State-Selective Electron Transfer in an Unsymmetric Acceptor−Zn(II

J. Phys. Chem. A 2010, 114, 1709–1721

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State-Selective Electron Transfer in an Unsymmetric Acceptor-Zn(II)porphyrin-Acceptor Triad: Toward a Controlled Directionality of Electron Transfer from the Porphyrin S2 and S1 States as a Basis for a Molecular Switch Staffan Wallin,† Cyrille Monnereau,‡ Errol Blart,‡ Jean-Richard Gankou,‡ Fabrice Odobel,*,‡ and Leif Hammarstro¨m*,† Chemical Physics Group, Department of Photochemistry and Molecular Science, Uppsala UniVersity, Box 523 SE-751 20 Uppsala, Sweden, and Chimie et Interdisciplinarite´: Synthe`se, Analyse, Mode´lisation (CEISAM) UMR CNRS 6230, 2, rue de la Houssinie`re - BP 92208 - 44322 Nantes 3, France ReceiVed: August 13, 2009; ReVised Manuscript ReceiVed: December 8, 2009

A series of Zn(II) porphyrin (ZnP) compounds covalently linked to different electron acceptor units, naphthaleneimide (NI) and naphthalenediimide (NDI), are reported. The aim was to demonstrate a stateselective direction of electron transfer, where excitation to the lowest excited S1 state of the porphyrin (Qband excitation) would give electron transfer to the NDI unit, while excitation to the higher S2 state (Soretband excitation) would give electron transfer to the NI unit. This would constitute a basis for an opto-electronic switch in which the direction of electron transfer and the resulting dipole moment can be controlled by using light input of different color. Indeed, electron transfer from the S1 state to NDI occurred in solvents of both high and low polarity, whereas no electron transfer to NDI was observed from the S2 state. With NI as acceptor instead, very rapid (τ ) 200-400 fs) electron transfer from the S2 state occurred in all solvents. This was followed by an ultrafast (τ ≈ 100 fs) recombination to populate the porphyrin S1 state in nearly quantitative yield. The charge-separated state ZnP+NI- was spectroscopically observed, and evidence was obtained that recombination occurred from a vibrationally excited (“hot”) ZnP+NI- state in the more polar solvents. In these solvents, the thermally relaxed ZnP+NI- state lies at lower energy than the S1 state so that further charge separation occurred from S1 to form ZnP+NI-. This resulted in a highly unusual “ping-pong” sequence where the reaction went back and forth between locally excited ZnP states and charge-separated states: S2 w ZnP+NI-“hot” w S1 w ZnP+NI- w S0. The electron transfer dynamics and its solvent dependence are discussed, as well as the function of the present molecules as molecular switches. 1. Introduction The concept of a molecular switch stimulates research on both a fundamental level and in the context of, for example, molecular sensors and logic circuits. Molecular switches may be based on various kinds of postsynthetic control of the function, using different types of input and output signals.1 It is recognized as an advantage, however, if the switch can be addressed with optical and/or electronic signals only, as this allows for a more rapid function and avoids “wet” chemical additions and waste products. Multicomponent molecular arrays that exhibit a controllable light-induced electron transfer between the components are interesting in that respect. The state populated by light then has a different dipole moment than the ground state, and the electrons and “holes” that are transferred may be utilized by further transport to a circuit. Furthermore, if the electronic response to light excitation in such an array can be varied by using different optical signals, an optoelectronic molecular switch with two different light-activated states can be achieved. Instead of a more common “on-off” switch, this would be a switch with “on1-on2-off” character for, for example, directing signals in two different ways. In this Article, we report attempts at constructing a molecular array of * Corresponding author. E-mail: [email protected]. † Uppsala University. ‡ Chimie et Interdisciplinarite´.

