The structures of 2-4 and 6a-b were additionally confirmed by single-crystal X-ray diffraction (Fig. and Fig. ). In all compounds, the bond lengths within the λ 5 -phosphinines are within expectations and attest the conjugated ylidic nature of the P-C bond (1.75 Å < d < 1.77 Å). The P-ring is globally planar (maximal deviation from the mean plane: 0.07 Å). As expected, a tilt angle is always observed between the two lateral λ 5 -phosphinines (26.6° for 6a and 32.5° for 6b) or between the λ 5phosphinines and its neighbouring aromatic ring (see Fig. ). The bond between the two λ 5 -phosphinines rings in 6a-b is a pure C-C single bond (d ∼ 1.49 Å), confirming that each λ 5 -phosphinine moiety keeps its aromatic nature. No intermolecular interactions are observed in the packing due to the bulkiness of the P-Ph exocyclic fragment. and). All dimers 2-6a-b display absorption spectra with an intense band at ca. 490 nm (Fig. and Table ) characteristic of DCNP derivatives, that is only slightly redshifted compared to pristine DCNP (λ = 458 nm). 9a Replacing the cyano in 6a by esters in 6b leads to a small redshift of 10 nm. In all these systems, this absorption band is assigned to π-π* excitations mainly centered on the λ 5 -phosphinine rings. However, in 4, the central benzothiadiazole unit plays the role of an acceptor (see theory below). The impact of the bridge is more striking on the luminescence (Fig. and Table ), with a strong redshift compared to pristine DCNP (λem = 491 nm) observed for all compounds. When aromatic spacers are introduced, a gradual redshift is observed in the 2/3, and 4 series leading to a large Stokes shift of 6300 cm -1 in 4. Interestingly, the directly connected dimers 6a and 6b display emission bands that are redshifted as compared to dimers with phenyl/fluorenyl (2/3) or with an alkyne (5). Finally, replacing the cyano groups by esters in 6b again leads to a small bathochromic shift of ca. 20 nm. This trend in luminescence wavelength comes with a general decrease in quantum yields where only 2-3 are efficient emitters (φ ∼ 0,45). This is in global agreement with the energy gap law though 4 still delivers a relatively bright emission (φ ∼ 0,05) for a dye emitting at nearly 700 nm in contrast to both 6a and 6b (Table ).