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Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study

Roy, Khokan; Kayal, Surajit; Kumar, Venkatraman Ravi; Beeby, Andrew; Ariese, Freek; Umapathy, Siva

Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study Thumbnail


Authors

Khokan Roy

Surajit Kayal

Venkatraman Ravi Kumar

Freek Ariese

Siva Umapathy



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.

Citation

Roy, K., Kayal, S., Kumar, V. R., Beeby, A., Ariese, F., & Umapathy, S. (2017). Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study. The Journal of Physical Chemistry A, 121(35), 6538-6546. https://doi.org/10.1021/acs.jpca.7b03893

Journal Article Type Article
Acceptance Date Apr 25, 2017
Online Publication Date Jun 30, 2017
Publication Date Jun 30, 2017
Deposit Date Oct 11, 2017
Publicly Available Date Mar 28, 2024
Journal The Journal of Physical Chemistry A
Print ISSN 1089-5639
Electronic ISSN 1520-5215
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
Volume 121
Issue 35
Pages 6538-6546
DOI https://doi.org/10.1021/acs.jpca.7b03893

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Copyright Statement
This document is the Accepted Manuscript version of a Published Work that appeared in final form in The journal of physical chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.jpca.7b03893





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