U.S.A.).92 Preliminary geometry optimization was done with molecular mechanics module93-96 using the UFF force field,97-100 while the final geometry was obtained using DFT method with B3PW91 functional and 6-311+G(d,p) basis set. Atomic charges were computed using NPO analysis. Molecular orbitals and surfaces were computed using the same theory level and tight convergence criteria. Electronic transitions were calculated using time-dependent DFT method with B3PW91 functional and 6-311+G(d,p) basis set. Vibrational frequencies were calculated using CAChe 7.5 package (Fujitsu, Japan, and FQS, Poland) with B88PW91 functional and DZVP basis set. 3. Results and Discussion 3.1. Structure and Photophysical Properties of Carminic Acid. Carminic acid (7-D-glucopyranosyl-3,5,6,8-tetrahydroxy1-methyl-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid, CA) is a red glucosidal hydroxyanthapurin dye naturally occurring in the cochineal insects. A CA molecule consists of two moieties: anthraquinone chromophore and glucopyranose ring connected via a glucoside bond. The glucopyranose ring is perpendicular to the anthraquinone planar framework (Figure 1), which is the chromophoric moiety and the metal-binding site. The frontier molecular orbitals are localized mostly within the anthraquinone moiety with only a small contribution from the sugar part. The HOMO-1 orbital (Figure 1) is delocalized over the A ring, two hydroxyl groups, and the sugar moiety.

Figure 2. Diffuse reflectance spectra of carminic acid (a) and carminic acid-titanium dioxide hybrid material (b). Gaussian components are shown along with the original spectra.

The HOMO orbital encompasses A and B rings and corresponding carbonyl and hydroxyl groups, while its density over the C ring is much smaller, which influences interactions of carminic acid with titanium dioxide (vide infra). The LUMO and LUMO+1 orbitals are delocalized over all three anthraquinonic rings and the carboxylic group. The carboxylic group bears small positive charge, while the sugar moiety is negatively charged (cf. Figure 1a). This electronic structure should induce large electric dipoles upon excitation thus resulting is high molar absorption coefficients. In the ground state, change distribution is rather uniform; a slight negative charge is observed within the sugar moiety, while the other terminus bears small positive charge. Low energy part of the absorption spectrum of carminic acid, both in acidified DMF solutions and in solid phase (deposited onto BaSO4 powder) shows intense asymmetric peak at 510 nm. Deconvolution reveals that this peak is composed from two broad Gaussian components at 18660 (∆ν1/2 ) 2970 cm-1) and 20 840 cm-1 (∆ν1/2 ) 4160 cm-1) (Figure 2a). This result is consistent with the TD-DFT calculations, which reveal the existence of two singlet excited states with similar energies. The lowest excited state involves electron transfer from HOMO to LUMO (20 410 cm-1), while the higher involves electron transfer from HOMO-1 to LUMO (22 995 cm-1) orbitals. Calculated energy difference between these states amounts 0.3 eV (2580 cm-1), while observed separation between two Gaussian components amounts 0.27 eV (2180 cm-1). Both transition energies and energy separation between two low energy excited states of carminic acid are in a relatively good agreement with the theoretical predictions. Analysis of the contour of the orbitals involved in these transitions (cf. Figure 1) indicates that the lowest energy excited state can be regarded as a local excited state (LE), while the higher one as the internal charge transfer state (ICT). Excitation of carminic acid at 510 nm results in weak asymmetric emission band centered at 590 nm (Φ ) 3.6 × 10-4), weakly resolved component at ∼630 nm can be also noticed. Excitation at 570 nm results in separation of the low energy component, while the intensity of the 590 nm component is much lower. Deconvolution of the emission spectrum also results in two distinct Gaussian components at 16 180 (∆ν1/2 ) 2140 cm-1) and 17 070 cm-1 (∆ν1/2 ) 1170 cm-1), which can be attributed to LE and ICT emissions,

19134 J. Phys. Chem. C, Vol. 112, No. 48, 2008

Gaweˆda et al.

Figure 4. Square wave voltammogram of carminic acid recorded at Pt disk electrode in DMF solution.

Figure 3. Fluorescence spectra of carminic acid (a) and carminic acid in the presence of TiO2 quantum dots (b) in DMF solutions acidified with HClO4. Gaussian components are shown along with the original spectra.

respectively (Figure 3a). Weak band at 14200 cm-1 can be attributed to the traces of carminic acid monoanion.84,101 The values derived above can be used for estimation of reorganization energy. In the simplest approach, the reorganization energy is related to the absorption and emission maxima and excited-state energy (∆E0) by eqs 2 and 3:102

hνabs max ) ∆E0 + χ

(2)

hνem max ) ∆E0 - χ

(3)

These yield total reorganization energy of 1240 and 1890 cm-1 and excited-state energies of 17 420 cm-1 (2.16 eV) and 18 960 cm-1 (2.35 eV) for LE and ICT excited states, respectively. These results clearly indicate that there are two different emissive excited states of carminic acid. Because of the character of molecular orbitals, the low energy state can be regarded as the local excited state (LE), while the higher can be regarded as the intramolecular charge transfer state (ICT). This assumption is confirmed by the difference in Stokes shifts observed for both transitions: 2480 cm-1 (0.31 eV) for LE and 3770 cm-1 (0.39 eV) for ICT state. Increased Stokes shift for high energy transition clearly indicates larger geometrical changes and significant reorganization of salvation sphere upon excitation, which is the consequence of the redustribution of electron density upon the excitation of carminic acid molecule. Similar photophysical phenomena were recently reported for isoquinoline N-oxide derivatives and utilized for molecular switching and information processing at molecular level.103 Quasi-reversible one-electron reduction of carminic acid in DMF solution proceeds at -0.73 V versus Ag/AgCl reference while oxidation of carminic acid takes place at +1.06 V versus Ag/AgCl. The difference between these two potentials constitutes a rough estimate of the HOMO-LUMO gap, as reduction process is localized at LUMO orbital and oxidation at HOMO. Electrochemically evaluated HOMO-LUMO gap amounts 1.79 eV (Figure 4). This figure is quite different from the value obtained from spectroscopic measurement (2.16 eV) and DFT calculations (2.59 eV). This indicates significant change in the

Figure 5. Photophysical diagram for singlet states of carminic acid (a) and energy levels of carminic acid derived by the TD-DFT calculations (b). Dashed potential energy curves correspond to the diabatic electronic states, while the solid lines correspond to the mixed (adiabatic) electronic states. Red and blue arrow represent the locally excited and charge transfer luminescence, respectively.

geometry upon excitation, large reorganizational energy, and probable involvement of proton exchange reaction upon redox process. This is also confirmed in large Stokes shifts of 2480 cm-1 (0.31 eV) and 3770 cm-1 (0.39 eV) for the first and the second excited state, respectively. On the basis of the spectroscopic investigation, DFT and TD-DFT calculations the photophysical diagram for carminic acid was constructed (Figure 5). Moreover, the charge distribution change upon excitation has been calculated. It can be clearly seen that the quinonoid ring B (cf Figure 1b) served as the electron acceptor, while the ring C is the electron donor. Dual emission observed in the case of carminic acid is a consequence of its peculiar electronic structure. Because of different spatial character of the S1 and S2 excited states, the internal conversion should be strongly hampered as it would require significant geometrical changes. This in turn results in high activation energy for S2fS1 transition. Therefore, dual emission is observed, as both S1 and S2 are luminescent. Differences in Stokes shifts for LE and ICT states clearly indicate the difference in geometrical changes upon excitation to the S1 and S2 states. These observations imply very weak configurational mixing between the S1 and the S2 excited states. Therefore, the adiabatic (mixed) state local minima are almost identical with their diabatic precursors. Similar energy of the S1 and S2 states and distinct difference in corresponding molecular structure allows resolution of emission components (cf. Figure 5a). Similar effects have been recently observed for MC and MLCT emissions of RuII 104,105 and CuI 106 complexes, MC and MMCT emissions of dinuclear chromium-ruthenium complexes,107-109 and for various organic chromophores.103,110-117 This behavior is usually observed in the systems with small energy separation between S1 and S2 excited states.118 This condition is fulfilled by carminic acid as shown in Figure 5b. Separation between calculated energies of S1 and S2 states amounts 0.3 eV. Furthermore, time-dependent DFT calculations exclude possibility of triplet state involvement in dual emission



Figure 7. Determination of optical indirect band gap of carminatemodified titanium dioxide.

