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Comparative study of the bimolecular electron transfer of fullerenes (C60 /C70 ) and 9,9-disubstituted fluorenes by laser flash photolysis† Mohamed E. El-Khouly* Received 7th December 2006, Accepted 22nd January 2007 First published as an Advance Article on the web 15th February 2007 DOI: 10.1039/b617814f Photoinduced bimolecular electron transfer processes of fullerenes (C60 /C70 ) with fluorene derivatives, namely 9,9-bis(4-amino-3-methylphenyl)fluorene (BAMF), 9,9-bis(4-amino-3-fluorophenyl)fluorene (BAFF) and 9,9-bis(3-amino-4-hydroxyphenyl)fluorene (BAHF) have been comparatively studied in benzonitrile by nanosecond laser photolysis technique in the visible/near-IR regions. By the selective excitation of C60 /C70 using 532 nm laser light, it has been proved that electron transfer takes place from the ground state fluorenes to the triplet excited state of C60 /C70 . The observed rates and efficiencies of the electron transfer processes are found to be significantly associated with the substitution patterns on the 9,9-bisphenylfluorenes, and to be correlative with the free energy changes on the basis of the Rehm–Weller relation.

Introduction Photoinduced electron transfer (ET) process has long been of interest in chemistry and biology because it is a universal and fundamental phenomenon in nature.1–4 Toward constructing the molecular electronic devices, fullerenes (C60 /C70 ) are particularly appealing as electron acceptors, because of their three-dimensional structure, delocalized p-electrons within the spherical carbon framework, low reduction potential, and absorption spectra extending over most of the visible region. These unique properties make fullerenes promising candidates for the investigation of photoinduced electron transfer process by mixing with electron–donor compounds.5–11 Among the electron donors, fluorene derivatives are of particular interest. The fluorene derivatives are known to have high two-photon absorptivity, high fluorescent, and good thermal and photochemical stability. The unique chemical and physical characteristics of fluorene derivatives make them essential and useful in a wide variety of applications ranging from the plastic solar cells and LED (light electroluminescent diode) devices to photodynamic therapy.12–20 In spite of enormous electron transfer investigations of fullerenes (C60 /C70 ) with various kinds of electron donors, there have been few studies concerning the electron transfer with fluorenes.21–28 The aim of this article is to study the effect of substitution on the intermolecular electron transfer between fullerenes (C60 /C70 ) and fluorene derivatives, namely 9,9-bis(4amino-3-methylphenyl)fluorene (BAMF), 9,9-bis(4-amino-3fluorophenyl)fluorene (BAFF) and 9,9-bis(3-amino-4-hydroxyphenyl)fluorene (BAHF) (Fig. 1). These substituents perform simultaneously the functions of solubilizing, suppressing aggregation and improving hole-injection of the fluorenes. The bimolecular electron-transfer processes were studied by employing the nanosecond laser photolysis with the visible/near-IR detectors.

Department of Chemistry, Faculty of Education, Kafr El-Sheikh, Tanta University, Egypt. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional spectroscopic data. See DOI: 10.1039/b617814f

Fig. 1 Molecular structures of C60 , C70 and fluorene derivatives.

Results and discussion Steady-state absorption studies The absorption spectra of the studied compounds were measured in benzonitrile at room temperature (Fig. 2). The absorption spectra of fullerenes (C60 and C70 ) exhibited absorption peak at around 355 and 470 nm, respectively, with a tail in the visible region. The absorption of C70 in the visible region is markedly stronger than that of C60 . Notably, the studied fluorene derivatives show much weaker absorptions in the visible region than the C60 /C70 . Upon

Fig. 2 Absorption spectra of C60 , C70 and BAMF in benzonitrile. The concentrations were 8.0 × 10−6 mol dm−3 .

