External Electric Field Effects on Absorption and Fluorescence Spectra

Superatom Compounds under Oriented External Electric Fields: Simultaneously Enhanced Bond ... Oriented electric fields as future smart reagents in che...
0 downloads 0 Views 179KB Size
Download PDF
20354

J. Phys. Chem. B 2006, 110, 20354-20361

External Electric Field Effects on Absorption and Fluorescence Spectra of a Fullerene Derivative and Its Mixture with Zinc-Tetraphenylporphyrin Doped in a PMMA Film Md. Wahadoszamen,† Takakazu Nakabayashi,† Soonchul Kang,‡ Hiroshi Imahori,*,‡,§ and Nobuhiro Ohta*,† Research Institute for Electronic Science (RIES), Hokkaido UniVersity, Sapporo 060-0812, Japan, Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ReceiVed: June 9, 2006; In Final Form: July 28, 2006

Electroabsorption and electrofluorescence spectra of a fullerene derivative, C60(C18)2, and its mixture with zinc-tetraphenylporphyrin (ZnTPP) have been measured by using electric field modulation spectroscopy. The change in dipole moment is significant in the electroabsorption spectra both of C60(C18)2 and of a complex composed of C60(C18)2 and ZnTPP, indicating that the excited states both of C60(C18)2 and of a complex between C60(C18)2 and ZnTPP have a large charge-transfer character. The fluorescence quantum yield of C60(C18)2 decreases in the presence of an electric field, which probably arises from the field-induced acceleration of the intramolecular nonradiative process of C60(C18)2 in the fluorescent state. In a mixture between ZnTPP and C60(C18)2, electrofluorescence spectra show the field-induced enhancement for the fluorescence of ZnTPP and the field-induced de-enhancement for the fluorescence both of C60(C18)2 and of the complex between ZnTPP and C60(C18)2. A theoretical analysis clearly shows that the field-induced enhancement of the ZnTPP fluorescence in a mixture results from the field-induced deceleration of the rate of the electron transfer from the excited ZnTPP to C60(C18)2. The standard free energy gap for the photoinduced electron-transfer process is estimated based on the theoretical simulation of the field-dependent fluorescence intensity.

1. Introduction Electron transfer between a redox pair has been extensively studied over the past three decades and still remains a major subject of growing interest in chemical physics. In recent years, a redox pair of fullerene and porphyrin has gained much attention among various types of redox pairs.1-13 Fullerene and porphyrin have strong electron-accepting14 and electron-donating15 ability, respectively, and their large π surfaces allow the release and uptake of an electron with minimal structural perturbation.1,9,10 Thus, the reorganization energy toward the photoinduced electron transfer is very small, which expedites the productive forward electron transfer and diminishes the unfavorable back electron transfer. These compounds also have strong and distinguishable absorption bands in the UV and visible region, harvesting the UV and visible light intensively.12 Fullerene and porphyrin therefore have emerged as an attractive donor-acceptor pair for building models of the photosynthetic reaction center as well as for developing supramolecular functional devices. The large π surfaces of porphyrin and fullerene are favorable to interact with each other, which results in the formation of a ground-state complex or cocrystalate.2,5-8,11 This property allows us to prepare various types of multidimensional supramolecular fullerene and porphyrin systems exhibiting photoinduced electron transfer and/or energy transfer. Fullerene compounds * Corresponding authors. E-mail: [email protected] (N.O.); [email protected] (H.I.). † Research Institute for Electronic Science (RIES), Hokkaido University. ‡ Graduate School of Engineering, Kyoto University. § Fukui Institute for Fundamental Chemistry, Kyoto University.

become soluble in organic solvents with an addition of long methylene chains, and the higher solubility enables us to construct supramolecular systems with desirable functionalities. The fullerene derivatives with long groups are also shown to exhibit a superior performance more than fullerene itself not only in photovoltaic applications16,17 but also in biological ones.18 Fullerenes as well as their derivatives form microscopic clusters in mixed solvents and the deposition of these clusters on nanostructured SnO2 electrodes results in remarkable photoelectrochemical activities.19-21 In this case, however, the incident photon-to-current efficiencies decrease with the addition of substituents.21 Thus, effects of substituents on physical properties of fullerenes remain unclear despite their importance in design of the supramolecular systems. For optimizing physical properties of supramolecular systems for specific applications, detailed knowledge on their electronic properties is essential. External electric field effects on optical spectra have been extensively applied in molecular spectroscopy for examining electronic properties and photoexcitation dynamics of molecules.22,23 The so-called electroabsorption and electrofluorescence spectra (plots of the electric-field-induced change in absorption intensity and fluorescence intensity as a function of wavelength, respectively) provide unique information on the differences in electric dipole moment and molecular polarizability between the ground state and the excited state. Measurements of these spectra are also very useful to clarify the mechanism of molecular dynamics following photoexcitation. These spectra are especially powerful for studying electrontransfer dynamics because of its high sensitivity to an electric field.24,25

10.1021/jp0635967 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006

Fullerene Derivative and Its Mixture with ZnTPP

Figure 1. Chemical structures of C60(C18)2 and ZnTPP.

