Non-classical donor–acceptor–donor chromophores. A strategy for high two-photon brightness

Abstract

Rod-like π-expanded bis-imidazo[1,2-a]pyridines were designed and synthesized. Strategic placement of a central electron-poor unit at position C3 of the imidazo[1,2-a]pyridine core resulted in donor–acceptor–donor quadrupolar systems. We further demonstrated the applicability of the Ortoleva–King–Chichibabin tandem process in the preparation of complex imidazo[1,2-a]pyridines. The monomer model heterocycles, differing by substitution at C2 and C3, displayed luminescence properties marginally dependent on the substitution pattern and conjugation extension. The highest luminescence quantum yield (ϕ_flca. 0.2) is shown by the model compounds without a substitution at C3, whereas lower values are measured for the others. Quantum yields in toluene for the quadrupolar systems varied from ϕ_fl = 0.7 to ϕ_fl > 0.9. Apart from bis-imidazo[1,2-a]pyridine possessing two methyl groups, with ϕ_fl = 0.69 in toluene and 0.35 in dichloromethane, the luminescence properties were only slightly affected by the change in the solvent. Most of the fluorescence lifetimes ranged from 1–2 ns and the radiative rate constants reached values of ca. 6 × 10⁸ s⁻¹. Intense phosphorescence spectra at 77 K with lifetimes in the second range (τ = 0.6–1.3 s) are recorded for the monomers, whereas for the bis-imidazo[1,2-a]pyridines, phosphorescence spectra are detected only after enhancing intersystem crossing by heavy atoms. Non-linear optical properties of bis-imidazo[1,2-a]pyridines revealed two-photon absorption cross-sections (σ₂) typically of the order of 500–800 GM. The remarkable fluorescence quantum yield, in combination with high a two-photon absorption cross-section, generated a two-photon brightness of ∼500 to 750 GM units within the biological window (700–800 nm).