Estimation of ground and excited state dipole moments of quinidine and quinidine dication: Experimental and numerical methods

Journal of Molecular Liquids 179 (2013) 88–93

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Estimation of ground and excited state dipole moments of quinidine and quinidine dication: Experimental and numerical methods Sunita Joshi a, Rituparna Bhattacharjee b, Tej Varma Y a, Debi D. Pant a,⁎ a b

Department of Physics, Birla Institute of Technology and Science, Pilani, 333031, Rajasthan, India Department of Chemistry, Birla Institute of Technology and Science, Pilani, 333031, Rajasthan, India

a r t i c l e

i n f o

Article history: Received 9 October 2012 Received in revised form 21 November 2012 Accepted 23 November 2012 Available online 10 December 2012 Keywords: Solvatochromic shift Dipole moment Absorption Fluorescence Quinidine and quinidine dication

a b s t r a c t Absorption and fluorescence spectra of quinidine (QD) and quinidine dication (QD2+) have been measured at room temperature in solvents of different polarities. Ground and excited state electric dipole moments are determined experimentally using solvatochromic shift method based on bulk solvent properties. Numerical calculations are also performed using B3LYP/6-31G(D) level of theory for ground state and CIS/6-31G(D) level of theory for first excited singlet state. From both experimental and numerical studies it has been observed that dipole moment values of excited states (μe) are higher than corresponding ground state value (μg), of QD and QD2+, which is attributed to the higher polarity of excited states of QD and QD2+ molecules. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The photochemistry of quinidine (QD) has been studied as a function of pH by Schulman et al. [1]. On changing pH, the various species derived from QD are dication (pH 2), monocation (pH 7) and neutral at higher pH values. The excited state dynamics of QD has been studied as a function of pH in steady state and nanosecond time resolved fluorescence experiments [2–5]. A detailed study on QD as a function of temperature under both steady state and transient conditions was carried out and it was suggested that there are two major relaxation process in this molecule, i.e., charge transfer process and solvent relaxation process in polar fluid medium at ambient temperature [2–5]. The basic chromophore responsible for the photophysical characteristics is the methoxyquinoline ring and vinyl group causes minor changes in the photophysics. Structures in the ground and excited states of organic molecules are responsible for their observed spectral properties. The knowledge of the solvent effect on absorption and fluorescence spectra is of particular importance. A change in solvent is accompanied by a change in polarity; therefore, different solvents (having different dielectric constant or polarizability) affect the ground and excited state differently. Since the polarities of the ground and excited states of a chromophore are characteristically different, a change in solvent results in different stabilisation of the ground and excited states, thereby altering the energy gap between the electronic states. So a systematic study on solvent effect proves to be informative for studying the excited state ⁎ Corresponding author. Tel.: +91 1596515513. E-mail address: [email protected] (D.D. Pant). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.11.023

behaviour of the molecule. The dipole moment of an electronically excited state of a molecule is an important property that provides information on the electronic and geometrical structure of the molecule in short-lived state. Knowledge of the excited-state dipole moment of electronically excited molecules is quite useful in designing nonlinear optical materials [6], elucidating the nature of the excited states and in determining the course of photochemical transformation. For a chromophore, the tunability range of the emission energy as a function of polarity of the medium is also determined by excited state dipole moment [7] and this can be useful in optimizing the efficiency or performance of dye lasers. A number of techniques e.g. electronic polarization of fluorescence, electric-dichroism, microwave conductivity and stark splitting [8,9] are available for determination of excited-state dipole moment, but their use is limited because they are considered equipment sensitive and studies have been related to very simple molecules. Several workers have made extensive experimental and theoretical studies on ground (μg) and excited state (μe) dipole moments using different techniques in variety of organic fluorescent compounds like coumarins [10,11], indoles [12,13], purines [14,15], exalite dyes [16,17], quinazolines [18], nile red dye, prodan, laurdan, badan, acrylodan [19–24], thiadiazole [25], bezimidazolone [26], phloroglucinol [27], chalcons [28], 546 laser dye [29], etc. The solvatochromic method is based on the shift of absorption and fluorescence maxima in different solvents of varying polarity. The solvent dependence of absorption and fluorescence maxima is used to determine the excited-state dipole moments of different molecules. Theoretical description of solvatochromism is based on the Onsager description of non-specific electrostatic solute–solvent interactions