In previous studies, we have measured the electroabsorption and electrofluorescence spectra of tetraphenylporphyrin (both free-base and its zinc analog), fullerene, and their mixtures in a poly(methyl methacrylate) (PMMA) film.26-28 We have evaluated the magnitudes of the changes in dipole moment and in molecular polarizability following photoexcitation and discussed the field-induced change in photoexcitation dynamics. The electroabsorption spectra of the fullerenes also indicate that the inversion center does not exist in C60 doped in a PMMA film in the ground state and/or in the excited state.26 Because of the very weak fluorescence intensity, however, the electrofluorescence spectrum of fullerene could not be observed within the experimental accuracy, and the field-induced change in photoexcitation dynamics of fullerene remains unresolved. In the present study, a derivative of C60 having long methylene chains has been prepared,21 and the electroabsorption and electrofluorescence spectra of the mixture between the C60 derivative and zinc-tetraphenylporphyrin doped in a PMMA film have been measured. The fluorescence of zinc-tetraphenylporphyrin is found to be enhanced by an electric field in a mixture, which arises from the field-induced decrease in the rate of the electron transfer from the excited zinc-tetraphenylporphyrin to the C60 derivative. We have theoretically analyzed the observed field-induced change in the electron-transfer rate by using the theoretical model proposed by Tachiya et al.29 The electroabsorption and electrofluorescence spectra of the C60 derivative and its complex with zinc-tetraphenylporphyrin have been also analyzed. 2. Experimental Section A fullerene derivative with long methylene chains was prepared according to the previously reported method.21 Zinctetraphenylporphyrin was purchased from Kanto Kagaku Kogyo and used without further purification. The chemical structures of the prepared C60 derivative and zinc-tetraphenylporphyrin are shown in Figure 1. Hereafter, the C60 derivative and zinctetraphenylporphyrin are denoted by C60(C18)2 and ZnTPP, respectively. PMMA (MW ) 120 000) was obtained from Aldrich and purified by a precipitation with a mixture of benzene and methanol and by an extraction with hot methanol. A certain amount of toluene solution of PMMA containing solutes was cast on an indium-tin-oxide (ITO) coated quartz substrate by a spin-coating method. Then the polymer film was dried in a vacuum, and a semitransparent aluminum (Al) film was deposited on the dried polymer film by a vacuum vapor deposition technique. The ITO and Al films were used as electrodes. The thickness of the film was typically 0.4 µm. The concentration of the solute was evaluated in the ratio to the monomer unit of PMMA. In the present study, the concentration of C60(C18)2 was 3 mol % and that of ZnTPP was 0.5 mol %.

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20355 A mixture was prepared with ZnTPP at 0.5 mol % and C60(C18)2 at 3 mol %. All the optical spectra were measured at room temperature under vacuum conditions. Electric-field-induced changes in absorption and emission spectra were measured using the same apparatus reported elsewhere.27,28,30 A sinusoidal ac voltage with a modulation frequency of 40 Hz was applied to a sample polymer, and the field-induced change in absorption intensity or emission intensity was detected with a lock-in amplifier at the second harmonic of the modulation frequency. A dc component of the transmitted light intensity or the fluorescence intensity was simultaneously observed. Electroabsorption and electrofluorescence spectra were obtained by plotting the fieldinduced change in absorption intensity and in fluorescence intensity as a function of wavenumber, respectively. Applied field strength, which is denoted hereafter by F, was estimated from the applied voltage divided by the polymer thickness. Hereafter, electroabsorption and electrofluorescence spectra are abbreviated as E-A and E-F spectra, respectively. Fluorescence decay measurements were carried out using a single-photon counting system.25 A second harmonic of the output from a mode-locked Ti:sapphire laser (Spectra Physics, Tsunami, pulse duration 80 fs) was used for excitation. Fluorescence from the sample was dispersed by a monochromator (Nikon, G-250) and then detected by a microchannelplate photomultiplier (Hamamatsu, R3809U-52). The instrumental response function had a pulse width of ∼ 60 ps (fwhm). 3. Theoretical Background 3.1. Electric Field Effects on Absorption and Fluorescence Spectra. When an electric field is applied to molecules, each energy level is shifted, which is known as the so-called Stark shift. The magnitude of each shift depends on electric dipole moment (µ) and molecular polarizability (R) of the state concerned. When the magnitude of µ or R in the excited electronic state is different from the one in the ground state, the absorption spectra as well as the fluorescence spectra are shifted because the magnitudes of the level shift in both states are different from each other. For an isotropic and immobilized sample, the presence of F will broaden an isolated transition due to the change in electric dipole moment following optical absorption, giving rise to the E-A spectrum, the shape of which is the second derivative of the absorption spectrum. If the change in molecular polarizability is significant, the shape of the E-A spectrum is the first derivative of the absorption spectrum. If the transition moment is affected by F, the shape of the E-A spectrum is the same as that of the absorption spectrum. In the present study, solute molecules can be regarded as randomly distributed in a PMMA film. On the assumption that the original isotropic distribution in a PMMA film is maintained in the presence of F, the E-A spectrum (∆A(ν)) is given by the following equation:31

∆A(ν) )

[

]

d{A(ν)/ν} d2{A(ν)/ν} (1) + C′ν dν dν2

(fF)2 A′A(ν) + B′ν

where F is the applied electric field and f is the internal field factor. The coefficient A′ depends on the transition moment polarizability and hyperpolarizability, and B′ and C′ are given by the following forms:

20356 J. Phys. Chem. B, Vol. 110, No. 41, 2006

B′ )

Wahadoszamen et al.

