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

Joscha Hoche,Lysanne Monika Dietrich,Frank Würthner,Tobias Brixner,Alexander Schulz,Alexander Humeniuk,Roland Mitric,David Schmidt,Matthias Stolte

doi:10.1039/c9sc05012d

Joscha Hoche, Lysanne Monika Dietrich + Show 7 more

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https://doi.org/10.1039/c9sc05012d

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Abstract

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.

Highlights

Understanding and predicting photophysical and electronic properties of uorophores are a prerequisite for many applications including biomolecular imaging,[1] organic lasers,[2] and lightemitting molecular devices.[3,4] The usefulness of many organic uorophores depends directly on the uorescence quantum yield, which is de ned by the competition of uorescence to all other non-radiative relaxation channels
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
We have synthesized a new dipolar merocyanine dye and measured its uorescence quantum yield as a function of the solvent polarity and temperature

Summary

Introduction

Understanding and predicting photophysical and electronic properties of uorophores are a prerequisite for many applications including biomolecular imaging,[1] organic lasers,[2] and lightemitting molecular devices.[3,4] The usefulness of many organic uorophores depends directly on the uorescence quantum yield, which is de ned by the competition of uorescence to all other non-radiative relaxation channels. The ability to uoresce is usually linked to a rigid, conjugated molecular structure which ensures that the excited state has a stable minimum.[5–7]. Non- uorescent chromophores have reactive excited states and decompose or decay to the. Edge Article a stable minimum close to the Franck–Condon point from a conical intersection, it is legitimate to assume that the reactions in the excited state are governed by the rules of thermodynamics, so that one can apply transition-state theory for estimating the temperature-dependent rates of non-radiative decay.[25,36]. The energetic position of the barrier and conical intersection can be tuned by substitutions as demonstrated for naphthalene derivatives.[27]. This situation is evidenced by the temperature dependence of the non-radiative rate

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