this type, in which the direction of electron transfer is controlled by the wavelength of the excitation light. Porphyrins have been used extensively for light-induced electron transfer reactions, but until relatively recently only the lowest excited singlet (S1) or triplet (T1) state has been involved,2 following a generalization of Kasha’s rule of photochemistry.3 In, for example, ZnII-tetraphenylporphyrin (ZnTPP), however, the higher excited state denoted S2 is relatively long-lived, τ ) 1-3 ps,4 and electron transfer reactivity from the S2 state to the dichloromethane solvent was suggested.4a The relatively long S2 lifetime was utilized to demonstrate ultrafast electron transfer from the S2 state in porphyrin-acceptor dyads, with a covalently bound or self-associated acceptor.5 Electron transfer studies from the S2 state in porphyrin-acceptor dyads have more recently been explored by a number of groups, with focus on the dependence on reaction free energy or linking position on the porphyrin.6 In the ZnTPP-Ru(bpy)32+ dyad of LeGourrie´rec et al., it was found that the rate of electron transfer to the Ru(bpy)32+ acceptor was 2 orders of magnitude faster from the porphyrin S2 state (τET ) 1.6 ps) than from the S1 state (τET ) 100 ps).5b The S2 state was populated by excitation into the Soret band around 420 nm and the S1 state by the usual Q-band excitation around 575 nm. Thus, the electron transfer reactivity of the dyad was very different depending on the wavelength of the excitation light, and we have proposed to use this as a basis for a molecular switch exhibiting different directions of the electron transfer.

10.1021/jp907824d  2010 American Chemical Society Published on Web 01/11/2010

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SCHEME 1: (Top) Schematic State Diagram and Reaction Properties of the Proposed Switch; (Bottom) Cartoon Illustrating How Light of Different Wavelengths Should Induce Electron Transfer in Different Directions

SCHEME 2: Structures of the Compounds

One way to achieve this is to link two different electron acceptor units on opposite sides of a ZnII-porphyrin. One of the acceptors should be strongly coupled to allow for rapid electron transfer from the short-lived S2 state, but have a low reduction potential so that electron transfer from the S1 state will not occur for energetic reasons (Scheme 1). The other acceptor should be easier to reduce to allow for electron transfer from the S1 state, but more weakly coupled so that electron transfer from the S2 state will be too slow to compete with electron transfer to the first acceptor. Thus, the direction of electron transfer would be opposite with Soret- versus Q-band excitation. The suggested switch would work with a single photon as input, which is different from other ultrafast molecular switches that have used a combination of two input photons.1b-e It would result in population of two different electronic states depending on the color of the input photon, states that have a very different spatial location of the excess electron and dipole moments of different directions. The compounds in Scheme 2 were designed on the basis of these ideas. The aromatic imides were chosen as acceptors because they form stable anions with characteristic absorption spectra, and it is possible to tune their reduction potentials by extending the π-system or with substituents.7 Compound ZI is a dyad consisting of a zinc porphyrin with a naphthalene imide

