Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)2 derivatives; where aryl refers to phenyl, anisyl, naphthyl, or pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that the sp3 hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615 and 1.636 A. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)2 compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent dependent and, at least in some cases (aryl = anisyl or pyrenyl), nonradiative decay from the first-excited singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state, in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer from pyrene or naphthalene residues to Bodipy is quantitative.
A molecular dyad has been synthesized in which free-base porphyrin and ruthenium(II) bis(2,2‘:6‘,2‘ ‘-terpyridine) subunits are linked via a meso-phenylene group. The distal terpyridine ligand bears a single
phenylethynylene group. Selective illumination into the metal complex is followed by rapid intramolecular
triplet−triplet energy transfer from the metal-to-ligand charge-transfer (MLCT) triplet to the lowest-energy
π,π* triplet state localized on the porphyrin. This process is characterized by a reorganization energy of
0.14 eV and an electronic coupling matrix element of 76 cm-1. Because of the alkynylene substituent, the
initially produced MLCT state is centered on the distal terpyridine. Therefore, triplet energy transfer must
cross the proximal terpyridine ligand. At low temperature, nuclear tunneling renders the rate of triplet energy
transfer activationless. Upon selective illumination into the lowest-energy singlet (S1) state localized on the
porphyrin, fast singlet−triplet energy transfer occurs, to populate the MLCT triplet with high efficiency. This
process happens by way of Dexter-type electron exchange at room temperature, and the MLCT triplet can be
identified as a reaction intermediate at low temperature. The activation energy for singlet−triplet energy
transfer is only 0.05 eV, because of the smaller energy gap, and the electronic coupling matrix element is
decreased to 11 cm-1, because energy transfer is spin-forbidden. At low temperature, dipole−dipole energy
transfer becomes the main mechanism for decay of the porphyrin S1 state. Excitation into the Soret band of
the porphyrin is followed by rapid internal conversion to S1 without energy or electron transfer to the appended
metal complex.
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