J. Phys. Chem. C 2008, 112, 19699–19709
19699
Energy- and Electron-Transfer Processes in Corrole-Perylenebisimide-Triphenylamine Array Mariusz Tasior,† Daniel T. Gryko,*,† Jing Shen,‡ Karl M. Kadish,*,‡ Thomas Becherer,§ Heinz Langhals,§ Barbara Ventura,| and Lucia Flamigni*,| Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, Warsaw, Poland, Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, Department of Chemistry, LMU UniVersity of Munich, Butenandstrasse 13, 81377 Munich, Germany, and Istituto ISOF-CNR, Via P. Gobetti 101, 40129 Bologna, Italy ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: October 15, 2008
A multichromophoric system, TPA-C-PI, possessing a corrole as the central unit linked to perylene bisimide and N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine was efficiently synthesized, as were the component dyads C-PI and C-TPA. The combination of known methodologies with the newest synthetic advances in Pdcoupling reactions enabled a six-step preparation of the final compounds in a convergent fashion. Several different synthetic strategies were investigated, and one with Sonogashira coupling involved in the final step was successful. Electrochemical and spectroscopic studies confirmed a weak coupling between the component units in the triad which was characterized by an extensive absorption throughout the 300-700 nm region. Photophysical measurements showed efficient energy transfer to occur from the diphenylacetylene substituted aromatic amine (99%) and perylene bisimide unit (100%) to the corrole and subsequent electron transfer from the corrole to perylene bisimide (97% efficient). The final charge separated state in TPA-C-PI was identified as TPA-C+-PI-, with a lifetime of 1.9 ns, similar to that of C+-PI- (2.3 ns). The charge separated state with charges on the extreme components, TPA+-C-PI-, is not formed for thermodynamic reasons. Introduction The crucial role played by photosynthesis in providing energy to all living organisms on Earth has induced researchers to study this process for some time.1 The natural photosynthetic system is regarded as one of the most elaborate nanobiological machines, and both crystallization of these systems2 as well as advanced model studies of energy transfer and charge separation continue to provide thorough in-depth insight into its mechanism.3-5 The core function of photosynthesis is a cascade of photoinduced energy and electron transfers between donors and acceptors in the antenna complexes and the reaction center. Although many diverse model systems have been synthesized and studied and significant progress has been achieved, numerous problems still remain.6,7 During the past decades, the challenge of creating artificial light-harvesting systems has prompted development of novel synthetic chromophores and led to a diverse collection of arrays capable of mimicking the key steps in photosynthesis. Among various systems, porphyrins are of particular importance as building blocks for the construction of models.8,9 Other porphyrinoids (as porphyrin tapes,10 phthalocyanines,11 chlorins,12 and porphyrins with expanded π-conjugated system13) have also been examined in these models although to less extent. Among different structural scaffolds which fulfill the basic spectroscopic and photophysical preconditions, corroles14,15 have become a popular subject of research.16 Recent advances in their synthesis17 has given an additional boost to the case of corroles in various fields, and these macrocycles have emerged as attractive molecular building †
Polish Academy of Sciences. University of Houston. § LMU University of Munich. | Istituto ISOF-CNR. ‡
blocks for incorporation into donor-acceptor ensembles where they function mainly as an electron donor. Our own laboratories and that of others have investigated energy and electron transfer phenomena in systems containing corroles covalently linked with chromophores such as porphyrins,18 napthaleneimides,19 and perylene bisimides.20,21 Both electron transfer and energy transfer were observed in these bichromophoric systems. Major challenges involved in the design of more complex systems involve the need for bettering the choice of light-absorbing components and the appropriate three-dimensional arrangement of the component units. The resulting array of chromophores must absorb