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J. Phys. Chem. B 2010, 114, 14495–14504

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Photoinduced Processes in Self-Assembled Porphyrin/Perylene Bisimide Metallosupramolecular Boxes† M. Teresa Indelli,*,‡,§ Claudio Chiorboli,| Franco Scandola,*,‡,§ Elisabetta Iengo,*,⊥ Peter Osswald,¶ and Frank Wu¨rthner*,¶ Dipartimento di Chimica, UniVersita` di Ferrara, 44100 Ferrara, Italy, ISOF-CNR, Sezione di Ferrara, 44100 Ferrara, Italy, Centro di Ricerca InteruniVersitario per la ConVersione Chimica dell’Energia Solare, Sezione di Ferrara, 44100 Ferrara, Italy, Dipartimento di Scienze Chimiche, UniVersita` di Trieste, 34127, Trieste, Italy, and Institut fu¨r Organische Chemie and Ro¨ntgen Research Center for Complex Material Systems, UniVersita¨t Wu¨rzburg, 97074 Wu¨rzburg, Germany ReceiVed: March 1, 2010; ReVised Manuscript ReceiVed: May 21, 2010

Two new supramolecular boxes, (ZnMC)2(rPBI)2 and (ZnMC)2(gPBI)2, have been obtained by axial coordination of N,N′-dipyridyl-functionalized perylene bisimide (PBI) dyes to the zinc ion centers of two 2+2 porphyrin metallacycles (ZnMC ) [trans,cis,cis-RuCl2(CO)2(Zn · 4′-cis-DPyP)]2). The two molecular boxes involve PBI pillars with different substituents at the bay area: the “red” PBI (rPBI ) N,N′-di(4-pyridyl)1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisimide) containing tert-butylphenoxy substituents and the “green” PBI (gPBI ) N,N′-di(4-pyridyl)-1,7-bis(pyrrolidin-1-yl)perylene-3,4:9,10tetracarboxylic acid bisimide) bearing pyrrolidinyl substituents. Due to the rigidity of the modules and the simultaneous formation of four pyridine-zinc bonds, these discrete adducts self-assemble quantitatively and are remarkably stable in dichloromethane solution. The photophysical behavior of the new supramolecular boxes has been studied in dichloromethane by emission spectroscopy and ultrafast absorption techniques. A different photophysical behavior is observed for the two systems. In (ZnMC)2(rPBI)2, efficient electron transfer quenching of both perylene bisimide and zinc porphyrin chromophores is observed, leading to a charge separated state, PBI--Zn+, in which a perylene bisimide unit is reduced and zinc porphyrin is oxidized. In the deactivation of the perylene bisimide localized excited state, an intermediate zwitterionic charge transfer state of type PBI--PBI+ seems to play a relevant role. In (ZnMC)2(gPBI)2, singlet energy transfer from the Zn porphyrin chromophores to the perylene bisimide units occurs with an efficiency of 0.7. This lower than unity value is due to a competing electron transfer quenching, leading to the charge separated state PBI--Zn+. The distinct photophysical behavior of these two supramolecular boxes is interpreted in terms of energy changes occurring upon replacement of the “red” rPBI by “green” gPBI. Introduction Porphyrins and metalloporphyrins have been extensively used as building blocks for the construction of supramolecular arrays designed to emulate at the molecular level light-induced functions that are typical of biological systems, such as antenna effect and photoinduced charge separation.1,2 By using a variety of binding motifs (e.g., covalent linkages, usually through the meso positions of the porphyrin ring,3 or coordinative bonds involving peripheral ligand functions and appropriate metal ion centers4), a large number of multiporphyrin arrays of different shapes and functions has been created in recent years. In our laboratories, a number of metal-ion-mediated arrays have been assembled by exploiting the coordination ability of mesosubstituted pyridyl porphyrin5 and semisynthetic zinc chlorin dyes.6 In particular, we have previously reported on the synthesis †

Part of the “Michael R. Wasielewski Festschrift”. * To whom correspondence should be addressed. E-mails: (M.T.I.) [email protected], (F.S.) [email protected], (E.I.) [email protected], (F.W.) wuerthner@ chemie.uni-wuerzburg.de. ‡ Universita` di Ferrara. § Centro Interuniversitario di Ricerca per la Conversione Chimica dell’Energia Solare. | ISOF-CNR. ⊥ Universita` di Trieste. ¶ Universita¨t Wu¨rzburg.

of 2+2 porphyrin metallacycles (“molecular squares”), including [trans,cis,cis-RuCl2(CO)2(Zn · 4′-cis-DPyP)]2 (4′-cis-DPyP ) 5,10-bis(4′-pyridyl) 15,20-diphenylporphyrin), ZnMC,7,8 and multiporphyrin sandwich adducts (“molecular boxes”), including (ZnMC)2(t-DPyP)2 obtained by axial coordination of two 4′trans-DPyP (t-DPyP) units (4′-trans-DPyP ) 5,15-bis(4′pyridyl)-10,20-diphenylporphyrin) to the zinc ion centers of two metallacycles (see Chart 1 for structures).7 These multiporphyrin discrete adducts self-assemble quantitatively in solution and show a high thermodynamic stability. The high stability is attained by the rigidity of the modules (ZnMC has been characterized in the solid state, showing a perfectly flat scaffold8) and by the contemporary formation of four pyridine-zinc bonds. (ZnMC)2(t-DPyP)2 has been fully characterized, both in solution and in the solid state.7 The photophysical behavior of the latter system, which exhibits mainly energy transfer from the peripheral zinc porphyrins to the central free base porphyrins, has also been studied.9,10 The most common strategy to achieve efficient photoinduced electron transfer from photoexcited porphyrins is based on socalled dyad molecules that consist of porphyrin and an electrondeficient chromophore.2 This design is inspired by natural photosynthetic reaction centers, which contain chlorophyll dyes as electron donor and quinones as electron acceptors. For

10.1021/jp101849m  2010 American Chemical Society Published on Web 06/07/2010

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CHART 1

artificial dyads, however, bisimide chromophores appear more attractive as electron acceptors, since they can be easily connected to various scaffolds via the imide nitrogens, and their optical and redox properties can be easily tuned over a wide range. Particularly, Wasielewski and co-workers have recognized these advantageous features of mono- and bisimide chromophores and exploited them for a broad variety of photophysical studies on dyad and triad molecules.11 In the recent years, perylene bisimides became one of the most popular building blocks for photophysical studies.12 Their excited state and redox properties can be easily tuned by chemical design, especially by appropriate choice of the substituents in the “bay” area. Thus, a large number of functional systems and assemblies based on perylene bisimides have been developed in the past decade.12-15 A number of mixed systems containing covalently linked porphyrins and perylene bisimides have also been studied, with particular emphasis on the occurrence of energy and electron transfer processes between the two types of molecular components16-19 and charge migration in higher-order supramolecular aggregates of such species.16 Previously, the synthesis and photophysics of a side-to-face assembly made of a perylene bisimide bearing two pyridine imide substituents that is axially connected to two ruthenium-tetraphenylporphyrins have been reported.20 We have now extended the concept of assembly of molecular boxes, initially realized for (ZnMC)2(t-DPyP)2,7 to new architecture in which the two axial ditopic ligands are now perylene bisimides equipped with two pyridine imide substituents.13a Thus, we have synthesized a new class of stable assemblies of general formula {[trans,cis,cis-RuCl2(CO)2(Zn · 4′-cis-DPyP)]2}2(µ-PBI)2 employing PBI derivatives with different substituents at the bay area (Chart 2). In particular, (ZnMC)2(rPBI)2 and (ZnMC)2(gPBI)2 were obtained using the “red” and “green” perylene bisimide ligands rPBI (N,N′-di(4-pyridyl)-1,6,7,12tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisimide) and gPBI (N,N′-di(4-pyridyl)-1,7-bis(pyrrolidin-1yl)perylene-3,4:9,10-tetracarboxylic acid bisimide), respectively. Due to the rigidity of the modules21 and the simultaneous formation of four pyridine-zinc ion bonds, these multichromophore assemblies are remarkably stable, even in dilute (>10-5

