Neutral push-pull chromophores for nonlinear optical imaging of cell membranes

A new class of push-pull molecules was recently identified, based on pyridine dicarboxamide as an electron acceptor group and bearing a fluorenethynyl π-conjugated bridge. The molecules present good second and third order nonlinear properties, with a static quadratic hyperpolarisability μβ (at 1.907 μm) of 320 × 10⁻⁴⁸ esu (μβ₀ = 249 × 10⁻⁴⁸ esu) and a maximum two-photon absorption cross-section of 1146 GM. Starting from this generic structure, we designed a series of eight amphiphilic nonlinear probes, varying the length of the lipophilic tail and the nature of the polar head, and tested their cell membrane affinity by nonlinear optical imaging. A good membrane affinity was observed with probes bearing short alkyl chains and carbohydrate moieties as the polar head, emphasizing the importance of the lipophilic–hydrophilic balance, as well as the role of the polar head. This original approach based on carbohydrate head groups is an important advancement in the design of membrane probes, since these highly versatile functional groups confer adequate hydrophilicity and yet conserve overall molecular neutrality.