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 borondipyrromethene (bodipy) dye is equipped with a 4-pyridine residue attached via the meso position. The strong fluorescence inherent to this class of dye is extinguished on protonation of the pyridine N atom. For the corresponding N-methylpyridinium derivative, fluorescence from the dye fragment is also extensively quenched due to the onset of a light-induced charge-shift reaction. The resultant charge-transfer state (CTS) is weakly fluorescent and decays primarily by way of population of the triplet excited state localized on the bodipy dye. Time-resolved spectral studies provide rate constants for all the steps involved in the forward and reverse charge-shift reactions. An interesting feature is that the lifetime of the CTS, around 1 ns, correlates with the viscosity of the solvent as might be expected if the rate-limiting step involves a substantial change in geometry. There is an unexpectedly small activation energy for the reverse charge-shift reaction, even allowing for the fact that this involves triplet formation. Local fluorescence is restored on cooling to 77 K.

A series of donor-spacer-acceptor triads has been synthesized and fully characterized. Both donor and acceptor units are built from boron dipyrromethene (BODIPY) dyes but they differ in their respective conjugation lengths, and thereby offer quite disparate optical properties. The spacer units comprise an oligomer of 1,4-phenylene-diethynylene repeat units and allow the boron-boron separation distance to be varied progressively from 18 to 38 A. A notable feature of this series is that each subunit can be selectively excited with monochromatic light. Highly efficacious electronic energy transfer (EET) occurs from the first-excited singlet state localized on the conventional BODIPY dye to its counterpart resident on the expanded BODIPY-based nucleus, but the rate constant follows a nonlinear evolution with separation distance. Overall, the rate of EET falls by only a factor of 4-fold on moving from the shortest to the longest spacer. This shallow length dependence is a consequence of the energy gap between donor and spacer units becoming smaller as the molecular length increases. Interestingly, a simple relationship exists between the measured electronic resistance of the spacer unit and the Huang-Rhys factor determined by emission spectroscopy. Both parameters relate to the effective conjugation length. Direct illumination of the spacer unit leads to EET to both terminals, followed by EET from conventional BODIPY to the expanded version. In each case, EET to the expanded dye involves initial population of the second-singlet excited state, whereas transfer from spacer to the conventional BODIPY dye populates the S(2) state for shorter lengths but the S(1) state for the longer analogues. The rate of EET from spacer to conventional BODIPY dye, as measured for the corresponding molecular dyads, is extremely fast (>10(11) s(-1)) and scales with the spectral overlap integral. The relative partitioning of EET from the spacer to each terminal is somewhat sensitive to the molecular length, with the propensity to populate the conventional BODIPY dye changing from 65% for N = 0 to 45% for N = 2. The most likely explanation for this behavior can be traced to the disparate spectral overlap integrals for the two dyes. These systems have been complemented by a molecular tetrad in which pyrene residues replace the fluorine atoms present on the conventional BODIPY-based dye. Here, rapid EET occurs from pyrene to the BODIPY dye and is followed by slower, long-range EET to the opposite terminal. Such materials are seen as highly attractive solar concentrators when dispersed in transparent plastic media and used under conditions where both inter- and intramolecular EET operate.