Wallin et al. (NI) directly attached at the meso position. Naphthalene monoimides are relatively difficult to reduce, so this unit could act as an acceptor unit in electron transfer from S2 but not from S1. The compounds DBZ and DPZ are dyads consisting of a zinc porphyrin with a naphthalene diimide (NDI) attached at the meso positions via a link. The two dyads differ in the type of linking unit, which is either a biphenyl (DBZ) or an ethoxyphenyl group (DPZ). The NDI is easier to reduce than the NI, but the longer linking unit than in ZI should give a weaker electronic coupling between the porphyrin and the NDI. Thus, NDI is designed to act as an acceptor unit in electron transfer from S1 but not from S2, as electron transfer would be too slow to compete with S2 deactivation by other reactions. Importantly, the energy of the lowest singlet excited states of the NDI and NI units is higher than the porphyrin S2 state energy, which prevents competing energy transfer reactions from the S2 state. As NDI and NI show no significant absorption of light in the visible, it is also feasible to excite the porphyrin unit selectively. DPZI is a triad consisting of a zinc porphyrin connected directly to a NI on one side and a NDI via a phenyl ethoxy link on the other side. The triad should combine the properties of the two dyads and thus have the potential to display the desired excitation wavelength-dependent direction of electron transfer. The energy of the different charge-separated (CS) states can be tuned by varying the solvent polarity, which allows for tuning of the function. The purpose of the present Article is to investigate if the design gave the desired function, as described above. On a more general level, these compounds also give the possibility to compare electron transfer from the first and the second excited states of the porphyrin in the same donor-acceptor molecule, which may give insight into state-dependent electron transfer processes and how to control them. We have examined the compounds in solvents of different polarity, and also in hydrogen-bonding solvents, to control the electron transfer processes. The results from emission and femtosecond transient absorption spectroscopy are rich and informative, and also revealed some unexpected reaction sequences. We describe and interpret the data in the Results. In the Discussion, we discuss the electron transfer processes, the observation of recombination from “hot” charge-separated states, and the function of these compounds as molecular switches of the type outlined above. 2. Results 2.1. Synthesis of the Compounds. The synthesis route for the preparation of the dyads 1 (ZI) and 2 (BDZ) is shown in Scheme 3. The known trisaryl porphyrin 58 was nitrated with the mixture silver nitrite and iodine with a slight modification of the conditions reported by Osuka.9 We found that it is essential to carry out this reaction at low temperature to limit the oxidative dimerization of the porphyrin. The nitro group of porphyrin 6 was then reduced by sodium borohydride in the presence of palladium on carbon as catalyst. Finally, the naphthalene monoimide (NI) was introduced by reacting amino porphyrin 7 with naphthalene dianhydride in quinoline in the presence of zinc acetate and gave the first dyad 1 in 86% yield. The synthesis of dyad 2 involves the initial preparation of naphthalene derivative 9, which was obtained through a Suzuki cross-coupling reaction between naphthalene diimide 810 and commercially available 4-bromo phenyl boronic acid. A second Suzuki cross-coupling reaction of the latter compound 9 with tetramethyl dioxaborolanyl porphyrin 1011 afforded the dyad 2 in a low yield (22%). Decomposition of the boronic ester of the porphyrin 10 is certainly the main reason of this low yield,

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SCHEME 3: Synthetic Route for the Preparation of Dyads ZI (1) and DBZ (2)a

a Reagents and conditions: (i) AgNO2, I2, CH2Cl2/CH3CN, 0 °C, 47%; (ii) NaBH4, Pd/C, CH2Cl2/MeOH, room tempeature, 91%; (iii) 1,8naphtalic anhydride, xylene, imidazole, Zn(OAc)2, 4H2O, 130 °C, 88%; (iv) DMF/H2O, Pd(PPh3)4, K2CO3, 62%; (v) 9, DME/H2O, Pd(PPh3)4, Ba(OH)2, 8H2O, 22%.

SCHEME 4: Synthetic Route for the Preparation of Dyad DPZ (3)a

a Reagents and conditions: (i) methane sulfonylchloride, CH2Cl2, Et3N, not isolated; (ii) LiBr, THF, reflux, overall 40%; (iii) PBr3, CH2Cl2, -20 °C, 90%; (iv) 13, K2CO3, DMF, 70 °C, 68%.

because a large amount of diarylporphyrin was formed during this reaction. The synthesis of dyad 3 (DPZ) was accomplished following the route depicted in Scheme 4. The preparation of the key NDI intermediate 12 starts from the unsymmetrical naphthalene diimide 11,12 in which the alcohol group was first esterified by methansulfonyl chloride and directly transformed into the bromo derivative 12 in 40% overall yield. Porphyrin 13 was prepared according to the Lindsey procedure13 and was subsequently demethylated with boron tribromide at room temperature. The phenol group of porphyrin 14 was deprotonated by potassium carbonate in DMF to undergo a Williamson reaction with the bromo group of the naphthalene derivative 12 to give the expected dyad 3 in a 68% yield. The preparation of the triad 4 (DPZI) was based on the synthetic methodologies developed for the dyads 1-3 and is

