NIR luminescence of gadolinium porphyrin complexes Gamal E. Khalil a, * , Elizabeth K. Thompson a , Martin Gouterman a , James B. Callis a , Larry R. Dalton a , Nicholas J. Turro b , Steffen Jockusch b a Department of Chemistry, The University of Washington, Bagley Hall, Box 351700, Seattle, WA 98195, United States b Department of Chemistry, Columbia University, New York, NY 10027, United States Received 5 October 2006; in final form 7 December 2006 Available online 16 December 2006 Dedicated to the memory of the late professor Alan Adler of Western Connecticut State University Abstract Most lanthanide porphyrin derivatives exhibit ‘normal’ absorption spectra with Q and B bands. The rare earth ions have little effect on the absorption spectra, which are much like those of other closed-shell metalloporphyrins. However, their emission spectra vary according to which lanthanide ion is present. We observe two types of emissions: (i) a line emission arising from the lanthanide ion f–f transitions and (ii) p-p * emission from the triplet state of porphyrin. In this Letter, we focus on the emission from gadolinium por- phyrin complexes. The oxygen sensitivity was measured in solution and polymeric film. Both intensity and lifetime Stern–Volmer plots were measured. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction Since the classic research of Weissman [1], Abramson [2] and of Crosby [3], lanthanide complexes have attracted many researchers. They have been investigated as laser materials, electroluminescent devices, biological indicators [4] and immunoassay sensors [5]. In general, the organic ligands of the complex absorb light and transfer the energy to the lanthanide ion through the triplet state of the ligand. Emission then occurs from the metal ion as an f–f transi- tion. Crosby elegantly reviewed the process of energy trans- fer from the triplet manifold of the complex to the lanthanide ion levels in 1966, and the weight of the litera- ture to date accepts this plausible mechanism. For example, all europium ion emission originates from the 5 D 0 energy level with the strongest emission being the 5 D 0 ! 7 F 2 tran- sition at approximately 615 nm. Complexes of ytterbium, neodymium, and erbium produce near-IR (NIR) lumines- cence at 980 nm, 1065 nm and 1520 nm, respectively from the f-shell. Lanthanide porphyrin complexes were first synthesized by Horrocks [6] as potential nuclear magnetic resonance probes and shift reagents. Adler [7] investigated lan- thanide-binding mechanisms by porphyrins. Buchler [8] synthesized a series of lanthanide porphyrins that have double-decker structures. These species produce near-infra- red charge transfer absorption bands whose frequencies are inversely proportional to the lanthanide ion radius. A great majority of publications on lanthanide porphyrins are focused on their medical applications as magnetic reso- nance imaging contrast agents [9]. Gadolinium texaphyrin, an expanded porphyrin ring species, is being investigated for cancer therapy as an efficient radiation sensitizer in addition to its role as a contrast agent [10]. The coordina- tion of lanthanide porphyrins with chiral amino acids was detected using circular dichroism spectroscopy [11]. The groups at Minsk [12,13] and Seattle [14] were the first to evaluate the luminescence spectral properties of lanthanide porphyrins. Both groups found that the lantha- nide porphyrins have similar absorption spectra with only 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.042 * Corresponding author. Fax: +206 685 8665. E-mail address: gkhalil@u.washington.edu (G.E. Khalil). www.elsevier.com/locate/cplett Chemical Physics Letters 435 (2007) 45–49