Synthesis and photophysical properties of a [60]fullerene compound with dimethylaniline and ferrocene connected through a pyrazolino group: a study by laser flash photolysiswz Juan L. Delgado, a Mohamed E. El-Khouly, bc Yasuyuki Araki, c Marı´a J. Go´mez- Escalonilla, a Pilar de la Cruz, a Fre´de´ric Oswald, a Osamu Ito* c and Fernando Langa* a Received 21st February 2006, Accepted 2nd August 2006 First published as an Advance Article on the web 14th August 2006 DOI: 10.1039/b602633h Pyrazolino[60]fullerene covalently-linked to ferrocene and N,N-dimethylaniline groups has been prepared and studied using time-resolved spectroscopic methods. The fluorescence quenching of the C 60 moiety indicates that charge-separation takes place via the singlet excited state of the C 60 moiety in both polar and non-polar solvents. The charge-separated state, in which an electron is localized on the C 60 sphere and a hole is located on the whole donor moieties of ferrocene, pyrazole, and N,N-dimethylaniline groups, has been confirmed by nanosecond transient spectra in the visible and near-IR spectral region. The lifetimes of the radical ion-pairs are as long as 30–50 ns in both polar and non-polar solvents. Introduction In the last decade, a great deal of research has been directed toward exploiting the photophysical 1 and electrochemical 2 properties of fullerenes, particularly in the field of artificial photosynthesis and molecular electronic devices. 3 For this reason, a variety of dyads containing C 60 covalently linked with electron donors have been prepared. 4 Remarkably small reorganization energies 5 associated with the reduction of the C 60 cage lead to efficient photoinduced electron transfer occurring, with the generation of relatively long-lived charge-separated (CS) states. One of the most attractive strategies for developing artificial photosynthetic systems is to link various electron donors in a covalent manner to the fullerene cage to attain efficient intramolecular electron transfer and to generate long-lived CS states. 6 This approach imitates the ‘‘electron hopping’’ in photosynthesis by generating a reduction gradient between the donor and acceptor units, a process that introduces additional electroactive moieties. Among the different donors, ferrocene (Fc) and N,N-dimethylaniline (DMA) groups have been shown to be excellent donors in C 60 -dyads, and efficient photoinduced electron transfer has been observed in some of these systems. 7 In terms of synthesis, the 1,3-dipolar cyclo- addition of nitrile imines has been proven to be a general and versatile method for preparing C 60 derivatives, termed 2- pyrazolino[60]fullerenes (PzC 60 ). In these materials, the first and second reduction potentials are anodically shifted (B100 mV) relative to other 1,2-disubstituted fullerenes, such as pyrrolidino[60]fullerenes, and they show similar reduction values to the parent C 60 . This shift is attributed to the electronic character of the nitrogen atom covalently linked to the C 60 core. 8,9 Moreover, in contrast to pyrrolidino[60]- fullerenes, 2-pyrazolino[60]fullerenes show efficient photoin- duced electron transfer from the pyrazoline ring to the fullerene sphere, 10 confirming the particular properties of this family of fullerene derivatives. In C 60 -donor-based systems, the pyrazoline ring can therefore provide an intermediate step in the photoinduced electron transfer process from the donor to the C 60 cage, participating in the potential gradient indi- cated above. Given the above information, we report here the synthesis and properties of triad 3 (Scheme 1), in which Fc and DMA groups attached to PzC 60 as a pair of buckhorns (ESI, Fig. S1z). The photophysical properties were assessed using the steady-state spectra, time-resolved fluorescence and nano- second transient absorption techniques in various solvents, expecting a synergistic effect of these donor groups on efficient photoinduced charge-separation and on the prolongation of the CS state. Results and discussion Synthesis and characterization The synthesis of compound 3 is depicted in Scheme 1. N- Anilino-2-pyrazolino[60]fullerene 2 was obtained in 72% yield a Facultad 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 b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980-8577 Sendai, Japan. E-mail: ito@tagen.tohoku.ac.jp; Fax: +81-22-217-5608 c Department of Chemistry, Faculty of Education, Kafr El-Sheikh, Tanta University, Tanta, Egypt. E-mail: mohamedelkhouly@yahoo.com w The HTML version of this article has been enhanced with colour images. z Electronic Supplementary Information (ESI) available: Molecular image, transient spectrum of 1; absorption spectra and transient absorption spectra of 3 in the presence of HCl; 1 H and 13 C-NMR spectra of compounds 2–4. See http://dx.doi.org/10.1039/b602633h 4104 | Phys. Chem. Chem. Phys., 2006, 8, 4104–4111 This journal is  c the Owner Societies 2006 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics