Effects of Photoinduced Electron Transfer on the Rational Design of

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Effects of Photoinduced Electron Transfer on the Rational Design of Molecular Fluorescence Switch Ruili Zhang,†,‡ Yishi Wu,† Zhongliang Wang,†,‡ Wei Xue,†,‡ Hongbing Fu,*,† and Jiannian Yao*,† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China, and Graduate School of Chinese Academy of Sciences, Beijing, 100049, P. R. China ReceiVed: October 15, 2008; ReVised Manuscript ReceiVed: December 18, 2008

Aiming at exploring the relationship between the spacer and fluorescence switch properties, we synthesized a series of new photoactive triads, in which one perylenetetracarboxylic diimide unit acting as the electron acceptor was attached to two ferrocene moieties through different spacers. This kind of donor-spacer-acceptor structure allows for tuning one of the key factors that governs photoinduced electron transfer, the distance between the electron donor and acceptor units. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as transient absorption techniques. It was found that fluorescence intensity of the solution of all triads 1-3 can be reversibly modulated by the electrochemical oxidation and reduction sequentially. More importantly, as the length of the spacer between the donor and acceptor increases, the background fluorescence increased proportionally, but the contrast ratio of the fluorescence decreases. Together these two factors determine the assay sensitivity, and therefore this work is helpful to provide a basis for the rational design of fluorescence switch by optimizing these factors above. Introduction In recent years, there has been considerable interest in efficient modulation of fluorescence in molecular systems because of their significant potential application in fluorescence probes,1 chemical or biological sensors,2 memory devices,3 logic gates,4 and so forth. A large class of these molecular systems utilizes a donor-spacer-acceptor (D-s-A) molecular framework facilitating systematical investigations of modulation of the fluorescence emission. Photoinduced electron transfer (PET)5-12 is a widely accepted mechanism for fluorescence quenching, in which electron transfer from the electron donor to the excited fluorophore diminishes the fluorescence of the fluorophore, and the fluorescence emission is “switched off”. When the electrondonating abilities of the donor are reduced, the PET reaction would be arrested, leading to fluorescence “on”. Although the effects of the nature of D-A separation, spacer, on the rate of PET have been studied intensely,13-18 few investigations are focused on how the D-A spacer influences the fluorescence on/off switching behavior. For practical applications, the contrast ratio of the fluorescence and background fluorescence are particularly important, because they define the optical sensitivity in applications such as microscopy imaging. Therefore, considering that the fluorescence properties of these switches are controlled by the PET, it is meaningful to design the fluorescence switch rationally on the basis of the modulation of photophysical parameters governing the electron transfer process, so as to optimize the fluorescence enhancement upon PET being “switched off”. Our strategy is to incorporate ferrocene (Fc) into a D-s-A molecular framework as the electron donor because of its * Corresponding authors. Tel: +86-10-8261-6517. Fax: +86-10-82616517. E-mail: [email protected]; [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.

attractive properties, such as high degree of chemical and thermal stability and robust and excellent reversible Fc/Fc+ redox couple.19 Meanwhile, perylenetetracarboxylic diimide (PDI) was chosen as the fluorophore and electron acceptor. In addition to its high fluorescence quantum yields which are near unity and its good photo and thermal stability,20 the following considerations were taken into account. First, the fluorescence spectrum of PDI (500-650 nm) is not significantly overlapped with the absorption spectrum of the Fc moiety (∼440 nm). Otherwise, intramolecular energy transfer would occur efficiently. Second, the lowest excited singlet state of the Fc unit is higher in energy than that of the PDI subunit.21 Thus, the reverse electron transfer from 1PDI* to Fc•+ cannot take place. Moreover, by proper functionalization of PDI, the desired optical and electronic properties, which are required for the application of PDI, may be achieved. In this work, we aim to determine whether there is an association between the behavior of the fluorescence switch and the spacer. For this, we designed three systems 1-3 (Scheme 1), which contain the PDI acceptor covalently linked to two Fc moieties as the donor. An analysis of photoinduced electron transfer kinetics of such PDI-Fc system as a function of the distance between the electron donor and acceptor units was obtained using UV-vis absorption, steady-state and timeresolved emission, and femtosecond transient absorption spectroscopy. Combining the above data, we further investigated the influences of spacer on the fluorescence on/off switching behavior. This work offers a basis for developing novel fluorescence switches with high contrast ratio of the fluorescence predictably. Experimental Section Materials. The compound 3,4,9,10-perylenetetracarboxylic dianhydride was purchased from Acros Organics. The 4-aminophenol and 6-amino-1-hexanol were purchased from Alfa

