Article pubs.acs.org/JPCC
Femtosecond Transient Absorption Study of Supramolecularly Assembled Metal Tetrapyrrole−TiO2 Thin Films Habtom B. Gobeze, Sushanta K. Das, and Francis D’Souza* Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States S Supporting Information *
ABSTRACT: Photoexcited electron injection and back electron transfer dynamics of metal tetrapyrrole bound to TiO2 nanoparticle surfaces via the metal−ligand axial coordination approach have been investigated using femtosecond pump−probe transient spectroscopic technique. The employed metal tetrapyrroles include zinc and magnesium metalated meso-tetraarylporphyrins having halogen substituents on the peripheral aryl groups, perfluorinated zinc phthalocyanine derivatives, and zinc naphthalocyanine. The employed metal tetrapyrroles covered absorption at different portions of the visible and near-IR region of the spectrum with excited-state reduction potentials ranging between −0.61 eV and −1.34 V, that is, having energy higher than the TiO2 conduction band edge (−0.57 V vs NHE). Two linkers, pyridine and phenylimidazole, have been employed to visualize electronic coupling between the dye and metal oxide surface for optimal electron injection and back electron transfer dynamics. In agreement with the previously reported photocurrent generation of dyesensitized solar cells constructed using this self-assembly approach (J. Am. Chem. Soc. 2009, 131, 14646), spectral evidence for electron injection from the excited metal tetrapyrrole to the TiO2 nanoparticle in the form of a tetrapyrrole radical cation has been obtained. The time profile of the π-cation radical of tetrapyrroles revealed the occurrence of ultrafast electron injection (time constant = 470−700 fs), while the back electron transfer processes were found to be complicated due to the intricate environment of the metallotetrapyrrole−TiO2 interface.
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INTRODUCTION Light-to-electricity conversion using dye-sensitized solar cells (DSSCs) has been a widely studied topic in recent years as a renewable alternative to fossil fuel based energy sources.1−12 In typical DSSCs, nanocrystalline semiconductors (TiO2, SnO2, ZnO, etc.) decorated with photosensitizer molecules inject electrons upon photoexcitation into the conduction band of the semiconductor, achieving light energy conversion.13−42 The dye molecules are often covalently immobilized on the semiconductor metal oxide surfaces using carboxylic acid anchoring groups.43−53 Using ruthenium polypyridyl or porphyrin-based dyes in conjunction with mesoporous TiO2 semiconductor light-to-electricity conversion efficiencies (η) over 12% has been accomplished,54−56 making DSSCs an economic reality for replacing expensive Si-based solar cells and fossil fuels. Recently, by exploiting the metal−ligand coordination chemistry of metallotetrapyrroles, we reported on the design of supramolecular solar cells as an alternate method to the covalent immobilization of dye molecules.57,58 Here, the nitrogen donor ligands were functionalized with a carboxyl acid anchoring group that bound to TiO2 and served as the coordinating linker between the metallotetrapyrrole and TiO2 surface. A key advantage of this modular assembly approach was that it allowed employing different sensitizers (both metalloporphyrin and metallophthalocyanine derivatives) having different redox and spectral properties and permitted us to verify their ability of photocurrent generation.57,58 In addition, © 2014 American Chemical Society
using this strategy it was possible to immobilize complex photosynthetic model compounds for improved cell performance. For example, by using zinc porphyrin−ferrocene, a donor1−donor2 type dyad, improved DSSC performance (compared to a cell made out of only zinc porphyrin) was achieved due to a possible electron transfer−hole transfer mechanism.57 By immobilizing a zinc porphyrin−zinc phthalocyanine dyad,59 better photocurrent was accomplished due to a photoinduced energy transfer followed by an electron injection mechanism. Anion-binding-induced photocurrent enhancement in the case of a zinc porphyrin−oxoporphyrinogen dyad immobilized solar cell (anions bind to oxoporphyrinogen imino hydrogens), due to redox modulation of oxoporphyrinogen, was demonstrated.60 Thus, this versatile supramolecular self-assembly approach has provided us an opportunity to explore mechanisms for improving light energy conversion which otherwise would be difficult to accomplish. The interfacial electron transfer including electron injection and back electron transfer are critical to the overall efficiency of the DSSC, among other factors.61−67 Using femtosecond transient spectroscopy, the electron transfer dynamics of both ruthenium polypyridyl/TiO261−67 and porphyrin/TiO268−72 Special Issue: Michael Grätzel Festschrift Received: December 26, 2013 Revised: January 22, 2014 Published: January 31, 2014 16660
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Scheme 1. Thin-Film MP/TiO2 on a Fluorine-Doped Tin Oxide (FTO) Surface Held by Metal−Ligand Axial Coordination via (a) Pyridine (Py) and (b) Phenylimidazole (PhIm) Ligand Spacersa
a
See Scheme 2 for MP structures and abbreviations.
Scheme 2. Structure of the Tetrapyrrole Sensitizers Employed to Decorate the TiO2 Surface via Metal−Ligand Axial Coordinationa
a
Abbreviations: (TPP)Mg = magnesium meso-tetraphenylporphyrin, (TTP)Zn = zinc meso-tetratolylporphyrin, ((CF3)4TPP)Zn = zinc meso-tetra(4trifluoromethylphenyl)porphyrin, (F20TPP)Zn = zinc meso-tetra(pentafluorophenyl)porphyrin, (Pc)Zn = zinc tetra-t-butyl-phthalocyanine (tBu= terbutyl), (F8Pc)Zn = zinc octafluorophthalocyanine, (F16Pc)Zn = zinc hexadecafluorophthalocyanine, and (Nc)Zn = zinc tetra-t-butylnaphthalocyanine.
revealed an intense Soret band in the 420 nm range and two Qbands in the 500−650 nm range.73 Among the porphyrin derivatives, (F20TPP)Zn revealed the highest blue-shifted Soret band located at 416 nm due to the presence of 20 electronwithdrawing fluorine atoms on the meso-aryl substituents, while the electron-rich (TPP)Mg revealed the highest red-shifted Soret band at 424 nm and Q-bands at 563 and 604 nm. A similar trend was also observed in the fluorescence spectrum of the porphyrin derivatives (Figure 1b). While the emission bands of the blue-shifted (F20TPP)Zn were located at 582 and 638 nm, the corresponding bands of red-shifted (TPP)Mg were located at 607 and 661 nm, respectively. The absorption and emission spectral changes for the perfluorinated zinc phthalocyanine derivatives were much larger due to direct attachment of fluorines to the phthalocyanine macrocycle (Figures 1c and d). Substitution of eight fluorines to the phthalocyanine macrocycle red-shifted the (Pc)Zn Q-band from 680 to 712 nm, while substitution of 16 fluorines shifted the visible band even further to 732 nm. The Q-band of (Nc)Zn appeared at 768 nm due to extended
systems have been reported in a few studies. In the present study, we report the interfacial electron transfer dynamics of metal tetrapyrrole/TiO2 assembled using the metal−ligand coordination approach (Scheme 1). Two ligands, viz., 4carboxypyridine and 4-carboxyphenylimidazole, are utilized to link TiO2 and metal tetrapyrroles. The former linker would place the TiO2 and metal tetrapyrrole about 6.2 Å apart, while the latter would place them about 8.5 Å apart, thus allowing us to probe the effect of nanoparticle−dye distance on the kinetics of electron transfer. Eight metal tetrapyrroles including few high potential zinc porphyrins and zinc phthalocyanines have been employed as photosensitizers (Scheme 2). Structuredynamics properties of the metal tetrapyrrole/TiO2 interface, in relation to the free energy of electron injection, are systematically investigated.
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RESULTS AND DISCUSSION Figure 1a shows the absorption spectrum of the employed metalloporphyrins in dichloromethane. Irrespective of whether Zn or Mg is in the central cavity, the porphyrin derivatives 16661
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Figure 1. Optical absorbance (a and c) and emission (b and d) of utilized metalloporphyrins (a and b) and zinc phthalocyanine/naphthalocyanine (c and d) in toluene. The concentrations ranged between 6 and 7 μM.
designed, and the occurrence of photoinduced electron/energy transfer events has been demonstrated.76−84 In noncoordinating solvents, the binding constants for the 1:1 complexes range between 7.0 × 103 and 5.0 × 105 M−1 depending upon the nature and substituents on the macrocycle periphery.74,75 Additionally, for electron-deficient tetrapyrroles, such as the fluorinated tetrapyrroles discussed here, even higher binding constants have been reported.70 The binding constant for pyridine and phenylimidazole, the coordinating spacer used in the present study, varies between 5 × 103 M−1 and 3 × 105 M−1 depending upon the nature of the ligand, metal macrocycle, and solvent polarity.83,84 Further, electrochemical studies were performed using cyclic and differential pulse voltammetric techniques to evaluate the redox potentials of the employed metal tetrapyrroles.85 The presence of fluorines on the porphyrin and phthalocyanine macrocycles revealed a strong influence; that is, easier reduction and harder oxidation with an increase in fluorine substituents was observed. In agreement with literature data,86 within the potential window of the solvent, two one-electron oxidations and two one-electron reductions were observed for each of the employed metallotetrapyrroles. Further, the excited-state reduction potential and free-energy change for electron injection for the employed metallotetrapyrroles were calculated according to the Rehm−Weller approximation,87 using eqs 1 and 2.
conjugation of the macrocycle. Such a trend was also observed in the fluorescence emission spectra. While the emission maxima of (Pc)Zn was located at 692 nm, the corresponding peak maxima for (F8Pc)Zn and (F16Pc)Zn were at 727 and 744 nm, respectively. The latter two derivatives also revealed diminished fluorescence intensity. The emission maxima of (Nc)Zn was located at 770 nm. This study showed absorption and emission features of the employed sensitizers covering different portions of the visible/near-IR portions of the spectrum. The information gathered here was further used in calculating the singlet energy of the employed sensitizers (vide infra). The lifetimes of singlet excited states of the employed tetrapyrroles were also determined using the time-correlated single photon counting method in dichloromethane by employing nanoLED of appropriate wavelengths for excitation. The lifetimes calculated from the monoexponential decay were found to be (TPP)Mg = 5.1 ns, (TTP)Zn = 1.85 ns, ((CF3)4TPP)Zn = 1.91 ns, (F20TPP)Zn = 1.22 ns, (Pc)Zn = 3.14 ns, (F8Pc)Zn = 2.85 ns, (F16Pc)Zn = 2.84 ns, and (Nc)Zn = 2.42 ns. That is, fluorine substitution on both porphyrin and phthalocyanine macrocycles reduced the excited-state lifetime to some extent. The binding ability of zinc and magnesium porphyrins and zinc phthalocyanines to nitrogenous ligand(s) is a very wellknown phenomenon.74,75 Using this binding concept, several elegant supramolecular donor−acceptor assemblies have been 16662
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E o(MP•+/1MP*) ∼ E o(MP•+ /MP*) − E0 − 0 /e
(1)
ΔGinj = E o(MP•+ /1MP) − ECB(TiO2 )
(2)
o
