Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 85–91

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Fluorescence characteristics of aryl boronic acid derivate (PBA) S.S. Patil a, G.V. Muddapur b, N.R. Patil b,⇑, R.M. Melavanki c,⇑, R.A. Kusanur d a

Department of Physics, Government R G S P U College, Kittur 591115, Karnataka, India Department of Physics, B V B College of Engg & Tech., Hubli 580031, Karnataka, India c Department of Physics, M S R Institute of Technology, Bangalore 560054, Karnataka, India d Department of Chemistry, R V College of Engineering, Bangalore 560059, Karnataka, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A bathochromic shift is observed in

the chemical system.  The used molecule shows

solvatochromism.  It is observed that the excited state is

more polar than the ground state.  The angle between the ground and

excited state is found to be 70°.  Hydrogen bond acceptor influence is

more than hydrogen bond donor for ma & mf .

a r t i c l e

i n f o

Article history: Received 30 September 2014 Received in revised form 9 November 2014 Accepted 12 November 2014 Available online 20 November 2014 Keywords: Solvatochromic shift DFT Boronic acid Kamlet–Taft Angle between dipole moments

a b s t r a c t The absorption and fluorescence spectra of newly synthesized aryl boronic acid derivative namely Phenyl boronic acid (PBA) have been recorded in various solvents of different polarities. The ground state dipole moment of PBA was obtained from quantum chemical calculations. Solvatochromic correlations were used to estimate the ground state (lg) and excited state (le) dipole moments. The excited state dipole moments are observed to be greater than the ground state dipole moments. Further, the ground and excited state dipole moments are not parallel but subtend by an angle of 70°. The changes in dipole moment (Dl) were calculated both from solvatochromic shift method and microscopic solvent polarity parameter (ENT ), and the values are compared. Solvent effects on the absorption and fluorescence spectra were quantified using Reichardt’s and bulk solvent polarity parameters were complemented by the results of the Kamlet–Taft treatment. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Boronic acids and their derivatives establish a family of dyes which are applicable in different fields of science and technology. Boronic acids have potential applications and they are very impor-

⇑ Corresponding authors. Mobile: +91 9902351732 (N.R. Patil). Mobile: +91 8951478172 (R.M. Melavanki). E-mail addresses: [email protected] (N.R. Patil), [email protected] (R.M. Melavanki). http://dx.doi.org/10.1016/j.saa.2014.11.028 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

tant in synthetic organic, materials, bioorganic, and medicinal chemistry as well as chemical biology. In organic chemistry, boronic acids are very important in Suzuki–Miyaura coupling, aromatic functionalization (such as amination) with a heteroatom containing functional group and protection of diols. In materials chemistry, boronic acids are important in crystal engineering, construction of polymers with reversible properties, building unique molecular architects, functionalization of nanostructures, separation and purification of glycosylated products and feed-back controlled drug delivery (glucose). In bioorganic chemistry, boro-

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nic acid is a commonly used recognition moiety for the design and synthesis of sensors for carbohydrates, amino acids, in medicinal chemistry, boronic acids are important for the preparation of inhibitors of hydrolytic enzymes in boron neutron capture therapy (BNCT), quorum sensing inhibition, antifungal agent development, and the inhibition of other enzymes. Among all the biologically active boronic acids, bortezomib is an FDA-approved anticancer agent. In chemical biology, boronic acids are used in the detection and sensing of peroxides, recognition and sensing of the tetraserine motif in protein, development of new MRI contrast agents [1]. Further, Boronic acids exhibit strong fluorescence in the UV and VISIBLE region which makes them suitable to be used as colorants, dye laser media and as nonlinear optical chromospheres [2]. Solvent effect on the absorption and fluorescence characteristics of organic compounds has been a subject of interesting investigation [3,4]. Knowledge of ground and electronically excited state dipole moments of a molecule are important properties that provide valuable information about the electronic and geometrical structure of the molecule in short lived state. Among the different methods of determining excited state dipole moment (le) solvatochromic method [5,6] is the most widely used one. This method has been based on a linear correlation between the wave numbers of the absorption and fluorescence maxima and a solvent polarity function. Such correlations have been derived from quantum mechanical second order perturbation and Onsager’s reaction field theories for development of solvent polarity functions. The most employed solvent polarity functions are those obtained by Lippert–Mataga [7], Bakhshiev [8], Kawski– Chamma–Viallet [9,10], which use both dielectric constant (e) and refractive index (n) of the medium as empirical parameters. Several workers have made extensive experimental and theoretical studies on ground state (lg) and excited state (le) dipole moments using different techniques in variety of organic fluorescent compounds like coumarins [11–15], thiophene Carboxamides [16], ketocyanine dyes [17], and in some laser dyes [18,19] etc. Because of the tremendous importance of boronic acids, there is an increasing interest in finding ways to increase their structural diversity. This paper reports the theoretically computed results from ab initio calculations using DFT and systematic study of effects of solvent on absorption, emission spectra and the solvatochromic shift. Further, the photophysical properties of Phenyl boronic acid (PBA) molecule in different solvents have been analyzed using microscopic and bulk solvent polarity parameters and rationalized using the Kamlet–Taft treatment. However, there are no reports available in literature on the estimation of ground and excited state dipole moments for phenyl boronic acid. This has prompted us to carry out the present work. Theory Theoretical calculations of ground state dipole moments The ground state dipole moment of the PBA is calculated using quantum chemical calculations. All the computations were carried out using Gaussian 09 program on a Pentium- 4 PC and the basis set level used is B3LYP/6-31g⁄. Experimental calculation of ground and excited state dipole moments The three independent equations used for the estimation of excited state dipole moment of boronic acid derivatives are as follows Lippert’s equation [7],

a  m f Þ ¼ m1 F 1 ðe; nÞ þ Constant ðm

ð1Þ

Bakhshiev’s equation [8]

a  m f Þ ¼ m2 F 2 ðe; nÞ þ Constant ðm

ð2Þ

Bilot–Kawski’s equation [9,10]



ma þ mf

 ¼ m3 F 3 ðe; nÞ þ Constant

2

ð3Þ

a and m f are absorption and emission maxima wave numwhere m a  m f Þ is Stokes shift, 1=2ðm a þ m f Þ is arithmetic bers in cm1, ðm mean of absorption and emission wave number, e and n are dielectric constant and refractive index of solvents respectively. The expressions for (Lippert’s polarity equation) F1(e, n), (Bakhshiev’s polarity equation) F2(e, n), and (Bilot–Kawski’s polarity equation) F3(e, n) are given as





e  1 n2  1  2e þ 1 2n2 þ 1    e  1 n2  1 2n2 þ 1  2 F 2 ðe; nÞ ¼ e þ 2 n þ 2 ðn2 þ 2Þ     3 n4  1 2n2 þ 1 e  1 n2  1 þ  F 3 ðe; nÞ ¼ 2 2ðn2 þ 2Þ e þ 2 n2 þ 2 2ðn2 þ 2Þ

F 1 ðe; nÞ ¼

ð4Þ ð5Þ ð6Þ

a  m f ) versus F1(e, n), ðm a  m f Þ From Eqs. (4)–(6) it follows that (m a þ m f Þ versus F3(e, n) should give linear versus F2(e, n) and 1=2ðm graphs with slopes m1, m2 and m3 respectively and are given as

2



le  lg

m1 ¼

hca 2

le  lg hca

2

3



m2 ¼

2

3



m3 ¼

2

l2e  l2g hca



3

where lg and le are the ground and excited state dipole moments of the solute molecules, h is Planck’s constant and c is velocity of light, ‘a’ is the radius of the solute molecule and the value was calculated from the molecular radius of the molecule [20]. If the ground state and excited states are parallel, the following expressions are obtained on the basis of Eqs. (8) and (9) [21,22].

