Steady state and time-resolved fluorescence behavior of coumarin153 (C153) has been investigated in two ionic liquids (ILs), namely 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MOEMPL][FAP]) and 1-(2-methoxyethyl)-1-methylmorpholinium tris(pentafluoroethyl)trifluorophosphate ([MOEMMO][FAP]) in order to find out the viscosity-diffusion decoupling during solvation and rotational relaxation of C153. Thermophysical studies have also been carried out to understand the physicochemical properties of the media. At 293 K, the measured viscosity of [MOEMMO][FAP] is 8 times higher than that of [MOEMPL][FAP]. The data obtained from steady state and time-resolved fluorescence measurements show the deviation of average solvation and rotation times from conventional hydrodynamics. The decoupling of solute and solvent dynamics from medium viscosity is manifested through fractional viscosity dependence (η) of the measured average solvation (<τ(s)>) and rotation (<τ(r)>) times: <τ(x)> [proportionality] (η/T)(p) (x denotes solvation or rotation and T is the temperature) covering the p value 0.69 < p < 0.85 for solvent relaxation and 0.48 < p < 1.10 for solute rotation. The excitation wavelength dependent fluorescence studies have been performed to correlate the experimental findings with the heterogeneity of the medium. While the excitation wavelength dependent time-resolved fluorescence studies of coumarin153 reveal a very similar variation of average solvation time with a change in excitation wavelengths for both the ionic liquids, the steady state excitation wavelength dependent measurements of 2-amino-7-nitrofluorene (ANF) show a higher (630 cm(-1)) shift of the fluorescence maximum for highly viscous ionic liquid as compared to that (430 cm(-1)) of another much less viscous ionic liquid. The recent theoretical (Chem. Phys. Lett.2011, 517, 180) and experimental (J. Chem. Phys.2012, 136, 174503) findings and the outcome of the excitation wavelength dependent fluorescence measurements in the present case seem to suggest that both static and dynamic heterogeneity may play an important role in the observed viscosity-diffusion (d-η) decoupling for highly viscous ionic liquid.

Rotational relaxation dynamics of nonpolar perylene, dipolar coumarin 153, and a negatively charged probe, sodium 8-methoxypyrene-1,3,6-sulfonate (MPTS), have been investigated in a dicationic ionic liquid, 1,6-bis-(3-methylimidazolium-1-yl)hexane bis-(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), and a structurally similar monocationic ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6MIm][NTf2]), to have a comprehensive and a quantitative understanding on the solute-solvent interaction in these media. Analysis of the rotational relaxation dynamics data by Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that perylene rotation is found to be the fastest compared to the other two probes and shows slip to sub-slip behavior, coumarin 153 rotation lies between the stick and slip boundary, and MPTS shows a superstick behavior in [C6MIm][NTf2]. Interestingly, MPTS exhibits a normal SED hydrodynamics in dicationic [C6(MIm)2][NTf2]2, in spite of the fact that dicationic ionic liquid contains two cationic sites bearing acidic hydrogen (C2-H) which may be available to form stronger interaction with the negatively charged MPTS. The difference in the rotational diffusion behavior of these three probes is a reflection of their location in different distinct environments of these ILs. Superstick behavior of MPTS in monocationic IL has been attributed to its specific hydrogen bonding interaction with the corresponding imidazolium cation. The relatively faster rotational behavior of MPTS in dicationic IL has been explained by resorting to mass spectrometry. Mass spectral analysis demonstrates that positively charged (imidazolium) sites in dicationic IL are strongly associated with negatively charged bis-(trifluoromethylsulfonyl)amide anion (NTf2(-)), which in turn makes it difficult for imidazolim cation to have stronger hydrogen bonding interaction with bulkier negatively charged molecule MPTS.

The solvation and rotational relaxation dynamics of coumarin 153 have been investigated in a dicationic ionic liquid (IL), 1,6-bis-(3-methylimidazolium-1-yl)hexane bis-(trifluoromethylsulfonyl)amide ([C6(MIm)2][NTf2]2), for the first time to provide a comprehensive and a quantitative understanding of the nature of the ionic fluid and its influence on the average solvation and rotational relaxation time. On several occasions, the photophysical data obtained in the present dicationic IL have also been compared with the monocationic imidazolium-based ionic liquid so as to find out the difference between their behaviors. The dicationic ionic liquid has been synthesized via a two step process and subsequently characterized by conventional spectroscopic methods. Steady state absorption and fluorescence measurements reveal that the polarity of the ionic liquid is close to that of dichloromethane. Steady state excitation wavelength dependent fluorescence measurement indicates the micro-heterogeneous nature of the ionic liquid. However, the steady state excitation wavelength dependent fluorescence response is found to be similar for both the dicationic and a structurally similar monocationic ionic liquid. In the time-resolved fluorescence studies, contrary to the monocationic imidazolium-based ionic liquid, no missing ultra-fast component of solvation has been observed in the present dicationic IL. Excitation wavelength dependence of the average solvation and rotation times also indicates the micro-heterogeneous nature of these media. When viscosity dependence (η) of the measured average solvation 〈τs〉 and rotation 〈τr〉 times are verified by the relation: 〈τx〉 ∝ (η/T)(p) (where 'x' is solvation or rotation, p is the exponent and T is the temperature), the fractional dependence of both average solvation and rotational times with the medium viscosity have been observed. The recent findings (J. Chem. Phys., 2012, 136, 174503; Chem. Phys. Lett., 2011, 517, 180; ChemPhysChem, 2012, 13, 2761) and the outcome of the present study suggest that the observed viscosity-diffusion (η-D) decoupling for the present dicationic ionic liquid is due to the dynamic heterogeneity of the medium.

