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

Article abstract of DOI:10.1039/c4tc00234b

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).