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viscosity from 202 to 262 to 381 mPa s, no correlation in regard to
the PD conformation was observed (Fig. S3, ESI ). An inverse
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correlation was evident for [C9-mim]NO3 and [C9-mim]PF6 ILs
(Fig. 3D, see Fig. S4, ESI for overlaid spectra): with increasing
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viscosity of IL, the planar conformation started to dominate.
It is plausible that in ILs specific solvent–solute interactions (as
well as the structure of ILs) could control the conformation of a
small molecule, rather than the viscosity of the media. Unlike
molecular solvents, which interact with solutes via dipole–dipole
interactions and hydrogen bonding, ILs have the added ability to
interact via ion–, and ion–ion interactions, i.e., via electrostatic
interactions. It was also suggested that a significant amount of
solute stabilization by ILs comes from the cation,30 which might
explain the apparent cation effect observed here.
To gain further support for the notion that viscosity of ILs
might not be the dominant factor in controlling PD’s conforma-
tion in ILs, we examined the relationship between viscosity and
conformational bias of PD as a function of temperature. Arguably,
if the viscosity were the main factor that modulated the
conformation of PD, then changing the temperature (and as a
result changing the viscosity of the solvent) should result in a
linear correlation between the viscosity and the percent of the
twisted conformation of PD. Also, the slope should be similar to
that observed for solvents of various viscosities at a fixed
temperature. Conversely, if the slope of the viscosity (obtained at
different temperatures) as a function of PD conformation would
be different from the slope obtained for solvents of various
viscosities at a fixed temperature, some specific interactions
between the solvents and PD might be present.
To test this hypothesis, we examined the conformation of PD in
several molecular solvents (Tables S4–5; Fig. S5, ESI ) and ILs
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(Tables S6–8; Fig. S6, ESI ) at various temperatures and
consequently viscosities (Fig. 4).
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Fig. 4 Effect of solvent (A: molecular solvents; B: ILs) viscosity on % of twisted PD
measured at 20–60 uC range (see ESI for additional information3); conditions: lex
= 470 nm for A, lex = 475 nm for B, [PD] = 1 mM, all mixtures contain 0.1% DMSO
(v/v).
The slope of viscosity as a function of the twisted PD in EtOH–
glycerol (2 : 8 v/v) mixture as well as tetraethylene glycol (Fig. 4A)
appeared to be very similar to that obtained for various molecular
solvents (Fig. 2C). Arguably, this indicated that viscosity of
molecular solvents was the major factor that controlled the
conformation from that observed upon variation of temperature
(Fig. 4B) as well as the structure of ILs (Fig. 3D). It is worth
pointing out that in water alone, PD was found to be non-
fluorescent.20 Consistent with this assumption, we observed a
linear correlation between the viscosity of [C4-mim]NO3 with
various amounts of water and the conformation of PD (Fig. 5).
However, the slope was found to be drastically different, i.e.,
7.7892 as compared to 5.1156, from that found for the
temperature effect of the PD emission in [C4-mim]NO3 (Fig. 4B),
for example.
conformational bias of PD (See Fig. S7, ESI for overlaid data
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points).
On the contrary, in ILs no apparent correlation among several
different ILs was observed (Fig. 4B). Although for PF6-containing
ILs ([C4-mim]PF6 and [C9-mim]PF6) somewhat similar slopes were
obtained, the two data sets were off-set. When the ILs with the
same cation were compared ([C4-mim]NO3 and [C4-mim]PF6), the
corresponding slopes were found be distinctly different. Therefore,
it is plausible that in ILs, solute–solvent specific interactions are
playing a significantly more prominent role than viscosity. This is
in contrast to the observation noted for the molecular solvents.
Finally, we decided to investigate the effect of water in [C4-
Conclusions
Overall, the structure of ILs appeared to have an effect on the
conformation of PD. ILs promoted the opposite conformation as
molecular solvents with similar viscosities, i.e., planar vs. twisted,
respectively, and an order of magnitude larger viscosity of ILs was
required to promote a similar amount of twisted PD. Specifically,
the viscosity range at which twisted conformation becomes
mim]NO ILs on the viscosity of the IL (Table S9, ESI ) as well as
3
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the conformational preference of PD. Following the aforemen-
tioned rationale on the effect of temperature on viscosity and the
conformation of PD, we reasoned that the presence of water
should alter the PD–[C4-mim]NO3 interactions, which should
produce a distinct correlation between IL viscosity and PD
This journal is ß The Royal Society of Chemistry 2013
RSC Adv., 2013, 3, 18300–18304 | 18303