Synthesis and photophysical studies of an indigo derivative: N-octyl-7,7′-diazaindigo

In this paper, we explore the synthesis, characterization, and photophysical properties of a novel indigo derivative, N-octyl-7,7′-diazaindigo, being the first time that diazaindigos have been studied as photophysically-active chemical entities. Reduction of the neutral “keto-form” to the so-called “leuco-form” changes the global spectroscopic and photophysical behaviors. Both species have been investigated by different photophysical studies, such as analysis of absorption and emission spectra, fluorescence quantum yields (Φ(F)) and lifetimes. Finally, to appraise in depth the deactivation of the excited state of the keto form, femtosecond transient absorption (TA) experiments and Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations were performed. In an organic aprotic solvent (N,N-dimethylformamide), TA experiments showed a fast deactivation channel (τ(1) = 2.9 ps), which was ascribed to solvent reorganization, and a longer decay component (τ(2) = 86 ps) associated with an internal conversion (IC) process to the ground-state, in opposition to the excited state proton transfer (ESPT) mechanism that takes place in the indigo molecules but in protic solvents. A comparative study was also carried out on the parent molecule, 7,7′-diazaindigo, corroborating the previous conclusions obtained for the alkyl derivative. In agreement with experimental observations, DFT and TD-DFT calculations revealed that the deactivation of the S(1) state of the keto form takes place through an internal conversion process.