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Photochem. Photobiol. Sci., 2007, 6, 539–544 | 539

photo-excitation at 532 nm, the C60 /C70 was selectively excited to yield the singlet excited-states (1 C60 */1 C70 *), that in turn decayed to populate the triplet excited-states (3 C60 */3 C70 *) via the intersystem crossing process. The spectral features of the mixtures of C60 and fluorene derivatives are closely resemble the sum of the spectra of the components, suggesting that the interaction between the two components in the ground state is negligibly weak. Electrochemical studies The redox potentials have been studied using the differential pulse voltammetry (DPV) technique to evaluate the driving forces for the electron transfer (−DGet T ). By sweeping the voltage applied to the solutions containing a suitable electrolyte, the redox potentials of the studied compounds were measured in benzonitrile (Fig. 3). The first reduction potential (E red ) of C60 , C70 and the viologen dication (OV2+ ) was located at −0.76, −0.72 and −0.60 V vs. Ag/Ag+ , respectively. The remarkable observation is the low oxidation potential (E ox ) of BAHF (0.32 V vs. Ag/Ag+ ) compared to that of BAMF (0.58 V vs. Ag/Ag+ ) and BAFF (0.80 V vs. Ag/Ag+ ).

where E ox is the first oxidation potential of the fluorene derivatives, E red is the first reduction potential of C60 /C70 , E T is the triplet energy of 3 C60 */3 C70 * (35.0 kcal mol−1 ), and e2 /es d cc is the Coulomb energy term (approximately 1.4 kcal mol−1 in the polar benzonitrile). The free-energy change (−DGet T ) values via 3 C60 * were estimated as 13.0, 6.9 and 1.8 kcal mol−1 for BAHF, BAMF and BAFF, respectively. Since the first E red of C70 is slightly lower than C60 , the calculated DGet T values via 3 C70 * are slightly more negative than the corresponding values via 3 C60 *. In general, the DGet T values via 3 C60 */3 C70 * are negative suggesting that the quenching process should be close to the diffusion-controlled limit (kdiff ).31 The electrochemical measurements are in line with the picture of the electronic structure of substituted fluorenes (Fig. 4) which emerges from the ab initio molecular orbital calculations performed at B3LYP/3-21G levels. As listed in Table 1, the calculated HOMO (highest occupied molecule orbital) energy in BAHF is much lower (more negative) than the other fluorene derivatives, which is in good agreement with electrochemical measurements. It is remarkable that the HOMO energy of BAHF increases (−4.72 eV) by changing the positions of the NH2 groups (from meta to para) and of the OH groups (from para to meta), suggesting that the effect of the NH2 groups in the para position is stronger than in the meta position. Electron transfer from fluorenes to 3 C60 */3 C70 * in benzonitrile By photo-excitation of C60 (1 × 10−4 mol dm−3 ) in the presence of BAHF (4 × 10−4 mol dm−3 ) in Ar-saturated benzonitrile using 532 nm laser photolysis, the transient spectra exhibit the characteristic band of 3 C60 * at 760 nm. With the decay of 3 C60 *, the concomitant rise of the C60 radical anion (C60 • − ) at 1080 nm32–36 was observed (Fig. 5). Similar to C60 –BAHF, the characteristic bands of 3 C60 * and C60 • − were clearly observed in the mixtures of C60 – BAMF and C60 –BAFF in benzonitrile (Fig. 4, lower panel). The

Fig. 3 Cyclic voltammograms of the studied compounds in benzonitrile. Scan rate = 100 mV s−1 .

Table 1 The oxidation potentials (vs. Ag/Ag+ ) and calculated HOMO energies of fluorene derivatives

The feasibility of the electron transfer process from the ground state fluorenes to 3 C60 */3 C70 * is controlled by the free-energy change (DGet T ), which can be expressed by the Rehm–Weller relation.29,30 DGet T (kcal mol−1 ) = 23.06(E ox − E red ) − E T − e2 /es d cc

Meta

Para

BAHF

NH2 OH CH3 F

OH NH2 NH2 NH2

BAMF BAFF a

(1)

Compounds

E ox /V

HOMO/eV

0.32

−5.48 −4.72 −5.06 −4.90

a

0.58 0.80

Not recorded.

