Optical Properties and Excited-State Dynamics of Safranin O Monomers and Dimers in Aqueous Solution

Phenazines, phenoxazines, and phenothiazines are a structurally similar set of compounds that exhibit antibiotic and photocatalytic activity and that have been used as histological stains and optical sensors. Safranin O (SO) is a phenazine that has been found useful to quantify polysaccharides in cartilage and that has been shown to have a unique ability to inhibit ice growth. In both of these applications, the high concentration of SO can lead to aggregation, which can result in drastic changes in optical properties and excited-state dynamics. In this work, we characterize the optical properties, structure, and excited-state dynamics of SO monomers and dimers in dilute and concentrated aqueous solution. We use a combination of steady-state and time-resolved absorption and fluorescence spectroscopies along with density functional theory (DFT) and time-dependent DFT. We find that the monomer exhibits an appreciable absorption strength of ∼7 D and the aggregates, which we assign to dimers, form at concentrations exceeding 10 μM. The dimer absorption spectrum exhibits a modest blueshift with respect to the monomer, which we attribute to face-to-face (so-called H-aggregate) packing later confirmed by DFT. The monomer exhibits a large Stokes shift (∼2300 cm^-1), small fluorescence quantum yield (∼0.05), and appreciable singlet excited-state lifetime (∼1 ns). The dimer exhibits a drastically reduced lifetime of ∼4 ps. Our work adds to an existing body of work suggesting that aggregation-induced quenching may be a general phenomenon in phenazines, phenoxazines, and phenothiazines, which is why these compounds may have not found widespread use in applications involving fluorescence emission and why aggregates of these compounds may potentially be advantageous for applications involving rapid, local heat generation.