Singlet and Triplet Energy Transfers in Tetra-(meso-truxene)zinc(II)- and Tetra-(meso-tritruxene)zinc(II) Porphyrin and Porphyrin-Free Base Dendrimers

The synthesis, optical properties, and energy transfer features of four dendrimers composed of meso-tetrasubstituted zinc(II) porphyrin (ZnP) or a free base (P) central core, where the substituents are four truxene (Tru) or four tritruxene dendrons (TriTru), TruP, TriTruP, TruZnP, and TriTruZnP, are reported. Selective excitation of the truxene donors results in a photoinduced singlet energy transfer from the truxenes to the porphyrin acceptor. The rates for singlet energy transfer (k(ET)), evaluated from the change in the fluorescence lifetime of the donors (Tru and TriTru) in the presence and absence of the acceptor (P or ZnP) for TruP, TruZnP, TriTruP, and TriTruZnP, are 5.9, 1.2, 0.87, and 0.74 (ns)(-1) at 298 K and 2.6, 2.6, 2.7, and 1.2 (ns)(-1) at 77 K, respectively. A slow triplet-triplet energy transfer from tnixene to porphyrin cores in glassy 2MeTHF at 77 K is also reported with rates of 1.3 x 10(3) and 0.10 X 10(2) s(-1) for TruZnP and TriTruZnP, respectively. Tithe Dexter mechanism for the triplet energy transfers is considered, these slow rates are easily explained by a poor orbital overlap between the truxene and porphyrin pi systems. The fluorescence quantum yields (Phi(F)) are 0.20 and 0.16 for TruP and TriTruP and 0.08 and 0.10 for TruZnP and TriTruZnP, respectively at 298 K At 298 K, a phosphorescence from TruZnP at 795 nm was also observed and is associated with the ZnP chromophore.