Pyrene-Fullerene C60 Dyads as Light-Harvesting Antennas

A series of pyrene-fullerene C60 dyads bearing pyrene units (PyFC12, PyFPy, Py2FC12 and PyFN) were synthesized and characterized. Their optical properties were studied by absorption and fluorescence spectroscopies. Dyads were designed in this way because the pyrene moeities act as light-harvesting molecules and are able to produce “monomer” (PyFC12) or excimer emission (PyFPy, Py2FC12 and PyFN). The fluorescence spectra of the dyads exhibited a significant decrease in the amount of pyrene monomer and excimer emission, without the appearance of a new emission band due to fullerene C60. The pyrene fluorescence quenching was found to be almost quantitative, ranging between 96%–99% depending on the construct, which is an indication that energy transfer occurred from one of the excited pyrene species to the fullerene C60.


Introduction
Photovoltaics based on fullerene C 60 derivatives is one of the most active research areas in materials science due to the great performance of these molecules in emerging technologies for solar energy conversion [1][2][3][4][5][6]. However, the main disadvantage of fullerene C 60 and other carbon allotropes is their poor solubility in organic solvents [7]. Fullerene C 60 also mainly absorbs in the UV region of the electromagnetic spectra [7]. Since a wide absorption range (UV and visible) is desirable for organic photovoltaics [1], several strategies to covalently and non-covalently modify fullerene C 60 have been employed. Among these, the incorporation of porphyrins [8], TTF [9,10], and azobenzenes [11] has been tested. One of the aims of any fullerene modification is to confer solubility in organic solvents and to improve its absorption in the UV-Vis region without affecting the photophysical and electrochemical properties of the raw fullerene [7]. Axial chromophores attached to fullerene C 60 also serve to produce long life charge separated species in a fullerene C 60 cage [12,13] and to isolate fullerene from the environment [14]. Fullerene-C 60 hybrid derivatives have a wide range of applications in other fields such as chemosensors [15], biological probes [16], senzitizers and photocatalysis [17][18][19][20][21][22][23], etc. In addition, testing of chemical and non-covalent modification of fullerene C 60 can be scaled to other carbon allotropes such as carbon nanotubes (CNT) and graphene [24,25]. Despite the great variety of fullerene dyads mentioned in the literature, relatively little information about pyrene-fullerene C 60 hybrids can be found [26][27][28][29][30][31][32]. Pyrene is a very attractive chromophore due to its intrinsic optical properties such as high quantum yield, long luorescence lifetime and its ability to form excimers [33]. Pyrene has been claimed to be "by far, the most studied fluorescent probe in macromolecules" [33] and for this reason is a very useful analytical probe to test different dynamic processes in solution which occur in the pyrene lifetime regime [34][35][36][37]. Other interesting and emerging qualities of pyrene are its application as an antenna chromophore in dyads for solar energy conversion applications and as exfoliation agent for CNT and graphene dispersions [38].
Our research group has explored the photophysical properties of some pyrene model compounds [39]. We have also incorporated pyrene into several macromolecular architectures in order to design functional materials with interesting optical behaviors [40,41]. In our interest to study pyrene dynamics in macromolecules, in a previous work, we studied the photophysical properties of a new family of dendronized porphyrins labelled with pyrene units [42]. Thus, we reported a very efficient Fluorescence Resonance Energy Transfer (FRET) process from the excited state pyrene units to the ground state porphyrin. Taking advantage of the photophysical characteristics of this chromophore we were able to determine the quantitative quenching of fluorescence due to FRET, but we also obtained valuable information about pyrene dynamics, namely, the precluded formation of pyrene excimers due to the most competitive FRET phenomenon with higher rate constant. However, as dendron generation increases the excimer formation mechanism is favored and becomes a competitive process with FRET. Therefore, we concluded that pyrene acts as an efficient donor for FRET when it is combined with a porphyrin. Furthermore, pyrene can donate energy in a dual mode: monomer or excimer emission, depending on the molecular structure of the macromolecule [42].
In this work, we extended the study of pyrene excimer formation to hybrid materials that exhibit a complex photophysical behavior. It is worth pointing out that pyrene and fullerene C 60 show a very small spectral overlap between pyrene emission and fullerene absorption, which precludes the possibility of an efficient FRET from pyrene to fullerene C 60 . Nevertheless, it was observed that efficient photoinduced energy transfer takes place [27,28]. The main goal of this project is the synthesis and characterization of novel pyrene-fullerene C 60 dyads as well as the study of the resulting energy transfer process by means of steady-state fluorescence.