Various configurations were investigated to find the most stable structures of glycine-(water)3 complex. Five different optimized conformers of glycine-(water)3 complex are obtained from density functional theory calculations using 6-311++G* basis set. Relaxation energy and many body interaction energies (two, three, and four body) are also calculated for these conformers. Out of the five conformers, the most stable conformer has the BSSE corrected total energy -513.917 967 7 Hartree and binding energy -27.28 Kcal/mol. It has been found that the relaxation energies, two body energies and three body energies have significant contribution to the total binding energy whereas four body energies are very small. The chemical hardness and chemical potential also confirmed the stability of the conformer having lowest total energy.

Steady state and time-resolved spectroscopic measurements have been carried out in two different series of neat ionic liquids where in one group (1-ethyl-3-methylimidazolium alkylsulfate) a systematic variation in alkyl chain length in their anionic moiety has been maintained keeping the cation fixed and in another series variation remains in the cation (morpholinium, pyrrolidinium and imidazolium) and anion (tris(pentafluoroethyl)trifluorophosphate) remains constant. The investigations have been carried out to get a better idea about the role of ionic constituents influencing different structural organisation in ILs. Even though observation of emission wavelength dependent decay behaviors of ILs may indicate the electronic energy transfer (EET) among different associated structures, absence of any rise-time during decay measurements certainly raises questions about the EET process. Emission wavelength dependent time-resolved data do not provide any straight forward linear correlation between these data and structural organisation of RTILs. Interestingly, significant fluorescence has also been observed even in neat non-aromatic RTILs resembling viscous glycerol.

Analysis of time-resolved fluorescence anisotropy data in light of the Stokes-Einstein-Debye hydrodynamic description reveals significant decoupling of rotational motion of the solute and the viscosity of the medium for a hydroxyl-functionalized ionic liquid (IL). This behavior and NMR experiments indicate that the hydroxyl-functionalized IL is more heterogeneous than other structurally similar ILs. Considering that recent theoretical investigations have demonstrated that the jump dynamics and hydrogen-bond fluctuations are closely related in viscous media, in such a case the hydrodynamic description can provide inconsistent results, and the present inapplicability of the hydrodynamics description in explaining solute rotation in a viscous hydroxyl-functionalized IL perhaps provides experimental support to the role of orientational jumps and hydrogen bond formation in that event.

Steady development on photophysical behaviors for a variety of organic fluorophores inspired us to generate anthracene-based fluorescent molecules with a strong acceptor and a significantly weak donor through a π-spacer. Such molecules are found to have substantial twisted conformational orientations in the solid state and enhanced apolar character because of the attachment of tolyl or mesityl group with an anthracenyl core. Upon exposure to a variety of solvents, strong solvatochromism was noticed for 4-nitro compound (84 nm red shift) in contrast to the cyano analogue (18 nm red shift). Both these probes were highly emissive in apolar solvents while nitro-analogue, in particular, could discriminate the solvents of E T (30) (a measure of microscopic solvent polarity) ranging from 31 to 37. Thus, 4-nitro compounds can be successfully employed to detect and differentiate the apolar solvents. On the contrary, the 2-nitro analogue is almost nonemissive for the same range of solvents perhaps because of favorable excited-state intramolecular proton-transfer process. The fundamental understanding of solvatochromic properties through the formation of twisted intramolecular charge-transfer (TICT) state is experimentally analyzed by synthesizing and studying the π-conjugates linked to only benzene in place of nitro or cyanobenzene, which exhibits no solvatochromism and that helped finding the possible emission, originated from the locally excited state. Moreover, the molecular structures for these compounds are determined by the single-crystal X-ray diffraction studies to examine the change in emission properties with molecular packing and alignment in the aggregated state. The measurement of dihedral angles between the substituents and anthracenyl core was helpful in finding the possible extent of electronic conjugations within the system to decipher both solvatochromism and aggregation enhanced emission (AEE)-behavior. The cyano analogue exhibited prominent AEE-behavior, whereas nitro analogues showed the aggregation-caused quenching effect. The reason behind such dissimilarity in solvatochromism and AEE-behavior between cyano- and nitro-linked anthracenyl π-conjugates are also addressed through experimental outcomes.

Steady state, time-resolved fluorescence and NMR experiments are carried out to gain deeper insights into the structure-property correlation in structurally similar monocationic and dicationic room-temperature ionic liquids (RTILs). The excitation wavelength dependent fluorescence response of fluorophore in 1-methy-3-propyllimidazolium bis(trifluoromethylsulfonyl)amide [C3MIm][NTf2] is found to be different from that of 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide [C6(MIm)2][NTf2]2 and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [C6MIm][NTf2]. The outcomes of the present solvent dynamics study in [C3MIm][NTf2] when compared with those in [C6(MIm)2][NTf2]2 and in [C6MIm][NTf2] from our previous studies (Phys. Chem. Chem. Phys. 2014, 16, 12918-12928) indicate the involvement of dipolar rotation of imidazolium cation during solvation. To correlate the findings of solvation dynamics study with the dipolar rotation of the imidazolium ring, pulsed-field gradient (PFG)-NMR technique for translational diffusion coefficient measurement and (1)H as well as (19)F spin-lattice relaxation measurements are employed. NMR investigation reveals that an ultrafast component of solvation can be related to the dipolar rotation of imidazolium cation; hence, the role of dipolar rotation of cations in governing the dynamics of solvation in ILs cannot be ignored. Analysis of the rotational relaxation dynamics data by the Stokes-Einstein-Debye hydrodynamic theory unveils distinctive features of solute-solvent interaction in [C3MIm][NTf2] and [C6(MIm)2][NTf2]2.