CHAPTER II
EVALUATION OF
GROUND AND EXCITED
STATE DIPOLE
MOMENTS
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
III.1.
INTRODUCTION
The nature and the energy of electronically excited state of a dye
molecule determine its photophysical properties. In quantum calculations, the
molecule is assumed absolutely to be isolated, as if it were in the gas phase at
very low pressure. Since we are concerned with molecules in condensed
phases, liquids, viscous liquids and solids, the effect of electronic states of
solute molecules, is therefore, of substantial importance in photophysics [1]
and photochemistry [2]. A molecule has a number of electronic states, which
are defined by their energy E, and wavefunctionψ , according to Shrödinger
equation Eψ = Hψ . The wavefunction signifies the nature of the state, as
different from its energy; it describes, in particular, the electron distribution
within the molecule, which is closely related to its chemical reactivity. In
quantum calculations, these states have the most stable nuclear configuration
(molecular geometry, i.e., bond length and bond angles) for the relevant
electron distribution. Transitions between such states cannot be observed since
the heavy nuclei take a long time to rearrange to their most stable geometry
after the rapid change in the electron distribution; the nuclear configuration of
the initial state is preserved in the Frank-Condon (FC) state reached directly
after the transition (in time <10-16 s) [3].
A radiative transition (absorption or emission) connects a relaxed initial
state to a FC final state. When the molecule is surrounded by a liquid solvent or
solid matrix (the medium) each state is stabilized (or destabilized, as the case
maybe) by an energy Es, known as the solvation energy. The difference in the
solvation energies of the final and initial states in a various solvents is referred
to as solvatochromic shift and is the main experimental evidence for this
energy difference. Therefore, the absorption and emission spectra are more
33
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
informative in this respect as they are related to the energy of relaxed excited
state. These experimental observations with solvent dependence of the energy
of excited states gives rise to various theories of solvatochromic shifts and try
to narrate the electronic distribution of the molecule in its excited
state, especially to the total dipole moment (µ) and/or to the average
polarizability (α).
Determination of the ground and exited-state dipole moments of the
molecules is imperative, since the values of dipole moments provide
information about the change in the electronic charge distribution upon
excitation. The simple physical meaning of the dipole moment allowed
application to a variety of structural, in particular, stereochemical-problem,
even at the early stage of its development. There are various techniques of
determining excited state dipole moment µe with electro optical methods like
electrochromism of absorption and fluorescent bands [4], Stark splitting of
rotational levels of the 0-0 vibrational bands and the effect of an external
electric field on the fluorescence anisotropy [5, 6], nevertheless the solventshift method based on the analysis of salvatochromism of absorption and
fluorescence maxima is the simplest and the most widely used. Though these
methods are considered to be more accurate, they are restricted only to
relatively simple molecules. In order to study the changes in electronic
distribution in excited states of solute molecules, the solvent-induced shifts of
electronic bands of molecules has been used extensively [7].
It is known that the loss of energy of an excited molecule in a solution,
both by radiative and non-radiative processes characteristically depend upon
the molecular structure and the environment of the molecules. Therefore, not
34
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
only the structure of solute molecules but the solvent also plays an important
role in determining its radiative and non-radiative properties. The strong
emission of dyes results from the polar character of low-lying excited states. It
is identified that the electronic spectra of these molecules are affected by their
milieu. The solvent effects are of particular importance, among the principal
environmental factors affecting the electronic spectra. The dipole moments of a
molecule in the ground state and an excited state depend on the electron
distribution in these states. A change of solvent when it produces alteration in
both the excitation and fluorescence wavelength indicates that the solvent, in
some way, is able to interact with the solute in the ground state [8]. However,
when solvent produces a shift in fluorescence wavelength alone, it indicates an
interaction between the solvent and the solute molecule in the excited state.
Also, when a molecule is excited, its dipole moment changes and it no more
remains in equilibrium with its immediate environment. As the molecule
relaxes and tries to attain equilibrium with its surroundings, some energy is
dissipated in the form of non-radiative energy and the fluorescence emission
wavelength gets shifted according to Frank-Condon principle [9]. In addition to
this, sometimes large-shifts are produced by specific short-range interactions,
e.g., hydrogen-bonding, complex formation, etc. A systematic solvent effect
analysis is therefore, useful to understand the effect of the environment and
various mechanisms of de-excitation and relaxation of an excited molecule in
solution. These properties have been studied as a function of concentration of
the dye, viscosity, polarity and polarizability of the solvents, as well as the pH
35
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
of the solution. The solvent shifts can be accounted in terms of the overall
effect of the interaction forces (which are mainly of van der Waals type) on the
π-electron system of the molecule. It is also known that as the π-electron
system becomes less localized, the transition energy becomes smaller resulting
in a bathochromic shift (red shift) and its opposite effect gives rise to a
hypsochromic shift (blue shift). In order to assign the electronic transitions as
π→π* or n→π* the solvatochromic technique is found quite informative. It is
known that π→π* bands show a red shift in the solvents of increasing polarity
while n→π* bands show a blue shift. Thus a change of solvent affects the
ground and excited states differently because of the difference in the solvation
energies. It may be noted that the solute-solvent interaction affects the
fluorescence spectra in several ways viz., shift in the fluorescence spectrum,
quenching of fluorescence by intersystem crossing (ISC) and formation of nonfluorescent complex or by ionization to non-fluorescent species.