shown in Scheme 5. Monobromoporphyrin 158 was reacted with 4-methoxy phenyl boronic acid to give trisarylporphyrin 16 following a Suzuki cross-coupling reaction in very good yield. The resulting trisaryl porphyrin 16 was nitrated with the mixture silver nitrite-iodine according to the conditions developed for porphyrin 5. Reduction of the nitro group by sodium borohydride and then imidation with naphthalene dianhydride afforded porphyrin 18. Demethylation of methoxy group of 18 could be achieved using boron tribromide at room temperature to afford 20 in 55% yield after purification. Finally, porphyrin 20 was coupled with the naphthalene derivative 12 following a Williamson reaction to give the expected triad 4. 2.2. Absorption and Emission Spectroscopy. The absorption spectra in the 300-700 nm region were similar for all compounds (Figure 1). In the visible region, the absorption spectra in all solvents and for all compounds show characteristic

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SCHEME 5: Synthetic Route for the Preparation of Triad DPZI (4)a

a Reagents and conditions: (i) 4-methoxybenzene boronic acid, DME, H2O, Pd2(dba)3CHCl3, PPh3, Ba(OH)2, 8H2O, 90 °C, then Zn(OAc)2, CH2Cl2/MeOH, room temperature, 96%; (ii) AgNO2, I2, CH2Cl2/CH3CN, -20 °C, 70%; (iii) NaBH4, Pd/C, CH2Cl2/MeOH, room temperature, 94%; (iv) 1,8-naphtalic anhydride, xylene, imidazole, Zn(OAc)2, 4H2O, 180 °C, 67%; (v) PBr3, CH2Cl2, room temperature, 55%; (vi) 12, K2CO3, DMF, 110 °C, then Zn(OAc)2, 4H2O, CH2Cl2/MeOH, room temperature, 18%.

Figure 1. Absorption spectra of ZnTPP (-), DBZ (- - -), and ZI (×) in toluene. Inset: Zoom of the 520-620 nm region.

porphyrin features,2,14 that is, a Soret band located around 420 nm, which corresponds to a transition to the S2 state, and two Q-bands between 540 and 600 nm that correspond to transitions to the S1 state. Below 400 nm, some additional absorbance from the imide and diimide units is observed. The absorption spectrum of DPZ (not shown) is nearly identical to that of DBZ, and the absorption spectrum of DPZI at λ > 400 nm (not shown) is nearly identical to that of ZI. As compared to ZnTPP, the Soret- and Q-band peaks are red-shifted by ∼60 cm-1 in DBZ and DPZ and by 300-500 cm-1 in ZI and DPZI (see the Supporting Information; note that the difference between the tri- and tetraaryl porphyrin absorption in DBZ and DPZ is negligible). There is also a slight increase in amplitude of the Q(0,0) transition in DBZ and DPZ and a somewhat larger increase in ZI and DPZI. These effects indicate a very small porphyrin-diimide interaction and a somewhat stronger porphyrin-imide interaction. The effect on the absorption spectra of changing solvent is small and equal in all compounds over a large range of solvent polarities. This

indicates that there is no significant charge transfer character of the excited state in DBZ or ZI. Thus, the initially formed excited states of the dyads and the triad are the normal porphyrin-localized states usually denoted S1 (Q-band excitation) and S2 (Soret-band excitation). The shape of the emission spectra also shows typical porphyrin features with maxima at 640-650 nm and is similar for all compounds and in all solvents. A weak S2 fluorescence could also be seen very close to the Soret-band absorption. The energy of the S1 and S2 states can be estimated from the midpoint between the absorption and emission peaks for the (0,0) transitions. For DBZ and DPZ, this gives an S1 energy varying from of 2.11 in toluene to 2.07 eV in DMF, and an S2 energy from 2.93 to 2.92 eV. The corresponding values for ZI and DPZI are 2.07-2.06 eV for S1 and 2.92-2.89 eV for S2. There are some difficulties in measuring the position of the S2 emission peak due to reabsorption, and accurate measurements suggest that the Stokes shifts measured as done here are overestimated by about 100 cm-1.15 Thus, the S2 state energy could be underestimated only by about 0.01 eV, and for the purpose of the present study this effect is not important. The quantum yields for fluorescence varied greatly between the compounds and with a change of solvent. Quantum yields of fluorescence, relative to that for ZnTPP, are given in Table 1. Three different relative quantum yields are given: the S1 fluorescence yield with Q-band excitation and with Soret-band excitation, and the S2 fluorescence yield with Soret-band excitation. This can give important, complementary information of the quenching pathways for the two states. The results for DBZ show that in this compound there is little or no quenching of the S2 fluorescence, while the S1 fluorescence is quenched significantly in both toluene and DMF. The similar S1 fluorescence yields with Soret- and Q-band excitation show that S2 to S1 internal conversion is near quantitative, as expected in the absence of S2 quenching. The results for DPZ are very similar, with only a somewhat stronger quenching of the S1 fluorescence. The results for ZI are instead very different. The quantum yields