∆R j /2 + (∆Rm - ∆R j )(3 cos2 χ - 1)/10 hc

(2)

[5 + (3 cos2 ξ - 1)(3 cos2 χ - 1)] C′ ) |∆µ|2 30h2c2

(3)

where ∆µ and ∆R are the differences in electric dipole moment and molecular polarizability, respectively, between the ground state (g) and the excited state (e), i.e., ∆µ ) µe - µg and ∆R j denotes the trace of ∆R, i.e., ∆R j ) (1/3)Tr) Re - Rg. ∆R (∆R). ∆Rm is the diagonal component of ∆R with respect to the direction of the transition dipole moment, χ is the angle between the direction of F and the electric vector of the light, and ξ is the angle between the direction of ∆µ and the transition dipole moment. The internal field factor f is estimated to be 1.87 from the Lorentz field correction.32 At a magic angle condition of χ ) 54.7°, the coefficients B′ and C′ are reduced, respectively, as

B′ )

∆R j 2hc

(4)

C′ )

|∆µ|2 6h2c2

(5)

From eqs 4 and 5, the values of |∆µ| and ∆R j can be obtained from the first and second derivative components of the E-A spectrum, respectively. The E-F spectrum (∆IF(ν)) is also given by the following equation:

[

k(r, F) )

4π2 h

J2(r)

x4πλ(r)kBT

{

}

(∆G(r) + λ(r) - µ‚F)2 4λ(r)kBT (7)

exp -

where r and µ are the distance between donor and acceptor and the dipole moment of a generated radical-ion pair, respectively. The transfer integral, J(r), and the reorganization energy, λ(r), are given as follows:

J2(r) ) J02 exp{-β(r - d - a)} λ(r) )

(

)(

(8)

)

1 1 1 2 e2 1 + - + λi 2 op s a d r

(9)

where J0 and β are the constants appropriate for each donoracceptor pair, and d and a are the radii of donor and acceptor, respectively. op and s are the optical and static dielectric constants of PMMA, respectively, and λi stands for the vibrational reorganization energy. ∆G(r) is the energy gap at zero field:

∆G(r) ) ∆G0 -

e2 sr

(10)

where ∆G0 is the standard Gibbs free energy gap in the reaction. The average of the reaction rate in eq 7 is taken over the full distance r and over the full orientation of the dipole moment of the radical-ion pair with respect to F. Then, the survival probability P(t, F) that the excited donor will survive at time t in the presence of F is given by

P(t, F) ) exp(-tk0) exp{-2πc

∞ π r2 dr∫0 sin θ d θ[1 - e-tk(r,F)]} ∫a+d

(11)

∆IF(ν) ) (fF)2 AIF(ν) + Bν3

]

d {IF(ν)/ν } d{IF(ν)/ν } + Cν3 (6) dν dν2 3

2

3

The first and second derivative components arise from ∆R and ∆µ, respectively. The zeroth derivative component, which corresponds to the field-induced change in emission quantum yield, arises from the field-induced change in radiative decay rate and/or in the nonradiative rate at the fluorescent state. Thus electric field effects on photoexcitation dynamics can be evaluated from the zeroth component of the E-F spectrum. 3.2. Electric Field Effects on Fluorescence Intensity Due to Photoinduced Electron Transfer. When the electron transfer takes place between donor and acceptor, the radical-ion pair with a large dipole moment is generated, and its energy level is shifted in the presence of F. As a result, the rate of the electron transfer is expected to be significantly influenced by F. In the polymer film used in the present study, donor and acceptor molecules are randomly distributed, and electron transfer occurs in donor-acceptor pairs having different distance and different orientation. Tachiya et al.29 have reported the expression of the electric field effects on the photoinduced electron transfer rate when donor and acceptor molecules are randomly distributed within the rigid matrix. In their model, the rate constant for electron transfer from an excited donor in the presence of F, k(r, F), is represented by29

where θ is the angle between the direction of F and the dipole moment of the radical-ion pair and k0 is the decay rate constant of the excited donor in the absence of acceptor. The fluorescence decay of the donor in a mixture in the absence of F can be simulated by using P(t, 0). In the presence of F, the truncation of the expansion of the exponential function in eq 11 to the second-order term in F yields

{

P(t, F) ) P(t, 0) 1 -

c|F|2 6

}

∞ M(t, r) e-tk(r,0)r2 dr ∫a+d

(12)

M(t, r) ) πk(r, 0)t|µ|2 {[∆G(r) + λ(r)]2[1 - tk(r, 0)] 2 [kBTλ(r)] 2λ(r)kBT} (13) The second-order term in F is known to be sufficient to describe electric field effects on fluorescence.23-30 The steady-state fluorescence intensity IF(F) in the presence of F is obtained by integrating the survival probability over time t,

IF(F) ) kf

∫0∞ P(t, F) dt

(14)

where kf is the radiative rate constant. The field-induced change in fluorescence intensity ∆IF(F) relative to the fluorescence intensity without F, IF(0), is given by the following equation:

Fullerene Derivative and Its Mixture with ZnTPP

∆IF(F) IF(0)

) |F|2η(∆G0)

c η(∆G0) ) 6

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20357

(15)

∞ M(t,r) exp[-tk(r,0)]r2 dr dt ∫0∞ P(t, 0) ∫a+d ∫0∞ P(t, 0) dt

(16)

