CC BY
10Фталоцианины Phthalocyanines
МаКрОГЭТЭрОЦМКЛЫ
http://macroheterocycles.isuct.ru
Перспективы Highlight
Towards Collective Physical Properties in Supramolecular Organized Phthalocyanine-C60 Fullerene Conjugates®
Giovanni Bottari@1 and Tomás Torres@2
Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco 28049 Madrid, Spain @lCorresponding author E-mail: [email protected] @2Corresponding author E-mail: tomas. [email protected]
The organization of phthalocyanine-C60 fullerene conjugates is emerging as a powerful tool for the development of novel collective physical properties. This highlight presents the pioneering approaches on this topic.
Keywords: Donor-acceptor systems, fullerene, phthalocyanine, supramolecular organization.
Phthalocyanines (Pcs)[1,2] are aromatic macrocycles which present a rich redox chemistry coupled to an intense absorption in the red/near infrared (IR) region of the solar spectrum with high extinction coefficients and fluorescence quantum yields.
The unique physicochemical features of these macrocycles make them ideal molecular components for the preparation of photoactive, donor-acceptor (D-A) ensembles. In such systems, the Pcs' role is twofold: first, they function as antennas, due to their excellent optical absorption in the visible region of the emission solar spectrum; and second, once photoexcited, they act as an electron donor.[3,4]
Among the acceptor moieties that have been chosen as molecular partner for Pcs, C60 fullerene has a privileged position. This spherical carbon nanostructure in fact possesses an extraordinary electron acceptor property,[5-7] which coupled with its small reorganization energy and its ability for promoting fast charge separation and slow charge recombination, have prompted its incorporation in Pc-based materials.
In this context, a few molecular and supramolecular architectures incorporating both Pc and C60 fullerene moieties have been prepared[8-10] and the photophysical properties of these systems studied both in solution[11,12] and in the solid state. In the majority of the cases the formation of long-lived charge-separated species was observed for these photo- and electroactive systems, thus paving the way for the utilization of these materials as active components of devices such as organic solar cells.[13-15]
However, the successful utilization of these systems into devices is strongly related to the possibility to control the spatial arrangement of these molecules with respect to each other, since a high degree of order of these molecules at the molecular level is often accompanied by an improvement in the device's performance.
To achieve this goal then, it is necessary to carefully design and prepare Pc-C60 systems capable to self-organize
spontaneously into highly-ordered supramolecular structures.
Although Pcs, owing to their flat aromatic surface, can spontaneously interact with each other by n-n stacking interactions,[16-20] the formation of long-range, ordered Pc-based systems, either in the term of discrete supramo-lecular architectures or infinite ill-defined aggregates, still remains a challenging task[21,22] and often requires the introduction of additional recognition motifs such as the cation complexation as in crowned-Pcs,[23-25] the use of hydrogen-bonding[26-30] and metal-ligand[31,32] interactions and/or the introduction of adequate substituents at the Pc periphery to promote a mesomorphic behavior.[33-36]
Somehow more complicate is the case when such organization is sought for Pc-C60 conjugates, probably due to the presence of the bulky spherical carbon moiety, as reflected by the scarce number of reports on this topic.
Up to date, few examples have been reported in which a Pc-C60 dyad has been organized through supramolecular interactions.
The first example is represented by Pc-C60 dyad 1 (Scheme 1).[37] The synthesis of this dyad starts with a statistical crossover condensation of 4-tert-butylphthalonitrile and 4-hydroxymethylphthalonitrile (the latter one prepared via reduction of 4-formylphthalonitrile)
® This contribution is dedicated to professor Vasilij Fedorovich Borodkin on the оccasion of his 100th Anniversary. ® Статья посвящена 100-летнему юбилею профессора Василия Фёдоровича Бородкина.
_ .. ____ __ 4 R = (CH2)2NHCOC(CH,k
1 R = (CH2)2NH3+ TFA"
Scheme 1. General reaction conditions for the preparation of the amphiphilic Pc-C60 dyad 1. i) Zn(OAc)2, dimethylaminoethanol, reflux. ii) SO3-pyridine complex, NEt3, dry dimethylsulfoxide, argon, 50 °C. iii) C60 fullerene, tert-butylcarbamate-protected, N-functionalized glycine, toluene, reflux. iv) trifluoroacetic acid, CH2Cl2, RT.
