Two-photon absorption spectra of Salen dye complexes with azo dyes U.M. Neves a , L. De Boni a , Zhihong Ye b , Xiu R. Bu b , C.R. Mendonc ¸a a, * a Instituto de Fı ´sica de Sa ˜o Carlos, Universidade de Sao Paulo, Caixa Postal 369, 13560-970 Sa ˜o Carlos, SP, Brazil b Department of Chemistry and NASA Center for High Performance Polymers and Composites, Clark Atlanta University, Atlanta, GA 30314, USA Received 9 April 2007; in final form 27 April 2007 Available online 10 May 2007 Abstract This work reports the degenerate two-photon absorption spectrum for a series of Salen complexes possessing two azo dye units. The observed spectra peak at 1000 nm, twice the wavelength of the linear absorption peak, and increase monotonically to the blue due to resonant enhancement of the non linearity. The two-photon absorption properties of these molecules were found to be an additive effect of chromophores and independent of the metal ions. The two-photon absorption spectra were described with a sum-over-states model, from which the transition dipole moment and dipole moment change were determined and compared with quantum chemistry calculation. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Two-photon absorption (2PA) is an attractive subject for different fields of application. It occurs when two-pho- tons are simultaneously absorbed by a material, in such a way that the sum of the two-photons energy is equal to the molecule transition energy. The 2PA quadratic depen- dence on the pulse intensity improves the spatial resolution of light-matter interaction, which can be exploited in sev- eral technological applications such as 3D optical storage [1,2], biological imaging [3–5], micro fabrication [6], optical limiting [7,8] and two-photon photodynamic cancer ther- apy [9,10]. In this perspective, the development of mole- cules with unique 2PA properties is essential [11,12]. Besides the development of pure organic compounds, metal organics molecules have also been exploited [13–16]. The incorporation of metal ions adds new features such as ther- mal stability, redox chemistry, and coordination chemistry to nonlinear materials. This work presents a systematic study on the two-pho- ton absorption of a novel class of metal-organic azoaro- matic dyes, where Salen complexes are tethered with two azo dye units. The azocompounds optical properties have been investigated due to their potential use in a variety of applications [17–19], some of them exploiting the process of 2PA [20–25]. From both, technological and fundamental points of view, the development of strategies to improve the 2PA properties of azocompounds is a relevant subject, since their 2PA cross-section are relatively small [24,25]. In this direction, this work reports not only a molecular sys- tem with enhanced 2PA properties through an additive property but also fundamental aspects of 2PA in azochromophores. 2. Experimental The compounds studied in this work, whose molecular structures are shown in Fig. 1, were designated as Salen Dye (S-Dye), Salen Cu (S-Cu), Salen Ni (S-Ni) and Salen VO (S-VO). These compounds were synthesized specifically to provide a straightforward analysis of the effect of incor- porating metals in the nonlinear optical property [26,27]. The samples were dissolved in N-methylpyrrolidone (NMP, from Aldrich), with concentrations of approximately 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.05.021 * Corresponding author. Present address: Department of Physics and School of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge, MA 02138, USA. E-mail address: crmendon@if.sc.usp.br (C.R. Mendonc ¸a). www.elsevier.com/locate/cplett Chemical Physics Letters 441 (2007) 221–225