- Email: [email protected]
Excited state electric dipole moment of 5-hydroxy indole and 5-hydroxy indole 3-acetic acid through solvatochromic shifts
Journal of Electron Spectroscopy and Related Phenomena 182 (2010) 1–3
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Excited state electric dipole moment of 5-hydroxy indole and 5-hydroxy indole 3-acetic acid through solvatochromic shifts G. Neeraja Rani a , Narasimha H. Ayachit b,∗ a b
Department of H&S, TRREC, Patancheru, Hyderabad, India Department of Physics, BVB College of Engineering & Technology, Vidyanagar, Hubli, India
a r t i c l e
i n f o
Article history: Received 15 March 2010 Received in revised form 29 April 2010 Accepted 3 May 2010 Available online 11 May 2010 Keywords: Excited state electric dipole moment Solvatochromic shifts Indoles Fluorescence shifts Absorption shifts
a b s t r a c t The determination of excited state electric dipole moment through solvatochromic shifts is one of the easiest approaches to understand the molecular structure in the excited state. These studies have gained importance due to their application in photo science, especially if they are of biological importance. In view of this the excited state electric dipole moments of two substituted indoles which are of biological importance are determined and reported here. The fluorescence shifts have been used and the results found seem to be more consistent in comparison with the one calculated through absorption shifts. The results presented are also discussed. A qualitative estimate of the orientation of the dipole moments in ground and excited state are also presented and discussed. The method proposed by Ayachit and Neeraja Rani is used in view of the several advantages it has. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The determination of excited state electric dipole moment through solvatochromic shifts is one of the easiest approaches to understand the molecular structure in the excited state [1–20]. These studies have gained importance due to their application in photo science, especially if they are of biological importance. The dipole moment values obtained using the solvatochromic shifts can improve by the proper choice of solvents and a method. For example, the use of slightly polar solvents, which are having almost same refractive indices, and use of fluorescence shifts rather than absorption data should be preferred wherever available. The fluorescence data in different solvents is expected to give better estimates of dipole moments in the excited state than the absorption data in view of the fact that fluorescence shifts are larger compared to those in absorption. The improved estimate of the dipole moment means a better understanding of the electron charge distribution, which plays an important role in giving insight into photochemical reaction. The estimation of the excited state electric dipole moments of substituted indoles has found to be of great importance due to the biological relevance they have. The biological activities of these compounds mainly depend on their molecular structure. The
∗ Corresponding author. E-mail addresses: [email protected] (G.N. Rani), [email protected] (N.H. Ayachit). 0368-2048/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.05.002
insight about this can be had from their ground and excited state dipole moment values. Thus, a better estimation of dipole moment means a better understanding of molecular structure. Sharma et al. [9] have estimated the excited electric dipole moment of some substituted indoles using combination of fluorescence and absorption data and without taking into account the change in the orientation of dipole moment on excitation. In the present paper the method proposed by Ayachit et al. [12–17] is employed for both absorption shifts and emission shifts separately and the results are presented. Further, it is shown that the results obtained through fluorescence shifts seem to be more consistent in comparison with the one calculated through absorption shifts. The results presented are also discussed. A qualitative estimate of the orientation of the dipole moments in ground and excited state is also presented and discussed. Of the several methods proposed, the one proposed from Ayachit [14], Ayachit et