Photophysical, photochemical, and electrochemical properties of dendrimers with a dimethoxybenzil core

Abstract

Three dendrimers consisting of a dimethoxybenzil core and branches that contain two (G0), four (G1), and eight (G2) naphthalene units at the periphery and zero (G0), two (G1), and six (G2) dimethoxybenzene units in the branches have been synthesized and their photophysical, photochemical, and electrochemical properties have been investigated. For comparison purposes, the properties of dimethoxybenzil (MB) and of a dendron containing four naphthalene and three dimethoxybenzene units (D2) have also been studied. The properties of the dendrimers in the ground state (absorption spectra and electrochemical behavior) are those expected for their noninteracting component units. The excited state properties, however, are substantially controlled by electronic interactions between the dimethoxybenzil core and the naphthalene units contained in the branches. In dichloromethane–chloroform 1 : 1 (v/v) solution at 298 K, energy transfer from the lowest excited state (S₁) of the naphthalene units to the lower lying S₁ excited state of the dimethoxybenzil core takes place with high efficiency. In a rigid matrix at 77 K, selective excitation of the dimethoxybenzil chromophore yields an emission band that exhibits a spectral evolution: in the millisecond time scale it shows a spectral profile very similar to the dimethoxybenzil phosphorescence, whereas in the second time scale it is very similar to the naphthalene-type phosphorescence. Energy transfer from the T₁ excited state of the dimethoxybenzil core to the T₁ excited state of the naphthalene units takes place at 77 K, but not at 298 K, because the T₁ excited state of the dimethoxybenzil core moves to energy lower than that of the naphthalene chromophore. The photochemical results show that the dimethoxybenzil core maintains its intrinsic photoreactivity toward dioxygen, and that on increasing dendrimer generation a photoreaction between core and branches predominates.