On the ground and excited state dipole moments of dansylamide from solvatochromic shifts of absorption and fluorescence spectra N. Tewari a,1 , N.K. Joshi a , R. Rautela a , R. Gahlaut a , H.C. Joshi b , S. Pant a, ⁎ a Photophysics Laboratory, Department of Physics, D.S.B. Campus, Kumaun University, Nainital-263001, India b Institute for Plasma Research, Bhat, Gandhinagar-382428, India abstract article info Article history: Received 19 April 2010 Received in revised form 3 March 2011 Accepted 16 March 2011 Available online 23 March 2011 Keywords: Dipole moments Solvatochromic shift method Dansylamide Ground and excited state dipole moments of dansylamide were obtained by using solvatochromic shift method. Bakhshiev and Kawski–Chamma–Viallet correlations based on bulk solvent polarity parameters were applied. The results were further verified by using the dimensionless microscopic solvent polarity parameter E T N . Higher dipole moment is obtained for the excited state as compared to the ground state. The results indicate a more polar excited state, which may be due to the charge transfer character. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Fluorescence spectra are usually shifted with respect to absorption (Stokes shift) due to vibrational relaxation, redistribution of electrons in the solvent molecules induced by altered dipole moment of the excited fluorophore, reorientation of the solvent molecules around the excited state dipole, and specific interaction between the fluorophore and the solvent [1]. The ground and excited state dipole moments of a molecule are important parameters to decipher the dynamical processes that molecule undergoes during excitation. The study of these parameters reveals information on the electronic and geometrical structure of the molecule in the excited state. A number of techniques e.g. electronic polarization of fluorescence, electric-dichroism, microwave conductivity and stark splitting [2–4] are available for the determination of the excited state dipole moment. These are generally considered to be accurate, but their use is limited because of being equipment intensive and restricted to relatively smaller molecules [5]. However, the experimental determination of this parameter based on the analysis of the solvatochromism of absorption and fluorescence is quite popular. In this technique, variation of Stokes shift vis-a-vis the bulk solvent polarity described in terms of dielectric constant (ε) and refractive index η is exploited to determine the dipole moments [5–17]. Knowing the value of the excited state dipole moments of organic molecules having large hyper polarizabilities is important in designing efficient non-linear optical materials [18]. 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. Also, the experimental data on dipole moments are useful in parameterization of quantum chemical methods [7]. Dansylamide (5-dimethylamino-1-naphthalenesulfonamide) is a biologically important probe molecule exhibiting solvatochromism and high emission quantum yields. It finds application in fluorescence quenching measurements, energy transfer and molecular motion studies [19–21]. Dansylamide is a core-structure present in many fluorescent biosensors, labels and supramolecular structures; besides, dansylamide nanoparticles are finding application in nano biotech- nology research [22–26]. It has been found that the excited state dynamics of dansylamide is complex as it is governed by the transitions from nearly degenerate 1 L a and 1 L b excited states. On going from nonpolar to polar environment a level inversion of these states has been suggested. In polar solvents 1 L a state, which has a charge transfer character, interacts more strongly with the medium than does the 1 L b state. The relaxation of the solvent cage possibly shifts the 1 L a state from slightly above 1 L b to just below 1 L b state [27,28]. In this report we have calculated the ground and excited state dipole moments of dansylamide molecule by solvatochromic shift method using Bakhshiev [29] and Kawski–Chamma–Viallet [30,31] correlations. To the best of our knowledge, and as literature survey suggests, the dipole moments of here studied probe have not been ascertained by simultaneously applying the above-mentioned equa- tions. The molecular structural formula of dansylamide is given in Fig. 1. Journal of Molecular Liquids 160 (2011) 150–153 ⁎ Corresponding author. Tel.: + 91 9411198359. E-mail address: sanjaypent@yahoo.com (S. Pant). 1 Present address: Lajpat Rai Post Graduate College, Sahibabad, India. 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.03.008 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq