Length Dependence for Intramolecular Energy Transfer in Three- and Four-Color Donor-Spacer-Acceptor Arrays

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 angstrom. 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, 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.