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The influence of temperature on C153 steady-state absorption and fluorescence kinetics in hydrogen bonding solvents.

Dobek K, Karolczak J - J Fluoresc (2012)

Bottom Line: 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.We show that while such bonds do not affect the transition probability, they do change the S(0) an S(1) 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.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Physics, Adam Mickiewicz University, Poznań, Poland. dobas@amu.edu.pl

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 S(0) an S(1) 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.

No MeSH data available.


Temperature dependencies of the radiative, (a) kF, and non-radiative, (b) knr, rates in ClH (filled circles), PPN (empty circles), EtOH (filled triangles) and TFEtOH (empty triangles)
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Fig8: Temperature dependencies of the radiative, (a) kF, and non-radiative, (b) knr, rates in ClH (filled circles), PPN (empty circles), EtOH (filled triangles) and TFEtOH (empty triangles)

Mentions: The independence of ∆vS of temperature in 1-chloroalkanes is unexpected as the emitting state is more strongly stabilized than due to the reorientational solvation of the solvent taking place when C153 is relaxed. The energy of this additional stabilizing interaction should increase with decreasing temperature, which in turn should lead to an increase in ∆vS. However, the Onsager model (Eqs. 1 and 2 in [2]) reveal that the theoretical Stokes shift in ClP and ClH changes only by 90 cm-1 within the 120 K temperature interval studied in this work—a value in the range of the error of the points in Fig. 7 for C153 in 1-chloroalkanes. In PPN, EtOH and TFEtOH even a smaller ∆vS change is expected from the Onsager model. In contrast, clear ∆vS (T) dependencies in these solvents are observed. In TFEtOH ∆vS(T) dependence reflects the above-described effects. In PPN and EtOH the increase in ∆vS must be related to an additional stabilization of C153 in state, rising in energy with decreasing temperature. This conclusion is in accordance with the results shown in [2]. The difference between the expected positions of the C153 emission spectra in methanol, PPN and TFEtOH and the experimental ones have been found to show temperature dependencies with the same slope signs as observed in Fig. 7 for ∆vS in the same two solvents. The analysis of the results given in [2] and in this work is based on the continuum solvent Onsager model, thus their reliability can be questioned by those who believe that this model fails in solvation description. Additionally, we have found the absorption spectrum full width at half of the maximum (fwhm) to be higher than that of the emission spectrum in all solvents, at each temperature. According to [18] such an observation can be a manifestation of a nonlinear dependence of the local solute potential on the solute dipole moment. This observation would however need a much deeper study, as absorption and emission spectra fwhm are strongly dependent on the frequency of the most active vibrational mode in a selected type of transition. Our results, shown later, and these presented in [15] reveal this frequency to be higher in the absorbing ground state, , than in the emitting . Nevertheless, the experimental ∆vS(T) dependence obtained for C153 in PPN compared to that found in 1-chloroalkanes gives a very strong argument for the presence of specific interactions between C153 in and PPN molecules. Thus, the results show that C153 can act as a hydrogen donor. The presence of this type of H-bonding, however, does not influence C153 τF(T) dependence, as can be deduced from results presented in Fig. 5. These results show that the protic character of the solvent has a significant impact on the τF(T) dependence, when compared to non-protic solvents (ClP as well [7]). Overall, the shortest lifetime is observed in the most protic TFEtOH, longer in EtOH, ClH, and the longest in PPN. In alcohols a slight modulation of the linear τF(T) dependence can be observed, while in ClH and PPN the values of τF changes with temperature in a similar way as in ClP, that is the slope of τF(T) changes in sign at a temperature in the range 260÷280 K. In both protic solvents a small modulation of τF(T) takes place in the same temperature range in which the change in sign of the τF(T) dependence occurs in ClH and PPN. Thus, we can assume that H-bonding with protic solvents dilutes C153 τF(T) dependence resulting from pure intramolecular deactivation observed in ClH and PPN. Non-negligible is also the retarded solvation, which however as shown for C153 in EtOH is expected to lead to an increase in τF. The τF(T) dependence in neat solvents cover a narrow range of τF values as can be seen in Fig. 6. Such subtle changes in τF are also a manifestation of the narrow ranges in which ϕF changes in all solvents. Together these quantities gives the radiative, kF, and non-radiative, knr, rates. Figure 8 show their temperature dependencies for all four solvents.Fig. 8


The influence of temperature on C153 steady-state absorption and fluorescence kinetics in hydrogen bonding solvents.

