6533b823fe1ef96bd127e1bf

RESEARCH PRODUCT

Dendron to central core S1-S1 and S2-S(n) (n1) energy transfers in artificial special pairs containing dendrimers with limited numbers of conformations.

subject

description

Two dendrimers consisting of a cofacial free-base bisporphyrin held by a biphenylene spacer and function- alized with 4-benzeneoxomethane (5-(4-benzene)tri-10,15,20-(4-n-octyl- benzene)zinc(II)porphyrin) using either five or six of the six available meso-positions, have been synthesized and characterized as models for the an- tenna effect in Photosystems I and II. The presence of the short linkers, -CH2O-, and long C8H17 soluble side chains substantially reduces the number of conformers (foldamers) compared with classic dendrimers built with longer flexible chains. This simpli- fication assists in their spectroscopic and photophysical analysis, notably with respect to fluorescence resonance energy transfer (FRET). Both steady- state and time-resolved spectroscopic measurements indicate that the cofacial free bases and the flanking zinc(II)- porphyrin antennas act as energy ac- ceptor and donor, respectively, follow- ing excitation in either the Q or Soret bands of the dendrimers. The rate con- stants for singlet electronic energy transfer (kEET) extracted from the S1 and S2 fluorescence lifetimes of the donor in the presence and absence of the acceptor are � (0.1-0.3) � 10 9 and ~ 2�1 0 9 s � 1 for S1!S1 (range from a bi- exponential decay model) and about 1.5 � 10 12 s � 1 for S2!Sn (n > 1). Com- parisons of these experimental data with those calculated from Fcrster theory using orientation factors and donor-acceptor distances extracted from computer modeling suggest that a highly restricted number of the many foldamers facilitate energy transfer. These foldamers have the lowest energy by molecular modeling and con- sist of one or at most two of the flank- ing zinc porphyrin antennas folded so they lie near the central artificial spe- cial pair core with the remaining anten- nas located almost parallel to and far from it.