A family of molecular switches based on the indanylidene–pyrroline (IP) framework is designed to mimic the photoisomerization in retinal proteins. The IP framework can also be regarded as a conformationally locked derivative of the retinal protonated Schiff base. The relevant features of these systems are:

• the indanylidene and pyrroline units are conformationally rigid and connected by a single exocyclic double bond.
• the indanylidene can be functionalized with protected carboxyl and amino groups at the same quaternary carbon. This allows the switch to be integrated in a protein backbone.
• the pyrroline unit can host a permanent zwitterion with a dipole moment orthogonal to the double bond. Hence, it can be used as an effective dipole moment switc

The main design principles for a molecular switch are:

1. Efficient energy funneling into the isomerization mode vibrational coherence
2. Photochromism
3. Unidirectional isomerization (chirality)

Recently, we have studied the origin of the 1. principle, namely the vibrational coherence. For this purpose we have used nonadiabatic QM/MM molecular dynamics simulations. Based on these simulations we have identified a double-bond rotation coupled to ring inversion that induced a periodic modulation of the overlap of the pi-orbitals. The video below shows one QM/MM trajectory of a successful isomerization.