A Multitechnique Physicochemical Investigation of Various Factors

ARTICLE pubs.acs.org/JPCC

A Multitechnique Physicochemical Investigation of Various Factors Controlling the Photoaction Spectra and of Some Aspects of the Electron Transfer for a Series of PushPull Zn(II) Porphyrins Acting as Dyes in DSSCs Alessio Orbelli Biroli,‡ Francesca Tessore,† Maddalena Pizzotti,*,† Cinzia Biaggi,† Renato Ugo,† Stefano Caramori,§ Alessandro Aliprandi,§ Carlo Alberto Bignozzi,§ Filippo De Angelis,|| Giacomo Giorgi,|| Emanuela Licandro,^ and Elena Longhi^ †

)

Dipartimento di Chimica Inorganica Metallorganica e Analitica dell’Universita degli Studi di Milano “Lamberto Malatesta”, Unita di Ricerca dell’INSTM, via G. Venezian 21, 20133 Milano, Italy ‡ Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), via C. Golgi 19, 20133 Milano, Italy § Dipartimento di Chimica dell’Universita di Ferrara, Via Luigi Borsari 46, 44121 Ferrara, Italy Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), c/o Dipartimento di Chimica, Via Elce di Sotto 8, 06123 Perugia, Italy ^ Dipartimento di Chimica Organica e Industriale, Via C. Golgi 19, 20133 Milano, Italy

bS Supporting Information ABSTRACT: A multitechnique physicochemical comparative investigation involving TDDFT theoretical calculations, steadystate and time-resolved electronic absorption spectra, and electrochemical and photoelectrochemical investigations was carried out on a family of pushpull porphyrinic sensitizers ([5-(40 -carboxyphenylethynyl)-15-(400 -methoxy-phenylethynyl)-10,20-bis(3,5-ditert-butylphenyl)porphyrinate]Zn(II) (1) and [5-(40 -carboxy-phenylethynyl)-15-(400 -N,N-dimethylamino-phenylethynyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrinate]Zn(II) (2) and the new fluorinated porphyrinic dye [5-(40 -carboxy-20 ,30 ,50 ,60 -tetrafluorophenylethynyl)-15-(400 -N,N-dimethylamino-phenylethynyl)-10,20-bis(3,5-di-tert-butylphenyl)porphyrinate]Zn(II) (3)) with the aim of identifying the structurally related electronic properties at the basis of efficient interfacial charge separation. We found for all dyes a photoconversion nearly twice more effective for the B band than for the Q band, which could not be explained only by considerations based on the electron collection efficiency but also by a more energetically favorable electron injection from the S2 excited state. The lower photoconversion of the fluorinated dye 3, when compared to dyes 1 and 2, was explained not only by a more difficult absorption on the TiO2 photoanode but also by a lower electron injection efficiency and a less successful hole transfer to the electrolyte, leading to increased charge recombination.

’ INTRODUCTION The search for alternative energy sources is actually characterized by an increasing interest toward the theoretical design, synthesis, and optical, photophysical, and electrochemical investigations of molecular systems applied in the so-called organic photovoltaic cells.1 In particular, dye-sensitized solar cells (DSSCs) are attracting much attention over the recent years because they promise a satisfactory efficiency and a low cost, thanks to the involvement of a large band-gap nanostructured semiconductor, such as TiO2.25 An efficiency (the power conversion of the incident light) of 11% has been reached with the N3 photosensitizer [Ru(II)(cis-dithiocyanate-bis(2,20 -bypiridine-4,40 -dicarboxylate)],6 in combination with a liquid electrolyte containing the I/I3 couple as a redox system for the regeneration of the oxidized dye.3 Many investigations have been r 2011 American Chemical Society

devoted to the search for new photosensitizers based on Ru(II)polypyridyl complexes that may show better properties.2f For instance, Ru(II) complexes with dipyridyl ligands carrying πdelocalized substituents2f,7 show an increased intensity and a red shift of the relatively weak metal-to-ligand charge-transfer (MLCT) bands typical of Ru(II)polypyridyl complexes.2,8,9 Moreover, various Fe(II),10 Pt(II),11 Os(II),12 Re(I),13 and Cu(I)14 complexes have been also investigated as dyes for DSSCs, but for these complexes, a relatively poor efficiency has been reported.2f Metal porphyrins, characterized by very strong absorption bands around 450 nm (Soret band) and 600700 nm (Q band), Received: April 1, 2011 Revised: October 10, 2011 Published: October 11, 2011 23170

dx.doi.org/10.1021/jp2030363 | J. Phys. Chem. C 2011, 115, 23170–23182

The Journal of Physical Chemistry C are potentially interesting photosensitizers; therefore, in the recent years, some research efforts have been devoted to their application in DSSCs.2f,9,15,16 Recently, some of us have investigated the second-order nonlinear optical properties of 10,20-diarylmetalporphyrins, carrying in the meso 5- and 15-positions an aromatic ring connected by a triple bond to the porphyrin ring.17 These metal porphyrins act as a pushpull system by introduction of an electron donor group on one aromatic ring and an electron acceptor group on the other one, as first reported by Therien and co-workers.18 The significant quadratic hyperpolarizability17,18 and the strong two-photon absorption19 of this family of pushpull metal porphyrins have been attributed mainly to a directional charge-transfer process, as confirmed clearly for the first time by an ab initio TDDFT investigation.20 In fact, the HOMOLUMO transition produces a directional electron transfer from the donor site to the acceptor site along the pushpull system, the origin of a quite strong Q absorption band at about 650680 nm, which gives a significant contribution to the quadratic hyperpolarizability. Moreover, this TDDFT investigation has evidenced that the LUMO level of this family of pushpull metal porphyrins is slightly higher than the level of the TiO2 conduction band and that the HOMO level is at energies satisfying the energy level requirements of the I/I3 redox system used in the DSSC.25 This evidence prompted us to investigate the application of Zn(II) porphyrins of this pushpull family in DSSCs. However, at the same time, other groups have carried out and recently published detailed investigations on the synthesis and application in DSSCs of these pushpull porphyrinic systems reaching interesting efficiencies.2123 We report here the results of a physicochemical investigation on the factors affecting the contribution of the B and Q absorption bands to the photoaction spectra, with particular reference to the charge-transfer processes at the interface of the TiO2 photoanode of a DSSC involving the pushpull metal porphyrins as dye. In particular, we analyze the origin of the poor efficiency observed by introduction of four fluorine atoms in the aromatic ring, of the ethynyl moiety carrying the anchoring carboxylic group. The fluorination of the aromatic ring was introduced with the aim of investigating the effect of an increase of the acceptor properties of the ethynyl aryl moiety and of a fluorous barrier that could reduce the parasite electron/ electrolyte recombination process theoretically more facile for porphyrinic dyes due to the tendency of large π-conjugated systems to interact with I2.24 Finally, we also report on the successful substitution of platinum with the much less expensive PEDOT (polyethylenedioxythiophene) as a counter electrode in the electrolytic process involving the I/I3 redox system.

’ EXPERIMENTAL SECTION General. All reagents and solvents used in the synthesis were purchased from Sigma Aldrich and used as received, except for Et3N and Et2NH (freshly distilled over KOH) and THF (freshly distilled from Na/benzophenone under a nitrogen atmosphere). Silica gel (Geduran Si 60, 63200 μm) was purchased from Merck. Transparent titanium dioxide was either purchased from Dyesol (DSL NR18 NT) or prepared according to literature procedures by acidic hydrolysis of titanium tetraisopropoxide.25

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Opaque TiO2 paste was purchased from Dyesol (DSL NR18AO). Nanocrystalline ZrO2 was prepared by acidic hydrolysis (HNO3 working at pH ∼ 2) of zirconium tetraisopropoxide 70% (w/w) in isopropanol solution (Aldrich). Conductive glass (FTO TEC 8, 8 Ω/sq) was purchased form Hartford Glass, Indiana. Reagent grade chenodeoxycholic acid, iodine, lithium iodide, and 4-tertbutylpyridine and electrochemical grade tetrabutylammonium hexafluorophosphate [(TBA)PF6] and methoxypropionitrile (MPN) were purchased from Aldrich; MPN was redistilled under reduced pressure prior to use. Glassware was flame-dried under vacuum before use when necessary. 10,20-Bis(3,5-di-tert-butylphenyl)porphyrin was prepared as reported in the literature,26 [5-bromo-15-iodio-10,20-bis(3,5-di-tertbutylphenyl)porphyrinate]Zn(II) (4) was prepared following the synthesis reported in the literature for the analogous [5-bromo-15iodio-10,20-diphenylporphyrinate]Zn(II),27 and 4-ethynylbenzoic acid was prepared as reported in the literature.28 NMR spectra were recorded on a Bruker Avance DRX-400 and on a Bruker AMX 300 with CDCl3 as the solvent with addition, in the case of metal porphyrins, of a drop of pyridine-d5 or in THF-d8 (Cambridge Isotope Laboratories, Inc.). Mass spectra were obtained with a Bruker-Daltonics ICR-FTMS APEX II with an electrospray ionization source (ESI). Highresolution mass spectra were recorded on a Vg Analytical 7070 EQ spectrometer. Photophysical Measurements. Electronic absorption spectra in solution were measured on a Jasco V-530 spectrophotometer. Diffuse reflectance spectra on TiO2 thin films were recorded on a Jasco V570 UVvis-NIR spectrophotometer equipped with an ISN 470 integrating sphere. Emission spectra were obtained with a Jobin Yvon Spex spectrofluorimeter equipped with a R 3896 phototube, by using a band pass of 2 nm, an integration time of 0.5 s, and an excitation wavelength of 450 nm. The excited-state oxidation potential was estimated from the relation EGS E00, where EGS is the formal oxidation potential of the ground state of the dye (dye/dye+), corresponding to the E1/2 or to the peak potential (in the case of irreversible processes) of the oxidative wave observed by cyclic voltammetry at anodic potentials, and E00 is the 00 spectroscopic energy, determined from the maximum of the vibronic band at highest energy. Emission spectra of optically diluted (absorbance of the Q-band maximum < 0.3) transparent dye-sensitized TiO2 or ZrO2 films were recorded by using a thin film holder that allowed a fixed excitation angle of 33. An excitation wavelength of 450 nm, a spectral bandwidth of 10 nm, and an integration time of 1 s were employed. The resulting emission spectra were background-subtracted, integrated, and divided by the film light-harvesting efficiency (LHE = 110A450), where A450 is the absorbance at 450 nm of the transparent film. For all the metal porphyrins, the value of LHE was very close to unity. The quenching ratio was defined by (IZrO2)/(ITiO2), where ITiO2 and IZrO2 are the integrated emission intensities calculated as just specified. Lifetimes of the fluorescent emission at about 690 nm were determined in THF solution by time-correlated single-photon counting (TCSPC) by using the PicoQuant Picoharp 300 apparatus equipped with subnanosecond LED sources (280, 380, 460, and 600 nm; 500700 ps pulsewidth) powered with a PicoQuant PDL 800-B variable (2.540 MHz) pulsed power supply. The resulting decays (Supporting Information) were analyzed by means of PicoQuant FluoFit Global Fluorescence Decay Analysis Software and satisfactorily fitted with monoexponential functions. 23171

dx.doi.org/10.1021/jp2030363 |J. Phys. Chem. C 2011, 115, 23170–23182

The Journal of Physical Chemistry C

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Triplet lifetimes and differential transient state spectra in THF solution were obtained by nanosecond transient absorption spectroscopy by using the 355 nm excitation generated by a pulsed Q-switched Nd:YAG laser Continuum Surelite II (fwhm = 7 ns, 8 Hz) laser. The probe beam, orthogonal to the excitation pulse, generated by a pulsed Xe lamp, was focused on an Acton Spectra Pro 2300i triple grating monochromator and detected with an Hamamatsu R 3896 photomultiplier. Transient spectroscopy experiments on the metal porphyrins absorbed on the TiO2 photoanodes were carried out by using the 532 nm laser excitation defocused with a plano-concave lens to obtain pulse energies of about 10 mJ/cm2. To obtain a satisfactory S/N ratio, oscilloscope traces were averaged over 30 laser shots. To avoid direct TiO2 band-gap excitation, the analysis beam was filtered with a 420 nm cutoff filter. A 532 nm notch filter prevented scattered laser stray light from reaching the photomultiplier, thus eliminating artifacts and blinding. Electrochemical Measurements. Cyclic voltammetry was performed in THF with an EcoChemie PGSTAT 30/2 electrochemical workstation without dynamic Ohmic drop compensation at 20 mV/s by using a glassy carbon working electrode and a Pt wire counter electrode. Potentials were referred to SCE (saturated calomel electrode). The supporting electrolyte was (TBA)PF6 (0.1 M). Prior to each measurement, the working electrode was accurately cleaned by brushing the carbon surface on an alumina (0.5 μm) pad, followed by sonication and rinsing with acetone. Photoelectrochemical Measurements. Photon-to-current action spectra were collected by focusing the monochromatic light generated by an Applied Photophysics monochromator (spectral bandwidth = 10 nm) onto the photoanode of a sandwich-type photoelectrochemical cell. The illuminated area was 0.25 cm2. Shortcircuit photocurrents were measured with an Agilent 34401A multimeter. IPCE was calculated according to eq 1 IPCE % ¼ 1:24 103 ðVnmÞ

JðμA cm2 Þ λðnmÞPðW m2 Þ

ð1Þ

where J is the monochromatic photocurrent density, P is the radiant power density, and λ is the wavelength of the incident light. To test the wavelength dependence of the electron collection efficiency (ηcoll), IPCE spectra were recorded by illuminating the cell through the back contact (FTO) or through the transparent sputtered platinum counter electrode (electrolyte side). For these experiments, a lightly Pt sputtered counter electrode was preferred over PEDOT counter electrodes to minimize light losses through the first interface. IPCE spectra under variable white light bias were obtained by superimposing, by means of optical fibers, a weak modulated (0.5 Hz) monochromatic excitation to the white light generated by a calibrated 150 W Xe lamp (Newport). The resulting signal was preamplified and fed to a virtual lock-in amplifier (National Instruments) and the resulting IPCE calculated by means of a custom-made Labview program. JV curves were obtained under AM 1.5 G simulated solar illumination generated by an ABET sun simulator by using an EcoChemie PGSTAT 30/2 electrochemical workstation controlled by the Ecochemie GPES 4.9 software. The linear potential sweep rate was 10 mV/s. Incident white light irradiance was measured with a Molectron Powermax 5200 Power-meter. Electrochemical impedance spectroscopy (EIS) was performed under solar illumination and open-circuit conditions in the 105101

Hz frequency range by applying a single sine perturbation of 10 mV amplitude with an EcoChemie FRA 2 frequency response analyzer controlled by the NOVA 1.5 software. The resulting impedance plots were fitted by the Zsimp Win 3.22 software (the maximum relative standard error was e10%), in terms of equivalent electric circuits of the type R(RctQ)(Q0 (Rct0 O)), where Q and Q0 are the constant phase elements (essentially nonideal capacitances) of the counter electrode/electrolyte and TiO2/electrolyte interfaces, Rct is the charge-transfer resistance at the cathode, Rct0 is the charge-transfer resistance at the TiO2/electrolyte interface, and O is the Nernst diffusion element. Cell Preparation. TiO2 or ZrO2 gels were cast onto wellcleaned FTO substrates by doctor blading. The resulting film was gently dried in warm air and fired at 450 C for 45 min. The TiO2 or ZrO2 layer so obtained (about 6 μm thick) was then treated with TiCl4 according to the literature3 and annealed at 450 C before being immersed, still hot, in the dye solution. Dye absorption on the TiO2 (or ZrO2) surface was carried out in the dark from a THF solution containing 4 mM chenodeoxycholic acid; 1 and 2 can be absorbed effectively also from a EtOH/THF 9:1 solution, whereas under these conditions, 3 was poorly absorbed. In general, for all the dyes investigated, a very limited absorption was observed starting from THF solutions, without addition of chenodeoxycholic acid. PEDOT counter electrodes were obtained from 102 EDOT (ethylenedioxythiophene) solution in 0.1 M LiClO4 by potentiostatic deposition on well-cleaned FTO substrates at 1.6 V vs SCE for 30 s. The resulting deep blue coated electrodes were rinsed with acetonitrile and ethanol, dried, and stored in the dark. Parafilm sealed DSSCs were built by pressing the TiO2 sensitized photoanode against a PEDOT counter electrode equipped with a Parafilm frame used to confine the liquid electrolyte (PMII 0.6 M/ LiI 0.1M/I2 0.02 M in methoxypropionitrile) inside the cell. The thickness of the liquid layer corresponded roughly to the thickness of the frame borders (about 120 μm). Theoretical DFT and TDDFT Calculations. DFT groundstate geometries of dyes 13 were optimized by using the B3LYP exchange-correlation functional,29 along with a 6-311G* basis set. For sake of simplicity, the tert-butyl groups linked to the phenyl groups have been replaced by hydrogen atoms and a Cs symmetry was used for geometry optimizations. TDDFT excited-state calculations were performed on the DFT-optimized geometries at the same B3LYP/6-311G* level of theory, both in vacuo and in solution. The lowest 15 excitation energies were calculated, spanning approximately 3.5 eV. All the calculations have been performed by the Gaussian 03 program package.30 The nonequilibrium C-PCM solvation model31 was employed for TDDFT calculations in solution.

’ RESULTS AND DISCUSSION Synthesis. To synthesize the asymmetrical pushpull Zn(II) porphyrinates 13 reported in Figure 1, three different strategies have been exploited, taking into account that the synthesis of 1 and 2 has already been reported without a comparison of different synthetic methods.21 The first strategy consists of a regiospecific synthetic approach first described by some of us17b (method A; see the Supporting Information, page S3) in which the donor and acceptor ethynyl aromatic fragments have been linked to the porphyrinic ring with a two-step process based on the Sonogashira coupling,32 starting from the asymmetrically dialogenated species [5-bromo-15-iodo-10, 23172


The Journal of Physical Chemistry C 20-bis(3,5-di-tert-butylphenyl)porphyrinate]Zn(II) as an intermediate (4) (Scheme 1). The higher reactivity of the iodinecarbon bond allows, as the first step, the selective coupling by the Sonogashira reaction32 of the 4-ethynylbenzoic acid, more reactive if compared to an ethynyl aromatic moiety carrying a donor group,33 thus obtaining the intermediate 5 in 70% yield. However, the reaction of 5 with 4-ethynylanisole to obtain the Zn(II) porphyrinate 1, carried out by the standard Sonogashira reaction conditions,32 was initially not successful due to the formation of a significant amount of the homocoupling byproduct of the acetylenic reagent. Thus, to minimize the homocoupling byproduct, this step was carried out under a hydrogen atmosphere and in a rather polar solvent, such as MeCN, since these reaction conditions should facilitate the reactivity of the brominecarbon bond.34 The Zn(II)

Figure 1. Zn(II) pushpull porphyrinates investigated in this work.

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porphyrinate 1 was thus obtained with 15% yield, still too low to be considered satisfactory. By a statistical two-step approach (method B) (Supporting Information, page S6) starting from the symmetrical [5,15diiodo-10,20-bis(3,5-di-tert-butylphenyl)porphyrinate]Zn(II) (7), as described by Therien and co-workers for the analogous [5,15-dibromo-10,20-bis(phenyl)porphyrinate]Zn(II),18a it was possible to introduce selectively, by the Sonogashira reaction,32 first the donor 4-ethynylanisole moiety, to give the intermediate 8 with 43% yield. Still by the Sonogashira reaction of 8 with 4-ethynylbenzoic acid, the Zn(II) porphyrinate 1 was then obtained with about 43% yield. The overall yield of method B is higher (17%) than that of method A (11%). In addition, the synthesis of the starting product 7 is easier than that of 4. Finally, the one-step statistical approach, which is totally new, was found to be the most advantageous for the synthesis of both 1 and 2 (method C) (Supporting Information, page S6). The reaction was carried out by introducing simultaneously equimolar amounts of the donor and acceptor ethynyl aryl reagents so that the final products 1 and 2 were obtained, after purification by chromatography, with 40% and 30% yields, respectively. The lower yield for 2 can be attributed to the higher tendency of 4-ethynyl-N,N-dimethylaniline to give some homocoupling byproduct, as mentioned above.34 However, this one-step statistical approach cannot be considered a general method for the synthesis of this family of push pull Zn(II) porphyrinates, because, in the case of 3, with an ethynyl reagent with a totally fluorinated aromatic ring carrying the carboxylic group, we could not obtain the pure product 3 even after column chromatography. Therefore, it was necessary to obtain 3 by the statistical two-step process (method A),

Scheme 1. Synthesis of 1 Following the Three Different Methods A, B, and C

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Table 1. Electronic Absorption and Emission Bands of 13 in THF Solution and Absorption Bands when 13 Are Loaded onto TiO2 dye 1

λmax (nm)

ε (log ε)

