Polymethine cyanines are organic compounds in which a charge (either positive such A, Fig 1, or negative) is fully delocalized between two heteroatoms (or heterocycles) over an odd number of sp 2 carbon atoms. This particular structure, called the ideal polymethine state (IPS) leads to a series of hallmark properties: (i) a bond length alternation (BLA) equal to zero as all the C-C bonds display a "one and a half" bond length, (ii) a sharp and particularly intense absorption band with a vibronic shoulder at high energy, and (iii) luminescence with a very small Stokes shift. The molecular engineering of the structure (polymethine chain length, nature of the heterocycle, counter-ion, central substituent) allowed to tune the absorption/emission of these dyes from the visible to the near infrared. Polymethine cyanines thus represent a fascinating platform to study the effect of the molecular structures on electronic delocalization as their exceptional characteristics can vanish when the "cyanine limit" is crossed. Thanks to these hallmark properties, cyanine dyes have been used in many applications in chemical detection, bio-imaging 4 and material sciences 5 . So far, the vast majority of cyanine dyes display capping groups encompassing nitrogen (A, Fig. ), even though it was shown that substituting it by oxygen-or sulfur-heterocycles significantly redshifts the absorption/emission. During the last decades, systems in which the nitrogen is replaced by phosphorus, its higher group 15 analogue, became very attractive compounds. Such chromophores benefit from the particular structural and electronic features of the P atom, e.g., a reactive lone pair, a pyramidal geometry preventing staking, and the possibility to display - conjugation. 7 Surprisingly, while phospha-cyanines have sporadically appeared in the literature since the 1950s, no systematic structure-properties relationship has been established on an entire family (B, Fig. ). 9 Interestingly whilst several articles devoted to the reactivity of organophosphorus derivatives report P-compounds fulfilling the structural requirements of phospha-cyanines (cationic compounds in which a charge is delocalized between two P over an odd number of sp 2 carbon atoms), this aspect is not commented in the articles nor the optical properties reported. In this context, we wish to highlight Yamada's approach, which studied the reaction of perfluorocyclopentene with various nucleophiles, including a phosphonium ylide, and reported a compound that can be structurally characterized as a phospha-pentamethine cyanine (Fig. , R = Ph). In the present contribution, we adapt this synthetic method to prepare a full family of dyes and systematically study their structures, and optical properties using a joint experimental and theoretical approach. From this study, we can conclude that these "phospha-cyanines" possess all the structural and spectroscopic properties of cyanines in their IPS. . Here, a library of methyl-phosphonium 1-6[X] are prepared from commercially available phosphanes and then converted to the corresponding phospha-cyanines 7- highlighting the stability of the platform. Nevertheless, slow oxidation to the corresponding phosphine oxide was observed when the compound was stored several days in solution under ambient conditions (Fig. ). Scheme 1: Synthesis of 7-12[X] 7-11[X] are characterized by X-ray diffraction performed on single crystals (see Fig. , Fig. and Table and). In addition, to confirm the molecular structure of the synthesized compounds, the crystallographic structure provides the opportunity to determine the BLA and therefore the "cyanine" character of the dyes.