Juan José
Oviedo
a,
Mohamed E.
El-Khouly
cd,
Pilar
de la Cruz
a,
Laura
Pérez
a,
Javier
Garín
b,
Jesús
Orduna
b,
Yasuyuki
Araki
c,
Fernando
Langa
*a and
Osamu
Ito
*c
aFacultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, 45071, Toledo, Spain. E-mail: Fernando.LPuente@uclm.es; Fax: +34 902 204 130; Tel: 34 925 268 843
bDepartamento de Química Orgánica, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain. E-mail: jorduna@unizar.es; Fax: +34 976761 194; Tel: 34 976 761 194
cInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai, 980-8577, Japan. E-mail: ito@tagen.tohoku.ac.jp; Fax: +81-22-217-5608
dDepartment of Chemistry, Faculty of Education, Kafr El-Sheikh, Tanta University, Egypt. E-mail: elkhouly@tagen.tohoku.ac.jp
First published on 11th November 2005
Two novel ruthenocene-C60 dyads, with a 2-pyrazoline ring or a pyrrolidine ring as a linker, have been synthesized with the aim of providing a simple model of natural photosynthesis. The photophysical properties of the two ruthenocene-C60 dyads have been investigated by steady-state absorption and fluorescence, time-resolved fluorescence and nanosecond transient measurements in polar and non-polar solvents. The charge separation takes place in the ruthenocene-pyrazolino[60]fullerene more efficiently than in the ruthenocene-pyrrolidino[60]fullerene dyad. The lifetimes of the charge-separated states of the ruthenocene-pyrazolino[60]fullerene and the ruthenocene-pyrrolidino[60]fullerene dyads are 100 ns in PhCN. It was found that the ruthenocene-[60]fullerenes have an ability to prolong the charge-separated states compared with those for ferrocene-[60]fullerenes.
Nevertheless, combinations of other metallocenes, such as ruthenocene, with C60 remain unexplored. Recently, we described the synthesis and preliminary studies on ruthenocene-C60 dyads.9 The first ionization potentials (IPs) for ferrocene and ruthenocene are 6.86 and 7.45 eV, respectively.10,11 Thus, ferrocene is a stronger donor in an electron-transfer sense. In many electron-donor (D) and electron-acceptor (A) systems, however, there is a little evidence to distinguish between the donor strengths of ferrocene and ruthenocene in systems with strong electron-acceptors. Indeed, there is electrochemical evidence that ruthenocene is the stronger donor, despite the fact that it has a lower energy HOMO than ferrocene.12 The superior donating properties of ruthenocene in some D–A dyads can be attributed to the more extensive d-orbitals of ruthenium,13 which lead to increase direct or indirect spatial overlap with the acceptor.14 It seems that this difference in donor strength between ferrocene and ruthenocene is most evident when η6-fulvene is a significant resonance form. These findings stimulated us to investigate in detail the electrochemical and optical properties of ruthenocene-fullerene dyads.
In the work described here, two new ruthenocene-C60 (Rc∼C60) dyads 3 and 4 were synthesized with different linking bridges to the fullerene cage. In case of dyad 3, a pyrazoline ring was used whereas a pyrrolidine ring was incorporated in dyad 4. The photophysical behavior of both dyads was investigated using different photochemical techniques in polar and non-polar solvents.
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Scheme 1 (i) 4-Nitrophenylhydrazine and (ii) NBS, C60 and Et3N. |
On the other hand, pyrrolidino[60]fullerene system 4 was prepared in 26% yield by a 1,3-dipolar cycloaddition reaction18 between the ruthenocenecarboxaldehyde (1), α-aminophenyl- acetic acid and C60 in chlorobenzene under microwave irradiation19 at 210 W for 45 minutes in a focused microwave oven (Scheme 2). 1′-(4-Nitrophenyl)-3′-(ferrocenyl)pyrazolino [60]fullerene (5) (Chart 1) was prepared as previously described.23
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Scheme 2 |
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Chart 1 Reference compounds 5 and 6. |
The structures of dyads 3 and 4 were confirmed by spectroscopic analysis (NMR, FT-IR and MS). The 1H NMR spectra of C60 derivatives exhibited all expected signals for the organic addends. In the case of compound 3, the 4-nitrophenyl group signals were observed between 8.00 and 8.50 ppm and the ruthenocene protons appeared as a singlet around 4.50 ppm for the unsubstituted cyclopentadienyl ring and as two multiplets, between 4.50 and 5.50 ppm, for the C5H4 moiety. Similar characteristic signals for the ruthenocene moiety were also observed in the 1H NMR spectrum of compound 4, together with signals for the pyrrolidine ring, which appear as singlets at 5.91 and 5.58 ppm, and the signals for the protons of the phenyl group between 7.26 and 8.00 ppm. The 13C NMR spectra are also consistent with the proposed structures, with the signals from the ruthenocene fragment between 70.0 and 75.0 ppm and those due to the corresponding sp2 carbons of the sphere between 160.0 and 134.0 ppm. In addition, the two sp3 carbon signals for the functionalized C60 cage were observed at around 89.0 and 76.0 ppm. The MALDI-MS analyses for 3 and 4 revealed the molecular ion peaks to be in agreement with the calculated ones. Finally, in the UV-Vis spectra, measured in CH2Cl2, the characteristic absorption of C60 monoadducts was observed at around 430 nm.
