Unconventional Nonlinear Input-Output Response in a Luminescent Molecular Switch by Inner Filtering Effects

Nonlinear input-output relations are at the basis of the regulation of biochemical processes in living organisms and are important for the development of digital logic circuits based on molecules. In this article we show that a linear change of a chemical input can be translated into an exponential change of a luminescence output in a simple fluorescent acid-base switch based on 8-methoxyquinoline. Such unconventional behavior arises from the fact that part of the light emitted by the switch in its basic form is reabsorbed by the acid form, and is made possible by the particular spectroscopic properties of the two forms. Systems of this kind could act as noise filters in analog-to-digital conversion, and as control elements to increase the functional complexity of artificial molecular devices.