Fig. 4 Ab initio B3LYP/3-21G optimized structure of BAHF (right) and frontier HOMO orbitals (left).

540 | Photochem. Photobiol. Sci., 2007, 6, 539–544

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By photo-excitation of C70 (1 × 10−4 mol dm−3 ) in the presence of BAHF (4 × 10−4 mol dm−3 ) in Ar-saturated benzonitrile using a 532 nm laser light (Fig. 6), the transient absorption spectra showed the characteristic absorption bands of 3 C70 * and C70 • − at 980 and 1380 nm, respectively.37,38 These observations indicate that the electron transfer takes place via 3 C70 * as shown in Scheme 1.

Fig. 6 Transient absorption spectra of C70 (1 × 10−4 mol dm−3 ) in the presence of BAHF (4 × 10−4 mol dm−3 ) in benzonitrile.

Fig. 5 Transient absorption spectra of C60 (1 × 10−4 mol dm−3 ) in the presence of BAHF (upper) and BAFF (lower) in benzonitrile. The concentrations of BAHF and BAFF were 4 × 10−4 mol dm−3 .

decay of 3 C60 * and rise of C60 • − seem to match each other. These observations indicate that the electron transfer process takes place via 3 C60 * as shown in Scheme 1. In oxygen-saturated solutions, an intermolecular energy transfer from 3 C60 * to oxygen emerges, suppressing the electron transfer events between 3 C60 * and fluorene derivatives. The kO2 was evaluated to be 8.0 × 109 mol−1 dm3 s−1 when assuming [O2 ] = 1 × 10−3 mol dm−3 .

The rate constants of the bimolecular quenching (kq ) were evaluated under the pseudo-first-order conditions by monitoring the decay of 3 C60 * or 3 C70 * as a function of the concentration of the fluorene derivatives.39 The efficiencies of the electron transfer process (U et ) were evaluated from the ratio of the maximum concentration of the generated radical ions of fullerenes to the initial concentration of triplet fullerenes.40 An examination of Table 2 reveals the following: (1) The rate constants of the bimolecular quenching (kq ) were estimated as 3.0 × 109 and 1.0 × 108 mol−1 dm3 s−1 for 3 C60 *–BAHF and 3 C60 *–BAFF, respectively. (2) The efficiencies of the electron transfer process (U et ) were estimated as 0.43 and 0.05 for 3 C60 *–BAHF and 3 C60 *– BAFF, respectively (Fig. 7). From these findings, it is clear that kq , ket , and U et are associated with the substitution patterns on the 9,9-bisphenylfluorenes, where the electron transfer rates and efficiencies are higher for 3 C60 *–BAHF and 3 C60 *–BAMF compared to those for 3 C60 *–BAFF. (3) It is remarkable that the U et values of 3 C70 *–fluorenes are higher than those of 3 C60 *–fluorenes, which may be explained by considering the more negative DGet T of 3 C70 *–fluorenes systems compared to that of 3 C60 *–fluorenes ones.

Fig. 7 Dependences of electron-transfer quantum yields on the concentration of the fluorenes for 3 C60 *–fluorenes in Ar-saturated benzonitrile. Scheme 1 Electron transfer and electron mediating processes of fullerenes–BAHF mixtures in benzonitrile.

