Unique temperature-dependent syn-anti conformational switching in bis- and
monozinc ethane-bridged porphyrin dimers takes place in nonpolar solvents c
ontaining a small amount of alcohol (1-5%). These dimers adopt a syn confor
mation 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 res
ults in the gradual shift of the conformational equilibrium toward the anti
conformer. Apparently the mechanism of this unprecedented phenomenon is ba
sed 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 eva
luated using a van't Hoff type analysis. Strong exciton coupling between th
e B transitions of the anti dimers are observed at low temperatures.