APPLIED ORGANOMETALLIC CHEMISTRY Appl. Organometal. Chem. 2007; 21: 246–254 Published online in Wiley InterScience (www.interscience.wiley.com) DOI:10.1002/aoc.1188 Materials, Nanoscience and Catalysis Molecular photochromic systems: a theoretical and experimental investigation on zinc(II) dithizonate L. Armelao 1 *, G. Bandoli 2 , D. Barreca 1 , G. Bottaro 1 , E. Tondello 3 , A. Venzo 1 and A. Vittadini 1 1 ISTM-CNR and INSTM, Dipartimento di Scienze Chimiche, Universit ` a di Padova, 35131 Padova, Italy 2 Dipartimento di Scienze Farmaceutiche, Universit ` a di Padova, 35131 Padova, Italy 3 Dipartimento di Scienze Chimiche, Universit ` a di Padova and INSTM, 35131 Padova, Italy Received 20 November 2006; Accepted 20 November 2006 Zinc(II)-dithizone based molecular systems [Zn(HDz) 2 ] are intriguing candidates for the development of optical devices thanks to their interesting photochromic and nonlinear optical properties. In the present work, the behavior of Zn(HDz) 2 in different solvents was investigated by a combined theoretical and experimental approach. In particular, solutions of both dithizone (H 2 Dz) and Zn(HDz) 2 were analyzed by optical absorption spectroscopy and nuclear magnetic resonance (NMR) techniques, with particular attention to structure–properties relationships. Density functional and time-dependent density functional calculations were performed on the stable and the activated forms of the complex, obtaining information on the energetics of their interconversion, as well as on the nature of their electronic excitations. Copyright  2007 John Wiley & Sons, Ltd. KEYWORDS: Zinc(II) dithizonate; optical properties; NMR; density functional theory INTRODUCTION Diphenylthiocarbazone, or dithizone [1,5-diphenyl-3-forma- zathiol, H 2 Dz], is a diprotic organic acid which forms strongly colored complexes with most transition metals. 1 In particular, different coordination compounds of the monoanionic HDz − species in tetrahedral or square-planar geometries [M(HDz) 2 ,M = Zn, Cd, Hg, Pt, etc.] have been previously synthesized and characterized. 2 The interest towards such compounds has been notably stimulated by their photochromism, i.e. the reversible interconversion between two molecular forms endowed with different absorption properties, observed both under UV–vis steady illumination 3,4 and laser pulse photolysis. 5 The color change from red to violet blue is usually regarded as an inherent property of the HDz − ligand, since the involved electronic transitions are essentially localized on its molecular orbital system, whereas the nature of metal centers seems to *Correspondence to: L. Armelao, ISTM-CNR and INSTM, Diparti- mento di Scienze Chimiche, Universit` a di Padova, 35131 Padova, Italy. E-mail: armelao@chin.unipd.it Contract/grant sponsor: National Research Council. Contract/grant sponsor: University of Padova. Contract/grant sponsor: INSTM. influence only the speed of the interconversion process. 6 The photochromic interconversion opens appealing possibilities for the use of the above complexes in various fields. In fact, solid-state systems containing photochromic molecules have displayed photoinduced birifrangence and dichroism phenomena 7 and can be proposed for the conception of data storage and opto-optical switching devices. 8–10 Besides, M(HDz) 2 molecular systems display remarkably large first and second molecular hyperpolarizabilities, which, in connection with the high absorption coefficients of the two forms involved in the photochromic process, play a prominent role for potential applications in nonlinear optics (NLO). In particular, the coupling between photochromism and NLO properties leads to specific material features that might be advantageous for several technological applications, such as reversible information storage, holography and all-optical modulators. 8–10 Further interesting perspectives for advanced applications might be offered by the exploitation of NLO properties for both the forms involved in the photochromic interconversion. In this context, the tetrahedral Zn(II) dithizonate complex has been the subject of thorough investigations regarding its structure and chemical properties. 3,4,11,12 Fundamental studies on this system are simplified by the d 10 closed shell of Copyright  2007 John Wiley & Sons, Ltd.