Synthesis of mesogenic phthalocyanine-C60 donor–acceptor dyads designed for molecular heterojunction photovoltaic devices

A series of phthalocyanine-C60 dyads 2a–d was synthesized. Key steps in their synthesis are preparation of the low symmetry phthalocyanine intermediate by the statistical condensation of two phthalonitriles, and the final esterification of the fullerene derivative bearing a free COOH group. Structural characterization of the molecules in solution was performed by NMR spectroscopy, UV–vis spectroscopy and cyclic voltammetry. Preliminary studies suggest formation of liquid crystalline (LC) mesophases for some of the prepared dyads. To the best of our knowledge, this is the first example of LC phthalocyanine-C60 dyads.


Introduction
Among sustainable energy technologies, photovoltaic (PV) conversion of solar energy is considered as a promising solution. Although currently the market is dominated by inorganic PV devices, development of organic PV materials is driven by their compatibility with solution processing and hence the potentially low cost of large-scale production by printing technologies [1][2][3]. One of the most popular concepts in the design of organic PV devices is the "bulk heterojunction" architecture, featuring blends of the two immiscible materials: a donor and an acceptor of electrons [4][5][6][7]. After absorption of a photon, an initially formed exciton is dissociated at the donor/acceptor interface into a positive charge (hole) and a negative charge (electron), which are transported to the electrodes. Hence, a critical issue in bulk heterojunction PV devices is the control of morphology of materials, in order to provide both the efficient exciton generation and the rapid charge carrier transport. The logical step in the development of this architecture is "molecular heterojunction", that is, creation of covalent link- ages between donor and acceptor components [6]. The main idea is that in material well-organized on the molecular level, donor and acceptor moieties will create the separate pathways for the transport of holes and electrons, respectively. One can intuitively expect that for such high order of self-organization, liquid crystalline (LC) materials will be beneficial, because they combine order and fluidity, which allows the self-healing of structural defects [8].
Phthalocyanines (Pc) have found a number of industrial applications as dyes and pigments due to their bright blue or green colors combined with extraordinary thermal and photochemical stability [30]. Phthalocyanines bearing flexible peripheral substituents form columnar mesophases [8,31], which demonstrate very efficient charge transport along the columns [32,33]. A unique combination of properties makes phthalocyanines excellent candidates as active materials for photovoltaic devices.
Within the concept of a "molecular heterojunction", a number of phthalocyanine-fullerene dyads and triads were synthesized [34][35][36][37][38] and long-lived photoinduced charge transfer was demonstrated in these type of materials [39]. Although examples of mesogenic fullerene derivatives have been previously reported [40][41][42][43][44], mesogenic Pc-C 60 dyads are unknown, most probably due to the tedious purification of the unsymmetrically substituted phthalocyanines bearing long peripheral substituents. A possible elegant solution to this problem was demonstrated by Torres and co-workers, who obtained LC mesophases of several phthalocyanine-C 60 dyads by blending them with a mesogenic phthalocyanine derivative [45]. In this paper we report the synthesis of the first example, to the best of our knowledge, of mesogenic phthalocyanine-fullerene dyads.

Results and Discussion
The structure of dyads 2a-d is depicted in Figure 1. We decided to combine in one molecule the phenylcyclopropa [6,6]C 60 moiety of PCBM as an acceptor and a phthalocyanine bearing three swallow-tail alkoxy groups as a donor. The choice of the substituents on the phthalocyanine core was dictated by the previous finding that the pseudosymmetrical phthalocyanine 3 bearing four such substituents is very soluble in many organic solvents, forms columnar mesophases over a broad temperature range and, at the same time, possesses a reasonably low clearing point (ca. 180 °C) [46]. Finally, a critical issue in obtaining a mesogenic phthalocyanine-C 60 dyad can be a difficulty in accommodating a bulky C 60 molecule in the columnar LC Scheme 1: Synthesis of low-symmetry phthalocyanines 4a-d.
mesophases [45]. To this end, we envisaged the variation of the linker between the fullerene and the phthalocyanine unit.