Synthesis of Pyrene-Fullerene C 60 Dyads
Fulleropyrrolidine Py 2 FN was synthesized by reacting 3,5-bis(4-(pyren-1-yl)butoxy)benzaldehyde and N-methylglycine (sarcosine) in refluxing toluene (Scheme 1). Py 2 NF possesses one fullerene unit and two axially attached pyrene units. The non-functionalized homologue compound Py 2 OH has been reported [42] to very efficiently produce pyrene-type excimer emission via diffusion of pyrenes in the media, as it was demonstrated by steady-state fluorescence and excitation spectra. Inclusion of the fullerene C 60 moiety in the Py 2 NF derivative is supposed to generate different pyrene fluorescence deactivation pathways which are able to compete with excimer formation. The aim of this work was to elucidate at which degree the incorporation of fullerene C 60 can preclude excimer formation phenomena. Fulleropyrrolidine was characterized by NMR spectroscopy (Figure 1) and the structural elucidation was confirmed by means of mass spectrometry using the MALDI-TOF technique with α-cyano-4-hydrocinnamic acid as matrix. The mass spectrometry analysis showed a peak corresponding to the molecular ion M + with m/z = 1,398.024, which matches well with the calculated value (M + m/z = 1398.51).
It is worth noticing that the PyFPy, Py2NF and Py 2 FC 12 dyads are able to form excimers via diffusion of pyrene units, however, in the PyFPy molecule pyrene diffusion is blocked by the substitution of the protons in the malonate bridge by a fullerene C 60 cage as it was reported in the literature [26]. In contrast, compounds Py 2 NF and Py 2 FC 12 readily form excimers, since the fullerene C 60 unit is not in the diffusive pathway of the pyrene moieties. By tuning the excimer formation mechanism via well-controlled structural characteristics of the dyads, we can study the dynamics of excimer formation and also the possible deactivation pathways of this process, namely FRET or charge transfer phenomena.

Absorption Spectra of Pyrene-Donor Molecules and Pyrene-C 60 Dyads
Absorption spectra in toluene solution of PyMPy, PyMC 12 and Py 2 MC 12 ( Figure 2) exhibited the typical absorption bands of pyrene-derivatized compounds with a maximum absorption band at  = 346 nm due to the S 0 S 2 transition. According to Duhamel et al. [34−37] if the peak-to-valley ratio at the S 0 S 2 transition band shows a value lower than 2.5 there is pyrene preassociation in the ground state which leads to the formation of static excimers. Inspection of the electronic spectra shows that the numerical values of peak-to-valley ratio for the S 0 S 2 transition (A 346 /A 336 ) of PyMPy, PyMC 12 and Py 2 MC 12 are lower than 2.5. Therefore, there is weak pyrene preassociation in the ground state for these molecules. It is important to highlight this feature because it suggests that if the fluorescence spectra shows excimer-type emission, the nature of the excimer formation would rise from pyrene diffusion in media, i.e., a dynamic process [34−37]. On the other hand, absorption bands at 346 nm do not show remarkable shift respect to pyrenebutanol, used as model compound, which show that the electronic properties of pyrene moieties in PyMPy and PyMC 12 remain alike to those of pyrenebutanol.
The electronic spectra of dyads PyFPy, PyFC 12 and Py 2 FC 12 ( Figure 3) in toluene solution exhibited similar spectral features to those of pyrenebutanol on the top of the fullerene C 60 absorption background. PyFPy dyad shows an intense absorption band at 330 nm due to the high extinction coefficient of the fullerene C 60 cage, followed by a band at 346 nm attributed to the pyrene moieties as well as an extended absorption in the visible region from 400-700 nm, which is entirely attributed to fullerene C 60 cage. It is noticeable the extremely low extinction coefficient of the absorption at 682 nm due to forbidden S 0 S 1 transition of fullerene. In contrast, in PyFC 12 and Py 2 FC 12 the pyrene bands are more resolved due to the higher pyrene local concentration in the molecules. These compounds showed similar features as those discussed above. A unique pyrene unit is present in PyFC 12 , this precludes the possibility of intramolecular excimer formation. On the contrary, PyFPy and Py 2 FC 12 which contain two pyrene units in their structure can lead to excimer formation. However, in PyFPy pyrene interactions are blocked by the presence of the fullerene moiety. Surprisingly, excimer formation is still possible in Py 2 FC 12 since the fullerene unit is not in the middle of the pyrene diffusion pathway. It is remarkable that the peak shifts of the fullerene and pyrene transitions were not noticeable, suggesting that there is no remarkable electronic interaction between both chromophores, as discussed earlier; this trend was also confirmed by NMR spectroscopy.

