Conformationally locked salicylideneaniline derivatives with strong ESIPT fluorescence

Three conformationally locked salicylideneaniline derivatives, 1a–1c, were synthesized and characterized by single-crystal X-ray diffraction. Compound 1a possesses a triply locked configuration, i.e., the intramolecular five-membered-ring C–H⋅⋅⋅N and six-membered-ring O–H⋅⋅⋅N hydrogen bonds and five-membered-ring C(1-6-7-8-9) cyclization, from which the excited-state intramolecular proton transfer takes place, resulting in a record high tautomer emission quantum yield of 0.28 in the solid state. Compared with salicylideneaniline, a substantial increase in the emission quantum yield is also observed for 1b and 1c. Furthermore, compound 1a shows pH-dependent optical properties and a highly reversible response to pH, which makes it good candidate for potential applications in pH sensing. Time-dependent density functional theory calculations are reported on these salicylideneaniline derivatives in order to rationalize their electronic structure and optical properties.