Decay kinetics of intramolecular charge transfer states in azafluoranthene derivatives bearing an amino group

Abstract Azafluoranthene derivatives bearing an amino group exhibit fluorescence in the visible region and have been shown to be incorporated into cells, suggesting their potential as fluorescent probes. In our previous study, the Stokes shifts of these derivatives with an amino group were found to vary depending on the polarity of the solvent, indicating that intramolecular charge transfer (ICT) from the amino group to the azafluoranthene frame occurs in their first excited (S 1 ) state. In this study, the excited state dynamics of azafluoranthene derivatives with and without an amino group were investigated by determining radiative, internal conversion (IC), and intersystem crossing (ISC) rate constants. The conformation of the amino group in the S 1 state was examined based on the solvent viscosity dependence of fluorescence intensity and quantum chemical calculations. It was found that the S 1 states of the azafluoranthene derivatives with an amino group in polar solvents were consistent with planar ICT (PICT) states, which are emissive, or twisted ICT (TICT) states, which are essentially non‐emissive. These findings provide a quantitative picture of how ICT states are reflected in excited state decay kinetics and offer a basis for the rational design of environment‐sensitive fluorescent probes.