Electronic Structure and Excited-State Dynamics of Safranin-O Monomers and Dimers
Abstract
Safranin-O (SO) is a cationic dye commonly used as a biological stain in histology and as a redox indicator in analytical chemistry. At sufficiently high concentrations, SO has recently been shown to inhibit ice growth with an activity similar to antifreeze glycoproteins. In concentrated solutions, monomers of SO assemble into aggregates, such as dimers, that exhibit distinct, emergent properties. Here, the electronic structure and dynamics of SO monomers and dimers were investigated using a combination of steady-state and time-resolved optical spectroscopies and quantum chemical calculations. UV-Vis absorption spectroscopy showed a slight blueshift in concentrated SO solutions compared to dilute SO solutions, which was consistent with the formation of aggregates that exhibit face-to-face packing (so called H-aggregates). Time-correlated single photon counting (TCSPC) measured an excited-state lifetime of the monomer of 1.06 ns. Transient absorption (TA) confirmed this value with an estimated lifetime of 1.01 ns and showed that the dimer lifetime was drastically reduced to ~5 ps. Guided by the dimer spectrum, density functional theory (DFT) calculated a nearly ideal H-aggregate structure for the dimer, which was consistent with the packing observed in the single crystal. This work represents an important first step toward understanding how the rapid generation of heat by SO aggregates may play a role in various applications, including ice inhibition.
Electronic Structure and Excited-State Dynamics of Safranin-O Monomers and Dimers
Safranin-O (SO) is a cationic dye commonly used as a biological stain in histology and as a redox indicator in analytical chemistry. At sufficiently high concentrations, SO has recently been shown to inhibit ice growth with an activity similar to antifreeze glycoproteins. In concentrated solutions, monomers of SO assemble into aggregates, such as dimers, that exhibit distinct, emergent properties. Here, the electronic structure and dynamics of SO monomers and dimers were investigated using a combination of steady-state and time-resolved optical spectroscopies and quantum chemical calculations. UV-Vis absorption spectroscopy showed a slight blueshift in concentrated SO solutions compared to dilute SO solutions, which was consistent with the formation of aggregates that exhibit face-to-face packing (so called H-aggregates). Time-correlated single photon counting (TCSPC) measured an excited-state lifetime of the monomer of 1.06 ns. Transient absorption (TA) confirmed this value with an estimated lifetime of 1.01 ns and showed that the dimer lifetime was drastically reduced to ~5 ps. Guided by the dimer spectrum, density functional theory (DFT) calculated a nearly ideal H-aggregate structure for the dimer, which was consistent with the packing observed in the single crystal. This work represents an important first step toward understanding how the rapid generation of heat by SO aggregates may play a role in various applications, including ice inhibition.