Spectral Dynamics of Nitro Derivatives of Xanthione in Solutions

Nitro derivatives of xanthione, 2,7-dinitro-9 H-xanthene-9-thione and 2,4,7-trinitro-9 H-xanthene-9-thione, have been first synthesized and their stationary and transient spectra have been measured. The stationary spectra show that the attachment of the nitro groups to the xanthione scaffold leads to strong quenching of S₂ → S₀ fluorescence and the decrease of the oscillator strength of the S₂ ← S₀ electronic transition. Analysis of the transient absorption spectra uncovers the ultrafast stimulated emission quenching from the second excited state, S₂, in the both derivatives. A kinetic scheme has been suggested to rationalize the complex spectral dynamics of the transient absorption signal. The kinetic scheme is deduced from the analysis of the transient spectra and supported by the quantum-chemical calculations, which predict the existence of a dark state and S₂ state splitting into two close levels. The ultrafast transitions between S₂ state sublevels and the transition into the dark state play a crucial role in spectral dynamics. These new features discovered in the nitro derivatives of xanthione distinguish essentially their spectral dynamics from that observed in xanthione.