all of the excited-state reduction potentials are more negative (higher in energy) than the conduction band edge of TiO2, thus electron injection is possible from this state. However, the potentials of the (F20TPP)Zn and (F16Pc)Zn are very close to the conduction band edge potential; in such a case, electron injection into TiO2 could occur through S2 excitation of the sensitizer (vide infra). Generally, the S2 excited states are 0.8− 1.2 eV higher in energy compared to the S1 excited state of the tetrapyrroles,71 providing the necessary driving force. Femtosecond Pump−Probe Transient Absorption Studies. Time-resolved transient absorption spectroscopy is a proven method to probe electron injection and recombination dynamics of the semiconductor dye/nanoparticle interface.61−72 In this technique, femtosecond laser pulses of about 90−100 fs are used to excite the interface while monitoring the formation of transient species characteristic of electron injection products, in this case, the π-radical cation of metallotetrapyrrole and reduced TiO2 species. The π-radical cation of metallotetrapyrrole reveals absorption in the visible/ near-IR portion of the spectrum, while the reduced TiO2 reveals absorption in the infrared region88 beyond the spectral range of the current instrumental setup. Thus, it was important to know the absorption spectral features of π-radical cation species of each of the employed metallotetrapyrroles. To secure this information, the metallotetrapyrroles were chemically oxidized by the addition of one equivalent of nitrosonium tetrafluoroborate in methanol/dichlorobenzene which oxidized the metallotetrapyrroles except for (F20TPP)Zn and (F16Pc)Zn due to their higher oxidation potential demand (see Table 1). The formation of π-radical cation species of ((CF3)4TPP)Zn and (F 8 Pc)Zn, as examples of typical porphyrin and phthalocyanine, is shown in Figure 3, while for the rest of the tetrapyrroles, the spectral changes are shown in the Supporting Information (Figures S1 and S2). In the case of ((CF3)4TPP)Zn (Figure 3a), new bands at 605, 683, and 868 nm and diminished in intensity accompanied by red-shifted Soret and visible bands were also observed. Similar observations were also made to other metalloporphyrins (Figure S1, Supporting Information). In the case of (F8Pc)Zn (Figure
•+ 1
where E (MP / MP*) is the excited-state potential for the porphyrin radical cation/singlet excited porphyrin couple; Eo(MP•+/MP) is the ground-state potential for the porphyrin radical cation/porphyrin couple; E0−0 is the estimated porphyrin ground-state to porphyrin singlet-state transition energy (S0 − S1 energy difference); and e is the elementary charge of an electron. ΔGinj is the free-energy change for electron injection, and ECB is the conduction band edge potential of TiO2. The values of the measured excited-state reduction potentials and free-energy change for electron injection are relevant since they determine the thermodynamic feasibility of photoinduced electron transfer from the excited MP into the TiO2 conduction band. These values are listed in Table 1, while Figure 2 presents Table 1. Electrochemical Oxidation Potential, Singlet Excited-State Reduction Potential, and Free Energy of Electron Injection into TiO2 of the Metallotetrapyrrole Sensitizers MP (TTP)Zn (TPP)Mg (F20TPP)Zn ((CF3)4TPP) Zn (Pc)Zn (F8Pc)Zn (F16Pc)Zn (Nc)Zn
Eox (V) vs NHE
ECB(TiO2) (V) vs NHE
E0−0 (eV)
Eo(P•+/1P*) (eV)
ΔGinj (eV)
0.92 0.76 1.42 1.13
−0.57 −0.57 −0.57 −0.57
2.11 2.10 2.13 2.11
−1.19 −1.34 −0.71 −0.98
−0.62 −0.77 −0.14 −0.41
0.64 1.00 1.06 0.51
−0.57 −0.57 −0.57 −0.57
1.79 1.74 1.67 1.61
−1.15 −0.74 −0.61 −1.10
−0.58 −0.17 −0.04 −0.53
an energy level diagram showing the excited-state reduction potential of the employed metallotetrapyrroles in relation to the conduction band of TiO2. As seen from this data and figure,
Figure 2. Energy level diagram relevant to electron injection involving the different metal tetrapyrroles. For numerical values of excited-state reduction potential, refer to Table 1. For TiO2, only the position of the conduction band edge (at neutral pH) is shown. 16663
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Figure 3. Spectral changes observed during increasing addition of nitrosonium tetrafluoroborate oxidant (0.2 equiv of each addition) to a solution of (a) ((CF3)4TPP)Zn and (b) (F8Pc)Zn in methanol/dichloromethane.