16
© ISUCT Publishing
Макрогетероциклы /Macroheterocycles 2010 3(1) 16-18
G. Bottari and T. Torres
in the presence of Zn(OAc)2 leading to the hydroxymethyl-functionalized ZnnPc 2, which was isolated in 22% yield. Pc 2 was then subjected to an oxidative treatment with an excess of a SO3-pyridine complex in the presence of NEt3 in dry dimethylsulfoxide, affording formylPc 3 in 84% yield.
Finally, the 1,3-dipolar cycloaddition reaction of an azomethine ylide, generated in situ from formylPc 3 and a tert-butylcarbamate-protected, ^-functionalized glycine, to C60 fullerene (also known as "Prato reaction"), afforded Pc-C60 dyad 4. Such reaction, which results in the formation of a pyrrolidine group, represents one of the most used synthetic strategies used nowadays for the functionalization of fullerenes. The carbamate protecting group in 4 was finally converted into an ammonium group by treatment
with an excess of trifluoroacetic acid to afford Pc-C„ salt
60
1. Due to the presence of the positively charged end groups, compound 1 could be dispersed in aqueous solutions giving rise to the formation of aggregates as demonstrated by UV-vis analysis. Transmission electron microscopy (TEM) analysis revealed that the amphiphilic Pc-C60 dyad 1 is able to form, when dispersed in water, perfectly ordered 1-D nanotubules due to a combination of solvophobic and n-n stacking interactions (Figure 1).
Figure 1. TEM image of the supramolecular nanotubules formed by dispersing Pc-C60 dyad 1 in water. The TEM image is reprinted with permission from reference 37. Copyright 2005, American Chemical Society.
A considerable change in the photophysical properties of the one-dimensional, self-organized Pc-C60 salt 1 with respect to the molecularly dispersed compound was also observed. Photophysical studies (i.e., steady-state/time-resolved fluorescence experiments and transient absorption measurements) in fact revealed that the photoreactivity of these tubular nanostructures is remarkable, in terms of both ultrafast charge separation (i.e., ~ 1012 s-1) and ultraslow charge recombination (i.e., ~ 103 s-1), and it results in an impressive stabilization of the charge separated species Pc^+-C60" 1 of six orders of magnitude relative to the Pc-C60 dyad 4.
Somehow different, and probably to some extent more complex, is the case when such organization of Pc-C60 conjugates is sought on substrates, since in this latter case the interactions between the molecules and the surface itself should also be taken into account.
Up to date, the only example of a Pc-C60 conjugate organized on a surface is represented by dyad 5 (Scheme 2).[38]
G-
Scheme 2. Synthesis of the structurally-rigid, Pc-C60 conjugate 5. i) 4-ethynylbenzaldehyde, Cul, PdCl2(PPh3)2, NEt3, dry toluene, reflux. ii) C60 fullerene, N-methylglycine, dry toluene, reflux.
The structurally-rigid Pc-C60 dyad 5 was prepared starting from the iodo-containing Pc 6 which was then coupled to 4-ethynylbenzaldehyde via a palladium-catalyzed Sonogashira coupling reaction affording the formylPc 7 in 92% yield. This latter compound was then reacted with C60 fullerene and N-methylglycine affording dyad 5 in 36% yield. The organization properties of the covalently-linked conjugate 5 on highly ordered pyrolytic graphite (HOPG) and graphite-like surfaces were also investigated by using atomic force microscopy (AFM). These studies showed that, on such surfaces, the D-A ensemble 5 is able to self-organize forming fibers and films (Figure 2). The electrical properties of these self-assembled nanostructured architectures were also probed by conductive-AFM (C-AFM). C-AFM is a powerful tool for measuring electrical properties in nanostructured architectures, since it allows the electrical mapping (usually with a spatial resolution of the order of the C-AFM tip radius, ca. 30 nm) of a sample as a metal-coated AFM tip is passed over it.
Figure 2. AFM topographic image of Pc-C60 dyad 5 drop-casted on highly oriented pyrolytic graphite (HOPG). The AFM image is reprinted with permission from reference 38. Copyright 2008, Wiley-VCH.
These C-AFM studies revealed outstanding electrical conductivity values for both supramolecular fibers and films, which resulted to be related to the extremely high degree of molecular order of the Pc-C60 conjugates within the nanostructures.