strongly across the spectrum of incident light and enable excited-state energy migration among components. Dyads comprised of perylene bisimide and corrole showed interesting characteristics,20 and we therefore decided to explore this further via incorporation of an accessory chromophore as an energy donor at position 5 and 15 of the corrole ring. Presented below are the results of this investigation. Experimental Section General Synthetic Section. All chemicals were used as received unless otherwise noted. Reagent grade solvents (CH2Cl2, hexanes, cyclohexane) were distilled prior to use. All reported 1H NMR and 13C NMR spectra were recorded on AM 500 or 400 MHz spectrometer. Chemical shifts (δ ppm) were determined with TMS as the internal reference; J values are given in Hz. UV-vis spectra were recorded in toluene. Chromatography was performed on silica (200-400 mesh), or dry column vacuum chromatography (DCVC)22 was performed on preparative thin layer chromatography silica. Mass spectra were obtained via EI, field desorption (FD), or electrospray MS (ESI-MS). The purity of all new corroles was established based on 1H NMR spectra and elemental analysis. The following
10.1021/jp8065635 CCC: $40.75 2008 American Chemical Society Published on Web 11/17/2008
19700 J. Phys. Chem. C, Vol. 112, No. 49, 2008 compounds were synthesized according to literature procedures: 1,23 7,20 and 8.24 The detailed experimental procedures and analyses of compounds 2-5, C, C-TPA, C-PI, TPA-C-PI, and TPA are in Supporting Information. Electrochemistry. Cyclic voltammetry was carried out with an EG&G model 173 potentiostat/galvanostat. A homemade three-electrode electrochemistry cell was used and consisted of a platinum button or glassy carbon working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). The SCE was separated from the bulk of the solution by a fritted-glass bridge of low porosity which contained the solvent/supporting electrolyte mixture. All potentials are referenced to the SCE. Spectroelectrochemistry. UV–visible spectroelectrochemical experiments were performed with a home built optically transparent platinum thin-layer electrode. Potentials were applied with an EG&G Model 173 potentiostat/galvanostat. Timeresolved UV–visible spectra were recorded with a Hewlett Packard Model 8453 diode array rapid-scanning spectrophotometer. Spectroscopy and Photophysics. Spectrophotometric grade toluene (C. Erba) was used without further purification. Standard 10 mm fluorescence cells were used at 295 K whereas experiments at 77 K made use of capillary tubes dipped in a homemade quartz Dewar filled with liquid nitrogen. Due to these geometrical conditions at 77 K, the absolute quantum yield cannot be determined with confidence and only qualitative information can be derived. Solutions were air-equilibrated. A Perkin-Elmer Lambda 950 UV/vis spectrophotometer was used to measure absorption spectra. A Spex Fluorolog II spectrofluorimeter and a Perkin-Elmer LS-50 spectrofluorimeter were used to acquire fluorescence and phosphorescence spectra, respectively. The reported luminescence spectra are uncorrected unless otherwise specified. Emission quantum yields were determined after correction for the photomultiplier response, with reference to an air-equilibrated toluene solution of TPP with a Φfl ) 0.1125 for C and corrole based luminescence, and to quinine sulfate in air-equilibrated 1 N H2SO4 with φfl ) 0.54626 for TPA and TPA based luminescence. The perylene based luminescence of the arrays was determined with reference to PI0 in dichloromethane (Φfl ) 0.9927). The phosphorescence lifetime of TPA in the microsecond-millisecond scale was measured by using a Perkin-Elmer LS-50 spectrofluorimeter equipped with a pulsed Xenon lamp and variable repetition rate and was elaborated with standard software fitting procedures. Luminescence lifetimes in the nanosecond range were obtained with an IBH single photon counting equipment with excitation at 331, 465, or 560 nm from pulsed diode sources (resolution 0.3 ns). For determination of emission lifetimes in the picosecond range an apparatus based on a Nd:YAG laser (35 ps pulse duration, 532 nm, 1.5 mJ) and a Streak Camera with overall resolution of 10 ps was used.28 Transient absorbance in the picosecond range made use of a pump and probe system based on a Nd:YAG laser (Continuum PY62/10, 35 ps pulse). The second harmonic (532 nm) at a frequency of 10 Hz and an energy of ca. 3.5 mJ/pulse was used to excite the samples whose absorbance at the excitation wavelength was ca. 0.6. More details on the apparatus can be found elsewhere.16a Results and Discussion Design and Synthesis. The major objective of the current research was to study the effect of an additional energy donor (implementing antennae motif harvesting UV-photons to the system) on the character and efficiency of photophysical
