We present a joint theory–experiment study investigating the excitonic
absorption of spiropyran-functionalized carbon nanotubes. The
functionalization is promising for engineering switches on a molecular level,
since spiropyrans can be reversibly switched between two different
conformations, inducing a distinguishable and measurable change of optical
transition energies in the substrate nanotube. Here, we address the question
of whether an optical read-out of such a molecular switch is possible.
Combining density matrix and density functional theory, we first calculate the
excitonic absorption of pristine and functionalized nanotubes. Depending on
the switching state of the attached molecule, we observe a red-shift of
transition energies by about 15 meV due to the coupling of excitons with the
molecular dipole moment. Then we perform experiments measuring the absorption
spectrum of functionalized carbon nanotubes for both conformations of the
spiropyran molecule. We find good qualitative agreement between the
theoretically predicted and experimentally measured red-shift, confirming the
possibility for an optical read-out of the nanotube-based molecular switch