Design of donor-acceptor small molecules based on diazaisoindigo unit: Synthesis, theoretical calculations and photophysical studies

Two donor-acceptor-donor (D–A–D) and two donor-acceptor (D-A) small molecular systems involving alkyl bithiophene and thiophene derivatives as donor blocks and 5,5′- and 5-linked 7,7′-diazaisoindigo (DAIID) as acceptor blocks have been synthesized and investigated with theoretical calculations and photophysical studies. The incorporation of the thiophene groups led to significant changes in the UV–vis absorption spectra and electronic structure of the diazaisoindigo. Bithiophene and thiophene derivatives did not show fluorescence, in contrast to their dibrominated and brominated precursors. This fact was attributed to both a decrease of the radiative rate constant and an increase of the non-radiative deactivation associated to the incorporation of the thiophenyl groups. Additionally, picosecond transient absorption experiments revealed short time decays for the thiophenyl derivatives indicating a faster non-radiative deactivation associated to a rotation of the central double bond of 7,7′-diazaisoindigo in the excited state.