10.1021/jp809135j CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

Effect of D-A Spacer on Fluorescence Switch

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SCHEME 1: Syntheses of Triads 1-3

Aesar. Ferrocenecarboxamide was purchased from the Yixing Weite Petrochemical Additives Plant. The (aminomethyl)ferrocene and the reference compound (N,N′-di(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic acid diimide, 4) were prepared according to the literature.22,24 All other reagents and solvents (standard grade) were used as received unless otherwise stated. Measurements. The 1H NMR spectra were recorded on a Bruker DMX 300 MHz instrument and a Bruker AV 400 instrument. The TOF-MS spectra were determined using a BEFLEX III. Elemental analyses were performed on a CarloErba-1106 instrument. The UV-visible absorption spectra were measured with a Perkin-Elmer Lambda 35 spectrophotometer. The steady-state fluorescence measurements were carried out with a Hitachi

F-4500 fluorospectrometer, using a Xe lamp as the excitation source with a 1-cm quartz cell. The fluorescence lifetime measurements were carried out on the picosecond time-resolved fluorescence spectrometer. The 800-nm laser pulses generated from a Ti:sapphire regenerative amplifier (Spitfire, Spectra Physics) were sent to an optical parametric amplifier (Spectra Physics, OPA-800CF) and used as the excitation at 530 nm. The pulse energy was about 100 nJ/pulse at the sample. Fluorescence gathered with the 90° geometry was dispersed by a polychromator (250is, Chromex) and collected with a photon-counting type streak camera (C5680, Hamamatsu Photonics). The data detected by digital camera (C4742-95, Hamamatsu) were routinely transferred to PC for analysis with HPDTA software. The spectral resolution was 2

