Accurate Analysis of Spectrophotometric Data to Quantify the Photoswitching Performance of a Biomimetic E/Z Light-Induced Molecular Switch.

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Photoswitches are widely investigated molecules because upon exposure to selected light irradiation, they are able to undergo structural, and hence optical, changes. To fully exploit their responsiveness to irradiation, the quantum efficiency of the forward and back reactions is a fundamental parameter, whose accurate determination is critical. In this work, the spectral evolution of a biomimetic switch, which undergoes E/Z photoinduced isomerization, is spectrophotometrically examined. The minimal spectral differences between the E and Z forms inhibit the accurate determination of the quantum efficiency of the E-Z and Z-E photoconversions. We present a kinetic analysis of the E-Z spectral evolution, which enables us to determine quantum yields of the forward and back reactions; the data show that the back reaction has a higher efficiency (0.60) than the forward process (0.49), evidencing that irradiation conditions have to be carefully selected to accumulate the Z isomer. The method has been further used to determine the E-Z and Z-E quantum yield in the deprotonated form of the biomimetic switch; the obtained values demonstrate that under basic conditions, the back reaction is relatively more efficient than the forward one even for the deprotonated species. The detailed analysis enables us to establish the conditions to monitor the fluorescence of the E and Z forms, setting the basis for a more sensitive detection of the photoswitch and its transformation.