Synthesis, Photophysical Behavior, and Electronic Structure of Push-Pull Purines

Push-pull purines have been synthesized by the introduction of electron-accepting functional groups (A = CN, CO2Me, and CONHR) to the heterocyclic C(8) position to complement typical electron-donating substituents at C(2) (D-1) and C(6) (D-2). The donor-acceptor purines show significantly altered, and overall improved photophysical properties relative to their acceptor-free precursors (A = H); these include red-shifted (20-50 nm) absorption maxima, highly solvatochromic emission profiles (em lambda(max) from 355-466 nm depending on substitution pattern and solvent) with excellent linear correlations between emission energy and solvent polarity (E-r(N)), improved photochemical stability upon continuous irradiation, and enhanced (up to 2500%) fluorescence quantum yields. Comprehensive structure-property studies show how the absorption/emission maxima and quantum yields depend on donor and acceptor structure, relative donor position (C(2) or C(6)), and solvent (1,4-dioxane, dichloromethane, acetonitrile, methanol, and in some cases water). Further insight regarding electronic structure comes from a quantitative treatment of the solvent-dependent emission data (that provides Delta(mu go) values ranging from 1.9 to 3.4 D) and DFT (B3LYP/6-311++G**) electronic structure calculations. X-ray crystal structures of several derivatives showcase the molecular recognition capabilities of the donor-acceptor chromophores that overall have photophysical and structural properties suitable for applications in biosensing and materials.