some of the electrodes are also shown. The absorbance for the Q-bands was around 0.8−0.9 for porphyrins (slightly redshifted due to axial coordination) and 1.1−1.3 for phthalocyanines revealing sufficient amounts of bound dye on the surface. It may be mentioned here that the dye adsorption was significantly less (OD = 0.1−0.2) for unmodified pristine TiO2, signifying the importance of the coordinating ligand for dye immobilization. The electrodes once prepared were stored in dark prior to performing spectral measurements. Prior to performing transient absorption studies on the thinfilm MP/TiO2 surfaces, femtosecond transient spectral studies for all of the metallotetrapyrroles in toluene were performed, and the details are summarized elsewhere.85 To a large extent, the spectral features of fluorinated porphyrins and phthalocyanines were similar to that observed for (TTP)Zn and (Pc)Zn derivatives in the literature.69,72,85 Excitation by the laser light instantly populated the S1 and S2 states of the porphyrins with a maximum in the ∼470 nm range. At the higher wavelength region, two depleted signals in the Q-band region for zinc and magnesium porphyrins and one band for zinc phthalocyanines, opposite mimics of the ground-state absorption of the Q-bands were observed. With time, these bands revealed a small red-shift and appeared in the region of the Q(0,0) band of steady-state emission. Collectively, these results point out the occurrence of depleted absorption of the S0 → S1 and S0 → S2 transitions and stimulated emission of the S1 → S0 in the transient absorption spectra with lifetime close to the earlier discussed TCSPC results within the monitored time window (3 ns), and populating the triplet excited states was observed. Figure 5 shows the transient spectra of metalloporphyrin on PhIm surface modified TiO2 thin films. Generally, recovery of the visible bands and the depleted emission bands for TiO2 surface bound metalloporphyrins were much faster than that observed for the metalloporphyrin probe alone in solution indicating the occurrence of additional photochemical events. Importantly, in the visible region of 630−750 nm and in the near-IR region of 800−1100 nm, characteristic absorption bands of [(P)Mg]•+ and [(P)Zn]•+, as established from earlier discussed chemical oxidation, were observed providing evidence for electron injection from photoexcited metalloporphyrin to TiO2. The spectral features for (TPP)Mg and (TTP)Zn were almost identical (Figures 5a and b) indicating the π-cation radical nature of the species whose optical signature was independent of redox neutral magnesium or zinc metal ions in the cavity. The π-cation radical signature generated for ((CF3)4TPP)Zn and (F20TPP)Zn was much better defined in
3b), complete bleaching of the Q-band and the appearance of new bands at 577, 743, and 865 nm of the π-radical cation species were observed. The appearance of these bands in the transient spectrum will serve as a direct proof of electron injection from excited metallotetrapyrrole to TiO2. The preparation of TiO2 thin films decorated with metallotetrapyrrole via metal−ligand axial coordination is given in the Experimental Section. In brief, first, a thin film of TiO2 (anatase, 18 NRT, Dyesol, about 6 μM thick) on FTO was formed by a doctor-blade technique, followed by treating the electrodes with either pyridine carboxylic acid or 4imidazolylbenzoic acid, to provide the axial ligand functionality. Next, the electrodes were dipped in a dichlorobenzene solution of the desired metallotetrapyrrole (3−5 h). The electrodes were then rinsed with copious amounts of dichlorobenzene to remove unbound dye molecules. Figure 4 shows the spectrum of representative ((CF3)4TPP)Zn and (Pc)Zn on the TiO2 surface modified with 4-imidazolylbenzoic acid. Pictures of
Figure 4. Absorption spectrum of (i) ((CF3)4TPP)Zn and (ii) (Pc)Zn on the thin-film TiO2 surface modified with 4-imidazolylbenzoic acid. Top: pictures of the thin-film TiO2 surface modified with 4imidazolylbenzoic acid bound to (A) (TTP)Zn, (B) (TPP)Mg, (C) (F20TPP)Zn, (D) ((CF3)4TPP)Zn, (E) (Pc)Zn, (F) (F8Pc)Zn, and (G) (Nc)Zn. 16664
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Figure 5. Femtosecond transient absorption spectra of metalloporphyrin on PhIm surface modified TiO2 thin films at different delay times: (a) (TPP)Mg, (b) (TTP)Zn, (c) ((CF3)4TPP)Zn, and (d) (F20TPP)Zn. The samples were excited with 400 nm femtosecond (90 fs) laser pulses. The figure inset shows the time profile of the π-cation radical peak monitored at 700 nm for (TPP)Mg, 660 nm for (TTP)Zn, 700 nm for ((CF3)4TPP)Zn, and 700 nm for (F20TPP)Zn.