CHART 2

M) solution. A full characterization in solution as well as the X-ray crystal structures of (ZnMC)2(rPBI)2 and (ZnMC)2(gPBI)2 will be reported elsewhere.22 Here, we report a detailed photophysical study of the new molecular boxes aimed at the characterization of the photoinduced processes taking place in these systems. Experimental Section Materials. The metallacycle [trans,cis,cis-RuCl2(CO)2(Zn · 4′cis-DPyP)]2 (ZnMC) was prepared as previously reported.7 N,N′Di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4: 9,10-tetracarboxylic acid bisimide and N,N′-di(4-pyridyl)-1,7bis(pyrrolidin-1-yl)perylene-3,4:9,10-tetracarboxylic acid bisimide were synthesized according to literature procedure.13a The solvents for spectroscopic and photophysical measurements were of spectroscopic grade (Aldrich) and were used as received. Apparatus and Procedures. 1H and 2D NMR spectra were recorded at 400 MHz on a JEOL Eclipse 400 FT instrument. All spectra were measured at room temperature in CDCl3 (Aldrich) unless otherwise stated. Proton peak positions were

Bisimide Metallosupramolecular Boxes referenced to the peak of residual non deuterated chloroform set at 7.26 ppm. UV-vis absorption spectra were recorded with a Jasco U-570 spectrophotometer. Emission spectra were taken on a Spex Fluoromax-2 spectrofluorometer equipped with Hamamatsu R3896 photomultiplier by using 1 mm optical path cells and a 30° excitation-emission geometry. Nanosecond emission lifetimes were measured using a TCSPC apparatus (PicoQuant Picoharp300) equipped with subnanosecond LED sources (280-600 nm range, 500-700 ps pulsewidth) powered by a PicoQuant PDL 800-B variable (2.5-40 MHz) pulsed power supply. The measurements were performed in 1 mm optical path cells by using a 45° excitation-emission geometry. The decays were analyzed by means of PicoQuant FluoFit global fluorescence decay analysis software. The time resolution of the system after the deconvolution procedure is 300 ps, and estimated errors are 10% for lifetime values. Femtosecond timeresolved experiments were performed using a pump-probe setup based on the Spectra-Physics Hurricane Ti:sapphire laser source and the Ultrafast Systems Helios spectrometer. The 560 nm pump pulses were generated with a Spectra Physics 800 OPA. Probe pulses were obtained by continuum generation on a sapphire plate (useful spectral range, 450-800 nm). Effective time resolution ∼300 fs, temporal chirp over the white light 450-750 nm range ∼200 fs, temporal window of the optical delay stage 0-1000 ps. The time-resolved spectral data were analyzed with the Ultrafast Systems Surface Explorer Pro software. More details on the apparatus can be found elsewhere.23 All the photophysical experiments were performed in freshly prepared solutions. The stability of the solutions was spectrophotometrically checked before and after each experiment. The solvent (CH2Cl2) was saturated with potassium carbonate before measurements to remove traces of acidity. Results and Discussion Synthesis and Characterization. The details of the synthesis of the molecular boxes (ZnMC)2(rPBI)2 and (ZnMC)2(gPBI)2 and their characterization in solution will be given elsewhere.22 Here, we describe briefly the NMR spectroscopic study of the self-assembly of (ZnMC)2(gPBI)2 in CDCl3 (a very similar spectral behavior is observed for (ZnMC)2(rPBI)2). 1H NMR spectroscopy, together with conventional 2D NMR experiments (H-H COSY, NOESY, and ROESY), of a stoichiometric mixture of [trans,cis,cis-RuCl2(CO)2(Zn · 4′-cis-DPyP)]2 ZnMC and gPBI convincingly establish the formation of a discrete assembly with a 1:1 ratio of the two constituents. Typical upfield shifts for the protons of the axial ligands are observed as a consequence of the shielding cones of the zinc porphyrins of ZnMC that are now in a cofacial disposition. Each resonance of the meso aromatic rings of Zn · 4′cis-DPyP in (ZnMC)2(gPBI)2 is relatively broad at room temperature and splits into two sharp signals of equal intensity on lowering the temperature below -20 °C (Figure 1). Below -20 °C, the rotation of the porphyrin meso substituents is slow on the NMR time scale, and hence, the aromatic protons of the phenyl and pyridyl moieties experience two different magnetic environments, depending on their orientation toward the inside or the outside of the molecular box. These observations, together with relative integrals and symmetry, are in proper agreement with the quantitative formation of the molecular box (ZnMC)2(gPBI)2. The overall NMR spectral feature is similar to that of (ZnMC)2(t-DPyP)2.7 A detailed discussion of the solid state structures of (ZnMC)2(rPBI)2 and (ZnMC)2(gPBI)2 will be reported elsewhere.22 It has to be mentioned here, however, that the crystal

