Resonant Nonlinear Absorption in Zn-Phthalocyanines L. De Boni, † E. Piovesan, † L. Gaffo, ‡ and C. R. Mendonc ¸a* ,† Instituto de Fı ´sica de Sa ˜o Carlos, UniVersidade de Sa ˜o Paulo, Caixa Postal 369, 13560-970 Sa ˜o Carlos, SP, Brazil, and Centro de Cie ˆncias Exatas, Departamento de Quı ´mica, UniVersidade Estadual de Maringa ´, AV. Colombo 5790, 87020-900 - Maringa, PR, Brazil ReceiVed: June 05, 2008; ReVised Manuscript ReceiVed: June 20, 2008 In this work, we investigate the nonlinear absorption dynamics of Zn phthalocyanine in dimethyl sulfoxide (DMSO). We used single pulse and pulse train Z-scan techniques to determine the dynamics and absorption cross-sections of singlet and triplet states at 532 nm. The excited singlet state absorption cross-section was determined to be 3.2 times higher than the ground state one, giving rise to reverse saturable absorption. We also observed that reverse saturable absorption occurs from the triplet state, after its population by intersystem crossing, whose characteristic time was determined to be 8.9 ns. The triplet state absorption cross-section determined is 2.6 times higher than the ground state one. In addition, we used the white light continuum Z-scan to evaluate the singlet excited state spectrum from 450 to 710 nm. The results show two well-defined regions, one above 600 nm, where reverse saturable absorption is predominant. Below 600 nm, we detected a strong saturable absorption. A three-energy-level diagram was used to explain the experimental results, leading to the excited state absorption cross-section determination from 450 nm up to 710 nm. Introduction Phthalocyanines are organic macrocyclic compounds that present excellent stability, and whose optical and electrical properties can be tuned by varying the peripheral groups and the central metal ion. Such features make phthalocyanines promising candidates for a broad range of applications, from organic solar cells 1–3 to photodynamic therapy of cancer. 4–7 Macrocyclic compounds, such as phthalocyanines, exhibit large optical nonlinearities in general. 8–11 Thanks to the intense reverse saturable absorption (RSA) exhibited by these compounds, 12 they have been employed in a number of applications with emphasis on optical limiting. 13,14 For instance, lead 12,14 and indium 15 phthalocyanines are two of the most successful materials for optical limiting operating with the RSA process. As mentioned before, these materials can be synthesized in a wide variety, with the attachment of functional groups. In this way, fine-tuning of the physical and chemical properties can be achieved with the molecular control of the compound itself, or by the way the molecules are assembled. The RSA process occurs when the absorption cross-section of the excited state exceeds that of the ground state. Usually, for macrocyclic compounds RSA follows from an intersystem crossing from a higher excited singlet state to an excited triplet state. The RSA efficiency depends upon the properties of excited states, such as excited states relaxation times, intersystem crossing time and cross-sections. Therefore, to select the most suitable candidates for a given application, the knowledge of the compound’s excited state properties is of foremost importance. In this work, we used single pulse 16 and pulse train Z-scan (PTZ-Scan) 17 techniques, both at 532 nm, to investigate the dynamics and excited state properties of phthalocyanines containing Zn, namely ZnPc, whose molecular structures is presented in Figure 1. To gain further insight into the character and processes of the excited states of ZnPc, we also carried out nonlinear absorption spectrum measurements from 450 to 710 nm using the WLC Z-scan technique. 18 The resonant nonlinear absorption spectrum of ZnPc presents saturable absorption (SA) and reverse saturable absorption (RSA), depending on the excitation wavelength. The understanding of the influence of chemical structure on the excited state spectroscopic parameters, may lead us to the development of compounds with better optical properties. Experiments To measure the nonlinear optical process of ZnPc, we prepared DMSO solution in a concentration of 1.2 × 10 17 molecules/cm 3 (∼2.0 × 10 -4 mol/L). The UV-vis absorption, at room temperature, was measured with a Cary-17A spectrom- eter, using a solution concentration 6 times smaller than that * Corresponding author. E-mail: crmendon@if.sc.usp.br. † Universidade de Sa ˜o Paulo. ‡ Universidade Estadual de Maringa ´. Figure 1. Molecular structure of ZnPc. 6803 10.1021/jp8049735 CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008 2008, 112, 6803–6807