Very recently, the first example of mesogenic Pc-C60 dyads has been reported.[39] In such systems, a favourable combination of the length of the linker with the length of peripheral substituents on the Pc, allows the bulky C60 moiety to be accommodated in the liquid-crystalline phase.
Макрогетер0циmbl /Macroheterocycles 2010 5(1) 16-18
17
Phthalocyanine-C60 Fullerene Conjugates
Detailed analysis of this supramolecular organization, as well as deeper insight in the structure-property relationships of these dyads, will be the subject of a separate publication.
The above mentioned studies clearly demonstrate that the supramolecular organization of Pc-C60 systems, either in solution or a solid substrate, is able to considerably affect some of the dyad's physical properties, giving rise to the appearance of collective physical phenomena.
In this context, the preparation of easy-to-assemble, supramolecular nanostructures in which photo- and redox-active units, such as Pcs and C60 fullerenes, respectively excellent donor and acceptor moieties, are molecularly self-assembled across multiple length scales is extremely promising, since it opens up the possibility of using these D-A conjugates for relevant technological applications such as nano-optoelectronics and photovoltaics. It can be easily envisaged that both scanning force and tunneling microscopies will play an important part in the study and implementation of such systems, providing some valuable guidelines about these systems' supramolecular organization, information that are of paramount importance for the optimization of resulting devices.
Acknowledgements. Funding from MEC (CTQ2008-00418/ BQU), ESF-MEC (MAT2006-28180-E, SOHYDS), COST Action D35, CAM (S-0505/PPQ/000225) is acknowledged. G. B. thanks the Spanish MEC for a "Ramón y Cajal" contract.
References
1. de la Torre G., Claessens C.G., Torres T. Chem. Commun. 2007, 2000-2015.
2. de la Torre G., Bottari G., Hahn U., Torres T. Struct. Bond. in press
3. D'Souza F., Ito O. Coord. Chem. Rev. 2005, 249, 1410-1422.
4. D'Souza F., Ito O. Chem. Commun. 2009, 4913-4928.
5. Guldi D.M. Chem. Soc. Rev. 2002, 31, 22-36.
6. Fullerenes: Principles and Applications (Langa F., Nierengarten J.-F., Eds.) The Royal Society of Chemistry: Cambridge, 2007 (Nanoscience and Nanotechnology Series).
7. Guldi D.M., Illescas B.M., Atienza C.M., Wielopolski M., Martin N. Chem. Soc. Rev. 2009, 38, 1587-1597.
8. The Electrochemical Society Proceedings; Electrochemical Society (Bottari G., González-Rodríguez D., Torres T., Eds.) ECS, Pennington, NJ, 2004, Vol. 14, 282-297.
9. Gonzalez-Rodriguez D., Bottari G. J. Porphyrins Phthalocy-anines 2009, 13, 624-636.
10. Bottari G., de la Torre G., Guldi D.M., Torres T. Chem. Rev. in press
11. Guldi D.M., Gouloumis A., Vázquez V., Torres T. Chem. Commun. 2002, 2056-2057.
12. de la Escosura A., Martínez-Díaz M.V., Guldi D.M., Torres T. J. Am. Chem. Soc. 2006, 128, 4112-4118.
13. Loi M.A., Denk P., Hoppe H., Neugebauer H., Winder C., Meissner D., Brabec C., Sariciftci N.S., Gouloumis A., Vazquez P., Torres T. J. Mater. Chem. 2003, 13, 700-704.
14. Neugebauer H., Loi M.A., Winder C., Sariciftci N.S., Cerullo G., Gouloumis A., Vazquez P., Torres T. Sol. Energy Mater. Sol. Cells 2004, 83, 201-209.
15. Gouloumis A., de la Escosura A., Vazquez P., Torres T., Kahnt A., Guldi D. M., Neugebauer H., Winder C., Drees M., Sariciftci N. S. Org. Lett. 2006, 8, 5187-5190.
16. Elemans J.A.A.W., Rowan A.E., Nolte R.J.M. J. Mater. Chem. 2003, 13, 2661-2670.
17. de la Escosura A., Martínez-Díaz M.V., Thordarson P., Rowan A.E., Nolte R.J.M., Torres T. J. Am. Chem. Soc. 2003, 125, 12300-12308.