CHART 2: Molecular Structure of the Model Compound, Dihydroxytitanium Carminate

Figure6. Adsorptionisoterms(a)andthecorrespondingLineweaver-Burk plots (b) for carminic acid (red circles) and salicylic acid (blue diamonds, based on data from ref 90) on the surface of titanium dioxide at 298 K.

of carminic acid, as T1 and T2 phosphorescence should be observed with the same probability upon S1 and S2 excitations (Figure 5b). 3.2. Interaction of Carminic Acid with Titanium Dioxide. Impregnation of TiO2 powders with DMF solutions of carminic acid results in stable purple colored materials. Binding of carminic acid to the surface of TiO2 is very strong, and resulted materials are stable in contact with water, acetic acid, and DMF. Slow hydrolysis and CA desorption can be observed only in alkaline solutions (pH > 10). The surface binding of carminic acid should strongly resemble binding of salicylic acid to the TiO2 surfaces, which is stabilized via formation of chelate ring and the modifier is bound as a dianion.90,119,120 As in the case of salicylic acid, one should expect formation of a new absorption band resulting from LMCT (salicylate f TiIV) charge transfer.120,121 Surprisingly, the chemisorption of carminic acid onto titanium dioxide only insignificantly changes the spectrum of the organic chromophore (Figure 2b). The small bathochromic shift (510 nm for pure CA, 538 nm for CA@TiO2 material) is observed only in the case of large crystallites; chemisorption of CA onto TiO2 quantum dots does not change its absorption spectrum at all. Adsorption of carminic acid from DMF solution onto the surface of titanium dioxide follows the Langmuir isotherm (Figure 6a), while salicylic acid adsorption is best described by high affinity multisite isotherm.90 Fitting of the LineweaverBurk equation (eq 4) to the experimental data allows calculation of maximal coverage and adsorption equilibrium constants (Figure 6b).

1 1 1 + ) Γ Γmax Γmax · K · ceq

(4)

The maximal coverage (Γmax) for carminic acid reaches 0.65 ( 0.02 µmol m-2, which is somewhat lower than that of salicylic acid (∼14.7 µmol m-2). These figures correspond to average

packing of 0.4 and 8.4 molecules per nm2 for carminic acid and salicylic acid, respectively. This is consistent with a much larger size of CA as compared with salicylic acid; specifically, the bulky sugar moiety may cause strong steric hindrance. The carminic acid Langmuir binding constant is relatively large (2.5 × 10-2), while salicylic acid is bound only weakly (K ) 2.3 × 10-4). This explains very high stability of the carminatemodified titania against hydrolysis. Large area occupied by single carminic acid molecule substantiates the lack of any excitonic interactions122 in contrary to the folic acid-TiO2 system, where formation of J-aggregates was observed.65 The structure of the main absorption band remains unchanged; the two Gaussian components are observed at 18 030 (∆ν1/2 ) 3100 cm-1) and 20 610 cm-1 (∆ν1/2 ) 4730 cm-1). The most significant difference consists of a significant increase of the high energy bandwidth (4730 vs 4160 cm-1 for pure carminic acid). Much more significant changes are observed in fluorescence spectra (Figure 3b). The luminescence intensity is significantly decreased (Φ ) 2.2 × 10-4 vs 3.6 × 10-4 for neat carminic acid). The high energy component is completely quenched, but the low energy component retains most of its intensity and is bathochromically shifted to 15 780 cm-1 (∆ν1/2 ) 1940 cm-1). The quenching of the ICT component may be a result of photoinduced electron transfer from the excited chromophore to the conduction band of the semiconductor. On the basis of spectroscopic data presented above, the value of the reorganization energy associated with the LE state equals 1125 cm-1, and the energy of the LE state is 16905 cm-1. Because of complete quenching of the ICT luminescence, it is impossible to calculate directly the χ and ∆E0 values. It can be assumed, however, that the reorganization energy is not changing significantly upon binding to the TiO2 surface. On the basis of this assumption, the reorganization energy for the ICT state should approximate 1900 cm-1, which in turn implies the ICT energy equals 18 700 cm-1. In contrary to the cyanoferrate-based semiconducting systems,52 deposition of carminic acid onto titanium dioxide powders does not change the band gap energy. Optically determined indirect band gap energy amounts 3.25 eV for both neat and modified samples, which is in perfect agreement with a previously reported value (Figure 7).52


Gaweˆda et al.

