Forward (singlet–singlet) and backward (triplet–triplet) energy transfer in a dendrimer with peripheral naphthalene units and a benzophenone core

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Abstract
The photochemical and photophysical behaviour of two dendrimers consisting of a benzophenone core and branches that contain four (4) and eight (5) naphthalene units at the periphery has been investigated in CH2Cl2 solution (298 K) and in CH2Cl2/CHCl3 1 ∶ 1 v/v rigid matrix (77 K). For comparison purposes, the photophysical properties of dimethoxybenzophenone (1), 2-methylnaphthalene (2) and of a dendron containing four naphthalene units (3) have also been studied. In both dendrimers 4 and 5, excitation of the peripheral naphthalene units is followed by fast (1.1 × 109 s−1 at 298 K, > 2.5 × 109 s−1 at 77 K for 4; 2.9 × 108 s−1 at 298 K, 7 × 105 s−1 at 77 K for 5) singlet–singlet energy transfer to the benzophenone core. On a longer time scale (>1 × 106 s−1 at 298 K, >6 × 103 s−1 at 77 K for 4; 3.1 × 107 s−1 at 298 K, ca. 3 × 102 s−1 at 77 K for 5) a back energy transfer process takes place from the triplet state of the benzophenone core to the triplet state of the peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet–triplet energy transfer to the peripheral naphthalene units. In hydrogen donating solvents, the benzophenone core is protected from degradation by the presence of the naphthalene units. In solutions containing Tb(CF3SO3)3, sensitization of the green Tb3+ luminescence is observed on excitation of both the peripheral naphthalene units and the benzophenone core of 5. Upon excitation of the naphthalene absorption band (266 nm) with a laser source, intradendrimer triplet–triplet annihilation of naphthalene excited states leads to delayed naphthalene fluorescence (λmax = 335 nm), that can also be obtained upon excitation at 355 nm (benzophenone absorption band). The results obtained show that preorganization of photoactive units in a dendritic structure can be exploited for a variety of useful functions, including photosensitized emission, protection from undesired photoreactions, and energy up-conversion.