S. Joshi et al. / Journal of Molecular Liquids 179 (2013) 88–93

and the solvent is described as a dielectric continuum hosting solute molecules into Onsager type cavities. The surrounding solvent molecules get polarized due to the electric dipole moment of the solute molecule. As a result, the solute itself experiences an electric field, the reaction field, which is proportional to the dipole moment of the solute. In our recent papers [30,31], we have reported the ground and excited state dipole moments of quinine sulfate (QS) and quinine sulfate dication (QS 2+) using solvatochromic shift method (where values of ground state dipole moment were theoretically calculated). We observed a large change in dipole moment of QS and QS 2+ in the excited state compared to the ground state. QD and QD 2+ are diastereomers to QS and QS 2+ respectively [32]. Since diastereomers are non-equivalent in their physical and chemical properties, the dynamic properties of the diastereomers in different environments become essential for understanding the effects of the conformational change of the diastereomers on their chemical reactivities. To the best of our knowledge, the dipole moments of QD and QD 2+ molecules have not been reported. In this report we have estimated the ground and excited state dipole moments of QD and QD 2+ experimentally by solvatochromic shift method using Bakhshiev and Kawski–Chamma–Vallet correlations and theoretical calculations were also performed using B3LYP/6-31G(D) level of theory for ground state and CIS/6-31G(D) level of theory for first excited singlet state.

89

(A)

N

O H

OH N

(B)

H+ N

O 2. Experimental

H

Quinidine obtained from Sigma Aldrich having 99% purity and used as such without further purification. All the samples of quinidine (QD) and quinidine di-cation (QD2+) were prepared by dissolving the appropriate concentration of quinidine in different solvents. The molecular structure of QD and QD2+ is shown in Fig. 1. In dilute sulphuric acid solutions (1N H2SO4), QD is present as a dicationic species. All the solvents were either of spectroscopic grade or were checked for their purity. Absorption spectra were taken with the help of dual beam JASCO V-570 UV/Vis/NIR spectrophotometer and fluorescence spectra were recorded with the help of Shimadzu, RF-5301PC Spectrofluorometer. The data were analyzed using related software. The spectral shifts obtained with different sets of samples were identical in most of the cases and values were within ±1.0 nm. Data were analyzed and were fitted to a straight line using Origin 6.1 Software. Density of the probe was estimated by ACD/Chemsketch software. The concentration of quinidine in all the solutions prepared in different solvents was 10−4 M. For all spectral measurements, the samples were taken in 1 cm ×1 cm quartz cells.

OH NH+

Fig. 1. Molecular structure of (A) quinidine (QD), and (B) quinidine dication (QD2+).

F1 the bulk solvent polarity function and S1 the slope are defined as follows:

F 1 ðε; ηÞ ¼

" # 2η2 þ 1 ε−1 η2 −1 − η2 þ 2 ε þ 2 η2 þ 2

ð2Þ

and

S1 ¼

 2 2 μ e −μ g hca0 3

ð3Þ

2.1. Method Most theories of solvent effect on the location of absorption and fluorescence spectra, in spite of different assumptions, lead to similar expressions for the Stokes shift. We have used the following two formulae to determine the excited singlet state dipole moment (μe) and ground state dipole moment (μg) by the solvatochromic method. These equations have been obtained by employing the simplest quantum-mechanical second order perturbation theory and taking into account the Onsager reaction field for a polarisable dipole. Bakhshiev's formula [33]

v a −v f ¼ S1 F 1 ðε; ηÞ þ const:

ð1Þ

Here v a and v f are the wavenumbers of the absorption and emission maxima respectively.

