Microwave-assisted synthesis and spectroscopic properties of 4'-substituted rosamine fluorophores and naphthyl analogues

A series of rosamine fluorophores was obtained by reaction of the appropriate benzaldehyde and 3-(diethylamino)phenol followed by an oxidation step. Both conventional heating and microwave irradiation methods were utilized, and higher yields in a remarkably shorter period of time (10 min) were achieved by using the microwave irradiation protocol under closed vessel conditions (80 °C). The nitro-substituted rosamine was further converted into the corresponding amino derivative through Pd/C-catalysed hydrogenation. The synthetic protocol used for rosamines was also successfully employed to synthesise naphthyl analogues by using 1-and 2-naphthaldehyde. In the latter case, a novel hydroxy analogue was also obtained as a minor product. The photophysical behaviour of all the rosamines was studied in different solvents to rationalize the influence of the substituent groups on the electronic distribution, the effect of steric hindrance caused by the naphthyl ring, and the effect of the solvent. The results demonstrate that the introduction of different substituents in the periphery of the rosamine framework (in particular NO ₂ and NH ₂ groups) alters the fluorescence properties of the molecule such as emission wavelength and fluorescence quantum yield. All the rosamines are shown to be more fluorescent in dichloromethane than in more polar solvents such as ethanol. The spectroscopic properties of the 4'-carboxy-substituted rosamine, which is particularly attractive for labelling purposes, was also studied at physiological pH.