Spectrochimica Acta Part A 67 (2007) 636–642

Photoinduced intermolecular electron transfer process of fullerene (C60) and amine-substituted fluorenes studied by laser flash photolysis Mohamed E. El-Khouly Department of Chemistry, Faculty of Education, Kafr El-Sheikh, Tanta University, Egypt Received 6 June 2006; received in revised form 16 August 2006; accepted 17 August 2006

Abstract Photoinduced intermolecular electron transfer process of fullerene (C60 ) with 9,9-bis(4-triphenylamino)fluorene (BTAF) and 9,9-dimethoxyethyl2-diphenylaminofluorene (DAF) in toluene and benzonitrile has been investigated by nanosecond laser photolysis technique in the visible/near-IR regions. By the selective excitation of C60 using 532 laser light, it has been proved that the electron transfer takes place from the ground states BTAF and DAF to the triplet excited state of C60 (3 C60 * ) by observing the radical anion of C60 and radical cation of BTAF and DAF. It was observed that the electron transfer of BTAF/3 C60 * is more efficient than DAF/3 C60 * reflecting the effect of amine-substitutents of the fluorene moiety on the efficiency of the electron transfer process. On addition of a viologen dication (OV2+ ), the electron of the anion radical of C60 mediates to OV2+ yielding the OV•+ . These results proved that the photosensitized electron-transfer/electron-mediating processes have been confirmed by the transient absorption spectral method. © 2006 Elsevier B.V. All rights reserved. Keywords: Fullerene (C60 ); Fluorene; Electron transfer; Laser flash photolysis

1. Introduction Photoinduced electron transfer (PET) process of donor– acceptor systems has long been of interest in chemistry and biology because it is a universal and fundamental phenomenon in nature [1–8]. Toward constructing the molecular electronic devices, fullerene (C60 ) is particularly appealing as electron acceptor, because of its three-dimensional structure, delocalized ␲-electrons within the spherical carbon framework, small reorganization energy, low reduction potential, and absorption spectra extending over most of the visible region. These unique properties make C60 promising candidate for the investigation of photoinduced electron transfer processes by mixing with electron-donor compounds [9–16]. Among the electron donors, fluorene and its derivatives are of particular interest, because of their thermal and chemical stability along with their desirable photoluminescence and electroluminescence properties [17–20]. The unique chemical and physical characteristics

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of blue-emitting fluorenes make them essential and accessible in a wide variety of applications ranging from the electroluminescent devices, plastic solar cells, and photodynamic therapy [21–27]. Herein, we report the intermolecular electron transfer between fullerene (C60 ) and two amine-substituted fluorenes, namely 9,9-bis(4-triphenylamino)fluorene (BTAF) and 9,9-dimethoxyethyl-2-diphenylaminofluorene (DAF), in which triphenylamine and diphenylamine units are facilely introduced as substituents at the 9-position, respectively (Fig. 1). These substituents perform simultaneously the functions of solubilizing, suppressing aggregation and improving hole-injection. Comparison of the donor ability of BTAF with DAF may be interesting to reveal the effect of aromatic amines attached with the fluorene on the efficiency of photoinduced electron transfer processes. The electron-mediating process from C60 •− to octylviologen dication (OV2+ ) was also investigated to prove a photosensitized electron-transfer/electron-mediating processes of fullerene/fluorene/octylviologen dication. The forward/backward electron-transfer and electron mediating processes were studied by employing nanosecond laser photolysis with the visible/near-IR detectors (Fig. 1).

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Fig. 1. Molecular structures.

2. Experimental

3. Results and discussion

2.1. Materials

3.1. Molecular orbital of BTAF and DAF

C60 (>99%) was purchased from Texas Fullerene Corp. 9,9bis(4-triphenylamino)fluorene (BTAF) and 9,9-dimethoxyethyl -2-diphenylaminofluorene (DAF) were synthesized according to the procedure published elsewhere [28]. Benzonitirle and toluene (99.9% HLPC grade, Aldrich) were used as received. Octylviologen dication (OV2+ ) with chloride anion as a counter ion was employed as an electron-mediating reagent, which was prepared from commercially available octylviologen dication with chloride anions.