Figure 8. Molecular structure and contours of the frontier molecular orbitals of carminic acid dianion as calculated on the B3PW91/ 6-311+G(d,p) level of theory. Figure 10. Determination of pH0 for neat TiO2 (squares) and CAmodified TiO2 (triangles).

Figure 9. Molecular structure and electrostatic potential distribution of dihydroxytitanium carminate model complex in its ground state (a) and the change in electrostatic potential upon S2 excitation (b) together with the contours of the frontier molecular orbitals (c) as calculated on the B3PW91/6-311+G(d,p) level of theory.

In order to elucidate the details of interactions between carminic acid and titanium dioxide, a simple molecular model has been constructed (Chart 2). It was shown recently than complexes containing di- and trihydroxytitanium(IV) fragments are good models of the semiconductor-surface molecule interactions.51,52,123-125 As only minor perturbation of spectroscopic properties of CA on binding to TiO2 surface are observed, it can be assumed that the TiIV ion is bound to the carboxylic and hydroxy groups thus forming the six-membered ring. This binding mode was confirmed by DFT calculations on carminate dianion (Figure 8). The HOMO-1 and HOMO orbitals are localized mainly on the C ring with significant contribution of hydroxylate and carboxylate groups. On the basis of TD-DFT calculations in the model system, the electronic transitions of the CA@TiO2 (Figure 9c) can be assigned to LE and ICT states, respectively (vide supra), similar to that of neat carminic acid. The calculated transition energies amount 20 260 and 22 880 cm-1 for the LE and ICT transition, respectively. Measured separation of these two excited states amounts 2580 cm-1, while the calculated difference amounts 2620 cm-1, which is in perfect agreement with experimental value. This result indicates a slight increase in energetic separation of these two excited states upon interaction of carminic acid with titanium dioxide. The only difference consists of a small contribution of titanium d orbitals in LUMO orbital of the model complex. In contrast to various transition metal complexes51-55,61,123-127 and enediol ligands58,59,61,128-132 deposited onto titanium dioxide, the carminic acid-titanium dioxide system exhibits only weak electronic coupling between the organic chromophore and the semiconductor particle. This observation is supported by optical spectroscopy: no charge transfer bands are observed upon complexation of titanium dioxide with carminic acid. Efficient fluorescence quenching, in turn, presumably proceeds via the photoinduced electron transfer pathway. Interestingly, charge distribution in the ground (Figure 9a) and excited state (Figure 9b) of titanium carminate

model complex are much the same as calculated for neat carminic acid. There is no evidence for direct photoinduced electron transfer between the anthraquinone chromophore and titanium ions, which is consistent with spectroscopic observations. 3.3. Photoelectrochemical Properties of CA-TiO2 Material. Photoelectrochemical applications require a detailed knowledge of the positions of semiconductors band edges since this gives information on the reductive and oxidative power of photogenerated electrons and holes, respectively. These data can be obtained from the quasi-Fermi level measurements 89,133 (i.e., the Fermi level potential measured for a semiconducting material under irradiation, EqF), which together with the spectral characteristics of the synthesized materials allow the determination of the redox properties of the excited semiconducting materials. The modified method of the quasi-Fermi level determination,133 described by Roy89 et al., was applied. This method is based on the pH-dependency of the valence- and conduction-band edge potentials of the semiconductor particles. In the case of titanium dioxide with increasing pH, the cathodic shift of EqF by 59 mV per one pH unit was reported (k ) 0.059 V, eq 5).

ECBE(pH) ) E ° + k(pH0- pH)

(5)