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14497 structures of the two molecular boxes are distinctly different; in particular, regarding the distance between the PBI axial ligands. For (ZnMC)2(gPBI)2, the structure is similar to the one depicted in Chart 1 with the four zinc centers forming a rectangle and the two axial PBI units at a mean distance of 13.8 Å, reasonably close to the Zn-Zn distance of metallacycle ZnMC.8 On the other hand, in (ZnMC)2(rPBI)2, the axial PBI units are found in a π-stacked rotationally displaced arrangement at a mean distance of 3.9 Å, consequently, with a strong distortion of the two facing zinc porphyrin metallacycles.22 A representative cartoon for the two solid-state structures is depicted in Figure 2. No information is available as to whether such structural differences are maintained in solution (vide infra). Stability in Solution. All the measurements were carried out in CH2Cl2 solutions.24 The stability of the assemblies in solution was checked with spectrofluorometric dilution experiments, in which dissociation of the molecular assembly is sensitively detected by tracing the rise of the fluorescence of free PBI (strongly quenched in the assembly, see below). These experiments, performed by measuring PBI fluorescence intensity in the assembly relative to optically matched (585 nm) solutions of free PBI as a function of assembly concentration (see Figures S2 and S3 in Supporting Information), showed that at concentrations >6 × 10-5 M, both assemblies are completely associated. All photophysical experiments were thus performed in freshly prepared solutions at concentrations higher than 6 × 10-5 M. The solutions were photochemically stable, as checked spectrophotometrically before and after each experiment. Properties of Molecular Components. The present assemblies are composed of two types of molecular subunits: dipyridyl perylene bisimide dyes (rPBI and gPBI) and zinc porphyrin metallacycles (ZnMC). These PBI compounds, available as free molecular species, are taken as appropriate models for the corresponding subunits in the assemblies. This is justified because our earlier investigations showed that the ligation of pyridine ligands to metal ion centers is accompanied by only modest changes of the absorption spectra (95%) quenched relative to those of the rPBI and ZnMCpy model compounds. As to the quenching mechanism, Fo¨rster energy transfer can be easily ruled out, given the almost isoenergetic situation of the two excited states. On the basis of the energy level diagram (Figure 6), two l-electron transfer mechanisms can be proposed for the quenching of the *PBI-Zn singlet state: (i) electron transfer leading to the PBI--Zn+ charge-separated state (1*PBI-Zn f PBI--Zn+) and (ii) electron transfer process to the PBI--PBI+ state (1*PBI-Zn f PBI--PBI+). By contrast, for the quenching of the PBI-*Zn singlet excited state, the only process available is an electron transfer to yield the PBI--Zn+ charge transfer state. In this case, the process PBI-1*Zn f PBI--PBI+, although energetically allowed, is kinetically forbidden, this being a three-molecule two-electron process. Ultrafast absorption experiments were performed to obtain information on the quenching mechanism. When excitation was carried out at 585 nm, where light is predominantly absorbed by the PBI subunits (85%), the spectral changes shown in Figure 7 were observed. The ultrafast spectroscopic behavior is clearly triphasic with different spectral changes taking place in the 0-10 (Figure 7a),

