Syn-anti conformational changes in zinc porphyrin dimers induced by temperature-controlled alcohol ligation

Unique temperature-dependent syn-anti conformational switching in bis- and monozinc ethane-bridged porphyrin dimers takes place in nonpolar solvents containing a small amount of alcohol (1-5%). These dimers adopt a syn conformation at any studied temperatures in the absence of alcohol added, while, in the presence of alcohol, a decrease in temperature from 310 to 183 K results in the gradual shift of the conformational equilibrium toward the anti conformer. Apparently the mechanism of this unprecedented phenomenon is based on the enhanced alcohol ligation to zinc porphyrins at low temperature which is capable of destroying the strong pi-pi interporphyrin interactions. This process is monitored by variable temperature (VT) UV-vis and VT H-1 NMR spectral methods and the corresponding thermodynamic parameters are evaluated using a van't Hoff type analysis. Strong exciton coupling between the B transitions of the anti dimers are observed at low temperatures.