Synthesis and Photophysical Properties of Borondipyrromethene Dyes Bearing Aryl Substituents at the Boron Center

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 Å. 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.