Donor-acceptor architectures of tetraphenylethene linked aza-BODIPYs: Synthesis, crystal structure, energy transfer and computational studies

This work is focused on the design and synthesis of donor-acceptor pairs comprising of aza-BODIPY and tetra-phenylethene (TPE) fluorogens with large pseudo Stokes shifts (386-424 nm). Femtosecond transient absorption spectroscopy, single crystal X-ray technique and TD-DFT studies were used to get more insight about their spectral behavior. As compared to the tetra-phenylaza-BODIPY, the four aza-BODIPYs exhibited 50-90 nm red shifted absoprtion and emission spectra. When the donor-acceptor architectures were excited at 300 nm, the aza-BODIPY unit emitted strong fluorescence in red region. This suggested an intramolecular energy transfer between donor (TPE) and the aza-BODIPY acceptor; with 81-90% energy transfer efficiencies. Femtosecond transient absorption studies also supported energy transfer process in the molecules, with 3.7 ps time constant associated with it. Aggregation studies in THF/n-heptane solution showed hypsochromic shifts in the aggregated form; Xray packing structures indicated H-type aggregates (slip angle > 54.7 degrees) in the solid state. Scanning electron microscopy (SEM) measurement revealed 200-700 nm size sphere shaped structures in the aggregated forms.