Finally, emission could be recorded for some compounds (Table and Fig. , Fig. ). If the associated luminescence quantum yield remains poor ( < 0.5%), the measured Stokes shift is small, again fitting the cyanine structure. The trend observed in luminescence wavelengths follows the one observed in absorption. We have used theory to obtain some first hints on this absence of emission, or very low . First, optimization of the lowest excited-state of 7 + yields a bright state with an oscillator strength larger than 0.5 with both TD-DFT and CC2, indicating that there is no dark state quenching appearing in the excited state, consistent with the experimentally measured small Stokes shifts (for the emissive dyes). Second, given the presence of "heavy" phosphorous atoms, we envisaged the possibility of intersystem crossing (ISC). Theory returns only one triplet below the lowest singlet (on the S1 optimal geometry), but this triplet is both strongly energetically far (ST = 0.77 eV) and weakly coupled (SOC of 0.22 cm -1 ) with the corresponding singlet, which phosphanes. The impact of the P-substituents and the counterions on the structural and optical properties is investigated using a joint experimental/theoretical approach. All the phosphacyanines present a BLA close to zero according to the crystallographic structures, confirming their Ideal Polymethine State (IPS). All these compounds display a sharp UV-vis absorption band with a vibronic shoulder, characteristic of a cyanine in its IPS. The low luminescence quantum yield was rationalized on the basis of TD-DFT calculations. Interestingly, the optical properties of these phospha-cyanines are independent of both the solvent and the counterion. This is contrast in to its N-analogs and enables predictive engineering of the phospha-cyanines regardless of the medium in which they are used (biological media or materials) which is a clear advantage for potential applications. Finally, this study also paves the way toward the preparation of unprecedented cyanines dyes featuring heavier main-group elements, as those heteroatoms were recently described to induce unusual structural/electronic properties to chromophore/fluorophores. ASSOCIATED CONTENT Supporting Information. Synthetic procedure, complete characterizations, X-ray crystallographic data and CIF files, computational details and Cartesian coordinates. The following files are available free of charge.