With these equations, the values of ∆G0, J0, and β for the electron transfer can be obtained by fitting the fluorescence decay and the field-induced change in fluorescence intensity of donor in the mixture. 4. Results and Discussion 4.1. Electric Field Effects on Absorption and Fluorescence Spectra of the Mixture of ZnTPP and C60(C18)2. Parts a and d of Figure 2 show the absorption and E-A spectra of a mixture of ZnTPP at 0.5 mol % and C60(C18)2 at 3.0 mol % doped in a PMMA film. These spectra were obtained as difference spectra by subtracting the spectrum of C60(C18)2 from the one of the mixture, with a special attention to the Soret absorption band of ZnTPP. The absorption and E-A spectra of ZnTPP are also shown by a dotted line. The E-A spectra were measured with a field strength of 0.75 MV cm-1 at the magic angle condition for χ () 54.7°). The Soret absorption band undergoes red-shift and broadening in the mixture, indicating the formation of the complex between ZnTPP and C60(C18)2 in a PMMA film2,33,34 (see Figure S1 for the absorption and E-A spectra of ZnTPP and C60(C18)2 in Supporting Information). The absorption and E-A spectra of the resultant complex can be evaluated by subtracting the absorption spectra of the unperturbed C60(C18)2 and ZnTPP from that of the mixture. As mentioned above, the difference spectra in Figure 2a and d were obtained by subtracting the C60(C18)2 spectrum from the mixture one. These difference spectra are regarded as a superposition of the spectra of ZnTPP and the complex. Actually, as shown in Figure 2b, the difference spectrum is well fitted by the ZnTPP absorption band and two Gaussian curves of G1 and G2. The sum of the two Gaussian curves, i.e., G1 + G2, can be regarded as the absorption spectrum of the complex. The E-A spectrum of the complex was also evaluated by subtracting the contribution of the E-A spectrum of ZnTPP from the difference E-A spectrum of the mixture. The result is shown in Figure 2e. Figure 2c shows the absorption spectrum of the complex and its first and second derivative spectra. By using the first and second derivatives of the complex absorption spectrum, the E-A spectrum of the complex is well reproduced, as shown in Figure 2e. The second derivative component is dominant in the E-A spectrum of the complex, indicating that the complex exhibits a charge-separated character between C60(C18)2 and ZnTPP following the absorption in the Soret band region.33,34 The magnitude of |∆µ| is evaluated to be 2.0 D. The angle (ξ) between ∆µ and the direction of the transition dipole moment can be evaluated from the angle (χ) dependence of E-A spectrum (see eq 3). The magnitude of ξ is related to the molecular structure of the sample measured. In the present study, the ξ value of the complex was determined to be 34° by comparing the E-A spectra of the complex with the magic angle and with 90.0° for χ (see Figure 2S in Supporting Information). The obtained angle is almost half as large as that obtained for the complex between ZnTPP and C60 (∼65°).27 The direction of the transition dipole of the Soret band is parallel to the porphyrin ring and ∆µ arises from the charge transfer from ZnTPP to fullerene. Thus, the fullerene molecule in the complex

Figure 2. (a) Difference absorption spectrum (solid line) obtained by subtracting the absorption spectrum of C60(C18)2 from that of the mixture, and absorption spectrum (dotted line) of ZnTPP. (b) Difference absorption spectrum (solid line) together with the decomposed ZnTPP absorption spectrum and two Gaussian profiles. (c) Absorption spectrum of the complex (G1 + G2) and its first and second derivative spectra. (d) Difference E-A spectrum (solid line) obtained by subtracting the E-A spectrum of C60(C18)2 from that of the mixture, and E-A spectrum of ZnTPP (dotted line). (e) E-A spectrum (shaded line) of the complex together the simulated one (dotted line). All the E-A spectra were measured with a field strength of 0.75 MV cm-1 at the magic angle condition.

Figure 3. Schematic illustration of the relationship between the angle ξ and the structure of the complex. ξ is defined as the angle between the direction of ∆µ and the transition dipole moment. ZnTPP is represented by a tetragon for simplicity.

should be located on the porphyrin plane when the ξ value is around 90°. The decrease in magnitude of ξ in the present study suggests that the position of the C60(C18)2 molecule in the complex slightly shifts from the porphyrin plane because of the steric hindrance due to the long methylene groups, as shown in Figure 3. Figure 4a shows the fluorescence spectrum of a mixture of ZnTPP at 0.5 mol % and C60(C18)2 at 3.0 mol % in a PMMA film. The fluorescence spectrum of the mixture is characterized by a structured fluorescence of ZnTPP together with a broad fluorescence in the lower energy region. Inset of Figure 4a shows the difference fluorescence spectrum obtained by subtracting the fluorescence spectrum of ZnTPP from that of the

20358 J. Phys. Chem. B, Vol. 110, No. 41, 2006

Wahadoszamen et al.

Figure 5. Fluorescence decays of a mixture between ZnTPP at 0.5 mol % and C60(C18)2 at 3.0 mol % (solid line) and ZnTPP at 0.5 mol % (dotted line) doped in a PMMA film. Excitation and emission wavelengths are 425 and 600 nm, respectively. The decay profiles simulated by using eq 11 are also shown by open circles for the mixture and open triangles for ZnTPP. Figure 4. (a) Fluorescence spectra of a mixture between ZnTPP at 0.5 mol % and C60(C18)2 at 3.0 mol % (solid line), and ZnTPP at 0.5 mol % (dotted line) doped in a PMMA film. The inset of (a) shows the difference fluorescence spectrum obtained by a subtraction of the fluorescence spectrum of ZnTPP from that of the mixture. Decomposed fluorescence spectra of C60(C18)2 (dashed line) and the complex (dotdashed line) are also shown in the inset of (a). (b) E-F spectrum (shaded line) of a mixture between ZnTPP at 0.5 mol % and C60(C18)2 at 3.0 mol % together with the simulated E-F spectra in the ZnTPP fluorescence region (dotted line), in the C60(C18)2 fluorescence region (dashed line) and in the complex fluorescence region (dot-dashed line). Applied field strength was 1.0 MV cm-1.

mixture. The difference fluorescence spectrum is reproduced by a linear combination of the fluorescence spectrum of C60(C18)2 and a broad fluorescence spectrum, as shown in inset of Figure 4a. The broad fluorescence spectrum is considered to arise from the complex.28,33,34 The fluorescence excitation spectrum of the mixture measured near the peak position of the fluorescence spectrum of the complex is very similar in shape to the absorption spectrum of the mixture, suggesting that the fluorescence of the complex arises not only from the direct excitation of the complex but also from the excitation energy transfer following the excitation of ZnTPP. It is also found from the fluorescence excitation spectrum that the C60(C18)2 fluorescence of the mixture arises from the excitation both of C60(C18)2 and of ZnTPP. The E-F spectrum of the mixture is shown in Figure 4b. Applied field strength was 1.0 MV cm-1 and the excitation wavelength was 425 nm, where the field-induced change in absorption intensity was negligible. The E-F spectrum in the ZnTPP and the complex fluorescence region is fitted by a linear combination of the fluorescence spectrum and its first derivative spectrum, while the E-F spectrum in the C60(C18)2 fluorescence region is fitted by a linear combination of the fluorescence spectrum and its first and second derivative spectra. The E-F spectrum of the mixture exhibits the field-induced enhancement for the ZnTPP fluorescence and the field-induced de-enhancement both for the C60(C18)2 and for the complex fluorescence. The magnitude of the field-induced enhancement of the ZnTPP fluorescence is 0.2% at 1.0 MV cm-1 and those of the fieldinduced quenching of the C60(C18)2 and the complex fluorescence are 2.5 and 2.7% at 1.0 MV cm-1, respectively. Note that the magnitude of the field effect is proportional to the square of the applied field strength. The observed field-induced change in fluorescence intensity can be ascribed to the field-induced change in dynamics following photoexcitation. A similar fieldinduced enhancement of the ZnTPP fluorescence was observed for a mixture of ZnTPP and C60 28 and for a linked compound of ZnTPP and C60.12 It is known that the electron transfer from ZnTPP and C60 occurs after photoexcitation of ZnTPP in the

mixture of the ZnTPP + C60 system, which strongly affects the fluorescence intensity of ZnTPP. Thus, it can be concluded that the field-induced increase in fluorescence intensity of ZnTPP arises from the deceleration of the electron transfer from the excited ZnTPP to C60(C18)2 in the presence of F. The values of ∆G0, J0, and β for the electron transfer from the excited ZnTPP to C60(C18)2 were estimated by fitting the fluorescence decay of ZnTPP by using P(t, 0) of eq 11 and by fitting the field-induced change in the ZnTPP fluorescence intensity by using eqs 15 and 16. Figure 5 shows the fluorescence decay of the mixture between ZnTPP and C60(C18)2, together with that of ZnTPP doped in a PMMA film at 0.5 mol %. It is clearly seen that the fluorescence lifetime of ZnTPP decreases in the presence of C60(C18)2, indicating the existence of the electron transfer from the excited state of ZnTPP to C60(C18)2. Both fluorescence decays could be fitted by a biexponential decay, i.e., ∑i Ai exp(-t/τi), where Ai and τi denote the preexponential factor and lifetime of component i () 1 and 2), respectively. The obtained lifetime and the preexponential factor of the mixture are as follows: τ1 ) 0.60 ns, τ2 ) 1.49 ns and A1 ) 0.78, A2 ) 0.22. The corresponding values of ZnTPP are determined as follows: τ1 ) 1.43 ns, τ2 ) 2.65 ns and A1 ) 0.82, A2 ) 0.18. Then, the average fluorescence lifetime (τjf) in a polymer film, defined by ∑i Aiτi/∑i Ai, is estimated to be 0.80 ns for the mixture and 1.65 ns for ZnTPP. We used the decay rate constant of k0 ) 6.06 × 108 s-1 from the average fluorescence lifetime of ZnTPP in the absence of acceptor. The radii of donor and acceptor molecules are assumed to be 5.0 and 4.4 Å, respectively. λi is connected with the average skeletal vibrations of donor and acceptor and is assumed to be 0.3 eV.35 For the physical properties of PMMA, the refractive index n is 1.489, the static dielectric constant s is 3.6, and the optical dielectric constant is given by op ) 1.05 n2. From these values, we simulated the fluorescence decay of ZnTPP by using eq 11 and the field-induced enhancement of the ZnTPP fluorescence in the mixture by using eqs 15 and 16. Open circles in Figure 5 show the simulated curve for the fluorescence decay of ZnTPP in the mixture. We adopted the nonlinear least-squares method for the simulation. The observed decay is satisfactorily reproduced by the simulation, as shown in Figure 5, and the parameters of the electron transfer from the excited state of ZnTPP to C60(C18)2 were determined as follows: ∆G0 ) -0.534 eV, J0 ) 5.29 × 10-4 eV, and β ) 5.92 × 109 m-1. Figure 6 shows the field-strength dependence of ∆IF(F)/IF(0) calculated with eqs 15 and 16 for different values of the free energy change, ∆G0, ranging from -0.70 to -0.40 eV. All the curves show the quadratic dependence of the fieldinduced change on the field strength, but the magnitude of the