al. [12] and Ayachit and Neeraja Rani [13] is used in view of the several advantages it has. 2. Theory and procedure The displacement of electronic absorption and luminescence spectra are related to solvent interaction. These interactions can be non-specific, when they depend only on multiple and polarizability properties of solute and solvent molecules. When a liquid solvent surrounds a molecule, each state of it is stabilized by an energy known as salvation energy. The medium, i.e., solvent can affect the solute molecule by its viscosity and also by its polarity. The number of electronic states which are defined for a molecule are determined
2
G.N. Rani, N.H. Ayachit / Journal of Electron Spectroscopy and Related Phenomena 182 (2010) 1–3
through Schrodinger wave equation E = H , where being the wave function. The wave function gives the information regarding the nature of a state. By employing the quantum mechanical second order perturbation theory and using Onsager model, McRae [1] has given an equation for the frequency of maximum solute absorption/fluorescence in a solvent, s and is as below:
vs = v0 + (A + B + C)f (ns ) + E[f (Ds ) − f (ns )] + F[f (Ds ) − f (ns )]2 (1) where 0 is the corresponding vapor phase frequency of s , f (ns ) = (n2s − 1)/(2n2s + 1) is a function of solvent refractive index (ns ), f(Ds ) = (Ds − 1)/(2Ds + 1) is a function of solvent dielectric medium (Ds ), (A + B) is a measure of dispersive effect, and h and c are Planck’s constant and velocity of light respectively. C is either (2e − 2g )/hca30 or (2g − 2e )/hca30 depending on whether s is measured through absorption spectra or emission spectra, respectively. Similarly, E is either (g g−e )/hca30 or (e e−g )/hca30 depending upon the spectra under study, with e , g being dipole moment in the excited state and ground state, respectively with e−g = e − g and a0 is the Onsager cavity radius. The last term in Eq. (1) and (A + B) can be neglected due to their small contribution towards shift. With this, Suppan [3] formulated the equation cited below for the difference in frequency in absorption for two different solvents 1 and 2, which is:
−v1−2 =
g g−e
[f (Ds ) − f (ns )]1−2 +
hca30
2e − 2g
f (ns )1−2
hca30
The above equation for fluorescence will be (with ae being the cavity radius in the excited state):
−v1−2 =
e g−e hca3e
[f (Ds ) − f (ns )]1−2 +
2g − 2e hca3e
f (ns )1−2
The above equation was written by Ayachit et al. in the form:
X C1
+
Y C2
=1
(2)
Here, X = [f(Ds ) − f(ns )1−2 ]/−1−2 and Y = [f(ns )1−2 ]/−1−2 , with C1 = hca30 /e g−e and C2 = hca30 /(2e − 2g ) in case of absorption studies and C1 = hca3e /e g−e and C2 = hca3e /(2g − 2e ) in florescence studies. The values of C1 and C2 can be obtained using either a graphical method or at least square fit. The values of X and Y are of the order of exp(−4) and hence it forces to plot the graph of quantities which are greater by a factor exp(−4), which may lead to points being scattered leading erroneous values of C1 and C2 . Hence in the present study C1 and C2 have been calculated using a least squares fit. From the values of C1 and C2 , e and the angle between e and g namely can be determined by writing e . g−e = e g cos − (e )2 . 3. Result and discussion The values of excited state electric dipole moments determined for the two indoles in the present work using absorption shifts and fluorescence shifts are presented in Table 1 along with the available dipole moment values in ground state and excited state as reported by Sharma et al. [9]. In this table are also given the Onsagar cavity radius. From the table it can be seen that the values of excited state dipole moments are much less than the reported earlier however greater than the ground state dipole moments of the molecules. The larger values observed in the earlier work may be because of the fact that:
