Abstract |
The synthesis of a novel series of 5,5 -sulfonamide thioindigo derivatives, including a new synthetic methodology (via microwave assisted synthesis), together with a comprehensive electronic spectral and photophysical characterization is described. The study made in solution, as a function of the solvent polarity, showed-rationalized by TDDFT calculations- that in nonpolar solvents the high efficiency of the radiative deactivation channel (50\% in dioxane and 70\% in benzene) is due to an S-1 (of pi,pi* origin) in the trans form, whereas in polar solvents the low fluorescence quantum yield (1\% or less) is due to the adiabatic stabilization of the trans and saddle S-1 forms, making the sloped type CI (Conical Intersection) with S-0 energetically accessible thus becoming an efficient competitive decay channel to the radiative pathway. It is shown that thioindigo and all the 5,5 -sulfonamide thioindigo derivatives exhibit on-off fluorescence that can be tuned by the solvent polarity. The presence of a dark cis form with an S-1 (of n,pi* origin) was confirmed by the appearance of a new absorption band, upon irradiation, assigned by TDDFT to an S-2 (of pi, pi* origin) state. |