2596 J. Phys. Chem. C, Vol. 113, No. 6, 2009 nm, and the temporal resolution was 2-100 ps, depending on the delay time range setting. The fluorescence quantum yields (ΦF) of compounds were calculated with dilute solutions (A < 0.05) by the steady-state comparative method using Rhodamine 6G as a standard. Cyclic voltammetric measurements were carried out on a CHI660A electrochemical workstation using a scan rate of 50 mV/s in CH2Cl2 with a thin plate of platinum as the working electrode, a platinum wire as the counter electrode, a Ag wire as a reference electrode (-0.430 V vs Fc/Fc+), and n-Bu4NPF6 (0.1 M) as the supporting electrolyte. The electrochemical experiments mentioned above were carried out under an atmosphere of dry N2. Femtosecond Transient Absorption Measurements. A Ti: sapphire femtosecond laser system provided laser pulses for the femtosecond transient absorption measurements by using the pump-probe technique. A regenerative amplifier (Spectra Physics, Spitfire) seeded with a mode-locked Ti:sapphire laser (Spectra Physics, Tsunami) delivered laser pulses at 800 nm (120 fs, 1 kHz), which were then divided into two components by using a beam splitter. The major component was sent to an optical parametric amplifier (Spectra Physics, OPA-800CF) generating the pump pulses (530 nm, 130 fs, 1 kHz). The whitelight continuum, generated by focusing the minor component into a 5-mm sapphire plate, was then divided into a probe beam and a reference beam. The probe beam was overlapped with the pump beam at the sample. The reference beam was used to eliminate the influence of the laser beam fluctuations and subsequently enhance the signal-to-noise ratio. The optical delay times between the pump and probe beams were achieved through a computer-controlled motorized translation stage (LTS200, Σ-koki). A magic-angle (54.7°) scheme was used in the pump-probe measurement to cancel out the orientation effects on the measured dynamics. The cross correlation function between the pump and probe pulses was determined to be about 150 fs by taking advantage of the technique based on the nonresonant optical Kerr effect. The transmitted light was detected by a liquid nitrogen-cooled CCD (Spectrum One, JY). Cuvettes with a 1-mm-path length were used, and the samples were irradiated at 530 nm with 0.2-0.5 µJ/pulse. The typical optical density of the sample at the excitation wavelength was 0.2-0.6/mm. Steady-state UV-vis absorption spectra were carried out before and after the transient absorption measurements to confirm that there was no photodegradation of the samples. Analysis of the kinetic data was performed using a nonlinear least-squares fit to a general sum of exponentials function with convolution of a Gaussian instrument response function of 150 fs. All spectroscopic measurements were carried out at room temperature. Quantum Chemical Calculations. All calculations were performed using the Gaussian 03 package.25 DFT in its threeparameter hybrid B3LYP functional26 together with the standard all electron 6-311++G**27 basis sets were used in the structural optimization for triads 1-3. The simplified model systems consisting of the PDI derivates without Fc moiety were chosen to reduce computation time. This removal of the Fc groups should have insignificant influence on the parallel comparison of the spacer in triads 1-3 because there were negligible intramolecular charge-transfer interactions for triads 1-3 in their ground state (vide infra). Results and Discussion Synthesis and Structural Characterization. The studied FcPDI arrays were synthesized according to Scheme 1. Condensa-

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Figure 1. Cyclovoltammogram of triads 1 (a), 2 (b), and 3 (c) in CH2Cl2 (scanning rate 50 mV/s) with a thin plate of platinum as the working electrode, platinum wire as the counter electrode, Ag wire as the reference electrode (-0.430 V vs Fc/Fc+), and n-Bu4NPF6 (0.1 M) as the supporting electrolyte.

TABLE 1: Redox Properties of Triads 1-3 and Reference Compound 4 (vs Fc/Fc+) 1 2 3 4

Ered (PDI-/PDI2-)

Ered (PDI/PDI-)

Eox (Fc/Fc+)

-1.13 -1.15 -1.22 -1.12

-0.96 -0.97 -1.04 -0.95

0.12 0.29 0.23 not determineda

a Value could not be determined owing to the irreversible oxidation process of the PDI moiety.

tion of 3,4,9,10-perylenetetracarboxylic dianhydride 5 with (aminomethyl)ferrocene, which was prepared by reduction of ferrocenecarboxamide22 in the presence of Zn(OAc)2, led to the triad 1 in an overall yield of 43% after purification with column chromatography. Triads 2 and 3 were synthesized through an analogous two-step procedure. First, compound 6 or 7 was prepared by the condensation of 4-aminophenol or 6-amino-1hexanol with 5. The purified 6 or 7 was then reacted with chlorocarbonylferrocene23 through an esterification reaction. After purification by chromatography on silica gel, triads 2 and 3 were obtained as red solids in yields of 56 and 45%, respectively. The reference compound 4 was synthesized by the reaction of 5 with 2-ethylhexylamine.24 Electrochemistry. The redox properties of compounds 1-3 were investigated by cyclic voltammetry at room temperature in CH2Cl2 (vs Fc/Fc+). The data obtained are summarized in Table 1. In the cathodic direction, triads 1-3 all give rise to two quasi-reversible reduction waves, which correspond to the first and the second one-electron stepwise reductive process of the perylene moieties (Figure 1). Although the reduction potential values are sensitive to the properties of electrondonating or electron-withdrawing appending to the PDI position,28 we found that Fc substituents in the imide nitrogen position of PDI do not significantly affect the first reduction potential as compared to that of 4, suggesting the absence of ground-state interactions between the Fc and the PDI moieties. In the anodic region, no oxidation potential is observed for reference 4, so a one-electron reversible oxidation wave of 1 observed at 0.12 V can be attributed to the oxidation of Fc units. For 2 and 3, the oxidation potentials observed at 0.29 and 0.23 V were increased as compared to that of 1. This result shows that the oxidation potentials vary with increasing distances between the Fc and PDI moieties.



