Photoluminescent properties of fullerene derivatives
''Photoluminescent properties of Fullerene Derivatives'' Ümit TAYFUN CHEM758 CHEMISTRY OF OPTOELECTRONIC SYSTEMS Polymer Science & Technology M.E.T.U.
Photoluminescence <ul><li>Photoluminescence ( PL ) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates them. </li></ul><ul><li>In photoluminescence,the electrons are promoted to excited states by absorption of photons of light energy. The emitted light is the result of those electrons which relax back to the ground state by emitting photons. </li></ul><ul><li>Sources of excitation of emission (fluorescence) of photoluminescent materials may be daylight, luminescent and incandescent lamps, UV-emitters. </li></ul><ul><li>They convert UV- and visible light into various coloured luminescent emission with afterglow duration from minutes to several hours. </li></ul>
Fullerene derivatives <ul><li>Fullerenes and their derivatives show a strong absorption around 3.7 eV typical of a p-conjugated organic molecular system. </li></ul><ul><li>Due to the high molecular symmetry, the HOMO-LUMO transition of fullerenes is dipole forbidden, exhibiting only very weak luminescence at room temperature. </li></ul><ul><li>However, their PL increases as they are cooled to low temperature because of reduction of thermal quenching of excited states. </li></ul><ul><li>A t very low temperature, as for instance the liquid helium temperature, can PL be clearly detected. </li></ul>
<ul><li>The highly conjugated nature of fullerene molecules leads to </li></ul><ul><li>interesting electronical properties. A close packed film of C60 </li></ul><ul><li>and many of its derivatives shows direct bandgap semiconducting </li></ul><ul><li>behaviour with a symmetry forbidden valence band–conduction </li></ul><ul><li>band transition. </li></ul><ul><li>In the higher fullerenes like C70 and C84, this high degree of symmetry is broken and the low energy transitions become more prominent. </li></ul>The chemical structures of some fullerene derivatives are depicted in Figure
Resonant energy transfer <ul><li>Important factors are the distance between donor and acceptor, the overlap and strength of the donor emission and acceptor absorption as well as the orientation of the dipoles and the dielectric surrounding. </li></ul><ul><li>For a system of specific chromophores inside a matrix usually the leading dependency is in the distance. </li></ul><ul><li>Upon illumination, the excitation energy is transferred from the donor to the acceptor molecule. This leads to a quenching of the donor emission and—in contrast to the charge transfer. </li></ul>Energy level diagram illustrating the resonance condition for energy transfer.
Fullerenes as energy transfer acceptors <ul><li>Fullerene derivatives have an absorption range reaching to over 700 nm, as can be seen in Figure for the example of the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester. </li></ul><ul><li>But the symmetry restrictions with the resulting low oscillator strength of absorption for energies below 2.5eV limits the characteristic radius of the energy transfer from even a highly luminescent donor. </li></ul>
Fullerenes as energy transfer donors <ul><li>Compared with many photoactive organic substances, fullerenes have a relatively low energy gap of about 1.8 eV. </li></ul><ul><li>Additionally, they show only a weak luminescence due to the symmetry forbidden S1->S0 transition and a fast intersystem crossing to the triplet state. </li></ul><ul><li>Therefore, systems in which fullerenes can act as energy transfer donors with significant efficiency are very rare. </li></ul>The triplet state of C60 can undergo a very efficient triplet energy transfer to a ground state triplet oxygen molecule transferring it into a very reactive singlet oxygen molecule.
Energy transfer from an organic molecule to a fullerene <ul><li>Some combinations of fullerenes and organic molecules nevertheless clearly yield energy transfer instead of charge transfer upon excitation. </li></ul><ul><li>A near equal proportion of charge and energy transfer of C60 linked to a perylene derivative via a pyrrolidine linker system, as can be seen from luminescence and ultrafast pump–probe measurements [Martini et al. ]. </li></ul><ul><li>A predominance of energy transfer in photoluminescence studies additionally claim that they see evidence for the presence of a weak charge transfer in photoconductivity studies on a similar system [Hua et al. ]. </li></ul>These works underline the competing nature of the two processes that are often simultaneously possible in a system of two organic chromophores in close contact.