" # 3 1=2 m3  m2 hca 2 2m2 " # 3 1=2 m þ m2 hca le ¼ 3 2 2m2   m2 þ m3 l and le ¼ m3  m2 g

lg ¼

ð7Þ

ð8Þ for m3 > m2

ð9Þ

If ground state (lg) and excited state (le) dipole moments are not parallel to each other, which form an angle /, then / can be calculated using Eq. (10).

cos / ¼

1 2lg le





l2g þ l2e 

 m3  2 le  l2g m2

ð10Þ

Molecular-microscopic solvent polarity parameter (ENT ) The empirical polarity parameter ENT proposed by Reichardt [23] gave towering results with solvatochromic shift of dipolar molecule. The results correlate better with microscopic solvent polarity ENT rather than the traditionally used bulk solvent polarity functions involving dielectric constant (e) and refractive index (n) as

S.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 85–91

in the later error estimation of Onsager cavity radius ‘a’ has been minimized. This method is based on empirical solvent polarity parameter ENT to estimate excited state dipole moment. It also correlates the spectral shift better than the traditionally used bulk solvent polarity function. In this method the problem associated with the estimation of Onsager cavity radius has been minimized and also this polarity scale includes intermolecular solute/solvent hydrogen bond donor/acceptor interactions along with solvent polarity. The theoretical basis for the correlation of the spectral band shift with ENT was proposed by Reichardt and developed by Ravi et al. [24] is according to Eq. (11):

"

ma  mf ¼ 11307:6

Dl Dlb

2  3 # aB ENT þ constant a

ð11Þ

where Dlb = 9D and aB = 6.2A0 are the change in dipole moment on excitation and Onsager cavity radius of molecule respectively and Dl and ‘a’ are the corresponding quantities for the solute molecule of interest. A dimensionless normalized scale ENT was introduced in order to avoid the use of non SI unit kcal/mol in ET (30) solvent polarity scale and is defined by Eq. (12), using water (ENT ¼ 1) and tetramethylsilane (TMS = ENT = 0) as extreme reference solvents [20,24].

ENT ¼

ET ðSolv ent Þ  ET ðTMSÞ ET ðSolv entÞ  30:7 ¼ ET ðWaterÞ  ET ðTMSÞ 32:4

ð12Þ

The change in dipole moment (Dl) can be evaluated from the slope of the stokes shift versus ENT plot and is given by Eq. (13)

Dl ¼





le  lg ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mX81

ð6:2=aÞ3 11307:6

ð13Þ

a  m f Þ where ‘m’ is the slope obtained from the plot of Stokes shift ðm versus microscopic solvent polarity (ENT ) using Eq. (11). The microscopic solvent polarity parameter (ETN ) values of solvents were taken from literature [23]. Kamlet–Taft solvatochromic parameters To find the information about the individual contribution of different solvent effects, a multiparametric approach, known as solvatochromic comparison method or linear solvation energy relationship (LSER) proposed by Kamlet, Taft and others [25,26], has been used. In the general expression (14),

m ¼ m0 þ aa þ bb þ sðp þ ddÞp

ð14Þ

where mo is the vapor phase wave-number (independent of solvent effects) and the values of m are absorption/fluorescence band maxima in a solvent. The empirical parameters a b and p⁄ are solvent’s hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) capacities and a measure of the polarity/polarizability character of the solvent respectively [27–29]. The coefficients a, b and s are interpreted as solute properties. The coefficient, ‘a’ describes its tendency to accept hydrogen bond from the solvent, ‘b’ measures its property to donate hydrogen bond to the solvent and s is related to the solute polarity/polarizability character. The coefficients a, b and s indicate the susceptibility of m to a change in the corresponding parameter. The term d used in Eq. (14) is a ‘polarizability correction term’, whose value depends on the class of solvents used; for aromatic solvents d = 1, for polyhalogenated solvents d = 0.5 and for all other solvents d = 0 [30]. The term d is interpreted as an indicator of the change in direction of the molecular dipole moment in going from initial to transition state, is zero for all spectra that are shifted bathochromically with increasing solvent dipolarity [31]. So, the Kamlet and Taft Eq. (14) reduce to Eq. (15).