In the present study our main aim is to obtain ground and excited state
dipole moments using different experimental techniques viz., solvatochromic
shift method and theoretically by density functional theory (DFT) using
Gaussian03 program. For this purpose, absorption and fluorescence
characteristics of two dipolar fluorescent dyes namely, 2-(2, 7-dichloro-6hydroxy-3-oxo-3H-xanthen-9-yl) benzoic acid (Fluorescein 27) and N-[6diethylamino)-9-(2,4-disulfophenyl)-3H-xanthen-3-ylidene]-N-ethylhydroxid
(Sulfarhodamine B) in various solvents have been undertaken in the present
study. The molecular structures of these are shown in Fig.3.1. An
36
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
understanding of excited state properties of fluorescent dyes helps not only in
the design of new molecules but also for the optimum performance for specific
applications. The spectroscopic properties of fluorescein dyes are well known
with the dyes having applications ranging from dye lasers [10] to tracers in
flow visualization and mixing studies [11-14]. Fluorescein dyes are very
attractive due to their low cost, non-toxicity, low-staining potential, high
quantum yield (φ > 0.9), water-solubility, and easy optical accessibility with
commercial lasers. On the other hand, the SRB has been used to measure druginduced cytotoxicity and cell proliferation for large-scale drug-screening
applications [15]. The solvatochromic shifts have also been used for the
determination of the excited state dipole moments of some polar dyes and
coumarins [16-23]. In the following section we present the two different
theoretical methods used to evaluate the ground and excited state dipole
moments followed by their results and discussion.
III.2.
THEORETICAL BACKGROUND
III.2.1.
Solvatochromic shift method
The expression most frequently used in fluorescence spectroscopy is,
however, the somewhat simplified equation, first developed by Lippert [24, 25]
and Mataga [26, 27]. It is based on the Onsager’s reaction field theory, which
assumes that the fluorophore is a point dipole residing in the centre of a
spherical cavity with radius ( a ) in a homogeneous and isotropic dielectric with
relative permitivity (ε). The so called Lippert-Mataga equation is no longer
applicable when, in addition to the non-specific interactions, specific
fluorophore/solvent interactions such as hydrogen bonding or electron-pair
37
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
donor/electron-pair acceptor interactions also contribute significantly to the
overall solute-solvent interaction. A further limitation results from the cavity
radius, which is difficult to estimate for elongated molecules with an ellipsoidal
shape [28].
For quantitative assessment of the solvent-solute interactions, multiparameter solvent polarity scale and spectroscopic shifts can be used. Effect of
solvent polarity on the spectral aspects of the solute can be interpreted by
means of linear solvation energy relationship (LSER) concept that can be
formulated as Kamlet-Abbound-Taft and Katritzky Equations (eqns.3.1 and 3.2
respectively) [29, 30].
(νa-νf ) = (νa-νf )0 + ࣵ⋅α +b ⋅β +s⋅ π*
(3.1)
 n2 −1 
ε −1 
(νa-νf ) = (νa-νf )0 + ࣵ⋅ 
+b
⋅
 2  +s⋅ ET (30)

 2ε + 1 
 2n + 1
(3.2)
where π* is a measure of the solvent polarity/polarizability [31], α is the scale
of the solvent hydrogen solvent bond donor (HBD) acidities [32], β is the scale
of the solvent hydrogen solvent bond acceptor (HBA) basicities [33]. ε, n and
ET (30) are relative permittivity, refractive index and empirical polarity
parameter of the solvent, respectively. Eqn. (3.2) estimates independent
contributions of solvent dipolarity, polarizibility, and other specific interactions
like hydrogen bonding and extra π – π interaction. All the parameters should be
re-normalized and re-scaled for Eqn. (3.2), to have comparable ࣵ, b and s
values. (νa-νf )0 is the regression value of the solute property in the reference
solvent. The regression coefficients ࣵ, b and s in these equations measure the
38
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
relative susceptibilities of the solute property, such as absorption, fluorescence
and other spectroscopic parameters.