Tasior et al. processes. Among the different possibilities we decided on a modified triphenylamine derivative as the auxiliary energy donor. N-[4-(Phenylethynyl)phenyl]-N,N-diphenylamine (TPA, Chart 1)29 displays many attractive spectroscopic and chemical features that make it a good candidate as an accessory chromophore. These are an absorption maximum around 350 nm which extends the absorption into the UV, amenability to a modular building block synthesis and good solubility in organic solvents. The design of the structure of the final multichromophoric array must also take into account the following points. (1) Corroles of trans-A2B type are the most convenient architecture for the introduction of two different substituents at the meso positions of the macrocycle. (2) The stability of the dyad plays a crucial role in its performance and is greatly enhanced by electron-withdrawing groups around the periphery of the corrole as shown in a previous publication.20 Because we planned to introduce perylene bisimide moiety, a previously investigated electron acceptor, at position 10 of the corrole, this meant that the linkers at positions 5 and 15 should possess an electronwithdrawing character. (3) Consequently, the substituents placed at positions 5 and 15 cannot be simple 4-substituted phenylenes. Indeed they have to possess additional withdrawing groups (like CN or F). From the point of view of availability of the building blocks it seemed to us that 2,3,5,6-tetrafluorophenylene moiety is the most convenient spacer. Synthesis of complex corroles can be accomplished via the preparation of complex aldehydes followed by the corroleforming reaction30,31 or via a postfunctionalization of simple corroles bearing desired functional groups (NH2, COOH, OH, etc.).32 From the point of view of minimalization of operations on the corroles, the first route is more attractive. On the other hand, this procedure leads to a significant loss of precious substrates. It initially seemed to us that the combination of both approaches would be the best solution. The synthesis of the multichromophoric system TPA-C-PI and model compounds C, C-PI, C-TPA, and TPA followed the strategy outlined on Schemes 1 and 2.33-38 The detailed description is presented in Supporting Information. Electrochemical Studies. Electrochemical studies of TPA, C-TPA, C-PI, and TPA-C-PI were performed in order to confirm the magnitude of the coupling in all assemblies. The reduction and oxidation potentials are summarized in Tables 1 and 2 where assignments of the electron transfer site in the case of the corrole unity are given in ref 39. As seen in Table 1, reduction at the corrole macrocycle, either (Cor)H3 or [(Cor)H2]-, shows that the potentials are only slightly shifted in comparison with previously characterized meso-substituted corroles having simple aryl substituents.39 Also, similar to what was observed for simple meso-triaryl corroles, the first reduction of C-TPA leads a corrole π-anion radical at Epc ) -1.14 V. The product of the first (Cor)H3 reduction is [(Cor)H2]- which is then reduced at E1/2 ) -1.71 V or oxidized at E1/2 ) 0.15 V on the reverse scan. The same reduction processes occur for C-PI and TPA-C-PI, but these compounds exhibit two additional reductions at E1/2 ) -0.54 to -0.55 V and -0.73 to -0.75 V. Both reductions are assigned to the perylene bisimide moiety on the basis of data in the literature for similar comounds.40 A comparison of E1/2 values for the first two reduction of C-PI and TPA-C-PI with potentials for reduction of simple N-phenyl or N-alkyl substituted perylene bisimides (-0.52 and -0.75 V) shows basically no difference.40 Oxidation of N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine (TPA) occurs at E1/2 ) 1.01 V in CH2Cl2 (see Table 1). Similar
