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Solvent effect on the spectroscopic properties of 6MAMC and 7MAMC
Journal of Molecular Liquids 158 (2011) 105–110
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Solvent effect on the spectroscopic properties of 6MAMC and 7MAMC Raveendra M. Melavanki a,⁎, N.R. Patil b, S.B. Kapatkar b, N.H. Ayachit b, Siva Umapathy c, J. Thipperudrappa d, A.R. Nataraju e a
Department of Physics, M S Ramaiah Institute of Technology, Bangalore 560054, Karnataka, India Department of Physics, B V B College of Engineering and Technology, Hubli 580031, Karnataka, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, Karnataka, India d Department of Physics, BNM Institute of Technology, Bangalore 560070, Karnataka India e Department of Physics, Seshadripuram, P. U. College (Main), Bangalore 560020, Karnataka, India b c
a r t i c l e
i n f o
Article history: Received 1 August 2010 Received in revised form 2 October 2010 Accepted 7 November 2010 Available online 11 November 2010 Keywords: Solvatochromic shift method Ground-state dipole moments Excited-state dipole moments 6MAMC and 7MAMC
a b s t r a c t The absorption and emission spectra of two dyes namely 6MAMC and 7MAMC have been recorded at room temperature in solvents of different polarities. The ground-state dipole moments (μg) of these two were determined experimentally by Guggenheim method and were compared with theoretical values obtained using quantum chemical method. The exited state (μe) dipole moments were estimated from Lippert's, Bakhshiev's and Chamma-Viallet's equations by using the variation of the Stokes shift with the solvent dielectric constant and refractive index. The ground and excited-state dipole moments were calculated by means of the solvatochromic shift method and also the excited-state dipole moments are determined in combination with ground-state dipole moments. It was observed that dipole moments of excited state were higher than those of the ground state, indicating a substantial redistribution of the π-electron densities in a more polar excited state for these two dyes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The effect of solvent on the absorption and fluorescence characteristics of organic compounds has been a subject of interesting investigation [1–3]. Excitation of a molecule by photon causes a redistribution of charges leading to conformational changes in the excited state. This can result in an increase or decrease of dipole moment of the excited sate as compared to ground state. 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 the short-lived state. Knowledge of the excited-state dipole moment of electronically excited molecules is quite useful in designing nonlinear materials, elucidating the nature of the excited states and in determining the course of a photochemical transformation. The excited-state dipole moments of fluorescent dye molecules such as those studied here also determine the tunability range of the emission energy as a function of the polarity of the medium. All the methods available so far for the determination of
⁎ Corresponding author. Tel.: +91 80 23600822, +91 9008228982 (mobile); fax: +91 80 23603124. E-mail address: [email protected] (R.M. Melavanki). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.11.002
singlet excited-state dipole moment are based on the spectral shift caused either externally by electrochromism or internally by solvatochromism. The electrooptic methods such as electronic polarization of fluorescence, electric-dichroism, microwave conductivity and stark splitting are generally considered to be very accurate, but their use is limited because they are considered equipment sensitive and the studies have been restricted to relatively very simple molecules. The solvatochromic method is based on the shift of absorption and fluorescence maxima in different solvents of varying polarity. Koutek has shown that under suitable conditions, the solvatochromic method yields fairly satisfactory results [4]. The solvent dependence of absorption and fluorescence maxima is used to estimate the excitedstate dipole moments of different molecules. Several workers have made extensive experimental and theoretical studies on ground-state (μg) and excited-state (μe) dipole moments using different techniques in variety of organic fluorescent compounds like coumarins [5], indoles [6], purines [7], exalite dyes [8], acridine dyes [9], fluorescein [10], hemicyanine dyes [11], PRODAN, BADAN and ACRYLODAN [12], acridines and phenazines [13] and in some laser dyes [14–20] etc. Coumarins and their derivatives establish a family of dyes which are applicable in different fields of science and technology. They exhibit strong fluorescence in the UV and VISIBLE region which makes them suitable for used as colorants, dye laser media and as nonlinear optical chromospheres. In medicine, coumarin derivatives are used as anticoagulants, as
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a fluorescent indicators for the physiological pH region and as fluorescent probes to determine the rigidity and fluidity of living cells and its surrounding medium. They also possess distinct biological activity and have been described as potential agents for anticancer. In the present work we report the effects of solvent on absorption and emission spectra, and estimation of ground- and excited-state dipole moments of these two dyes namely 6-methyl 4-azidomethyl coumarin (6MAMC) and 7-methyl 4-azidomethyl coumarin (7MAMC) by solvatochromic shift method and also theoretical computed results from ab initio calculations using DFT. However, there are no reports available in literature on the determination of μg and μe values of these dyes investigated. This prompted us to carry out the present work. 2. Materials and methods
be measured with the help of Forbes Tinsley (FT) 6421 LCR Data Bridge at 10 KHz frequency. The refractive indices of various dilute solutions of the solute for sodium D line were determined by using Abbe's refractometer. All these measurements were carried out at room temperature (300 K). 2.3. Determination of the dielectric constants and refractive indices The capacitance of air, the solvent and the solution have been used to measure dielectric constant. By measuring the capacitance of different concentrations of the solute in toluene the dielectric constant of the solution (ε12) was calculated using the expression ε12 =
2.1. Chemicals used The 6MAMC and 7MAMC were synthesized in our laboratory using standard methods [21–23]. The molecular structures of these dyes are given in Fig. 1. The solvents used in the present study namely toluene (TL), acetonitrile (AN), tetrachloroethane (TCE), dioxane (DX), diethyl ether (DEE), n-butyl alcohol (n-BA), carbon tetrachloride (CCl4), ethyl acetate (EA), tetrahydrofuran (THF) and benzene (BZ) were obtained from S-D-Fine Chemicals Ltd., India, and they were of spectroscopic grade. The required solutions were prepared at fixed concentration of solutes 1 × 10− 4M/L in each solvent.
C2 −CX C1 −CX
ð1Þ
where C2, CX and C1 represent the capacitances due to leads, solution, and air respectively. The values of dielectric constants thus determined for 6MAMC and 7MAMC using toluene. The values of refractive
2.2. Spectroscopic measurements The absorption spectra were recorded using Hitachi 50–20 UV–Vis spectrophotometer. The fluorescence spectra were recorded using Hitachi F-2000 fluorescence spectrophotometer. The dielectric constants of the dilute solutions were measured in a suitably fabricated cell of usually small capacitance where the accurate determination of small changes in the capacitance would be possible. The small capacitance can
N3 H3C
O
O
6-methyl-4-azidomethyl coumarin (6MAMC)
N3
H3C
O
O
7-methyl-4-azidomethyl coumarin (7MAMC) Fig. 1. The molecular structures of 6MAMC and 7MAMC.
Fig. 2. Ground-state optimized molecular geometries of 6MAMC and 7MAMC. The arrow indicates the direction of dipole moment.
R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110
107
Table 1 Solvatochromic data of 6MAMC and 7MAMC along with the calculated values of polarity functions in different solvents. − − − − − − 1 Dyes Solvents νa nm νf nm ν a (cm−1) νf (cm−1) νa − νf Þ (cm−1) =2 νa + νf Þ (cm−1) 6MAMC
7MAMC
TCE BZ DX n-BA TL DEE EA CCL4 AN BZ DX DEE TCE THF TL
323 326 322 274 324 272 270 330 275 320 315 315 330 275 320
416.24 407.07 409.30 385.84 407.74 389.22 385.33 405.55 380.1 384.24 380.62 389.14 393.09 343 386.71
30,959.7 30,674.8 31,055.9 36,496.3 30,864.2 36,764.7 37,037.0 30,303.0 36,363.63 31,250.00 31,746.03 31,746.03 30,303.03 36,363.63 31,250.00
24,024.6 24,565.8 24,432.0 25,917.5 24,525.4 25,692.4 25,951.8 24,657.9 26,308.86 26,025.01 26,272.92 25,697.69 25,439.46 29,154.52 25,859.17
indices (n12) of solutions were determined for 6MAMC and 7MAMC using toluene respectively.
6935.1 6109.0 6623.9 10,578.8 6338.8 11,072.3 11,085.2 5645.1 10,054.77 5224.99 5473.11 6048.34 4863.57 7209.11 5390.83
27,492.15 27,620.3 27,743.95 10,578.8 27,694.8 31,228.55 31,494.4 27,480.45 31,336.24 28,637.50 29,009.47 28,721.86 27,871.24 32,759.07 28,554.58
F1(ε,n)
F2(ε,n)
F3(ε,n)
0.035 0.032 0.013 0.192 0.022 0.132 0.174 0.024 0.280 0.032 0.013 0.132 0.035 0.159 0.022
6.357E−4 6.29E−3 0.04400 0.5613 0.02900 0.37100 0.49300 0.02300 0.79700 6.29E−3 0.04400 0.37100 6.357E−4 0.51200 0.02900
0.341 0.341 0.308 0.5517 0.350 0.425 0.499 0.323 0.633 0.341 0.308 0.425 0.341 0.585 0.350
Bakshiev's equation [27] −
−
νa − vf = −m2 F2 ðε; nÞ + Constant:
ð4Þ
3. Theory Chamma–Viallet's equation [28] 3.1. Theoretical calculations of ground-state dipole moments
− νa
The ground-state dipole moments (μ g) of these two molecules were calculated by quantum chemical calculations. All the computations were carried out using the Gaussian 03 program [24] on a Pentium-4 PC. The basis sets at the levels of theory B3LYP/6–31 g* were used for calculations and corresponding optimized molecular geometries are shown in Fig. 2. The values of ground-state dipole moments obtained from ab initio calculations using DFT. 3.2. Experimental calculations of ground-state dipole moments
# 27KT Δ X = 4πNðε1 + 2Þ n21 + 2 C
2
= m3 F3 ðε; nÞ + Constant:
ð5Þ
The expressions for F1(ε,n) [Lippert's polarity function], F2(ε,n) [Bakshiev's polarity equation] and F3(ε,n) [Chamma–Viallet's polarity equation] are given as " F1 ðε; nÞ =
2
ε−1 n −1 − 2ε + 1 2n2 + 1
#
# 2 2 2n + 1 ε−1 n −1 2 − F2 ðε; nÞ = ε + 2 n2 + 2 n +2
ð6Þ
"
The ground-state dipole moments (μ g) of these two dyes were estimated experimentally using Guggenheim's method [25]. According to this the expression for ground-state dipole moment is given by "
2 μg
−
+ νf
ð2Þ
" # 4 3 n −1 2n2 + 1 ε−1 n2 −1 − F3 ðε; nÞ = 2 2 n + 2 ε + 2 n2 + 2 2 n2 + 2 2
ð7Þ
ð8Þ
−
where − νa and νf are absorption and fluorescence maxima wavelength in cm−1 respectively. The other symbols ε and n are dielectric constant and refractive index respectively. From Eqs. (6–8) it follows that − − − − νa + νf − − versus νa − νf Þ versus F1 (ε, n), νa − νf Þ versus F2 (ε, n) and 2
where 2 Δ = ðε12 −n12 Þ− ε1 −n1
F3 (ε, n) should give linear graphs with slopes m1, m2 and m3 respectively and are given as where K is the Boltzmann constant, T is the absolute temperature in Kelvin, N is Avogadro's number, ε12 is the dielectric constant and n12 is refractive index of the solution,ε1and n1 are the dielectric constant and refractive index of the pure solvent and C the concentration of the solute in given solvent. The estimated values of the ground-state dipole moments (μ g) using Eq. (2) for 6MAMC and 7MAMC respectively.