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Fig. 1 Optimized geometries (B3PW91/LanL2DZ) of compounds 3 (left) and 4 (right). |
On the other hand, the lack of conjugation between the ruthenocene group and the pyrrolidine ring in compound 4 allows free rotation around the bond linking these units and the ruthenocene group is arranged away from the C60 spheroid. The distances (RCC) from the Rc center to the center of the C60 moiety were estimated to be 7.55 and 8.40 Å, respectively, for compounds 3 and 4.
The HOMO and LUMO of compounds 3 and 4 are depicted in Fig. 2. It can be seen that the LUMO is localized on the C60 moiety whereas the main contribution to the HOMO is from ruthenium d orbitals in 4 and the HOMO in dyad 3 spreads from ruthenocene to the para-nitrophenyl group. These orbital topologies indicate that the charge-separated state has a structure like Rc˙+∼C60˙−, but the positive charge in 3 is also partially supported by the pyrazoline ring and the para-nitrophenyl group attached to it. It should also be noted that there is a significant difference in the calculated HOMO–LUMO gaps, which are 1.86 eV and 2.30 eV for 3 and 4, respectively.
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Fig. 2 The HOMO and LUMO of dyads 3 (top) and 4 (bottom). |
The cyclic voltammogram (CV) of compound 4 in DCB/acetonitrile (4 ∶ 1) is shown in Fig. 3. Three reversible peaks can be seen in the negative potentials down to −2.5 V vs. Ag/Ag+ and these are attributed to the reduction potentials (Ered) of the C60 core. In the positive potential, an irreversible potential is observed at 0.71 V vs. Ag/Ag+ and this corresponds to the oxidation potential (Eox) of the Rc moiety. A similar cyclic voltammogram was obtained for 3, but a new non-reversible reduction wave at −1.82 V, assigned to the para-nitrophenyl moiety, also appeared. It can be seen that the Eox value for 3 is less positive by 0.13 V than that of 4, indicating the better electron-donor capability of the Rc connected to the pyrazoline ring. On the other hand, the Ered value of 3 is less negative than that of 4 by 0.10 V, indicating that the C60 moiety with the pyrazoline ring (3) is more electron-negative than that with pyrrolidine ring (4) as reported previously.17 The data obtained are summarized in Table 1 together with comparison data for C60, ruthenocene, formylruthenocene (1) and hydrazone 2.
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Fig. 3 Cyclic voltammogram of compound 4 in ortho-dichlorobenzene/acetonitrile (4 ∶ 1) (V vs. Ag/AgNO3; scan rate was 100 mV s−1). |
E 11/2(red) | E 21/2(red) | E 31/2(red) b | E 41/2(red) | E 1ox b | E 2ox b | HOMO–LUMO | |
---|---|---|---|---|---|---|---|
a V vs. Ag/AgNO3; GCE as working electrode; 0.1 M (n-C4H9)4NClO4; scan rate was 100 mV s−1. b Irreversible. c Assigned to the para-nitrophenyl group. | |||||||
1 | 0.68 | 1.17 | |||||
2 | −1.78c | 0.61 | |||||
3 | −1.01 | −1.41 | −1.82c | −1.96 | 0.58 | 1.42 | 1.59 |
4 | −1.11 | −1.52 | −2.12 | 0.71 | 1.82 | ||
C60 | −0.99 | −1.40 | −1.92 | ||||
Rc | 0.73 |
The experimental HOMO(Rc)–LUMO(C60) gap, determined as the difference between Ered and Eox, is significantly lower for 2-pyrazoline-based dyad 3 (1.59 eV) than that for pyrrolidine derivative 4 (1.82 eV) according to theoretical calculations. This difference suggests an easier photoinduced electron transfer in 3 than in 4.
