The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

In some molecular systems, such as nucleobases, polyenes or the active ingredients of sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale, raising questions such as: where does this energy go or among which degrees of freedom it is being distribut...

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Bibliographic Details
Published in:Chemical science (Cambridge) 2019-05, Vol.10 (18), p.4792-4804
Main Authors: Balevičius, Jr, Vytautas, Wei, Tiejun, Di Tommaso, Devis, Abramavicius, Darius, Hauer, Jürgen, Polívka, Tomas, Duffy, Christopher D P
Format: Article
Language:English
Subjects:
Carotenoids
Cooling
Excitation
Excitation spectra
Heating
Molecular dynamics
Organic chemistry
Restoration
Solvents
Subsystems
Sun screens
Vibration monitoring
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Summary:In some molecular systems, such as nucleobases, polyenes or the active ingredients of sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale, raising questions such as: where does this energy go or among which degrees of freedom it is being distributed at such early times? Here we use transient absorption spectroscopy to track excitation energy dispersing from the optically accessible vibronic subsystem into the remaining vibrational subsystem of the solute and solvent. Monitoring the flow of energy during vibrational redistribution enables quantification of local molecular heating. Subsequent heat dissipation away from the solute molecule is characterized by classical thermodynamics and molecular dynamics simulations. Hence, we present a holistic approach that tracks the internal temperature and vibronic distribution from the act of photo-excitation to the restoration of the global equilibrium. Within this framework internal vibrational redistribution and vibrational cooling are emergent phenomena. We demonstrate the validity of the framework by examining a highly controversial example, carotenoids. We show that correctly accounting for the local temperature unambiguously explains their energetically and temporally congested spectral dynamics without the postulation of additional 'dark' states. An immediate further application of this approach would be to monitor the excitation and thermal dynamics of pigment-protein systems.
ISSN:2041-6520
2041-6539
DOI:10.1039/c9sc00410f