m1 =
m2 =
3.3. Experimental calculations of excited-state dipole moments The three independent equations used for the estimation of excited-state dipole moments of two dyes are as follows Lippert's equation [26] − νa
−
− νf = m1 F1 ðε; nÞ + Constant:
ð3Þ
m3 =
2 2 μ e −μ g hca3 2 2 μ e −μ g hca3 2 2 μe2 −μ g2 hca3
ð9Þ
ð10Þ
ð11Þ
where μ g and μ e are the ground and excited-state dipole moments of the solute molecules. The symbols h and c are Planck's constant and velocity of light in vacuum respectively, ‘a’ is the Onsager radius of the solute molecule. If the ground state and excited states are parallel,
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Fig. 3. The variation of Stokes shift with F1 (ε, n) using Lippert's equation for 6MAMC and 7MAMC.
the following expressions are obtained on the basis of Eqs. (11–12) [17,18] μg =
μe = and μe =
" # 3 1 m3 −m2 hca =2 2 2m2
ð12Þ
" #1 m3 + m2 hca3 =2 2 2m2
ð13Þ
m2 + m3 μ m3 −m2 g
ð14Þ
forðm3 N m2 Þ
4. Results and discussion − − − − The spectral shifts νa − νf Þ and 1 =2 νa + νf Þ of two dyes and solvent polarity function values F1(ε,n), F2(ε,n) and F3(ε,n)for various solvents are presented in Table 1. We have used eight and seven solvents with dielectric constants varying from 2 to 21 for 6MAMC − − and 7MAMC Figs. 3–5 show the graph of νa − νf Þ versus − respectively. − − − F1(ε,n), νa − νf Þ versus F2(ε,n)and 1 =2 νa + νf Þ versusF3(ε,n) respectively. A linear progression was done and the data was fit to a straight line, corresponding the values of the slopes are given in Table 2. − − − − In most cases νa − νf Þ versus F1(ε,n) and νa − νf Þ versus F2(ε,n) correlation is established for a larger number of solvents than − − 1 =2 νa + νf Þ versus F3(ε,n) correlation. In most cases the correlation coefficients are larger than 0.92 and indicate a good linearity for m1, m2 and m3 with selected number of Stokes shift data points. Generally, the deviation from linearity may be due to specific solute solvent interactions. The literature survey shows that m2 is usually negative but for our chemical systems, it is positive which is in agreement with findings of several other workers [7,10,11,16]. The ground-state dipole moments of two dyes were estimated by using the Guggenheim method [25]. The values obtained from this
method are 5.962D and 5.891D for 6MAMC and 7MAMC respectively and also ground-state (μ g) dipole moment values obtained from Eq. (12) are presented in Table 3. The values of the Onsager cavity radii of the 6MAMC and 7MAMC molecules were calculated by molecular volumes and the Parachor [29] and are listed in Table 3. The excited-state (μ e) dipole moments of the two dyes, estimated by computing the values of ground-state (μ g) dipole moments obtained from the Guggenheim method, in Eqs. (9–11) are presented in Table 3. Also the (μ g) and (μ e) values were obtained from Eqs. (12), and (13). The ratio of (μ g) and (μ e) obtained from Eq. (14) are presented in Table 3. The experimental (from Eq. (2)) and theoretically calculated (ab initio calculations using DFT) values are presented in Table 3. The experimental and theoretical ground-state (μ g) dipole moment results are good in agreement for our used chemical systems as shown in Table 3 [15,24]. The difference in the ground-state dipole moment is due to the necessity of knowing the radius of the solute molecule in Eq. (12) as compared to experimental and theoretical values obtained from Eq. (2) and ab initio calculations using DFT. It may be noted that the measured values of (μ g) and (μ e) for 6MAMC and 7MAMC differ from each other. The higher values of (μ e) in the case of 6MAMC may be attributed to the structural difference between the molecules. It may be noted that the discrepancies occur between the estimated values of (μ e) for the two coumarin derivatives. These differences between the values of (μ e) may be in part, due to the various assumptions and simplifications made in the use of Lippert's, Bakshiev's and Chamma– Viallet's correlations [26–28]. The large magnitude of the 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 an increase in 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 π–π*, n–π*, etc. It can be noticed from Table 1 that, with increase in the solvent polarity, the fluorescence emission peak undergoes a bathochromic shift, confirming a π–π* transition. The shift of the fluorescence wavelengths towards longer wavelengths could be caused, if the excited-state charge distribution in the solute is markedly different
R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110
109
Fig. 4. The variation of the Stokes shift with F2 (ε, n) using Bakshiev's equation for 6MAMC and 7MAMC.
from the ground-state charge distribution, and is such as to give a stronger interaction with polar solvents in the excited state. 5. Conclusion We have studied the spectroscopic behavior of 6MAMC and 7MAMC. It has been found that excited-state dipole moment (μ e) is greater than ground-state dipole moment (μ g) for both the dyes. The increase in dipole moment in the excited singlet states range between
about 2.5 to 3 D. This demonstrates that these two dyes are more polar in excited states than in ground states for all the solvents studied. The ground-state dipole moments results are correlated (experimental and theoretical) in our used chemical systems. It may be noted that there is a difference in the ground-state and excited-state dipole moment as estimated from Eqs. (12) and (13). It is worthwhile to stress that the discrepancies observed may be due to approximations made in both methods to estimate ground-state and excited singlet state dipole moments for two dyes. Also Eq. (14) can be used to
Fig. 5. The variation of arithmetic means of the Stokes shift with F3 (ε, n) using Chamma–Viallet's equation for 6MAMC and 7MAMC.
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Table 2 Statistical treatment of the correlations of solvents spectral shifts of 6MAMC and 7MAMC. Dyes
6MAMC 7MAMC
Slope
Correlation coefficient
Number of data
A
B
C
A
B
C
A
B
C
23,015.146 13,685.468
9477.105 5463.844
19,451.904 12,128.445
0.96 0.94
0.95 0.95
0.91 0.89
8 7
8 7
8 7
A — Lippert's equation; B — Bakshiev's equation; and C — Chamma–Viallet's equation.
Table 3 Ground- and excited-state dipole moments of 6MAMC and 7MAMC. Dyes
Radius μ ga ‘a’ (Ao) (D)
6MAMC 3.576 7MAMC 3.576
μ gb (D)
μ gc (D)
μ ed (D)
μ ee (D)
μ ef (D)
μ eg (D)
(μ e/ μ g)h
5.962 6.121 3.454 16.188 12.524 11.133 10.02 2.900 5.891 6.086 3.039 13.777 10.873 9.477 8.02 2.639
a The experimental ground-state dipole moments calculated from the Guggenheim method. b Calculated by B3LYP functional with 6–31 g* basis using DFT. c The ground-state dipole moments calculated using Eq. (12). d The experimental excited-state dipole moments calculated from Lippert's equation. e The experimental excited-state dipole moments calculated from Bakshiev's equation. f The experimental excited-state dipole moments calculated from Chamma–Viallet's equation. g The excited states dipole moments calculated using Eq. (13). h The ratio of excited-state and ground-state dipole moments values calculated using Eq. (14).
estimate the value of excited-state dipole moment by pre-knowledge of the value of ground-state dipole moment, without the necessity of knowing the Onsager radius of the solute. Acknowledgements The author (RMM) is grateful to Dr. K. Rajanikanth, Principal, M.S.R.I.T, Dr. K. Venkatesh, H O D, Department of Physics, M.S.R.I.T and The Management of M.S.R.I.T, Bangalore. The authors (NRP, SBK and NHA) are grateful to Principal Dr. Ashok. S. Shettar B V B College of Engineering and
Technology, Hubli and The Management of K.L.E Society, Belgaum for their encouragement and support. Authors also thank Prof. M.V. Kulkarni, Department of Chemistry, Karnatak University, Dharwad for providing the necessary coumarin compounds used in the present work.
References [1] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, New York, 1991. [2] A. Elassyry, B. Benali, Z. Lazar, K. Elblidi, B. Lakhrissi, M. Massoui, D. Mondieig, J. Mol. Liq. 128 (2006) 46. [3] B. Benali, Z. Lazar, K. Elblidi, B. Lakhrissi, M. Massoui, A. Elassyry, C. CazeauDubroca, J. Mol. Liq. 128 (2006) 42. [4] B. Koutek, Collect. Czech. Chem. Commun. 43 (1978) 2368. [5] R. Giri, M.M. Bajaj, Curr. Sci. 62 (1992) 522. [6] N. Sharma, K. Sapan, R.C. Rastogi, Spectrochim. Acta, Part A. 66 (2007) 171. [7] J.J. Aaron, M.D. Gaye, C. Parkanyi, N.S. Cho, Von SzentpalyL. , J. Mol. Struct. 156 (1987) 119. [8] S.R. Inamdar, Y.F. Nadaf, B.G. Mulimani, J. Mol. Struct. THEOCHEM 47 (2003) 624. [9] N.P. Srividya, P. Ramasamy, V.T. Radhakrishanan, Spectrochim. Acta A 53 (1997) 1743. [10] B. Acemioglu, M. Arık, H. Efeoglu, Y. Onganer, J. Mol. Struct. THEOCHEM 548 (2001) 165. [11] J. Kabatc, B. Osmialowski, Paczkowski, J. Spectrochim. Acta A 63 (2006) 524. [12] A. Kawski, B. Kuklinski, P. Bojarski, Z. Naturforsch. 57A (2002) 716. [13] J.J. Aaron, M. Maafi, C. Parkanyi, C. Boniface, Spectrochim. Acta 51A (1995) 603. [14] J. Thipperudrappa, D.S. Biradar, S.R. Manohara, S.M. Hanagodimath, S.R. Inamadar, J.R. Manekutla, Spectrochim. Acta A 69 (2008) 991. [15] B.R. Gayathri, J.R. Mannekutla, S.R. Inamadar, J. Mol. Struct. 889 (2008) 383. [16] D.S. Biradar, B. Siddlingeshwar, S.M. Hanagodimath, J. Mol. Struct. 875 (2008) 108. [17] A. Kawaski, in: J.F. Rabeck (Ed.), Photochem. And Photophys, vol.5, CRC Press, Boca Raton, Ann Arbor Boston, 1992, p. 1. [18] A. Kawaski, Acta Phys. Polon 29 (1966) 507. [19] R.M. Melavanki, R.A. Kusanur, M.V. Kulakarni, J.S. Kadadevaramath, J. Lumin. 129 (2009) 1298. [20] R.M. Melavanki, R.A. Kusanur, M.V. Kulakarni, J.S. Kadadevaramath, J. Lumin. 128 (2007) 573. [21] Raviraj A. Kusanur, Manohar V. Kulakarni, Indian Council of Chemists. 22nd Conference, Roorkee, Uttaranchal, 2003. [22] B.B. Dey, Y. Shankaranaraayan, J. Ind. Chem. Soc. 11 (1934) 687. [23] S. Rangaswamy, T.R. Sheshadri, V. Venkateshwaralu, Proc. Ind. Acad. Sci. 13A (1941) 316. [24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian, Revision B, Gaussian, Inc., Pittsburg, PA, 2003. [25] E.A. Guggenheim, Trans. Faraday Soc. 47 (1951) 573. [26] LippertE. , Z. Naturforsch. 10 A (1955) 541. [27] N.G. Bakshiev, Opt. Spectrosc. 16 (1964) 821. [28] ChammaA. , VialletP. , C.R. Acad. Sci. Paris, Ser. C 270 (1970) 1901. [29] J.T. Edward, Molecular Volumes and Parachor, Chem. Ind, London, 1956, p. 774.
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Solvent effect on the spectroscopic properties of 6MAMC and 7MAMC Raveendra M. Melavanki a,⁎, N.R. Patil b, S.B. Kapatkar b, N.H. Ayachit b, Siva Umapathy c, J. Thipperudrappa d, A.R. Nataraju e a
Department of Physics, M S Ramaiah Institute of Technology, Bangalore 560054, Karnataka, India Department of Physics, B V B College of Engineering and Technology, Hubli 580031, Karnataka, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, Karnataka, India d Department of Physics, BNM Institute of Technology, Bangalore 560070, Karnataka India e Department of Physics, Seshadripuram, P. U. College (Main), Bangalore 560020, Karnataka, India b c
a r t i c l e
i n f o
Article history: Received 1 August 2010 Received in revised form 2 October 2010 Accepted 7 November 2010 Available online 11 November 2010 Keywords: Solvatochromic shift method Ground-state dipole moments Excited-state dipole moments 6MAMC and 7MAMC
a b s t r a c t The absorption and emission spectra of two dyes namely 6MAMC and 7MAMC have been recorded at room temperature in solvents of different polarities. The ground-state dipole moments (μg) of these two were determined experimentally by Guggenheim method and were compared with theoretical values obtained using quantum chemical method. The exited state (μe) dipole moments were estimated from Lippert's, Bakhshiev's and Chamma-Viallet's equations by using the variation of the Stokes shift with the solvent dielectric constant and refractive index. The ground and excited-state dipole moments were calculated by means of the solvatochromic shift method and also the excited-state dipole moments are determined in combination with ground-state dipole moments. It was observed that dipole moments of excited state were higher than those of the ground state, indicating a substantial redistribution of the π-electron densities in a more polar excited state for these two dyes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The effect of solvent on the absorption and fluorescence characteristics of organic compounds has been a subject of interesting investigation [1–3]. Excitation of a molecule by photon causes a redistribution of charges leading to conformational changes in the excited state. This can result in an increase or decrease of dipole moment of the excited sate as compared to ground state. 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 the short-lived state. Knowledge of the excited-state dipole moment of electronically excited molecules is quite useful in designing nonlinear materials, elucidating the nature of the excited states and in determining the course of a photochemical transformation. The excited-state dipole moments of fluorescent dye molecules such as those studied here also determine the tunability range of the emission energy as a function of the polarity of the medium. All the methods available so far for the determination of
⁎ Corresponding author. Tel.: +91 80 23600822, +91 9008228982 (mobile); fax: +91 80 23603124. E-mail address: [email protected] (R.M. Melavanki). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.11.002
singlet excited-state dipole moment are based on the spectral shift caused either externally by electrochromism or internally by solvatochromism. The electrooptic methods such as electronic polarization of fluorescence, electric-dichroism, microwave conductivity and stark splitting are generally considered to be very accurate, but their use is limited because they are considered equipment sensitive and the studies have been restricted to relatively very simple molecules. The solvatochromic method is based on the shift of absorption and fluorescence maxima in different solvents of varying polarity. Koutek has shown that under suitable conditions, the solvatochromic method yields fairly satisfactory results [4]. The solvent dependence of absorption and