In combination with a pH-independent reversible redox pair (here 1,1′-dimethyl-4,4′-bipyridinium dichloride, MV2+, EMV2+/+ · ) -0,45V vs NHE), the electrons from the conduction band of the irradiated semiconductor may or may not reduce the redox couple, depending on the pH value and relative potentials. The method of Roy yields the pH0 values for TiO2 and carminic acid modified TiO2 of 4.50 and 6.55, respectively. In neutral solution (pH ) 7), it results in the CB edge potential equal to -0.58 and -0.47 V for neat and modified TiO2, respectively (Figure 10). Modification of the surface of titanium dioxide with carminic acid results in a significant anodic shift of the conduction band edge potentials. This observation is consistent with covalent modification of the TiO2 surface.52,65,125,134,135 Unmodified titanium dioxide generates anodic photocurrent within a wide potential window upon UV illumination. At very high negative polarization, cathodic photocurrent is observed because of the reduction of some surface TiIV to TiIII. CA-modified titanium dioxide is a suitable material for the preparation of photoelectrodes using ITO glass as conducting support. The material is photostable, and no photodecomposition of the surface complex was observed, even after prolonged irradiation with ultaviolet or visible light. Photocurrent generation by transparent ITO electrodes covered with CA@TiO2 was investigated as a function of the incident-light wavelength at various electrode potentials in the absence and presence of molecular oxygen in aqueous electrolytes at pH ) 7. To have a full overview of photocurrent generation as a function of applied potential and irradiation wavelength, a set


Figure 11. Photocurrent action spectra recorded at CA@TiO2 photoelectrodes as a function of electrode potential in the absence (a) and the presence of oxygen (b).

of action spectra was collected at constant potentials (at every 50 mV). In this way, three-dimensional photocurrent maps were obtained (Figure 11). In the absence of oxygen, generation of anodic photocurrents at positive polarization is observed while negative photoelectrode polarization results in cathodic photocurrents of low intensity (Figure 11a). A good photosensitization is observed between 400 and 580 nm at cathodic polarization, while it is much less efficient at anodic polarization of the photoelectrode (400-450 nm). In the presence of oxygen, cathodic photocurrents of high intensity are observed within the absorption spectrum of the semiconductor (300-400 nm), and cathodic photocurrents of lower intensity are observed within the absorption band of the surface chromophore (400-600 nm). In air-equilibrated and oxygen saturated solutions, photoelectrode generates anodic photocurrents only at positive polarization of the photoelectrode (Figure 11b), while at negative polarization, high intensity cathodic photocurrents are observed (Figure 11b) It can be concluded that cathodic photocurrents are generated because of an efficient reduction of molecular oxygen present in the electrolyte solution. The cathodic photocurrent intensities are higher than anodic ones. The phenomenon of photocurrent polarization switching upon changes of photoelectrode potential and incident light wavelength can be a platform for construction of various logic devices.123,135 Furthermore, at low oxygen concentration, the potential-induced photocurrent switching does not proceed as sharply as that in oxygen saturated solutions. This results in potential-dependent wavelength switching, which is especially beneficial in the context of optoelectronic applications.136,137 Addition of iodide (0.01 M) to the electrolyte solution results in increase of photocurrent intensities both in anodic and cathodic regimes. Furthermore, the efficiency of photosensitization is greatly improved. Final confirmation of the electron injection mechanism was found using laser flash photolysis. Excitation of CA@TiO2 quantum dots suspended in DMF with 532 nm light results in immediate (within laser pulse, ∼7 ns) decrease in absorbance at 520 nm and increase at 400 and 620 nm (Figure 12). The former can be associated with oxidation of carminate anions


Figure 12. Transient differential absorption spectra recorded for carminic acid-modified titanium dioxide quantum dots upon 532 nm excitation in DMF. The first spectrum is recorded 1 µs, while subsequent spectra are recorded at 2, 5, 12, 42, and 90 µs upon laser pulse, respectively (a). Differential absorption spectrum recorded during electrochemical reduction of carminic acid in the presence of TiO2 particles at 1100 mV vs Ag/AgCl (b).

TABLE 1: Rate Constants Associated with Transient Species Photogenerated in the Carminic Acid-Titanium Dioxide System k/105 s-1 wavelength/nm

