Abstract
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In nature, light is harvested by photoactive proteins to drive
a range of biological processes, including photosynthesis, phototaxis,
vision, and ultimately life. Bacteriorhodopsin, for example, is a
protein embedded within archaeal cell membranes that binds the chromophore
retinal within its hydrophobic pocket. Exposure to light triggers
regioselective photoisomerization of the confined retinal, which in
turn initiates a cascade of conformational changes within the protein,
triggering proton flux against the concentration gradient, providing
the microorganisms with the energy to live. We are inspired by these
functions in nature to harness light energy using synthetic photoswitches
under confinement. Like retinal, synthetic photoswitches require some
degree of conformational flexibility to isomerize. In nature, the
conformational change associated with retinal isomerization is accommodated
by the structural flexibility of the opsin host, yet it results in
steric communication between the chromophore and the protein. Similarly,
we strive to design systems wherein isomerization of confined photoswitches
results in steric communication between a photoswitch and its confining
environment. To achieve this aim, a balance must be struck between
molecular crowding and conformational freedom under confinement: too
much crowding prevents switching, whereas too much freedom resembles
switching of isolated molecules in solution, preventing communication.
In this Account, we discuss five classes of synthetic light-switchable
compounds—diarylethenes, anthracenes, azobenzenes, spiropyrans,
and donor–acceptor Stenhouse adducts—comparing their
behaviors under confinement and in solution. The environments employed
to confine these photoswitches are diverse, ranging from planar surfaces
to nanosized cavities within coordination cages, nanoporous frameworks,
and nanoparticle aggregates. The trends that emerge are primarily
dependent on the nature of the photoswitch and not on the material
used for confinement. In general, we find that photoswitches requiring
less conformational freedom for switching are, as expected, more straightforward
to isomerize reversibly under confinement. Because these compounds
undergo only small structural changes upon isomerization, however,
switching does not propagate into communication with their environment.
Conversely, photoswitches that require more conformational freedom
are more challenging to switch under confinement but also can influence
system-wide behavior.
Although we are primarily interested in
the effects of geometric
constraints on photoswitching under confinement, additional effects
inevitably emerge when a compound is removed from solution and placed
within a new, more crowded environment. For instance, we have found
that compounds that convert to zwitterionic isomers upon light irradiation
often experience stabilization of these forms under confinement. This
effect results from the mutual stabilization of zwitterions that are
brought into close proximity on surfaces or within cavities. Furthermore,
photoswitches can experience preorganization under confinement, influencing
the selectivity and efficiency of their photoreactions. Because intermolecular
interactions arising from confinement cannot be considered independently
from the effects of geometric constraints, we describe all confinement
effects concurrently throughout this Account.
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