10-180 (Figure 7b), and 180-1000 ps (Figure 7c) time ranges. The initial spectrum of Figure 7a, taken immediately after the excitation pulse (t ) 1 ps), is the typical spectrum of the PBI singlet excited state, as shown by comparison with that of the rPBI model compound (Figure 4a). It shows, in addition to positive absorption in the long (>675 nm) and short ( 660 nm and for the zinc porphyrin units at 560 nm. Energy LeWels. The energy level diagram can be obtained as a superposition of those of the respective molecular subunits ZnMCpy and gPBI (Figure 8). Also in this case, the diagram includes, in addition to a charge transfer state, in which a perylene bisimide unit is reduced and a zinc porphyrin chromophore is oxidized (PBI--Zn+), a further charge transfer state in which one PBI unit is oxidized and the other unit is reduced (PBI--PBI+) at a relatively low energy. The energies of the charge transfer states (obtained from the known redox potentials of the model compounds with appropriate correction for the work term42) are 1.52 and 1.35 eV for PBI--Zn+ and PBI--PBI+, respectively. Photophysical BehaWior. In steady-state experiments, the assembly exhibits the typical fluorescence of the PBI subunit, slightly red-shifted (10 nm) with respect to that of free gPBI molecules. Upon selective excitation of PBI unit (720 nm), the fluorescence intensity is about half that of molecular gPBI (Φ0/Φ ) 2). The lifetime of 2.1 ns is also shorter than that measured for free gPBI (3.1 ns). Considering the observed spectral shift, these differences in fluorescence intensity and lifetime can probably be attributed to a perturbation effect due to coordination, rather than to a true quenching process. From the energy level diagram of Figure 8, a substantially exergonic electron transfer pathway involving the charge separated state PBI--PBI+ is available for excited-state deactivation and could, in principle, contribute to the observed modest lifetime decrease. If present,43 however, such a process must be very slow. Possible reasons for this behavior, which contrasts sharply with that of the (ZnMC)2(rPBI)2 analogue, are discussed in Conclusions section. When excitation is carried out at 566 nm, where light is selectively (>90%) absorbed by the Zn porphyrin units, the

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Figure 9. Ultrafast spectroscopy of (ZnMC)2(gPBI)2 in CH2Cl2 (excitation at 560 nm): (a) t e 100 ps; (b) t ) 100-1000 ps; (c) kinetic analysis at 720 nm.

emission spectrum consists of the perylene bisimide fluorescence (λmax ) 771 nm), whereas the zinc porphyrin fluorescence is completely quenched with respect to that of the model compound ZnMCpy. The strong quenching of the zinc porphyrin fluorescence and the sensitization of the PBI fluorescence indicates the occurrence of singlet energy transfer from the zinc porphyrin metallacycles to the perylene bisimide units. The excitation spectrum of the PBI fluorescence (Figure S5 in Supporting Information) displays the typical two absorption bands of the zinc porphyrin component, indicating clearly that the excitation of the zinc porphyrin subunits populates with high efficiency the fluorescent singlet state of the PBI component. To evaluate the efficiency of this process, relative emission intensity measurements were performed with solutions of the assembly optically matched at λ ) 566 nm (95% Zn-porphyrin excitation) and λ ) 714 nm (100% PBI excitation). From these experiments, making the appropriate correction for the difference in light intensity at these two wavelengths, a value of η ) 0.7 for the efficiency of energy transfer process has been obtained. Looking at the energy level diagram of Figure 8, this lower than unity efficiency can plausibly be attributed to competing electron transfer pathways for deactivation of the PBI-*1Zn state. Ultrafast absorption experiments were carried out by excitation at 560 nm, where light is selectively absorbed by the Zn porphyrin chromophore (Figure 9). The ultrafast spectroscopic behavior is clearly biphasic with different spectral changes taking place in the t e 100 ps (Figure 9a) and t g 100 ps (Figure 9b) time ranges. The initial spectrum in Figure 9a (t ) 1 ps) is the typical one for the zinc porphyrin singlet excited state (see, for comparison, the spectrum obtained for ZnMCpy, Figure 4c), with typical bleach of the Q-band at 520 nm and stimulated emission at 520 and 670 nm.

