Chapter 17
Very High Spin Polyradicals
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Andrzej Rajca Department of Chemistry, University of Nebraska, P.O. Box 880304, Lincoln, NE 68588-0304
I. Introduction Synthesis and study of mesoscopic size molecules which are relevant to properties at the interface between the condensed phase and small molecule is one of the interdisciplinary frontiers in organic chemistry (1). Recent advances in organic synthesis and characterization allow for preparation and study of large organic molecules with large numbers of "unpaired" electrons. Such molecules address fundamental aspects of electronic structure and are relevant to search for novel materials (2-6). This chapter is focused on macrocyclic high-spin polyradicals. Well-defined molecular mesoscopic-size high-spin polyradicals will test the possibility of maintaining strong magnetic coupling over large distances in organic molecules and will provide novel opportunities for studying magnetic phenomena on nanometerscale within a single molecule. Such magnetic phenomena in mesoscopic-size systems are of both fundamental and future technological interest (7-10). Macrocyclic polyradicals are building blocks (cross-linking units) for network polyradicals. Development of synthetic routes toward two- and three-dimensional network polyradicals is a key stepping stone to organic ferromagnetism with high Curie temperature (T ) (2). c
n. Prerequisites for a Very-High-Spin Polyradical (2) Spin coupling in 7C-conjugated polyradicals is primarily controlled by molecular connectivity (77,72), which may be discussed in terms of ferromagnetic coupling units (fCU's), antiferromagnetic coupling units (aCU's), and spin sites (2,3). The well-known spin coupling units are 1,3-phenylene (fCU) and 1,4-phenylene (aCU); e.g., the triplet ground state Schlenk Hydrocarbon may be viewed as two triarylmethyl spin sites connected to a fCU and the singlet ground state Thiele Hydrocarbon contains two spin sites connected to an aCU (Figure 1). The energy difference between the singlet and triplet states (AE^) in a diradical measures the strength of the spin coupling. Selected organic diradicals possess very
0097-6156/96/0644-O258S15.00/0 © 19% American Chemical Society
In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by NORTH CAROLINA STATE UNIV on September 14, 2013 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch017
17. RAJCA
Very High Spin Polyradicals
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strong ferromagnetic coupling (e.g., in trimethylenemethane, AE^ = 15 kcal/mol) (13); for a relatively "spin-diluted" Schlenk Hydrocarbon, an example of 1,3connected polyarylmethyl diradical, AE^ > 1 kcal/mol is estimated (2). 3,4'-Biphenylene-based polyarylmethyl diradicals are expected to possess ferromagnetic coupling (triplet ground state) (14); their fCU may be viewed as sequential connection of a fCU and an aCU (Figure 2) (2). Application of an empirical relationship between spin density distribution and strength of spin coupling predict that strength of ferromagnetic coupling will decrease by factor of 5 - 6, when 1,3-phenylene is replaced with 3,4-biphenylene in a polyarylmethyl diradical (2,75). The conceptual extension from diradicals to high-spin polyradicals is attained by connecting, in an alternating mode, the fCU's and the spin sites (Figure 3). [In particular, fCU's coordinated with three spin sites may correspond to 1,3,5-phenylyne or 3,5,4-biphenylyne.] This approach was applied to construct series of polyarylmethyls, from diradicals with two spin sites to polyradicals with 31 spin sites, using either star-branched or dendritic connectivity (2). [The Iwamura group used analogous approach to construct linear and star-branched polycarbenes (76).] Preparation of very-high-spin polyradicals may be affected by defects, which we define as those spin sites where an "unpaired" electron has failed to be generated. Probability of having polyradicals with defects increase rapidly with the number of spin sites. Therefore, it is essential to maintain ferromagnetic coupling between the "unpaired" electrons in the presence of defects, if very-high-spin polyradicals are to be attained. In acyclic systems, only a single pathway links any pair of spin sites; therefore, even a single defect at an inner site may interrupt ^-conjugation (and ferromagnetic coupling), leading to a mixtures of spin systems with drastically lower than expected spin values (Figure 4) (77). The recently synthesized macrocyclic polyradicals, which possess at least two spin coupling pathways between any pair of spin sites, show great promise in addressing the problem of defects (18). ID. Polymacrocyclic Polyradicals Annelation of macrocytic polyradicals provides polymacrocyclic structures with greater resistance to defects. The resistance to defects may be evaluated using parameter "Q". We define "Q" as percentage of those polyradicals with all "unpaired" electrons ferromagnetically coupled; i.e., each of those polyradicals consists of a single spin system. Assuming that the yield per site for generation of an "unpaired" electron is 95 %, which is a typical value for non-dendritic polyarylmethyl polyradicals, the calculated values of "Q" decrease rapidly with the number of sites for macrocyclic polyradicals, but very slowly for polymacrocyclic polyradicals. Therefore, polymacrocyclic structures are promising targets for very-high-spin polyradicals (Figure 5). IV. Elongated Shape Very-High-Spin Molecular Polyradicals A fascinating feature of mesoscopic polyradicals (> 10 spin-coupled electrons) is the possibility of observing magnetic phenomena associated with anisotropy barrier (E ) in a single molecule (e.g., superparamagnetism). The elongated shape of the polyradical would increase E by magnetic dipole-dipole interactions (79). Estimates of this effect in polyarylmethyl polyradicals can be found in our previous work (77). A
A
In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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MOLECULE-BASED MAGNETIC MATERIALS
P
Downloaded by NORTH CAROLINA STATE UNIV on September 14, 2013 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch017
Schlenk Hydrocarbon
n
Ph Ph
Figure 1. Spin coupling in Schlenk and Thiele Hydrocarbons.
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Figure 2. Spin coupling in 3,4'-biphenylene-based diradical.
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