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Understanding ultrafast dynamics of conformation specific photo-excitation : a femtosecond transient absorption and ultrafast Raman loss study.

Roy, Khokan and Kayal, Surajit and Kumar, Venkatraman Ravi and Beeby, Andrew and Ariese, Freek and 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). pp. 6538-6546.

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.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1021/acs.jpca.7b03893
Publisher 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
Date accepted:25 April 2017
Date deposited:11 October 2017
Date of first online publication:30 June 2017
Date first made open access:No date available

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