Ar

O2

420 520 620

3.7 2.3 9.4; 1.2

5.5 4.4 13.0; 1.8

deposited onto the TiO2 surface, while the latter can be associated with carminate cation radical (420 mn) and free electrons within the conduction band (600-700 nm).138-140 The 420 and 520 nm signals decay monoexponentially with rate constants of 105 s-1 order of magnitude (Table 1), while the 620 nm transient absorption decays biexponentially (Table 1). Upon oxygenation of the solutions, the rates associated with carminic acid increase by a factor of 2, while the decay of trapped electrons is only slightly accelerated. Quantitatively analogous spectral changes were observed during spetroelectrochemical measurement. Oxidation of both neat carminic acid and carminic acid deposited onto TiO2 nanoparticles results in decrease of CA absorption band intensity, while absorption within 400-450 nm slightly increases. Therefore, the positive absorption change around 420 nm can be attributed to formation of carminic acid cation radical (Figure 12b). All of the above observations allow elucidation of the photocurrent switching mechanism. Irradiation of photoelectrodes prepared from CA@TiO2 hybrid material results in generation of photocurrent in which the characteristics are very different from those recorded for neat titanium dioxide.135 In air-equilibrated and oxygen saturated solutions, the photoelectrode generates anodic photocurrents only at positive polarization of the photoelectrode (Figure 11a). Negative polarization of the photoelectrode results in reversal of the photocurrent direction. Interestingly, the cathodic photocurrent intensities are higher than the anodic ones. Such anomalous behavior was previously observed for cyanoferrate modified TiO2,51 ferrocene-TiO2


Gaweˆda et al.

Figure 13. Latimer-type diagrams of oxidation and reduction potentials of the ground and two lowest excited states of the carminic acid molecule in DMF solution (a) and on the surface of titanium dioxide (b). All of the potentials are referenced to the NHE.

composites,52 and other surface-modified semiconductors,65 called the PEPS effect (photoelectrochemical photocurrent switching).135,136 Redox potentials of carminic acid in the first excited state can be calculated from ground-state redox potentials and energy of the excited state as follows (Figure 13):141-144

E1⁄2(D+ ⁄ D*) ) E1⁄2(D+ ⁄ D) - E0,0

(6)

E1⁄2(*A ⁄ A-) ) E1⁄2(A ⁄ A-) + E0,0

(7)

where E0,0 is the energy of the excited state of interest. These data, together with previously determined semiconductor band gap and conduction band edge potential, are the basis for the energy diagram of the studied system. It can easily be noticed that carminic acid in the excited state is a good electron donor (-0.82 to -1.05 V vs NHE) and a good electron acceptor (1.57 - 1.80 V vs NHE). The excited state of carminic acid is a stronger reducing agent than the conduction band electrons (-0.47 V) of the surface modified TiO2 (vide supra). On the other hand, photogenerated holes in the valence band are the strongest oxidants in the system (2.78 V, Figure 13). The free enthalpy of the electron transfer process between the photoexcited carminic acid and the conduction band on the semiconductor for LE and ICT states can be formulated as follows (eqs 8 and 9):

∆GLE ) ECA+⁄CA - ECBE - ∆ELE 0 + ECoul

(8)

∆GICT ) ECA+⁄CA - ECBE - ∆EICT 0 + ECoul

(9)

where ECoul is the energy of Coulomb interaction between charged products of the electron transfer reaction at the distance r according to the Rehm-Weller theory (eq 10):145-147

ECoul )

e2 4πεε0r

(10)

The value of the Coulombic component was estimated as 400-1730 cm-1 (0.05-0.21 eV), depending on the assigned donor-acceptor distance (780 pm from the center of the CA molecule to the TiO2 surface, but only 180 pm from the phenolate oxygen atom to the neatest titanium atom). Thus, the ∆GLE falls within -0.14 to -0.30 eV, while the ∆GICT falls within -0.37 to -0.53 eV. This rough estimation shows that

