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The scaling of the cubic nonlinearity gamma with chain length in polyenic molecules has received considerable theoretical attention. Earlier experimental investigations have been restricted to oligomers with fewer than 20 double bonds because of problems associated with the synthesis and solubility of conjugated molecules. These synthetic difficulties have been overcome in the present study by the use of modern living polymerization techniques. Solution measurements of gamma as a function of chain length in long-chain (up to 240 double bonds) model polyene oligomers are reported. A saturation of the increase of gamma with chain length is observed, and the onset of this saturation occurs for chain lengths considerably longer than predicted from theory.

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The diradical 1,1′,5,5′-tetramethyl-6,6′-dioxo-3,3′-biverdazyl, 6, crystallizes in a monoclinic unit cell with a ) 4.224(1), b ) 17.378(2), c ) 7.529(1) Å, ) 96.02(1)°, space group P2 1 /c (No. 14). The crystal structure of 6 shows local D 2h symmetry. ESR measurements in a frozen chloroform solution indicate that isolated 6 has a singlet ground state with a triplet excited state 760 cm -1 (0.094 eV) above the ground state with zerofield splitting parameters D ) 0.038 cm -1 and E ) 0.0016 cm -1 . Semiempirical calculations (AM-1) suggest that 6 is twisted in solution. Crystalline 6 shows a temperature-activated ESR signal with no features characteristic of an isolated triplet. Strong intermolecular π-stacking interactions prevent the analysis of this temperature activation in terms of intermolecular and intramolecular exchange parameters.