Two-photon induced emissive thiophene donor–acceptor systems as molecular probes for in vitro bio-imaging: synthesis, crystal structure, and spectroscopic properties

Abstract

Two new classes of organic two-photon absorption (TPA) chromophores, D–π–A (Bt₁1/Tt₁1) and D–π–A–π–D (Bt₂2/Tt₂2), were designed and synthesized using a Sonogashira coupling reaction between 4-ethynyl-N,N-dimethylaniline and 5′-bromo-2,2′-bithiophene/2,2′:5′,2′′-terthiophene or 5,5′-dibromo-2,2′-bithiophene/2,2′:5′,2′′-terthiophene, respectively. At room temperature, all of the compounds had single-photon fluorescent and solvatochromatic properties, and could produce two-photon-induced luminescence upon excitation at 700 nm (Ti:sapphire femtosecond laser, 150 fs pulses). The corresponding TPA cross-section values ranged from 81 to 563 GM (10⁻⁵⁰ cm⁴ s photon⁻¹ molecule⁻¹). The quadrupolar-type (D–π–A–π–D) chromophores, Bt₂2 and Tt₂2, exhibited a larger TPA cross-section (σ₂) value than their dipolar analogues (D–π–A), Bt₁1 and Tt₁1, whereas the bithiophene chromophores, Bt₁1 and Bt₂2, exhibited a larger σ₂ value than their terthiophene counterparts, Tt₁1 and Tt₂2. All of the compounds were used for bio-imaging in living human cervical carcinomas (HeLa). We found that Bt₁1 and Tt₁1 were able to pass through the cell membrane and to selectively stain the cell cytoplasm and lysosome, respectively. The results of these studies could provide new molecular-design strategies for two-photon imaging in vitro.