Figure 6. Femtosecond transient absorption spectra of metalloporphyrin on Py surface modified TiO2 thin films at different delay times: (a) (TTP)Zn, (b) (F20TPP)Zn, and (c) ((CF3)4TPP)Zn. The samples were excited with 400 nm femtosecond (90 fs) laser pulses. The corresponding time profile of the π-cation radical peak monitored at 660 nm for (TTP)Zn, and 700 nm for both (F20TPP)Zn and ((CF3)4TPP)Zn is shown below.
The spectral features for metalloporphyrin on Py surface modified TiO2 thin films were similar to the corresponding
the near-IR region with the appearance of a band at 920 nm for [((CF3)4TPP)Zn]•+ and 910 nm for [(F20TPP)Zn]•+. 16665
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Table 2. Rise Time and Electron Injection Rate, Decay Component Times, and Back Electron Transfer Rates of Investigated Supramolecularly Decorated MP/TiO2 Thin-Film Surfaces MP (TPP)Mg (TTP)Zn ((CF3)4TPP) Zn (F20TPP)Zn (Pc)Zn (Nc)Zn (F8Pc)Zn (F16Pc)Zn a
ΔGinj, eV
liganda
peak λ, nm
rise time, fs
kEI, s−1
decay time-1, ps
kBET‑1, s−1
12
decay time-2, ps
kBET‑2, s−1
−0.77 −0.62 −0.62 −0.41
PhIm Py PhIm Py
700 660 660 700
560 720 639 587
1.78 1.38 1.56 1.70
× × × ×
10 1012 1012 1012
1.20 4.10 3.39 2.92
8.32 2.44 2.95 3.43
× × × ×
10 1011 1011 1011
16.1 22.9 24.0 16.9
6.20 4.40 4.16 5.93
× × × ×
10 1010 1010 1010
−0.41 −0.14 −0.14 −0.58 −0.53 −0.53 −0.17 −0.17 −0.04
PhIm Py PhIm PhIm Py PhIm Py PhIm PhIm
700 700 700 852 900 900 920 920 975
640 587 667 560 480 507 474 507 487
1.56 1.70 1.51 1.78 2.08 1.97 2.11 1.97 2.05
× × × × × × × × ×
1012 1012 1012 1012 1012 1012 1012 1012 1012
2.16 1.98 1.66 1.71 0.77 0.80 1.36 2.39 1.10
4.62 5.05 6.02 5.83 12.5 5.83 7.35 4.18 9.09
× × × × × × × × ×
1011 1011 1011 1011 1011 1011 1011 1011 1011
16.7 22.5 27.8 25.2 6.34 7.88 9.40 22.0 28.0
5.97 4.45 3.60 3.97 15.8 12.7 10.6 4.54 3.56
× × × × × × × × ×
1010 1010 1010 1010 1010 1010 1010 1010 1010
11
decay time-3, ps
kBET‑3, s−1
200 433
5.00 × 109 2.31 × 109
10
Py = pyridine carboxylic acid and PhIm = 4-carboxyphenyl imidazole modified TiO2 surfaces.
Figure 7. Femtosecond transient absorption spectra of zinc phthalocyanine and zinc naphthalocyanine on PhIm surface modified TiO2 thin films at different delay times: (a) (Pc)Zn, (b) (F8Pc)Zn, (c) (F16Pc)Zn, and (d) (Nc)Zn. The samples were excited with 400 nm femtosecond (90 fs) laser pulses. The figure inset shows the time profile of the π-cation radical peak monitored at 852, 920, 975, and 900 nm, respectively, for (Pc)Zn, (F8Pc)Zn, (F16Pc)Zn, and (Nc)Zn.
PhIm surface modified TiO2 thin films (see Figure 6). That is, clear evidence of electron injection into the TiO2 surface in the form of π-cation radical transient species of the sensitizer was observed in this closely spaced sensitizer−TiO2 system. The kinetic analysis of the π-cation radical peak at one or two of their prominent wavelengths was performed, and the kinetic trace for each of the investigated metalloporphyrin/TiO2 is shown as an inset for each studied system in Figure 5 and below the corresponding transient spectra in Figure 6. The rise time for the development of the π-cation radical ranged
between 560 and 700 fs, which was slightly higher than the duration of the femtosecond laser pulse, being about 90 fs with