The Rehm–Weller behavior is observed in the plot of kq vs. DGet T (Fig. 8), where closed circles are used to indicate the C60 –fluorenes

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Table 2 Electron transfer parameters of C60 /C70 with fluorene derivatives in benzonitrile System

−DGet T a /kcal mol−1

kq b /mol−1 dm3 s−1

U et c

ket d /mol−1 dm3 s−1

3

13.0 6.9 1.8 11.8 5.7 0.7

3.0 × 109 2.2 × 109 1.8 × 108 1.8 × 109 1.4 × 109 1.6 × 108

0.43 0.33 0.05 0.59 0.43 0.18

1.3 × 109 7.3 × 108 9.0 × 106 1.0 × 109 6.0 × 108 2.8 × 107

C60 *–BAHF C60 *–BAMF 3 C60 *–BAFF 3 C70 *–BAHF 3 C70 *–BAMF 3 C70 *–BAFF 3

DGet T (kcal mol−1 ) = 23.06(E ox − E red ) − E T − e2 /es d cc , where E ox , E red , E T , and e2 /es d cc are the oxidation potential of the fluorenes, reduction potential of C60 /C70 , the triplet energy of the exciting molecule (3 C60 */3 C70 *), and the Coulomb term, respectively. b kq values are calculated by monitoring the decay of 3 C60 * as a function of the fluorene concentration. c U et values are calculated from the ratio of the maximum concentration of the generated radical ions to the initial concentration of 3 C60 *, employing the reported molar extinction coefficients: e(3 C60 *) = 16000 mol−1 dm3 cm−1 at 760 nm, e(C60 • − ) = 12000 mol−1 dm3 cm−1 at 1080 nm, e(3 C70 *) = 6500 mol−1 dm3 cm−1 at 980 nm, and e(C70 • − ) = 4000 mol−1 dm3 cm−1 at 1380 nm. d ket = kq U et

a

Fig. 8 Plot of kq vs. DGet T for the electron transfer process via 3 C60 * and 3 C70 * in benzonitrile. The solid line was calculated from the Rehm–Weller equation.

systems and open circles are used to indicate the C70 –fluorenes ones. From Fig. 8 it can be seen that for both C60 –fluorenes and C70 –fluorenes systems, the quenching rate constants correlate well with the free energy changes for the electron transfer process.41 The bimolecular electron transfer reactions under diffusive conditions, as in the present case, can be represented by Scheme 2. According to this scheme, A* is the excited acceptor (C60 /C70 ), D is the ground-state donor (fluorenes), (A* · · · D) is the acceptor–donor encounter complex, (A− · · · D+ ) is the ion-pair state formed following electron transfer, kd and k−d are the diffusion-controlled formation and dissociation rate constants for (A* · · · D), and ket and k−et are the forward and back electron transfer rate-constants.

Scheme 2

Electron-mediating process from C60 • − to viologen dication (OV2+ ) Electron-mediating process can be proved by the excitation of C60 (1 × 10−4 mol dm−3 ) with BAHF (4 × 10−4 mol dm−3 ) in the presence of an appropriate electron acceptor such as OV2+ (4 × 10−4 mol dm−3 ). As it can be seen from the transient spectra in Fig. 9, the absorption band of OV• + was observed at 620 nm which parallels with the decay of C60 • − at 1080 nm.42–44 These observations indicate that in the presence of BAHF and OV2+ , laser irradiation of C60 induces, at first, an electron transfer from BAHF to 3 C60 *. The C60 • − produced successively donates its excess 542 | Photochem. Photobiol. Sci., 2007, 6, 539–544

Fig. 9 Transient absorption spectra obtained using 532 nm laser excitation of C60 (1 × 10−4 mol dm−3 ) with BAHF (4 × 10−4 mol dm−3 ) in the presence of OV2+ (4 × 10−4 mol dm−3 ) in benzonitrile.

