Chloride anion controlled molecular 'switching'. Binding of 2,5,7-trinitro-9-dicyanomethylenefluorene-C60 by tetrathiafulvalene calix[4]]pyrrole and photophysical generation of two different charge-separated states

The binding of the snake-like trinitrodicyanomethylenefluorene-C60 derivative (TNDCF-C60) to the dynamic receptor, tetrathiafulvalene calix[4]pyrrole (TTF-calix[4]pyrrole), may be controlled via the use of a chloride anion as an external trigger. Whereas, in the absence of a chloride anion, the TNDCF "tail" of the trinitrodicyanomethylenefluorene-C60 substrate binds to the TTF−calix[4]pyrrole in a 2:1 (substrate/receptor) stoichiometry in CH2Cl2 solution, addition of a chloride anion (yellow) leads the TNDCF tail to be displaced in favor of a bound C60 "head", a process that leads to the formation of a complex with overall 1:2:2 substrate/receptor/chloride anion stoichiometry. These chemical switching events are reflected in easy-to-visualize color changes, as well as in the production of two different kinds of charge-separated states following selective femtosecond photoexcitation.