Examples <ul><li>Fullerene-doped polymer films: </li></ul><ul><li>Due to the special optical and electrical properties of fullerenes, fullerene-doped polymer films have attracted much interest in the past few years. </li></ul><ul><li>PL of C60 doped polyvinylcarbazole PVK films showed that the efective charge and energy transfer processes happened in these films. </li></ul>
Examples <ul><li>Fullerene C60 or C70-doped PMMA films display intense visible PL. </li></ul><ul><li>Normally, C60 HOMO–LUMO transition is forbidden due to the high symmetry of its structure. Under laser irradiation in air, C60 undergoes some photochemical reactions with molecular oxygen and polymer to form oxidized C60-polymer adducts, with oxygen atoms attached to the fullerene cage. </li></ul><ul><li>The low symmetry of C60-polymer or oxidized C60-polymer adducts strengthens the HOMO–LUMO transition. </li></ul><ul><li>So, PL of fullerene-doped polymer could be increased largely by laser irradiation in air. </li></ul>
<ul><li>PL behavior of the solution looks like that of the blends of PS and C60. </li></ul><ul><li>Although C60 is well known as a strong quencher, it is not a predominant factor for the observed PL quenching of the PS segments because the PL of pure PS is also strongly quenched in chlorobenzene. </li></ul>Examples
Examples <ul><li>H ighly photoluminescent fullerene-silica nanoparticles(FSNP ): </li></ul><ul><li>Monodisperse, spherical nanoparticles showed an excellent PL intensity and it was successfully employed as bioimaging material increased photoluminescence, compared to both C 60 and silica nanoparticles, ar i se from the C-O-Si linkages that seems to be formed in the sol-gel process . </li></ul><ul><li>C60 in Zeolites: </li></ul><ul><li>The incorporation into the zeolites of isolated C60 results in a clear modification of the fullerene electronic structure. </li></ul><ul><li>The C60-matrix interaction gives novel characteristics in particular PL emission of white light at room temperature and shifts in the optical absorption peaks. </li></ul>
References <ul><li>Increase of photoluminescence from fullerenes-doped poly (alkyl methacrylate) under laser irradiation, G.Z. Li, N. Minami / Journal of Luminescence 104 (2003) 210 207–213 </li></ul><ul><li>Photoinduced charge and energy transfer involving fullerene derivatives, R. Koeppe and N. S. Sariciftci/Photochemical & Photobiological Sciences, (2006) </li></ul><ul><li>Photoluminescence from fullerene-doped polyvinylcarbazole (PVK) prepared by solution casting under laser irradiation, G.Z. Li, N. Minami / Chemical Physics Letters 331 (2000) 26-30 </li></ul><ul><li>Photoluminescence Characteristics of a Highly Soluble Fullerene-Containing Polymer, Jungahn Kim, Young Chul Kim, Macromol. Res., Vol. 10, No. 5, 2002 </li></ul><ul><li>Photoluminescence of fullerene-doped copolymers of methyl methacrylate during laser irradiation G. Z. Li, C. U. Pitmann Jr., Journal of Mat. Sci. (2003) 3741 – 3746 </li></ul><ul><li>Photoconductivity of fullerene-doped polymers, Y. Wang, Nature, 1992, 356, 585–587 </li></ul><ul><li>Ultrafast competition between energy and charge transfer in a functionalized electron donor/fullerene derivative, I. B. Martini, B. Ma, T. Da Ros, R. Helgeson, F. Wudl and B. J. Schwartz, Chem. Phys. Lett., 2000, 327(5–6), 253–262 </li></ul><ul><li>Resonance energy transfer from organic chromophores to fullerene molecules, J. Appl. Phys., Y. Liu, M. A. Summers, S. R. Scully and M. D. McGehee, 2006, 99, 093521 </li></ul><ul><li>Ultrafast competition between energy and charge transfer in a functionalized electron donor/fullerene derivative, I. B. Martini, B. Ma, T. Da Ros, R. Helgeson, F. Wudl and B. J. Schwartz, Chem. Phys.Lett. , 2000, 327(5–6), 253–262 </li></ul><ul><li>Novel soluble and thermally-stable fullerene dyad containing perylene, J. L. Hua, F. S.Meng, F. Ding, F. Y. Li and H. Tian, J. Mater. Chem. , 2004, 14(12), 1849–1853 </li></ul><ul><li>White Light Emission from C,, Molecules Confined in Molecular Cage Materials , Hamilton, B.; Rimmer, J. S.; Anderson, M.; Leigh, D. Adv. Mater. 1993, 5, 583. </li></ul><ul><li>Fullerene-Based Organic-Inorganic Nanocomposites and Their Applications, Plinio Innocenzi* and Giovanna Brusatin , Chem. Mater. 2001, 13, 3126-3139 </li></ul>
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