18. Li X., Links L.E., Rybtchinsky B., Wasielewski M.R., J. Am. Chem. Soc. 2004, 126, 10810-10811.
19. Sly J., Kasák P., Gomar-Nadal E., Rovira C., Górriz L., Thordarson P., Amabilino D.B., Rowan A.E., Nolte R.J.M. Chem. Commun. 2005, 1255-1257.
20. Gouloumis A., González-Rodriguez D., Vázquez P., Torres T., Liu S., Echegoyen L., Ramey J., Hug G.L., Guldi D.M. J. Am. Chem. Soc. 2006, 128, 12674-12684.
21. Yoshimoto S., Itaya K. J Porphyrins Phthalocyanines 2007, 11, 313-333.
22. Martínez-Díaz M.V., Bottari G. J. Porphyrins Phthalocyanines 2009, 13, 471-480.
23. Sielcken O.E., van Tilborg M.M., Roks M.F.M., Hendriks R., Drenth W., Nolte R.J.M. J. Am. Chem. Soc. 1987, 109, 42614265.
24. van Nostrum C.F., Picken S.J., Nolte R.J.M. Angew. Chem., Int. Ed. Engl. 1994, 33, 2173-2175.
25. van Nostrum C.F., Picken S.J., Schouten A.-J., Nolte R.J.M. J. Am. Chem. Soc. 1995, 117, 9957-9965.
26. Lützen A., Starnes S.D., Rudkevich D.M., Rebek J.Jr. Tetrahedron Lett. 2000, 41, 3777-3780.
27. Martínez-Díaz M.V., Rodríguez-Morgade M.S., Feiters M.C., van Kan P.J.M., Nolte R.J.M., Stoddart J.F., Torres T. Org. Lett. 2000, 2, 1057-1060.
28. Sessler J.L., Jayawickramarajah J., Gouloumis A., Pantos, G.D., Torres T., Guldi D.M. Tetrahedron 2006, 62, 2123-2131.
29. Torres T., Gouloumis A., Sanchez-Garcia D., Jayawickramarajah J., Seitz W., Guldi D.M., Sessler J.L. Chem. Commun. 2007, 292-294.
30. Seitz W., Jiménez A.J., Carbonell E., Grimm B., Rodríguez-Morgade M.S., Guldi D.M., Torres T. Chem. Commun., 2010, DOI: 10.1039/b921363e.
31. Rodríguez-Morgade M.S., Torres T., Atienza-Castellanos C., Guldi D.M. J. Am. Chem. Soc. 2006, 128, 15145-15154.
32. Fischer M.K.R., López-Duarte I., Wienk M.M., Martínez-Díaz M.V., Janssen R.A.J., Bäuerle P., Torres T. J. Amer. Chem. Soc. 2009, 131, 8669-8676.
33. van Nostrum C.F., Bosman A.W., Gelinck G.H., Schouten P.G., Warman J.M., Kentgens A.P.M., Meijerink A., Picken S.J., Devillers M.A.C., Sohling U., Schouten A.-J., Nolte R.J.M. Chem. Eur. J. 1995, 1, 171-180.
34. Duro J.A., de la Torre G., Barberá J., Serrano J.L., Torres T. Chem. Mater. 1996, 8, 1061-1066.
35. Claessens C.G., Blau W.J., Cook M.J., Hanack M., Nolte R.J.M., Torres T., Wöhrle D. Monatsh. Chem. 2001, 132, 3-11.
36. García-Frutos E.M., Bottari G., Vázquez P., Barberá J., Torres T. Chem. Commun. 2006, 3107-3109.
37. Guldi D.M., Gouloumis A., Vazquez P., Torres T., Georgaki-las V., Prato M. J. Am. Chem. Soc. 2005, 127, 5811-5813.
38. Bottari G., Olea D., Gomez-Navarro C., Zamora F., Gomez-Herrero J., Torres T. Angew. Chem. Int. Ed. 2008, 47, 20262031.
39. Geerts Y.H., Debever O., Amato C., Sergeyev S. Beilstein J. Org. Chem. 2009, 5 (49). DOI:10.3762/bjoc.5.49.
Received 01.12.2009 Accepted 10.12.2009 First published on the web 12.02.2010
18
Макрогетероциклы /Macroheterocycles 2G1G S(1) 16-18