here h denotes the Planck's constant, c is the velocity of light in vacuum, μg is the dipole moment in the excited singlet state, a0 is the Onsager cavity radius, ε is the solvent dielectric constant and η is the solvent refractive index. Bilot–Kawski formula [34,35] v a þ v f ¼ −S2 F 2 ðε; ηÞ þ const: 2

ð4Þ

here the meaning of symbols is same as given above except for F2 and S2 which are defined as follows " # " # 2η2 þ 1 ε  1 η2  1 3 η4  1   2 F 2 ðε; ηÞ ¼  2 þ  2 2 2 η þ2 2 η þ2 εþ2 η þ2

ð5Þ

90

S. Joshi et al. / Journal of Molecular Liquids 179 (2013) 88–93

and

S2 ¼

  2 μ e 2 −μ g 2 hca0 3

ð6Þ

The parameters S1 and S2 are the slopes which can be calculated from Eqs. (1) and (4) respectively. Assuming the ground and excited states are parallel, the following expressions are obtained using Eqs. (3) and (6) [35] " #1=2 S2 −S1 hca0 3 2 2S1

ð7Þ

" #1=2 S1 þ S2 hca0 3 μe ¼ 2 2S1

ð8Þ

μg ¼

and μe ¼

jS1 þ S2 j μ jS2 −S1 j g

ð9Þ

The value of solute cavity radius (a0) was calculated from the molecular volume according to suppan's equation [36]  a0 ¼

3M 4πδN

1 =

3

ð10Þ

where δ is the solid state density of solute molecule, M is the molecular weight of the solute and N is the Avogadro's number. 3. Numerical calculations Density functional theory (DFT) [37,38] is emerging out rapidly as a computationally cost-effective general procedure for studying physical properties of molecules. Several research groups are actively involved in systematic comparisons of DFT theories with experiment and also with HF and Møller-Plesset (MP2) treatments [39]. In our study, geometry optimizations were performed at B3LYP/6-31G(D) [40–42] level of theory for ground states of QD and QD 2+ and dipole moment values (μg) were determined. Frequencies are simultaneously checked while optimizing the geometry (until there is no imaginary frequency in the optimized geometry indicating true energy minima). The basis set employed here is 6-31G (D) which is already widely used in studies of moderately large organic molecules. (This basis set has polarization functions on nonhydrogen atoms.) First excited singlet state dipole moment (μe) of quinidine were computed by single point calculations (preceded by geometry optimization at ground state at B3LYP/ 6-31G(D) level at CIS/6-31G(D)) [43] level of theory. The SCF density is used for ground state whereas CI One-particle and CI density are used for excited state calculations. All calculations were carried out in gas phase using Gaussian 03 suite of program [44]. The optimized structures are given in Fig. 2. 4. Results and discussion The structural formula of QD and QD2+ is shown in Fig. 1. In dilute sulphuric acid solutions (1 N H2SO4), quinidine (QD) is present as a dicationic species, which is quite stable [4]. The nitrogens are adequately bound as the ammonium salt by strong acid of the medium effectively preventing n, π* and n, σ* transitions from occurring. The quinoline ring is a main fluorophore responsible for the absorption; methoxy group is very sensitive to the surrounding environment and responsible for anomalous photophysical behavior of QD and QD 2+ [2–5]. The steady state absorption and fluorescence measurements were made in different solvents of different polarities at room temperature. The

Fig. 2. Optimised geometry at B3LYP/6-31G(D) level of (A) quinidine (QD) and (B) quinidine dication (QD2+) in gas phase.