(1) The solvents of very high dielectric constants have been used in the earlier work and hence neglecting the higher terms or considering them as constant may not be suitable. (2) When high dielectric constant solvents are used non-specific interactions come in to effect whose contribution cannot be neglected. (3) Orientation of the dipole moment has to be taken in to account. (4) Not much reliable absorption data are used. In view of the above, the dipole moments determined in the present work seems to be reliable representing a * transition. The solvatochromic shifts are the experimental evidence for the change in energies of levels of a solute due to change in the matrix. Emission spectra are usually more informative than the absorption spectra as emission state arise out of energy of more relaxed excited states. In view of this, the quantities determined using solvatochromic shifts become more important for the description of the intramolecular charge transfer in molecular excited states [4–14,21,22] and inter-molecular charge transfer in exciplexes. [23,24]. The shifts observed are dependent on the nature of solvent (polar or non-polar), dielectric constant, etc. This is evident from the fact that, in certain cases the shift in fluorescence spectrum is not necessarily accompanied by a shift in the absorption spectra, if exists it need not be in proportion to the one observed in fluorescence. There are examples for shifts in fluorescence to occur with no shifts in the absorption spectrum of a solute [25]. The estimations carried out using absorption shifts will thus not be that reliable compared with that calculated with emission spectra. If the difference of excited state of a solute in vacuum and the excited state of a polar solvent is greater than the difference of excited state of a solute molecule in vacuum and the excited state of non-polar solvent, the fluorescence as one goes from polar to non-polar shows a blue shift (*– transition) and if it is reverse the shift will be red (*–n transition). In the present study, the fluorescence shifts for two indoles indicate that the transitions involved are of the first type. The values determined for dipole moments in the excited state in the earlier work through absorption and in this work (presented later in the text) are in conformity with this observation. When a molecule gets electronically excited (by having different dipole moment and orientation), the electron will be raised to a new electronic level in much less time than it takes for the whole molecule to rearrange itself with solvent environment. Thus, immediately upon excitation, the molecule can be assumed to be in the same environment in the excited state as in the ground state and hence one can safely assume ae = a0 . The values of orientation between ground and excited state, estimated for the two indoles are given in Table 1; clearly indicate the large change in the molecular structure in the excited state. These orientations are very sensitive to the value of a0 and hence cannot be given much importance quantitatively unless the method is free from a0 , which is not the case here and hence the values of orientation between ground and excited state reported here are to be considered as indicative of the trend.
Table 1 Results for 5-hydroxy indole (5HI) and 5-hydroxy indole 3-acetic acid (5HI3AA). Parameters
in D e (abs) in D e (fluo) in D (fluo) in◦ e in D (earlier work)
Compounds 5HI
5HI3AA
3.02 3.01 3.7 21 5.75
4.05 4.13 4.76 34 5.17
G.N. Rani, N.H. Ayachit / Journal of Electron Spectroscopy and Related Phenomena 182 (2010) 1–3
4. Conclusion Since the excited state of a molecule is much different from its ground state, it may be more or less depending upon the transition involved, the ground state dipole moment may slightly change from solvent to solvent, but the excited state may be much different. Thus it is expected that little changes in absorption while large in the fluorescence spectrum with change in solvent. In the different equations used for the determination of excited state dipole moments one or more assumptions are made in obtaining the simplified equation from the original McRae’s equation. The estimations carried out using absorption shifts will thus not be that reliable compared with that calculated with emission spectra. Acknowledgement The authors thank the Principal and management of their respective colleges for encouragement shown. References [1] [2] [3] [4] [5]
E.G. McRae, J. Phys. Chem. 61 (1957) 562. M.B. Ledger, P. Suppan, Spectrochim. Acta 23A (1967) 641. P. Suppan, J. Chem. Soc. 4 (1968) 3125. M. Ito, K. Inuzuka, M. Imanishi, J. Am. Chem. Soc. 82 (1960) 1317. P. Suppan, C. Tsiams, Spectrochim. Acta 36A (1980) 364.
