Tracking Ultrafast Fluorescence Switch-On and Color-Tuned Dynamics in Acceptor-Donor-Acceptor Chromophore

Understanding how the conformational change of conjugated molecules with acceptor-donor-acceptor (A-D-A) architecture affects their physical and optoelectronic properties is critical for determining their ultimate performance in organic electronic devices. Here, we utilized femtosecond transient absorption, time-resolved upconversion photoluminescence spectroscopy, and tunable femtosecond-stimulated Raman spectroscopy, aided by quantum chemical calculations, to systematically investigate the excited state structural dynamics of the intramolecular charge transfer of the tetramethoxy anthracene-based fluorophore 2,3,6,7-tetramethoxy 9,10-dibenzaldehydeanthracene (AnDA) and its derivative 2,3,6,7-tetramethoxy 9,10-diphenylanthracene (TMDPAn) in chloroform. In the AnDA molecule, the tetramethoxy anthracene and benzaldehyde moieties exhibit a strong ability to donate and withdraw electrons. Upon photoexcitation, AnDA shows intriguing ultrafast fluorescence switch-on and red shift dynamics on charge transfer states, and the temporal evolution of AnDA recorded by ultrafast spectroscopy reveals a dynamic picture of two-step intramolecular charge transfer assisted by ultrafast conformational changes and solvation processes. Removing the aldehyde group from TMDPAn significantly decreases the electron pulling capacity of the phenyl unit and disables charge transfer characteristics.