Phthalonitriles with heterocyclic substituents via direct CC bond formation: synthesis, spectral-luminescent properties and DFT study

A novel, efficient, and ligand-free synthetic methodology for the preparation of heterocycle-substituted phthalonitriles via Pd-catalyzed Suzuki-Miura cross-coupling has been developed. The optimized reaction conditions employ 5 mol % PdCl 2 as the catalyst, KOAc as the base, and DMA as the solvent at 100 °C, affording target in excellent to good yields (74 to 93%). A series of mono- and di-heterocycle-substituted phthalonitriles bearing furan and thiophene moieties, along with their nitro and 4-propylphenoxy derivatives, were successfully synthesized and fully characterized by ATR-FTIR, GC-MS, NMR spectroscopy, and DSC/TGA analyses. Direct C H functionalization attempts were unsuccessful due to competing cyclotetramerization. Monoheteryl-substituted phthalonitriles exhibit high fluorescence quantum yields (up to 39.2%) attributed to their rigid, nearly planar molecular geometry, whereas bis -heterocycle derivatives show significantly reduced quantum yields due to steric congestion. Nitro-substituted compounds display pronounced bathochromic shifts (20 to 35 nm) arising from intramolecular charge transfer from the electron-donating heterocycle to the nitro acceptor, alongside substantial fluorescence quenching. The 4-propylphenoxy derivatives exhibit bimodal fluorescence spectra, consistent with two distinct, partially independent fluorophore systems. DFT and TDDFT calculations, including GIAO NMR chemical shift predictions, and potential energy surface analysis, reveal that bis -furyl and bis -thienyl substituted phthalonitriles may exist as syn - and anti -conformers with moderate rotation barriers (11 to 13 kJ/mol). The calculated rotation rate constants (∼10 10 s −1 ) explain the observation of averaged sharp signals in NMR and broad unresolved bands in UV–Vis spectra, providing comprehensive insight into the structure property relationships of these compounds.