A combined experimental and computational investigation of solvatochromism of nonpolar laser dyes: Evaluation of ground and singlet excited-state dipole moments G.H. Pujar a , M.N. Wari a , B. Steffi c , H. Varsha c , B. Kavita c , C. Yohannan Panicker b , C. Santhosh c , Ajeetkumar Patil d , Sanjeev R. Inamdar a, ⁎ a Laser Spectroscopy Programme, Department of Physics and UGC-CPEPA, Karnatak University, Dharwad 580003, India b Department of Physics, Fatima Mata National College, Kollam 691 001, India c Centre for Atomic and Molecular Physics, Manipal University, Manipal, India d BioSyM, Singapore-MIT Alliance for Research and Technology (SMART), Singapore 138602, Singapore abstract article info Article history: Received 25 July 2017 Received in revised form 18 August 2017 Accepted 22 August 2017 Available online 24 August 2017 In the present work, the solvatochromism of two large nonpolar laser dyes Exalite 384 (E384) and Exalite 416 (E416) has been studied both experimentally and computationally. The steady state absorption and fluorescence spectra have been measured in a series of polar protic, polar aprotic and non-polar/weakly polar solvents to in- vestigate their solvatochromism and determine dipole moments. Various solvent correlation techniques, like Lippert–Mataga, Bilot–Kawski, Kawski–Chamma–Viallet, Bakhshiev and Reichardt methods were used to evalu- ate the singlet excited and ground state dipole moments. Kamlet–Taft and Catalan solvent parameters were used by means of multiple linear regression (MLR) method to analyze specific and non-specific solute-solvent interac- tions. Computational studies were carried out to optimize ground and excited state geometries using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), respectively, in vacuum. This study also extends to evaluate the electronic transition energies from ground to first electronic excited state of solvated laser dyes employing semiempirical wave function model. In particular, the semiempirical method ZINDO has been combined with integral equation formalism of polarizable continuum model (IEF-PCM) to cal- culate solute-solvent interaction potential which is comparatively studied with E T (30) polarity scale along with experimental. The intramolecular charge transfer and hybridization is demonstrated by natural bond orbital analysis (NBO). The ground (μ g ) and excited state dipole moments (μ e ) of these dyes computed and those deter- mined experimentally are compared and the results are discussed. © 2017 Elsevier B.V. All rights reserved. Keywords: Exalite laser dyes ZINDO-PCM Natural bond orbital Dipole moments Solvatochromism DFT and TD-DFT 1. Introduction The organic probes with poly-p-phenylenes have generated great in- terest in the field of optics through the last couple of decades due to their spectroscopic properties, absorption and photoluminescence, promising nonlinear optical coefficients, and lasing abilities [1– 6].These explorations have led to their use as laser dyes, fluorescent dyes and for other spectroscopic applications. Several laser dyes having p-phenylene as subunit are commercially available from Exciton Chem- ical Co., USA. These dyes are highly fluorescent and are used as laser dyes mainly in the UV and blue region [7–9]. The exalite series dyes have been known as one of the successful dye laser sources and caught considerable interest in applications ranging from dermatology to mo- lecular spectroscopy [10] as well as in single crystal analysis [11]. In particular the titled compounds, exalite 384 (E384) and exalite 416 (E416) are widely used as fluorescent laser dyes in solution and exhibit efficient photophysical properties (absorption/fluorescence‚ λ max values of E384 are 324/378 nm and of E416 are 353/413 nm in 1,4-diox- ane, respectively [12,13]). They have high absorption coefficients at 355 nm and excellent operating lifetimes in 1,4-dioxane solvent, mak- ing them potential candidates for pumping with the third harmonic of a Nd:YAG laser as well as under XeCl (308 nm) pumping. E384 and E416 are class of para-quaterphenylenes and their molecular structures were optimized to maximize the performance of the compounds as fluorescent dyes. There are four and six aromatic rings in E384 and E416 dyes, respectively (Fig. 1). The effects of these substituted rings have reflected on the optical properties [12,13]. The determination of dipole moments of laser dyes play an impor- tant role in research as it provides an insight into electron density, charge distribution around the probes, electronic and geometrical struc- ture of a dye in bulk solution. In addition, the knowledge of excited state Journal of Molecular Liquids 244 (2017) 453–463 ⁎ Corresponding author. E-mail address: him_lax3@yahoo.com (S.R. Inamdar). http://dx.doi.org/10.1016/j.molliq.2017.08.078 0167-7322/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq