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Durham University

Department of Chemistry

Publication details for Prof. Andrew Beeby

Roy, Khokan, Kayal, Surajit, Kumar, Venkatraman Ravi, Beeby, Andrew, Ariese, Freek & Umapathy, Siva (2017). Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study. The Journal of Physical Chemistry 121(35): 6538-6546.

Author(s) from Durham

Abstract

Excited state ultrafast conformational reorganization is
recognized as an important phenomenon that facilitates light-induced
functions of many molecular systems. This report describes the
femtosecond and picosecond conformational relaxation dynamics of
middle-ring and terminal ring twisted conformers of the acetylene π-
conjugated system bis(phenylethynyl)benzene, a model system for
molecular wires. Through excitation wavelength dependent, femtosecond-transient
absorption measurements, we found that the middlering
and terminal ring twisted conformers relax at femtosecond (400−600
fs) and picosecond (20−24 ps) time scales, respectively. Actinic pumping
into the red flank of the absorption spectrum leads to excitation of
primarily planar conformers, and results in very different excited state
dynamics. In addition, ultrafast Raman loss spectroscopic studies revealed
the vibrational mode dependent relaxation dynamics for different excitation wavelengths. To corroborate our experimental
findings, DFT and time-dependent DFT calculations were carried out. The Franck−Condon simulation indicated that the
vibronic structure observed in the electronic absorption and the fluorescence spectra are due to progressions and combinations of
several vibrational modes corresponding to the phenyl ring and the acetylenic groups. Furthermore, the middle ring torsional
rotation matches the room-temperature electronic absorption, in stark contrast to the terminal ring torsional rotation. Finally, we
show that the middle-ring twisted conformer undergoes femtosecond torsional planarization dynamic, whereas the terminal rings
relax on a few tens of picosecond time scale.