From the first Eox and Ered values, the free-energies (ΔGRIP) of the radical ion-pairs of Rc˙+∼C60˙− were calculated using the Rehm–Weller equation (eqn (1)).21
ΔGRIP = Eox(Rc) − Ered(C60) − ΔGS | (1) |
ΔGS = e2/(4πε0εRRD–A) | (2) |
Compound | Solvent | ΔGRIP/eV | ΔGCS/eV via1C60* | ΔGCS/eV via3C60* |
---|---|---|---|---|
a E 00 = 1.75 eV for 1C60*, and E00 = 1.50 eV for 3C60*. | ||||
3 | PhCN | −1.38 | −0.37 | −0.15 |
Toluene | −1.97 | 0.22 | 0.44 | |
4 | PhCN | −1.61 | −0.14 | 0.08 |
Toluene | −2.20 | 0.45 | 0.67 |
From these ΔGCS values, the charge-separation process of 3 and 4via the excited singlet state of C60 (1C60*) is sufficiently exothermic in benzonitrile. In toluene, the CS process seems to be slightly endothermic, taking into consideration that the Weller model in toluene is likely an oversimplification, and the exact values quoted above are probably not completely reliable. The charge-separation process via the excited triplet state of C60 (3C60*) is exothermic for 3 in PhCN, but endothermic in toluene.
−ΔGCS = ΔGCR − E00 | (3) |
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Fig. 4 (top) Steady-state absorption spectra of compounds 3 (a, ![]() ![]() |
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Fig. 5 Steady-state fluorescence spectra of (top) dyad 3 and (bottom) dyad 4 in different organic solvents; λex = 400 nm. |
The emission peak at 653 nm is typical of a pyrazolino-C60 moiety attached to the Rc moiety. In polar solvents, these emission intensities were almost completely quenched compared with those in toluene. These observations suggest efficient quenching of the singlet excited state of the C60 moiety by the appended Rc moieties in polar solvents. The close spatial separation between the fullerene and the Rc group is expected to lead to efficient deactivation of the fullerene singlet excited state. In toluene the 720 nm emission of 3 was weak compared with that for 4, suggesting a weak quenching process for 3 even in toluene.
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Fig. 6 Time resolved spectra of (top) dyad 3 and (bottom) dyad 4 in different solvents; λex = 400 nm. |
In these experiments a streak-scope was used as the detector. The observed emission spectra in toluene are similar to those of the steady-state measurements, supporting the reliability of our measurements for these weak emissions. For compound 3 in DCB, the emission band at 653 nm disappeared but the remaining 700–720 nm emission is consistent with the steady-state measurement thus providing evidence of efficient quenching in polar solvents.
The fluorescence decay-time profiles at 700 nm for dyads 3 and 4 are shown in Fig. 7. The fluorescence time profile of the reference C60 exhibited a single exponential decay with a lifetime (τf0) of 1.3 ns. The attachment of donor moieties introduces a new quenching pathway for the lowest excited singlet state of the C60 moieties in dyads 3 and 4. In polar and non-polar solvents, the recorded fluorescence lifetimes monitored at 720 nm for both dyads are shorter than the (τf0) value, suggesting that the fluorescence quenching occurs from the 1C60* moiety. The fluorescence decay time profile can be fitted with two exponential components for 3, from which two lifetimes can be evaluated (see Table 3).
