Singlet-Singlet, Triplet-Triplet, and 'Optically-Controlled' Energy Transfer in Polychromophores. Preliminary Models for a Molecular Scale Shift Register

摘要

The photophysics of two trichromophoric molecules have been studied by a combination of absorption, fluorescence, phosphorescence, and laser flash photolysis. TRI-1 consists of phenanthrene and naphthalene terminal chromophores joined to a central biphenyl group by methyl ester bridges. Intramolecular singlet-singlet energy transfer (SSET) between the biphenyl and terminal chromophores occurs efficiently with k_(SSET) > 6 * 10~(10) S~(-1). Longer range SSET from the naphthalene moiety to the phenanthrene group takes place with a lower rate, k ～ 2.5 * 10~8 s~(-1). In TRI-2, the biphenyl moiety is replaced by a benzophenone group, SSET here occurs from the phenanthrene chromophore to the central benzophenone chromophore although with a significantly lower rate than the biphenyl → phenanthrene rate in TRI-1. This is likely due to the change in configuration (ππ~* to nπ~*) of the excited singlet states involved in the energy transfer. Triplet-triplet (TTET) energy transfer between biphenyl and the terminal chromophores is not observed as a result of a low biphenyl triplet population. That both the phenanthrene and naphthalene triplets are observed following laser photolysis and have different lifetimes and different excitation wavelength concentration ratios indicates that there is no significant TTET between these two groups on the time scale at which the triplets are decaying. This is attributed to the large interchromophore distance (～12.5 A) as indicated by modeling studies. In TRI-2, TTET from benzophenone to the terminal chromophores is indicated by both phosphorescence and laser flash photolysis results. Two-laser flash photolysis of the phenanthrene triplet in TRI-1 results in the production of the naphthalene triplet by the following suggested route: (i) production of the upper excited triplet state of the phenanthrene group, (ii) energy transfer to the central biphenyl moiety, and (iii) further energy transfer to the naphthalene chromophore. The utility of this two-laser behavior as the basis for the operation of an optically coupled molecular scale shift register is discussed.