electron to OV2+ yielding OV• + as indicated by the decay of C60 • − and rise of OV• + . A possible rationale for this finding involves the lower reduction potential of OV2+ (−0.58 V vs. Ag/Ag+ ) compared to C60 (−0.76 V vs. Ag/Ag+ ), as shown earlier. In this case C60 acts as an electron mediator as well as a photosensitizing electron acceptor. The whole photosensitized electron-transfer/electronmediating processes are summarized in Scheme 1. Solvent effect The transient absorption spectra obtained using 532 nm laser excitation of C60 in the presence of BAMF in toluene, showed only the characteristic band of 3 C60 * at 760 nm without observing C60 • − , suggesting that no electron transfer process is taking place. Kinetics analysis of the 3 C60 *–BAMF system in a mixture of toluene and benzonitrile affords information about the quenching mechanism in both solvents. In the region of toluene-rich content, only the characteristic absorption band of 3 C60 * was observed, from which the kq value was derived as shown earlier. Thus, it is presumed that the non-polar solvent retard the dissociation of contact ions to the free ion radicals in solution. When the % of benzonitrile in the mixture exceeds 20%, the ion radicals are formed by observing the absorption band of C60 • − (see the electronic supplementary information (ESI), Fig. S3).† As listed in Table 3, the rates and efficiencies of the electron transfer increase gradually with increasing the ratio of benzonitrile to toluene. When the radical ions are formed from neutral reactants, polar solvent molecules rapidly surround each ion and screen the electrostatic interactions and consequently prevent the electron

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Table 3 The kq , U et and ket values for 3 C60 *–BAMF in benzonitrile– toluene mixtures Benzonitrile–toluene ratio

kq /mol−1 dm3 s−1

U et

ket /mol−1 dm3 s−1

0 : 100 25 : 75 50 : 50 75 : 25 100 : 0

8.2 × 108 2.1 × 109 2.6 × 109 2.8 × 109 2.2 × 109

a

a

0.06 0.17 0.29 0.33

1.2 × 108 4.5 × 108 8.2 × 108 7.3 × 108

a

No electron transfer was observed in toluene.

return. In contrast, the partners in an ion pair formed in toluene have a tendency to stay in close proximity, favoring electron return. Escape from the solvent cage is indeed a key feature of electron transfer in solution.45–48

Conclusion The bimolecular electron transfer of fullerenes (C60 /C70 ) with 9,9disubstituted fluorenes was studied by observing the transient absorption spectra of their ion radicals (C60 • − /C70 • − ) in the visible and near-IR region. By the selective excitation of C60 /C70 using 532 nm laser light, it has been proved that the electron transfer takes place from the fluorenes to the triplet excited state of C60 /C70 in benzonitrile, but not in toluene. The observed rates and efficiencies of the electron transfer process are found to be significantly associated with the substitution patterns on the 9,9bisphenylfluorenes. The change in the electron transfer rates and efficiencies seems to correlate well with the free energy changes (DGet T ). On addition of a viologen dication, the photosensitized electron-transfer/electron-mediating processes were established.

Experimental Materials C60 /C70 (>99%) were purchased from Texas fullerene corporation; 9,9-bis(4-amino-3-methylphenyl)fluorene (BAMF), 9,9bis(4-amino-3-fluorophenyl)fluorene (BAFF), and 9,9-bis(3amino-4-hydroxyphenyl)fluorene (BAHF) are newly produced by Tokyo Kaesei Kogoyo Co. Ltd. (Japan). Benzonitrile and toluene (99.9% HLPC grade, Aldrich) were used as received. Octylviologen dication (OV2+ ) with perchlorate anion as a counter ion was employed as an electron-mediating reagent, which was prepared from commercially available octylviologen dication with chlorine anion. Methods Steady-state absorption spectra were measured using an optical cell (0.2–1.0 cm) with a JASCO V-570 spectrophotometer. Cyclic voltammetry (CV) measurements were performed by using a BAS CV-50W voltammetric analyzer (Japan). All measurements were carried out under argon in benzonitrile solutions containing tetra-n-butylammonium perchlorate (0.1 M) as the supporting electrolyte and AgCl as the reference electrode. The transient absorption spectra and time profiles were measured using 532 nm laser light as the excitation source. An InGaAs-PIN photodiode (Hamamatus photonics) was employed as detectors in the nearIR region and visible region, to monitor the steady light from

continuous Xe lamp (150 W). More details of the experimental set-up were described elsewhere.49 Laser photolysis experiments were performed in a rectangular quartz cell with a 10 mm optical path. All the measurements were carried out at 23 ◦ C using freshly prepared argon-saturated solutions to eliminate the influence of oxygen.