Fullerene Derivative and Its Mixture with ZnTPP

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20359

Figure 6. Plots of the field-induced change in fluorescence intensity ∆IF(F)/IF(0) against field strength calculated with eqs 15 and 16 for different values of the free energy change ∆G0.

field-induced change strongly depends on ∆G0. In the present study, the ∆IF(F)/IF(0) value of the ZnTPP fluorescence in the mixture is evaluated to be 2.0 × 10-2 at 1.0 MV cm-1 (see Figure 4). The magnitude of the observed change is very small by comparing the simulated curves in Figure 6 and is reproduced when the ∆G0 value is in the range of -0.52 to -0.54 eV. This result is consistent with the decay analysis mentioned above. The magnitude of the field-induced enhancement of the ZnTPP fluorescence in the mixture of ZnTPP and C60(C18)2 is about three times smaller than that in the ZnTPP and C60 mixture.28 This indicates that the magnitude of the field-induced decrease in electron transfer rate is largely reduced in the ZnTPP and C60(C18)2 mixture. As shown in Figure 6, the difference in the magnitude of the field-induced change may come from the difference in the value of the free energy change ∆G0: the ZnTPP and C60 mixture has a smaller magnitude of ∆G0 than the ZnTPP and C60(C18)2 one. The fluorescence of C60(C18)2 in the mixture decreases in the presence of F. The C60 fluorescence of the mixture arises not only from the direct photoexcitation of C60(C18)2 but also from the energy transfer following photoexcitation of ZnTPP. As will be mentioned later, the field-induced quenching of the C60(C18)2 fluorescence is also observed without ZnTPP; however, the magnitude of the field-induced quenching is much smaller than that of the mixture. It is therefore suggested that the field-induced quenching of the C60(C18)2 fluorescence of the mixture mainly comes from the field-induced decrease in formation yield of the fluorescent state of C60(C18)2 following the photoexcitation of ZnTPP. The field-induced quenching of the complex fluorescence suggests that the nonradiative process at the emitting state of the complex is accelerated by F. The quenching mechanism of the complex fluorescence can be attributed both to the fieldassisted dissociation of the radical-ion pair produced between ZnTPP and C60(C18)2 and to the field-induced acceleration of back electron transfer. Note that the fluorescent state of the complex is regarded as a kind of charge-separated state, as shown in the E-A spectra of the complex. The quenching of the complex fluorescence in the ZnTPP and C60(C18)2 mixture is smaller in magnitude than that in the ZnTPP and C60 mixture.28 The magnitude of the decrease in the rate of the nonradiative process is reduced in the complex of ZnTPP and C60(C18)2. 4.2. Electric Field Effects on Absorption and Fluorescence Spectra of C60(C18)2. Parts a and e of Figure 7 show the absorption and E-A spectra of C60(C18)2 doped in a PMMA film, respectively. The E-A spectrum was measured with a field strength of 0.75 MV cm-1 at the magic angle condition for χ. The absorption spectrum of C60 has two distinct bands with peaks at 258 and 331 nm.26,36 The absorption spectrum of C60-

Figure 7. (a) Absorption spectrum (shaded line) of C60(C18)2 at 3 mol % doped in a PMMA film together with the simulated absorption spectrum and the decomposed Gaussian profiles. (b-d) Absorption spectra (solid line), first derivative spectra (dotted line), and second derivative spectra (dashed line) of G2, G3, and G4, respectively. (e) E-A spectrum (shaded line) of C60(C18)2 at 3 mol % doped in a PMMA film, together with the simulated E-A spectrum (dotted line). The E-A spectrum was measured with a field strength of 0.75 MV cm-1 at a magic angle condition for χ.

(C18)2 also exhibits two absorption bands with peaks at 271 nm (36 900 cm-1) and 331 nm (30 211 cm-1). A broad tail in the 400-600 nm region is only observed in the C60(C18)2 absorption spectrum (see Figure S3 for the absorption and E-A spectra of C60 in Supporting Information). To perform the detailed analysis of the E-A spectrum of C60(C18)2, we first tried to reproduce the E-A spectrum with a linear combination of the zeroth, first, and second derivatives of the total absorption spectrum according to eq 1. However, a large disagreement between the simulated and the observed E-A spectra was obtained, suggesting that the magnitudes of the fieldinduced change in absorption bands are different from each other. Therefore, we have deconvoluted the absorption spectrum of C60(C18)2 into several components. The fitted result for the absorption spectrum of C60(C18)2 at 3.0 mol % is shown in Figure 7a. The fit to the absorption spectrum is based on a model composed of four Gaussian curves of G1, G2, G3, and G4. The G1 component corresponds to a part of the absorption band around 271 nm, the G2 component to the absorption band around 331 nm, and the combination of the G3 and G4 components to the broad absorption feature in the 400-600 nm region. Figure 7b-d shows the zeroth, first, and second derivatives of the G2, G3, and G4 components, respectively, and the E-A spectrum simulated by these derivatives is shown in Figure 7e. The E-A spectrum is reproduced largely by a linear combination of the first and second derivatives of G3 and G4 and the second derivative of G2.

20360 J. Phys. Chem. B, Vol. 110, No. 41, 2006

Wahadoszamen et al.