λmax (nm)

in THF

(104 M1 cm1)

on TiO2

λem (nm) 667

451 (B)

30.5 (5.48)

451

659 (Q)

4.8 (4.68)

649

2

456 (B)

17 (5.23)

450

680

3

671 (Q) 454 (B)

4.9 (4.69) 10 (5.00)

658 444

694

673 (Q)

3.3 (4.51)

642

introducing, first, the 4-ethynyl-2,3,5,6-tetrafluorobenzoic acid acceptor moiety, with 39% yield, and then the 4-ethynyl-N, N-dimethylaniline donor moiety, with 72% yield. The pure product 3 was thus obtained with an overall yield of 28%. In general, when an ethynyl aryl moiety is carrying a too strong acceptor group, the method C is not the best one; for instance, when using the 4-ethynyl nitrobenzene reagent, the homologue of the Therien’s pushpull Zn(II) porphyrinate18 was not obtained with acceptable yields. Electronic Absorption and Emission Spectra. Electronic absorption spectra of dyes 13 have been recorded in THF solution (Table 1). As shown in Table 1, dyes 13 exhibit, in THF solution, the typical absorption bands of metal porphyrins:35 a strong B (Soret) band at about 450460 nm and a weaker, although still quite strong, Q band at about 660670 nm. All the three dyes satisfy the LambertBeer law in a range of concentrations, thus showing that, in THF solution, intermolecular interactions are efficiently hindered. It appears that the introduction of four fluorine atoms in the aromatic acceptor part of the molecule has a limited effect on the energy of the electronic transitions. However, moving from dye 1 to 2 and 3, the molar extinction coefficient, ε, of the B band significantly decreases from 30.5 104 M1 cm1 for 1 to 17 104 M1 cm1 for 2 and finally to 10.0 104 M1 cm1 for 3 (see Table 1), whereas the intensity of the Q band, quite unaltered by moving from 1 to 2 (ε = 4.8 104 M1 cm1 for 1 and 4.9 104 M1 cm1 for 2), sizably diminishes in the case of 3 (ε = 3.3 104 M1 cm1). The emission spectra of a THF solution of 1 and 2 are in agreement with those reported by Lin and co-workers.21 In the case of 3, we have a significant red shift of the emission of about 14 nm if compared to 2 (Table 1), although its intensity is similar. Dyes 13 exhibit similar singlet lifetimes ranging from 1.52 ns (1) to 1.24 ns (3) (Table 1 and Figure S1 in the Supporting Information). The long-lived triplet state (τ of about 2.32.5 μs in deareated solution; see the Supporting Information, Figure S2 and Table S1) is also populated within the instrumental resolution of our laser apparatus (kisc > 1.5 108 s1). The dyes 13 also show similar features in the triplet transient difference absorption spectra, which could be summarized by an intense absorption in the 500650 nm interval with a maximum around 550 nm, by the bleaching of the Q band centered at 700 nm and by a new strong low energy T1 Tn absorption extending to the NIR (Figure S3 of the Supporting Information). All three dyes, when absorbed on TiO2, show a significant blue shift of the energy of the B and, in particular, of the Q band if compared to the spectra in THF solution, with the only exception of the B band of 1 (Table 1). In particular, the Q band of 3

Figure 2. Diffuse reflectance spectra (transformed by the Kubelka and Munk method) of dyes 13 absorbed on opaque TiO2 from a THF solution added with 4 mM chenodeoxycholic acid.

shows a significant blue shift of 31 nm, whereas such a shift is only 13 nm for 2 and 10 nm for 1. It thus appears that, among the three dyes, 3 shows the most perturbed absorption spectrum. Interestingly, if the absorption on TiO2 is carried out from a THF solution without addition of chenodeoxycholic acid, even after prolonged heating (12 h), the intensity of the B band and, in particular, the Q band of 3 is very weak, indicating a poor absorption. The positive effect of the addition of chenodeoxycholic acid on the absorption of 3 was unexpected due to a competitive interaction with TiO2, generally observed with metalpolypyridine dyes. This fact suggests that the decreased hydrophyllic character of the TiO2 surface, coated with the low polar chenodeoxycholic acid, favors the impregnation of the fluorinated porphyrin (see Figure 2). It must be noticed finally that addition of 4-tert-butylpyridine to a THF solution of 1 produces a slight red shift of the Q band, mirrored by a more relevant red shift of the emission band, as reported for various Zn(II) porphyrinates in the presence of a pyridinic donor base.36 Computational Investigation. A computational DFT investigation on the structural and ground-state electronic properties of 1 and 2 has been already carried out by Lin and co-workers.21 Thus, we introduce here a DFT-TDDFT30 approach in order to take into account also the excited states and, therefore, to analyze correctly the electronic level origin of the B and, in particular, of the Q absorption bands of 13 and to highlight the possible differences by introduction of four fluorine atoms in the aromatic ring carrying the carboxylic group, as in the case of 3. For all the dyes 13, the planar geometry of the pushpull system21 is confirmed (Figure 3A), although for 3, an isoenergetic tilted geometry (Figure 3B) is also found, which is missing in the cases of 1 and 2. Starting from this tilted geometry and replacing the fluorine by hydrogen atoms, the geometry effectively returns to be strictly planar. A schematic energy diagram, reported in Figure 4, shows both the HOMO and LUMO energies and the combination of ground- and excited-state properties calculated for the planar pushpull system of dyes 13. The HOMO is calculated to lie below 5.0 eV, therefore, sufficiently positive compared to the redox potential of the I3/I electrolyte for efficient regeneration of 23174



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Figure 3. Optimized geometries of the planar (A) and tilted (B) structures of dye 3. Dyes 1 and 2 only adopt the planar geometry of the pushpull system.

the dye to take place.25 Sizable variations are observed in the HOMO energy by varying the nature of the donor group, with 1, which bears a weaker electron donor OMe group compared to the NMe2 group of 2, showing the more negative energy, corresponding to the highest oxidation potential. The impact, on the HOMO energy, of the substitution of the four hydrogen atoms with four fluorine atoms in the aromatic acceptor moiety as in 3 produces a small shift below 0.1 eV at more negative energy if compared to 2, but still less negative than in the case of 1. The variation in the LUMO energy of 13 is within 0.12 eV. For dye 3, the tilted geometry has HOMO and LUMO values similar to, within 0.04 eV, those of the planar geometry. The HOMO is, in all cases, localized on the donor moiety of the pushpull system and partly on the porphyrin macrocycle, whereas the LUMO extends from the porphyrin macrocycle to the acceptor moiety bearing the carboxylic acid anchoring group. The first excited state of all the three dyes is located at slightly lower energy with respect to the LUMO level; still, 2 shows the highest energy value, followed by 1 and 3, respectively (Figure 4). The charge-transfer character along the pushpull system of the transition composing the Q band is clearly shown by the electron density difference maps reported in Figure 5. In particular, we have a less pronounced charge transfer from the OMe than from the NMe2 group, while the increased acceptor properties of the aryl moiety carrying the anchoring carboxylic group by introduction of four fluorine atoms, as in 3, leads, as expected, to a substantial increase of the charge transfer toward the carboxylic group when compared to 2 (Figure 5). In summary, we have confirmed that, in dyes 13, a sizable electron density is transferred by excitation to the anchoring carboxylic group, at an energy level that ensures a dye excited state coupled with the unoccupied 3d states of the TiO2 conduction band.21 The more relevant accumulation of electron density, by excitation, on the anchoring carboxylic group in the dye 3 could suggest a more facile anchoring and a potentially increased electron transfer from the dye 3 to the TiO2 semiconductor, although the excited-state energy of 3 is the one closer to the calculated level of the TiO2 conduction band37 (Figure 4). We mention, however, that our calculations do not take into account the possible effect of the interaction of the dye with the TiO2 surface and, in particular, of the deprotonation due to the anchoring, which should be controlled by the strength of the carboxylic group. Such effects could lead to slightly different values