3
[6] D.K. Deshpande, M.A. Shashidhar, K. Suryanarayana, Z. Rao, Phys. Chem. Leipzig 262 (1981) 588. [7] L.S. Prabhumirashi, D.K. Narayan Kutty, A.S. Bhide, Spectrochim. Acta 39A (1983) 663; L.S. Prabhumirashi, Spectrochim. Acta 39A (1983) 91. [8] K.M.C. Davis, in: R. Foster (Ed.), Molecular Association, Academic Press, New York, 1975, p. 151. [9] N. Sharma, S.K. Jain, R.C. Rastogi, Spectrochim. Acta Part A 66 (2007) 171–176. [10] L.S. Prabhumirashi, S.S. Kunte, Spectrochim. Acta 42A (1986) 441. [11] P. Suppan, Chem. Phys. Lett. 94 (1983) 272. [12] N.H. Ayachit, D.K. Deshpande, M.A. Shashidhar, K. Suryanarayana Rao, Spectrochim. Acta 42A (1986) 585; N.H. Ayachit, D.K. Deshpande, M.A. Shashidhar, K. Suryanarayana Rao, Spectrochim. Acta 42A (1986) 1405. [13] N.H. Ayachit, G. Neeraja Rani, Phys. Chem. Liq. 45 (2007) 41. [14] N.H. Ayachit, Chem. Phys. Lett. 164 (1989) 272. [15] N.H. Ayachit, Proc. Indian Acad. Sci. (Chem. Sci.) 101 (1989) 269. [16] N.H. Ayachit, Asian Chem. Lett. 3 (1999) 167. [17] N.H. Ayachit, Asian J. Phys. 9 (2000) 349. [18] P. Suppan, J. Photochem. Photobiol. 50 (1990) 293. [19] Y.F. Nadaf, D.K. Deshpande, A.M. Karaguppikar, S.R. Inamdar, J. Photosci. 9 (2002) 29. [20] M.S. Masoud, A.E. Ali, M.A. Shaker, M.A. Ghani, Spectrochim. Acta 61A (2005) 3102. [21] T. Abe, I. Iweibo, Bull. Chem. Soc. Jpn. 58 (1985) 3415; T.A. Bull, Chem. Soc. Jpn. 41 (1985) 1260. [22] N.H. Ayachit, G. Neeraja Rani, Phys. Chem. Liq. 45 (2007) 615. [23] Z.R. Crabowski, K. Rothiewicz, A. Siemiarezuk, D.J. Cowley, W. Baumann, Nouv. J. Chem. 3 (1979) 443. [24] H. Beans, H. Knibbe, A. Weller, J. Chem. Phys. 47 (1967) 1183. [25] A.J. Pesce, C.G. Rosen, T. Pasby, Fluorescence Spectroscopy, Marcel Dekker, New York, 1971.
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Excited state electric dipole moment of 5-hydroxy indole and 5-hydroxy indole 3-acetic acid through solvatochromic shifts G. Neeraja Rani a , Narasimha H. Ayachit b,∗ a b
Department of H&S, TRREC, Patancheru, Hyderabad, India Department of Physics, BVB College of Engineering & Technology, Vidyanagar, Hubli, India
a r t i c l e
i n f o
Article history: Received 15 March 2010 Received in revised form 29 April 2010 Accepted 3 May 2010 Available online 11 May 2010 Keywords: Excited state electric dipole moment Solvatochromic shifts Indoles Fluorescence shifts Absorption shifts
a b s t r a c t The determination of excited state electric dipole moment through solvatochromic shifts is one of the easiest approaches to understand the molecular structure in the excited state. These studies have gained importance due to their application in photo science, especially if they are of biological importance. In view of this the excited state electric dipole moments of two substituted indoles which are of biological importance are determined and reported here. The fluorescence shifts have been used and the results found seem to be more consistent in comparison with the one calculated through absorption shifts. The results presented are also discussed. A qualitative estimate of the orientation of the dipole moments in ground and excited state are also presented and discussed. The method proposed by Ayachit and Neeraja Rani is used in view of the several advantages it has. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The determination of excited state electric dipole moment through solvatochromic shifts is one of the easiest approaches to understand the molecular structure in the excited state [1–20]. These studies have gained importance due to their application in photo science, especially if they are of biological importance. The dipole moment values obtained using the solvatochromic shifts can improve by the proper choice of solvents and a method. For example, the use of slightly polar solvents, which are having almost same refractive indices, and use of fluorescence shifts rather than absorption data should be preferred wherever available. The fluorescence data in different solvents is expected to give better estimates of dipole moments in the excited state than the absorption data in view of the fact that fluorescence shifts are larger compared to those in absorption. The improved estimate of the dipole moment means a better understanding of the electron charge distribution, which plays an important role in giving insight into photochemical reaction. The estimation of the excited state electric dipole moments of substituted indoles has found to be of great importance due to the biological relevance they have. The biological activities of these compounds mainly depend on their molecular structure. The
