Heavy-atom effects on metalloporphyrins and polyhalogenated porphyrins Em  ılia G. Azenha a , Arm enio C. Serra b , Marta Pineiro a , Mariette M. Pereira a , J. Seixas de Melo a , Luis G. Arnaut a, * , Sebasti~ ao J. Formosinho a,1 , A.M.d’A. Rocha Gonsalves a a Chemistry Department, University of Coimbra, 3049 Coimbra Codex, Portugal b Universidade Cat olica Portuguesa, P-3080 Figueira da Foz, Portugal Received 23 January 2002; in final form 18 March 2002 Abstract The photophysics of halogenated and metallated tetrakisphenylporphyrins is investigated using single-photon counting, photoacoustic calorimetry and luminescence techniques. The radiationless transition rates in these and related molecules are interpreted with a quantum-mechanical tunnelling model modified to include the effect of spin–orbit coupling in the intersystem crossing rates. It is shown that tetrakisphenylporphyrins with halogens in the ortho positions have long-lived triplet states that are formed in high yields. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Porphyrins; Radiationless transitions; Tunnel-effect theory; Heavy-atom effect 1. Introduction Porphyrin derivatives represent an important class of pigments in the storage of energy in both natural and artificial photosynthesis [1–4], on the making of molecular energy devices [5,6], catalysts [7], and more recently as sensitisers of specific products (e.g., the singlet 1 D g state of molecular oxygen) in photodynamic therapy (PDT) [8–10] and in the photodynamic destruction of viruses [11,12]. The present work was motivated by the need for better PDT sensitisers based on porphy- rins, chlorins and bacteriochlorins [13–15], that is, sensitisers with high absortivity in the photother- apeutic window (600–900 nm) that generate singlet oxygen in high yields. The most important pre- cursor of the O 2 ð 1 D g Þ state is the triplet state T 1 of the sensitiser. Therefore, an ideal sensitiser must have a long-lived triplet state ðs T > 5 lsÞ formed in high yields ðU T  1Þ,andatripletenergyatleast 20 kJ mol 1 above that of the O 2 ð 1 D g Þ state ðE T > 115 kJ mol 1 Þ. In order to maximise the quantum yield for triplet formation one usually resorts to an internal heavy-atom effect [16]. A quite extensive research Chemical Physics 280 (2002) 177–190 www.elsevier.com/locate/chemphys * Corresponding author. Tel.: +351-39-822826; fax: +351-39- 827703. E-mail address: lgarnaut@ci.uc.pt (L.G. Arnaut). 1 Present address: Escola Superior de Ci^ encias e Tecnologia, Universidade Cat olica Portuguesa, 3500 Viseu (Portugal). 0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0301-0104(02)00485-8