**3.2.1 Rotational dynamics of non-polar probes**

The molecular structures of the non-polar probes exalite 404 (E404), exalite 417 (E417) and exalite 428 (E428) chosen for the study are shown in Fig.2.The absorption and fluorescence spectra of the probes in methanol are shown in Fig.3. These probes are approximated as prolate ellipsoids (Inamdar et al., 2006) with molecular volumes 679, 837 and 1031 Å3, respectively, for E404, E417 and E428. The rotational reorientation times (τ*<sup>r</sup>*) calculated using Eqn. (4.43), are tabulated in Table 1 and 2, respectively.

Fig. 2. Molecular structures of (a) E404, (b) E417 and (c) E428

Fig. 3. Absorption and Fluorescence spectra of E404

Rotational Dynamics of Nonpolar and Dipolar

0

**(b)**

0

**ii. Rotational reorientation times of Exalite 417 (E417)**  The rotational reorientation times of E417 scale linearly with

**iii. Rotational reorientation times of Exalite 428 (E428)** 

η

increase linearly with

900

1800

τ

r/ ps

2700

3600

400

800

τ

r/ ps

1200

**(a)**

Molecules in Polar and Binary Solvent Mixtures 203

**Stick**

**Slip**

0.0 0.7 1.4 2.1 2.8

η/ mPa s

**Stick**

**Slip**

0 3 6 912

η/ mPa s

η

from methanol to butanol and follows slip boundary condition, and

(Fig. 5) and exhibits subslip

*<sup>r</sup>* values for E428

τ

Fig. 4. Plot of rotational reorientation times of E404 as function of viscosity in (a) alkanes and (b) alcohols. The symbols (○,●) represent experimentally measured reorientation times. The stick and slip lines calculated using hydrodynamic theory are represented by solid lines.

behavior in alcohols. A large nonlinearity is observed on increasing solvent viscosity. In alkanes, the rotational reorientation times follow slip hydrodynamic boundary condition, similar to E404. GW theory is unable to explain experimental results while DKS theory is in

from pentanol to decanol a large deviation from the linearity is observed resulting in subslip behavior (Fig. 6). However, in alkanes the measured reorientation times, clearly follow slip hydrodynamics up to tridecane, whereas in higher alkanes pentadecane and hexadecane

GW and DKS theories are represented using the symbols Δ and respectively.

fairly good agreement with experiment and slip hydrodynamics in case of alkanes.

E428 is the largest probe studied so far in literature. In alcohols the


a Viscosity data is from Inamdar et al., 2006

Table 1. Rotational reorientation times (τ*<sup>r</sup>*) of Exalites in alkanes at 298K


a Viscosity data is from Inamdar et al., 2006

Table 2. Rotational reorientation times (τ*<sup>r</sup>*) of Exalites in alcohols at 298K

### **i. Rotational reorientation times of Exalite 404 (E404)**

Fig. 4 gives the plot of τ*r* vs η in alkanes and alcohols for E404 shows that τ*<sup>r</sup>*values increase linearly with η both in alkanes and alcohols, following slip hydrodynamic and subslip behavior, respectively. This clearly indicates that the rotational dynamics of E404 follows SED hydrodynamics with slip boundary condition. Further, E404 rotates slower in alkanes compared to alcohols by a factor of 1 to 1.3. It may be recalled that E392A followed SED hydrodynamics near stick limit in alkanes (Inamdar et al., 2006). E404 is larger than E392A by a factor of 1.1, and exhibits an opposite behavior to that of E392A following slip behavior in alkanes. Interestingly, the rotational dynamics of both these probes follow subslip behavior in higher alcohols.

Theoretical justification for this approach is provided by the microfriction theories of Geirer-Wirtz (GW) and Dote-Kivelson-Schwartz (DKS) wherein the solvent size as well as free spaces is taken into account. However, there is a large deviation of experimentally measured reorientation times from those calculated theoretically.

τ

τ

*<sup>r</sup>*) of Exalites in alkanes at 298K

*<sup>r</sup>*) of Exalites in alcohols at 298K

τ

*<sup>r</sup>*values increase

in alkanes and alcohols for E404 shows that

behavior, respectively. This clearly indicates that the rotational dynamics of E404 follows SED hydrodynamics with slip boundary condition. Further, E404 rotates slower in alkanes compared to alcohols by a factor of 1 to 1.3. It may be recalled that E392A followed SED hydrodynamics near stick limit in alkanes (Inamdar et al., 2006). E404 is larger than E392A by a factor of 1.1, and exhibits an opposite behavior to that of E392A following slip behavior in alkanes. Interestingly, the rotational dynamics of both these probes follow subslip

Theoretical justification for this approach is provided by the microfriction theories of Geirer-Wirtz (GW) and Dote-Kivelson-Schwartz (DKS) wherein the solvent size as well as free spaces is taken into account. However, there is a large deviation of experimentally measured

both in alkanes and alcohols, following slip hydrodynamic and subslip

a Viscosity data is from Inamdar et al., 2006 Table 1. Rotational reorientation times (

a Viscosity data is from Inamdar et al., 2006 Table 2. Rotational reorientation times (

Fig. 4 gives the plot of

η

behavior in higher alcohols.

linearly with

**i. Rotational reorientation times of Exalite 404 (E404)** 

reorientation times from those calculated theoretically.

τ*r* vs η

Fig. 4. Plot of rotational reorientation times of E404 as function of viscosity in (a) alkanes and (b) alcohols. The symbols (○,●) represent experimentally measured reorientation times. The stick and slip lines calculated using hydrodynamic theory are represented by solid lines. GW and DKS theories are represented using the symbols Δ and respectively.
