Structure and wavelength modification in retinylidene iminium salts

The spectroscopic and structural properties of the perchlorate and triflate salts of N-n-butyl-retinylidene imine, 2 and 3, have been examined 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, lambda(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 crystallography. The cations in each of the salts were shown to have very similar conformations, detailed structures, and packing in their crystal lattices. The differences in the spectroscopic properties of the salts in the solid state could not be accounted for on the basis of any structural differences in the cations themselves. In terms of cation-anion 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 suggestive that the wavelength and positive charge delocalization in retinylidene iminium salts are controlled by variation of the distance between the anion and the proton bonded to the Schiff base nitrogen atom. The work reported here represents the first examples of secondary retinylidene iminium salts containing an N-alkyl substituent to be successfully analyzed by X-ray crystallography. The relationship of these observations in the solid state to the spectroscopic properties of the natural visual pigments is explored.