Figure 2. (a) Steady-state UV-vis absorption spectra (A ) 0.45) and (b) fluorescence emission spectra of triads 1-3 and 4 in CH2Cl2. Excitation wavelength was 490 nm.

TABLE 2: Optical Properties of Compounds 1-4 in CH2Cl2 UV/vis absorption compound λmax (nm) λmax (nm) 1 2 3 4

526 526 525 524

539 538 537 537

fluorescence emission Φfl

τ (ns)

0.0039 0.14 (61%) 3.66 (39%) 0.034 0.13 (54%) 3.50 (46%) 0.38 0.31 (40%) 4.25 (60%) 0.85 4.77

Steady-State Absorption and Fluorescence Spectra. The UV-vis spectra of triads 1, 2, and 3 in CH2Cl2 are shown in Figure 2a. As one would expect, the spectra are essentially dominated by strong bands with maxima at λ ) 537 nm associated with PDI units. The absorption spectra are similar for all compounds (Table 2), and no new absorption bands or distinctive spectropic shoulders were detected in the UV-vis spectra of triads 1-3 as compared to that of the reference compound 4. This fact indicates that there were negligible intramolecular charge-transfer interactions for triads 1-3 in their ground state. Figure 2b shows the fluorescence emission spectra of triads 1-3. For comparison, the fluorescence spectrum of compound 4 is also included. Clearly, except for the intensity difference the fluorescence spectra of triads 1-3 are very similar to that of compound 4. Their fluorescence spectra with three bands at about λ ) 537, 577, and 625 nm are indeed the mirror images of UV-vis absorption spectrum of compound 4 (Figure 2). These results imply that the fluorescence of triads 1-3 is due to the PDI unit. As compared to that of compound 4 without Fc substituents, the fluorescence intensities of triads are reduced obviously. Moreover, on comparison with triad 3, triads 1 and 2 show much lower fluorescence intensities. This behavior is