Figure 4. Energy diagram showing the HOMO and LUMO (dashed lines) energies, and also the lowest transition energies and the estimated excited-state oxidation potentials for dyes 13, compared to the energy levels calculated for TiO2.37

for the real excited-state energies, in particular, of 3, when the dye is absorbed on the surface of TiO2. By combining the ground- and excited-state energies calculated for dyes 13, an estimate of the excited-state oxidation potential for the dyes can be obtained according to a procedure developed by some of us.38 The excited-state oxidation potentials calculated for dyes 13 have been thus compared to the energy levels of the TiO2 conduction band edge calculated for a TiO2 cluster of nanometric dimensions37 (Figure 4). All the dyes 13 have an excited-state oxidation potential substantially higher than the TiO2 conduction band, implying thus favorable energetics for electron injection. The order of the excited-state potentials is 2 > 1 > 3, with a difference between 3 and 1 and 2 of 0.03 and 0.11 eV, respectively, consistent with the trend of energy of the Q band (Table 1). Moreover, this trend nicely correlates with the electrochemical data (see below). In Table 2, a comparison of the calculated versus experimental absorption maxima for dyes 13 is reported. Here, we resort to values calculated in vacuo since this approach was shown to yield results in closer agreement with the experimental trends for structurally related pushpull metal porphyrins.17b,20 23175



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Figure 5. Electron density difference plots between the ground and lowest excited state for dyes 13. A blue (yellow) color signals a charge depletion (accumulation) in a given molecular region. For the sake of simplicity, in position 10,20, simple phenyl substituents, not carrying two tert-butyl groups in the meta position, were taken into account in the calculations.

Table 2. Comparison between Calculated and Experimental Absorption Maxima Energies (eV) for Dyes 13 1

2

3

theoret

exptl

theoret

exptl

theoret

exptl

Emax (Q)

1.98

1.88

1.91

1.85

1.87

1.85

Emax (B)

2.92

2.75

2.99

2.72

3.00

2.74

The agreement between calculated and experimental values of the energy of the B and Q bands is rather good, with maximum deviations within 0.25 eV mainly for the B band. For the Q band, the values are comparable within less than 0.1 eV, nicely reproducing the experimental trend of the energy shifts, pointing at the overall accuracy of the computational procedure. However, we did not find the substantial intensity reduction of both the Q and the B bands experimentally observed moving from 2 to 3 (Table 1). Our calculations on the planar structures indeed provide essentially the same intensity, within the Q or B band, for dyes 13, possibly suggesting that the presence of other structural forms of 3 might be responsible for the lower intensity of B and Q bands of the dye 3 in THF solution (Table 1) and also (for the Q band) when it is adsorbed on TiO2 (Figure 2). To investigate the possible origin of these differences among dyes 2 and 3, we estimated the acidity of the carboxylic groups in 2 and 3. The latter, due to the presence of four fluorine atoms in the aromatic ring, is indeed expected to be considerably more acidic than the former. By taking the energy differences between the protonated and deprotonated dyes, we calculated dye 3 to be 2.4 pKa units more acidic than dye 2. Because of the increased acidity of 3, we have thus investigated the possible presence in THF solution of a zwitterionic form of 3, with the NMe2 group protonated and the carboxylic groups deprotonated (Figure 6A).

This zwitterionic form of dye 3 was computed to lie 11.3 kcal/ mol above that of the neutral form, whereas for dye 2, the zwitterionic form was calculated to be 14.8 kcal/mol less stable than the neutral form. These data seem to exclude a possible role of a zwitterionic species in determining the observed differences in the electronic absorption spectra, because the energy gap between the two forms is definitely too high. In light of the increased acidity of dye 3, we looked also for hydrogen-bonded dimers, where the carboxylic proton only weakly interacts, without protonation, with the nitrogen NMe2 group. An optimized stable structure for such a hydrogen-bonded dimer is reported in Figure 6B. This structure was calculated to be favored by 6.4 kcal/mol in vacuo but, in solution, to be essentially isoenergetic with the two noninteracting molecules. It follows that the main possible difference between dye 3 and dyes 1 and 2 is the existence in the former of an almost isoenergetic tilted structure (Figure 3B) or of a rather stable hydrogenbonded dimer (Figure 6B). The impact of these two forms on the electronic absortion spectrum was investigated by performing TDDFT calculations on both the tilted structure and the hydrogen-bonded dimer and, for the sake of completeness, also on the zwitterionic form of dye 3. In the two neutral forms, the lowest electronic transition, corresponding to the Q band, is found to be slightly blue shifted and of diminished intensity, in accordance with the trend of the experimental absorption spectrum (Table 1). In particular, for the tilted geometry, the energy of the Q band shifts from 1.87 to 1.91 eV (experimental, 1.85 eV) with an associated decrease of the oscillator strength from 1.11 to 0.91. Similar values were retrieved for the hydrogenbonded dimer, whereas a more drastic blue shift of the Q band, up to more than 2.1 eV, and a significant reduction of the oscillator strength characterizes the zwitterionic form of dye 3. Overall, we may suggest that the observed reduction of the absorption intensity of the Q band for dye 3 could be due to an 23176



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Figure 6. Optimized geometry of the zwitterionic form of dye 3 (A) and of the hydrogen-bonded dimer of dye 3 (B). For the sake of simplicity, in position 10,20, simple phenyl substituents, not carrying two tert-butyl groups in the meta position, were taken into account in the calculations.

almost isoenergetic tilted form or to the presence in solution of a strong hydrogen-bonded dimer, stabilized by the increased acidity of the carboxylic group. Electrochemical Behavior in Solution. The investigation of the redox behavior of the Zn(II) porphyrinates 13 is useful for the experimental determination of the ground- and excited-state oxidation potentials necessary to evaluate the processes of charge injection and dye regeneration. The cyclic voltammetry in THF/(TBA)PF6 at 20 mV/s is characterized by quasi reversible waves (peak separation about 100 mV), indicative of a relatively fast heterogeneous electron-transfer kinetic. For dye 2, carrying a NMe2 donor group, two monoelectronic oxidation waves with an E1/2 of 0.72 and 0.87 V vs SCE are reasonably well-resolved (Figure 7) and quite in agreement with the peaks at 0.78 and 0.96 V vs SCE obtained by working with differential scanning (DPV). By comparison with dye 1, carrying an OMe donor group, the first wave is ascribed to the oxidation of the NMe2 group, as first suggested by some of us for other related Zn(II) porphyrinates39 and later for this family of pushpull Zn(II) porphyrinates by other authors,21,22 while the second more positive oxidation wave is characteristic of the oxidation of the porphyrinic macrocycle, since 1 shows only one oxidation wave at 0.91 V vs SCE. However, for the dye 3, which also carries a NMe2 group, the two waves are poorly resolved since the oxidation of the NMe2 group is strongly anodically shifted with respect to 2 so that it appears as a shoulder, quite difficult to be detected, of a very broad oxidation wave with E1/2 about 0.98 V vs SCE. This latter observation is in support for dye 3 of a higher ground-state oxidation potential, possibly due to a perturbation