Dobek K, Karolczak J - J Fluoresc (2012)

Temperature dependencies of the radiative, (a) kF, and non-radiative, (b) knr, rates in ClH (filled circles), PPN (empty circles), EtOH (filled triangles) and TFEtOH (empty triangles)
© Copyright Policy
Related In: Results  -  Collection

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Fig8: Temperature dependencies of the radiative, (a) kF, and non-radiative, (b) knr, rates in ClH (filled circles), PPN (empty circles), EtOH (filled triangles) and TFEtOH (empty triangles)
Mentions: The independence of ∆vS of temperature in 1-chloroalkanes is unexpected as the emitting state is more strongly stabilized than due to the reorientational solvation of the solvent taking place when C153 is relaxed. The energy of this additional stabilizing interaction should increase with decreasing temperature, which in turn should lead to an increase in ∆vS. However, the Onsager model (Eqs. 1 and 2 in [2]) reveal that the theoretical Stokes shift in ClP and ClH changes only by 90 cm-1 within the 120 K temperature interval studied in this work—a value in the range of the error of the points in Fig. 7 for C153 in 1-chloroalkanes. In PPN, EtOH and TFEtOH even a smaller ∆vS change is expected from the Onsager model. In contrast, clear ∆vS (T) dependencies in these solvents are observed. In TFEtOH ∆vS(T) dependence reflects the above-described effects. In PPN and EtOH the increase in ∆vS must be related to an additional stabilization of C153 in state, rising in energy with decreasing temperature. This conclusion is in accordance with the results shown in [2]. The difference between the expected positions of the C153 emission spectra in methanol, PPN and TFEtOH and the experimental ones have been found to show temperature dependencies with the same slope signs as observed in Fig. 7 for ∆vS in the same two solvents. The analysis of the results given in [2] and in this work is based on the continuum solvent Onsager model, thus their reliability can be questioned by those who believe that this model fails in solvation description. Additionally, we have found the absorption spectrum full width at half of the maximum (fwhm) to be higher than that of the emission spectrum in all solvents, at each temperature. According to [18] such an observation can be a manifestation of a nonlinear dependence of the local solute potential on the solute dipole moment. This observation would however need a much deeper study, as absorption and emission spectra fwhm are strongly dependent on the frequency of the most active vibrational mode in a selected type of transition. Our results, shown later, and these presented in [15] reveal this frequency to be higher in the absorbing ground state, , than in the emitting . Nevertheless, the experimental ∆vS(T) dependence obtained for C153 in PPN compared to that found in 1-chloroalkanes gives a very strong argument for the presence of specific interactions between C153 in and PPN molecules. Thus, the results show that C153 can act as a hydrogen donor. The presence of this type of H-bonding, however, does not influence C153 τF(T) dependence, as can be deduced from results presented in Fig. 5. These results show that the protic character of the solvent has a significant impact on the τF(T) dependence, when compared to non-protic solvents (ClP as well [7]). Overall, the shortest lifetime is observed in the most protic TFEtOH, longer in EtOH, ClH, and the longest in PPN. In alcohols a slight modulation of the linear τF(T) dependence can be observed, while in ClH and PPN the values of τF changes with temperature in a similar way as in ClP, that is the slope of τF(T) changes in sign at a temperature in the range 260÷280 K. In both protic solvents a small modulation of τF(T) takes place in the same temperature range in which the change in sign of the τF(T) dependence occurs in ClH and PPN. Thus, we can assume that H-bonding with protic solvents dilutes C153 τF(T) dependence resulting from pure intramolecular deactivation observed in ClH and PPN. Non-negligible is also the retarded solvation, which however as shown for C153 in EtOH is expected to lead to an increase in τF. The τF(T) dependence in neat solvents cover a narrow range of τF values as can be seen in Fig. 6. Such subtle changes in τF are also a manifestation of the narrow ranges in which ϕF changes in all solvents. Together these quantities gives the radiative, kF, and non-radiative, knr, rates. Figure 8 show their temperature dependencies for all four solvents.Fig. 8

Bottom Line: 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.We show that while such bonds do not affect the transition probability, they do change the S(0) an S(1) 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.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Physics, Adam Mickiewicz University, Poznań, Poland. dobas@amu.edu.pl

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 S(0) an S(1) 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.

No MeSH data available.