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.