The subsequent changes, in the 1-100 ps time range, lead to the disappearance of the PBI-1*Zn features and to the formation of a transient spectrum very similar to that of the PBI singlet excited state (see for comparison the spectrum of gPBI singlet excited state in Figure 4b), with a new broad, intense bleaching centered at 700 nm, corresponding to overlapping bleach of ground state absorption (λmax ) 720 nm) and stimulated emission (λmax ) 770 nm). These spectral changes are therefore fully consistent with the occurrence of a singlet energy transfer process from the Zn porphyrin chromophores to the perylene bisimide units (PBI-*1Zn f *1PBI-Zn), as already indicated by the fluorescence sensitization experiments. Kinetic analysis of the spectral changes in Figure 9a at 720 nm yields a time constant for the energy transfer process of 14 ps (Figure 9c). This time constant is consistent with the predictions of Fo¨rster theory.44 In the longer time scale, Figure 9b (100-1000 ps), relatively small further transient changes take place, with decrease and red shift of the bleach in the 700-750 nm range. The time constant of this process is ∼0.3 ns. The hypothesis that the lower than unity efficiency of energy transfer quenching (η ) 0.7) is due to competing electron transfer (see above) implies that a Zn+-PBI- charge transfer state should be formed with ∼30% efficiency, simultaneous to the perylene bisimide singlet excited state formation. We attribute the slower spectral changes (Figure 9b) to charge recombination from this charge transfer state. The photophysical mechanism of (ZnMC)2(gPBI)2 is summarized in the energy level diagram of Figure 8. Conclusions Our present results show that different substituents in the bay area of the perylene bisimide pillars cause a sharp difference in

