Stable Free Radicals. 12. New Aspects of the Behavior of 1 -Alkyl-4-(carboalkoxy)- and 1 -Alkyl-4-carbamidopyridinyl Radicals in Solution and in Thin Films

New techniques (thin-film spectroscopy, spectroelectrochemistry) have led to (a) revised absorption coefficients and maxima for 1 -alkyl-4-(carbomethoxy)pyridinyl radicals (4.), (b) stable solutions of 1 -alkyl-4-carbamidopyridinyl radicals [(CONH₂)4] (in the presence of LiC10₄), (c) variation of the visible π-mer absorption with alkyl group size for 4 in thin films at 77 K, (d) reversible dimerization of 4 in thin films (dimerization, >-100 °C; dissociation of dimer, >-54 °C), (e) photodissociation of covalent dimers of 4 to monomeric radicals, and (f) ester conformations as the origin of the “splitting” in the 395-nm band observed for 4 radicals. A novel vacuum variable-path-length UV-vis-near-IR cell (“VV-cell”) is described in detail along with its application to the problem of the concentration dependence of the absorption spectra of pyridinyl radicals in solution. The results provide a paradigm for the study of stable but reactive intermediates in concentrated form (thin films) or in solution.