m ¼ m0 þ aa þ bb þ sp

ð15Þ

87

The corresponding parameters for different solvents were taken from literature [12,32]. All least-squares fit analyses were carried out with Microsoft EXCEL. Solvatochromatic effect has been used to determine the magnitude of the solute–solvent interactions such as the polarizability/dipolarity parameter, p⁄, of the solvent, as well as giving information about hydrogen bond donor (HBD), a and/or acceptor (HBA), b ability of the solvent which can be evaluated by multi linear regression analysis. The Kamlet–Taft approaches [32] have been applied to separate the influence of non-specific interactions, from specific interactions. The nonspecific interactions are expressed by Catalan’s SP parameter (solvent polarizability) as well as by the SdP parameter (both of which represent a combination of the solvent dipolarity and polarizability). The signs of aa and bb coefficients vary from one compound to another, and in most cases they present weaker values than c coefficients, which indicate that the ability of the solvent to donate or accept hydrogen bonds is weaker than the solute–solvent dipole– dipole interactions. Materials and methods Materials The Phenyl boronic acid (PBA) was synthesized in our laboratory using standard methods [33]. The molecular structure of molecule is given in Fig. 1. The solvents used in the present study namely pentane, hexane, heptane, decane, toluene, ethyl acetate, acetonitrile and water were obtained from S-D-Fine Chemicals Ltd., India and they were of spectroscopic grade. Measurement of absorption and fluorescence spectra The absorption spectra and fluorescence spectra of PBA were recorded using Lab India UV/VIS 3000 spectrophotometer over a wavelength range 200–700 nm and Hitachi F-2700 FL Spectrophotometer respectively at Dept. of Physics, M.S.R.I.T., Bangalore. The concentration was chosen to be 1  104 M for all the organic solvents. Each time fresh homogeneous solutions were prepared to record both absorption and fluorescence spectra. All these measurements were carried out at room temperature. The uncertainty in the measured wavelength of absorption and fluorescence maxima is ±1 nm. Results and discussion Solvent effect on absorption and fluorescence emission spectra The normalized absorption and emission spectra of PBA in different solvents are given in Figs. 2 and 3 respectively. The absorption spectra show a maxima around 222–234 nm which shows a shift with respect to the dielectric constants of solvents used. Since only the longer wavelength is sensitive to solvent polarity, the absorption shifts with solvents has been reported. The emission spectra are recorded by exciting the sample at its longest absorp-

Fig. 1. Molecular structure of PBA.

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The magnitudes of Stokes shift vary between 7199.981 and 16268.762 cm1. The values of the Stokes shift are also indicative of the charge transfer transition. On changing solvents from a low polar one to a high polar one show a difference in Stokes shift of about 9068.781 cm1, which is again an indicative of charge transfer transition. The large magnitude of Stokes shift indicates that the excited state geometry could be different from that of the ground state. The general observation is that there is an increase in the Stokes shift with increasing solvent polarity which shows that there is an increase in the dipole moment on excitation. The solvatochromic data can be used to identify the spectra, namely n ? p⁄, p ? p⁄ etc. It can be noticed from the Table 1 that, with an increase in the polarity of the solvent, the Stokes shift undergoes a bathochromic shift, which confirms p ? p⁄ transition. Theoretical and experimental ground-state dipole moments Theoretical calculations were performed using DFT/B3LYP level on a Pentium IV/2.8 GHz personal computer using Gaussian 09W program package. The optimized geometry and ground state optimized molecular geometry for the title molecule are given in Fig. 4. The experimental ground state dipole moment is estimated using solvatochromic shift method. It can be seen that there is a slight variation between theoretical and experimental values of ground state dipole moments. This difference in the ground state dipole moment is due to the necessity of knowing the radius of the solute molecule in Eq. (7) as compared to experimental and theoretical values obtained from ab initio calculations using DFT. However, the difference in the value may be due to discrepancies in the experimental and theoretical values of ground-state dipole moments. This may be due to the fact that, the experimental methods take solvent and environmental effects into account, where as ab initio calculations gives lg value only for molecule in a gas phase. However, no other experimental data on lg could be obtained from literature for comparison purposes.

Fig. 2. Normalized absorption spectra of PBA.

Experimental excited-state dipole moments Fig. 3. Normalized emission spectra for PBA.

tion maxima. The excitation maxima coincide with the longest wavelength absorption band and this longest wavelength absorption band has been assigned as the intermolecular charge transfer transition. The absorption and emission maxima, Stokes shift and arithmetic mean of absorption and emission peak values of the test molecule in different solvents are given in Table 1. The emission spectra show a maxima around 268–372 nm. The emission spectra show a greater shift as compared with the absorption spectra. The more pronounced emission shift with solvents implies that the ground state energy distribution is not affected to a greater extent possibly due to the less polar nature of the molecule in the ground-state rather than in the excited sate.

The solvent polarity function values F1(e, n), F2(e, n), and F3(e, n) of various solvents for the test molecule are given in Table 2. From a  m f ) versus F1(e, n), ðm a  m f Þ versus Eqs. (1)–(3) it follows that, ðm a þ m f Þ versus F3(e, n) should be linear with slopes F2(e, n) and 1=2ðm m1, m2 and m3 respectively. ‘a’ is the radius of the solute molecule and the value was calculated from the molecular volume of molea  m f ) versus F1(e, n), (m a  m f ) versus cule [20]. The graphs of (m a þ m f Þ versus F3(e, n) are given in Figs. 5–7 F2(e, n) and 1=2ðm respectively. The correlation coefficients and slopes m1 (Lippert’s), m2 (Bakshiev) and m3 (Bilot–Kawski’s) of the fitted lines are given in Table 3. The excited-state dipole moments of the molecule are estimated using Eqs. (1)–(3). Assuming that the symmetry of the investigated molecule remains unchanged upon electronic transition, the ground and excite state dipole moments are parallel to each other. The value

Table 1 Solvatochromic data of PBA in different solvents. Solvents

ma (cm1)

mf (cm1)

a  m f (cm1) ¼m Dm

a þ m f Þ (cm1) 1=2ðm

Pentane Hexane Heptane Decane Toluene Ethyl acetate Acetonitrile Water

45146.726 44480.028 44165.709 44881.289 42793.564 43131.335 42949.792 43109.356

36145.449 37280.047 36686.647 36472.390 35395.724 31304.783 27329.872 26840.594

9271.277 7199.981 7479.062 8408.899 7397.840 11826.552 15619.920 16268.762

40646.088 40880.038 40426.178 40676.839 39094.644 37218.059 35139.832 34974.975

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S.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 85–91

Fig. 4. The optimized geometry and ground state optimized molecular geometries of PBA. The arrow indicates the direction of the dipole moment.

Table 2 Various solvent parameters used in the solvatochromic studies. Solvents

e

n

F1(e, n)

F2(e, n)

F3(e, n)

EN T

a

b

p⁄

Pentane Hexane Heptane Decane Toluene Ethyl acetate Acetonitrile Water

1.800 1.890 1.900 2.000 2.400 6.080 37.500 80.400

1.358 1.375 1.388 1.408 1.497 1.372 1.346 1.333

0.006 0.033 0.003 0.002 0.014 0.174 0.305 0.320

0.011 1.01E03 0.006 0.004 0.030 0.493 0.863 0.914

0.238 0.255 0.267 0.279 0.349 0.499 0.667 0.683

0.009 0.009 0.012 0.009 0.099 0.228 0.460 1.000

0.00 0.00 0.00 0.00 0.00 0.00 0.19 1.17

0.00 0.00 0.00 0.00 0.11 0.45 0.40 0.47

0.08 0.04 0.08 0.03 0.54 0.55 0.75 1.09

Fig. 5. The variation of Stoke’s shift with F1(e, n) using Lippert equation for PBA.

Fig. 6. The variation of Stoke’s shift with F2(e, n) using Bakshiev’s equation for PBA.

of lg obtained from theoretical, the experimental ground (lg) and excited state (le) dipole moments are estimated using Eqs. (7) and (8), the values of Dl and the ratio of le and lg for the PBA are presented in Table 4. It is observed that the excited state dipole moments are greater than ground state dipole moment. An increase in dipole moment of a molecule on excitation and this change in dipole moment on excitation can be explained in terms of nature of emitting state or intra molecular charge transfer. It is interesting to note from Table 4 that, small differences are observed between the estimated values of le for the test molecule. The le value obtained by Bilot–Kawski’s method is large compared to other methods. In literature one may find that large numbers of investigators have used solvatochromic shift method (Eq. (8)) to estimate excited state dipole moment. These differences between the values of le may be in part due to the various assumptions and simplifications made in the use of Lippert’s, Bakshiev’s, and Kawski–Chamma–Viallet’s correlations [7,8,10].