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TABLE 1: Relative Fluorescence Quantum Yields with ZnTPP as Standarda compound

solvent

DBZ

toluene DMF DMF toluene THF butanol ethanol methanol DMF toluene THF butanol DMF

DPZ ZI

DPZI

Ex S1, Em S1 Ex S2, Em S1 Ex S2, Em S2 0.37 0.26 0.15 0.79 0.87 0.03 0.03 0.03 0.12 0.53 0.54 0.05 0.07

0.32 0.27 0.14 0.54 0.78 0.03 0.02 0.02 0.10 0.51 0.59 0.03 0.06

∼0.8 ∼1.0 ∼1.0 ∼0.16 ∼0.10 ∼0.07 ∼0.10 ∼0.10 ∼0.13 ∼0.12 ∼0.10 ∼0.09 ∼0.12

a Excitation was in the maximum of either the Soret band (“Ex S2”) or the Q-bands (“Ex S1”); emission was detected from the S2 and S1 states (“Em Ss” and “Em S1”, respectively).

show that the S2 fluorescence in ZI is quenched by about 90% in all solvents, while quenching of S1 fluorescence is strongly solvent dependent. In the alcohols, the S1 fluorescence is almost completely quenched; in DMF, it is quenched by about 90%, while there is very little quenching in THF and toluene. It is interesting to note that while the S2 fluorescence is quenched, the quantum yield for S1 fluorescence is nevertheless nearly the same when exciting in the Soret and Q-bands. This shows that most of the initially populated S2 state converts to the S1 state, despite the S2 quenching reaction. The quenching of the S2 state is therefore either because of an enhanced rate of direct internal conversion to S1 or because it populates a state that in turn reacts further with a high yield to form the S1 state. The transient absorption data in section 2.4 show that the latter is the case. For DPZI, the quenching of both the S2 and the S1 fluorescence in butanol is very similar to the results for ZI, indicating that quenching in this solvent is mainly due to the NI unit. In THF, the quenching of the S1 emission is somewhat stronger in DPZI as compared to ZI, suggesting that there is additional quenching by the NDI unit. Finally, in DMF the S1 emission yields for DPZ and ZI are quite similar, showing that both the imide and the diimide quench the S1 excited state. Consequently, two quenching pathways of similar importance are expected for DPZI in this solvent. This gives the following expected relation of the S1 fluorescence quantum yields (see the Supporting Information):

1 1 1 + -1 ) φDPZI φDPZ φZI

(1)

Insertion of values for ΦZPD ) 0.15 and ΦZI ) 0.12 into eq 1 gives ΦDPZI ) 0.07. This is in excellent agreement with the measured value, which indicates that the two acceptors in DPZI react independently of each other, with the same rates as in the corresponding dyads. 2.3. Transient Absorption Measurements on DBZ and DPZ. Transient absorption measurements were performed to observe the different charge-separated (CS) states and to follow the dynamics of their formation and decay. The samples were excited near the Soret- or Q(1,0)-band maximum, populating the S2 and S1 state, respectively. In toluene, DBZ decays from S2 to S1 with a time constant of 1.6 ps with no indication of a CS state (Figure 2). These results are identical within experimental uncertainty to that for the reference ZnTPP (see below).

