Solvatochromism and thermochromism of the electronic spectra of an indocarbocyanine dye Ce Âsar A.T. Laia * , Silvia M.B. Costa Centro de Quõ Âmica Estrutural, Complexo 1, Instituto Superior Te Âcnico, 1049-001 Lisbon, Portugal Received 31 August 2000; revised 23 October 2000; accepted 23 October 2000 Abstract Solvent effects in the electronic spectra of a cyanine dye (1,1 0 -diethyl-3,3,3 0 ,3 0 -tetramethylindocarbocyanine, diIC 2 (3)) were studied. Solute/solvent interactions originate from dispersion forces but contributions arising from the solvent Stark effect were also detected, showing signi®cant changes of both dipole moment and polarizability of the dye on the electronic transition. The absorption and emission spectra of diIC 2 (3) in glycerol at different temperatures were obtained and analyzed in the framework of the coordinate con®guration model, caused by the slow solvent dynamics. It is shown that this model describes the dye thermochromism at high temperatures well, but the internal nuclear coordinate plays an important role at lower temperatures. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Cyanines; Solvatochromism; Thermochromism 1. Introduction Cyanine dyes are a class of molecules with a wide range of applications. Recently, these molecules have been used as probes of solvation dynamics [1,2]. The use of these dyes highlighted the importance of understanding the solvatochromic effects on the elec- tronic transitions in these molecules [1]. Several reports attempted to clarify how solvents can alter the energy of such transitions [1,3±6]. In particular, the solvatochromism of indocarbocyanines was studied [1,3,6]. It was observed that the solvent polarizability (solute/solvent dispersive interactions) plays a major role on the electronic transitions of cyanine dyes. Similar effect is also observed in carotenoids [7], and it is attributed to the apolar character and to the polymethine chain of these molecules. In this work, the solvatochromism of the electronic absorption band of diIC 2 (3) (Fig. 1) was studied in a wide series of polar solvents. The shift and shape of the same band in glycerol at different temperatures are analyzed and compared with the con®guration coordinate model [8]. 2. Experimental The dye 1,1 0 -diethyl-3,3,3 0 ,3 0 -tetramethylindo- carbocyanine (diIC 2 (3)) was purchased from Molecular Probes (99% purity) and used as received. All experiments were made at a concentration of 1 mM. The solvents used were methanol, ethanol, 1-propanol, 1-butanol, 1-hexanol, 1-decanol, 2-pro- panol, 2-butanol, cyclohexanol, ethylene glycol, Journal of Molecular Structure 565±566 (2001) 83±86 0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0022-2860(00)00883-8 www.elsevier.nl/locate/molstruc * Corresponding author. Tel.: 1351-1-841-9000; fax: 1351-1- 846-4455/7. E-mail address: cesar.laia@alfa.ist.utl.pt (C.A.T. Laia).