absorption spectrum in cyclohexane and fluorescence spectrum in two different representative solvents dichloromethane (aprotic polar) and cyclohexane (nonpolar) are shown in Fig. 3(A) for QD and Fig. 3(B) for QD2+. Absorption maximum of quinidine (QD) in different solvents was obtained around 334 nm with ±1 nm experimental error and of quinidine dication (QD2+) absorption peak for all solvents was obtained at 346 ± 1 nm experimental error and emission maximum shifts towards lower frequencies with increase in polarity of the solvent as shown in Fig. 3(A) and (B) respectively for QD and QD2+. The emission spectrum has only a structure-less broad band in all the solvents studied. The absorption maxima for different solvents studied remains almost constant with polarity function, whereas the emission maximum shifts toward lower frequencies with the increase in solvent polarity. The fluorescence spectrum is more red shifted in the case of polar aprotic solvents as compared to non-polar solvents. This trend in the fluorescence spectra is a bathochromic shift with increase in polarity [45] and is an indication of π, π* transition. The constant absorption spectrum with solvent polarity implies that the ground state energy distribution is not affected possibly due to less polar nature of the molecule in the ground state. On the other hand, a large red shift of the emission maximum with the solvent polarity indicates greater stabilization of excited state in polar solvents. Solvent polarity functions F1 (ε, η) and F2 (ε, η) have been calculated in order to ascertain the ground and excited state dipole moments of the molecule and are given in Tables 1 and 2. The spectral shifts   v a −v f versus the solvent polarity function F1 (ε, η) and v a þ2 v f versus F2 (ε, η) are shown in Figs. 4 and 5, respectively for QD and QD2+. The linear behaviour of Stokes shift versus solvent polarity function indicates

S. Joshi et al. / Journal of Molecular Liquids 179 (2013) 88–93

(A) Emission

1.0

(b)

Absorbance

Absorption

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 250

300

350

Table 2 Different solvent parameters and spectral data of QD2+ (quinidine dication) in different solvents. Solvent

Normailzed Intensity

(a)

1.0

Dimethylformamide Dichloromethane Ehylacetate Chloroform Diethylether Carbontetrachloride Cyclohexane

Wavelength (nm)

(B)

Absorbance

emission

b)

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

Normalized Intensity

a)

1.0 absorption

v a þ vf 2

ε

η

F1

F2

v a −v f (cm−1)

(cm−1)

38 8.9 6 4.8 4.3 2.2 2

1.4305 1.4241 1.3724 1.4458 1.3520 1.4610 1.4240

0.8387 0.5903 0.4891 0.5903 0.3738 0.0144 −0.0065

0.7114 0.5830 0.4979 0.4872 0.4269 0.3193 0.2845

6596.2 6630.0 6396.9 6645.5 6500.3 6277.3 6111.8

25520.3 25586.7 25619.9 25495.7 25651.6 25763.1 25762.6

spectra with the solvent polarity and continuous variation of emission spectra suggests that the excited state is more sensitive to solvent polarity than the ground state and the change in fluorescence peak position with solvent polarity is responsible for the observed Stokes shift in QD and QD 2 +. Theoretical calculations for QD and QD 2 + also revealed that the excited state dipole moment value is higher than ground state (Table 4) due to high polarity of excited state. Molecular dipole moment is an experimental measure of the charge distribution in a molecule. It is difficult to evaluate accurately the global electron distribution in a molecule because it involves all the multipoles. The calculated values of dipole moment based on densities computed by different methods (SCF, CI-one-particle and CI) are usually higher than the values of experimental dipole

0.0 450

400

91

(A)

2200 2

4

2100 350

400

450

500

550

0.0 600

2000

Wavelength (nm) Fig. 3. Normalized absorption spectra (in cyclohexane) and fluorescence spectra of QD in (a) cyclohexane (b) dichloromethane. (A) QD and (B) QD2+, respectively.

general solvent effects as a function of dielectric constant and refractive index. Origin 6.1 software was used for the data fitted to a straight line. Using ACD/ChemSketch software the density of the molecule was found to be 1.21± 0.1 g/cm3. Thus using Eq. (10), value of solute cavity radius (a0) was obtained 4.74A0. Slope values found from Figs. 4 and 5(A) and (B) are given in Table 3. The value of μe obtained is different compared to the ground state dipole moment. All the data related to experimental dipole moments are summarized in Table 3. Clearly the dipole moment in excited state is significantly larger than the dipole moment in the ground state of the probe molecules QD and QD 2 +. The dipole moments of a molecule in the ground and excited states are different due to changes in electron densities in these states. The excited state dipole moment value is higher than ground state indicating more polar excited state as compared to ground state. The invariance of absorption

Table 1 Different solvent parameters and spectral data of QD in different solvents.