Figure 2. Transient absorption spectra of DBZ in toluene after Soretband excitation at 422 nm at 0.2 ps (-), 20 ps (- · - · -), and 2 ns (×).

Figure 3. Transient absorption traces of DBZ in toluene after Soretband excitation at 422 nm and probing at 450 nm (3), 475 nm (O), and 645 nm (×). The lines are double-exponential fits to the data.

The S2 state has a spectrum similar to that for the S1 state,5 but with somewhat lower amplitude. The S2 to S1 conversion is thus seen as a general rise in the 425-600 nm region, especially in the region of stimulated S2 emission (ca. 435 nm), and as a development of the S1 stimulated emission band around 645 nm. The S1 state in the ZnTPP reference has a lifetime of ca. 2 ns and forms the triplet T1 state in a high yield (ca. 0.82a,14,16). For DBZ instead the S1 state population converts to the CS state with a time constant of 400 ps, as seen by the appearance of the reduced diimide (NDI-) absorption peaks around 474 and 605 nm (ε ) 26 000 and 7200 cm-1 M-1, respectively)7 and the ZnP+ absorption around 450 nm (Figure 2). These absorption features then decay with a time constant around 10 ns, see Figures 2 and 3, indicating charge recombination to re-form the ground-state reactants. Nanosecond flash photolysis measurements (not shown) reveal a biexponential decay of the transient absorption around 475 nm with time constants of 12 ns and 0.35 µs. The 12 ns component is attributed to CS state recombination, while the 0.35 µs component is attributed to the decay of a fraction of the compounds that have undergone direct intersystem crossing from the S1 to the T1 state, in competition with charge separation. The 0.35 µs lifetime agrees with the triplet lifetime of ZnTPP in aerated solution (0.42 µs). No indication of triplet formation via charge recombination was observed in this or any other experiment of this study.

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TABLE 2: Time Constants for Charge Separation (τET1) and Recombination (τCR1) after Excitation to the S1 State compound

solvent

εsa

τET1 (ps)

τCR1 (ps)

DBZ

toluene DMF DMF toluene THF butanol ethanol methanol DMF toluene THF butanol DMF

2.4 37 37 2.4 8 16 24 33 37 2.4 8 16 37

400 350 250 b b 28 39 25 170 600d 400d 30e 120f

12 × 103 c c b b 120 34 12 38 c c 120 c

DPZ ZI

DPZI

a Static dielectric constant. b Not detected. c Not measurable, τCR , τCS. d Electron transfer to the NDI acceptor. e Electron transfer to the NI acceptor. f Electron transfer to both acceptors in parallel.

The charge separation and recombination are much slower than those of a very similar dyad where the NDI was linked via a single phenyl group to the Zn-porphyrin, for which τET ) 9 ps and τCR ) 70 ps.9 The ca. 100-fold slower ET and CR rates for DBZ are consistent with the expected rate decrease with one extra phenyl group.17 In the more polar solvent DMF instead, the DBZ signals decay monoexponentially from S1 to the ground state with a time constant of about 350 ps, with no sign of a CS state. This can be explained by forward electron transfer followed by fast ( 1 ps after S2 excitation in methanol. The time constants in all solvents are given in Table 2. The S1 lifetime relative to that of the ZnTPP reference agrees very well with the relative S1 fluorescence yield after S1 excitation in Table 1, that is, τ/τZnTPP ) Φfl/Φfl, ZnTPP. The relative emission yields indicate an ET time constant of around 40 ps in the alcohols and 160 ps in DMF, assuming that the intrinsic S1 lifetime in unaffected by the imide substitution. The good agreement with the steady-state data gives support for simple irreversible, single-exponential quenching kinetics and also means that the level of impurities in the form of unquenched

porphyrins is very low ( CS (ethanol) > CS (methanol) > CS (DMF), which is what would be expected from the recombination rates that increase throughout this series of solvents. In toluene and THF, no electron transfer from S1 could be detected on the time scale of experiment (