The synthesis of dyads 2a-d is summarized in Scheme 1 and Scheme 2. The key step, which also represents a bottle-neck in terms of yield (see below), is the preparation of the lowsymmetry phthalocyanines 4a-d. To this end, we originally decided to explore the statistical condensation of phthalonitriles 5 and 6a-d (Scheme 1). Previously, synthesis of 4-alkoxyphthalonitriles such as 5 via aromatic nucleophilic substitution in 4-nitrophthalonitrile with the corresponding alcohols in the presence of LiOH in DMSO was reported [46]. However, the yields of phthalonitriles 5 were moderate (below 50%) and their separation from multiple side products was cumbersome. Hence, we preferred to prepare 5 (80% yield) by Williamson reaction between phenol 7 and bromide 8. The latter was synthesized from the corresponding commercially available alcohol by the well-known general method (CBr 4 /PPh 3 ) [47]. A similar procedure was then used to synthesize phthalonitriles 6a-d (80-86% yield) starting from the corresponding commercially available ω-bromoalcohols.
Several strategies for the synthesis of low-symmetry A 3 B phthalocyanines (where A and B refer to the chemically different isoindole units comprising the phthalocyanine core) were reported earlier, including expansion of subphthalo-cyanines or solid-phase synthesis on polymer supports [48]. However, operationally simplest and shortest route is the statistical condensation of the two different phthalonitriles. This method inevitably produces a mixture of the desired A 3 B phthalocyanines with the products of compositions A 4 , A 2 B 2 , AB 3 and B 4 . In theory, 3 : 1 molar ratio of the two dinitriles is required to access A 3 B products. However, using larger excess of one of dinitriles (often 9 : 1 stoichiometry is used) is more practical. Although the amount of the A 4 phthalocyanine in the reaction mixture grows, the formation of cross-condensation products other than A 3 B is essentially suppressed (see Figure  SI2 in the Supporting Information for the graphical presentation of the product distribution). Hence, the isolation of the desired A 3 B is limited to the separation from the A 4 . Another factor that facilitates the separation by chromatography is rather different polarity of the substituents on A and B units. In our case, the oligomethylene linker with a terminal OH group is much more polar than the bulky swallow-tail alkyl group, and we did not expect difficulties in the separation of condensation products.
However, attempted condensation of 5 and 6a-d gave no desired phthalocyanine derivative. We reasoned that the unprotected OH group in one of the reaction components may affect the condensation, and converted alcohols 6a-d into acetals 9a-d (84-93% yield) by treatment with an excess of meth- oxyethoxymethyl chloride (MEMCl) in the presence of i-Pr 2 EtN as a base. The general choice of acetal as a protective group was dictated by its excellent stability in the presence of the strong bases, required for the cyclotetramerization of phthalonitriles. Among various popular acetal protective groups, the relatively polar MEM moiety was chosen to facilitate the chromatographic separation of the unsymmetrical phthalocyanines 10a-d from the major side product 3: the latter bears only relatively apolar, bulky swallow-tail alkoxy groups. In addition, the signal of the terminal methyl group of the MEM moiety serves as a convenient "marker" in rather complex 1 H NMR spectra of phthalocyanines 10a-d (see below). Condensation of 5 and 9a-d in n-C 5 H 11 OH/n-C 5 H 11 OLi upon reflux afforded the desired A 3 B phthalocyanines 10a-d (13-24% after column chromatography) together with pseudo-symmetrical A 4 phthalocyanine 3 (35-51%). The seemingly modest yields of 10a-d actually correspond to those reported earlier for other A 3 B-type phthalocyanines: yields higher than 20% are rare in such reactions [45,[49][50][51]. The MEM protective group in 10a-d was then quantitatively removed to give 4a-d upon treatment with pyridinium tosylate (PPTS) in t-BuOH according to previously reported general method [52].
