Photosensitization and Photocurrent Switching in Carminic Acid/Titanium Dioxide Hybrid Material

Photosensitization and Photocurrent Switching in Carminic Acid/Titanium Dioxide HybridMaterial

Sylwia Gaweda, Grazyna Stochel, and Konrad Szaciłowski*Uniwersytet Jagiellonski, Wydział Chemii, ul. Romana Ingardena 3, 30-060 Krakow, 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 phenolategroups; the anchorage is strengthened by formation of the six-membered chelate ring. Photoelectrochemicalstudies reveal strong photosensitization of the new material toward visible light: The photoelectrodes comprisedof 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 byapplication of appropriate photoelectrode potential, changes in electrolyte composition, and incident lightwavelength. Geometry, electronic structure, and the photophysics of the neat chromophore and its modelcomplex with TiIV oxo-species are studied in detail using the density functional theory method. Photoelec-trochemical and spectroscopic studies supplemented with quantum-chemical modeling allow elucidation ofphotocurrent switching mechanism. The carminic acid-titanium dioxide material constitutes an efficientplatform for light harvesting antennae, optoelectronic switches, and other optoelectronic devices.

1. Introduction

Constantly growing demand for clean energy sources hasturned the attention of the chemical community toward lightharvesting and solar energy conversion. There are numerousapproaches toward utilization of light in the production of energyand/or useful chemicals, but the most successful seems to bethe photosensitized solar cells. These devices can be regardedas “biomimetic” or “bioinspired” because their principle ofoperation is closely related to photosynthetic reactions. Lightabsorbed by semiconducting electrodes generates couples ofcharge carriers: electrons and holes. They in turn are transferredto the conducting substrate (photovoltaics)1 or involved insecondary chemical reactions with the electrolyte components(photocatalysis).2 Various semiconducting materials have beenused for construction of the photocells, but the most successfulappears 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 absorbsonly UV light. Therefore, unmodified titanium dioxide-baseddevices cannot harvest more than 10% of total solar light flux.3

The efficiency of the photovoltaic device can be greatlyenhanced by photosensitization toward visible light. One strategyconsists of doping TiO2 with transition metal ions (V,4 Cr,5,6

Ni,7 Pt,7-11 etc.). This results in formation of intrabandgapelectronic states close to the conduction or valence band edges,respectively, which in turn induces visible light absorption uponsub-band gap irradiation. This material usually suffers fromphotocorrosion and enhanced recombination of photogeneratedcharges. Recently, doping of TiO2 with main group elementslike 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 yieldsstable photocatalysts and photovoltaic materials.2 The mostcommonly used technique of TiO2 photosesitization involvessurface modification. Photosensitization of UV absorbing semi-

conductors to the longer wavelengths of the electromagneticspectrum can be achieved by electron injection from visible lightabsorbing molecules which are adsorbed (via formation ofcovalent or ionic bonds between semiconductor surface andchromophoric molecules) onto the surface of semiconductingmaterial. This photosensitization protocol usually involvesorganic dyes (porphyrins,27-32 phtalocyanines,33 thiacarbo-cyanine dyes,34,35 natural dyes36-43), transition metalcomplexes,36,37,44-46 or low bandgap semiconductor quantumdots.47 This approach has been very successful in the case ofthe regenerative dye-sensitized solar cells (“Gratzel cells”48).These systems show no electron delocalization between surfacechromophore and semiconductor bands. Photoexcitation of thesesystems results in indirect electron injection from photoexcitedsurface chromophores into the conduction band according tothe Sakata-Hashimoto-Hiramoto model (SHH).49,50 Con-versely, some dyes and other surface species strongly interactwith the TiO2 surface, and in these cases, strong electroniccoupling is observed. In these cases, optical properties of surfacechromophores are strongly perturbed by interaction with semi-conductor, and new allowed optical transitions are observed.Cyanoferrates51-55 and catechols56-59 are standard examples ofsuch systems. Photoexcitation results in direct electron injectionfrom the surface species into the semiconductor conductionband.52,60,61

An inspiration of light harvesting molecules and antennasystems capable of titania photosensitization comes from variousbiosystems.62 There are numerous trials of solar cell constructionwhich are based on biomolecules and supramolecular systems,for instance, chlorophylls,63,64 porphyrins,27-31,33 phtalocya-nines,33 and other natural or bioinspired dyes.36-40,42,65 Hybridmaterials incorporating biomolecules immobilized on conductingor semiconducting surfaces are unique systems combiningcollective properties of solids with structural diversity ofmolecules,66-68 which besides photosensitization show otherunique electrochemical and catalytical properties.69-74 Strongand stable chromophores derived from anthraquinone framework

* Corresponding author. E-mail: [email protected]. Phone: +4812 663 2208. Fax: +48 12 634 0515.