also observed for the fluorescence quantum yields in CH2Cl2 (Table 2). The fluorescence emission data thus indicate an additional process taking place for triads 1-3 that strongly quenches the PDI emission upon attachment of the Fc moiety. Thus, the electron-rich Fc acts as a fluorescence quencher for the electron-deficient PDI chromophore. The quantum yields of triads 1-3 and compound 4 were determined for the PDI emission by exciting the perylene units at 490 nm. As can be seen from Table 2, the emission quantum yields of the perylene unit are significantly lower when compared to the reference compound 4 (Φf ) 0.85 in CH2Cl2). Fluorescence Lifetime Measurements. Furthermore, the fluorescence lifetimes of all compounds were determined in CH2Cl2, and the obtained values are summarized in Table 2. The fluorescence lifetime of compound 4 exhibited monoexponential decay with a lifetime (τ0) of 4.77 ns, which is in reasonable agreement with the value of τ ≈ 4 ns reported in the literature for a similar compound in CH2Cl2.29,30 In contrast, for triads 1, 2, and 3, drastically quenched fluorescence lifetimes compared with the value of compound 4 are found. The fluorescence time profiles of 3 could be fitted satisfactorily with biexponential decay functions, from which the fluorescence lifetime (τf) of the short-lived component was evaluated as 0.31 ns (40%), as listed in Table 2. The shorter τf value of the 1PDI* moiety of Fc-spacer-PDI can be attributed predominantly to charge separation between the Fc and PDI moieties, while the longer τf value (4.25 ns, 60%) remained similar to the intrinsic fluorescence lifetime of the PDI chromophore.20d,31 Compared with that of 3, the fluorescence lifetimes of 2 were significantly shortened, giving the short-lived component as 0.13 ns (54%), suggesting that the initial charge-separation process take place via 1PDI* in triad 2, which is faster than that in 3. As for triad 1, the decay consisted of a fast decay component (τf ) 0.14 ns, 61%) and a slow component. From the shorter lifetime compared with that of 4, appreciable increase in the decay rate of triad 1 was observed. Apparently, attaching the Fc moiety to PDI introduced a new quenching pathway to reduce the lifetimes of the 1PDI* in CH2Cl2, which was in agreement with the steadystate results. This quenching was due to the photoinduced charge separation between excited singlet states of PDI and Fc units to yield charge-separated states (Fc•+-spacer-PDI•-), and the lifetime of PDI moiety is strongly shortened upon attachment of two Fc substituents. Such changes in fluorescence lifetimes thus relate to the observations made for the fluorescence quantum yields. Gibbs Energy of Photoinduced Electron Transfer. To further examine the feasibility of a photoinduced charge separation, it is instructive to consider the themodynamics of such a process. Therefore, the Gibbs free energy of an intramolecular charge-separated state (∆GCS) in a covalently bonded donor-acceptor system can be calculated for triads 1-3 using eq 1, where Eox (Fc) and Ered (PDI) represent the oxidation potential of the Fc donor and the first reduction potential of the PDI acceptor, respectively, E00 is the energy of the S0 f S1 excited state, RDA refers to the center-to-center distance between the donor and acceptor units (1: RDA ) 9.38 Å; 2: RDA ) 13.51 Å; 3: RDA ) 16.69 Å), and r+ and r- represent the effective ionic radii of the donor and acceptor radical cation and anion, respectively. r+ can be estimated to be 3.2 Å using X-ray diffraction data for ferrocene at room temperature,32 and r- was estimated to be 4.71 Å on the basis of the X-ray crystallographic data of N,N′-dimethylperylene-3,4:9,10-tetracarboxylic bisim-

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TABLE 3: Calculated Free-Energy Change of Charge Separation (∆GCS) and Charge Recombination (∆GCR) via 1PDI* for Triads 1-3 in CH2Cl2 compound

∆GCS (eV)

∆GCR (eV)

kCS (s-1)

1 2 3

-1.43 -1.20 -1.16

-0.91 -1.14 -1.18

2.68 × 10 4.43 × 1010 3.80 × 1010 11

kCR (s-1)

τCS (ps)

τCR (ps)

1.65 × 10 9.15 × 108 1.27 × 109

3.85 22.57 28.76

60.74 1093.33 784.78

10

ide.33 εref denotes the reference solvent dielectric constant used in electrochemistry, and εS is the respective solvent dielectric constant:34

∆GCS ) e[Eox(Fc) - Ered(PDI)] E00 -

(

)(

e2 e2 1 1 1 1 + 4πε0εSRDA 8πε0 r+ r- εref εS

)

(1)

