Abstract
In a recent paper (J Fluoresc (2011) 21:1547–1557) a temperature induced modulation of Coumarin 153 (C153) fluorescence lifetime and quantum yield for the probe dissolved in the polar, nonspecifically interacting 1-chloropropane was reported. This modulation was also observed in temperature dependencies of the radiative and nonradiative rates. Here, we show that the modulation is also observed in another 1-chloroalkane—1-chlorohexane, as well as in hydrogen bonding propionitrile, ethanol and trifluoroethanol. Change in the equilibrium distance between S 0 an S 1 potential energies surfaces was identified as the source of this modulation. This change is driven by temperature changes. It leads to a modulation of the fluorescence transition dipole moment and it is the primary source of the experimental effects observed. Additionally, we have found that proticity of the solvent induces a rise in the fluorescence transition dipole moment, which leads to a shortening of the fluorescence lifetime. Hydrogen bonds are formed by C153 also with hydrogen accepting solvents like propionitrile. We show that while such bonds do not affect the transition probability, they do change the S0 an S1 energy gap which in turn implies a change in non-radiative transition rate in a similar way as in protic solvents, as well as in the fluorescence spectrum position. Finally, the influence of temperature on the energies of hydrogen bonds formed by C153 when acting as hydrogen donor or acceptor is reported.
Highlights
The thermochromism of simple dye molecules is a good indicator of changes in the dye environment polarity following changes in temperature
A subtle difference in the shape of the spectra recorded at the two temperatures can be noted for C153 dissolved in 1chloropropane (ClP) and ClH
These results show that the protic character of the solvent has a significant impact on the τF(T) dependence, when compared to nonprotic solvents (ClP as well [7])
Summary
The thermochromism of simple dye molecules is a good indicator of changes in the dye environment polarity following changes in temperature. For a dye interacting with the solvent molecules, temperature influence on the energy of this interaction can affect much more significantly the scale of thermochromic shifts than for a dye showing nonspecific interaction, related to n(T) and ε(T) dependencies. This additional shift can be of the sign the same as or the opposite to that resulting from nonspecific interactions.
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