Collisional relaxation of apocarotenals: identifying the S state with vibrationally excited molecules in the ground electronic state S-0

2015 | journal article. A publication with affiliation to the University of Göttingen.

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​Collisional relaxation of apocarotenals: identifying the S state with vibrationally excited molecules in the ground electronic state S-0 ​
Ehlers, F.; Scholz, M.; Schimpfhauser, J.; Bienert, J.; Oum, K. & Lenzer, T.​ (2015) 
Physical Chemistry Chemical Physics17(16) pp. 10478​-10488​.​ DOI: https://doi.org/10.1039/c4cp05600k 

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Authors
Ehlers, Florian; Scholz, Mirko; Schimpfhauser, Jens; Bienert, Juergen; Oum, Kawon; Lenzer, Thomas
Abstract
In recent work, we demonstrated that the S signal of beta-carotene observed in transient pump-supercontinuum probe absorption experiments agrees well with the independently measured steady-state difference absorption spectrum of vibrationally hot ground state molecules S-0 in solution, recorded at elevated temperatures (Oum et al., Phys. Chem. Chem. Phys., 2010, 12, 8832). Here, we extend our support for this "vibrationally hot ground state model' of S by experiments for the three terminally aldehyde-substituted carotenes beta-apo-12'-carotenal, beta-apo-4'-carotenal and 3',4'-didehydro-beta,Psi-caroten-16'-al ("torularhodinaldehyde') which were investigated by ultrafast pump-supercontinuum probe spectroscopy in the range 350-770 nm. The apocarotenals feature an increasing conjugation length, resulting in a systematically shorter S-1 lifetime of 192, 4.9 and 1.2 ps, respectively, in the solvent n-hexane. Consequently, for torularhodinaldehyde a large population of highly vibrationally excited molecules in the ground electronic state is quickly generated by internal conversion (IC) from S-1 already within the first picosecond of relaxation. As a result, a clear S signal is visible which exhibits the same spectral characteristics as in the aforementioned study of b-carotene: a pronounced S-0 -> S-2 red-edge absorption and a "finger-type' structure in the S-0 -> S-2 bleach region. The cooling process is described in a simplified way by assuming an initially formed vibrationally very hot species S-0 which subsequently decays with a time constant of 3.4 ps to form a still hot S-0 species which relaxes with a time constant of 10.5 ps to form S-0 molecules at 298 K. beta-Apo-4'-carotenal behaves in a quite similar way. Here, a single vibrationally hot S-0 species is sufficient in the kinetic modeling procedure. S-0 relaxes with a time constant of 12.1 ps to form cold S-0. Finally, no S-0 features are visible for beta-apo-12'-carotenal. In that case, the S-1 -> S-0 IC process is expected to be roughly 20 times slower than S-0 relaxation. As a result, no spectral features of S-0 can be found, because there is no chance that a detectable concentration of vibrationally hot molecules is accumulated.
Issue Date
2015
Status
published
Publisher
Royal Soc Chemistry
Journal
Physical Chemistry Chemical Physics 
ISSN
1463-9084; 1463-9076

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