fluorescence maxima is used to estimate the excitedstate dipole moments of different molecules. Several workers have made extensive experimental and theoretical studies on ground-state (μg) and excited-state (μe) dipole moments using different techniques in variety of organic fluorescent compounds like coumarins [5], indoles [6], purines [7], exalite dyes [8], acridine dyes [9], fluorescein [10], hemicyanine dyes [11], PRODAN, BADAN and ACRYLODAN [12], acridines and phenazines [13] and in some laser dyes [14–20] etc. Coumarins and their derivatives establish a family of dyes which are applicable in different fields of science and technology. They exhibit strong fluorescence in the UV and VISIBLE region which makes them suitable for used as colorants, dye laser media and as nonlinear optical chromospheres. In medicine, coumarin derivatives are used as anticoagulants, as
106
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a fluorescent indicators for the physiological pH region and as fluorescent probes to determine the rigidity and fluidity of living cells and its surrounding medium. They also possess distinct biological activity and have been described as potential agents for anticancer. In the present work we report the effects of solvent on absorption and emission spectra, and estimation of ground- and excited-state dipole moments of these two dyes namely 6-methyl 4-azidomethyl coumarin (6MAMC) and 7-methyl 4-azidomethyl coumarin (7MAMC) by solvatochromic shift method and also theoretical computed results from ab initio calculations using DFT. However, there are no reports available in literature on the determination of μg and μe values of these dyes investigated. This prompted us to carry out the present work. 2. Materials and methods
be measured with the help of Forbes Tinsley (FT) 6421 LCR Data Bridge at 10 KHz frequency. The refractive indices of various dilute solutions of the solute for sodium D line were determined by using Abbe's refractometer. All these measurements were carried out at room temperature (300 K). 2.3. Determination of the dielectric constants and refractive indices The capacitance of air, the solvent and the solution have been used to measure dielectric constant. By measuring the capacitance of different concentrations of the solute in toluene the dielectric constant of the solution (ε12) was calculated using the expression ε12 =
2.1. Chemicals used The 6MAMC and 7MAMC were synthesized in our laboratory using standard methods [21–23]. The molecular structures of these dyes are given in Fig. 1. The solvents used in the present study namely toluene (TL), acetonitrile (AN), tetrachloroethane (TCE), dioxane (DX), diethyl ether (DEE), n-butyl alcohol (n-BA), carbon tetrachloride (CCl4), ethyl acetate (EA), tetrahydrofuran (THF) and benzene (BZ) were obtained from S-D-Fine Chemicals Ltd., India, and they were of spectroscopic grade. The required solutions were prepared at fixed concentration of solutes 1 × 10− 4M/L in each solvent.
C2 −CX C1 −CX
ð1Þ
where C2, CX and C1 represent the capacitances due to leads, solution, and air respectively. The values of dielectric constants thus determined for 6MAMC and 7MAMC using toluene. The values of refractive
2.2. Spectroscopic measurements The absorption spectra were recorded using Hitachi 50–20 UV–Vis spectrophotometer. The fluorescence spectra were recorded using Hitachi F-2000 fluorescence spectrophotometer. The dielectric constants of the dilute solutions were measured in a suitably fabricated cell of usually small capacitance where the accurate determination of small changes in the capacitance would be possible. The small capacitance can
N3 H3C
O
O
6-methyl-4-azidomethyl coumarin (6MAMC)
N3
H3C
O
O
7-methyl-4-azidomethyl coumarin (7MAMC) Fig. 1. The molecular structures of 6MAMC and 7MAMC.
Fig. 2. Ground-state optimized molecular geometries of 6MAMC and 7MAMC. The arrow indicates the direction of dipole moment.
R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110
107
Table 1 Solvatochromic data of 6MAMC and 7MAMC along with the calculated values of polarity functions in different solvents. − − − − − − 1 Dyes Solvents νa nm νf nm ν a (cm−1) νf (cm−1) νa − νf Þ (cm−1) =2 νa + νf Þ (cm−1) 6MAMC
7MAMC
TCE BZ DX n-BA TL DEE EA CCL4 AN BZ DX DEE TCE THF TL
323 326 322 274 324 272 270 330 275 320 315 315 330 275 320
416.24 407.07 409.30 385.84 407.74 389.22 385.33 405.55 380.1 384.24 380.62 389.14 393.09 343 386.71
30,959.7 30,674.8 31,055.9 36,496.3 30,864.2 36,764.7 37,037.0 30,303.0 36,363.63 31,250.00 31,746.03 31,746.03 30,303.03 36,363.63 31,250.00
24,024.6 24,565.8 24,432.0 25,917.5 24,525.4 25,692.4 25,951.8 24,657.9 26,308.86 26,025.01 26,272.92 25,697.69 25,439.46 29,154.52 25,859.17
indices (n12) of solutions were determined for 6MAMC and 7MAMC using toluene respectively.