The first reduction potential Ered(PDI) of 1 in CH2Cl2 (εS ) 9.1) was used, and Eox(Fc) ) 0.12 V. The RDA was estimated from an energy-minimized structure to be RDA ) 9.38 Å for triad 1. All electrochemical values were obtained in CH2Cl2, and therefore εref equals εS; thus, the last solvent-related term in eq 1 vanishes. According to eq 1, the energy of the intramolecular charge-separated state in CH2Cl2 is estimated to be ∆GCS ) -1.43 eV, which is lower than the energy of respective S0 f S1 excited state for triad 1 (E00 ) 2.34 eV). This negative value shows that electron transfer from the Fc substituent to the photoexcited moiety in triad 1 is an exergonic reaction. Similarly, the free-energy changes of the chargeseparation process (∆GCS) via 1PDI* in triads 2 and 3 were calculated, as listed in Table 3. Therefore, it can be concluded that the generation of a chargeseparated state consisting of the radical cation of the Fc moiety and the radical anion of the perylene unit is energetically feasible for triads 1-3. Thus, the strongly decreased fluorescence quantum yields and lifetimes for triads 1-3 upon attachment of two Fc moieties compared with that of reference compound 4 should be most likely attributed to a rapid photoinduced electron transfer process (Table 2). Femtosecond Transient Absorption Spectroscopy. To shed more light on this quenching process, all compounds were investigated with femtosecond transient absorption spectroscopy in CH2Cl2 solution. The spectral data for 1-4 are shown in Figures 3-6. For all four compounds upon photoexcitation, an intense bleaching due to the depopulation of the ground-state molecules in the probe area is observed at 490 and 530 nm. These bands are related to the S0 f S1 transitions of the orange PDI chromorephore. Furthermore, an intense negative signal is present in all spectra of 1-4 at around 530 and 575 nm, which can be assigned to the stimulated emission of the orange PDI chromophore. These features are accompanied by a strong positive absorption band with a maximum centered at ∼705 nm for compounds 1-3, which can be assigned to the characteristic absorption of the PDI radical anion.35-37 Therefore, a charge-separated state is produced by electron transfer upon excitation of the PDI moiety in compounds 1-3. When the PDI unit in 1 was excited at 530 nm in CH2Cl2, the femtosecond transient absorption spectra yielded a strong positive absorption band with a maximum centered at 705 nm, as shown in Figure 3a. This band was assigned to the characteristic absorption of the PDI radical anion, which demonstrated that a charge-separated state Fc•+-spacer-PDI•is produced by electron transfer from the Fc to the 1PDI* moiety in compound 1. Since the absorption band of Fc•+ was reported to have a small molar absorption coefficient (ε ) 450 M-1 cm-1 at λmax ) 617 nm),38 this band may be hidden under the other

Figure 3. (a) Femtosecond transient absorption spectra of 1 in CH2Cl2 upon excitation at 530 nm at different delay times. (b) Time profiles of the absorption at 705 nm. Inset: Corresponding time profiles at a shorter time delay. Fits are also shown in solid lines.

huge absorptions. Therefore, it is difficult to confirm Fc•+. Further analysis of the time profile of the absorption at 705 nm revealed that two decay components were involved with a time constant of 3.74 and 60.74 ps, respectively (Figure 3b). Note that the value of the fast decay was comparable with the kinetic behavior of the stimulated emission signal at around 575 nm. By using the reference compound 4, we found that a positive signal around 705 nm due to the S1 f Sn absorption of the PDI chromophore also appears (Figure 6)29,39 and decay in a similar way as the stimulated emission at 575 nm. Therefore, the fast decay at 705 nm for 1 is due to S1 f Sn absorption of the PDI chromophore. However, the decay of the S1 f Sn absorption of the PDI chromophore in 1 is much faster than that in reference compound 4. This is because the electron transfer upon photoexcitation in 1 accelerates the deactivation process of the S1 excited state of the PDI chromophore. Furthermore, the slow decay at 705 nm for 1 was ascribed to the PDI radical anion because of the charge-recombination (CR) process. The rate constants of charge separation (kCS)40 and charge recombination




