Synthesis, stability in solutions, and spectral and thermal properties of alkyl-substituted 3,3'-bis(dipyrromethene) hydrobromides

3,3′-Bis(dipyrromethene) dihydrobromides (H2L · 2HBr) with various types of alkylation have been synthesized. On the basis of IR, 1H NMR, and electronic absorption spectra, spectrophotometric titration, and thermogravimetry, conclusions have been drawn about the influence of structural factors on the optical and thermal properties and salt stability in solutions. An increase in the degree of alkylation causes a significant high-frequency shift of the N-H stretching vibration band in the IR spectra, an upfield shift of the 1H NMR signals of NH protons, a decrease in the auxochromic effects of protons on the aromatic system of the chromophore, and an increase in the salt stability in solutions in the presence of nucleophilic reagents and in the thermal stability in air and argon.