∗ Corresponding author. E-mail addresses: [email protected] (G.N. Rani), [email protected] (N.H. Ayachit). 0368-2048/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.05.002
insight about this can be had from their ground and excited state dipole moment values. Thus, a better estimation of dipole moment means a better understanding of molecular structure. Sharma et al. [9] have estimated the excited electric dipole moment of some substituted indoles using combination of fluorescence and absorption data and without taking into account the change in the orientation of dipole moment on excitation. In the present paper the method proposed by Ayachit et al. [12–17] is employed for both absorption shifts and emission shifts separately and the results are presented. Further, it is shown that the results obtained through fluorescence shifts seem to be more consistent in comparison with the one calculated through absorption shifts. The results presented are also discussed. A qualitative estimate of the orientation of the dipole moments in ground and excited state is also presented and discussed. Of the several methods proposed, the one proposed from Ayachit [14], Ayachit et al. [12] and Ayachit and Neeraja Rani [13] is used in view of the several advantages it has. 2. Theory and procedure The displacement of electronic absorption and luminescence spectra are related to solvent interaction. These interactions can be non-specific, when they depend only on multiple and polarizability properties of solute and solvent molecules. When a liquid solvent surrounds a molecule, each state of it is stabilized by an energy known as salvation energy. The medium, i.e., solvent can affect the solute molecule by its viscosity and also by its polarity. The number of electronic states which are defined for a molecule are determined
2
G.N. Rani, N.H. Ayachit / Journal of Electron Spectroscopy and Related Phenomena 182 (2010) 1–3
through Schrodinger wave equation E = H , where being the wave function. The wave function gives the information regarding the nature of a state. By employing the quantum mechanical second order perturbation theory and using Onsager model, McRae [1] has given an equation for the frequency of maximum solute absorption/fluorescence in a solvent, s and is as below:
vs = v0 + (A + B + C)f (ns ) + E[f (Ds ) − f (ns )] + F[f (Ds ) − f (ns )]2 (1) where 0 is the corresponding vapor phase frequency of s , f (ns ) = (n2s − 1)/(2n2s + 1) is a function of solvent refractive index (ns ), f(Ds ) = (Ds − 1)/(2Ds + 1) is a function of solvent dielectric medium (Ds ), (A + B) is a measure of dispersive effect, and h and c are Planck’s constant and velocity of light respectively. C is either (2e − 2g )/hca30 or (2g − 2e )/hca30 depending on whether s is measured through absorption spectra or emission spectra, respectively. Similarly, E is either (g g−e )/hca30 or (e e−g )/hca30 depending upon the spectra under study, with e , g being dipole moment in the excited state and ground state, respectively with e−g = e − g and a0 is the Onsager cavity radius. The last term in Eq. (1) and (A + B) can be neglected due to their small contribution towards shift. With this, Suppan [3] formulated the equation cited below for the difference in frequency in absorption for two different solvents 1 and 2, which is:
−v1−2 =
g g−e
[f (Ds ) − f (ns )]1−2 +
hca30
2e − 2g
f (ns )1−2
hca30
The above equation for fluorescence will be (with ae being the cavity radius in the excited state):
−v1−2 =
e g−e hca3e
[f (Ds ) − f (ns )]1−2 +
2g − 2e hca3e
f (ns )1−2
The above equation was written by Ayachit et al. in the form:
X C1
+
Y C2
=1
(2)