The electron density of the highest occupied molecular orbital (HOMO) for the electron donors was obtained by density functional method (DFT) at the B3LYP/3-21G level as shown in Fig. 2. In BTAF and DAF, the majority of the electron distribution of HOMO was located in the triphenylamine and diphenylamine, respectively. From this finding, it seems that the holes are mainly distributed over the substituted-amine groups. 3.2. Steady-state absorption studies

2.2. 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 applying BAS CV-50W Voltammetric Analyzer (Japan). All measurements were carried out under argon in benzonitrile solution containing 0.1 M tetra-n-butylammonium percholarate (Bu4 NClO4 ) as the supporting electrolyte and AgCl reference electrode. The transient absorption spectra and time profiles were measured using 532 nm laser light as an excitation source. In the near-IR region (600–1700 nm), an InGaAs-PIN photodiode (Hamamatus photonics) was employed as detectors in the near-IR region and visible region, to monitor the steady light from continuous Xe-lamp (150 W). All the measurements were carried out at 23 ◦ C using freshly prepared argon-saturated solutions to eliminate the influence of oxygen effect.

The absorption spectra of C60 , BTAF and mixture solution of C60 + BTAF were measured in benzonitrile at room temperature (Fig. 3). The absorption spectrum of C60 exhibited absorption peak around 355 nm with tail in the visible region. The spectra of the mixture are closely resembles the sum of the spectra of the C60 and BTAF moieties. The absorption spectra show that the molar absorption coefficient of C60 is much higher than that of BTAF in the visible region leading to predominant excitation of C60 by the laser light at 532 nm. 3.3. Cyclic voltammetric studies The redox potentials of the studied compounds have been evaluated using differential pulse voltammetry (DPV) technique. Through DPV technique it is possible to derive the driving forces for the electron transfer via the triplet state (−GTet ). By

Fig. 2. HOMO of the BTAF and DAF calculated by density functional method (DFT) at the B3LYP/3-21G level.

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Fig. 3. Absorption spectra of C60 , BTAF and C60 + BTAF mixture in benzonitrile. The concentrations kept at 0.04 mM.

sweeping an applied voltage to the solutions with a suitable electrolyte, the oxidation potentials of BTAF and DAF and the reduction potentials of C60 and OV2+ in benzonitrile were measured as shown in Fig. 4. The first oxidation potential (Eox ) of BTAF and DAF was located at 0.50 and 0.64 V versus Ag/Ag+ , respectively. From this finding, BTAF shows higher donor ability compared with DAF. On the other hand, the first reduction potential (Ered ) of the C60 and OV2+ were located at −0.79 and −0.63 V versus Ag/Ag+ , respectively. The feasibility of the electron transfer character from the ground state fluorene to the triplet excited state of C60 is directed by the free-energy change (GTet ) for the electron transfer process, which can be expressed by the Rehm–Weller equation (Eq. (1)) [29]: GTet (eV) = Eox − Ered − Ec − ET

(1)

where Eox , Ered , ET and Ec are oxidation potentials of BTAF and DAF, reduction potential of C60 , the triplet energy of the exciting molecule (3 C60 * ), and the coulombic term, respectively. The GTet values are estimated as −0.30 and −0.16 eV for 3 C60 * /BTAF and 3 C60 * /DAF, respectively. These negative GTet values imply that the rates of the bimolecular quenching (kq ) from fluorenes to 3 C60 * are close to the diffusion-controlled limit (kdiff ) [30]. 3.4. Electron transfer from 9,9-bis(4-triphenylamine)fluorene (BTAF) to 3 C60 * in non-polar and polar solvents By the use of nanosecond laser pulse, the electron transfer process via the triplet states can be observed in real time. By photoexcitation of C60 (0.1 mM) in both benzonitrile and toluene using 532 nm laser photolysis, the transient absorption spectrum immediately after the laser pulse exhibited only an absorption band at 760 nm, which unambiguously assigned to the triplet state of C60 (3 C60 * ).

Fig. 4. Cyclic voltammograms of DAF, BTAF, C60 and OV2+ in benzonitrile, 0.1 M (TBA)ClO4 . Scan rate = 100 mV/s.

