Application of 13C chemical shifts and relaxation times to the study of charge distribution, aggregation, and conformation in carbocyanine dyes. Part I

Abstract

13 C N.m.r. spectra are reported for a variety of symmetrical carbocyanine dyes. Changes in concentration and solvent have minor effects on chemical shifts relative to the total spectral width; chemical shifts can be accounted for primarily in terms of internal dye properties. Qualitative agreement within each dye of chemical shifts with calculated charge densities was found, and the chemical shifts of C-2 could be ordered according to the Brooker dye basicities except for a reversal of 2,2′-diethyloxacarbocyanine and 2,2′-diethylthiacarbocyanine. Preferential tumbling of monomeric thiacarbocyanine about its long axis is indicated by a comparison of the spin–lattice relaxation times of carbons bearing one proton. Higher concentrations and lower temperatures shortened the relaxation times as would be expected for a higher degree of dye dimerization or aggregation. Possible implications about the structure of dye dimers are discussed. Single-frequency decoupling experiments permitted a reassignment of signals in the proton spectrum.