J . Phys. Chem. 1993,97, 3396-3400
3396
Effect of a-System Spacers on Exchange Couplings and End-to-End Encounter Rates in Flexible Biradicals Malcolm D. E. Forbes Department of Chemistry, Venable and Kenan Laboratories, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received: October 5, 1992; In Final Form: December 14, 1992
Time-resolved electron paramagnetic resonance (EPR)studies of flexible biradicals containing meta- and para-linked aromatic spacers (aryl ethers) between the unpaired spins are reported. Spectral simulation allows determination of the scalar electronic spin-spin coupling J and the end-to-end encounter rate kenbetween the two radical centers. The biradicals have different J couplings compared to saturated polymethylene chain biradicals of similar length. The value of ken obtained for the para isomer was an order of magnitude lower than that of the saturated system, while for the meta isomer it was comparable. Both J and ken show large changes with temperature for the para isomer (50 O C vs 105 “C). The results are discussed in terms of through-bond (TB) and through-solvent (TS) coupling mechanisms. Semiempirical MO calculations of overlap integrals support the hypothesis that the TB component should be weaker for the meta isomer. It is concluded that the TS component of the J coupling is dominant for both biradicals at these elevated temperatures.
SCHEME I
Introduction
Electronic interactions in high-spin molecules are a subject of continued interest,’ especially because species such as biradicals are simple model compounds for molecules undergoing more complicated kinetic processes involving weakly coupled orbitals such as electron and energy transfer.2 Biradicals are easily produced in solution from ketones,diazo compounds, or sulfones3 and can be detectedby a growing array of experimentaltechniques with both fast time response and high structural res~lution.~ Flexible biradicals have been under intensestudy in our laboratory and in several others during the past decade,3v5and we have made extensive use of time-resolved magnetic resonance methods to observe such intermediates in liquid solutions and in real time.6 Detection of these species provides a unique opportunity to examine the kinetic and magnetic properties of highly reactive flexible chains and at the same time gain insight into the relative role of the two mechanisms by which scalar exchange coupling can take place, Le., through-bond (TB) or through-solvent (TS). In a separate manuscript, we have shown that these mechanisms have opposite temperature coefficients, allowing a separation of the chain length dependence of each mechanism in saturated polymethylene chain biradicals.’ Also, we have recently demonstrated that the presence of localized unsaturation in flexible biradicals can substantially alter the relative contribution of TB and TS couplings.8 Our magnetic resonance studies of novel biradicals have now been extended to more rigid systems containing aromatic spacers between the unpaired electrons. We report here the results of time-resolved electron paramagnetic resonance (TREPR)experiments on such biradicals at different temperatures and compare their magnetic and kinetic properties to each other and to their saturated analogues. Aromatic spacers can affect both the chain dynamics (end-to-end encounter rates) and the scalar exchange couplings, and we will use these parameters to determine which of the two coupling mechanisms is dominant in these species. Two ketocyclophanes were synthesized,which upon photolysis yielded biradicals with aryl ether linkages between the unpaired electrons, as shown in Scheme I. Norrish I a-cleavage of the ketone triplet state of 1 or 2 leads to an acyl-alkyl biradical, which then rapidly decarbonylates to a bis(alky1) type biradical l a or 2a. Depending on the point of attachment (meta or para)
1
la
2
2a
the TB coupling may be very different, and we will show molecular orbital calculations below that support this. Additionally, a change in the position of attachment will alter the distribution of end-to-end distances, therefore the TS mechanism is also expected to be different for the two isomers. Based on previous studies of saturated biradicals, the relative change in the magnitude of the TS vs TB mechanismswill be different for each isomer at different temperatures.6.’ Quantitative investigation of the contribution from each coupling mechanism is the goal of our TREPR experiments. There is evidence that the TB interaction decreases with increasing temperature because of a larger number of conformers containing gauche but the TS interaction increases due to the contribution of shorter endto-end distances to the average exchange coupling ( J ) . Faster encounter rates, ken,are also expected at higher temperatures, although in previous studies this parameter was not found to be extremely sensitive to temperat~re.~By making the central portion of the molecule rigid as in these biradicals, such a sensitivity might be observed. The solution TREPR spectra of such ‘semirigid” biradicals obtained at different temperatures should therefore be informative with regard to both static ((J))and dynamic ( k e n )properties.
0022-3654/93/2091-3396$04.0~/~ 0 1993 American Chemical Society
Effect of *-System Spacers on Flexible Biradicals
The Journal of Physical Chemistry, Vol. 97, No. 13, 1993 3397
Experimental Section All starting materials were obtained from Aldrich and were used as received. Syntheses of the meta and para keto dioxycyclophanes were carried out by stirring equivalent amounts of 1,9-dibromo-4,4,6,6-tetramethylnonan-5-0ne~~ and either resorcinol or hydroquinone at high dilution (