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

10.1021/jp804700d CCC: $40.75 2008 American Chemical SocietyPublished on Web 11/11/2008

(e.g., carminic acid) have not been applied as photosensitizersof wide band gap semiconductors. Furthermore, spectral andphotophysical properties of carminic acid have been only brieflydescribed.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 industryand as a food colorant.77 Its identification code as food additiveis E-120. Furthermore, it has found some applications inanalytical chemistry for photometric determination of boron,78

beryllium,79 uranium,80 thorium,80 and osmium.81 It was alsoused recently for preparation of red silica glasses using sol-gelmethod.82 Because of its specific interactions with proteins,83

nucleic acids,84 and lipids,85 carminic acid also found numerousapplications in biochemical studies. Structure of the carminicacid molecule suggests strong covalent binding via carboxylicand/or hydroxyl functionalities. Furthermore, this interactionmay provide an efficient platform for electron delocalizationand formation of surface charge-transfer complexes, which inturn may result in strong photosensitization and photocurrentswitching effects. The latter phenomena may be in turn utilizedin molecular information processing, which is the ultimatemission of chemistry.86,87

This paper reports the synthesis and properties of carminicacid-modified titanium dioxide and its photophysical andphotoelectrochemical 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. Allother chemicals were supplied by Fluka and used as received.Twenty-five milligrams of carminic acid was dissolved in 5 mLof dimethylformamide (DMF). A small sample of TiO2 powder(150 mg) was suspended in the resulting solution and stirredfor 5 min in ultrasonic bath. Modified TiO2 was removed bycentrifugation and washed 3 times with DMF and then 3 timeswith distilled water. Upon drying in air, carminic acid-modifiedTiO2 was obtained as purple powder. This material was usedfor diffuse reflectance measurements and for all photoelectro-chemical studies. The ITO-coated glass slides (∼2 cm2, Aldrich)were etched for at least 12 h in 15% NaOH solution, washedwith distilled water and acetone, and air-dried. Modifiedsemiconductor nanopowder (50 mg) was suspended in distilledwater (1 mL) and sonicated for 60 s. An aliquot of resultingsuspension (50 µL) was cast onto cleaned surface of ITO-coatedglass slides (1.5 cm2). Upon drying in hot air, copper wire wasattached to the ITO surface using copper adhesive tape (Elfa,Sweden), and the whole junction area was protected with waterresistant self-adhesive insulation tape. Titanium dioxide nano-particles were synthesized from hydrolyzed isopropyl orthoti-tanate. A mixture prepared from 2.5 cm3 of titanate ester and45 cm3 of isopropyl alcohol was added dropwise over 1 h to450 cm3 of diluted perchloric acid solution of pH ) 1 at 1 °C.Whitish colloidal solution was stirred overnight in an ice bath

yielding a transparent solution of TiO2 nanoparticles. Thusobtained titanium dioxide quantum dots were used for lumi-nescence measurements and laser flash photolysis experiments.

2.2. Instrumentation. Absorption spectra were recorded onHP 8453 (Hewlett-Packard, U.S.A.) diode array spectropho-tometer. Fluorescence spectra were recorded on LS45 (Perkin-Elmer, U.S.A.) instrument operating with 5 nm bandpass.Quantum yields were measured against erythrosin B reference(Φ ) 0.02).88 Spectroelectrochemical measurements wereperformed in a 2 mm quartz cell equipped with Pt-Ir minigridworking electrode (5% Ir), auxiliary electrode (Pt wire) andreference Ag/AgCl FLEXREF electrode (World PrecisionInstruments, U.S.A.); 0.1 M n-Bu4NBF4 in DMF was used assupporting electrolyte. Laser photolysis was performed usinglaser 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 Fouriertransform IR spectrometer IFS 48 (Bruker, Germany). Thetypical three-electrode setup was employed for electrochemicaland photoelectrochemical measurements. Electrochemical mea-surements were performed in DMF solutions of 0.1 M tetrabu-tylammonium tetrafluoroborate as a supporting electrolyte. Allof the photoelectrochemical experiments were performed in 0.1M aqueous solution of KNO3 which was purged with oxygenor argon for at least 15 min prior to the measurement. Platinumand Ag/AgCl (3 M NaCl, E1/2 ) +209 mV vs NHE) were usedas auxiliary and reference electrodes, respectively. A 150 WXBO lamp (Osram, Germany) equipped with water cooledhousing and LPS 200 power supply (Photon TechnologyInternational, U.K.) was used as a light source. The workingelectrodes were irradiated from the backside (through the ITO-glass) in order to minimize the influence of thickness of thesemiconductor layer on the photocurrent. An automaticallycontrolled monochromator and a shutter were applied to choosethe appropriate energy of radiation. Dark electrochemicalmeasurements 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 wererecorded 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 lightintensity. Diffuse reflectance spectra were recorded on Lambda15 (Perkin-Elmer, U.S.A.) spectrophotometer equipped with anintegrating sphere of 5 cm diameter. Barium sulfate of spectralpurity was used as a reference material. Conduction band edgepotential was determined using modified Roy′s procedure.89 A40 mg sample of semiconductor powder was suspended in 70cm3 of 0.1 M aqueous potassium nitrate and sonicated for 5min. Thirty milligrams of methylviologen bis(hexafluorophos-phate) was added, and the resulting mixture was acidified with1 cm3 of concentrated perchloric acid. Suspension was placedin rectangular glass vessel equipped with combined pH elec-trode, 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 andirradiated with full light of an HBO 200 mercury high pressurelamp. 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 syringesand a custom-built interface. Potential of the platinum electrodewas measured using BM-811 digital multimeter (Brymen,Taiwan).