(kCR) in CH2Cl2 were thus accordingly determined as 2.68 × 1011 and 1.65 × 1010 s-1, respectively (Table 3). This ultrafast charge-separation process is in good agreement with the almost complete quenching of the PDI emission and the presence of a shorter lifetime in the time-resolved fluorescence spectra. In the case of triad 2 in CH2Cl2, the femtosecond transient absorption spectra are quite similar to that of 1, revealing the characteristic absorption band of the PDI radical anion upon excitation of the PDI moiety at 530 nm, as shown in Figure 4a. This observation unambiguously confirmed the event of a charge-separated state. Further analysis of the time profile at 706 nm revealed temporal evolution of the PDI radical anion, where two decay components with a time constant of 19.27 and 1093.33 ps, respectively, were evaluated (Figure 4b). The kCS and kCR values of 2 in CH2Cl2 were equivalently estimated to be 4.43 × 1010 and 9.15 × 108 s-1, respectively. This rapid charge separation is consistent with the nearly complete fluorescence quenching of the PDI unit in 2. Similarly, the characteristic absorption band of the PDI radical anion was observed in femtosecond transient absorption spectra of triad 3 in solution by laser light excitation at 530 nm. By further analysis of the time profile at 705 nm, we obtained temporal evolution of the PDI radical anion, where two decay components with a time constant of 26.19 and 784.78 ps, respectively, were evaluated (Figure 5). The kCS and kCR values of 3 in CH2Cl2 were equivalently estimated to be 3.80 × 1010 and 1.27 × 109 s-1, respectively. Electrochemical Redox Switching of Fluorescence. To increase the understanding of photoinduced electron-transfer reactions, we paid attention to the electrochemistry of Fc/Fc+. One of the most interesting properties of Fc and its derivatives


Figure 6. Femtosecond transient absorption spectra of 4 in CH2Cl2 upon excitation at 530 nm at different delay times.

is their electrochemical reversibility, which reflects a necessary property of a molecular switch.41 Indeed, when one electron is removed from the electron donor Fc, radical cation thus formed presents accepting properties. Consequently, it was expected that the chemical or electrochemical oxidation of Fc (which could be achieved in triads 1-3 according to redox potentials of electroactive units) would hinder the PET from Fc to PDI units. Therefore, the fluorescence emission of the PDI moiety in triads should be restored.42

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Figure 7. Fluorescence spectra of triad 2 (4.3 × 10-5 M) in THF in the presence of different amounts of Fe(ClO4)3. Inset: Fluorescence intensity at 533 nm vs the molar equivalent of Fe(ClO4)3.

Figure 9. Stepwise oxidation and reduction cycles carried out in CH2Cl2 with (a) triads 1, (b) 2, and (c) 3 (3.2 × 10-4 M) by chronoamperometric analysis, following the changes by the evolution of the fluorescence intensity at 533 nm (λex ) 490 nm).

Figure 8. (a) Fluorescence spectra of triad 2 in CH2Cl2 (3.2 × 10-4 M) containing n-Bu4NPF6 (0.1 M) after an oxidation potential of +0.37 V (vs Fc/Fc+) was applied. (b) Fluorescence spectra of the solution of triad 2 that had been oxidized electrochemically for 400 s after a reduction potential of -0.45 V (vs Fc/Fc+) was applied. Excitation wavelength was 490 nm.

To test the possibility of 2 to act as a fluorescence switch, the chemical oxidation of the Fc unit was performed using Fe(ClO4)3 as an oxidant. As shown in Figure 7, the emission intensity of 2 was enhanced dramatically upon the addition of Fe(ClO4)3 to the solution of triad 2, and the fluorescence intensity grew with an increasing amount of Fe(ClO4)3. This result demonstrates the possibility of establishing a new redox fluorescence switch based on triad 2, as the Fc f Fc•+ transformation can be operated reversibly.