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37 H. Onodera, Y. Araki, M. Fujitsuka, S. Onodera, O. Ito, F. Bai, M. Zheng and J.-L. Yang, Photoinduced electron-transfer from mono/oligo-1,4-phenylenevinylenes containing aromatic amines to C60 /C70 and electron-mediating process to viologen dication in polar solution, J. Phys. Chem. A, 2001, 105, 7341–7349. 38 M. E. El-Khouly, M. Fujitsuka and O. Ito, Photoinduced electron transfer between metal octaethylporphyrins and fullerenes (C60 /C70 ) studied by laser flash photolysis: electron-mediating and hole-shifting cycles, Phys. Chem. Chem. Phys., 2002, 4, 3322. 39 The rate constants of the bimolecular quenching (kq ) were evaluated under the pseudo-first-order condition [3 C60 *]
[fluorenes] by monitoring the decay of 3 C60 * as a function of [fluorenes] as in the following equation; k1st = k0 + kq [fluorenes]; where k0 is the rate constant of 3 C60 * decay in the absence of quenchers. The decay time profiles of 3 C60 * obey first-order kinetics; each rate constant is referred to (k1st ). The linear concentration-dependence of the observed k1st values gives the kq values. 40 The efficiency of ET (F et ) was evaluated from the ratio of the maximum concentration of the generated radical ions to the initial concentration of 3 C60 *. The ratios of [C60 • − ]max /[3 C60 * ]max are plotted against [fluorenes]. The saturated values are attributed to the F et values. Furthermore, the (ket ) values were finally obtained from the following equation, ket = kq F et . 41 The solid line was obtained using the usual kinetic scheme for electron transfer quenching processes in solution and the parameters reported by Rehm–Weller for the quenching of several aromatic molecules by electron donors; kd = 5.6 × 109 mol−1 dm3 s−1 and DG*(0) = 0.07 eV. 42 M. M. Alam and O. Ito, Self-repairing photosensitized electron transfer from thiones to methyl viologen in aqueous media, J. Phys. Chem. A, 1999, 103, 1306–1310. 43 M. E. Milanesio, M. Gervaldo, L. A. Otero, L. Sereno, J. J. Sliber and E. N. Durantin, Synthesis and photophysical properties of Zn(II) porphyrin-C60 dyad with potential use in solar cells, J. Phys. Org. Chem., 2002, 15, 844–851. 44 S. Xiao, M. E. El-Khouly, Y. Li, Z. Gan, H. Liu, L. Jiang, Y. Araki, O. Ito and D. Zhu, Dyads and triads containing perylenetetracarboxylic diimide and porphyrin: efficient photoinduced electron transfer elicited via both excited singlet states, J. Phys. Chem. B, 2005, 109, 3658– 3667. 45 G. J. Kavarnos and N. G. Turro, Photosensitization by reversible electron transfer: theories, experimental evidence, and examples, Chem. Rev., 1986, 86, 401–449. 46 G. J. Kavarnos, Fundamentals of photoinduced electron transfer, VCH Publisher, New York, 1993, ch. 3, pp. 103–184. 47 N. Mataga, T. Okada and N. Yamamoto, Electronic processes in heteroexcimers and the mechanism of fluorescence quenching, Chem. Phys. Lett., 1967, 1, 119–121. 48 I. R. Gould and S. Farid, Dynamics of bimolecular photoinduced electron-transfer reactions, Acc. Chem. Res., 1996, 29, 522–528. 49 C. Luo, M. Fujitsuka, T. Akasaka and O. Ito, Photoinduced electron transfer from N,N-dimethylaniline to pyrrolidinofullerenes (C60 (C3 H6 N)R): emphasized substituent effects with solvent polarity change, J. Phys. Chem. A, 1998, 102, 8716–8721.

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