CHART 1: Molecular Structure of Carminic Acid

19132 J. Phys. Chem. C, Vol. 112, No. 48, 2008 Gaweda et al.

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

Γ) VS · mTiO2

· (ci - ceq) (1)

where V is the volume of the suspension, S is the specific surfacearea (here 51 m2 g-1), mTiO2 is the mass of the semiconductorsample, 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 ArgusLab4.0.1 (Planaria Software, U.S.A.).92 Preliminary geometryoptimization was done with molecular mechanics module93-96

using the UFF force field,97-100 while the final geometry wasobtained using DFT method with B3PW91 functional and6-311+G(d,p) basis set. Atomic charges were computed usingNPO analysis. Molecular orbitals and surfaces were computedusing the same theory level and tight convergence criteria.Electronic transitions were calculated using time-dependent DFTmethod with B3PW91 functional and 6-311+G(d,p) basis set.Vibrational frequencies were calculated using CAChe 7.5package (Fujitsu, Japan, and FQS, Poland) with B88PW91functional and DZVP basis set.

3. Results and Discussion

3.1. Structure and Photophysical Properties of CarminicAcid. Carminic acid (7-D-glucopyranosyl-3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-9,10-dihydroanthracene-2-carboxylic acid,CA) is a red glucosidal hydroxyanthapurin dye naturallyoccurring in the cochineal insects. A CA molecule consists oftwo moieties: anthraquinone chromophore and glucopyranosering connected via a glucoside bond. The glucopyranose ringis perpendicular to the anthraquinone planar framework (Figure1), which is the chromophoric moiety and the metal-bindingsite.

The frontier molecular orbitals are localized mostly withinthe anthraquinone moiety with only a small contribution fromthe sugar part. The HOMO-1 orbital (Figure 1) is delocalizedover the A ring, two hydroxyl groups, and the sugar moiety.

The HOMO orbital encompasses A and B rings and corre-sponding carbonyl and hydroxyl groups, while its density overthe C ring is much smaller, which influences interactions ofcarminic acid with titanium dioxide (vide infra). The LUMOand LUMO+1 orbitals are delocalized over all three an-thraquinonic rings and the carboxylic group. The carboxylicgroup bears small positive charge, while the sugar moiety isnegatively charged (cf. Figure 1a). This electronic structureshould induce large electric dipoles upon excitation thusresulting is high molar absorption coefficients. In the groundstate, change distribution is rather uniform; a slight negativecharge is observed within the sugar moiety, while the otherterminus bears small positive charge.

Low energy part of the absorption spectrum of carminic acid,both in acidified DMF solutions and in solid phase (depositedonto BaSO4 powder) shows intense asymmetric peak at 510nm. Deconvolution reveals that this peak is composed from twobroad Gaussian components at 18660 (∆ν1/2 ) 2970 cm-1) and20 840 cm-1 (∆ν1/2 ) 4160 cm-1) (Figure 2a). This result isconsistent with the TD-DFT calculations, which reveal theexistence of two singlet excited states with similar energies.The lowest excited state involves electron transfer from HOMOto LUMO (20 410 cm-1), while the higher involves electrontransfer from HOMO-1 to LUMO (22 995 cm-1) orbitals.Calculated energy difference between these states amounts 0.3eV (2580 cm-1), while observed separation between twoGaussian components amounts 0.27 eV (2180 cm-1). Bothtransition energies and energy separation between two lowenergy excited states of carminic acid are in a relatively goodagreement with the theoretical predictions. Analysis of thecontour of the orbitals involved in these transitions (cf. Figure1) indicates that the lowest energy excited state can be regardedas a local excited state (LE), while the higher one as the internalcharge transfer state (ICT). Excitation of carminic acid at 510nm results in weak asymmetric emission band centered at 590nm (Φ ) 3.6 × 10-4), weakly resolved component at ∼630nm can be also noticed. Excitation at 570 nm results inseparation of the low energy component, while the intensity ofthe 590 nm component is much lower. Deconvolution of theemission spectrum also results in two distinct Gaussian com-ponents 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,

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

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

Photosensitization and Photocurrent Switching J. Phys. Chem. C, Vol. 112, No. 48, 2008 19133

respectively (Figure 3a). Weak band at 14200 cm-1 can beattributed to the traces of carminic acid monoanion.84,101

The values derived above can be used for estimation ofreorganization energy. In the simplest approach, the reorganiza-tion energy is related to the absorption and emission maximaand excited-state energy (∆E0) by eqs 2 and 3:102

hνmaxabs )∆E0 + � (2)

hνmaxem )∆E0 - � (3)

These yield total reorganization energy of 1240 and 1890cm-1 and excited-state energies of 17 420 cm-1 (2.16 eV) and18 960 cm-1 (2.35 eV) for LE and ICT excited states,respectively.

