Journal of Solid State Chemistry 159, 460 } 465 (2001) doi:10.1006/jssc.2001.9179, available online at http://www.idealibrary.com on Polyarylmethyl Polyradicals as Organic Spin Clusters Suchada Rajca and Andrzej Rajca Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 E-mail: arajca1@unl.edu Received March 20, 2001; accepted March 21, 2001 IN DEDICATION TO THE LATE PROFESSOR OLIVIER KAHN FOR HIS PIONEERING CONTRIBUTIONS TO THE FIELD OF MOLECULAR MAGNETISM This overview describes the design for very high-spin organic polyradicals (polyarylmethyls), i.e., organic molecules and poly- mers with large values of spin quantum number S in the ground state.  2001 Academic Press Key Words: magnetic properties; radicals; macrocycles; polymers. 1. INTRODUCTION Design and synthesis of nanometer-sized molecules with magnetic properties is one of the most exciting research areas in magnetism. Over the past decade, steady progress has been made in the synthesis and characterization of transition metal ion clusters (molecules) with large values of spin quantum number S. The highest value of S for transition metal ion clusters is   (1). In several clusters with relatively large values of S and zero-"eld splitting parameter D, superparamagnetic blocking and/or quantum tunneling of magnetization at low temperatures were discovered (2). When the barrier for inversion of magnetization (super- paramagnetic blocking), related to S D , signi"cantly exceeds thermal energy at room temperature (k¹ ), such clusters could function as single-molecule memory bits. Purely organic molecules with unpaired electrons, i.e., polyradicals, o!er an alternative approach to molecules with very large values of S (3}6). In this approach, multistep organic syntheses serve as a powerful tool for a reliable (though laborious) implementation of rational designs for such molecules. 2. DESIGN OF VERY HIGH-SPIN ORGANIC MOLECULES AND POLYMERS The most fundamental prerequisite for a high-spin (S'  ) organic molecule is attaining ferromagnetic coupling be- tween unpaired electrons (7). Connecting two spin sites for an unpaired electron to a ferromagnetic coupling units (fCUs) gives rise to a high-spin diradical with S"1 ground state (8). An example is the Schlenk hydrocarbon (9), in which two arylmethyl spin sites are connected to a 1,3- phenylene fCU. The conceptual extension to a high-spin molecule relies on connecting in an alternating mode spin sites and fCUs (Fig. 1). The goal is to build up large macromolecules with very large numbers of interacting spin sites. Well- de"ned molecular sizes and shapes are very desirable. Fractals with well-de"ned dimensions, highly annelated polymacrocyclic molecules, and polymers are among fascin- ating targets. 2.1. Ensembles of Alternating Spin Sites and fcus Simple molecular designs for ensemble of alternating spin sites and fCUs are linear, dendritic, and macrocyclic struc- tures (Fig. 2) (3). These structures could be modi"ed with functional groups for polymerization. More sophisticated designs involve molecules with multiple dendritic and mac- rocyclic modules as well as annelated macrocycles (10). For the linear and dendritic structures, attempts to obtain molecules and polymers with S'5 were not successful. Increasing the number of sites for unpaired electrons be- yond 7}10 in such molecules and polymers leads to more complicated mixtures of spin systems with relatively low values of S; e.g., for a dendritic pentadecaradical with 15 sites for unpaired electrons, an average spin value of   }  was found (Fig. 3) (11). To obtain the expected values of spin, generations of unpaired electrons at the radical sites must be perfect. A fail- ure in generation of an unpaired electron (i.e., a defect) interrupts -conjugation, and consequently the spin coup- lings. In linear or dendritic structures, there is only one pathway for -conjugation between the spin sites. A defect at one of the sites in the interior of the molecule will cut the -conjugated system into noninteracting segments, with low values of S (Fig. 3). However, a defect at one of the terminal 460 0022-4596/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.