Electronic Energy-Transfer Rate Constants for Geometrical Isomers of a Bichromophoric Macrocyclic Complex

The rate of electronic energy transfer (EET) between a naphthalene donor and an anthracene acceptor in [ZnL3](ClO4)2 and [ZnL4](ClO4)2 was determined by time-resolved fluorescence measurements, where L3 and L4 are the geometrical isomers of 6-[(anthracen-9-ylmethyl)amino]-trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-13-amine (L2), substituted with either a naphthalen-1-ylmethyl or naphthalen-2-ylmethyl donor, respectively. The energy-transfer rate constant, kEET, was determined to be (0.92 ± 0.02) × 109 s-1 for the naphthalen-1-ylmethyl-substituted isomer, while that for the naphthalen-2-ylmethyl-substituted isomer is somewhat faster, with kEET = (1.31 ± 0.01) × 109 s-1. The solid-state structure of [ZnL3Cl]ClO4 has been determined, and using molecular modeling calculations, the likely distributions of solution conformations in CH3CN have been evaluated for both complexes. The calculated conformational distributions in the common trans-III N-based isomeric form gave Förster EET rate constants that account for the differences observed and are in excellent agreement with the experimental values. It is shown that the full range of conformers must be considered to accurately reproduce the observed EET kinetics.