These results clearly indicate that there are two differentemissive excited states of carminic acid. Because of the characterof molecular orbitals, the low energy state can be regarded asthe local excited state (LE), while the higher can be regardedas the intramolecular charge transfer state (ICT). This assump-tion is confirmed by the difference in Stokes shifts observedfor 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 energytransition clearly indicates larger geometrical changes andsignificant reorganization of salvation sphere upon excitation,which is the consequence of the redustribution of electrondensity upon the excitation of carminic acid molecule. Similarphotophysical phenomena were recently reported for isoquino-line N-oxide derivatives and utilized for molecular switchingand information processing at molecular level.103

Quasi-reversible one-electron reduction of carminic acid inDMF solution proceeds at -0.73 V versus Ag/AgCl referencewhile oxidation of carminic acid takes place at +1.06 V versusAg/AgCl. The difference between these two potentials consti-tutes a rough estimate of the HOMO-LUMO gap, as reductionprocess is localized at LUMO orbital and oxidation at HOMO.Electrochemically evaluated HOMO-LUMO gap amounts 1.79eV (Figure 4). This figure is quite different from the valueobtained from spectroscopic measurement (2.16 eV) and DFTcalculations (2.59 eV). This indicates significant change in the

geometry upon excitation, large reorganizational energy, andprobable involvement of proton exchange reaction upon redoxprocess. This is also confirmed in large Stokes shifts of 2480cm-1 (0.31 eV) and 3770 cm-1 (0.39 eV) for the first and thesecond excited state, respectively. On the basis of the spectro-scopic investigation, DFT and TD-DFT calculations the pho-tophysical diagram for carminic acid was constructed (Figure5). Moreover, the charge distribution change upon excitationhas been calculated. It can be clearly seen that the quinonoidring B (cf Figure 1b) served as the electron acceptor, while thering C is the electron donor.

Dual emission observed in the case of carminic acid is aconsequence of its peculiar electronic structure. Because ofdifferent spatial character of the S1 and S2 excited states, theinternal conversion should be strongly hampered as it wouldrequire significant geometrical changes. This in turn results inhigh activation energy for S2fS1 transition. Therefore, dualemission is observed, as both S1 and S2 are luminescent.Differences in Stokes shifts for LE and ICT states clearlyindicate the difference in geometrical changes upon excitationto the S1 and S2 states. These observations imply very weakconfigurational mixing between the S1 and the S2 excited states.Therefore, the adiabatic (mixed) state local minima are almostidentical with their diabatic precursors. Similar energy of theS1 and S2 states and distinct difference in correspondingmolecular structure allows resolution of emission components(cf. Figure 5a). Similar effects have been recently observed forMC and MLCT emissions of RuII 104,105 and CuI 106 complexes,MC and MMCT emissions of dinuclear chromium-rutheniumcomplexes,107-109 and for various organic chromophores.103,110-117

This behavior is usually observed in the systems with smallenergy separation between S1 and S2 excited states.118 Thiscondition is fulfilled by carminic acid as shown in Figure 5b.Separation between calculated energies of S1 and S2 statesamounts 0.3 eV. Furthermore, time-dependent DFT calculationsexclude possibility of triplet state involvement in dual emission

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

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

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


of carminic acid, as T1 and T2 phosphorescence should beobserved 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 carminicacid results in stable purple colored materials. Binding ofcarminic acid to the surface of TiO2 is very strong, and resultedmaterials are stable in contact with water, acetic acid, and DMF.Slow hydrolysis and CA desorption can be observed only inalkaline solutions (pH > 10).

The surface binding of carminic acid should strongly resemblebinding of salicylic acid to the TiO2 surfaces, which is stabilizedvia formation of chelate ring and the modifier is bound as adianion.90,119,120 As in the case of salicylic acid, one shouldexpect formation of a new absorption band resulting fromLMCT (salicylate f TiIV) charge transfer.120,121 Surprisingly,the chemisorption of carminic acid onto titanium dioxide onlyinsignificantly changes the spectrum of the organic chromophore(Figure 2b). The small bathochromic shift (510 nm for pureCA, 538 nm for CA@TiO2 material) is observed only in thecase 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 describedby high affinity multisite isotherm.90 Fitting of the Lineweaver-Burk equation (eq 4) to the experimental data allows calculationof maximal coverage and adsorption equilibrium constants(Figure 6b).