6935.1 6109.0 6623.9 10,578.8 6338.8 11,072.3 11,085.2 5645.1 10,054.77 5224.99 5473.11 6048.34 4863.57 7209.11 5390.83
27,492.15 27,620.3 27,743.95 10,578.8 27,694.8 31,228.55 31,494.4 27,480.45 31,336.24 28,637.50 29,009.47 28,721.86 27,871.24 32,759.07 28,554.58
F1(ε,n)
F2(ε,n)
F3(ε,n)
0.035 0.032 0.013 0.192 0.022 0.132 0.174 0.024 0.280 0.032 0.013 0.132 0.035 0.159 0.022
6.357E−4 6.29E−3 0.04400 0.5613 0.02900 0.37100 0.49300 0.02300 0.79700 6.29E−3 0.04400 0.37100 6.357E−4 0.51200 0.02900
0.341 0.341 0.308 0.5517 0.350 0.425 0.499 0.323 0.633 0.341 0.308 0.425 0.341 0.585 0.350
Bakshiev's equation [27] −
−
νa − vf = −m2 F2 ðε; nÞ + Constant:
ð4Þ
3. Theory Chamma–Viallet's equation [28] 3.1. Theoretical calculations of ground-state dipole moments
− νa
The ground-state dipole moments (μ g) of these two molecules were calculated by quantum chemical calculations. All the computations were carried out using the Gaussian 03 program [24] on a Pentium-4 PC. The basis sets at the levels of theory B3LYP/6–31 g* were used for calculations and corresponding optimized molecular geometries are shown in Fig. 2. The values of ground-state dipole moments obtained from ab initio calculations using DFT. 3.2. Experimental calculations of ground-state dipole moments
# 27KT Δ X = 4πNðε1 + 2Þ n21 + 2 C
2
= m3 F3 ðε; nÞ + Constant:
ð5Þ
The expressions for F1(ε,n) [Lippert's polarity function], F2(ε,n) [Bakshiev's polarity equation] and F3(ε,n) [Chamma–Viallet's polarity equation] are given as " F1 ðε; nÞ =
2
ε−1 n −1 − 2ε + 1 2n2 + 1
#
# 2 2 2n + 1 ε−1 n −1 2 − F2 ðε; nÞ = ε + 2 n2 + 2 n +2
ð6Þ
"
The ground-state dipole moments (μ g) of these two dyes were estimated experimentally using Guggenheim's method [25]. According to this the expression for ground-state dipole moment is given by "
2 μg
−
+ νf
ð2Þ
" # 4 3 n −1 2n2 + 1 ε−1 n2 −1 − F3 ðε; nÞ = 2 2 n + 2 ε + 2 n2 + 2 2 n2 + 2 2
ð7Þ
ð8Þ
−
where − νa and νf are absorption and fluorescence maxima wavelength in cm−1 respectively. The other symbols ε and n are dielectric constant and refractive index respectively. From Eqs. (6–8) it follows that − − − − νa + νf − − versus νa − νf Þ versus F1 (ε, n), νa − νf Þ versus F2 (ε, n) and 2
where 2 Δ = ðε12 −n12 Þ− ε1 −n1
F3 (ε, n) should give linear graphs with slopes m1, m2 and m3 respectively and are given as where K is the Boltzmann constant, T is the absolute temperature in Kelvin, N is Avogadro's number, ε12 is the dielectric constant and n12 is refractive index of the solution,ε1and n1 are the dielectric constant and refractive index of the pure solvent and C the concentration of the solute in given solvent. The estimated values of the ground-state dipole moments (μ g) using Eq. (2) for 6MAMC and 7MAMC respectively.
m1 =
m2 =
3.3. Experimental calculations of excited-state dipole moments The three independent equations used for the estimation of excited-state dipole moments of two dyes are as follows Lippert's equation [26] − νa
−
− νf = m1 F1 ðε; nÞ + Constant:
ð3Þ
m3 =
2 2 μ e −μ g hca3 2 2 μ e −μ g hca3 2 2 μe2 −μ g2 hca3
ð9Þ
ð10Þ
ð11Þ
where μ g and μ e are the ground and excited-state dipole moments of the solute molecules. The symbols h and c are Planck's constant and velocity of light in vacuum respectively, ‘a’ is the Onsager radius of the solute molecule. If the ground state and excited states are parallel,
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Fig. 3. The variation of Stokes shift with F1 (ε, n) using Lippert's equation for 6MAMC and 7MAMC.
the following expressions are obtained on the basis of Eqs. (11–12) [17,18] μg =
μe = and μe =
" # 3 1 m3 −m2 hca =2 2 2m2
ð12Þ
" #1 m3 + m2 hca3 =2 2 2m2
ð13Þ
m2 + m3 μ m3 −m2 g
ð14Þ
forðm3 N m2 Þ
4. Results and discussion − − − − The spectral shifts νa − νf Þ and 1 =2 νa + νf Þ of two dyes and solvent polarity function values F1(ε,n), F2(ε,n) and F3(ε,n)for various solvents are presented in Table 1. We have used eight and seven solvents with dielectric constants varying from 2 to 21 for 6MAMC − − and 7MAMC Figs. 3–5 show the graph of νa − νf Þ versus − respectively. − − − F1(ε,n), νa − νf Þ versus F2(ε,n)and 1 =2 νa + νf Þ versusF3(ε,n) respectively. A linear progression was done and the data was fit to a straight line, corresponding the values of the slopes are given in Table 2. − − − − In most cases νa − νf Þ versus F1(ε,n) and νa − νf Þ versus F2(ε,n) correlation is established for a larger number of solvents than − − 1 =2 νa + νf Þ versus F3(ε,n) correlation. In most cases the correlation coefficients are larger than 0.92 and indicate a good linearity for m1, m2 and m3 with selected number of Stokes shift data points. Generally, the deviation from linearity may be due to specific solute solvent interactions. The literature survey shows that m2 is usually negative but for our chemical systems, it is positive which is in agreement with findings of several other workers [7,10,11,16]. The ground-state dipole moments of two dyes were estimated by using the Guggenheim method [25]. The values obtained from this
