A.K. Vasu et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 55–62
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Upon an increase in concentration of the fluorophore, an initial
increase in intensity of the long wavelength band followed by
depending on the excitation wavelength, shifts in emission
wavelength were observed [43,44]. The interaction of ILs with
some dipolar molecules also leads to such excitation dependent
emission wavelength [23]. We therefore examined the excitation
wavelength (lex) dependence for dienes (1–5) in organic solvents
and IL media. As shown in Fig. S7 ESI, at lex of 350 nm, diene (1),
show two emission regions, one a structured emission band with a
maximum at ꢂ435 nm and another long wavelength band at
ꢂ592 nm in ethanol. As the excitation wavelength is increased
intensity drop was observed as
a result of concentration
quenching. In the case of diene (5), at very high optical density
(0.91 or 1.74) two clear emission bands one at 513 nm and another
at ꢂ670 nm are seen) [Fig. 4a]. If the emission is due to IL alone, a
drop in fluorescence intensity was not anticipated. Therefore we
attribute the short wavelength band due to the LE state and the
long wavelength band due to the CT state: characteristic dual
fluorescence behaviour. Other dienes also show comparable
concentration quenching behaviour. In all the cases, there is not
any appearance of a new emission band [Fig S5]. The emission at
ꢂ515 nm, for (5), closely correlates with the emission measured in
non-polar solvents such as heptane or methylcyclohexane [33].
Taking into account this emission data, one can hypothesize that
the fluorophore has preferential hydrophobic interaction with the
IL environment bearing predominantly hydrophobic alkyl side
chains. The longer wavelength band at ꢂ675 nm corresponds to the
interaction of the fluorophore with the polar environment [Fig. 4a]
of ionic liquids. In the case of diene (4), we can also notice two
emission bands at optical densities of 0.31 to 0.81 which upon
further increase in concentration results in the more dominant CT
band [Fig. 4b]. Therefore based on these observations it is likely
that fluorophores experience differential interaction with diverse
microenvironments of ionic liquids and contribute to dual
emission characteristics. This behaviour is also observed in organic
solvents such as ethanol but not seen in dioxane. At certain
concentrations dual emission behaviour, is also clearly noted
[Fig. 5] even in ethanol. It should be noted that solubility of dienes
in ionic liquids is not as quick as in organic solvents (dioxane,
ethanol or acetonitrile). Though the samples were sonicated prior
to the measurement, small contribution from the emission due to
aggregates may also occur.
[lex(420 nm)], the structured emission peaks decrease in intensity
and a stronger emission band appears at ꢂ592 nm. Dienes (2) and
(3) also have analogous observations and the spectral details are
given in ESI [Fig S7]. On the other hand, diene (4) containing a
methoxy donor and a nitro acceptor group show a different
excitation dependent emission pattern [Fig. 6a]. Upon excitation at
the blue end of the absorption spectrum (lex: 360 nm), structured
emission at ꢂ409, 435 and 458 nm was noted along with a broad
drooping shoulder. With increasing lex(380 nm, 400 nm, 420 nm) a
small but gradual shift in the emission in the range of
ꢂ545–565 nm was seen. Excitation at red-end (lex:440 nm) of
the absorption spectrum results in emission at ꢂ572 nm. Further
excitation at the extreme red end of the absorption spectrum
(lex:480 nm) results in a strongly red-shifted emission at ꢂ615 nm.
This is a surprising observation. Although one can ascribe this to
characteristic Red-Edge excitation (REE) phenomena, REE is most
often observed with membrane bound probes or in viscous media
[45,46]. Therefore observance in ethanol suggests that this large
red-shifted emission may be due to the solvent relaxation
involving the charge transfer state. This excitation dependence
was not observed in non-polar solvent such as a dioxane. In more
viscous IL media, such excitation dependent study reveals two
distinct emission regions one at ꢂ458 nm and another at ꢂ618 nm
corresponding to the LE and CT states respectively [Fig. 6b and
Fig. S8] for dienes (1–4). With increase in excitation wavelength
only the CT band is prominent in both organic and ionic liquid
media (Fig. 6).
Preferential H-bonding interaction with strongly basic amino
group in (5) could explain the observed weak CT emission band. No
such interactions are possible for other dienes due to lack of such
strong basic groups and therefore the long wavelength band
emission is preserved in ionic liquid media.
3.4. Excitation Spectra
3.3. Effect of excitation wavelength in ethanol and ionic liquids
As showed in Fig. 6, diene (4) in ethanol shows interesting REE
behaviour. To understand whether this is due to the presence of
different excited state species, we monitored excitation spectra for
ethanol solution of diene (4) at different emission wavelengths
Ideally, the emission peak is independent of the excitation
wavelength because of the time scale of the processes involved. But
as shown in Fig. S4, one of the key observations of ionic liquid
emission is its Red-edge excitation (REE) phenomena, where
(lem). The excitation spectra are shown in Fig. 7. The excitation
maxima, 395 nm obtained at lem of 610 nm (wavelength
Fig. 5. Concentration dependent emission of dienes (3) at lex = 370 nm and (4) at lex = 390 nm in ethanol. The spectra of other dienes are shown in in ESI (Fig. S6).
Vasu, Anuji K.