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CHART 1: Structures of Studied Molecules
oxidation potentials are seen for C-TPA and TPA-C-PI (Table 2), but for each free-base corrole, the initial oxidation is irreversible due to a chemical reaction following electron transfer, and this leads to a mixture of [(Cor)H2] and [(Cor)H4]+ as the product, each of which has its own redox properties as described in the literature for structurally related free-base corroles lacking the coupled electroactive groups.39 The first oxidation of the corrole at Epa ) 0.68 to 0.72 V generatesaπ-cationradical,andacomparisonofthecurrent-voltage curves for reduction of C-TPA to that of almost a dozen previously studied free-base corroles39 shows similar electrochemical behavior and similar potentials (due to the similar substituents). Neither the oxidation potentials of the donors nor the reduction potentials of the acceptors used to construct the dyads are significantly affected by the presence of covalent linkages between the two redox-active centers. The electrochemical data thus reinforce conclusions from analysis of the UV-vis spectra (see below) that ground-state electronic interactions between the components are weak. UV-visible spectral changes during the first oxidation of TPA at a controlled potential of 1.10 V were performed in a
thin-layer spectroelectrochemistry cell with an optically transparent platinum electrode. As shown in Figure 1a, the only band at 351 nm of neutral TPA in CH2Cl2 decreased in intensity during the first 25 s of the oxidation at an applied potential of 1.10 V as new bands appear at 422, 453, 537, and 869 nm. A second set of spectral changes is seen when the potential is applied for a longer periods (54 s) and under these conditions, a band at 510 nm appears in the spectrum of the final oxidized product (see Figure 1b). Spectroscopy and Photophysics. The investigated compounds shown in Chart 1 with the previously investigated reference compounds C3 and PI020 have been examined in toluene solutions. Absorption and emission spectra of model C and its simpler C3 analogue are compared in Figure 2. The molar absorption coefficient of C is slightly lower than that of C3 both on the Soret and on the Q bands, and the maxima are bathochromically shifted by ca. 6-8 nm. The shift to lower energies by 7 nm (663 nm with respect to 656 nm) is also apparent from the emission bands, and both the emission quantum yield and lifetime show slight changes with respect to C3 (Table 3).
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The absorption spectra of the arrays C-TPA, C-PI, and TPAC-PI (Figure 3a,b,c) is well reproduced by the simple superposition of the absorption spectra of component units C, TPA, and PI0. The spectra of the arrays closely match the sum of the spectra of the model components with only small discrepancies except for a sizable bathochromic shift (13-15 nm) of the band of the N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine unit in C-TPA and TPA-C-PI. This can be easily understandable in view of the stabilization induced by charge transfer interactions with the weakly electron-withdrawing tetrafluorophenyl substitutent. As a general observation the absorption data show a very weak electronic coupling and a localized description of the individual subunits can be adopted with confidence. Selective excitation of the corrole unit can be achieved at wavelengths > 400 nm in C-TPA, whereas in the perylene containing arrays selective excitation of the corrole is achieved
Tasior et al. at λ > 560 nm. N-[4-(Phenylethynyl)phenyl]-N,N-diphenylamine and perylene bisimide units cannot be selectively excited, but one can approximate nearly 90% of photons absorption by the perylene bisimide unit at λ ) 492 nm in C-PI and TPA-C-PI and about 70% of light absorption by the N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine units at λ ) 340 nm in C-TPA and TPA-C-PI. N-[4-(Phenylethynyl)phenyl]-N,N-diphenylamine (TPA) is a strong emitter with Φfl ) 0.43, lifetime of 1.0 ns, and emission maximum at 390 nm. This is different from triphenylamine which has a lower emission quantum yield (Φfl ) 0.0341). The luminescence properties of C, not so different from those of corresponding corrole C3, have been discussed above. The perylene bisimide model, PI0, has a structured fluorescence in the region 530-650 nm, with a quantum yield close to unity and a lifetime of 4.0 ns. The luminescence parameters of models and arrays are collected in Table 3. Selective excitation of corrole at 580 nm in optically matched solutions of C, C-TPA, and TPA-C-PI leads to the emission spectra displayed in Figure 4. The fluorescence yield of 0.16 for model C remains unaltered in C-TPA whereas reduces to ca. 3% in C-PI and TPA-C-PI, (Table 3). Excitation at 492 nm, where the perylene bisimide preferentially absorbs (93% in C-PI and TPA-C-PI), yields the emission spectra reported in Figure 5 a,b, where the spectra of the arrays are compared with those of the models. It is evident that the luminescence of the perylene bisimide is quenched to less that 1% both in the dyad and in the TPA-C-PI (Table 3). The luminescence of the corrole unit, upon excitation on the perylene unit, appears also quenched to 40% in C-PI and to 30% in TPA-C-PI (Table 3). It should be noted that the quenching in this case is lower than that occurring upon selective excitation of corrole in the same arrays, leading to a 3% residual luminescence. Excitation at 340 nm, where the N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine preferentially absorbs (ca. 75%-70% in C-TPA and TPA-CPI), leads to the results reported in Figure 6, where the spectra of the arrays are compared with those of the models absorbing the same numbers of photons as they do in the arrays. In the simple array C-TPA (Figure 6a) the luminescence of the N-[4(phenylethynyl)phenyl]-N,N-diphenylamine units appears completely quenched whereas the luminescence of the corrole unit is increased by a factor of 4, consistent with a quantitative sensitization of the corrole (absorbing ca. 25% of the light) by the peripheral excited N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine units. In TPA-C-PI (Figure 6b) a complete quenching of the N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine unit is observed as well as a nearly complete quenching (to 0.3%) of the perylene bisimide luminescence but, at variance with the dyad C-TPA, in TPA-C-PI also the luminescence of corrole is quenched to ca. 14% of the model luminescence. This last result indicates that in TPA-C-PI a further deactivation path is open to the corrole’s excited state. Time-resolved luminescence studies with picosecond resolution have been performed with excitation at 532 and 355 nm. The luminescence of the perylene bisimide unit, excited at 532 nm, decays faster than the instrumental resolution, indicating a lifetime < 10 ps, in both C-PI and TPA-C-PI. The luminescence of the N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine unit, excited at 355 nm, decays with a lifetime of ca. 10 ps, both in C-TPA and in TPA-C-PI. These results are in agreement with the quenching of the luminescence of these units detected in steady-state experiments. Corrole unit has a luminescence lifetime of 131 ps in C-PI and of 106 ps in TPA-CPI whereas the lifetime of the decay registered in C-TPA is
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SCHEME 2
TABLE 1: Reduction of Half-Wave Potentials (V vs SCE) of Free-Base Corroles in CH2Cl2, 0.1 M TBAPa cpd C-TPA C-PI TPA-C-PI
bis-imide -0.54 -0.55
-0.73 -0.75
TABLE 2: Oxidation of Half-Wave Potentials (V vs SCE) of Free-Base Corroles in CH2Cl2, 0.1 M TBAPa
(Cor)H3
[(Cor)H2]-
cpd
amine
-1.14b,c -1.15b,d -1.14b,e
-1.71 -1.68 -1.67
TPA C-TPA TPA-C-PI C-PI
1.01 1.01 1.01
a
Assignment of the reaction corrole species is given in ref 39. Peak potential at a scan rate of 0.1 mV/s. c A reversible couple at E1/2 ) 0.15 V was obtained on the reverse scan. d A reversible couple at E1/2 ) 0.21 V was obtained on the reverse scan. e A reversible couple at E1/2 ) 0.20 V was obtained on the reverse scan. b
3.0, the same as that of the model C (Figure 7). Excitation at 355 nm lead to luminescence lifetimes for corrole essentially identical to those detected upon excitation at 532 nm. Excitation spectra have been recorded with detection at 576 nm (perylene bisimide based emission) and 720 nm (corrole based emission). Figure 8 collects the data for the three arrays: part (a) shows the excitation spectrum of C-TPA recorded at
b
1.74b 1.50b
[(Cor)H4]+
(Cor)H3
0.84 0.87 0.92
0.68b 0.70b 0.72b
a Assignment of the reaction corrole species is given in ref 39. Peak potential at a scan rate of 0.1 mV/s.