Kawski [34-36] obtained a simple quantum mechanical second order
perturbation theory of absorption (νa) and fluorescence (νf) band shifts in
different solvents of varying permitivity (ε) and refractive index (n) relative to
the band position of a solute molecule, based on which the following equations
are obtained:
νa-νf =m1 f (ε, n) + const.
(3.3)
νa+νf =-m2 [ f (ε, n) + 2g(n)] + const.
(3.4)
where f (ε , n) =
2n 2 + 1  ε − 1 n 2 − 1 
−


n2 + 2 ε + 2 n2 + 2 
(3.5)
is the polarity of the solvent [37] and
3  n 4 −1 
g(n)= 

2  ( n 2 + 2) 2 


(3.6)
2
with
2 µ − µ 
e
g
m = 
1
3
hca
and
2 µ 2 − µ 2 
e
g
m = 
2
hca 3
(3.7)
(3.8)
h being Planck’s constant and c, the velocity of light in vacuum. The
parameters m1 and m2 can be determined from absorption and fluorescence
band shifts [Eqns. (3.3) and (3.4)], and the values of µg and µe from Eqns. (3.7)
and (3.8) can be given as [38]
m − m1
µg = 2
2
 hca 3 


 2m1 
1/ 2
(3.9)
39
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
m + m2
µe = 1
2
or
µe =
 hca 3 


 2m1 
1/ 2
(3.10)
m1 + m2
µ g ; (m2 > m1)
m2 − m1
(3.11)
The parameters m1 and m2 occurring for the difference (νa-νf) and the
sum (νa+νf) of the wave numbers are linear functions of the solvent polarity
parameters f (ε, n) and f (ε, n) +2g (n); and can be determined from the slopes
of the straight lines.
The method based on the empirical polarity scale proposed by Reichardt
[28] gave towering results with solvatochromic shift of dipolar molecules that
correlates much better with microscopic solvent polarity ETN rather than the
traditionally used bulk solvent polarity functions involving dielectric constant
(ε) and refractive index (n) as in the latter the error estimation of Onsager
cavity radius ‘a’ has been minimized. The theoretical basis for the correlation
as spectral shift with ETN has been developed by Ravi et al. [16] and
accordingly the excited state dipole moment is evaluated using the equation
 ∆µ
ν a − ν f = 11307.6
 ∆µ B
2
3
  aB   N
    ET + const.
  a  
(3.12)
where ∆µB =9 D and aB =6.2 Å are the dipole moment change on excitation and
Onsager radius, respectively, for betaine dye and also are the corresponding
quantities for the molecule of interest. ETN is defined using water and
tetramethylsilane (TMS) as extreme reference solvents with the following
equation [28]
ETN =
ET ( solvent ) − ET (TMS ) ET ( solvent ) − 30.7
=
ET ( water ) − ET (TMS )
32.4
40
(3.13)
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
The change in dipole moment is determined by
∆µ = µ e − µ g =
m × 81
(6.2 / a ) 3 × 11307.6
(3.14)
where m is the slope of the linear plot of ETN vs. Stokes shift.
III.2.2.
Computational method
Density functional theory (DFT) calculations were performed for the
three probes Fluorescein 27(F27) and Sulfarhodamine B (SRB) to calculate
ground and excited state dipole moments. The ground state geometries of
theses molecules were fully optimized using DFT with popular hybrid
B3LYP/6-31g* functional by Gaussian03 software [39]. However, information
regarding the excited state dipole moments was obtained by optimizing the
geometry in the excited state at CIS/6-31G* level using the ground state global
minima as reference [40, 41].
III.3.
EXPERIMENTAL
III. 3.1.
Chemicals used
The laser dyes F27 and SRB were of the highest available purity
procured from Lambda Physik GmbH, Germany and were used without further
purification. All the solvents used in the study were procured from Fluka
(HPLC grade) and were chosen as they are transparent and non-fluorescent in
the range of excitation and fluorescence emission.
III.3.2
Spectroscopic measurements
Absorption spectra were recorded using UV-visible double beam ratio
recording
spectrophotometer
(Hitachi,
Model
U-2800).
Fluorescence
spectrofluorometer (Hitachi, Model F-7000) was used to record the
41
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
fluorescence spectra. All the measurements were carried out at room
temperature (298K) keeping the dye concentration very low (10-5 ~10-6 M) in
order to avoid or minimize self-absorption and aggregation. The excitation
wavelength used for F27 and SRB are 490 nm and 550 nm, respectively. The
measurement of refractive index of probe solutions has been described in detail
in Chapter II (section II.3).
III.4.