Electrochemical oxidation of triad 2 was performed by applying an oxidation potential of +0.37 V (vs Fc/Fc+) to the solution of 2 in CH2Cl2 (3.2 × 10-4 M) containing n-Bu4NPF6 (0.1 M) as the supporting electrolyte. As shown in Figure 8a, the fluorescence intensity of 2 was gradually enhanced with the increasing time of electrochemical oxidation. Afte the oxidation potential was applied for 400 s, no obvious changes were observed in the emission spectrum (If). Interestingly, a subsequent application of a reduction potential of -0.45 V (vs Fc/ Fc+) to the oxidized solution resulted in the decrease of fluorescence, and the initial low fluorescence intensity (If0) of the solution (before applying an oxidation potential) was almost completely recovered after the reduction potential was applied for 600 s (Figure 8b). Oxidation of triad 2 and its subsequent reduction were carried out for several cycles without any fatigue, as shown in Figure 9b. The fluorescence enhancement or contrast ratio (If/If0) was about 2.1. This fluorescence change can be understood by reference to the cyclic voltammogram of 2 (Figure 1b), where the radical

Effect of D-A Spacer on Fluorescence Switch cation of the Fc unit was generated during the oxidation since the oxidation potential (+ 0.37 V vs Fc/Fc+) applied was higher than that of the oxidation potential of the Fc unit in triad 2. When a reduction potential was explored, the radical cation of the Fc unit would be reduced to the neutral state. Thus, the PET from the Fc subunit to the PDI unit can be modulated off and on sequentially, because of the different electron-donating ability of the neutral Fc unit and the Fc radical cation. As a result, a new reversible redox fluorescence switch can be constructed on the basis of triad 2 by taking advantage of the peculiar features of the Fc-type donor (i.e., the reversible modulation of the electron-donating abilities of the Fc unit and the PDI as a fluorescent readout unit). A similar result was also observed with triad 3 by electrochemical redox (Figure S2 in the Supporting Information). Applying a positive potential of 0.40 V (vs Fc/Fc+) led to a fast oxidation of the Fc moiety, and If increased gradually to a stable value during 400 s. A subsequent application of a reduction potential of -0.60 V (vs Fc/Fc+) to the solution resulted in the recovery of fluorescence of its neutral state (Figure S3 in the Supporting Information). The fluorescence was almost recovered after the reduction potential was applied for 400 s, and the If/If0 was about 1.7 (Figure 9c). We previously reported triad 1 suitable for the fluorescence switch that the fluorescence intensity of the solution of 1 can be reversibly modulated by the electrochemical redox.43 As shown in Figure 9a, the If/If0 is about 2.8 for triad 1. Conclusions In summary, we experimentally and theoretically investigated the distance dependence of PET in a series of Fc-PDI-based D-s-A systems. The difference in the electron-transfer rate observed may be explained by the difference in the length of the spacer. By measuring the fluorescence of 1-3 before and after oxidation, we found that the fluorescence intensity of the solution of all triads 1-3 can be reversibly modulated by the electrochemical oxidation and reduction sequentially. More importantly, the value of contrast ratio of the fluorescence (If/ If0) decreases with increasing distance between the donor and acceptor. For instance, If/If0 in triad 3 with the long spacer is about two times lower than that of triad 1 with the shorter methylene linker. These results suggest that the distance between the electron donor Fc and acceptor PDI units is an efficient mediator of contrast ratio of the fluorescence. Another useful feature of triads 1-3 is that the background fluorescence increased proportionally with the spacer length. The long spacer between Fc and fluorophore exhibits the enhanced background fluorescence. For applications, a lower background and higher contrast ratio of the fluorescence may be more beneficial. In this case, a shorter spacer, such as in triad 1, should be used preferably. These results are consistent with the idea that a PET process controls the fluorescence properties of the molecular switch. This, in turn, provides a way to fine-tune properties of the PET-based fluorescence switches, which may be desirable for certain applications predictably. Acknowledgment. We acknowledge the National Natural Science Foundation of China (Nos. 50221201, 90301010, 50502033), the Chinese Academy of Sciences, and the National ResearchFundforFundamentalKeyProjectNo.973(2006CB806200). Supporting Information Available: Synthesis and characterization of triads 1-3, the fluorescence quantum yields (ΦF), and electrochemical redox property of triad 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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