The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes

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...

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Bibliographic Details
Published in:Chemical science (Cambridge) 2019-12, Vol.1 (48), p.1113-1122
Main Authors: Hoche, Joscha, Schulz, Alexander, Dietrich, Lysanne Monika, Humeniuk, Alexander, Stolte, Matthias, Schmidt, David, Brixner, Tobias, Würthner, Frank, Mitric, Roland
Format: Article
Language:English
Subjects:
Channels
Chemical compounds
Chromophores
Crystallography
Decay
Dyes
Energy gap
Fluorescence
Low temperature
Optimization
Polarity
Solvents
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Summary: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-4 H -pyran ( DCM ), i.e. , merocyanine 4-(dicyanomethylene)-2- tert -butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4 H -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. An increasing activation energy barrier to a conical intersection was identified as the reason for higher fluorescence lifetimes and quantum yields for merocyanines in polar solvents.
ISSN:2041-6520
2041-6539
DOI:10.1039/c9sc05012d