⎯νa-⎯νf (cm-1)

300

1 7

1900

5

6

1800

3

1700 1600 8

1500 0.0

0.2

0.4

0.6

0.8

1.0

F1

(B) 29200

8

29150

(⎯νa +⎯νf )/2 (cm-1)

0.0

5

29100

3

29050 6

29000 2

1

28950 28900

4

28850 7

Solvent

ε

Η

F1

F2

v a −v f (cm−1)

v a þ vf 2 −1

Dimethylformamide Dichloromethane Ehylacetate Chloroform Diethylether O-xylene Carbontetrachloride Cyclohexane

38 8.9 6 4.8 4.3 2.4 2.2 2

1.4305 1.4241 1.3724 1.4458 1.3520 1.5054 1.4610 1.4240

0.8387 0.5903 0.4891 0.5903 0.3738 0.0277 0.0144 −0.0065

0.7114 0.5830 0.4979 0.4872 0.4269 0.3547 0.3193 0.2845

1960.0 2143.8 1771.0 2115.8 1861.0 1850.0 1906.8 1530.9

28960.0 28958.1 29054.5 28882.1 29099.5 29015.0 28808.6 29174.0

(cm

)

28800 0.3

0.4

0.5

0.6

0.7

F2 Fig. 4. (A) Plot for Stoke's shift versus solvent polarity function F1 for QD in (1) dimethylformamide, (2) dichloromethane, (3) ethylacetate, (4) chloroform, (5) diethylether, (6) O-xylene, (7) carbontetrachloride, and (8) cyclohexane. (B) Plot for arithmetic average of absorption and fluorescence wavenumbers versus solvent polarity function F2 for QD in (1) dimethylformamide, (2) dichloromethane, (3) ethylacetate, (4) chloroform, (5) diethylether, (6) O-xylene, (7) carbontetrachloride, and (8) cyclohexane.

92

S. Joshi et al. / Journal of Molecular Liquids 179 (2013) 88–93

(A)

Table 4 Dipole moment data of QD and QD2+ in ground and excited states by Theoretical method.

6700 4

2 1

⎯νa-⎯νf (cm-1)

6600 5

6500

3

6400 6300

7

6200 6100

8

0.0

0.2

0.4

0.6

0.8

1.0

F1

(B) 25800 6

7

-1

(⎯ν a +⎯ν f )/2 (cm )

25750 25700 5

25650

3

25600

2

25550 1 4

25500 25450

0.3

0.4

0.5

0.6

0.7

F2 Fig. 5. (A) Plot for stokes shift versus solvent polarity function F1 for QD2+ in (1) dimethylformamide, (2) dichloromethane, (3) ethylacetate, (4) chloroform, (5) diethylether, (6) carbontetrachloride, and (7) cyclohexane. (B) Plot for arithmetic average of absorption and fluorescence wavenumbers versus solvent polarity function F2 for QD 2+ in (1) dimethylformamide, (2) dichloromethane, (3) ethylacetate, (4) chloroform, (5) diethylether, (6) carbontetrachloride, and (7) cyclohexane.

moments (as observed in earlier theoretical studies as well) [46]. One of the limitations of quantum chemical calculations is attributed to the overestimation of uneven electronic distribution in a molecule and thus make it more polar compared to that of the real system. To compute theoretical dipole moments within 0.03 D of the experimental values requires the addition of extensive configuration interaction with a near-HF quality basis [47]. Moreover, all calculations in the present theoretical study were done in gas phase which could not take care of the solute-solvent interactions. A polar solvent polarizes a solute more significantly than a nonpolar solvent and hence it results in a larger charge separation which in turn leads to a higher dipole moment [48]. Even for the simple water molecule, the dipole moment of the gas phase water monomer is 1.85 D. When solvated in bulk water, the dipole moment of an individual water molecule is observed to be enhanced to the much larger value of 2.9 ± 0.6D [49].