720 nm superimposed with the absorption spectrum confirming the nearly complete sensitization of the corrole moiety when the N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine unit is excited; part (b) displays the excitation spectra of C-PI recorded at the corrole and perylenebisimide emission which demonstrate that excitation of the perylene bisimide unit is effective in producing with nearly unity yield corrole luminescence; and part (c) shows excitation spectra of TPA-C-PI where one can
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Figure 1. UV-visible spectral changes of TPA during the first oxidation in CH2Cl2 containing 0.1 M TBAP.
Figure 2. Absorption spectra of C (dash) and C3 (line) in toluene solutions; in the inset the room temperature emission is shown.
see that excitation of the perylene bisimide and N-[4-(phenylethynyl)phenyl]-N,N-diphenylamine units is effective in producing corrole luminescence with a nearly quantitative yield. One might also notice that excitation of the amine does not sensitize the perylene bisimide luminescence, which is reasonable if one takes into account that this would require a through space (Forster) type of energy transfer. As is well-known, this type of mechanism is maximized when overlap of donor luminescence and absorption of the acceptor is large, and this is not the case here; see Figures 3 and 6.42
Figure 3. Absorption spectra of components and arrays in toluene: (a) C-TPA (dash), C (line), and TPA multiplied by two (thick line); (b) C-PI (gray line), C (line), and PI0 (open circles); (c) TPA-C-PI (full circles), TPA multiplied by two (thick line), C (line), and PI0 (open circles).
Luminescence experiments have been performed also at 77 K in the glass, and the data are collected in Table 3. TPA displays both a strong fluorescence at 384 nm, slightly hypsochromically shifted with respect to room temperature data, and a strong phosphorescence at 501 nm with lifetimes of 0.9 ns and 751 ms, respectively. C shows an identical band maximum and a longer lifetime compared to the room temperature data,
TABLE 3: Fluorescence Data of Models and Arrays in Air-Equilibrated Toluene at 295 K and in Toluene Glass at 77 K 295 K state TPA
1
TPA TPA 1 PI 1 C3 1 C 1 C-TPA C-1TPA 1 C-PI C-1PI TPA-1C-PI TPA-C -1PI 1 TPA-C -PI
λmax, nm
a
Φ
b
Φ
390
77 K c
Φ
0.43
τ, ns 1.0
3
PI0 C3 C C-TPA C-PI TPA-C-PI
534, 574, 622 656, 716 663 664
0.14 0.16 0.16
0.92
661
0.0057
661
0.0048
0.69 ca. 10-3 0.062 0.0004 0.044 0.0008
0.023 ca. 10-3
4.0 3.8 3.1 3.0 0.010 0.131 700 nm (Figure 9 a). The bands are similar for C-TPA, except for a bathochromically shifted absorption (from 790 to 825 nm) and a higher absorption coefficient in the NIR region. The end of pulse spectrum of PI displays the typical perylene bisimide excited singlet spectrum,20 with bleaching below 550 nm, a stimulated emission signal around 575 and 625 nm, and positive absorption bands at 690 and 855 nm (Figure 9b). Both C-PI
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Figure 9. Transient absorbance detected in toluene solutions, A ) 0.6, after a 35 ps laser pulse (532 nm, 3 mJ) of (a) C (line) and C-TPA (dash); (b) C-TPA (dash), PI0 (gray), TPA-C-PI (thin line), and C-PI (thick line).
