Investigations of excited-state relaxation processes in 2′- hydroxychalcone analogues having pyrrole and indole moieties based on emission and transient absorption measurements

Abstract

Abstract Excited-state relaxation processes of five types of 2′-hydroxychalcone (HC) analogues having naphthol, pyrrole and indole chromophores were studied based on emission and transient absorption measurements. Fluorescence with large Stokes-shifts was observed from these compounds in solution and the solid state, indicating occurrence of excited-state intramolecular hydrogen atom transfer (ESIHT) as observed for HC in solution. These fluorescent species, were, therefore, considered to be the corresponding tautomer (cis-keto form) in the excited singlet (S1) states formed via ESIHT. The fluorescence quantum yields were larger by the magnitude of one order than that of HC. Upon 400 nm laser pulsing in benzene solution of these HC analogues, no transient absorption spectra were obtained for the analogues having the phenolic moiety (Ph@Py and Ph@Ind) whereas rise and decay profiles of transient spectra were recorded independently of the amount of the dissolved oxygen for HC analogues substituted with the naphthol moiety (Np@Py, Np@Ind and Np@Np). On the analogy of the deactivation processes of HC, the obtained transient species were suggested to be the corresponding isomer of the tautomer (trans-keto form) in the ground state due to the isomerization of the excited state cis-keto form. The deactivation processes in the excited states of these compounds were discussed by considering energetic reaction diagrams for the tautomers and isomers. Replacement of the phenol moiety of HC with the naphthol was demonstrated to affect the excited state properties of the tautomer.