Conspectus
The achievement of artificial systems capable
of being maintained
in out-of-equilibrium states featuring functional properties is a
main goal of current chemical research. Absorption of electromagnetic
radiation or consumption of a chemical species (a “chemical
fuel”) are the two strategies typically employed to reach such
out-of-equilibrium states, which have to persist as long as one of
the above stimuli is present. For this reason such systems are often
referred to as “dissipative systems”. In the simplest
scheme, the dissipative system is initially found in a resting, equilibrium
state. The addition of a chemical fuel causes the system to shift
to an out-of-equilibrium state. When the fuel is exhausted, the system
reverts to the initial, equilibrium state. Thus, from a mechanistic
standpoint, the dissipative system turns out to be a catalyst for
the fuel consumption. It has to be noted that, although very simple,
this scheme implies the chance to temporally control the dissipative
system. In principle, modulating the nature and/or the amount of the
chemical fuel added, one can have full control of the time spent by
the system in the out-of-equilibrium state.
In 2016, we found
that 2-cyano-2-phenylpropanoic acid (1a), whose decarboxylation
proceeds smoothly under mild basic conditions,
could be used as a chemical fuel to drive the back and forth motion
of a catenane-based molecular switch. The acid donates a proton to
the catenane that passes from the neutral state A to the transient
protonated state B. Decarboxylation of the resulting carboxylate (1acb), generates a carbanion, which, being a strong base,
retakes the proton from the protonated catenane that, consequently,
returns to the initial state A. The larger the amount of the added
fuel, the longer the time spent by the catenane in the transient,
out-of-equilibrium state. Since then, acid 1a and other
activated carboxylic acids (ACAs) have been used to drive the operation
of a large number of dissipative systems based on the acid–base
reaction, from molecular machines to host–guest systems, from
catalysts to smart materials, and so on. This Account illustrates
such systems with the purpose to show the wide applicability of ACAs
as chemical fuels. This generality is due to the simplicity of the
idea underlying the operation principle of ACAs, which always translates
into simple experimental requirements.