The structure of the phthalocyanines 4a-d was confirmed by 1 H and 13 C NMR, and HR-MALDI mass spectrometry. 1 H NMR spectra of low-symmetry phthalocyanines 4a-d and 10a-d in CDCl 3 closely resemble those of 3 and feature a series of three unstructured multiplets from the protons of 1,2,4-substituted benzene rings. Inner-core protons in 4a-d and 10a-d are observed as a broad and concentration-sensitive high-field signal. It should be noted that 3 (as well as any phthalocyanine derivative prepared from a 4-substituted phthalonitrile) represents a mixture of four regioisomers [48,53], while lowersymmetry 4a-d and 10a-d are actual mixtures of as many as eight regioisomers each. This greatly complicates their NMR spectra (see Supporting Information). Furthermore, each regioisomer is a mixture of different diastereoisomers due to the presence of the asymmetric carbon atom in every peripheral substituent. However, although 2-decyltetradecyl substituents in In the final synthetic step, reaction between 4a-d and the acid 11 (prepared by acid-catalyzed hydrolysis of the commercially available methyl ester 1, PCBM) [10,58] afforded the corresponding dyads 2a-d (Scheme 2). Although in general esterification appears as a trivial synthetic transformation, esterification of 11 is greatly complicated by its poor solubility in virtually all organic solvents. After some experimenting, we found that the classical dicyclohexylcarbodiimide/N,N-dimethylaminopyridine (DCC/DMAP) protocol gives the best results, provided the acid 11 was first sonicated in o-dichlorobenzene for two hours prior to reaction, and then an alcohol 4a-d, DCC and DMAP were added to the reaction mixture. After purification by column chromatography, the dyads 2a-d were isolated in yields up to 45%. Again, the relatively modest yields are comparable with or superior to those previously reported for the esterification of poorly soluble acid 11 with various alcohols [22,50,59].
The structure of dyads 2a-d was confirmed by NMR, MALDI MS and UV-vis absorption spectroscopy. 1 H NMR spectra of the dyads 2a-d essentially correspond to the superposition of the spectra of the phthalocyanine 4a-d and PCBM (1), with the exception of the signal of CH 2 OC=O in dyads 2a-d (ca. 4.10 ppm), which is shifted downfield vs. the signal of CH 2 OH in phthalocyanines 4a-d (ca. 3.80 ppm). 13 C NMR spectrum of the dyad 2c also essentially represents a superposition of the spectra of phthalocyanine 4c and PCBM (1). However, its detailed interpretation is greatly complicated because of a large number of signals, broadening of the signals due to the complex isomeric composition of phthalocyanine (see above), and overlaps of signals (see Supporting Information for the original NMR spectra). For this reason, only selected 13 C chemical shifts are given in the Experimental Part. UV-vis absorption spectra of the dyads 2a-d also appear as superposition of the spectra of 4a-d and 1: next to the intense Q-band (maxima at 670 and 707 nm) and B-band (between ca. 300 and ca. 450 nm) of phthalocyanine moiety, very strong high energy absorption of the cyclopropanated C 60 derivative with the maximum at ca. 260 nm is observed (Figure 2). Cyclic voltammetry measurements of dyads 2a and 2d in CH 2 Cl 2 produced essentially identical results. Wave potentials do not change upon elongation of the spacer between the donor and the acceptor (Table 1). Furthermore, the voltammograms effectively correspond to the superposition of those for PCBM (1) and the phthalocyanine 3, and do not display supplementary waves (Figure 3). The reduction part of the voltammograms of 2a,d displays five quasi-reversible reduction waves, three due to the C 60 moiety and two due to the phthalocyanine fragment. In the oxidation part of the voltammograms of 2a,d, several poorly resolved waves identical to those of phthalocyanine 3 are observed at potentials up to +1.6 V vs. saturated calomel electrode (SCE). These observations, together with UV-vis spectra, suggest the absence of intramolecular charge transfer in the ground state for dyads 2a-d.