TABLE 1: Magnitude of ∆r j and |∆µ| between the Franck-Condon Excited State and the Ground State of C60(C18)2 Doped in a PMMA Film absorption band

∆R j (Å3)

|∆µ| (D)

G2 G3 G4

0.0 -119.3 74.6

8.4 11.5 13.3

in the presence of F, indicating that the fluorescence quantum yield is reduced by F. The observed E-F spectrum can be well reproduced by a linear combination of the fluorescence spectrum and its first and second derivative spectra. The magnitudes of ∆R j and |∆µ| evaluated from the E-F spectrum are 10.2 Å3 and 1.9 D, respectively. The |∆µ| value obtained from the E-F spectrum is much smaller than those obtained from the E-A spectrum, suggesting that the electronic structure of the relaxed fluorescent state is significantly different from those of the Franck-Condon excited states. The observed field-induced decrease in fluorescence quantum yield indicates that the intramolecular nonradiative process from the fluorescent state is accelerated by F. The magnitude of the field-induced quenching of the C60(C18)2 fluorescence is evaluated to be 0.5% at 1.0 MV cm-1. The dominant nonradiative process from fluorescent states of fullerenes is known to be the intersystem crossing to a nearly resonant triplet state.39 The energy separation between the fluorescence state and the triplet state is shifted by F because of the difference in µ and R of both states. Such a change in energy gap may induce the field-induced increase in the rate of the intersystem crossing. 5. Conclusion

Figure 8. Fluorescence spectrum and its first and second derivative spectra and E-F spectrum of C60(C18)2 at 0.5 mol % doped in a PMMA film (from top to bottom). The simulated E-F spectrum is also shown in (d) by a dotted line. Applied field strength was 1.0 MV cm-1.

The magnitudes of |∆µ| and ∆R j evaluated from eqs 4 and 5 are shown in Table 1. The magnitude of |∆µ| is very large for all the absorption bands of G2, G3, and G4. E-A spectra do not provide the sign of ∆µ; however, the |µ| value in the excited state should be larger than that in the ground state because the ground state of C60(C18)2 is considered to have a small |µ| value. Thus, it is concluded that the dipole moments of the excited states of G2, G3, and G4 are much larger than that of the ground state. The large enhancement of the dipole moment following the photoexcitation indicates that these bands have large chargetransfer character in the excited state. The C60 molecule also exhibits the change in dipole moment following photoexcitation to the C transition at 331 nm; however, the |µ| value of the C transition of C60 is determined to be 4.4 D,26 which is about half as large as that of C60(C18)2. This indicates that the C transition excited state of C60(C18)2 has a greater charge-transfer character than that of C60. Owing to the very weak fluorescence intensity, the E-F spectrum of C60 in a film could not be obtained in our previous study.26 However, the fluorescence intensity of C60(C18)2 is larger than that of C60, and the E-F spectrum of C60(C18)2 has been observed with a sufficient signal-to-noise ratio, as shown in Figure 8. The fluorescence spectrum of C60(C18)2 and its first and second derivative spectra are also shown in this figure. Applied field strength was 1.0 MV cm-1. The excitation wavelength was 341 nm, where the field-induced change in absorption intensity is negligible. It is noted that both the shape and the peak position of the fluorescence of C60(C18)2 are almost the same as those of the fluorescence of C60.37,38 The E-F spectrum of C60(C18)2 shows a decrease in fluorescence intensity

We have measured the E-A and E-F spectra of a mixture of C60(C18)2 and ZnTPP in a PMMA film. The E-A spectrum of the complex between C60(C18)2 and ZnTPP in the Soret band region is similar in shape to the second derivative of its absorption spectrum, indicating that the complex in the FranckCondon excited state has a charge-transfer character. The angle between ∆µ and the direction of the transition dipole moment of the complex was determined to be 34°, suggesting that the position of the C60(C18)2 molecule in the complex slightly shifts from the porphyrin plane. In the mixture, the fluorescence of ZnTPP is enhanced, and the fluorescence both of the complex and of C60(C18)2 is de-enhanced by F. The field-induced enhancement of the ZnTPP fluorescence is attributed to the fieldinduced deceleration of the electron transfer from the photoexcited ZnTPP to C60(C18)2. Fluorescence decay as well as field-induced change in fluorescence intensity of ZnTPP in the mixture was simulated by using the theoretical model proposed by Tachiya et al. As a result, the standard free energy gap for the photoinduced electron-transfer process was estimated to be -0.53 ( 0.1 eV. The field-induced de-enhancement of the C60(C18)2 fluorescence intensity in a mixture mainly comes from the field-induced decrease in the formation yield of the C60(C18)2 fluorescent state following excitation of ZnTPP. The field-induced de-enhancement of the complex fluorescence is attributed to the acceleration of the rate of the nonradiative process of the complex in the fluorescent state. E-A and E-F spectra of C60(C18)2 have been also measured. The change in dipole moment following photoexcitation is significant in C60(C18)2, indicating that the intramolecular charge transfer occurs in C60(C18)2 following photoexcitation. The fluorescence of C60(C18)2 was found to be quenched by F, which probably results from the field-induced increase in the rate of the intramolecular intersystem crossing. The present information obtained for the electronic properties of porphyrins and fullerenes and their complexes will allow us to design new photoactive molecular devices including molecular photovoltaics. Acknowledgment. We thank Dr. Toshifumi Iimori at Hokkaido University for the simulation both of the fluorescence decay and of the field-induced change in fluorescence intensity. This work has been supported by Grant-in-Aid for Scientific