Bisimide Metallosupramolecular Boxes the photophysical behavior of the corresponding supramolecular boxes. Two main energy changes occur when “red” rPBI is replaced by “green” gPBI (Figures 6 and 8): (i) the energy of the PBI-localized singlet state is lowered, and (ii) the energy of the PBI--Zn+ charge transfer state is lifted. In the supramolecular (ZnMC)2(gPBI)2 box, singlet energy transfer from the zinc porphyrin to the perylene bisimide is exergonic and takes place efficiently. On the other hand, the PBI--Zn+ charge transfer state, while accessible from the singlet state of the zinc porphyrin unit, is too high in energy to provide an efficient deactivation channel for the singlet state of the perylene bisimide unit. In (ZnMC)2(rPBI)2, the PBI--Zn+ state is lower in energy and provides an efficient deactivation channel for the excited singlet states of both molecular components. The role of zwitterionic states of the type PBI+-PBI- in the photophysical process deserves some additional discussion. For (ZnMC)2(rPBI)2, a relevant role of this state is suggested by the transient spectral changes (Figure 7). The conversion of the PBI singlet excited state to the PBI--Zn+ charge transfer state clearly involves a fast intermediate step, and this step is proposed to involve the zwitterionic state (1*PBI-Zn f PBI--PBI+ f PBI--Zn+ f PBI-Zn). In (ZnMC)2(gPBI)2, however, the zwitterionic state does not seem to have any significant quenching effect on the PBI excited state. Thus, despite its similar driving force, the rate of photoinduced electron transfer between the PBI pillars seems to be very different (by a factor >103) in the two molecular boxes. As to possible reasons for such a difference, a hint could be provided by the X-ray structures of the two boxes.22 For the (ZnMC)2(gPBI)2 box, an almost perfect rectangular scaffold is observed with the two PBI units in a parallel configuration and at a distance largely determined by that between the zinc centers in the planar porphyrin metallacycles. The (ZnMC)2(rPBI)2box, on the other hand, exhibits a highly distorted twisted structure, with the two PBI units at a very short distance (3.9 Å) in a cofacial π-stacking arrangement. Although we do not have any independent evidence45 on whether these structural features are maintained in solution, it is tempting to assume that π-stacking plays a role in enabling very first photoinduced electron transfer between the perylene pillars in (ZnMC)2(rPBI)2. Excited-state symmetry breaking with fast formation of zwitterionic states was previously observed by Wasielewski and co-workers14a in perylene bisimide cofacial dimers. In conclusion, the self-assembly of new molecular boxes based on 2+2 porphyrin metallacycles and ditopic chromophoric units7,9 has now been implemented by using N,N′-dipyridyl perylene bisimides to give two discrete multichromophoric selfassembled structures with high stability. The distinct redox properties of these units evoke interesting types of photophysical behavior, largely controlled by the nature of the substituents in the bay area. The presence of two perylene bisimide units within the same assembly, with the associated zwitterionic states, can also be relevant to the photophysical mechanism. It is proposed that molecular boxes of this type could, in principle, be used to host a variety of species capable of establishing π-stacking or charge transfer interactions with the molecular units in the cavity. The possibility to easily modulate the nature of the molecular pillars is of interest toward the tailoring of these threedimensional supramolecular assemblies. Acknowledgment. The authors are grateful to A.-M. Krause (Wu¨rzburg) and L. Pozzetti and S. Fracasso (both Ferrara) for experimental assistance. E. Iengo has been supported by the EU Reintegration Grant ERG-LIGHT. Financial support by MIUR (PRIN project no. 2006030320) and DFG (project Wu