Preliminary investigation of the dyads 2a-d by polarized optical microscopy (POM) showed that the length of the spacer between the phthalocyanine and the fullerene moieties significantly affects the thermotropic properties of the material. Dyads 2a,b are liquid at room temperature, while dyads 2c,d display at room temperature fluidic birefringent textures confirming the occurrence of liquid crystalline (LC) mesophases. Upon heating, transitions to isotropic liquid were observed at 120 °C and 90 °C for 2c and 2d, respectively. For 2c this transition was only detectable by POM and not by differential scanning calorimetry (DSC), while for 2d DSC on heating revealed an endothermic peak at 90 °C [enthalpy 2.7 kJ/mol] in perfect agreement with POM observation. The small enthalpy value supports the transition from LC mesophase to isotropic liquid. However, assignment of the nature of LC mesophases for 1c,d from the available data was not obvious, since the textures observed by POM were not characteristic. The major conclusion is nonethe-  less that the length of the spacer in 2a-d between the C 60 fragment and the mesogenic phthalocyanine moiety decisively affects the supramolecular structure of the material.

Conclusion
We report the first example, to the best of our knowledge, of mesogenic phthalocyanine-C 60 dyads. The key step in their synthesis is the preparation of the low-symmetry phthalocyanines, bearing three mesogenic swallow-tail substituents and an OH-terminated polymethylene linker. These key intermediates were synthesized by experimentally simple statistical condensation of two phthalonitriles, giving comparatively high yields (up to 24%) for this type of reaction. Upon the favorable combination of the length of the linker with the length of peripheral substituents on the phthalocyanine, bulky C 60 moiety can be accommodated in the LC mesophase. Detailed analysis of this supramolecular organization, as well as deeper insight in the structure-property relationships of dyads 2, will be the subject of a separate publication. Other objectives of our ongoing research include: studies of macroscopic alignment in films of dyads 2, studies of charge carrier mobility in aligned films, and fabrication and evaluation of photovoltaic devices.

Experimental
General details. All chemicals were purchased from Aldrich, Acros or TCI and used without further purification. Acid 11 was prepared according to the published procedure [10]. TLC was performed on precoated plates with silica gel 60 F 254 (Merck), visualization with UV (λ = 254 nm). Column chromatography was performed on silica gel (0.040-0.063 mm, Macherey-Nagel). 1  Electrochemistry. Cyclic voltammetry experiments were performed with a computer controlled Autolab potentiostat. Measurements were carried out at room temperature in a threeelectrode single-compartment cell (10 mL), in CH 2 Cl 2 solutions (concentration 10 −4 M), containing Bu 4 NPF 6 (0.1 M) as supporting electrolyte, at a scan rate of 100 mV s −1 . A glassy carbon, polished by a slurry-suspension of alumina on microcloth and washed by Milli-Q water before each experiment, was used as a working electrode. A spiral platinum wire was employed as counter electrode and an Ag/AgCl/KCl(sat) used as reference electrode was connected to the cell solution via a salt bridge containing a saturated aqueous KCl solution. The Ag/AgCl electrode was checked against the ferrocene/ ferrocinium (Fc/Fc + ) couple (E°F c/Fc+ = 0.425 V vs. Ag/AgCl) before and after each experiment. All potentials are reported versus saturated calomel electrode (SCE) (E°F c/Fc+ = 0.405 V vs. SCE). Before each measurement, solutions were deaerated by 20 min nitrogen bubbling.

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The license is subject to the Beilstein Journal of Organic Chemistry terms and conditions: (http://www.beilsteinjournals.org/bjoc) The definitive version of this article is the electronic one which can be found at: doi:10.3762/bjoc.5.49