Figure 8. Excitation spectra (dashed) are overlapped with the array absorption spectra (line): (a) excitation registered at 720 nm for C-TPA; (b) excitation registered at 720 (black) and at 576 nm (gray) for C-PI; and (c) excitation registered at 720 (black) and at 576 nm (gray) for TPA-C-PI. The absorption spectrum is arbitrarily scaled.
and TPA-C-PI at 300 ps delay (end of a fast growing of the bands, see later) display an identical spectrum with maxima at 710, 800, and 960 nm previously identified as the reduced perylene bisimide residue.20 The presence of this species indicates the occurrence of an electron transfer from corrole to the perylene bisimide unit, with formation of a charge separated state (CS). Corrole oxidations lead to a weak absorption band around 650 nm39 which is overshadowed by the strong signal of the perylene bisimide anion. No other intermediate is detected: TPA radical cation which has a moderate absorption band around 870 nm (Figure 1) could be detected if present in TPA-C-PI. The identity of the transient absorption spectra in C-PI and TPA-C-PI (Figure 9 b) indicate the absence of such intermediate. The time evolution of the reduced radicals spectrum detected in C-PI and TPA-C-PI solutions is reported in Figure 10. It can be fitted in both cases by a double exponential with a rise of 130 ps for C-PI and 100 ps for TPA-C-PI and decay lifetimes of 2.3 and 1.9 ns for C-PI and TPA-C-PI, respectively. Whereas the fast rise is in agreement with the luminescence lifetime of corrole in the two arrays and represents the formation of the CS state, i.e., the charge separation, the decay corresponds to the lifetime of the CS state. Schematic energy diagrams for the arrays are shown in Figure 11; they display the excited-state energy levels, derived from
Figure 10. Time profile of the transient absorbance measured at λ ) 710 nm in C-PI (open circles) and TPA-C-PI (full circles). The fitting according to a double exponential is also reported.
the 77 K luminescence data of Table 3 and the CS energy levels derived from the electrochemical data of Tables 1 and 2 after corrections for the different solvents used.43 The CS state levels were calculated at 1.54 and 1.53 eV for C+-PI- and TPA-C+PI-; at 1.97 eV for TPA+-C-PI-, and at 2.75 eV for C--TPA+ and TPA+-C--PI. In the first two schemes are shown the dyads C-TPA and C-PI. In the former, excitation of TPA unit leads to an efficient (99%) energy transfer with a rate of 1011 s-1; the excited-state of the corrole formed by sensitization decays as in the isolated model. Formation of the CS state C--TPA+ which could be thermodynamically allowed is not detected. In C-PI, excitation of the perylene units is followed by a quenching with a rate higher than 1011 s-1. In principle, this might be due both to an energy transfer to the corrole or to a HOMO-HOMO electron transfer from corrole to the PI unit. The two reactions can be separated with some difficulty since the sensitized corrole leads to the same product as the electron transfer (see below); however, from the excitation spectra indicating quantitative sensitization of corrole from PI component (see Figure 8b) and from the slow rate of formation of CS detected by transient absorption spectroscopy (see Figure 10), energy transfer from the PI excited unit to the corrole prevails. Excitation of the
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Figure 11. Energy level diagram with main deactivation paths and reaction rates of the arrays.