Figure 7. Cyclic voltammetry of 2 and 3 in THF/TBAPF6 at 20 mV/s.

of its NMe2 group by hydrogen bonding with the carboxylic group of another molecule, as suggested by the above theoretical investigation, or to a more facile π delocalization between the donor NMe2 group and the porphyrin macrocycle with parallel more difficult charge-transfer along the pushpull system. The first excited-state oxidation state potential of these porphyrinic dyes, calculated from EGS E00, where EGS is the ground-state oxidation potential (obtained from this cyclic voltammetry investigation) and E00 is the 00 spectroscopic energy obtained from the spectroscopic data, varies in the order of 1.03 V vs 23177



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Figure 8. IPCE spectra of the dyes 13 absorbed on opaque TiO2 from a THF solution added with chenodeoxycolic (4 mM) (electrolyte: 0.6 M PMII/ 0.1 M LiI/0.02 M I2; PEDOT counter electrodes). In the inset, the IPCE spectrum of 2 absorbed on opaque TiO2 from a EtOH/THF (9/1) solution, done in the absence of chenodeoxycholic acid is reported.

SCE (2) > 0.92 (1) V vs SCE > about 0.8 V vs SCE (3), in agreement with the trend of TDDFT theoretically calculated values of the excited-state oxidation potentials, which decrease in the order of 3.21 eV (2), 3.29 eV (1),and 3.32 eV (3) (Figure 4). Photoelectrochemical Investigation. The investigation was extended to some aspects of the photoelectrochemical behavior of 13 in DSSCs.25 (see the Experimental Section). Dyes 13 have been absorbed on transparent or opaque TiO2 from a THF solution added with 4 mM chenodeoxycholic acid. A less relevant, but still significant, absorption of the dyes 1 and 2 occurs from a EtOH/THF (9:1) solution without addition of chenodeoxycholic acid, whereas the absorption from a THF solution is much less effective, in particular, for dye 3. The photoaction spectra of dyes 13 absorbed on the opaque TiO2 photoanode, working with the liquid electrolyte, 0.6 M PMII/0.1 M LiI/0.02 M I2 in methoxyproprionitrile (MPN), are in agreement (Figure 8) with the reflectance spectra of the absorbed dyes (Figure 2), with the two maxima of IPCE (incident photon-to-current conversion efficiency) in correspondence with the B Soret band (about 450 nm) and with the Q band (about 650 nm) with the intensity decreasing in the order of 2 > 1 . 3. The highest values of IPCE, of the order of 7080% at about 450 nm and of 60% in the 688700 nm region, are shown by the dye 2 together with a broader response and a significant red shift of the photoactivity, with the onset at 750 nm. The poorest values of IPCE have been found for dye 3 with an IPCE of less than 40% at about 450 nm and less than 20% at about 650 nm. It must be noted that, in all cases, the light-harvesting efficiency (LHE = 110A(λ)), in correspondence with Q and B band maxima, was essentially unitary. Although, in DSSCs, the electron collection efficiency, ηcol, is usually independent of the excitation wavelength,40 most data refer to ruthenium dyes in which ε(λ) is smaller and varies smoothly with the wavelength. On the contrary, porphyrins exhibit two sharp and strong bands with different absorption intensities. Thus, the higher photoconversion observed in correspondence with the very intense B band may also arise from an improved electron collection

efficiency, since the major fraction of incident photons in the 400500 nm interval would generate charge carriers in close proximity to the FTO electron collector. These latter have a greater probability to escape recombination, which, instead, is more likely to occur when the electrons have to diffuse over a long distance through the TiO2 network. The illumination in the backside (FTO collector) and frontside (electrolyte side, through the counter electrode) shows indeed meaningful differences. When taking into consideration optically diluted electrodes (Supporting Information, Figure S4, type B), one clearly sees that, while in backside illumination, the B over Q maximum conversion ratio (IPCE (B450)/IPCE (Q650)) approaches 3.5, it becomes essentially unitary in the frontside illumination (Supporting Information, Figure S5). In fact, in this latter illumination mode, 450 nm photons are 90% absorbed within ca. 2.5 μm from the outermost TiO2 surface and the photogenerated electrons have to diffuse through at least 3.5 μm of TiO2 before reaching the collector, thus having a larger probability to recombine. An even more dramatic effect is found on optically dense electrodes (Supporting Information, Figure S4, type A) since, in the backside illumination, the IPCE (B450)/ IPCE (Q650) value of 3.6 is reduced to about 0.6 in the frontside illumination and the wavelengths less efficiently absorbed (i.e., 500600 and 700800 nm intervals) give the most significant contribution to the IPCE (Figure S6, Supporting Information). This observation can be explained by recalling that, again, photons absorbed in correspondence with the Q and B bands are 90% absorbed within a TiO2 thickness of 2.5 and 1 μm, respectively, and have to traverse from 3.5 to 5 μm of insulating TiO2 nanoparticles before being collected. Moreover, the internal TiO2 layers, practically in dark conditions, may have also a negative effect on the electron transport, since it has been reported that the electron buildup following illumination increases anatase film conductivity.41 Hence, it is expected that a white light bias may be beneficial for increasing the values of IPCE, since it should generally increase film photoconductivity. The effect of the white light bias was quite relevant on improving the very low IPCE recorded in the frontside illumination 23178



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Table 3. Photovoltaic Parameters for DSSCs Based on Dyes 13 and N719 Jsc (mA/cm2)

Voc (V)

Pmax (mW)

η (%)

FF

1

7.4

0.36

1.31

1.6

0.49

2 3

10.3 4.5

0.46 0.38

2.50 0.94

3.1 1.2

0.53 0.55

N719

14

0.56

2.9

3.7

0.38

dye

Figure 9. JV curves of dyes 13 adsorbed on opaque TiO2 with PEDOT counter electrodes, working with the electrolyte 0.6 M PMII/ 0.1 M LiI/0.02 M I2.