Fullerene Derivative and Its Mixture with ZnTPP Research (Grant no. 15205001) and for Scientific Research on Priority Area “Molecular Nano Dynamics” from the Ministry of Education, Culture, Sports, Science, and Technology in Japan. Supporting Information Available: Absorption and E-A spectra of ZnTPP at 0.5 mol % and C60 at 2 mol % and angle dependence of absorption and E-A spectra of a mixture between ZnTPP at 0.5 mol % and C60(C18)2 at 3.0 mol %. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fukuzumi, S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000: Vol. 8, p 115. (2) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771. (3) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem. 1996, 100, 15926. (4) Armaroli, N.; Diederich, F.; Echegoyen, L.; Habicher, T.; Flamigni, L.; Marconi, G.; Nierengarten, J.-F. New J. Chem. 1999, 23, 77. (5) Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll, B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 7090. (6) Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.; Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc. 1999, 121, 10487. (7) Da Ros, T.; Prato, M.; Guldi, D.; Alessio, E.; Ruzzi, M.; Pasimeni, L. Chem. Commun. 1999, 635. (8) Armaroli, N.; Marconi, G.; Echegoyen, L.; Bourgeois, J.-P.; Diederich, F. Chem. Eur. J. 2000, 6, 1629. (9) Guldi, D. M. Chem. Commun. 2000, 321. (10) Imahori, H. Org. Biomol. Chem. 2004, 2, 1425. (11) Balbinot, D.; Atalick, S.; Guldi, D. M.; Hatzimarinaki, M.; Hirsch, A.; Jux, N. J. Phys. Chem. B 2003, 107, 13273. (12) Ohta, N.; Mikami, S.; Iwaki, Y.; Tsushima, M.; Imahori, H.; Tamaki, K.; Sakata, Y.; Fukuzumi, S. Chem. Phys. Lett. 2003, 368, 230. (13) Ohkubo, K.; Kotani, H.; Shao, J.; Ou, Z.; Kadish, K. M.; Li, G.; Pandey, R. K.; Fujitsuka, M.; Ito, O.; Imahori, H.; Fukuzumi, S. Angew. Chem., Int. Ed. 2004, 43, 853. (14) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 4364. (15) Kalyanasundaram, K. Photochemistry and Polypyridine and Porphyrin Complexes; Academic Press: London, 1992.

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20361 (16) Kraabel, B.; Hummelen, J. C.; Vacar, D.; Moses, D.; Sariciftci, N. S.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1996, 104, 4267. (17) Segura, J. L.; Go´mez, R.; Martin, N.; Luo, C.; Guldi, D. M. Chem. Commun. 2000, 701. (18) Da Ros, T.; Prato, M. Chem. Commun. 1999, 663. (19) Kamat, P. V.; Barazzouk, S.; Thomas, K. G.; Hotchandani, S. J. Phys. Chem. B 2000, 104, 4014. (20) Kamat, P. V.; Barazzouk, S.; Hotchandani, S.; Thomas, K. G. Chem. Eur. J. 2000, 6, 3914. (21) Hotta, H.; Kang, S.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Imahori, H. J. Phys. Chem. B 2005, 109, 5700. (22) Bublitz, G. U.; Boxer, S. G. Annu. ReV. Phys. Chem. 1997, 48, 213. (23) Ohta, N. Bull. Chem. Soc. Jpn. 2002, 75, 1637. (24) Ohta, N.; Koizumi, M.; Umeuchi, S.; Nishimura, Y.; Yamazaki, I. J. Phys. Chem. 1996, 100, 16466. (25) Tsushima, M.; Ushizaka, T.; Ohta, N. ReV. Sci. Instrum. 2004, 75, 479. (26) Ohta, N.; Tanaka, T.; Yamazaki, I. Res. Chem. Intermed. 2001, 27, 61 (27) Wahadoszamen, Md.; Nakabayashi, T.; Ohta, N. J. Photochem. Photobiol., A 2006, 178, 177. (28) Wahadoszamen, Md.; Nakabayashi, T.; Ohta, N. J. Chin. Chem. Soc. 2006, 53, 85. (29) Hilczer, M.; Traytak, S.; Tachiya, M. J. Chem. Phys. 2001, 115, 11249. (30) Jalviste, E.; Ohta, N. J. Chem. Phys. 2004, 121, 4730. (31) Liptay, W. In Excited States; Lim, E. C., Ed.; Academic Press: New York, 1974; p 129. (32) Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1975. (33) Tkachenko, N. V.; Guenther, C.; Imahori, H.; Tamaki, K.; Sakata, Y.; Fukuzumi, S.; Lemmetyinen, H. Chem. Phys. Lett. 2000, 326, 344. (34) Imahori, H.; Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.; Lemmetyinen, H.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105, 1750. (35) Yoshizawa, T.; Mizoguchi, M.; Iimori, T.; Nakabayashi, T.; Ohta, N. Chem. Phys. 2006, 324, 26. (36) Leach, S.; Vervloet, M.; Despre`s, A.; Bre´heret, E.; Hare, J. P.; Dennis, T. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Chem. Phys. 1992, 160, 451. (37) Kim, D.; Lee, M. J. Am. Chem. Soc. 1992, 114, 4429. (38) Sun, Y.-P.; Wang, P.; Hamilton, N. B. J. Am. Chem. Soc. 1993, 115, 6378. (39) Sassara, A.; Zerza, G.; Chergui, M.; Ciulin, V.; Ganiere, J.-D.; Deveaud, B. J. Chem. Phys. 1999, 111, 689.