Molecule

μg (D)

QD QD2+

3.58 5.55

a

μe (D) 5.02 8.41

b

μe (D) 4.58 7.33

c

Δμ (D)d

Δμ (D)e

1.44 2.86

1.00 1.78

a Ground State Dipole Moment at B3LYP/6-31G(D) (SCF) level of theory; density used for ground state is mentioned in bold in parentheses. b First Singlet Excited State Dipole Moment; geometries optimized at B3lyp/6-31G(D) followed by single point calculations at CIS/6-31G(D) (CI-one-particle) level of theory, density used for excited state is mentioned in bold in parentheses. c First Singlet Excited State Dipole Moment; geometries optimized at B3LYP/6-31 G(D) followed by single point calculations at CIS/6-31G(D) (CI) level of theory, density used for excited state is mentioned in bold in parentheses. d Δμ (D) calculated using b. e Δμ (D) calculated using c.

In the literature there are several examples of variation of dipole moments from the experimental results depending on the quantum chemical method used. In an extensive study, [50] it is found that “the DFT, RHF and MP2 predicted formamide dipole moments are essentially stable using the basis set 6-31G(D). It should be stressed that the RHF values are better than the MP2 ones but worse than the corresponding DFT results.” It is observed that dipole moment value in 6-31G basis set is maximum for several systems [51]. Therefore it is concluded that “maximum polarity is observed in this basis set.” In another study, [52] the equilibrium dipole moments have been calculated at the HF, MP2, CCSD and CCSD(T) levels. All electrons are correlated in the aug-cc-pVxZ basis sets with x = D, T, Q. Augmented basis sets have been used to ensure a flexible description of the outer-valence region. The excellent performance of the CCSD(T) among all methods relies on the fact that the chosen molecules are all dominated by a single configuration. When several configurations are important, the performance of the CCSD(T) model degrades. Thus it is worth mentioning here that the best method to evaluate dipole moment is yet to be explored. 5. Conclusions We have estimated and compared dipole moments of the molecules QD and QD 2+ in the electronic ground and excited states by experimental and numerical methods. We found that both the molecules possess higher dipole moment value in excited state than in the ground state. A comparison of the values indicates that there is no clear-cut preference for any of the methods as far as numerical values are concerned. Theoretically obtained values of excited state dipole moment are also higher than the values of ground state dipole moment of QD and QD 2+. This observation correlated well with the experimental study. Numerically higher theoretical values may be resulting from constrained use of basis set and quantum chemical methods due to limited computational facility. Solvation effect should also be incorporated to attain higher level of accuracy. A systematic theoretical study may be performed in future to investigate the proper reason behind the numerical differences between experimental and theoretical data, though both of them vouched for high polarity of excited state (resulting in higher values of dipole moment for excited state compared to that of ground state). Acknowledgements

Table 3 Dipole moment data of QD and QD2+ in ground and excited states by experimental method. Molecule

a0 (A0)

S1 (cm−1)

S2 (cm−1)

μg (D)

μe (D)

Δμ (D)

μe/μg

QD QD2+

4.74 4.74

330.83 528.88

586.04 303.00

0.7 0.5

2.46 1.76

1.76 1.26

3.5 3.5

UGC, New Delhi, is acknowledged for financial assistance. S.J. and T.V. are thankful to BITS-Pilani and UGC, New Delhi for research fellowships. R.B. is thankful to the Department of Science and Technology (DST), the Government of India, New Delhi, for financial support in the form of a Junior Research Fellowship (Project Ref. No. SR/S1/ PC-41/2008).

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