The estimated values of lg and le are 2.126 and 10.665 D. The difference in values of lg and le compared to respective values from other methods (Table 4) suggests that, lg and le are not parallel. This has prompted us to estimate the angle between lg and le according to Eq. (10) and the value is found to be 70° and it is shown in Table 4. Hence one can conclude that lg and le are not parallel to each other. Further, the positive value of Dl suggests that there is a considerable amount of charge transfer from the donor (OH group) to the acceptor moiety (Boron), which leads to an increase in the polarity of the excited state and consequently, the intensification of the donor–acceptor type of intramolecular interaction. Molecular-microscopic solvent polarity parameter (ENT ) The literature values of ENT for different solvents are presented in Table 2. The plot of Stoke’s shift as a function of ENT for all the solvents

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Fig. 7. The variation of arithmetic mean of absorption and emission wave number with F3(e, n) using Kawski–Chamma–Viallet’s equation for PBA.

Table 3 Statistical treatment of the correlations of solvents spectral shifts of PBA. Correlations

Slope

Correlation factor ‘r’

Number of data

Lippert correlation Bakhshiev’s correlation Kawski–Chamma–Viallet’s correlation

24915.52 8975.95 13446.05 9571.70

0.99 0.98 0.99 0.94

8 8 8 8

EN T correlation

Fig. 8. The variation of Stoke’s shift with ENT for PBA.

the prominent structure of boronic acids. Hence, the excited state for the molecule is more polar than the ground state due to intermolecular charge transfer. Kamlet–Taft solvatochromic parameters

for PBA is given in Fig. 8. The linear ENT dependence of Stoke’s shift indicates the existence of general type of solute–solvent interaction in which the Stoke’s shift depends on the dielectric constant and refractive index of the solvents. The excited state dipole moment is also calculated using microscopic solvent polarity parameter (ENT ) according to Eq. (11). The value of excited state dipole moment calculated from this method is represented as lge and is tabulated in Table 4. This value is slightly smaller than that obtained using Bakshiev’s equation and three times smaller than that obtained using Bilot–Kawski equation. This could be due to the fact that the methods based on Bakshiev’s and Bilot–Kawski equations do not consider specific solute solvent interactions such as hydrogen bonding effect, complex formation and also ignore molecular aspects of solvation, whereas these aspects are incorporated in the method based on ENT [18]. With an increasing solvent polarity, both absorption and emission bands undergo a bathochromic shift. This indicates that, ICT (intermolecular charge transfer) absorption of the less dipolar ground- state molecule leading to highly dipolar-excited state with

The formation of various specific hydrogen bonds between the solvent and the solvatochromic probe may possibly explain the UV–VIS shifts. The contribution of each of the specific interactions can have a different effect by changing the nature and geometry of the solvent [34]. This behavior explains the average correlation coefficients of the multiple square analyses, because each solvent most likely interacts in a different way with the probe. To get information about the various solute–solvent interactions that controls the solvent-induced spectral shifts, the effect of solvents on the absorption and emission energies is quantified by the multiple regressions. Hence, an individual contributions of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) a , m f and abilities of solvents on the spectroscopic properties m ¼m a  m f ) are correlated with solvatochromic parameters a, b (Dm and p⁄. The literature values of a, b and p⁄ are listed in Table 2. The multiple regression analysis data using Eq. (15) along with correlation coefficients are given in below equations.

ma ðcm1 Þ ¼ ð44513:23  210:39Þ þ ð1344:09  62:21Þa þ ð1831:87  83:1Þb þ ð3067:08  1060:88Þp ðr ¼ 0:93; a ¼ 5%; b ¼ 5%; c ¼ 35%Þ