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Fig. 7 Fluorescence decay profiles monitored at 720 nm of (top) 3 and (bottom) 4; λex = 400 nm. |
Compound | Solvent | τ FS/ps | k CS/s−1 | Φ CS |
---|---|---|---|---|
3 | PhCN | Very fast | >1.0 × 1010 | >0.90 |
DCB | 200(65%) 1550(35%) | 4.1 × 109 | 0.83 | |
Toluene | 750(70%) 2000(40%) | 5.6 × 108 | 0.42 | |
4 | PhCN | 150 | 5.8 × 109 | 0.87 |
DCB | 400 | 1.6 × 109 | 0.66 | |
Toluene | 700 | 5.9 × 108 | 0.41 |
The short lifetimes (τf) are in the 200 and 420 ps range in DCB and toluene, respectively. In PhCN, the fast part of the process was too fast to record. For compound 4, the lifetimes were evaluated by curve-fitting with a single exponential and gave values in the range 150–620 ps. The fluorescence lifetimes became shorter on increasing the solvent polarity, suggesting that the quenching process is due to the charge separation from the donor moieties to the C60 moiety to yield the charge-separated (CS) state. The rate constants for charge separation (kCS) were calculated using the eqn (4):
kCS = 1/τf − 1/τf0 | (4) |
ΦCS = (1/τf − 1/τf0)/1/τf | (5) |
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Fig. 8 Transient absorption spectra of compound 3 (0.1 mM) obtained by 355 laser light photolysis in Ar-saturated (top) PhCN and (bottom) toluene. |
The time profiles in the inset in Fig. 8 (top) show that the decay processes in the near-IR region give a decay rate constant of k(1020 nm) = 9.6 × 106 s−1, which is attributed to the charge-recombination rate constant (kCR) of Rc˙+∼C60˙−. The inverse of the kCR value gave the lifetime (τRIP) of Rc˙+∼C60˙− to be 100 ns in PhCN at room temperature. The decay-time profile at 720 nm shows dual decays; the initial decay rate is in good agreement with the kCR value, suggesting the absorption of Rc˙+∼C60˙− is overlapping in this region. The difference transient absorption spectrum obtained subtracting the transient spectrum at 500 ns from that at 50 ns is all attributed to Rc˙+∼C60˙−. The slow decay at 720 nm can be attributed to the lifetime of Rc∼3C60*. In toluene (Fig. 8 (bottom)) the predominant absorption peak appeared at 720 nm and this is due to the 3C60* moiety. The rapid decay at 720 nm can be attributed mainly to rapid decay of the 3C60* moiety due to the heavy atom effect.
The transient absorption spectra of 2 observed by exciting the Rc moiety with 532 nm light in PhCN are shown in Fig. 9, which exhibited the broad absorption spectra with maxima at 640, 950 and 1200 nm in the 50 ns spectrum. These bands might be attributed to the radical ion-pairs (Rc˙+∼PhNO2˙−). The lifetime of the radical-pairs was evaluated as 20 ns which is shorter than that of 3.
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Fig. 9 Transient absorption spectra of compound 2 (0.1 mM) obtained by 532 laser light photolysis in Ar-saturated PhCN. |
In the case of 5, the transient absorption spectra (supporting information; Fig. S2†) exhibited only the weak absorption band of triplet C60 at 720 nm without evidence of the forming of radical ion-pairs. This suggests that the charge recombination after efficient charge separation is fast, because of stronger donor ability of the Fc moiety. On the contrary, the charge-recombination of the Rc derivatives is slow, because the charge-recombination process may belong to the Marcus inverted region24 due to sufficiently negative ΔGRIP values in Table 2.
The transient absorption spectra of compound 4 in PhCN are shown in Fig. 10 (top). In the spectrum at 50 ns, the strong absorption appearing at 720 nm is due to the 3C60* moiety. In comparison to dyad 3, the absorbance of 4 at 720 nm is stronger by a factor of 10, suggesting the predominant generation of the 3C60* moiety. In the near-IR region, the broad absorption tail seems to be extended, although the relative intensity of the near-IR band of compound 4 is weak compared with that at 720 nm. The rapid-decay in the near-IR band can be attributed to Rc˙+∼C60˙−. On the other hand, the band at 720 nm can be attributed to the 3C60* moiety, although the decay was very fast, probably due to the heavy atom effect of Rc.
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Fig. 10 Transient absorption spectra of compound 4 (0.1 mM) obtained by 355 laser light photolysis in Ar-saturated (top) PhCN and (bottom) toluene. |
In the case of 4 in toluene, similar transient absorption spectra were observed and these are shown in Fig. 10 (bottom). Although the absorption shape is quite similar to that in PhCN, the time-profiles were quite different. The decay rates at 720 nm and the near-IR region were quite slow, which suggests that these bands are due to the 3C60* moiety. Indeed, the decay was accelerated on addition of O2 and this provides further evidence for this assignment. From the fluorescence quenching, the charge-separation takes place via the 1C60* moiety; however, the fast charge-recombination produces the 3C60* moiety.
