Spectroscopy and Photophysics of New Fullerene Derivatives

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During my post-doc I was engaged in a European TMR (Training and Mobility of Researchers) -network called "USEFULL" (Usable Fullerene Derivatives: Synthesis, Stabilisation, Spectroscopy and Systematics). People from all over Europe worked together on the synthesis and the photophysical characterisation of new fullerene derivatives having potential applications in optoelectronics. The network USEFULL was closely related to another network called BIOFULL which was concerned with the evaluation of the biological properties of fullerenes and new fullerene derivatives).

General Context: My work is dedicated to the exploration of possible applications of the new fullerene derivatives related to their interaction with light. The compounds are synthesised in the group of Prof. Harry Kroto at the University of Sussex. Finding new applications initially requires the full understanding of the spectroscopy and photophysics of the compounds.

One important aspect of the photophysics of fullerenes concerns the absorption of exited states, in particular that of the lowest lying triplet state. This absorption is situated in the visisble and is generally much stronger than the absorption of the ground state. The quasi-transparency of the lowest lying singlet state as well as the strong T_n¬T₁ transitions together with high quantum yields of singlet to triplet intersystem crossing [1,2] give rise to the possibility to construct optical limiters against destructive effects of intense light pulses. This application includes eye protection devices against strong laser pulses.

Given the capability of fullerenes to capture and stabilise up to 6 electrons, they are potentially useful as electron acceptor layers in photodiodes and photovoltaic cells.

Applications in medicine are particularly related to the capability of fullerenes to convert dissolved ground state oxygen into singlet state oxygen upon photoexcitation in the near UV. This opens up the opportunity to use fullerenes as photosensitisers in photodynamic therapy (PDT). The ability to capture electrons could lead to applications of fullerenes as protecting agents against oxidative stress in biological organisms [3]. However, the hydrophobicity of fullerenes is still a major problem in this research area [4].

Experiments performed and highlighting results (see also my publications) : The aryle [70] fullerenes C₇₀(C₆H₅)_2n and C₇₀(C₆H₄-R)_2n (n = 2-5, R = F, OCH₃) and similar derivatives have become available recently [5,6]. In these type of fascinating compounds two types of chromophores (phenyl moieties and the C₇₀ fullerene cage) are present. My work is aimed to reveal the intrinsic photophysical properties of the singlet and triplet states of the new C₇₀(C₆H₅)_2n species and also a series of different deca-functionalised C₇₀ compounds. The main objective is to systematically understand the modifications of the photophysical properties upon successive functionlisation of the fullerne cage. I used the Laser Flash Photolysis and Pulse Radiolysis equipment of the Free Radical Research Facility at the Paterson Institute for Cancer Research (Manchester, UK) in order to study the singlet and triplet state properties of the compounds mentioned above. To complete the measurements I performed stationary and time-resolved fluorescence studies at the Centre d'Etudes Nucléaires (CEN) at Saclay (near Paris).

The quantum yield of singlet to triplet intersystem crossing (F_T) diminishes linearly as the number of phenyl groups attached to the fullerene cage increases from n = 2 to 4: F_T = 1 ± 0.1 for C₇₀Ph₄, F_T = 0.5 ± 0.05 for C₇₀Ph₆, F_T = 0.18 ± 0.02 for C₇₀Ph₈ [7]. For C₇₀Ph₁₀ (n = 5), in which the conjugated p-system of the fullerene cage is divided into 2 separate aromatic-like components, F_T is again near unity. Singlet oxygen production from the triplet state occurs with a quantum yield F_D = F_T for all of the derivatives investigated. It is inferred that S_D, the fraction of triplet state molecules quenched upon collision with dissolved molecular oxygen and leading to singlet oxygen (¹D_g), is close to unity as found in the pristine fullerenes C₆₀ and C₇₀.

The triplet molar absorption coefficients at the maximum absorption wavelength, e_T (l_max), increase as n (the number of phenyl groups attached to the cage) increases from C₇₀Ph₄ [e_T = (5300 ± 500) M^-1cm^-1 at l= 900 nm] to C₇₀Ph₁₀ [e_T = (7300 ± 700) M^-1cm^-1 at l= 510 nm]. This is rationalised in terms of variation of charge on the atoms upon T_n¬T₁ excitation and supported by calculations of the electron density variation upon T_n¬T₁ excitations which show that phenyl contributions are important in the region of strong T_n¬T₁ transitions for all of the C₇₀Ph_2n compounds investigated [7].

[1] R.V. Bensasson, T. Hill, C. Lambert, E.J. Land, S. Leach, T.G. Truscott, Chem. Phys. Lett. 201 (1993), 326.

[2] R.V. Bensasson, T. Hill, C. Lambert, E.J. Land, S. Leach, T.G. Truscott, Chem. Phys. Lett. 206 (1993), 197.

[3] I.C. Wang , L.A. Tai, D.D. Lee, P.P. Kanakamma et al., J. Med. Chem. 42 (1999), 4614.

[4] R.V. Bensasson, E. Bienvenue, M. Dellinger, S. Leach, P. Seta, J. Phys. Chem. 98 (1994), 3492.

[5] A.G. Avent, P.R. Birkett, A.D. Darwish, H.W. Kroto, R. Taylor, D.M. Walton, Tetrahedron 52 (1996), 5235.

[6] P.R. Birkett, A.D. Darwish, H.W. Kroto, R. Taylor, D.R.M. Walton, J. Chem. Soc. Chem. Commun. 1995, 1869.

[7] R.V. Bensasson, M. Schwell, M. Fanti, N.K. Wachter_, J. Oviedo, J.M. Janot, P.R. Birkett, E.J. Land, S. Leach, P. Seta, R. Taylor and F. Zerbetto, submitted to Chem. Eur. J.