Optical Control of Conjugated Oligomer Planarity

Using a sequential photo-excitation mechanism we observe the ultrafast conformational planarization of a large fluorene oligomer at $\sim $ 60fs timescale. Novel non-adiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions has been used to rationalize this phenomenon. Simulation show the ultrafast relaxation of the photoexcited wavepacket toward the lowest electronic excited state along the torsional coordinate. The process effectively `locks' the oligomer into a planar state within 100~fs, with excess energy being dissipated into other vibrational modes. Ultrafast control of molecular conformation, as demonstrated here, could have impacts for molecular conformational switches for memory or molecular electronics.