Shape-persistent poly-porphyrins assembled by a central truxene: synthesis, structure, and singlet energy transfer behaviors

Four dyad systems composed of a central truxene and either one or three beta-substituted zinc(II) porphyrins (ZnP: TruZnP (7) and TruTriZnP (9)) or free-bases (H2P: TruP (6) and TruTriP (8)) have been prepared. The presence of beta-methyl groups minimizes pi-conjugation through the quasi right angle made by the porphyrin and the truxene planes, and renders these dyads relatively rigid. The position of the absorption and emission 0-0 peaks confirms the role of the truxene and porphyrin as the energy donor and acceptor, respectively. Selective excitation of the truxene results in an efficient singlet energy transfer (S-1 ET) from the truxene to the porphyrin unit. The rates for S-1 ET (k(ET)) are extracted from the change in the fluorescence lifetime of truxene in the presence and absence of the acceptor, and are temperature independent, (TruP (6), TruTriP (8), TruZnP (7) and TruTriZnP (9) are 5.0, 1.4, 1.0 and 1.4 at 298 K and 5.9, 1.3, 2.6, and 0.86 (ns)(-1) at 77 K, respectively), consistent with their relative rigidity. These k(ET)'s are similar to other related but more flexible systems reported by one of us (Inorg. Chem. 2011, 50, 11493-11505). The k(ET)'s time scale was assumed, based on modeling, to be related with hindered rotations about the truxene-porphyrin C-C bonds due to steric hexyl-hexyl interactions. This work confirms this earlier conclusion was correct.