STRUCTURE AND WAVELENGTH MODIFICATION IN RETINYLIDENE IMINIUM SALTS

The spectroscopic and structural properties of the perchlorate and tri flate salts of N-n-butyl-retinylidene imine, 2 and 3, have been examin ed in solution and solid phases. In solution these salts were found to exhibit very similar UV and NMR spectroscopic properties. However, in the solid state marked differences in their absorption spectra (2, la mbda(max) = 504 nm; 3, lambda(max) = 445 nm) and C-13 NMR spectra were found. The structures. of the two salts were determined by X-ray crys tallography. The cations in each of the salts were shown to have very similar conformations, detailed structures, and packing in their cryst al lattices. The differences in the spectroscopic properties of the sa lts in the solid state could not be accounted for on the basis of any structural differences in the cations themselves. In terms of cation-a nion interactions, a strong hydrogen bonding interaction was found in each case between the N-H proton and an oxygen atom of the counterion. However, there were significant differences between the two salts in terms of the N-H ... O internuclear distances (2, N-H ... O = 2.939(7) Angstrom and in 3, 2.85(1) Angstrom). The results are strongly sugges tive that the wavelength and positive charge delocalization in retinyl idene iminium salts are controlled by variation of the distance betwee n the anion and the proton bonded to the Schiff base nitrogen atom. Th e work reported here represents the first examples of secondary retiny lidene iminium salts containing an N-alkyl substituent to be successfu lly analyzed by X-ray crystallography. The relationship of these obser vations in the solid state to the spectroscopic properties of the natu ral visual pigments is explored.