(Supporting Information, Figure S8) but was less evident when the IPCE was collected in the conventional backside illumination geometry (Supporting Information, Figure S7). Referring to this latter case, although the IPCE of the Q band undergoes a relative 16% improvement by passing from a bias of 0.1 to 1 sun, suggesting a better collection, the IPCE (B450)/IPCE (Q650) ratio does not change much (1.77 at 0.1 sun compared to 1.60 at 1 sun). This trend does not come unexpected, given that, in optically dense electrodes, light excitation in both the B and the Q regions generates electrons close to the back contact, where the charge collection efficiency is high. Thus, the higher photoconversion observed in correspondence with the B band with respect to the Q one can only be partly explained by improved electron collection, but it has also to be ascribed to a more energetically favorable charge injection from the B band, which corresponds to the population of the S2 excited state. A significant, although lower, IPCE has been observed for 2 when absorbed from a EtOH/THF 9:1 solution in the absence of chenodeoxycholic acid (Figure 8). Since the addition of chenodeoxycholic acid is a way to prevent aggregation of the porphyrinic dye,2f,42 which may be the origin of excited-state selfquenching, it appears that the two tert-butyl groups on the phenyl substituents in position 10,20 of the porphyrin ring act as effective spacers, although not to the extent of preventing completely some aggregation. The much lower photoaction intensity of 3 is quite unexpected since the high excited dipole along the pushpull system (Figure 5) should theoretically favor an increased electron injection into the conduction band of TiO2. The lower photoaction activity of 3 can be thus due to some negative factors, such as self-aggregation by hydrogen bonding in the layer, and to a decreased tendency to be absorbed on the hydrophilic TiO2 surface, due to the strongly hydrophobic character of the fluorinated aryl moiety. Moreover, dye 3 shows the lower first excited-state oxidation potential that may be the origin of a less effective charge transfer to the empty conduction band of TiO2. Indeed, comparative steady-state emission studies on dyes 13, absorbed on transparent TiO2 and ZrO2 photoanodes, this latter being regarded as an electrochemically inactive material, corroborate the trend typical of the intensity of the IPCE spectra and of the first excited-state oxidation potential. In fact, these studies reveal that, on TiO2, the dye 2 undergoes the

most effective value of the quenching ratio (25) by electron transfer from the excited state to the TiO2 surface, whereas dye 1 exhibits an intermediate value (8.5), followed by 3, which was found to be the less effectively quenched (3.3) (see the Supporting Information, Figure S9). This trend can be originated by various factors, including the lower first excited-state oxidation potential of 3, which may set the charge injection process in critical kinetic competition with the other excited-state deactivation pathways. In summary, it appears that a theoretically higher directional electron transfer along the pushpull system is not so significant for an increased charge injection into the empty band of the TiO2 semiconductor, since other factors, such as low absorption on TiO2, aggregation of the absorbed dye, and insufficient driving force for an activationless charge injection, play a major role in determining the photoconversion efficiency of 3. The JV curves (Figure 9), under 0.8 AM 1.5G (0.08 W/cm2) illumination, follow the trend of the photoaction spectra. The calculated photovoltaic parameters of the DSSCs based on 13 adsorbed on opaque TiO2, operating with a 0.6 M PMII/0.1 M LiI/0.02 M I2 electrolyte and PEDOT as the counter electrode are reported in Table 3. The best efficiency (3.1%) is found for the DSSC based on 2, which, as already pointed out, combines a high and broad IPCE. This value compares relatively well with the efficiency of a set of equivalent N719-sensitized cells43 characterized by a 3.7 ( 0.3% efficiency measured under identical experimental conditions (ca. 6 μm thick TiO2 layer). In all cases, the fill factor was nearly constant (about 0.5), suggesting that the series resistance of the cells was essentially the same. It must be pointed out that this investigation was done with the aim of getting a comparative analysis of the photoelectrochemical properties of pushpull porphyrins 13, without trying to maximize the device efficiency. With the aim of getting further insights on the behavior of DSSCs based on this family of dyes under solar illumination, impedance spectroscopy experiments were performed, in order to allow the simultaneous investigation of the interfacial chargetransfer and mass-transfer processes at the two electrodes. The impedance spectroscopy of the DSSCs under illumination at Voc44 (Figure 10) clearly shows, in the complex plane, two kinetic loops at high and mid-frequency, originated by redox reactions at the PEDOT counter electrode and at the TiO2/ electrolyte interface. The straight line that curves toward the real axis at very low frequencies has been ascribed to the transport of the redox couple in the bulk and within the mesoporous TiO2 surface layer. Thus, equivalent circuits of the type R(RctQ)(Q0 (Rct0 O))45 were successfully employed to fit the experimental impedance plots, where Q and Q0 are the constant phase elements (essentially nonideal capacitances) of the PEDOT counter electrode/electrolyte and TiO2/electrolyte interfaces, respectively, Rct is the charge-transfer resistance at the cathode, Rct0 is the charge-transfer resistance at the TiO2/electrolyte 23179



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Figure 10. Experimental (black) and calculated (red) Nyquist (complex plane) plots for DSSCs (0.25 cm2 active area) based on dyes 2, 1, and 3 absorbed on opaque TiO2. Data are recorded under illumination at Voc. The standard maximum deviation is of the order of 10%.

Figure 11. Cyclic voltammetries (20 mV/s) of dyes 1 (red), 2 (blue), and 3 (black) adsorbed on opaque TiO2. The arrows indicate the two separate redox processes observable in 2.

interface, and O is the Nernst diffusion element. The chargetransfer resistance at the PEDOT counter electrode varies between 13 and 25 Ω/cm2 (Ω = ohm), testifying to the good catalytic activity of the PEDOT substrate as the counter electrode. The most reliable value is 13 Ω/cm2 obtained from the plot of the cell based on dye 2, which shows two well-separated loops. The most relevant differences are shown by the chargetransfer resistance at the TiO2/electrolyte interface (essentially the projection of the middle loop on the real axis), which varies in the order of (3) (120 Ω/cm2) > (1) (81 Ω/cm2) > (2) (73 Ω/ cm2), an order of magnitude comparable to that reported for interfaces sensitized by effective ruthenium dyes,2f with a trend that follows clearly what is expected from the IPCE and JV curves. It must be recalled that a small charge-transfer resistance at the TiO2/electrolyte interface is a requisite for a good quality DSSC since it also reflects the interaction between the dye and the electrolyte under steady-state conditions.44b,46 Such a trend cannot be simply explained on the basis of the free energy difference for iodide oxidation since the cyclic voltammetry experimental data obtained on dyes absorbed on opaque TiO2 (Figure 11) show that the ground-state oxidation potentials of the various absorbed dyes are indeed not too different, although an accurate measurement is complicated by the insulating nature of TiO2 at the anodic potential and by a slow electron-transfer kinetic. In fact, the dye 2 shows, as expected, two waves with the first broad oxidation wave corresponding to the oxidation of the NMe2 group slightly shifted at cathodic potentials (ca. 0.75 V vs SCE) with respect to 1, which shows only one wave related to the oxidation of the porphyrinic macrocycle at about 0.9 V vs SCE. Dye 3 is characterized by a more positive value (ca. 1.0 V vs SCE), but, as observed in solution, the first oxidation wave, related to the oxidation of the NMe2 group, cannot be easily separated from the second very broad wave (Figure 11). This observation confirms that, for dye 3, even when absorbed on TiO2, the localization of the first hole on the NMe2 group is more difficult to achieve, this oxidation process being essentially superimposed with that of the porphyrinic macrocycle. This latter observation would support the presence, in the adsorbed layer of 3, of hydrogen-bonding interactions involving the NMe2 groups to

