Photophysical and crystallographic study of three integrated pyrazolo[1,5-a]pyrimidine–triphenylamine systems

Abstract Three new intramolecular charge transfer (ICT) fluorophores having triphenylamine and pyrazolo[1,5-a]pyrimidine moieties 4a-c were synthesized, and their structures were solved by X-ray crystallography (XRC). Compounds 4a, 4b and 4c crystallize in the tetragonal P42/n, triclinic P-1 and monoclinic P21/c space groups with calculated CE-B3LYP structural energies of -104.3, -125.6 and -123.8 kJ/mol, respectively. Substituents effect on molecular and photophysical properties of 4a-c was studied in both solution and solid-state through fluorescence, UV–vis, XRC, and computational methods. The 2-phenyl (4b) and 2-anisyl (2c) derivatives showed better absorption coefficient (4b, e = 76400 → 119600 M-1cm-1 and 4c, e = 66200 → 89200 M-1cm-1) than 4a (2-Me, e = 9933 → 21667 M-1cm-1), while the relative quantum yield (ϕ) in solvents of diverse polarity is as high as ϕ = 0.98 for 4a, ϕ = 0.86 for 4b and ϕ = 0.83 for 4c. For these dyes, Lippert–Mataga correlation were used to estimate the difference between the excited and ground state dipole moments (Δμ), being 4b the one that suffer the bigger changes with a Δμ οf 26.9 D. Probe 4c is found to be useful as a fluorescent indicators for the qualitative sensing of water content in organic solvents. The solid-state emission data reveal that the antiparallel molecular packing of the crystal structure for 4a-c, with energy framework diagrams influenced mainly by dispersion forces, could disturbs the photophysical properties by changing the donor-acceptor intramolecular coupling. Therefore, the combination of these XRC and photophysical results may constitute in a key input for designing applications in material science.