An experimental and theoretical study of excited-state dipole moments of some flavones using an efficient solvatochromic method based on the solvent polarity parameter, ETN

doi:10.1016/S1386-1425(00)00372-3

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 57, Issue 2, February 2001, Pages 291-298

https://doi.org/10.1016/S1386-1425(00)00372-3 Get rights and content

Abstract

The electronic absorption and fluorescence spectra of some biologically active natural flavones have been recorded at room temperature (298 K) in solvents of different polarities. The effects of the solvents upon the spectral properties are discussed. Difference in fluorescence intensity of flavones has been explained on the basis of intersystem crossing and degree of non-planarity calculated theoretically using Austin Model 1 (AM1) method. Excited-state dipole moments have been determined using the solvatochromic method based on the microscopic solvent polarity parameter, E_T^N. A reasonable agreement has been observed between experimental and AM1 calculated dipole moment changes. Our results are found to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts with microscopic solvent polarity parameter, E_T^N is superior to that obtained using bulk solvent polarity functions for all the systems studied here.

Introduction

In continuation of our work on the determination of excited state dipole moments of some novel natural products [1], we have now examined few model compounds such as flavone, 3-hydroxyflavone(3HF), 7-hydroxyflavone(7HF), 7,8-benzofuranoflavone(78BFF) and 3,5,7-trihydroxy-8,4′-dimethoxyflavone(357THMF) (Fig. 1). Flavones [2] constitute an important group of biologically active compounds because of the diversity in their pharmaceutical uses arising from their anticancerous, antiinflammatory, antiallergic and antiinfective effects. Some interesting photophysical and photochemical characteristics of some prototype model flavones have been extensively investigated in recent years [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. These flavones have potential applications as highly sensitive environmental probes in normal and reverse micellar systems [13], [14]. In spite of all these spectral studies on flavones, no systematic study related to excited state dipole moments and solvent effects has been reported until now.

At present, there is no perfect theory that can correlate all the known solute and solvent interactions. It is very complicated to describe the solvatochromic shifts completely. However, some theoretical treatments using zeroth-order approximation and comparison of results with experimental data for different molecular systems are reported by various authors [15], [16]. All these theories are based on the Onsager model [17] of solution structure and correlate the dependent Stokes shifts to the macroscopic properties of the solvent (relative permitivities and refractive indices) and the permanent dipole moment in the two combining states of the fluorophore. The solvatochromic shifts of dipolar molecules correlate much better with microscopic solvent polarity parameter, E_T^N [1], [18] rather than the traditionally used bulk solvent polarity functions involving relative permitivities and refractive indices. Though, there are limitations to the use of solvatochromic method, it is better than other conventional methods based on electric dichroism [19], fluorescence polarisation [20], stark splitting of rotational levels [21], microwave conductivity [22] and thermochromic shift [23].

In the present paper, we have investigated the effects of solvents and substituents on the absorption and fluorescence spectra of some biologically important flavones. Further, we have used solvatochromic shift methods to determine excited state dipole moments experimentally. Increase in dipole moment on excitation has been explained in terms of nature of emitting state (ICT or TICT) and resonance structures. Our results are found to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts with microscopic solvent polarity parameter, E_T^N, is superior to that obtained using bulk solvent polarity functions in all the flavones studied here.

Section snippets

Experimental

The flavones used were synthesised according to the standard methods [24], [25], [26], [27]. Their structures and purity were confirmed on the basis of their known melting points and ¹H NMR spectra. Triple distilled water and spectroscopic grade solvents were used after checking their purity. The absorption and fluorescence spectra were measured on a Kontron UV-930 spectrophotometer and Jasco FP-770 spectrofluorimeter, respectively. Austin Model 1 (AM1) [28] semiempirical molecular orbital

Solvent effect on absorption and fluorescence spectra

The absorption and emission maxima values of all the compounds are given in Table 2. Absorption and fluorescence spectra of one of the compounds (78BFF) in different solvents are shown in Fig. 2, Fig. 3. A red shift is observed with increase in solvent polarity suggesting π-π* transition except for 357THMF where a blue shift is observed suggesting n-π* transition.

Flavone is found to be non-fluorescent [12] but the introduction of hydroxy and furano groups in flavone, renders it intensely

Conclusions

The excited state dipole moment of a molecule controls the tunability range of its emission energy as a function of solvent polarity. It is to be noticed that a large stokes shift with solvent polarity in case of all the flavones suggests their use as tunable dye lasers. However, the fluorescence quantum yields of flavones are probably too low as compared with umbelliferones [32] which has a known adverse effect on laser dye properties. Our experimentally calculated dipole moment changes are in

Acknowledgements

Sanjay Kumar and Sapan K. Jain gratefully acknowledge the financial assistance from University Grants Commission (UGC) and Council of Scientific and Industrial Research (CSIR), New Delhi. The authors also thank Prof. N.K. Ray, Department of Chemistry, University of Delhi for valuable discussions.

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An experimental and theoretical study of excited-state dipole moments of some flavones using an efficient solvatochromic method based on the solvent polarity parameter, ETN

Abstract

Introduction

Section snippets

Experimental

Solvent effect on absorption and fluorescence spectra

Conclusions

Acknowledgements

Spectrochim. Acta

J. Photochem. Photobiol. A: Chem.

J. Photochem. Photobiol. A: Chem.

J. Photochem. Photobiol. A: Chem.

J. Photochem.

Chem. Phys. Lett.

J. Phys. Chem.

J. Phys. Chem.

J. Chem. Soc. Faraday Trans. II

J. Am. Chem. Soc.

Proc. Natl. Acad. Sci. USA

J. Photochem. Photobiol. A: Chem.

Opt. Spektosk. (USSR)

C. R. Hebd. Seances Acad. Sci. Ser. France

An experimental and theoretical study of excited-state dipole moments of some flavones using an efficient solvatochromic method based on the solvent polarity parameter, E_T^N