Here, X = [f(Ds ) − f(ns )1−2 ]/−1−2 and Y = [f(ns )1−2 ]/−1−2 , with C1 = hca30 /e g−e and C2 = hca30 /(2e − 2g ) in case of absorption studies and C1 = hca3e /e g−e and C2 = hca3e /(2g − 2e ) in florescence studies. The values of C1 and C2 can be obtained using either a graphical method or at least square fit. The values of X and Y are of the order of exp(−4) and hence it forces to plot the graph of quantities which are greater by a factor exp(−4), which may lead to points being scattered leading erroneous values of C1 and C2 . Hence in the present study C1 and C2 have been calculated using a least squares fit. From the values of C1 and C2 , e and the angle between e and g namely can be determined by writing e . g−e = e g cos − (e )2 . 3. Result and discussion The values of excited state electric dipole moments determined for the two indoles in the present work using absorption shifts and fluorescence shifts are presented in Table 1 along with the available dipole moment values in ground state and excited state as reported by Sharma et al. [9]. In this table are also given the Onsagar cavity radius. From the table it can be seen that the values of excited state dipole moments are much less than the reported earlier however greater than the ground state dipole moments of the molecules. The larger values observed in the earlier work may be because of the fact that:
(1) The solvents of very high dielectric constants have been used in the earlier work and hence neglecting the higher terms or considering them as constant may not be suitable. (2) When high dielectric constant solvents are used non-specific interactions come in to effect whose contribution cannot be neglected. (3) Orientation of the dipole moment has to be taken in to account. (4) Not much reliable absorption data are used. In view of the above, the dipole moments determined in the present work seems to be reliable representing a * transition. The solvatochromic shifts are the experimental evidence for the change in energies of levels of a solute due to change in the matrix. Emission spectra are usually more informative than the absorption spectra as emission state arise out of energy of more relaxed excited states. In view of this, the quantities determined using solvatochromic shifts become more important for the description of the intramolecular charge transfer in molecular excited states [4–14,21,22] and inter-molecular charge transfer in exciplexes. [23,24]. The shifts observed are dependent on the nature of solvent (polar or non-polar), dielectric constant, etc. This is evident from the fact that, in certain cases the shift in fluorescence spectrum is not necessarily accompanied by a shift in the absorption spectra, if exists it need not be in proportion to the one observed in fluorescence. There are examples for shifts in fluorescence to occur with no shifts in the absorption spectrum of a solute [25]. The estimations carried out using absorption shifts will thus not be that reliable compared with that calculated with emission spectra. If the difference of excited state of a solute in vacuum and the excited state of a polar solvent is greater than the difference of excited state of a solute molecule in vacuum and the excited state of non-polar solvent, the fluorescence as one goes from polar to non-polar shows a blue shift (*– transition) and if it is reverse the shift will be red (*–n transition). In the present study, the fluorescence shifts for two indoles indicate that the transitions involved are of the first type. The values determined for dipole moments in the excited state in the earlier work through absorption and in this work (presented later in the text) are in conformity with this observation. When a molecule gets electronically excited (by having different dipole moment and orientation), the electron will be raised to a new electronic level in much less time than it takes for the whole molecule to rearrange itself with solvent environment. Thus, immediately upon excitation, the molecule can be assumed to be in the same environment in the excited state as in the ground state and hence one can safely assume ae = a0 . The values of orientation between ground and excited state, estimated for the two indoles are given in Table 1; clearly indicate the large change in the molecular structure in the excited state. These orientations are very sensitive to the value of a0 and hence cannot be given much importance quantitatively unless the method is free from a0 , which is not the case here and hence the values of orientation between ground and excited state reported here are to be considered as indicative of the trend.