Table 4 Ground and excited state dipole moments of PBA. Compound

Radius ‘a’ (A°)

PBA

3.21 30

lga (D)s

lgb (D)

lec (D)

led (D)

lee (D)

lef (D)

leg (D)

Dlh (D)

Dli (D)

(le/lg)j

/k

4.79

2.13

10.67

11.16

7.55

14.58

3.15

8.54

3.08

5.02

70°

18

cm = 10 esu cm. Debye (D) = 3.33564  10 a The ground states dipole moments calculated using Gaussian software. b The ground states dipole moments calculated using Eq. (7). c The excited states dipole moments calculated using Eq. (8). d The experimental excited state dipole moments calculated from Lippert’s equation. e The experimental excited state dipole moments calculated from Bakshiev equation. f The experimental excited state dipole moments calculated from Kawski–Chamma–Viallet equation. g The excited state dipole moments calculated from EN T equation. h The change in dipole moments for le and lg. i The change in dipole moments calculated from Eq. (13). j The ratio of excited state and ground state dipole moment. k The angle between ground state dipole moment and excited state dipole moment.

S.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 138 (2015) 85–91

mf ðcm1 Þ ¼ ð36697:08  760:10Þ  ð1940:75  224:76Þa þ ð13593:07  661:82Þb þ ð14650:88  3832:80Þp



ðr ¼ 0:98; a ¼ 12%; b ¼ 5%; c ¼ 26%Þ

91

B V B College of Engineering &Technology, Hubli, India and K.L.E Society Management Belgaum for their kind support and encouragement. And another author (RMM) is also grateful to the H.O.D., Dr. Suguna, the Principal and the Management of M.S.R.I.T. for their continuous encouragement.

Dm ðcm1 Þ ¼ ð7879:90  875:90Þ  ð3349:84  259:00Þa  ð14510:29  762:65Þb þ ð17653:8  4416:71Þp ðr ¼ 0:95; a ¼ 8%; b ¼ 6%; c ¼ 25%Þ From the above equations it is clear that, the non-specific dielectric interaction (p⁄) has the major solvent influence for title molecule. However, the contribution of HBD and HBA parameters cannot be neglected. From the above relations it is clear that HBA (b) influence a and m f . Whereas HBA (b) influence is less is more than HBD (a) for m . The negative values of a, and b indicate that than HBD (a) for Dm both the parameters a and b contribute to the stabilization of the ground state as well as the excited state [35]. Conclusions The fluorescence properties of PBA are reported and discussed by examining solvent effects on the absorption and fluorescence spectra. The test molecule shows positive solvatochromism with the increase in the polarity of the solvent. This study has brought into focus the single emission that originates from ICT excited states. The changes displayed by the photophysical properties of this molecule in different solvents can be explained in terms of electron distribution and hydrogen bonding participation in the stabilization of these molecules in ground and excited states. The ground state dipole moment results are correlated (experimental and theoretical) in our used chemical systems. It has been found that excited state dipole moment (le) is greater than ground state dipole moment (lg) for the molecule. This demonstrates that, the molecule is more polar in excited state than in the ground state for all the solvents studied. Further, the ground and excited state dipole moments are not parallel to each other but they are subtended by an angle of 70°. From Kamlet–Taft solvatochromic parameters, HBA (b) influa and m f . From the linear solvation ence is more than HBD (a) for m energy relationship (LSER) equation, parameters a, b and p⁄ play important roles in the mechanism of the interactions between these probes and solvents. The lowest singlet excited state of the molecule has larger dipole moment than that of the ground state. Based on multiparametric regression analysis, it appears that the excited state stabilization is mainly determined by the dipolar interactions and hydrogen bond accepting ability of solvents. Hence, dipolar interaction contributes usually to high value of dipole moment and le/lg. Acknowledgements The author (SSP) thank the Principal Government R G S P U College, Kittur, (GVM) and (NRP) express their thanks to the Principal

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Fluorescence characteristics of aryl boronic acid derivate (PBA).

The absorption and fluorescence spectra of newly synthesized aryl boronic acid derivative namely Phenyl boronic acid (PBA) have been recorded in vario...
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