method are 5.962D and 5.891D for 6MAMC and 7MAMC respectively and also ground-state (μ g) dipole moment values obtained from Eq. (12) are presented in Table 3. The values of the Onsager cavity radii of the 6MAMC and 7MAMC molecules were calculated by molecular volumes and the Parachor [29] and are listed in Table 3. The excited-state (μ e) dipole moments of the two dyes, estimated by computing the values of ground-state (μ g) dipole moments obtained from the Guggenheim method, in Eqs. (9–11) are presented in Table 3. Also the (μ g) and (μ e) values were obtained from Eqs. (12), and (13). The ratio of (μ g) and (μ e) obtained from Eq. (14) are presented in Table 3. The experimental (from Eq. (2)) and theoretically calculated (ab initio calculations using DFT) values are presented in Table 3. The experimental and theoretical ground-state (μ g) dipole moment results are good in agreement for our used chemical systems as shown in Table 3 [15,24]. The difference in the ground-state dipole moment is due to the necessity of knowing the radius of the solute molecule in Eq. (12) as compared to experimental and theoretical values obtained from Eq. (2) and ab initio calculations using DFT. It may be noted that the measured values of (μ g) and (μ e) for 6MAMC and 7MAMC differ from each other. The higher values of (μ e) in the case of 6MAMC may be attributed to the structural difference between the molecules. It may be noted that the discrepancies occur between the estimated values of (μ e) for the two coumarin derivatives. These differences between the values of (μ e) may be in part, due to the various assumptions and simplifications made in the use of Lippert's, Bakshiev's and Chamma– Viallet's correlations [26–28]. The large magnitude of the 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 an increase in 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 π–π*, n–π*, etc. It can be noticed from Table 1 that, with increase in the solvent polarity, the fluorescence emission peak undergoes a bathochromic shift, confirming a π–π* transition. The shift of the fluorescence wavelengths towards longer wavelengths could be caused, if the excited-state charge distribution in the solute is markedly different
R.M. Melavanki et al. / Journal of Molecular Liquids 158 (2011) 105–110
109
Fig. 4. The variation of the Stokes shift with F2 (ε, n) using Bakshiev's equation for 6MAMC and 7MAMC.
from the ground-state charge distribution, and is such as to give a stronger interaction with polar solvents in the excited state. 5. Conclusion We have studied the spectroscopic behavior of 6MAMC and 7MAMC. It has been found that excited-state dipole moment (μ e) is greater than ground-state dipole moment (μ g) for both the dyes. The increase in dipole moment in the excited singlet states range between
about 2.5 to 3 D. This demonstrates that these two dyes are more polar in excited states than in ground states for all the solvents studied. The ground-state dipole moments results are correlated (experimental and theoretical) in our used chemical systems. It may be noted that there is a difference in the ground-state and excited-state dipole moment as estimated from Eqs. (12) and (13). It is worthwhile to stress that the discrepancies observed may be due to approximations made in both methods to estimate ground-state and excited singlet state dipole moments for two dyes. Also Eq. (14) can be used to
Fig. 5. The variation of arithmetic means of the Stokes shift with F3 (ε, n) using Chamma–Viallet's equation for 6MAMC and 7MAMC.
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Table 2 Statistical treatment of the correlations of solvents spectral shifts of 6MAMC and 7MAMC. Dyes
6MAMC 7MAMC
Slope
Correlation coefficient
Number of data
A
B
C
A
B
C
A
B
C
23,015.146 13,685.468
9477.105 5463.844
19,451.904 12,128.445
0.96 0.94
0.95 0.95
0.91 0.89
8 7
8 7
8 7
A — Lippert's equation; B — Bakshiev's equation; and C — Chamma–Viallet's equation.
Table 3 Ground- and excited-state dipole moments of 6MAMC and 7MAMC. Dyes
Radius μ ga ‘a’ (Ao) (D)
6MAMC 3.576 7MAMC 3.576
μ gb (D)
μ gc (D)
μ ed (D)
μ ee (D)
μ ef (D)
μ eg (D)
(μ e/ μ g)h
5.962 6.121 3.454 16.188 12.524 11.133 10.02 2.900 5.891 6.086 3.039 13.777 10.873 9.477 8.02 2.639
a The experimental ground-state dipole moments calculated from the Guggenheim method. b Calculated by B3LYP functional with 6–31 g* basis using DFT. c The ground-state dipole moments calculated using Eq. (12). d The experimental excited-state dipole moments calculated from Lippert's equation. e The experimental excited-state dipole moments calculated from Bakshiev's equation. f The experimental excited-state dipole moments calculated from Chamma–Viallet's equation. g The excited states dipole moments calculated using Eq. (13). h The ratio of excited-state and ground-state dipole moments values calculated using Eq. (14).
estimate the value of excited-state dipole moment by pre-knowledge of the value of ground-state dipole moment, without the necessity of knowing the Onsager radius of the solute. Acknowledgements The author (RMM) is grateful to Dr. K. Rajanikanth, Principal, M.S.R.I.T, Dr. K. Venkatesh, H O D, Department of Physics, M.S.R.I.T and The Management of M.S.R.I.T, Bangalore. The authors (NRP, SBK and NHA) are grateful to Principal Dr. Ashok. S. Shettar B V B College of Engineering and
Technology, Hubli and The Management of K.L.E Society, Belgaum for their encouragement and support. Authors also thank Prof. M.V. Kulkarni, Department of Chemistry, Karnatak University, Dharwad for providing the necessary coumarin compounds used in the present work.
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