Spectrochimica Acta Part A 75 (2010) 739–744 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Photophysical properties of the 8-phenyl analogue of PM567: A theoretical rationalization Tandrima Chaudhuri a , Soumyaditya Mula b , Subrata Chattopadhyay b , Manas Banerjee a,∗ a Department of Chemistry, Burdwan University, Golapbag, Burdwan 713104, West Bengal, India b Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India article info Article history: Received 2 October 2009 Received in revised form 16 November 2009 Accepted 18 November 2009 Keywords: Pyrromethene dye Solvatochromism Ab initio Photophysical properties abstract The absorption and emission spectroscopic investigations of the C-8-phenyl substituted analogue of the pyrromethene dye PM567 in various polar, non-polar as well as protic and aprotic solvents are reported. The solvatochromic shifts of the spectral bands were studied in a multitude of polar, non-polar and protic, aprotic solvents followed by a multilinear regression in which several solvent parameters were simultaneously analysed. Comparison of the experimental results with those obtained by gas phase ab initio computation with CIS, TD-HF and TD-DFT theories using 6-31G* basis set reveal an overestimation of the experimentally measured excitation parameters by all these theoretical models. However, the trends in the experimental results agree with those calculated theoretically. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The pyrromethenes (PMs) with different substituents are highly fluorescent and useful laser dyes since they have low intersys- tem crossing (ISC) rate and low triplet excitation coefficients over the laser spectral region [1–3], and often possess triplet–triplet absorption coefficients about one-fifth of the rhodamine dyes [4,5]. Further, these dyes are also used as biolabels [6], artificial light har- vesters [7], sensitizers for solar cells [8], fluorescent sensors [9–11], molecular photonic wires [12,13], and electron-transfer reagents [14]. The PM ionic dyes show high polarizability, and exhibit good solubility in many organic solvents and even in solid matrices [15]. Quantum mechanical calculations indicate an increased charge separation for the PM dye, 4,4-difluoro-1,3,5,7,8-pentamethyl-2,6- diethyl-4-bora-3a,4a-diaza-s-indacene (common name PM567, 1a, Fig. 1) through its chromophore -system in polar solvents [16]. In order to obtain better insight into the photophysics of PM dyes, quantum mechanical calculations are now being applied to explore the geometry and electronic properties of the PM- chromophore [1,16]. In another study we found that compared to 1a, its C-8-phenyl structural analogue (4,4-difluoro-1,3,5,7- tetramethyl-2,6-diethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene, 1b) was photochemically more stable with enhanced lasing attributes [17]. It is well known that the lasing properties of PM dyes are a consequence of their photophysics. In fact, fluorescence ∗ Corresponding author. Tel.: +91 9434252709; fax: +91 3422530452. E-mail address: manasban@rediffmail.com (M. Banerjee). band maxima and lasing emission bands as well as fluorescence quantum yield and lasing efficiency for these PM dyes are deeply correlated [18,19]. The purpose of the present work has been to investigate the effect of nature of solvents on the photophysical properties of the lasing dye 1b. Solvent effect on the absorption and fluorescence bands has been analysed by a multicomponent linear regression in which several physicochemical properties of each solvent were simultaneously considered. Finally, the experimental results were rationalized through performing a wide range of high level ab initio calculations. Theoretical calculations were mainly focused on the evolution of absorption and emission characteristics of 1b. How- ever, this study could be considered as a work complementary to that of the photophysics of PM567 [19,20] which was studied well both experimentally and theoretically. 2. Experimental The PM dye 1b was synthesized by our own method [17]. All solvents (HPLC grade, Merck) were used without further purifica- tion. Absorption (UV–vis) spectral measurements were performed with a Shimadzu UV 1601 PC spectrophotometer fitted with an electronic temperature controller unit (TCC – 240 A). The steady state fluorescence emission and excitation spectra were recorded with a Hitachi F-4500 spectrofluorimeter equipped with a temper- ature controlled cell holder. The temperature was controlled within ±0.1 K by circulating water from a constant temperature bath (Heto Holten, Denmark). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.11.048