RESULTS AND DISCUSSION
To examine the effect of the solute shape on the dipole moments in the
excited state and the ground state, a number of solvents have been selected for
the present study. Firstly, the non hydrogen-bond donating solvents (non-HBD
solvents) such as acetone, acetonitrile, DMF and DMSO. Secondly, the
hydrogen-bond donating solvents (HBD solvents) such as formamide, glycerol
and alcohols are used which have been a subject of a number of studies related
to polar solvation [42 and references therein].
Solvents covering wide range from non-HBD to HBD have been
preferred. Thus, we have chosen various (i) aprotic (ε =21.01 to 47.24, from
acetone to DMSO) and (ii) protic solvents (ε =7.93 to 33.6, from decanol to
methanol, glycerol ε =42.5 and formamide ε =111).
III.4.1.
Evaluation of ground and excited state dipole moments F27
and SRB
The molecular structures of F27 and SRB are shown in Fig.3.1. The
values of absorption maxima λmax of F27 and SRB in the solvents used are
given in Table 3.1. From this table, one can observe that the absorption maxima
of the dyes are affected by the type of solvent and maximum shifts (∆λ) for F27
42
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
and SRB are 23 and 18 nm, respectively for the solvents used in this work.
Hence, this change in spectral position can be used as a probe for different
types of interactions between the solute and solvents. The study of solvent
effect based on spectral properties of dye solutions were carried out by using
the spectral position in the solvents mentioned here and correlating these with
the Kemlet-Taft solvent properties namely, π*, α, β, refractive index (n) and
dielectric constant (ε), obtained from literature [43, 44]. As the shift in λmax
values with solvent type reflects dye-molecule interactions, an attempt was
made to study this fact exhaustively. The solvent parameters essential for this
work are shown in Table 3.1. The spectral position of dye in a variety of
solvents has revealed interesting results. Since all the solvents used in this
study were polar in nature, one would expect that the solute would bind more
strongly to a more polar solvent and consequently cause the spectra to shift to
lower wavelengths. However, this is not observed from our results as λmax is
lowest in the case of glycerol for F27 and acetone for SRB. This may be due to
the reason that all other solvents used in this work are more polar than these
two solvents in which λmax is lowest and can engage more strongly in a solventsolvent type of interaction or their ability to interact with the dye molecules
reduces. These solvents, glycerol for F27 and acetone for SRB are less polar
and interact with dye molecules in terms of dipole-dipole interactions, leading
to a net stabilization of the ground state of the solute molecule and thus a
hypsochromic shift in the spectrum of these solvents is observed. Alternatively,
λmax value is shifted to lower energies in highly polar solvents such as
43
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
formamide because of strong solvent-solvent interaction or specific interaction
between the solvent and hydrogen present in the functional groups of the dye
molecule.
Figs. 3.2(a) and 3.2(b) illustrate the plot of λmax versus the dielectric
constant (ε) values in different non-HBD and HBD solvents for SRB dye. From
this Fig., it can be seen that with increase in ε values, the spectrum is shifted to
longer wavelengths. In case of F27, λmax of dye solution in DMSO (aprotic or
non-HBD) and glycerol (protic or HBD) were found to be at lower wavelengths
as compared to other solvents although their dielectric constants are higher
among respective non-HBD and HBD solvents. This might be due to the
formation of strong hydrogen bond between dye and solvent molecule. The
variation of λmax versus π* in different non-HBD and HBD solvents for SRB
shown in Fig. 3.3(a) and 3.3(b), wherein an increase in λmax values with π*
specifies that dye interaction changes with increasing capability of a given
solvent to form hydrogen bonds in solution.
The effect of solvents on the absorption and emission spectra of SRB in
aprotic (acetone to DMSO) and of F27 in protic (formamide and alcohols)
solvents are shown in Fig.3.4 (a) and 3.4(b), respectively. The uncertainties in
the measured wavelength of absorption and fluorescence maxima are ±0.5 nm
and ±1 nm, respectively. The observed absorption and emission spectra of these
two dyes are broad and they shift depending on the solvent used. The charge
transfer band shows a shift of about 4-23 nm in the absorption spectra on
changing the solvent from decanol to formamide for F27 and that in case of
44
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
SRB is 3-13 nm. A relatively larger spectral shift observed in the emission
spectra as compared to the absorption spectra. A lesser variation in the
absorption shift observed in all the solvents implies that the ground state energy
distribution is not affected to a greater extent probably due to the less polar
nature of these dyes in the ground state rather than the excited state. The
pronounced shift in the emission clearly indicates that the dipole moment of the
excited state is higher compared to that in the ground state. The large Stokes’
shift observed is also an indication of the charge transfer transition. The higher
magnitude of Stokes’ shift suggests that the excited state geometry could be
different from that of the ground state. A general observation is the increase in
Stokes’ shift values with increase in solvent polarity indicating that there is
increase in the dipole moment upon excitation. In such cases, the relaxed
excited state S1 will be energetically stabilized relative to the ground state S0
and hence, a significant red shift of the fluorescence will be observed.
The absorption data of dyes in different solvents was also analyzed in
terms of various polarity scales. One among such methods involves the
transformation of λmax of dyes in different solvents into molar transition
energies [ET (dye), kcal/mole] by using the following equation [45],
ET (dye) = 28591/ λmax
(3.15)
The ET (dye) values, obtained using equation (3.15), signify the
transition energies that reflect the stabilization of the dye in its ground state in
the respective solvent. This may perhaps be due to either hydrogen bond
formation or dye - solvent interaction. Hence, ET (dye) gives a direct empirical
45
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
measure of dye solvation behavior. One can notice from Table 3.3 that, ET
(dye) value is the highest as compared to other solvents in the case of glycerol
for F27 and acetone for SRB. The reason for this being the same as described
earlier.
The wave numbers of absorption and fluorescence emission maxima of
solutes along with the microscopic polarity scale ETN , are summarized in Table
3.3 for F27 and SRB. Consecutively, to estimate the ground state and excited
state dipole moments of the dye molecules, the solvent polarity f(ε, n) and f(ε,
n)+2g(n) parameters were calculated (Table 3.2). Figs.3.5(a) to 3.5(d) show the
respective spectral shifts νa-νf and νa+νf for both the dyes which are observed
in non-HBD solvents and HBD solvents against the polarity function f(ε, n) and
f(ε, n)+2g(n), respectively. A linear regression was done and the data were fit
to straight lines for both the plots whose slopes were taken as m1 and m2. For
polar solutes like F27 and SRB, the interaction with non-polar solvents depends
on the dipole-induced-dipole forces, while with aprotic (polar but non-HBD)
solvents, the solute-solvent interaction depends on the stronger dipole-dipole
forces. In HBD solvents (polar protic), in addition to dipole-dipole interaction,
specific interaction such as H-bonding may be effective as the intermolecular
charge transfer (ICT) character is favorable for H-bonding with hydroxyl
groups present in protic solvents. Hydrogen bonding interaction usually puts a
severe restriction on the validity of the eqns. (3.3) and (3.4). It is consequently
useful as pointed out by others also [46, 47] to use ET (30) function which is the
46
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
empirical measure of the solvent polarity [48] for understanding the
polarization dependence of spectral characteristics.
The solvent polarity parameter ET (30), which also considers other
interactions besides those of specific nature, was related with the absorption
values. ET (30) values were acquired from literature for different solvents used
in this study and are shown in Table 3.3. Fig. 3.6 depicts the correlation
between the absorption value (in wavenumber) and ET (30) for F27 dye. Linear
correlation of absorption energy over a range of ET (30) represents the presence
of specific interactions between the solute and solvents. Unfortunately, ET (30)
values have the dimension of kcal/mol, a unit which should be discarded in the
framework of SI units [28]. For that reason, the use of the so-called normalized
ETN values have been suggested, as defined in eqn. (3.13). Figs. 3.7(a) and
3.7(b) show the plots of Stokes’ shift as a function of ETN in all the solvents for
F27 and SRB, respectively. The linear ETN dependence of Stokes’ shift reveal
the existence of general type of solute-solvent interaction in which the Stokes’
shift depends on dielectric constant and refractive index of the solvents.
The Onsager’s cavity radius is calculated using both Edward’s
increment method [49] and from Suppan equation [50]. These two methods
collectively yield almost the same cavity radius of 4.15Å and 4.93Å for F27
and SRB, respectively. Using eqns. (3.9) and (3.10) the ground and excited
state dipole moments are evaluated and summarized in Table 3.4 along with the
slopes m1 and m2. The difference in dipole moment calculated from solvent
perturbation method and the one calculated using eqn. (3.14) are reasonably in
47
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
good agreement in case of protic solvents, obviously indicating the excited
state dipole moment to be higher compared to ground state.
The observed ground state dipole moment of F27 and SRB are 2.65D
and 6.85D in aprotic solvents 2.20D and 5.61D in protic solvents, respectively.
The disparity in the values of dipole moments of F27 and SRB can also be
explained on the basis of their possible resonance structures as shown in
Figs.3.8 (a) and 3.8 (b). In case of F27 dye, non-bonding electrons on the
oxygen of pyran ring, hydroxyl group, carbonyl group and carboxylic acid
group along with chlorine contribute towards the mobility of electrons on the
aromatic ring. Alcohols form strong hydrogen bonding, thus oxygen atom of –
OH can better contribute to the resonance structure. On excitation, the oxygen
atom of the carbonyl group and that of pyran ring also contribute towards the
mobility of electrons on the aromatic ring group by delocalizing their nonbonding electrons.
Whereas SRB (anion), is a model hydrophilic probe and an ionic
compound. The π electron mobility is more in SRB because of more electron
tendency of nitrogen atom. The nitrogen atom of diethylamine group in SRB
has lone pair of electrons. These non-bonding electrons on the nitrogen atom of
tertiary amino group –N (C2H5)2 contribute towards the mobility of electrons
on the aromatic ring. Since the nitrogen atom of tertiary amino group is sp3
hybridized, its electron donating tendency is more. Upon excitation the tertiary
amino group becomes strong electron donor. This is the reason why SRB
possesses higher dipole moment than F27.
48
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
The ground and excited state dipole moments of F27 and SRB probes
were also determined from ab initio calculations using DFT [51] and CIS
method, respectively (Table 3.4). The ground state optimized geometries of
these probes acquired using B3LYP functional with 6-31g* basis are shown in
Fig.3.9 (a) and 3.9 (b). The arrow in the Fig. indicates the direction of
transition dipole moment in the ground state. It is evident from Table 3.4 that
the ground state dipole moments evaluated from Salvatochromic shift method
are smaller than those from ab initio calculations for F27 and SRB probes.
Numerous reports in literature have shown disagreement between theoretical
and experimental values [52]. The excited state dipole moments were
computed using CIS method to estimate the minimum of the lowest excited
singlet state and optimized using 6-31g* .
The ground and excited state dipole moments obtained from ab initio
methods using DFT is shown in Table 3.4 for the comparison with the
experimental results. The ground state dipole moments, calculated using
B3LYP/6-31 g* basis set for F27 and SRB respectively, are 6.19 D and 12.12
D. The ground state optimized geometry of F27 and SRB along with total
atomic charge distribution is shown in Fig.3.9. In the calculation of excited
dipole moment, CIS/6-31 g* is used and are found to be 8.26 D and 14.64 D in
case of F27 and SRB, respectively. However, higher values in the excited state
dipole moments were noticed in case of theoretical values compared with
experimental one. These results were unanticipated. Several authors have
observed disagreement between theoretical and experimental values [52, 53,
54]. Aaron and Maafi [52] reported disagreement between experimental and
theoretical values in the excited state for acridine, acridine yellow and 949
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
aminoacridine. These differences found by Aaron and Maafi [52] and also by
Sharma et al. [8] could be partly due to the simplification used in Bakshiev and
Kawski-Chamma-Viallet’s method and to strong specific effects of the solvents
of different nature.
50
III. EVALUATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
III.5.
REFERENCES
[1]
K. C. M Davis, in: Foster (Eds.), Molecular Association, Vol. 1 (and
references therein), Academic Press, London, 1975
[2]
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54
Table 3.4.
Dipole moments, slopes (m1 and m2) and correlation factor (r) of F27 and SRB molecules:
µe/µg
m1 (cm-1)
m2 (cm-1)
r
2.69 (1.010*)
2.015
1023.39
3034.03
0.756:0.864
4.62
2.42 (2.596*)
2.100
824.60
2324.92
0.906:0.915
6.19a
8.26b
2.07
1.334
Aprotic
6.85
9.37
2.52 (1.101*)
1.370
536.31
3441.53
0.804:0.978
Protic
5.61
8.11
2.50 (2.115*)
1.446
521.86
2891.93
0.853:0.881
12.12a
14.64b
2.52
1.208
Molecules Radius (Å)
Solvents
µg(D)
µe(D)
F27
Aprotic
2.65
5.34
Protic
2.20
SRB
*
4.15
4.93
Calculated using equation 14.
Calculated from B3LYP functional with 6-31g* basis.
b
Excited state dipole moment using CI singles (CIS) method.
a
∆µ (D)
Table 3.3
Solvents
Acetone
Acetonitrile
DMF
DMSO
Formamide
Glycerol
Methanol
Ethanol
Propanol
Butanol
Pentanol
Hexanol
Heptanol
Octanol
Nonanol
Decanol
#
1
Some physical constants of solvents and spectral data of F27 and SRB
ET (dye)
1
F27
SRB
54.25
54.41
53.64
53.64
55.46
56.06
55.95
55.19
54.98
54.82
54.77
54.82
54.67
54.88
54.88
54.82
52.13
51.89
51.65
51.06
50.51
50.60
51.42
51.61
51.70
51.70
51.61
51.56
51.56
51.56
51.56
51.56
ET (30)
42.2
45.6
43.2
45.1
55.8
57.0
55.4
51.9
50.7
49.7
49.1
48.8
48.5
48.1
47.8
47.7
#1
ETN #
0.355
0.460
0.386
0.444
0.775
0.812
0.762
0.654
0.617
0.586
0.568
0.559
0.549
0.537
0.528
0.525
νa (cm-1)
νf (cm-1)
F27
SRB
F27
SRB
18975
19029
18762
18762
19399
19608
19569
19305
19231
19175
19157
19175
19120
19194
19194
19175
18232
18149
18067
17857
17668
17699
17986
18051
18083
18083
18051
18034
18034
18034
18034
18034
18215
18376
17986
17973
18362
18349
18622
18636
18382
18587
18553
18450
18546
18532
18546
18601
17476
17361
17259
17082
16920
16667
17247
17283
17337
17403
17319
17325
17331
17337
17379
17397
ET (30) values for both general and alcohols solvents are from Ref. [1,45, 46]. ETN values for both general and alcohols solvents are from Ref. [1].
