Solvatochromic characteristics of dansyl molecular probes bearing alkyl diamine chains

A series of dansyl-based fluorescent probes bearing linear alkyl-1,n-diamine chains of different length (DA_1.n, n = 2-8, 10, 12) was characterized in terms of the absorptive and emissive features in solvents of different polarity and hydrogen bond donor/hydrogen bond acceptor character. The probes show solvent-dependent absorption, a feature that is uncommon among dansyl derivatives. The dual emission of DA_1.n probes is strong in non-aqueous solvents and is influenced by the chain length and interactions with the solvent. Solvent effects on the spectral parameters were rationalized on the basis of the Kamlet-Taft and Catalán solvatochromic models, in order to quantify the degree of polarity-driven and hydrogen bonding interactions. A comparative discussion of the results predicted by the two models was made. In ground state, the DA_1.n probes act as hydrogen bond acceptors. In excited state, hydrogen bonding is less favoured, the solute-solvent interactions being governed by the increasing polarity of the solvent that results in a large bathochromic shift of the emission. A comparison was made with the spectral features previously reported for the corresponding series of bis-dansyl fluorescent probes (2DA_1.n).