Table 1 Results for 5-hydroxy indole (5HI) and 5-hydroxy indole 3-acetic acid (5HI3AA). Parameters
in D e (abs) in D e (fluo) in D (fluo) in◦ e in D (earlier work)
Compounds 5HI
5HI3AA
3.02 3.01 3.7 21 5.75
4.05 4.13 4.76 34 5.17
G.N. Rani, N.H. Ayachit / Journal of Electron Spectroscopy and Related Phenomena 182 (2010) 1–3
4. Conclusion Since the excited state of a molecule is much different from its ground state, it may be more or less depending upon the transition involved, the ground state dipole moment may slightly change from solvent to solvent, but the excited state may be much different. Thus it is expected that little changes in absorption while large in the fluorescence spectrum with change in solvent. In the different equations used for the determination of excited state dipole moments one or more assumptions are made in obtaining the simplified equation from the original McRae’s equation. The estimations carried out using absorption shifts will thus not be that reliable compared with that calculated with emission spectra. Acknowledgement The authors thank the Principal and management of their respective colleges for encouragement shown. References [1] [2] [3] [4] [5]
E.G. McRae, J. Phys. Chem. 61 (1957) 562. M.B. Ledger, P. Suppan, Spectrochim. Acta 23A (1967) 641. P. Suppan, J. Chem. Soc. 4 (1968) 3125. M. Ito, K. Inuzuka, M. Imanishi, J. Am. Chem. Soc. 82 (1960) 1317. P. Suppan, C. Tsiams, Spectrochim. Acta 36A (1980) 364.
3
[6] D.K. Deshpande, M.A. Shashidhar, K. Suryanarayana, Z. Rao, Phys. Chem. Leipzig 262 (1981) 588. [7] L.S. Prabhumirashi, D.K. Narayan Kutty, A.S. Bhide, Spectrochim. Acta 39A (1983) 663; L.S. Prabhumirashi, Spectrochim. Acta 39A (1983) 91. [8] K.M.C. Davis, in: R. Foster (Ed.), Molecular Association, Academic Press, New York, 1975, p. 151. [9] N. Sharma, S.K. Jain, R.C. Rastogi, Spectrochim. Acta Part A 66 (2007) 171–176. [10] L.S. Prabhumirashi, S.S. Kunte, Spectrochim. Acta 42A (1986) 441. [11] P. Suppan, Chem. Phys. Lett. 94 (1983) 272. [12] N.H. Ayachit, D.K. Deshpande, M.A. Shashidhar, K. Suryanarayana Rao, Spectrochim. Acta 42A (1986) 585; N.H. Ayachit, D.K. Deshpande, M.A. Shashidhar, K. Suryanarayana Rao, Spectrochim. Acta 42A (1986) 1405. [13] N.H. Ayachit, G. Neeraja Rani, Phys. Chem. Liq. 45 (2007) 41. [14] N.H. Ayachit, Chem. Phys. Lett. 164 (1989) 272. [15] N.H. Ayachit, Proc. Indian Acad. Sci. (Chem. Sci.) 101 (1989) 269. [16] N.H. Ayachit, Asian Chem. Lett. 3 (1999) 167. [17] N.H. Ayachit, Asian J. Phys. 9 (2000) 349. [18] P. Suppan, J. Photochem. Photobiol. 50 (1990) 293. [19] Y.F. Nadaf, D.K. Deshpande, A.M. Karaguppikar, S.R. Inamdar, J. Photosci. 9 (2002) 29. [20] M.S. Masoud, A.E. Ali, M.A. Shaker, M.A. Ghani, Spectrochim. Acta 61A (2005) 3102. [21] T. Abe, I. Iweibo, Bull. Chem. Soc. Jpn. 58 (1985) 3415; T.A. Bull, Chem. Soc. Jpn. 41 (1985) 1260. [22] N.H. Ayachit, G. Neeraja Rani, Phys. Chem. Liq. 45 (2007) 615. [23] Z.R. Crabowski, K. Rothiewicz, A. Siemiarezuk, D.J. Cowley, W. Baumann, Nouv. J. Chem. 3 (1979) 443. [24] H. Beans, H. Knibbe, A. Weller, J. Chem. Phys. 47 (1967) 1183. [25] A.J. Pesce, C.G. Rosen, T. Pasby, Fluorescence Spectroscopy, Marcel Dekker, New York, 1971.