222
S.-L. Wang et al. / Chemical Physics Letters 415 (2005) 217–222
Fig. 4. For an aprotic solvent the Kirkwood function
(fe = (e ꢁ 1)/(2e + 1)), a solvation scale to measure nonspe-
cific interaction, is used in Fig. 4a. For a protic solvent the
hydrogen bond acidity, a, developed by Taft is used to ex-
plore the influence of HB to the Stokes shift [20]. A good
practice the proton transfer process in the excited state.
For compounds 2-StT-NMe2 and 2-StN-NMe2, the value
of DpKa is too small to induce the ESDP process. Thus only
an ESDHB process, which possesses a smaller reaction bar-
rier than ESDP, was observed in this study. Using this
mechanism, we can probe the polarity and HB effect of
an interesting microenvironment at the same time.
linear correlation exists between the Stokes shift (cmꢁ1
)
and different solvent polarity functions, except for the pro-
tic solvent, as seen in Fig. 4a. But in Fig. 4b, a good linear
correlation exists between the Stokes shift and hydrogen
bond acidity. The correlation indicates that nonspecific
interaction possesses a dominant importance in aprotic sol-
vents, but HB interactions do exist between 2-StN-NMe2
molecules and protic solvents.
The value of the Stokes shift obtained in protic solvents
was higher than the one measured in aprotic solvents. The
HB interaction between N,N-dimethylaniline site and pro-
tic solvents was stronger in the ground state, leading to
hypsochromic shifts in the absorption maxima with
increasing a. This bond breaks after photoexcitation, so
the emission maxima are not seriously influenced by the
HB interaction, resulting in a large Stokes shift.
By calculating the deviation from linearity of protic sol-
vents, we can quantify the influence of HB, as shown in
Fig. 4a. These values are listed in Table 2. Unlike the pro-
tonation effect, the steric effect is significant for HB interac-
tions [12]. Table 2 shows that the HB interaction contributes
a positive Stokes shift only when the hydrogen bond acidity
is higher than the ethylene glycol. This means that the
ground-state HB interaction is not important for ethanol
because of its lower HB acidity and its bulky steric effect.
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4. Conclusion
Compounds of 2-StP-NMe2, 2-StQ-NMe2, 4-StQ-NMe2,
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