Survey of Factors Determining the Circularly Polarised Luminescence of Macrocyclic Lanthanide Complexes in Solution JAMES I. BRUCE, 1 DAVID PARKER, 1 * STEFAN LOPINSKI, 2 AND ROBERT D. PEACOCK 2 1 Department of Chemistry, University of Durham, Durham, UK 2 Department of Chemistry, University of Glasgow, Glasgow, UK ABSTRACT The development of emissive lanthanide complexes as structural or reactive probes to signal changes in their local chiral or ionic environment has been inhibited by the lack of understanding of correlating structural and electronic spectral information. The definition of relatively rigid enantiopure macrocyclic lanthanide complexes, whose inter- and intramolecular exchange dynamics have been defined, offers scope for remedying this situation. Chiral axially symmetric lanthanide complexes in solution give rise to large emission dissymmetry values (g em ) in CPL spectra. The sign and magnitude of g em are determined by the degree of twist about the principal axis, which is predicted to be a maximum at 22.5°, and by the site symmetry and local ligand field. In particular, the polarisability of the ligand donor atoms, especially for any axial donor, is very important. Examples of each case are discussed for structurally related cationic Eu(III) complexes. Chirality 14:562567, 2002. Ó 2002 Wiley-Liss, Inc. KEY WORDS: europium; polarisation; emission; helicity; square-antiprism; coordination; enantiopure The present understanding of the correlation of struc- tural and electronic spectral information for lanthanide complexes in solution remains poor and for the most part is empirical in nature. 1 To some extent this lack of knowledge has inhibited the development of emissive lanthanide complexes as structural or reactive probes to signal the nature of the local ionic or chiral environment. The reason why our understanding remains very limited, notwithstanding the promulgation of robust theories re- lating local lanthanide symmetry to the electronic and chiroptical features associated with f-f transitions, 24 is linked directly to the absence of well-defined complexes whose structure and solution exchange dynamics have been studied in detail. These exchange processes may be intermolecular in nature, for example, involving dissocia- tive solvent or ligand exchange, or may be purely intra- molecular as a result of the conformational mobility of the primary ligand itself. Such processes are generally slow with respect to the timescale of absorption spectros- copyÐleading to added complexityÐbut may be fast with respect to the emission timescale (Eu/Tb-ms; Nd, Yb-ls). This leads to time-averaged spectral information that may be difficult to correlate with a particular structural feature. In seeking to clarify this situation, structurally homolo- gous series of complexes are required which are intrinsi- cally more conformationally rigid and are hepta-, octa-, or nona-dentate so that ligand exchange processes are in- hibited. Such rigidity is a feature of the behaviour of cer- tain macrocyclic complexes in solution. The series of metal complexes involving octadentate ligands based on tetraazacyclododecane have proved to be remarkably diverse in the nature and scope of their chir- optical, structural, and functional behaviour. 5,6 The archetypal ligand in this series is 1,4,7,10-tetraazacyclo- dodecane-tetra-acetate or DOTA, 1, (Scheme 1), which upon complexation with a lanthanide ion forms four ster- eoisomeric complexes, de®ned by the sign of the torsion angles about the axially symmetric ring NCCN (preferably close to 60°) and pendant arm NCCO groups. Thus, two pairs of enantiomers exist with a d (+ sign for each NCCN torsion angle) or k con®guration in the ®ve-ring NLnN chelates and a D (+ sign for each NCCO torsion angle) or L layout of the four pendant arms. These complexes interconvert in solution by independent arm rotation and by a slower, cooperative ring inversion process (typically 40 s )1 at 298 K). The two main stereoisomeric complexes have been termed square antiprismatic (twist angle ca. 40°) and twisted square antiprismatic (29°) structures, with differing helicities about the principal axis. 6 Typically, the lanthanide coordination number is made up to nine by the addition of a solvent molecule, occupying an axial site and capping one of the antiprism faces. 6,7 The introduction of a substituent a to each ring N leads to further series of isomeric complexes of differing sta- bility, with the C-4 symmetric complexes proving the most Contract grant sponsor: EPSRC Contract grant number: GR/M04594. *Correspondence to: Professor David Parker, Department of Chemis- try, University of Durham, South Road, Durham, DH1 3LE, UK. E-mail: david.parker@dur.ac.uk. Received for publication 9 July 2001; Accepted 17 September 2001 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/chir.10092 Ó 2002 Wiley-Liss, Inc. CHIRALITY 14:562567 (2002)