In toluene as a non-polar solvent, the laser flash photolysis of C60 (0.10 mM) was carried out in the presence of BTAF (0.20 mM) as shown in Fig. 5 (upper panel). The transient absorption spectra in the visible and near-IR regions exhibited the absorption peak of 3 C60 * at 760 nm without any characteristic peaks of the radicals (BTAF•+ and C60 •− ) suggesting no contribution of the electron transfer process in the deactiviation process of 3 C60 * . Although the decay rate of 3 C60 * was accelerated on addition of BTAF as shown in the time profiles, triplet energy transfer from 3 C60 * to BTAF is also implausible due to energetic considerations. A collisional deactivation process of 3 C * with BTAF may take place via a weak interaction between 60 them. The rate constant of the deactivation process was evaluated as 3.2 × 109 M−1 s−1 by recording the decay rates of 3 C60 * with increasing the concentration of BTAF. In benzonitrile as polar solvent, the transient spectra (Fig. 5, lower panel) of excite C60 (0.10 mM) in the presence of BTAF

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Scheme 1. Schematic diagram showing the electron transfer process of C60 /BTAF mixture system in benzonitrile.

Fig. 5. Transient absorption spectra obtained by 532 nm laser light of C60 (0.1 mM) in the presence of BTAF (0.2 mM) in Ar-saturated toluene (upper panel) and benzonitrile (lower panel). Inset: time profiles at 760 and 1080 nm.

(0.20 mM) exhibit the characteristic peak of 3 C60 * at 760 nm. With the decay of 3 C60 * , the concomitant rise of the C60 radical anion (C60 •− ) at 1080 nm [9–16,31–36] was observed. Moreover, the absorption appearing in the 1000–1600 nm regions with a maximum centered around 1400 nm which assigned to the BTAF radical cation (BTAF•+ ). Interestingly, this wavelength is apparently much longer than non-substituted fluorene revealing the effect of the triphenylamine groups on the delocalization of the radical cation over 1000–1600 nm. The assignment of BTAF•+ at 1000–1600 nm was confirmed by recording the similar peak of BTAF•+ in case of mixture system of BTAF and fullerene (C70 ) (Supporting information; Fig. S2). The decay and rise time profiles seem to match each other. These observations indicate that the electron transfer process takes place via 3 C * as shown in Scheme 1. Furthermore, the contribution of 60 3 C * to electron transfer process was confirmed by oxygen60 effect on the yields of C60 •− and BTAF•+ . The decay of 3 C60 * was accelerated on addition of oxygen in the presence of BTAF, indicating that 3 C60 * was quenched owing to energy transfer to oxygen (rate constant is referred to kO2 ), yielding the sin-

glet oxygen (1 O2 ); kO2 was evaluated to be 2.0 × 1010 M−1 s−1 on assuming [O2 ] = 0.2 mM. Addition of oxygen obviously suppressed the formations of C60 •− and BTAF•+ , supporting the electron transfer process via 3 C60 * as shown in Scheme 1. A more detailed picture of the kinetic event is shown in Fig. 6 (left panel), where the rate constants of the bimolecular quenching (kq ) were evaluated under the pseudo-first-order condition [3 C60 * ]  [BTAF] by monitoring the decay of 3 C60 * as a function of [BTAF]. 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 value, which calculated as 3.6 × 109 M−1 s−1 . The efficiency of electron transfer process (Φet ) can be calculated from the ratio of the maximal concentration of the generated radical ions to the initial concentration of 3 C60 * . These value was calculated employing the reported molar extinction coefficients; ε (3 C60 * ) = 16000 M−1 cm−1 at 760 nm and ε (C60 •− ) = 12000 M−1 cm−1 at 1080 nm. The saturated values are attributed to the Φet value as evaluated as 0.56 (Fig. 6, right panel). The rate-constant of the electron transfer process (ket ) was finally obtained from the following equation, ket = kq × Φet . The ket value was evaluated as 2.0 × 109 M−1 s−1 which is near the diffusion-controlled limit (kdiff = 5.6 × 109 M−1 s−1 ) in benzonitrile [30]. 3.5. Electron transfer from 9,9-dimethoxyethyl-2-diphenylaminofluorene (DAF) to 3 C * in polar solvent 60 The transient spectra of DAF and C60 mixture system exhibited the decay of C60 triplet state at 760 as well as the rises of C60 •− at 1080 nm and DAF•+ at 860 nm [37] as shown in Fig. 7. Apparently, the recorded wavelength of DAF•+ is much shorter than BTAF•+ . The rate constants of the bimolecular quenching (kq ) was evaluated as kq = 1.4 × 109 M−1 s−1 . The efficiency of the electron transfer process (Φet ) was estimated as 0.36. From both kq and Φet , the rate of the electron-transfer was evaluated as 5.0 × 108 M−1 s−1 which is four times slower than the rate of

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Fig. 6. (Left panel) Decay of 3 C60 * at 760 nm and rise of C60 •− at 1080 nm with increasing [BTAF]. (Right panel) Efficiency of electron transfer [C60 •− ]/[3 C60 * ] vs. [fluorenes] in Ar-satruated benzonitrile.