Steady-State Fluorescence of Pyrene-Donor Molecules and Pyrene-C 60 Dyads
Fluorescence spectra of donor molecules PyMPy, pyrenebutanol and Py 2 OH were acquired in toluene solution at room temperature, exciting at 344 nm. The steady-state emission spectra of PyMPy and Py 2 OH show the characteristic pyrene monomer emission at 370 nm, followed by a broad excimer emission band centered at 478 nm. For PyOH, the fluorescence spectra show a uniquely monomer emission at 370 nm. The excimer emission intensity (I E ) relative to that of the monomer emission (I M ), namely the I E /I M ratio, increases with the number of pyrene groups in the construct from 0.6 for Py 2 OH and 2 for PyMPy. The compound PyMPy showed significantly higher excimer emission and lower monomer emission than Py 2 OH, even if they present the same local pyrene concentration. This is because PyMPy has a more flexible backbone and shows a better ability to diffuse in the media to efficiently form excimers. The nature of the excimers for Py 2 OH and 2 for PyMPy appears to be dynamic, since the absorption spectra overlap that of 1-pyrenebutanol. In addition, the excitation spectra of the pyrene monomer and excimer acquired at 398 and 478 nm, respectively, overlap for both constructs (Figure 4). The nature of the excimer formation was elucidated by excitation spectra of the donor molecules PyMPy and Py 2 OH. Analysis of the peak-to-valley ratio of S 0 S 2 transition in the electronic spectra suggested that the pyrene units are not pre-associated in the ground state. Thus, the excimers are dynamic in nature. This was confirmed by excitation spectra of the donor PyMPy and Py 2 OH recorded at the monomer (390 nm) and excimer (470 nm) emission. Since excitation spectra were identical in both samples for the monomer (390 nm) and excimer (470 nm) emission the dynamic nature of the excimers has been confirmed [34−37]. The fluorescence spectra of PyFC12, Py2FC12, PyFPy and Py2NF were acquired in toluene solution at room temperature exciting at 344 nm. Figure 5 shows the emission spectra of the pyrenelabeled constructs. They exhibited typical monomer-type pyrene fluorescence at 390 nm. Since fluorescence emission of the dyads is low, the emission profile is altered by the dispersion peaks of the solvent at the excitation wavelength. Previously, we reported a similar behavior in pyrene dendronized porphyrins [42]. However, in the present study, accurate fluorescence profiles were not retrieved after solvent correction, due to a possible photobleaching of the pyrene moieties after excitation. Fluorescence emission is almost quantitavely quenched in all the dyads with quenching values over 96% ( Figure 6 and Table 1). It is remarkable that compounds Py 2 FC 12 and Py 2 NF exhibited residual excimer-type emission at ca. 470 nm. A very small I E /I M ratio was calculated for Py 2 FC 12 and Py 2 NF, 0.14 and 0.24, respectively. The quite low excimer emission would rise from small fraction of pyrenes diffusing in media and also from those pyrene units whose fluorescence are not deactivated by fullerene units. It is interesting to remark the fact that the donor molecule PyMPy exhibited a high I E /I M ratio which suggests a very efficient dynamic excimer formation. In contrast, PyFPy does not show any excimer emission, due the presence of the fullerene unit which prevents the pyrene diffusion pathway. As could be expected, PyFC 12 does not exhibit any excimer emission.    After excitation at 344 nm in toluene solution, where absorption of pyrene and fullerene C 60 is observed, the pyrene emission at 390 nm is almost quantitatively quenched and no emission from the fullerene C 60 moiety was detected, even at 708 nm where the derivatized-fullerene emission usually appears. We carried out a comparison of the fluorescence of the fullerene C 60 (quantum yield lower than 0.1%) [21] and the obtained dyads and we observed that the emission of the latter is significantly lower. This dramatic quenching in toluene, which is a relatively low dielectric constant solvent, indicates that the fluorescence deactivation pathway is mainly due to a fast energy transfer from excited pyrene to fullerene C 60 . A similar behavior was observed in a similar fullerene C 60 dendritic molecule bearing 8 pyrene moieties previously reported in the literature [32]. The geometry of the molecule allows the - interactions of the pyrene moieties with the fullerene cage thereby favoring energy transfer mechanism.

