TY  - JOUR
A1  - Hoche, Joscha
A1  - Schulz, Alexander
A1  - Dietrich, Lysanne Monika
A1  - Humeniuk, Alexander
A1  - Stolte, Matthias
A1  - Schmidt, David
A1  - Brixner, Tobias
A1  - Würthner, Frank
A1  - Mitric, Roland
T1  - The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes
JF  - Chemical Science
N2  - Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM),i.e., merocyanine 4-(dicyanomethylene)-2-tert-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4H-pyran   (DCBT).   We   measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law.This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.
KW  - solvent-dependent fluorescence yield
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-198707
UR  - https://doi.org/10.1039/C9SC05012D
VL  - 10
ER  - 
TY  - INPR
A1  - Lambert, Christoph
A1  - Völker, Sebastian F.
A1  - Koch, Federico
A1  - Schmiedel, Alexander
A1  - Holzapfel, Marco
A1  - Humeniuk, Alexander
A1  - Röhr, Merle I. S.
A1  - Mitric, Roland
A1  - Brixner, Tobias
T1  - Energy Transfer Between Squaraine Polymer Sections: From helix to zig-zag and All the Way Back
T2  - Journal of the American Chemical Society
N2  - Joint experimental and theoretical study of the absorption spectra of squaraine polymers in solution provide evidence that two different conformations are present in solution: a helix and a zig-zag structure. This unique situation allows investigating ultrafast energy transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. We combine here femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy showing that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited helix segments to zig-zag segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of very small reorganization energy in these cyanine-like chromophores.
KW  - energy transfer dynamics
KW  - squaraine polymer
Y1  - 2015
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-159607
UR  - http://dx.doi.org/10.1021/jacs.5b03644
N1  - This document is the unedited Author's version of a Submitted Work that war subsequently accepted for publication in Journal of the American Chemical Society, copyright American Chemical Society after peer review. To access the final edited and published work see doi:10.1021/jacs.5b03644.
ER  -