In Table 4, the charge-recombination rate constants (kCR) of Rc˙+∼C60˙− evaluated from the decay at 1020 nm are summarized. The kCR value in PhCN is larger than that in toluene for 3. Lifetimes of the radical ion-pairs are as long as 100 ns in PhCN for both 3 and 4. Compared with 6 (kCR = 4.5 × 109 s−1 in PhCN),8 the kCR values for the Rc derivatives are quite smaller, probably because of the high hole trapping ability of the Rc moiety.
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Fig. 11 Energy diagram of 3 and 4 in benzonitrile. |
In the case of compound 3 in PhCN, the charge-separation process takes place mainly via1C60* moiety generating Rc˙+∼C60˙−. Minor contribution of forming Rc˙+∼C60˙− may arise via the electron migration (transfer) from Rc˙+∼PhNO2˙− to C60 to yield the final Rc˙+∼C60˙− in PhCN. As in Table 1, the energy level of Rc˙+∼PhNO2˙− is higher than that of Rc˙+∼C60˙−, because the reduction potential of PhNO2 group was more negative than that of C60 moiety. The charge-recombination of ion-pair, Rc˙+∼C60˙− may produce the ground states. In toluene, the charge-separation process of 3 is minor, because of the endothermic process; therefore, intersystem crossing takes place competitively with the charge-separation process. The charge-recombination of ion-pair, Rc˙+∼C60˙− may produce the 3C60* in addition to the direct production of the ground states.
In the case of dyad 4 in PhCN, the charge-separation process via the 1C60* moiety is a minor process. Similar results were observed for 6.8 The considerable 3C60* moiety is produced via intersystem crossing and via the charge-recombination process since the charge-separated state is higher than 3C60*. For the generation of 3C60* moiety in toluene, the contribution of the intersystem crossing increases compared with that of the charge-recombination process, if any.
The UV/Vis spectra were obtained using a Jasco model V570 DS spectrophotometer or a Shimatzu spectrophotometer. Steady-state fluorescence spectra were measured on a Shimadzu RF-5300 PC spectrofluorophotometer equipped with a photomultiplier tube with high sensitivity in the 700–800 nm region.
Cyclic voltammetry measurements were carried out on an Autolab PGSTAT 30 potentiostat using a BAS MF-2062 Ag/0.01 M AgNO3, 0.1 M (n-C4H9)4ClO4 in acetonitrile reference electrode, an auxiliary electrode consisting of a Pt wire, and a Metrohm 6.1247.000 conventional glassy carbon electrode (3 mm o.d.) as a working electrode directly immersed in the solution. A 10 mL electrochemical cell from BAS, Model VC-2, was also used. The reference potential was shifted by 290 mV towards a more negative potential compared with the Ag/AgCl scale. E1/2 values were taken as the average of the anodic and cathodic peak potentials. Scan rate: 100 mV s−1.
The picosecond time-resolved fluorescence spectra were measured by a single-photon counting method using a second harmonic generation (SHG, 410 nm) Ti:sapphire laser (Spectra-Physica, Tsunami 3950-L2S, 1.5 ps fwhm) and a streak-scope (Hamamatsu Photonics, C43334-01) equipped with a polychromator (Action Research, SpectraPro 150) as an excitation source and a detector, respectively. Lifetimes were evaluated with software attached to the equipment.
The nanosecond transient absorption measurements in the near-IR region were measured by means of laser-flash photolysis; 532 nm light from a Nd:YAG laser (Spectra-Physics and Quanta-Ray GCR-130, 6 ns fwhm) was used as an excitation source. For transient absorption spectra in the near-IR region (600–1200 nm), monitoring light from a pulsed Xe-lamp was detected with Ge-avalanche photodiode module (Hamamatsu Photonics, B2834). For spectra in the visible region (400–800 nm), a Si-PIN photodiode (Hamamatsu Photonics, S1722-02) was used as a detector. All the samples were placed in a quartz cell (1 × 1 cm) and were deaerated by bubbling Ar through the solution for 15 min.
The computational calculations were performed by ab initio B3LYP/3-21G(*) methods with the GAUSSIAN 03 software package on high-speed computers. The images of the frontier orbitals were generated from Gauss View-03 software.
Footnote |
† Electronic supplementary information (ESI) available: copies of 1H NMR, 13C NMR and MS spectra for compounds 2, 3 and 4 and transient absorption spectra of 5 in toluene and benzonitrile. See DOI: 10.1039/b509221c |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006 |