BTAF/3 C60 * mixture system. This difference can be explained by the lower donor ability of DAF compared to BTAF, which may arise from the effect of aromatic amines attached with the fluorene on the efficiency of electron transfer process. 3.6. Electron-mediating process from C60 •− to viologen dication (OV2+ ) Electron-mediating process can be proved by the excitation of C60 (0.1 mM) with BTAF (0.2 mM) in the presence of appropriate electron acceptor such as OV2+ (0.6 mM), as shown in Fig. 8. The transient spectra exhibit the characteristic bands of 3 C60 * at 760 nm, C60 •− at 1080 nm and BTAF•+ at 1000–1700 nm. In the spectrum at 5 ␮s, the absorption band of OV•+ was observed by build up of absorption at 500–700 nm [38–40] that parallels a concomitant decrease of the decay of C60 •− at 1080 nm. These observations indicate that in the presence of BTAF and OV2+ ,

Fig. 7. Transient absorption spectra obtained by 532 nm laser light of C60 (0.10 mM) in the presence of DAF (0.20 mM) in Ar-saturated benzonitrile. Inset: time profiles.

laser irradiation of C60 induces, at first electron transfer from BTAF to 3 C60 * . The C60 •− produced successively donates its excess 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+ (−0.63 V versus Ag/Ag+ ) compared to C60 (−0.79 V versus Ag/Ag+ ), as shown earlier. In this case C60 acts as an electron mediator in addition to a photosensitizing electron acceptor. The whole photosensitized electron-transfer/electron-mediating processes are summarized in Scheme 2. 3.7. Back electron transfer In long time-scale (200 ␮s), the C60 •− begins to decay slowly after reaching the maximal absorbance (Fig. 9). The decay time profile was fitted with second-order kinetics, suggesting that bimolecular back electron-transfer (BET) process from C60 •−

Fig. 8. Transient absorption spectra obtained by 532 nm laser light of C60 (0.10 mM) with BTAF (0.20 mM) in the presence of OV2+ (0.60 mM) in Arsaturated benzonitrile.

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Scheme 2. Electron transfer and electron mediation cycle of C60 /BTAF in the presence of OV2+ .

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effect of amine-substitutents on the efficiency of the electron transfer process. The tendency of the electron transfer rates and efficiencies correlate nicely with the free energy changes (−GTet ). (2) The study reveals the solvent effect by observing the radical ion-pairs in benzonitrile, but not in toluene. In the presence of fluorene, the triplet state of C60 was quenched in toluene without any evidence of energy transfer and/or electron transfer processes. By shift to the benzonitrile as polar solvent, dissociation of the contact ion-pair to solventseparated ion pair or free ion radical is promoted. (3) On addition of a viologen dication to a mixture of C60 /BTAF, the photosensitized electron transfer/electron mediating process was established. Acknowledgments The author is grateful to Prof. Osamu Ito (Tohoku University, Japan) for allowing me to measure the nanosecond transient spectra in his laboratory. Bunch of thanks to Prof. Yu Chen (East China University of Science and Technology, China) for providing BTAF sample. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2006.08.022. References

Fig. 9. Decay of C60 •− on long time scale produced under the same conditions as described in Fig. 5 (lower panel). Inset: second-order plot.

to BTAF•+ takes place. The rate constant of the back electrontransfer (kbet ) was evaluated from Eq. (2): 1 kbet 1 = t + Abs Abs0 ε

(2)

The kbet values were estimated from the slopes of the secondorder plots in the form of kbet /ε, where ε refers to the molar extinction coefficient of C60 •− . On employing the reported ε, the kbet value was evaluated as 7.5 × 109 M−1 s−1 . In general, the kbet value seem to be close to the diffusion-controlled limit (kdiff ) in benzonitrile, which means that C60 •− and BTAF•+ are long lived. Since the concentrations of C60 •− and BTAF•+ are considerably lower than that of the reactants [C60 and BTAF], the observed decay rate of the backward process are far smaller than that of the forward process, even though kbet  ket . 4. Conclusions Fullerene C60 acts as good electron acceptor in the presence of amine-substituted fluorenes. By summarizing the results of the present study, the following conclusions have been drawn: (1) It is shown that the intermolecular electron transfer of BTAF/3 C60 * is more efficient than DAF/3 C60 * reflecting the

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