Phospha-cyanines in their Ideal Polymethine State: Synthesis and Structure-Properties Relationships

We report the synthesis and full characterization of a family of phosphorus containing polymethine cyanines (phospha-cyanines). The compounds are easily prepared in two steps starting from readily available phosphanes. The impact of the P-substituents and the counterions on the structural and optical properties is investigated through a joint experimental/theoretical approach. Based on the study of the single crystal X-ray diffraction structures, all phospha-cyanines present a bond length alternation (BLA) close to zero, independently of the substituent and the counterions, which indicates an ideal polymethine state. All these compounds display the typical cyanine-like UV-vis absorption with an intense and sharp transition with a vibronic shoulder. TD-DFT calculations allowed to fully rationalize the optical properties (absorption/emission wavelengths, luminescence quantum yields). Interestingly, due to the tetrahedral shape of the P-atom, the optical properties are independent of the counterion, which is in marked contrast with N-analogs, which enables predictive engineering of the phospha-cyanines regardless of the medium in which they are used.