corrole unit in the same C-PI dyad leads with a rate of 7.3 × 109 s-1, 96% efficiency, to electron transfer to form the C+PI-, decaying with a lifetime of 2.3 ns, very similar to that determined in a strictly related dyad.20 The discussion of the processes in TPA-C-PI is simplified by comparison with the reference dyads. Excitation of the corrole unit leads to an efficient electron transfer (97%) with rate k ) 9.1 × 109 s-1, as can be determined by the quenching of corrole luminescence and the formation of the perylene bisimide radical anion bands. In analogy with the reference dyad C-TPA, excitation of the TPA unit leads to an efficient energy transfer to corrole with a rate k ) 1011 s-1, as derived by time-resolved luminescence experiments. The reaction is quantitative as can be assumed from the behavior of the model dyad C-TPA but also from the excitation spectrum in TPA-C-PI (Figure 8c); however, the sensitization of corrole luminescence is not detected due to a reaction which depletes the corrole excited state by electron transfer, as discussed above. Excitation in the perylene bisimide unit leads to an extremely fast quenching of the excited state of the perylene bisimide as detected in the corresponding dyad C-PI, and we assign this quenching to energy transfer to the corrole unit on the basis of the same arguments used for the dyad. The final CS state TPA-C+-PI- decays with a lifetime of 1.9 ns, slightly reduced with respect to that of the model C-PI. Also in this array no other intermediate CS state (see Figure 11) could be detected; in particular the formation of the CS state TPA+-C-PI- with charges on the extremities, which might display a much longer lifetime because of the poor electronic coupling of the terminal components, is not formed because of thermodynamic reasons. Finally, we considered the possibility of also carrying out photophysical studies of the array in a polar solvent, but this was rejected for a number of reasons. Polar solvents are known to stabilize the CS state and thus might affect the outcome of excited-state deactivation in complex arrays for charge separation. The use of a more polar solvent in the present case could decrease energies of the CS states TPAC+-PI- and TPA+-C-PI- but would not improve performance of the array since the charge separation step is already nearly 100% efficient in the apolar solvent toluene. Thus increasing the driving force for charge separation by lowering the CS energy level would not be of any use. In addition, the possibility to shift the charge from TPA-C+-PI- to TPA+-
C-PI- would not be affected by solvent polarity so that the final CS state would be the same TPA-C+-PI-. Furthermore, because of the decrease of the driving force for charge recombination in polar solvents, the CS lifetime would be shorter due to the well-known inverse relation for highly exoergonic reactions (Marcus inverted behavior).44 Finally, it is known that corroles have essentially the same spectroscopic and photophysical properties in polar and apolar solvents,16a,c but they display higher photolability in polar solvents.16a The latter reason, in addition to all the consideration above, was sufficient to discourage us from carrying out additional experiments in polar solvents. Conclusions In conclusion, we proved that complex corrole containing arrays can be prepared via the combination of various strategies. The most notable synthetic findings are as follows. (1) During the formation of corroles from 4-iodo-2,3,5,6-tetrafluorobenzaldehyde extensive deiodination occur probably via oxidation of C-I bond with DDQ. (2) trans-A2B-corrole synthesis in the H2O/MeOH/HCl system works well even for relatively large dipyrromethanes. (3) Fagnou direct coupling of electrondeficient arenes does not work with compounds possessing a free aldehyde group; however, it works well with acetals. (4) meso-Substituted corroles bearing nonactivated chloroarene moiety can be successfully coupled with alkynes under noncopper conditions. Both electrochemical and spectroscopic studies confirmed a weak coupling of the components in the resulting triad which is characterized by an intense and extended collection of light throughout the UV, vis, and NIR region of the spectra. Photophysical studies showed that the energy collected by the peripheral TPA units as well as that collected by the PI units can be efficiently (ca. 100%) funneled to the corrole and from this component an electron transfer reaction with a 97% yield leads to a CS state, TPA-C+-PI-, with a lifetime in the nanosecond range. Substitution of TPA with an amine with a lower oxidation potential should allow a further electron shift from the corrole to the terminal amine, providing thus a larger distance between charges and hence a longer lifetime to the ensuing CS. We are actually working in this direction. Acknowledgment. We thank Polish Ministry of Research and Higher Education, Volkswagen Foundation, CNR of Italy
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