| dc.description.abstract |
Most of chemical reactions require a suitable solvent media such as Volatile Organic
Compounds (VOCs) or water. The volatile organic solvents contribute a lot to the
environmental pollution due to their volatile nature. Furthermore, some of the solutes
are not soluble in water due to their poor solubility hence, new and alternative solvent
media are required so as to address environmental pollution and ensure sustainability.
Ionic liquids offers unique features including non-flammability, thermal stability
(high), melting point (low), ionic conductivity (good), interesting solvation properties
and their potential environmentally-benign nature. To ascertain the characteristics of
these new, alternative solvent media, we are using certain probe molecules possessing
significant absorbance and luminescence properties. The photo-physical characteristics
of such probes may be impacted by the surrounding solvent environment and hence
one can study the properties of the surrounding solvent media by using such probes.
This thesis thoroughly tests the concept that ILs and aqueous IL systems may result in
altered, preferred dye aggregation and prototropism when compared with conventional
organic solvents. The aggregates of cyanine dyes plays important role in various
applications including future light harvesting systems, sensors, photoconductors,
photography, biology, medicine, nanotechnology. Furthermore, the significance of
prototropism and Excited State Proton Transfer (ESPT) reactions can be manifested in
numerous applications.
The thesis entitled “Spectroscopic Investigation within Ionic Liquid Based
Media” deals with study of prototropic behaviour of some of the important prototropic
probes such as p-nitrophenol, carbazole and aggregation behaviour of technologically
important cyanine dye 1,1’-diethyl-4,4’-carbocynine iodide (DECI), often referred to
as kryptocyanine within neat Ionic Liquids (ILs) and IL added aqueous solutions to
better understand the unique role of these new and alternative solvent media. The
present study was carried out with the aim to control prototropic behaviour of various
prototropic probes and aggregation behaviour of cyanine dyes in a desired manner
with the help of ILs). The long-term objective of the research is to promote the use of
ILs to control dye aggregation and prototropism.
In the present work, the proton-transfer reaction of p-Nitrophenol (PNP) was
investigated and interestingly, addition of small amount of IL, 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] or 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] into PNP dissolved in high pH water causes protonation of
already present p-Nitrophenolate anion. Protonation of p-Nitrophenolate is attributed
to the presence of acidic C2-H protons in [bmim]+ cations associated with both ILs.
Furthermore, the extent of protonation induced by addition of [BF4]
− based IL is
found to be much pronounced as compared to [PF6]
− based IL having same cation due
iv
to difference in hydrolytic properties of [BF4]
− based ILs when compared with [PF6]
−
based ILs. These findings could contribute to technological applications for detection,
removal and sensing of environmentally hazardous p-Nitrophenol and its ionic forms
from water.
Similarly we have reported the unusual excited-state prototropism of an important
photoacid carbazole dissolved in water in presence of small amount of added ILs. We
have shown that a little amount of additional ILs can have an impact on the excited
state prototropic equilibria of carbazole dissolved in water. More specifically, the
acidity of added ILs may be used to control excited-state prototropism of carbazole.
More interestingly, we discovered that even in the presence of very small amount of
any of added ILs, like 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF4]),
1-butyl-1-methylpyrrolidinium bis(trifluromethansulphonyl)imide ([bmpyrr][Tf2N]),
& 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF6]), the fluorescence
emission from the neutral prototropic form of carbazole can be observed even in the
presence of very basic conditions (aqueous NaOH, pH 12.6) due to the excited state
protonation of the carbazole anion by the proton from the IL cation.
The aggregation behaviour of kryptocyanine dye was found to depend upon the
identity of Ionic Liquid (IL) present in the aqueous solutions of dye. The presence of
small amount of [BF4]
−, [PF6]
− and [OTf]− anions containing the ILs induced the
process of J-aggregation instantly and added ILs acts as promoters for J-aggregation
followed by time dependent precipitation. In contrast to [BF4]
−, [PF6]
− and [OTf]−
anions containing ILs, the presence of small amount of Tf2N
− anion containing ILs
induced the process of J- and H-aggregation simultaneously followed by the time
dependent conversion of H-aggregates to J-aggregates. These findings could contribute
to improve and broaden the applications of these new and alternative noble solvent
media for academia and industry. |
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