Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations
Type
ArticleAuthors
Munkerup, Kristin
Romanov, Dmitri A.
Bohinski, Timothy
Stephansen, Anne B.
Levis, Robert J.
Sølling, Theis Ivan
KAUST Department
Chemical Science ProgramKAUST Catalysis Center (KCC)
Physical Science and Engineering (PSE) Division
Date
2017-11-07Online Publication Date
2017-11-07Print Publication Date
2017-11-16Permanent link to this record
http://hdl.handle.net/10754/625962
Metadata
Show full item recordAbstract
Light harvesting via energy storage in azobenzene has been a key topic for decades, and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of non-ergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics, as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1 surface with little excess energy, at the equilibrium neutral geometry. These D1 ions are preferably formed because in this case D1 and D0 switch place in the presence of the strong laser field. The post-ionization dynamics is dictated by the potential energy landscape. The D1 surface is steep downhill along the cis/trans isomerization coordinate and towards a common minimum shared by the two isomers in the region of D1/D0 conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0 becomes populated with molecules that are energized mainly along the cis-trans isomerization coordinate, with the kinetic energy above the cis-trans inter-conversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0 surface, and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole-lone pair interaction that drives the molecule along the cis-trans isomerization coordinate to facilitate the transition between the involved electronic states.Citation
Munkerup K, Romanov DA, Bohinski T, Stephansen AB, Levis RJ, et al. (2017) Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations. The Journal of Physical Chemistry A. Available: http://dx.doi.org/10.1021/acs.jpca.7b09185.Sponsors
We gratefully acknowledge the generous support of the Villum foundation.Publisher
American Chemical Society (ACS)Additional Links
http://pubs.acs.org/doi/pdf/10.1021/acs.jpca.7b09185ae974a485f413a2113503eed53cd6c53
10.1021/acs.jpca.7b09185