Solvent polarity in the unit of kcal/mol.
Table 3.2.
Calculated values for solvent polarity parameters f (ε, n), g(n) and f (ε, n)+2g(n)
Solvents
f (ε, n)
2g (n)
f(ε, n)+2g(n)
Acetone
Acetonitrile
DMF
DMSO
Formamide
Glycerol
Methanol
Ethanol
Propanol
Butanol
Pentanol
Hexanol
Heptanol
Octanol
Nonanol
Decanol
0.792
0.861
0.839
0.841
0.895
0.830
0.857
0.812
0.781
0.749
0.716
0.686
0.652
0.614
0.588
0.553
0.489
0.469
0.583
0.648
0.606
0.661
0.448
0.491
0.524
0.542
0.557
0.568
0.575
0.582
0.589
0.593
1.281
1.330
1.423
1.489
1.501
1.491
1.305
1.303
1.305
1.291
1.273
1.254
1.227
1.196
1.177
1.146
Table 3.1
Solvents
Acetone
Acetonitrile
DMF
DMSO
Formamide
Glycerol
Methanol
Ethanol
Propanol
Butanol
Pentanol
Hexanol
Heptanol
Octanol
Nonanol
Decanol
#
Some physical constants of solvents and absorption maxima of F27 and SRB in various solvents
n#
1.359
1.344
1.430
1.479
1.447
1.489
1.329
1.361
1.385
1.399
1.410
1.418
1.424
1.429
1.434
1.437
ε#
21.01
36.64
38.25
47.24
111.00
42.5
33.70
24.30
20.60
17.40
14.80
13.00
11.30
9.80
9.00
8.00
π*#
0.71
0.75
0.88
1.00
0.97
0.62
0.60
0.54
0.52
0.47
0.40
0.40
-0.40
-0.45
α#
0.08
0.19
0.00
0.00
0.71
1.21
0.98
0.86
0.84
0.84
0.84
0.80
-0.77
-0.70
β#
0.43
0.40
0.69
0.76
0.48
0.51
0.66
0.75
0.90
0.84
0.86
0.86
-0.81
-0.82
λmax(nm)
F27
SRB
527
525.5
533
533
515.5
510
511
518
520
521.5
522
521.5
523
521
521
521.5
548.5
551
553.5
560
566
565
556
554
553
553
554
554.5
554.5
554.5
554.5
554.5
The values of π*, ε , α and β for general solvents and alcohols are from taken from Ref. [45, 46]. The values of n are taken from Fluka Catalogue
2004-05 for general solvents and alcohols.
Figure 3.9 (a) Ground state optimized structures of F27 molecules obtained using
B3LYP functional with 6-31g* basis is shown along with the
Distribution of charges. The arrow indicates the direction of
dipole moment.
Figure 3.9 (b) Ground state optimized structures of SRB molecules obtained using
B3LYP functional with 6-31g* basis is shown along with the
distribution of charges. The arrow indicates the direction of
dipole moment.
HO
O
Cl
O
HO
Cl
Cl
O
O
Cl
OH
OH
O
O
HO
O
Cl
O
HO
Cl
Cl
O
Cl
OH
OH
O
O
HO
O
Cl
O
O
HO
Cl
Cl
O
OH
O
Cl
OH
O
O
(a)
Figure 3.8 Possible resonance structures of (a) F27.
(C2H5)2N
N(C2H5)2
O
SO3
SO3Na
(C2H5)2N
O
N(C2H5)2
SO3
SO3Na
(b)
Figure 3.8 Possible resonance structures of (b) SRB.
1200
(a)F27
-1
νa-νf(cm )
800
400
NHBD (0.97297)
HBD (0.97226)
0
0.36
0.42
0.48
0.54
0.60
0.66
0.72
0.78
N
ET
1200
(b)SRB
-1
νa-νf(cm )
900
600
300
NHBD (R=0.96643)
HBD (R=0.96631)
0
0.36
0.42
0.48
0.54
0.60
0.66
0.72
0.78
N
ET
Figure 3.7
Plot νa-νf vs ETN of (a) F27 and (b) SRB in all solvents studied.
