Sonochemical synthesis, optical properties and DFT studies on novel (N-arylamino)phenothiazinium dyes suitable for fluorescence cells imaging

Novel (N-arylamino)phenothiazinium dyes containing meta-substituted-arylamine auxochrome units were successfully obtained by applying a sonochemical protocol designed for a more efficient energy usage in the preparation of methylene blue (MB) analogues. Single crystal X-ray diffraction analysis revealed the spatial arrangement in aggregated crystalline state of (N-(meta-bromoaryl)amino)phenothiazinium dye with minor variances induced by the nature of the halogenide counterion (iodide or chloride). The optical UV-vis properties of the novel (N-arylamino)phenothiazinium dyes were comparable to those of the parent MB, with the longest wavelength absorption maxima situated in the visible range (640-680 nm), large molar extinction coefficients (log ε = 4.5-5.1) and weak solvatochromism in polar solvents. Their fluorescence emission in solid state was evidenced by One Photon Excited Fluorescence Lifetime Imaging (OPE-FLIM) and Two Photon Excited Fluorescence Lifetime Imaging (TPE-FLIM) experiments. Theoretical calculations based on Time Dependent-Density Functional Theory (TD-DFT) at B3PW91 and CAM-B3LYP/def2-SV(P) level of theory predicted absorption and fluorescence emission wavelength maxima in reasonable agreement with experimental data. Computational results suggest that the electronic excitations imply a departure from the planar molecular ground state towards geometrically rearranged excited states disfavoring the vibronic couplings due to a high degree of flexibility induced by the conformational motion of the N-arylamino auxochromes. Preliminary studies regarding the dyes' relevance in biological environment indicated lipophilicity (log P octanol/water 0.5-2.3), no aggregation tendency in diluted solutions in the concentration range 10-50 microM and ability for cytoplasmatic staining of D407 human retinal pigment epithelial cells.