1Γ) 1

Γmax+ 1

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 salicylicacid (∼14.7 µmol m-2). These figures correspond to average

packing of 0.4 and 8.4 molecules per nm2 for carminic acidand salicylic acid, respectively. This is consistent with a muchlarger size of CA as compared with salicylic acid; specifically,the bulky sugar moiety may cause strong steric hindrance. Thecarminic 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 carminate-modified titania against hydrolysis. Large area occupied bysingle carminic acid molecule substantiates the lack of anyexcitonic 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 mostsignificant difference consists of a significant increase of thehigh energy bandwidth (4730 vs 4160 cm-1 for pure carminicacid). Much more significant changes are observed in fluores-cence spectra (Figure 3b). The luminescence intensity issignificantly decreased (Φ ) 2.2 × 10-4 vs 3.6 × 10-4 forneat carminic acid). The high energy component is completelyquenched, but the low energy component retains most of itsintensity and is bathochromically shifted to 15 780 cm-1 (∆ν1/2

) 1940 cm-1). The quenching of the ICT component may bea result of photoinduced electron transfer from the excitedchromophore to the conduction band of the semiconductor.

On the basis of spectroscopic data presented above, the valueof the reorganization energy associated with the LE state equals1125 cm-1, and the energy of the LE state is 16905 cm-1.Because of complete quenching of the ICT luminescence, it isimpossible to calculate directly the � and ∆E0 values. It can beassumed, however, that the reorganization energy is not chang-ing significantly upon binding to the TiO2 surface. On the basisof this assumption, the reorganization energy for the ICT stateshould approximate 1900 cm-1, which in turn implies the ICTenergy equals 18 700 cm-1.

In contrary to the cyanoferrate-based semiconducting sys-tems,52 deposition of carminic acid onto titanium dioxidepowders does not change the band gap energy. Opticallydetermined indirect band gap energy amounts 3.25 eV for bothneat and modified samples, which is in perfect agreement witha previously reported value (Figure 7).52

Figure6. Adsorptionisoterms(a)andthecorrespondingLineweaver-Burkplots (b) for carminic acid (red circles) and salicylic acid (bluediamonds, based on data from ref 90) on the surface of titanium dioxideat 298 K.

Figure 7. Determination of optical indirect band gap of carminate-modified titanium dioxide.

CHART 2: Molecular Structure of the ModelCompound, Dihydroxytitanium Carminate


In order to elucidate the details of interactions betweencarminic acid and titanium dioxide, a simple molecular modelhas been constructed (Chart 2). It was shown recently thancomplexes containing di- and trihydroxytitanium(IV) fragmentsare good models of the semiconductor-surface moleculeinteractions.51,52,123-125 As only minor perturbation of spectro-scopic properties of CA on binding to TiO2 surface are observed,it can be assumed that the TiIV ion is bound to the carboxylicand hydroxy groups thus forming the six-membered ring. Thisbinding mode was confirmed by DFT calculations on carminatedianion (Figure 8). The HOMO-1 and HOMO orbitals arelocalized mainly on the C ring with significant contribution ofhydroxylate and carboxylate groups.

On the basis of TD-DFT calculations in the model system,the electronic transitions of the CA@TiO2 (Figure 9c) can beassigned to LE and ICT states, respectively (vide supra), similarto that of neat carminic acid. The calculated transition energiesamount 20 260 and 22 880 cm-1 for the LE and ICT transition,respectively. Measured separation of these two excited statesamounts 2580 cm-1, while the calculated difference amounts2620 cm-1, which is in perfect agreement with experimentalvalue. This result indicates a slight increase in energeticseparation of these two excited states upon interaction ofcarminic acid with titanium dioxide. The only difference consistsof a small contribution of titanium d orbitals in LUMO orbitalof the model complex. In contrast to various transition metalcomplexes51-55,61,123-127 and enediol ligands58,59,61,128-132 de-posited onto titanium dioxide, the carminic acid-titaniumdioxide system exhibits only weak electronic coupling betweenthe organic chromophore and the semiconductor particle. Thisobservation is supported by optical spectroscopy: no chargetransfer bands are observed upon complexation of titaniumdioxide with carminic acid. Efficient fluorescence quenching,in turn, presumably proceeds via the photoinduced electrontransfer 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 neatcarminic acid. There is no evidence for direct photoinducedelectron transfer between the anthraquinone chromophore andtitanium ions, which is consistent with spectroscopic observations.

3.3. Photoelectrochemical Properties of CA-TiO2 Mate-rial. Photoelectrochemical applications require a detailed knowl-edge of the positions of semiconductors band edges since thisgives information on the reductive and oxidative power ofphotogenerated electrons and holes, respectively. These data canbe obtained from the quasi-Fermi level measurements 89,133 (i.e.,the Fermi level potential measured for a semiconducting materialunder irradiation, EqF), which together with the spectral char-acteristics of the synthesized materials allow the determinationof 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 basedon the pH-dependency of the valence- and conduction-band edgepotentials of the semiconductor particles. In the case of titaniumdioxide with increasing pH, the cathodic shift of EqF by 59 mVper 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 conductionband of the irradiated semiconductor may or may not reduce theredox couple, depending on the pH value and relative potentials.The method of Roy yields the pH0 values for TiO2 and carminicacid modified TiO2 of 4.50 and 6.55, respectively. In neutralsolution (pH ) 7), it results in the CB edge potential equal to -0.58and -0.47 V for neat and modified TiO2, respectively (Figure 10).Modification of the surface of titanium dioxide with carminic acidresults in a significant anodic shift of the conduction band edgepotentials. This observation is consistent with covalent modificationof the TiO2 surface.52,65,125,134,135

Unmodified titanium dioxide generates anodic photocurrentwithin a wide potential window upon UV illumination. At veryhigh negative polarization, cathodic photocurrent is observedbecause of the reduction of some surface TiIV to TiIII.

CA-modified titanium dioxide is a suitable material for thepreparation of photoelectrodes using ITO glass as conductingsupport. The material is photostable, and no photodecompositionof the surface complex was observed, even after prolongedirradiation with ultaviolet or visible light. Photocurrent genera-tion by transparent ITO electrodes covered with CA@TiO2 wasinvestigated as a function of the incident-light wavelength atvarious electrode potentials in the absence and presence ofmolecular oxygen in aqueous electrolytes at pH ) 7.

To have a full overview of photocurrent generation as afunction of applied potential and irradiation wavelength, a set

Figure 8. Molecular structure and contours of the frontier molecularorbitals of carminic acid dianion as calculated on the B3PW91/6-311+G(d,p) level of theory.

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

Figure 10. Determination of pH0 for neat TiO2 (squares) and CA-modified TiO2 (triangles).


of action spectra was collected at constant potentials (at every50 mV). In this way, three-dimensional photocurrent maps wereobtained (Figure 11). In the absence of oxygen, generation ofanodic photocurrents at positive polarization is observed whilenegative photoelectrode polarization results in cathodic photo-currents of low intensity (Figure 11a). A good photosensitizationis observed between 400 and 580 nm at cathodic polarization,while it is much less efficient at anodic polarization of thephotoelectrode (400-450 nm). In the presence of oxygen,cathodic photocurrents of high intensity are observed within theabsorption spectrum of the semiconductor (300-400 nm), andcathodic photocurrents of lower intensity are observed withinthe absorption band of the surface chromophore (400-600 nm).In air-equilibrated and oxygen saturated solutions, photoelec-trode generates anodic photocurrents only at positive polarizationof the photoelectrode (Figure 11b), while at negative polariza-tion, high intensity cathodic photocurrents are observed (Figure11b) It can be concluded that cathodic photocurrents aregenerated because of an efficient reduction of molecular oxygenpresent in the electrolyte solution. The cathodic photocurrentintensities are higher than anodic ones. The phenomenon ofphotocurrent polarization switching upon changes of photoelec-trode potential and incident light wavelength can be a platformfor construction of various logic devices.123,135 Furthermore, atlow oxygen concentration, the potential-induced photocurrentswitching does not proceed as sharply as that in oxygen saturatedsolutions. This results in potential-dependent wavelength switch-ing, which is especially beneficial in the context of optoelec-tronic applications.136,137 Addition of iodide (0.01 M) to theelectrolyte solution results in increase of photocurrent intensitiesboth in anodic and cathodic regimes. Furthermore, the efficiencyof photosensitization is greatly improved.

Final confirmation of the electron injection mechanism wasfound using laser flash photolysis. Excitation of CA@TiO2

quantum dots suspended in DMF with 532 nm light results inimmediate (within laser pulse, ∼7 ns) decrease in absorbanceat 520 nm and increase at 400 and 620 nm (Figure 12). Theformer can be associated with oxidation of carminate anions

deposited onto the TiO2 surface, while the latter can beassociated with carminate cation radical (420 mn) and freeelectrons within the conduction band (600-700 nm).138-140 The420 and 520 nm signals decay monoexponentially withrate constants of 105 s-1 order of magnitude (Table 1), whilethe 620 nm transient absorption decays biexponentially (Table1). Upon oxygenation of the solutions, the rates associated withcarminic acid increase by a factor of 2, while the decay oftrapped electrons is only slightly accelerated. Quantitativelyanalogous spectral changes were observed during spetroelec-trochemical measurement. Oxidation of both neat carminic acidand carminic acid deposited onto TiO2 nanoparticles results indecrease of CA absorption band intensity, while absorptionwithin 400-450 nm slightly increases. Therefore, the positiveabsorption change around 420 nm can be attributed to formationof carminic acid cation radical (Figure 12b).

