Energy transfer and concentration-dependent conformational modulation: a porphyrin-containing [3]rotaxane

A zinc porphyrin-containing [3]rotaxane A was synthesized through a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Energy donors and acceptor porphyrin were introduced to dibenzo[24]crown-8 (DB24C8) and dibenzyl ammonium (DBA) units of [3]rotaxane A to understand the intramolecular energy transfer process. Investigations of the photophysical properties of [3]rotaxane A demonstrated that the intramolecular efficient energy transfer readily occurred from the donors on the wheels to the porphyrin center on the axis. The fluorescence of energy donors in the region of 400 to 450 nm was efficiently absorbed by the porphyrin acceptor under irradiation at 345 nm, and finally a red light emission at about 600 nm was achieved. Further investigation indicated that the conformation of [3]rotaxane A was self-modulated by changing its concentration in CH(2)Cl(2). The triazole groups on the wheel coordinated or uncoordinated to Zn(2+) through intramolecular self-coordination with the change in the concentration of [3]rotaxane A in CH(2)Cl(2). Therefore, this conformational change was reversible in a non-coordinating solvent such as CH(2)Cl(2) but inhibited in a coordinating solvent such as THF. Such interesting behaviors were rarely observed in porphyrin derivatives. This self-modulation feature opens up the possibility of controlling molecular conformation by varying concentration.