Journal of Photochemistry and Photobiology A: Chemistry 155 (2003) 93–106 A study of the luminescent complexes formed by the dye 1,4-dihydroxyanthraquinone (quinizarin) and Ga(III) and In(III) Luisa Quinti a , Norman S. Allen a,∗ , Michele Edge a , Brian P. Murphy a , Angelo Perotti b a Department of Chemistry and Materials, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK b Dipartimento di Chimica Generale, Università di Pavia, Via Taramelli 12, 27100 Pavia, Italy Received 30 September 2002; received in revised form 25 October 2002; accepted 28 October 2002 Abstract Fluorescent species form when Ga(III) and In(III) ions interact with 1,4-dihydroxyanthraquinone (quinizarin), in aqueous and non-aqueous media. Time dependence, temperature, solvent, counterion and pH effects studies have been performed in order to interpret the behaviour of this ligand in the presence of Ga(III) and In(III) ions. Quinizarin by itself is not fluorescent, but both its absorbance and fluorescence intensity increase proportionally to the complexation with Ga(III) and In(III), thus suggesting the supramolecular nature of these complexes, which could be used as fluorescence sensors. The fluorescent quantum-yield values of the QNZ complexes with Al(III), Ga(III) and In(III) ions are negatively affected by the increase of the size of M(III) ions, suggesting that the increase in metal size weakens the supramolecular interaction with QNZ. Potentiometric studies, performed between pH 3 and 6, suggest the formation of 2:1 QNZ/In(III) species, having a high β formation constant. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Quinizarin; Hydroxyanthraquinones; Fluorescence; Supramolecular complexes 1. Introduction 1,4-Dihydroxyanthraquinone (quinizarin, QNZ) is accred- ited with properties that span from being used as a fungicide and pesticide, to being employed as a dye, a photoinitiator and an additive in lubricants [1]. It also serves as a model for anthracycline antitumour antibiotics [2]. Significantly, the fluorescent complexes of quinizarin with lithium and boron ions are used as spectrophotometric analytical reagents for these metals [3,4]. The literature on the speciation of quinizarin complexes is not mature [5,6]. Early reports suggest the formation of non-polymeric species, having 1:1, 1:2 or 1:3 metal/QNZ stoichiometry [6–8]. In contrast, recent reports cite the for- mation of polymeric complexes with 1:1 or 2:3 metal/QNZ ratio [1,5,9,10]. Recently, a number of fluorescent sensors and switches have been designed [11–14]. Most of them operate through photo-induced electron-transfer (PET) and consist of two-component systems. For example, an anthracene light-emitting moiety has been linked to an 18-membered ∗ Corresponding author. Tel.: +44-161-247-6520; fax: +44-161-247-1438. E-mail address: n.allen@mmu.ac.uk (N.S. Allen). azacrown, the latter acting as a sensor [12]. The uncom- plexed system does not fluoresce, the fluorophore being deactivated by an electron-transfer process taking place between the crown and the anthracene. However, the in- corporation of K + ions in the crown reinstates the typical anthracene emission by means of PET inhibition through the metal–ligand interaction. In this connection, we have studied the effect of the complexation of QNZ with metal ions that generate flu- orescence. Previously, we have investigated the lumine- scence-producing interaction between QNZ and Al(III) ions [15–17]. Here we report the studies on the systems consist- ing of QNZ and the trivalent metal ions Ga(III) and In(III). 2. Results and discussion 2.1. Preliminary solution studies The effect of Ga(III) ions was determined mainly using Ga(acac) 3 . This was found to be the best compound for solubility reasons. The use of GaCl 3 was restricted to those alcohols where no interaction between GaCl 3 and the solvent itself occurred. 1010-6030/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S1010-6030(02)00395-7