All of the above observations allow elucidation of thephotocurrent switching mechanism. Irradiation of photoelec-trodes prepared from CA@TiO2 hybrid material results ingeneration of photocurrent in which the characteristics are verydifferent from those recorded for neat titanium dioxide.135 Inair-equilibrated and oxygen saturated solutions, the photoelec-trode generates anodic photocurrents only at positive polarizationof the photoelectrode (Figure 11a). Negative polarization of thephotoelectrode results in reversal of the photocurrent direction.Interestingly, the cathodic photocurrent intensities are higherthan the anodic ones. Such anomalous behavior was previouslyobserved for cyanoferrate modified TiO2,51 ferrocene-TiO2

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

Figure 12. Transient differential absorption spectra recorded forcarminic acid-modified titanium dioxide quantum dots upon 532 nmexcitation in DMF. The first spectrum is recorded 1 µs, while subsequentspectra are recorded at 2, 5, 12, 42, and 90 µs upon laser pulse,respectively (a). Differential absorption spectrum recorded duringelectrochemical reduction of carminic acid in the presence of TiO2

particles at 1100 mV vs Ag/AgCl (b).

TABLE 1: Rate Constants Associated with TransientSpecies Photogenerated in the Carminic Acid-TitaniumDioxide System

wavelength/nm

k/105 s-1

Ar O2

420 3.7 5.5520 2.3 4.4620 9.4; 1.2 13.0; 1.8


composites,52 and other surface-modified semiconductors,65

called the PEPS effect (photoelectrochemical photocurrentswitching).135,136

Redox potentials of carminic acid in the first excited statecan be calculated from ground-state redox potentials and energyof 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. Thesedata, together with previously determined semiconductor bandgap and conduction band edge potential, are the basis for theenergy diagram of the studied system. It can easily be noticedthat 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 astronger reducing agent than the conduction band electrons(-0.47 V) of the surface modified TiO2 (vide supra). On theother hand, photogenerated holes in the valence band are thestrongest oxidants in the system (2.78 V, Figure 13). The freeenthalpy of the electron transfer process between the photoex-cited carminic acid and the conduction band on the semiconduc-tor for LE and ICT states can be formulated as follows (eqs 8and 9):

∆GLE )ECA+⁄CA -ECBE -∆E0LE +ECoul (8)

∆GICT )ECA+⁄CA -ECBE -∆E0ICT +ECoul (9)

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

ECoul )e2

4πεε0r(10)

The value of the Coulombic component was estimated as400-1730 cm-1 (0.05-0.21 eV), depending on the assigneddonor-acceptor distance (780 pm from the center of the CAmolecule to the TiO2 surface, but only 180 pm from thephenolate oxygen atom to the neatest titanium atom). Thus, the∆GLE falls within -0.14 to -0.30 eV, while the ∆GICT fallswithin -0.37 to -0.53 eV. This rough estimation shows that

the thermodynamic driving force for the electron transfer processof almost two-fold larger for the ICT excited state as comparedwith the LE excited state. There can be other factors not takeninto account (e.g., high frequency vibrational modes)102 whichmay render the LE-induced process thermodynamically disfa-vored. The most important vibration to be taken into accountseems to be the breathing mode of the anthraquinone moiety.148

Its energy according to DFT calculations should amount ∼1700cm-1. In fact, IR spectrum of carminic acid exhibits a veryintense peak at 1695 cm-1. Therefore, if a high frequencyvibrational contribution is taken into account (∼1700 cm-1),both energies (eqs 7 and 8) should increase by ∼0.21 eV. Inthis situation, the ICT-induced electron injection is still possible(∆G ≈ -0.16 to -0.32 eV); the LE-induced process mayalready be thermodynamically disfavored (∆G ≈ +0.07 to-0.09 eV). This estimation, although very rough, justifies thepeculiar luminescent and photoelectrochemical properties of theCA-TiO2 material and supports the suggested photosensitizationmechanism.

Excitation of the electrode within the absorption of thesemiconductor at positive polarization of the photoelectroderesults in photoanodic response according to the generallyaccepted mechanism (Figure 14a). Electron is promoted to theconduction band of the semiconductor and is subsequentlytransferred to the conducting electrode. The hole in the valenceband is in turn neutralized by the electron from oxidation ofthe solvent molecule (H+,OH•/H2O, 2.31 V at pH ) 7)149 oradded redox mediator (here iodide/triiodide couple, 0.535 V vsNHE).150 Excitation within the carminic acid absorption bandresults in similar processes. The excited state of carminic acid,which is a good electron donor, can inject an electron into theelectrode, most probably via the conduction band of thesemiconducting particle. The CA* state cannot oxidize freewater molecules directly, but formation of a hydrogen bondnetwork may support some oxidation processes, which resultin low intensity photocurrents, that is, very inefficient photo-sensitization. Upon addition of iodide, the CA* state can beeasily reduced, which results in much higher photocurrents(Figure 14b). This process can occur only at positive polarizationof the electrode; otherwise, the electrostatic barrier wouldprevent interfacial electron transfer and generation of anodicphotocurrents.