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14503 317/7) is gratefully acknowledged. This project was also supported by the COST D35 program. Supporting Information Available: Cyclic voltammogram of gPBI, stability curves of (ZnMC)2(rPBI)2 and (ZnMC)2(gPBI)2 as a function of dilution; femtosecond transient absorption spectroscopy of (ZnMC)2(rPBI)2 (550 nm excitation); excitation spectrum of (ZnMC)2(gPBI)2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Anderson, S.; Anderson, H. L.; Bashall, A.; McPartlin, M.; Sanders, J. K. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1096–1099. (b) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.; Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.; Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181–11193. (c) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.; Lindsey, J. S.; Moore, A.; Moore, T. A.; Gust, D. J. Am. Chem. 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Indelli et al. (23) Chiorboli, C.; Rodgers, M. A. J.; Scandola, F. J. Am. Chem. Soc. 2003, 125, 483–49. (24) Among common organic solvents, toluene did not dissolve the assemblies well. Chloroform and dichloromethane are good solvents for these assemblies. Dichloromethane was thus chosen, and the electrochemical potentials were measured in this solvent. (25) Gust, D.; Moore, T. A.; Moore, A. L.; Kang, H. K.; DeGraziano, J. M.; Liddell, P. A.; Seely, G. R. J. Phys. Chem. 1993, 97, 13637–13642. (26) Casanova, M.; Zangrando, E.; Iengo, E.; Alessio, E.; Indelli, M. T.; Scandola, F.; Orlandi, M. Inorg. Chem. 2008, 47, 10407–10418. (27) (a) Seybold, G.; Wagenblast, G. Dyes Pigm. 1989, 11, 303–317. (b) Gvishi, R.; Reisfeld, R.; Burshtein, Z. Chem. Phys. Lett. 1993, 213, 338–344. (c) Fron, E.; Schweitzer, G.; Osswald, P.; Wu¨rthner, F.; Marsal, P.; Beljonne, D.; Mu¨llen, K.; De Schryver, F. C.; Van der Auweraer, M. Photochem. Photobiol. Sci. 2008, 7, 1509–1521. (28) Calculated as the average between the energies of the first vibronic feature of the absorption and emission spectra. (29) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373–6380. (30) Because the oxidation process for the free dipyridyl perylene species rPBI is irreversible,13a the quoted oxidation potential was taken from the Pt(II)-coordinated reversibly oxidizable compound. (31) You, C.-C.; Dobrawa, R.; Saha-Mo¨ller, C. R.; Wu¨rthner, F. Top. Curr. Chem. 2005, 258, 39–82. (32) Wu¨rthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.sEur. J. 2001, 7, 2245–2253. (33) Berberich, M.; Krause, A. M.; Orlandi, M.; Scandola, F.; Wu¨rthner, F. Angew. Chem., Int. Ed. 2008, 47, 6616–6619. (34) Iengo, E.; Zangrando, E.; Bellini, M.; Alessio, E.; Prodi, A.; Chiorboli, C.; Scandola, F. Inorg. Chem. 2005, 44, 9752–9762. (35) Prodi, A.; Indelli, M. T.; Kleverlaan, C. J.; Scandola, F.; Alessio, E.; Gianferrara, T.; Marzilli, L. G. Chem.sEur. J. 1999, 5, 2668–2679. (36) Liu, R.; Holman, M. W.; Zang, L.; Adams, D. M. J. Phys. Chem. A 2003, 107, 6522–6526. (37) (a) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834–839. (b) Weller, A. Z. Phys. Chem. 1982, 133, 93–98. (38) The calculation of the electrostatic work terms requires knowledge of the distance between the donor and the acceptor in the assembly. For PBI--Zn+, electrostatic work term amounts to 0.14 eV at a center-to-center distance of 11.85 Å.22 For PBI--PBI+, the choice of the distance to be used is not obvious. In the highly distorted crystal structure, the two perylene bisimide units are in a p-stacked arrangement with a very short distance (3.9 Å). Because it is not known whether such a geometry is also maintained in solution, this should be taken as a lower limiting value, the maximum being the Zn-Zn distance in the porphyrin metallacycle (13.8 Å). Therefore, the work term for the PBI+-PBI- state may range from 0.12 to 0.42 eV. This translates in a range of energies (1.89-1.59 eV) to be used for the PBI+-PBI- state in the energy-level diagram of Figure 6. (39) The contribution of the zinc porphyrin radical cation to the overall spectral changes cannot be easily seen. In fact, the radical cation spectrum lacks distinctive features, exhibiting only a weak, broad band around 670 nm. (40) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L. Inorg. Chem. 2001, 40, 5507–5517. (41) The analogous side-to-face compound involving Zn porphyrin units is not sufficiently stable in solution. However, RuTPP(CO)py and ZnTPP have similar oxidation potentials (0.35 V20 and 0.42 V31 vs Fc/Fc+, respectively). (42) The work term amounts to 0.14 eV (center-to-center distance of 11.85 Å22) for PBI--Zn+ and 0.12 eV (center-to-center distance of 13.8 Å) for PBI--PBI+. (43) Experimental verification of this possibility is precluded, because nanosecond time-resolved spectroscopy with selective excitation of the perylene bisimide chromophore is not feasible with the available apparatus. (44) (a) Fo¨rster, T. Ann. Phys. 1948, 2, 55. (b) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7–17. (45) π-Stacking interactions between perylene bisimide units often give rise to spectral changes due to excitonic coupling.13,14 In the spectra of both molecular boxes (Figures 3 and 5), no significant difference with respect to the monomer spectrum of the respective PBI is observed.

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