Direct Observation of a Dark State in the Photocycle of a Light-Driven Molecular Motor

Controlling the excited-state properties
of light driven molecular
machines is crucial to achieving high efficiency and directed functionality.
A key challenge in achieving control lies in unravelling the complex
photodynamics and especially in identifying the role played by dark
states. Here we use the structure sensitivity and high time resolution
of UV-pump/IR-probe spectroscopy to build a detailed and comprehensive
model of the structural evolution of light driven molecular rotors.
The photodynamics of these chiral overcrowded alkene derivatives are
determined by two close-lying excited electronic states. The potential
energy landscape of these “bright” and “dark”
states gives rise to a broad excited-state electronic absorption band
over the entire mid-IR range that is probed for the first time and
modeled by quantum mechanical calculations. The transient IR vibrational
fingerprints observed in our studies allow for an unambiguous identification
of the identity of the “dark” electronic excited state
from which the photon’s energy is converted into motion, and
thereby pave the way for tuning the quantum yield of future molecular
rotors based on this structural motif.