At negative polarization of the photoelectrode, cathodicphotocurrents are observed, but only in the presence of efficientelectron acceptor, for example, molecular oxygen or triiodide.Neat titanium dioxide photoelectrodes almost do not yieldcathodic photocurrents. It is possible only upon doping,151

surface modification,51,135,136 or at high pH.152 The potentialdifference between the potential of the electron in conductionband and that of the O2

-/O2 couple is small, so there is no muchthermodynamic driving force for this reaction. Moreoverinterfacial electron transfer between titanium dioxide surfaceand molecular oxygen strongly depends on surface adsorptionof O2. At neat TiO2 surfaces, recombination is dominating overthe ET with dissolved oxygen. The presence of carminic acidon the surface facilitates oxygen reduction due to facilitatedelectron transfer resulting from the reactivity of carminate anionradicals. A hole within the valence band can easily be neutralizedby an electron from the carminic acid molecule, whichsubsequently is reduced electrochemically. As recombinationis hampered this way, the only possible process for an electronin the conduction band is to reduce the oxygen molecule eitherdirectly or via the ET process involving the carminic acidmolecule (Figure 14c). This is substantiated by much lower

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


cathodic photocurrent intensities as compared with anodic ones.The excited state of carminic acid is a much stronger electrondonor (-1.05 V), so it can very easily reduce oxygen; thusgenerated carminic acid cation radical can be easily reducedelectrochemically (Figure 14d). The strongly reducing excitedstate can be generated either directly (via excitation of surfaceCA molecules) or indirectly via energy transfer from excitedsemiconducting support (via electron-hole recombination). Thelatter process yields even higher photocurrent intensities ascompared with direct excitation, due to high absorptivity oftitanium dioxide as compared with monomolecular layer ofcarminic acid. Triiodide can also act as an efficient electronacceptor (Figure 14c,d).

The photocurrent switching mechanism is based mainly onthe electron donor and electron acceptor character of carminicacid molecule. Depending on photoelectrode potential, it maycontribute in anodic and cathodic photocurrent generation, whichis a quite unique feature among photosensitizers. Other words,the PEPS effect in the CA@TiO2 system is a direct consequenceof carminic acid electrochemical amphotericity. Such electro-chemically amphoteric molecules which offer stable reducedand oxidized forms within a feasible potential window areperfectly suited for molecular electronics applications.153,154

4. Conclusions and Outlook

Hybrid material obtained by immobilization of carminic acidonto the surface of nanocrystalline titanium dioxide has interest-ing photoelectrochemical properties. It exhibits pronouncedphotosensitization toward visible light. Photoelectrodes builtfrom CA@TiO2 generate photocurrent within the 300-650 nmwindow. The polarity of the photocurrent can be changed via achange in photoelectrode potential. The mechanism of thisprocess was described in terms of photoinduced electron transferinvolving both carminic acid molecule and titanium dioxidesupport. Carminic acid acts here as an electron buffer, donatingor accepting electrons if necessary. Spectroscopic investigationsand DFT calculations are in favor of the Sakata-Hiramoto-Hashimoto model.49,50

The photocurrent switching observed here is closely relatedto PEPS effect (photoelectrochemical photocurrent switching)observed in the other system, folic acid immobilized onto theTiO2 surface.65 Similar to other switchable photoelectrochemicalsystems,52,65,123-125,135,155-159 the CA@TiO2 hybrid material canbe applied for construction of optoelectronic switches, logicgates, and other functional devices. Incident light wavelengthand photoelectrode potential can be regarded as input channels,while photogenerated currents can be regarded as outputchannels. In this regime, various devices as simple switches,logic gates, and more complex combinatorial circuits can be

implemented without any obstacles.123,125 Furthermore, becauseof the ability of carminic acid to form a series of hydrogen bondswith a wide variety of substrates, the CA@TiO2 system can bea platform for various photoelectrochemical sensors and otherdevices. Because of catalytic activity toward photoreduction ofmolecular oxygen, carminic acid-titanium dioxide photoelec-trodes may also be a good starting point for developmentphotofuel cells69 and as a model system to study interfacialelectron transfer processes in nanoscale.160-162

Acknowledgment. Authors thank Mr. Andrzej Karocki forassistance during laser flash photolysis experiments and Dr.£ukasz Orzeł for valuable discussions on luminescence spec-troscopy. This work was supported by Polish Ministry ofEducation and Science (Grants PB1283/T09/2005/29 and PBZ-KBN-118/T09/8). DFT calculations were performed in theAcademic Computer Centre CYFRONET AGH within com-putational Grant MEiN/SGI3700/UJ/085/2006.

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