Chapter 4: Enhancement of H-bond mediated photoinduced electron transfer of upon
4.2. Results
4.2.2. Time-resolved measurements
To ensure the effect of H-bonding on the observed fluorescence quenching of C102- AN, we have used DMA which has no H-bonding ability. Figure 4.4 displays the absorption and emission spectra of C102 in cyclohexane-DMA mixture at various mole fractions of DMA. Upon addition of DMA to cyclohexane we have observed that both absorption and emission spectra of C102 remains unchanged. Yoshihara and co-workers has also observed no significant fluorescence quenching of C102 in neat DMA.18 Hence, the observed fluorescence quenching in the case of the C102–aniline system in a nonpolar solvent may proceed via a H-bond assisted mechanism.
Figure 4.4: Absorption (left panel) and emission (right panel) spectra of C102 in cyclohexane-DMA mixture at different mole fractions of DMA, XDMA.
fluorescence observed in the steady-state measurement was also manifested in the lifetime measurements. The fluorescence decays of C102 measured at corresponding steady-state emission maxima at different mole fractions of aniline in cyclohexane-aniline and toluene- aniline mixtures are shown in figure 4.5 and figure 4.6, respectively. The average fluorescence decay time of C102 decreased gradually up to the same critical mole fraction where the emission intensity was minimum. After that, the trend reversed and the average fluorescence lifetime increased with a further increase in XAN. Fluorescence decays were found to be bi-exponential in the solvent mixtures with two distinct time components (Table 4.2 and Table 4.3). The amplitude of the faster component (70–170 ps) increased gradually up to the critical mole fraction after that decreases with further increase in the mole fraction.
The fast component may be due to the PET of the H-bonded complex. On the other hand the slower component may be due to either a non-H-bonded complex or an improperly oriented H-bonded complex.
Figure 4.5: Fluorescence decays of C102 in the cyclohexane-aniline mixture at different mole fractions of aniline, XAN. Left panel represents the decay of C102 upto 0.075 mole fraction of aniline which becomes gradually faster. Right panel represents the decay of C102 from 0.075 to 1.0 mole fraction of aniline which becomes gradually slower with enrichment of aniline. Fluorescence decays were measured at corresponding emission maxima of C102.
Figure 4.6: Fluorescence decays of C102 in the toluene-aniline mixture at different mole fractions of aniline, XAN. Left panel represents the decay of C102 upto 0.13 mole fraction of aniline which becomes gradually faster. Right panel represents the decay of C102 from 0.13 to 1.0 mole fraction of aniline which becomes gradually slower with enrichment of aniline.
Fluorescence decays were measured at corresponding emission maxima of C102.
Figure 4.7: Fluorescence decays of C102 in the cyclohexane-aniline mixture at different mole fractions of aniline, XAN. Arrows indicates mode of the lifetime variation with increase in XAN. The decays were measured at 410 nm.
In addition, to understand if there exists any micro influence of solvation dynamics
wavelengths corresponding to the emission maxima of C102 in cyclohexane; 460 nm for aniline) (
anomalous trend of the fluorescence
For all these three wavelengths, fluorescence decays become faster upon the aniline to the non-interacting solvent (cyclohexane
and afterwards becomes gradually slower upon further addition.
Figure 4.8: Fluorescence decays of C102 in the cyclohexane mole fractions of aniline, XAN
in XAN. The decays were measured at 460 nm.
However, comparison of
wavelength decays are faster but progressive
4.9). This wavelength dependence may be due to the solvation dynamics solvent mixture. For a mixture of non
understand if there exists any micro-heterogeneity
influence of solvation dynamics, we have also measured the fluorescence decays at the to the emission maxima of C102 in neat solvents
aniline) (Figure 4.7 and Figure 4.8). We
fluorescence lifetime remains invariant on the detection wavelengths.
three wavelengths, fluorescence decays become faster upon the
interacting solvent (cyclohexane or toluene) up to the critical concentration becomes gradually slower upon further addition.
: Fluorescence decays of C102 in the cyclohexane-aniline mixture at different
AN. Arrows indicates mode of the lifetime variation with increase re measured at 460 nm.
comparison of the decays at the critical mole fraction showed that at shorter but progressively become slower at higher wavelengths (
wavelength dependence may be due to the solvation dynamics
solvent mixture. For a mixture of non-polar and polar solvents, solvation dynamics is usually heterogeneity or there is any the fluorescence decays at the neat solvents (410 nm for ). We observed that the invariant on the detection wavelengths.
three wavelengths, fluorescence decays become faster upon the addition of or toluene) up to the critical concentration
aniline mixture at different . Arrows indicates mode of the lifetime variation with increase
showed that at shorter become slower at higher wavelengths (Figure wavelength dependence may be due to the solvation dynamics of C102 in the polar solvents, solvation dynamics is usually
much slower particularly at a low amount of polar component compared to a neat polar solvent.161-162 Yoshihara and co-workers reported that the average solvation time in neat aniline is 13.2 ps.84 We may expect that solvation dynamics in a cyclohexane–aniline or toluene–aniline mixture is much slower particularly at low aniline content.
Figure 4.9: Fluorescence decays of C102 in the cyclohexane-aniline (left panel) and toluene- aniline (right panel) mixtures at three different emission wavelengths. Mole fractions of aniline, XAN in cyclohexane-aniline and toluene-aniline mixtures are 0.075 and 0.13, respectively.