General
All the reagents involved in the synthesis were purchased from Aldrich (Mexico D.F.) and employed as received. The solvents used in the reactions were purified by simple distillation. 3,5-bis-Dodecylbenzyl alcohol [43], monomalonyl ester C 12 MCOOH [43] and 3,5-bis(4-(pyren-1yl)butoxy)-benzyl alcohol [42] were obtained according to previously reported procedures. FTIR spectra of the intermediates, dendrons and pyrene-fullerene dendritic systems were recorded on a Spectrum 100 spectrometer (Perkin Elmer, Waltham, MA, USA) in solid state. 1 H and 13 C-NMR spectra of these compounds in CDCl 3 solution were recorded at room temperature on a Bruker Avance 400 MHz spectrometer (Billerica, MA, USA), operating at 400 MHz and 100 MHz for 1 H and 13 C, respectively. MALDI-TOF measurements were obtained in a Bruker-Microflex spectrometer. For UV-Vis and fluorescence spectroscopies, tetrahydrofuran (THF) was purchased from Aldrich (spectrophotometric grade). Prior to use, the solvent was checked for spurious emission in the region of interest and found to be satisfactory. The absorption spectra of the final compounds in solution were recorded on a Varian Cary 1 Bio UV/vis spectrophotometer (Palo Alto, CA, USA) using 1 cm quartz cells and solute concentrations of (1 × 10 −6 -3 × 10 −6 ) M for all compounds. It has been verified that the Beer-Lambert law applies for the used concentrations. Fluorescence spectra corrected for the emission detection were recorded on a Photon Technology International LS-100 steady-state fluorimeter (New Jersey, NJ, USA) having a continuous Ushio UXL-75Xe Xenon arc lamp (New Jersey, NJ, USA) and a PTI 814 photomultiplier detection system. Each solution was excited at 344 nm using a 1 cm quartz cell. For all compounds, a pyrene concentration of less than 1.25 × 10 −6 M was used to ensure that the solutions would have an absorbance of 0.05 at 344 nm and to avoid any inner filter effect [44].

General Procedure for DCC Esterification
DCC (2.2 equiv) was added to a stirred solution of monomalonyl acid (1 equiv), the appropriate alcohol (2 equiv), and DMAP (0.5 equiv) in CH 2 Cl 2 at 0 °C. After 12 h, the mixture was allowed to reach room temperature (about 1 h), then it was filtered and evaporated at reduced pressure. The crude product was then purified as outlined below.

General Procedure for Fullerene C 60 Dyads by Bingel-Hirsch Reaction
DBU (5 equiv) was added to a stirred solution of C 60 (1 equiv), I 2 (2.5 equiv) and the appropriate bismalonate (1.1 equiv) in toluene. The solution was stirred for 24 h, then filtered through a short plug of SiO 2 , eluting first with toluene (to remove unreacted fullerene C 60 ) and then with DCM:hexanes to yield the corresponding product.

Conclusions
The optical properties of pyrene model molecules PyMPy and Py 2 OH were studied by absorption spectroscopy and steady-state fluorescence. PyMPy and Py 2 OH form excimers very efficiently as it was demonstrated by steady-state fluorescence. Pyrene excimer formation is dynamic in nature which implies a diffusion pathway. In this work, pyrene was appended to a fullerene moiety cage using different motifs that allowed pyrene excimer formation via intermolecular diffusion. By employing this strategy absorption of the fullerene unit in the far UV region was increased. Pyrenes act as antennae molecules able to harvest UV radiation in a very efficient way. Also axially appended pyrenes are free to be used as binding motifs to disperse carbon allotropes as it has been previously reported in the literature. Furthermore, pyrene excimer diffusion was tested in this model compounds since molecular control of the pyrene dynamics was achieved by the incorporation of fullerene C 60 in different positions. In molecules PyFPy, Py 2 FC 12 and Py 2 NF, pyrene diffusion was possible in all cases, however, fullerene C 60 blocked dramatically the pyrene diffusion pathway in PyFPy. This phenomenon was evident since residual pyrene excimer-type emission was precluded in PyFPy. In Py 2 FC 12 and Py 2 NF pyrene excimer formation was favored since fullerene C 60 did not interfere in the pyrene diffusion pathway. This was also confirmed by the increase in the residual pyrene excimer-type emission.