On the Low-Lying Excited States of sym-Triazine-Based Herbicides Josep M. Oliva, [a] M. Emilia D. G. Azenha, [b] Hugh D. Burrows, [b] Rita Coimbra, [b] J. Serxio Seixas de Melo, [b] MoisØs Canle L.,* [c] M. Isabel Fernµndez, [c] J. Arturo Santaballa, [c] and Luis Serrano-AndrØs [d] Introduction sym-Triazines are widely used as pest-control agents, particu- larly as systemic herbicides that act as photosynthesis inhibi- tors. [1–3] Intensive agricultural use of these compounds has led to their increasing unwanted occurrence in natural waters, [4–9] especially groundwaters. [9–11] As a consequence, public concern has grown about the long-term environmental and health ef- fects of such pollutants. [12, 13] Many different techniques, including chlorination, [14] ozona- tion, [15] H 2 O 2 -promoted oxidation, [16] biological degradation, [17] Fenton oxidation, [18] radiolysis, [19] photocatalysis, [20] photosensi- tization, [21] and reduction, [22] have been attempted with ques- tionable success to promote the degradation or elimination of these pollutants. A possible alternative is direct photodegrada- tion, for which a number of studies on photoproducts have been performed, but little mechanistic information is availa- ble. [23–27] Proper modeling of the environmental fate of these agrochemicals will be difficult unless detailed mechanistic un- derstanding becomes available. Indeed, a comprehensive study of the photodegradation process must involve proper knowledge of the nature and role of their different low-lying excited states. The characteristics and properties of the excited states of azines have attracted the interest of chemists for at least 50 years. A wide variety of both theoretical (semiempirical to ab initio level) and experimental studies have been carried out on their electronic transitions. [28–39] However, there are still many unanswered questions regarding the nature of the excit- ed states. Therefore, within the framework of a wider project aiming to elucidate the molecular mechanisms of photodegra- dation of different water pollutants, we report the results of a joint computational and luminescence study of the low-lying singlet and triplet states of three different sym-triazines (see Scheme 1), and discuss their role in the photodegradation mechanism. Fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were mea- sured for the three compounds, while quantum chemical cal- culations on the low-lying singlet and triplet excited states at different levels of theory were performed to rationalize some of the experimental findings. [a] Dr. J. M. Oliva Instituto de Química Física “Rocasolano” Consejo Superior de Investigaciones Científicas Serrano 119, 28006 Madrid (Spain) [b] Dr. M. E. D. G. Azenha, Prof. Dr. H. D. Burrows, R. Coimbra, Dr. J. S. Seixas de Melo Departamento de Química, Universidade de Coimbra 3004-535 Coimbra (Portugal) [c] Dr. M. Canle L., Dr. M. I. Fernµndez, Dr. J. A. Santaballa Departamento de Química-Física e EnxeÇería Química I Universidade da CoruÇa Rffla Alejandro de la Sota, 1, 15008 A CoruÇa (Spain) Fax: (+ 34) 981-167-065 E-mail : mcanle@udc.es [d] Dr. L. Serrano-AndrØs Departamento de Química-Física e Instituto de Ciencia Molecular Universitat de Valncia Dr. Moliner 50, Burjassot, 46100 Valencia (Spain) Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N 2 -ethyl-N 4 -iso- propyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio- N 2 -ethyl-N 4 -isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of 1 and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spec- tra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different ab- sorption and emission features. Scheme 1. Molecular formula of the species studied in this work: 1,3,5-triazine (1;X = R 1 = R 2 = H), atrazine (2 ;X = Cl, R 1 = NHEt, R 2 = NHiPr), and ametryn (3 ; X = SCH 3 ,R 1 = NHEt, R 2 = NHiPr). 306  2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/cphc.200400349 ChemPhysChem 2005, 6, 306 – 314