the thermodynamic driving force for the electron transfer process of almost two-fold larger for the ICT excited state as compared with the LE excited state. There can be other factors not taken into account (e.g., high frequency vibrational modes)102 which may render the LE-induced process thermodynamically disfavored. The most important vibration to be taken into account seems to be the breathing mode of the anthraquinone moiety.148 Its energy according to DFT calculations should amount ∼1700 cm-1. In fact, IR spectrum of carminic acid exhibits a very intense peak at 1695 cm-1. Therefore, if a high frequency vibrational contribution is taken into account (∼1700 cm-1), both energies (eqs 7 and 8) should increase by ∼0.21 eV. In this situation, the ICT-induced electron injection is still possible (∆G ≈ -0.16 to -0.32 eV); the LE-induced process may already be thermodynamically disfavored (∆G ≈ +0.07 to -0.09 eV). This estimation, although very rough, justifies the peculiar luminescent and photoelectrochemical properties of the CA-TiO2 material and supports the suggested photosensitization mechanism. Excitation of the electrode within the absorption of the semiconductor at positive polarization of the photoelectrode results in photoanodic response according to the generally accepted mechanism (Figure 14a). Electron is promoted to the conduction band of the semiconductor and is subsequently transferred to the conducting electrode. The hole in the valence band is in turn neutralized by the electron from oxidation of the solvent molecule (H+,OH•/H2O, 2.31 V at pH ) 7)149 or added redox mediator (here iodide/triiodide couple, 0.535 V vs NHE).150 Excitation within the carminic acid absorption band results in similar processes. The excited state of carminic acid, which is a good electron donor, can inject an electron into the electrode, most probably via the conduction band of the semiconducting particle. The CA* state cannot oxidize free water molecules directly, but formation of a hydrogen bond network may support some oxidation processes, which result in low intensity photocurrents, that is, very inefficient photosensitization. Upon addition of iodide, the CA* state can be easily reduced, which results in much higher photocurrents (Figure 14b). This process can occur only at positive polarization of the electrode; otherwise, the electrostatic barrier would prevent interfacial electron transfer and generation of anodic photocurrents. At negative polarization of the photoelectrode, cathodic photocurrents are observed, but only in the presence of efficient electron acceptor, for example, molecular oxygen or triiodide. Neat titanium dioxide photoelectrodes almost do not yield cathodic photocurrents. It is possible only upon doping,151 surface modification,51,135,136 or at high pH.152 The potential difference between the potential of the electron in conduction band and that of the O2-/O2 couple is small, so there is no much thermodynamic driving force for this reaction. Moreover interfacial electron transfer between titanium dioxide surface and molecular oxygen strongly depends on surface adsorption of O2. At neat TiO2 surfaces, recombination is dominating over the ET with dissolved oxygen. The presence of carminic acid on the surface facilitates oxygen reduction due to facilitated electron transfer resulting from the reactivity of carminate anion radicals. A hole within the valence band can easily be neutralized by an electron from the carminic acid molecule, which subsequently is reduced electrochemically. As recombination is hampered this way, the only possible process for an electron in the conduction band is to reduce the oxygen molecule either directly or via the ET process involving the carminic acid molecule (Figure 14c). This is substantiated by much lower



Figure 14. Mechanism of photocurrent generation (the PEPS effect): anodic photocurrent is generated at positive potentials upon excitation of the inner part of the TiO2 particle (a) and surface carminic acid (b), while cathodic photocurrents are generated at negative potentials upon excitation of TiO2 (c) or carminic acid (d). Shaded vertical bar indicates the photoelectrode potential. The LE excited states are omitted for the sake of clarity.

cathodic photocurrent intensities as compared with anodic ones. The excited state of carminic acid is a much stronger electron donor (-1.05 V), so it can very easily reduce oxygen; thus generated carminic acid cation radical can be easily reduced electrochemically (Figure 14d). The strongly reducing excited state can be generated either directly (via excitation of surface CA molecules) or indirectly via energy transfer from excited semiconducting support (via electron-hole recombination). The latter process yields even higher photocurrent intensities as compared with direct excitation, due to high absorptivity of titanium dioxide as compared with monomolecular layer of carminic acid. Triiodide can also act as an efficient electron acceptor (Figure 14c,d). The photocurrent switching mechanism is based mainly on the electron donor and electron acceptor character of carminic acid molecule. Depending on photoelectrode potential, it may contribute in anodic and cathodic photocurrent generation, which is a quite unique feature among photosensitizers. Other words, the PEPS effect in the CA@TiO2 system is a direct consequence of carminic acid electrochemical amphotericity. Such electrochemically amphoteric molecules which offer stable reduced and oxidized forms within a feasible potential window are perfectly suited for molecular electronics applications.153,154 4. Conclusions and Outlook Hybrid material obtained by immobilization of carminic acid onto the surface of nanocrystalline titanium dioxide has interesting photoelectrochemical properties. It exhibits pronounced photosensitization toward visible light. Photoelectrodes built from CA@TiO2 generate photocurrent within the 300-650 nm window. The polarity of the photocurrent can be changed via a change in photoelectrode potential. The mechanism of this process was described in terms of photoinduced electron transfer involving both carminic acid molecule and titanium dioxide support. Carminic acid acts here as an electron buffer, donating or accepting electrons if necessary. Spectroscopic investigations and DFT calculations are in favor of the Sakata-HiramotoHashimoto model.49,50 The photocurrent switching observed here is closely related to PEPS effect (photoelectrochemical photocurrent switching) observed in the other system, folic acid immobilized onto the TiO2 surface.65 Similar to other switchable photoelectrochemical systems,52,65,123-125,135,155-159 the CA@TiO2 hybrid material can be applied for construction of optoelectronic switches, logic gates, and other functional devices. Incident light wavelength and photoelectrode potential can be regarded as input channels, while photogenerated currents can be regarded as output channels. In this regime, various devices as simple switches, logic gates, and more complex combinatorial circuits can be

implemented without any obstacles.123,125 Furthermore, because of the ability of carminic acid to form a series of hydrogen bonds with a wide variety of substrates, the CA@TiO2 system can be a platform for various photoelectrochemical sensors and other devices. Because of catalytic activity toward photoreduction of molecular oxygen, carminic acid-titanium dioxide photoelectrodes may also be a good starting point for development photofuel cells69 and as a model system to study interfacial electron transfer processes in nanoscale.160-162 Acknowledgment. Authors thank Mr. Andrzej Karocki for assistance during laser flash photolysis experiments and Dr. £ukasz Orzeł for valuable discussions on luminescence spectroscopy. This work was supported by Polish Ministry of Education and Science (Grants PB1283/T09/2005/29 and PBZKBN-118/T09/8). DFT calculations were performed in the Academic Computer Centre CYFRONET AGH within computational Grant MEiN/SGI3700/UJ/085/2006. References and Notes (1) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (2) Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2007, 111, 5259. (3) Thompson, T. L., Jr. Chem. ReV. 2006, 106, 4428. (4) Martin, S. T.; Morrison, C. L.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13695. (5) Radecka, M.; Wierzbicka, M.; Komornicki, S.; Rekas, M. Physica B 2004, 348, 160. (6) Dvoranova, D.; Brezova, V.; Mazur, M.; Malati, M. A. Appl. Catal. B: EnViron. 2002, 37, 91. (7) Zang, L.; Macyk, W.; Lange, C.; Maier, W. F.; Antonius, C.; Meissner, D.; Kisch, H. Chem. Eur. J. 2000, 6, 379. (8) Macyk, W.; Kisch, H. Chem. Eur. J. 2001, 7, 1862. (9) Kisch, H.; Burgeth, G.; Macyk, W. AdV. Inorg. Chem. 2004, 56, 241. (10) Burgeth, G.; Kisch, H. Coord. Chem. ReV. 2002, 230, 40. (11) Macyk, W.; Burgeth, G.; Kisch, H. Photochem. Photobiol. Sci. 2003, 2, 322. (12) Lu, N.; Quan, X.; Li, J.; Chen, S.; Yu, H.; Chen, G. H. J. Phys. Chem. C 2007, 111, 11836. (13) Reyes-Garcia, E. A.; Sun, Y.; Raftery, D. J. Phys. Chem. C 2007, 111, 17146. (14) Lettmann, C.; Hildenbrand, K.; Kisch, H.; Macyk, W.; Maier, W. F. Appl. Catal. B: EnViron. 2001, 32, 215. (15) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (16) Ohno, T.; Tsubota, T.; Toyofuku, M.; Inaba, R. Catal. Lett. 2004, 98, 255. (17) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (18) Morikawa, T.; Asahi, R.; Ohwaki, T.; Aoki, K.; Taga, Y. Jpn. J. Appl. Phys. 2001, 40, L561. (19) Sakthivel, S.; Kisch, H. ChemPhysChem 2003, 4, 487. (20) Silveyra, R.; De La Torre Saeńz, L.; Antuńez Flores, W.; Martıńez, V. C.; Elgue´zabal, A. A. Catal. Today 2005, 107-108, 602. (21) Kisch, H.; Sakthivel, S.; Janczarek, M.; Mitoraj, D. J. Phys. Chem. C 2007, 111, 11445. (22) Yu, J. C.; Ho, W.; Yu, J.; Yip, H.; Wong, P. K.; Zhao, J. EnViron. Sci. Technol. 2005, 39, 1175.

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Photosensitization and Photocurrent Switching in Carminic Acid

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