19800
F27
19200
-1
Wavenumber (cm )
19500
18900
18600
18300
18000
R=0.93165
17700
42
45
48
51
54
57
ET(30)
Figure 3.6
Plot of absorption value of F27 (in wave number) vs. ET (30) in all solvents
studied.
f(ε,n)+2g(n)
1.28
1.30
1.32
1.34
1.36
1.38
1.40
1.42
1.44
1.46
1.48
1.50
39000
(a) F27NHBD
2250
38250
37500
-1
36750
1350
ν a+ν f(cm )
-1
ν a-ν f(cm )
1800
36000
900
35250
450
34500
0.81
0.84
0.87
f(ε,n)
f(ε,n)+2g(n)
1.14
1.17
1.20
1.23
1.26
1.29
(b) F27 HBD
38400
3375
37200
-1
νa+νf(cm )
-1
νa-νf(cm )
2700
36000
2025
1350
34800
675
33600
0.55
0.60
0.65
0.70
0.75
0.80
0.85
f(ε,n)
Figure 3.5
Plot of νa-νf vs f(ε, n) and νa+νf vs f(ε, n)+2g(n) of F27 in (a) non-HBD (b)
HBD solvents.
f(ε,n)+2g(n)
1.30
2750
1.35
1.40
1.45
1.50
(c) SRB NHBD
36000
2200
-1
-1
νa-νf(cm )
33600
1650
νa+νf(cm )
34800
32400
1100
31200
550
30000
0.80
0.82
0.84
0.86
f(ε,n)
f(ε,n)+2g(n)
1.15
1.20
1.25
1.30
1.35
1.40
1.45
1.50
36000
3325
(d) SRB HBD
2850
34800
2375
-1
ν a+νf(cm )
-1
νa-νf(cm )
33600
1900
32400
1425
950
31200
475
30000
0.54
0.60
0.66
0.72
0.78
0.84
0.90
f(ε,n)
Figure 3.5
Plot of νa-νf vs f(ε, n) and νa+νf vs f(ε, n)+2g(n) of SRB in (c) nonHBD and (d) HBD solvents studied.
Absorption
1.0
(a) SRB NHBD
12 3 4
1 23 4
1.Acetone
2.DMF
3.Acetonitrile
4.DMSO
0.5
0.0
500
600
λ(nm)
(b)F27 HBD
21345
23451
1.Formamide
2.Methanol
3.Ethanol
4.Propanol
5.Octanol
0.5
0.0
450
500
550
Fluorescence(a.u.)
Absorption
1.0
600
λ (nm)
Figure 3.4
Absorption and fluorescence spectra of (a) SRB in non-HBD and (b) F27
in HBD solvents.
1.1
(a)SRB NHBD
4
1.0
3
π∗
0.9
0.8
1
2
1.Acetone
2.Acetonitrile
3.DMF
4.DMSO
0.7
0.6
546
549
552
555
558
561
λ (nm)
0.96
(b)SRB HBD
0.88
0.80
π∗
0.72
0.64
0.56
0.48
552
556
560
564
λ (nm)
Figure 3.3.
(a) Absorption shift of SRB dye solution as a function of solvent
polarizibility(π*) in non-HBD solvents.
(b) Absorption shift of SRB dye solution as a function of solvent
polarizibility(π*) in HBD solvents.
(a)
Dielectric constant
100
80
60
40
20
0
545
550
555
560
565
570
Wavelength(nm)
35
(b)
Dielectric constant
30
25
20
15
10
5
0
552
553
554
555
556
Wavelength(nm)
Figure 3.2:
(a) Absorption shift of SRB dye solution as a function of dielectric constant
in non-hydrogen-bond donating solvents.
(b) Absorption shift of SRB dye solution as a function of dielectric
constant in hydrogen-bond donating solvents.
HO
O
O
Cl
Cl
COOH
(a)
(H5C2)2N
+
O
N (C2H5) 2
SO 3
-
SO 3Na
(b)
Figure 3.1:
Molecular structures of (a) Fluorescein 27 (b) Sulforhodamine B
100
(a)SRB NHBD
Dielectric constant
80
60
4
2
40
20
3
1.Acetone
2.Acetonitrile
3.DMF
4.DMSO
1
0
545
550
555
560
Wavelength(nm)
120
(b) SRB HBD
105
Dielectric constant
90
75
60
45
30
15
0
552
555
558
561
564
567
Wavelength(nm)
Figure 3.2.
(a) Absorption shift of SRB dye solution as a function of dielectric
constant in non-hydrogen-bond donating solvents.
(b) Absorption shift of SRB dye solution as a function of dielectric
constant in hydrogen-bond donating solvents.