form aggregates with a decreased electron density of the nitrogen atom. These indications are confirmed by the analysis of the transient dye recovery kinetics on dye adsorbed on TiO2 (Supporting Information, Figure S10), which reveals that, within the series under investigation, 2 is able to establish the most favorable interaction with the electron mediator of the electrolytic solution (PMII/LiI), showing a 4-fold acceleration in regeneration of the dye (t2/3 = 0.18 μs, Table 4), resulting in an almost complete disappearance of the oxidized dye on a 2 μs time scale. Such facile regeneration is most probably due to the hole transfer to the nitrogen atom of the amino group, which, by establishing a good coupling with the electron donor I, promotes the subsequent electron transfer. In accordance, both 3 and 1, which, as evidenced by the electrochemical investigation, have either limited or no chance of hole localization on the amino donor group, are less effectively regenerated by the electrolyte, showing only about a 1.5 fold shortening of the dye cation lifetime and, therefore, a limited acceleration in regeneration (Table 4). This latter evidence is in support of the hypothesis, based on the electrochemical investigation, that the NMe2 group in dye 3 is strongly coupled with the electron system of the porphyrin ring. Finally, it must be noticed that the electron lifetime calculated at Voc from the frequency corresponding at the central peak of the imaginary part of the impedance (Z00 ) versus frequency plot44 (Figure 12) is, for the dyes 2 and 3, essentially identical (5.6 ms) and about 20% longer than that of 1 (4.2 ms), suggesting that the recombination kinetic of the electrons from the TiO2 working electrode to the oxidized dye is more efficient when the dye does not display, as in 1, the possibility to localize the hole on the amino group, more away from the TiO2 surface with respect to a hole located on the porphyrinic macrocycle. The similar lifetime of 2 and 3 is motivated by a slight decreased photoanode capacitance (from 3.1 104 F for 2 to 1.9 104 F for 3), which compensates for the increased charge-transfer resistance of the mid-frequency loop, accounting for the simplified transmission line of the TiO2 photoelectrode47 sensitized by 23180



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Table 4. Dye Cation Lifetime τ2/3a

τ2/3a

τ2/3a (LiClO4)/

dye

(LiClO4)b (μs)

(LiI/PMII)c (μs)

τ2/3a (LiI/PMII)d

1

0.49

0.32

1.53

2

0.68

0.18

3.8

3

1.28

0.86

1.48

a τ2/3 corresponds to the time at which 2/3 of the initial ΔA amplitude has been recovered. b In the presence of 0.1 M LiClO4. c In the presence of 0.1 M LiI/0.6 M PMII electron donor. d Lifetime ratio.

NMe2 group. The increased acidity of the carboxylic group and the hydrophobic character of the fluorinated aromatic ring of dye 3 may also contribute to explain this evidence, since they would result in a difficult hole delocalization and in a lower permeation of the hydrophilic TiO2 surface by the dye. Finally, we have confirmed that electrochemically deposited PEDOT electrodes may act as an efficient counter electrode48 for the cathodic reduction of I3.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed synthetic procedures, photophysical measurements in solution and on solid thin films, and additional photoelectrochemical measurements under different illumination geometries. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +39 02-50824363. E-mail: [email protected].

Figure 12. Imaginary part of impedance (Z00 ) vs frequency for the dyes 1 (red), 2 (blue), and 3 (black).

the dye 3, due to a lower steady-state electron density paralleled by a decreased conductivity of the semiconductor.

’ CONCLUSIONS A comparative study between three different pushpull Zn(II) porphyrinic dyes has been carried out with the aim of identifying how their structural and electronic properties are controlling the efficiency of the photon-to-electron conversion when acting as dyes in DSSCs. In particular, the new dye 3 with a strong electron-withdrawing character of the ethynyl aryl moiety carrying four fluorine atoms and the acceptor carboxylic group has been investigated and compared with the known dyes 1 and 2.21 We have thus shown that the IPCE of dyes 13, when adsorbed on the TiO2 semiconductor, is nearly twice more effective for the singlet excited state S2 involving the B absorption band than for that (S1) involving the Q band, although TDDFT calculations show for this latter band a singlet excited state characterized by a strong charge-transfer character directed along the pushpull system toward the anchoring carboxylic group. Although the substitution of four hydrogen atoms with four fluorine atoms in the aryl group of the ethynyl aryl acceptor moiety should theoretically enhance this latter charge-transfer process along the pushpull system, it lowers the first excitedstate oxidation potential of the dye, resulting in a lower charge injection efficiency. Moreover, the transient dye recovery kinetics shows, consistently with the electrochemical indications, a more facile regeneration of dye 2 with respect to 3, as a probable consequence of a more successful hole delocalization on the

’ ACKNOWLEDGMENT We thank the Fondazione CARIPLO (2008: Progettazione e utilizzo di nuovi materiali organometallici o di coordinazione per celle solari organiche di terza generazione) and the Italian MIUR (PRIN 2008: Progettazione e sviluppo di nuovi componenti per celle solari a semiconduttori sensibilizzati ad alta efficienza) for financial support. F.D.A. thanks Fondazione Istituto Italiano di Tecnologia, Project SEED 2009 “HELYOS” for financial support. We thank Dr. William Galbiati for precious collaboration in the synthesis of some dyes. The technical assistance of Dr. Rita Boaretto (UNIFE) is also acknowledged. ’ REFERENCES (1) (a) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (b) Pagliaro, M.; Palmisano, G.; Ciriminna, R. Flexible Solar Cells; Wiley-VCH: Weiheim, Germany, 2008. (c) Barabec, C., Scherf, U., Dyakonov, V., Eds. Organic Photovoltaic Materials, Device Physics and Manufacturing Technologies; Wiley-VCH: Weiheim, Germany, 2008. (2) (a) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (b) Hagefeldt, A.; Gr€atzel, M. Chem. Rev. 1995, 95, 49–68. (c) Rehm, J. M.; McLendon, G. L.; Nagasawa, Y.; Yoshihara, K.; Moser, J.; Gr€atzel, M. J. Phys. Chem. B 1996, 100, 9577–9578. (d) Gr€atzel, M. Nature 2001, 414, 338–344. (e) Nazeeruddin, M. K., Ed. Coord. Chem. Rev. 2004, 248, 1161. (f) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595–6663 and references therein. (3) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; M€uller, E.; Liska, P.; Vlachopoulos, N.; Gr€atzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (4) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Le, C.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gr€atzel, M. J. Am. Chem. Soc. 2001, 123, 1613–1624. (5) Wang, P.; Zakeeruddin, S. M.; Moser, J.; Nazeeruddin, M. K.; Sekiguchi, T.; Gr€atzel, M. Nat. Mater. 2003, 2, 402–407. (6) (a) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B; Gr€atzel, M. J. Am. Chem. Soc. 2005, 127, 16835–16847. (b) De Angelis, F.; Fantacci, S.; Selloni, A.; Nazeeruddin, M. K. Chem. Phys. Lett. 2005, 415, 115–120. 23181


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