We have used Time-Dependent Density Functional theory (TD-DFT) to probe the nature of the excited states of these compounds (see the ESI for details). First, the ground-state geometries of all dyes investigated herein belong to the highest point group that can be reasonably envisaged (up to C2h for 2 and 5 and D2 for both 6a and 6b). For absorption, the presence of two identical weakly-coupled chromophores and this symmetry typically yields two low-lying excited states with almost identical energies but different transition probabilities (see Table ). If one considers the most intense absorption transition, the relative shifts for the new compounds as compared to DCNP reasonably fit experiment, with, e.g., TD-DFT values of +21 nm for 2 and +50 nm 6b, respectively, that agree with the measured values of +28 nm and +46 nm. Importantly, both theory and experiment find that all new systems 2-6a-b absorb at rather similar wavelengths. In Fig. , we provide electron density difference (EDD) plots for selected transitions (see Fig. for all cases). In most cases, these EDD indicate that the excitedstate has an almost additive character, with two DNCP-like excitations. 9d This is clear for both 3 and 6b shown in Fig. . 4 constitutes a clear exception as the lowest excitation involves a charge-transfer (CT) from the two lateral λ 5 -phosphinines moieties towards the central benzothiadiazole core. This CT state is however appearing at a very close energy to the DNCPlocalized transition (Table ) and it is likely that the observed absorption band encompasses three close-lying excited states (Table ). For the emission, the relaxation of the excited-state geometries sometimes leads to symmetry breaking, making the analysis more complex, see Table in the ESI. TD-DFT predicts that 4 leads to the most redshifted fluorescence (656 nm with a very large f indicating a large radiative rate), consistent with the data of Table that shows a non-trifling emission at 688 nm. In contrast for both 6a and 6b, the minimization of the excited-state structure yields similar fluorescence wavelengths (534 and 554 nm, respectively) but associated with negligible transition probabilities, explaining the very low quantum yields that have been observed. For both 2 and 3, the relaxation effects are much less marked leading to similar fluorescence wavelengths for the two systems and also smaller Stokes shift, again consistent with experimental trends. In short, the fact that the absorption bands in Fig. are all similar whereas much stronger variations across the series are found for the emission bands in Fig. is related to the presence of two nearly equivalent λ 5 -phosphinine moieties and the emergence of symmetry breaking in several cases. The electrochemical behaviour of 2-6ab was investigated by cyclic voltammetry in dichloromethane solution (Fig. , Fig. ). Dimers featuring aromatic spacers 2-4 display only a single two electron reversible oxidation at ca + 1 V vs DmFc + /DmFc (decamethylferrocenium/decamethylferrocene) (see Fig. and Table ). Such values fit with the oxidation reported for parasubstituted DCNP derivatives. As observed in extended MV derivatives, the presence of aromatic π-linkers leads to a single redox process in DCNP dimers, indicating the absence of significant electronic communication between the two end-groups. Note that the presence of the electro-withdrawing benzothiadiazole in 4 leads to the presence of a quasi-reversible reduction, making 4 an amphoteric redox system (Fig. ). The presence of the alkyne linker in 5 surprisingly leads to a fully irreversible oxidation process (Fig. ). Finally, the directly connected DCNP dimers 6a and 6b display two distinctively separated reversible oxidation waves at low potential (e.g., Eox1(6a) = +0.85 V vs DmFc + /DmFc and Eox2(6a) = +1.05 V vs DmFc + /DmFc, Fig. and Table ), characteristic of Weitz-type redox systems. This finding suggests the successive formation of a radical cation and a dicationic specie. Replacing the cyano groups of 6a by esters in 6b leads to a decrease in oxidation potential of 400 mV, consistent with the redshift observed in UV-vis absorption. This univocally validates our design strategy of preparing λ 5 -phosphinine dimers as electron rich multi-stage redox systems. To gain deeper insights into the properties of these novel multi-stage electron donors, spectroelectrochemical and electron paramagnetic resonance (EPR) measurements were performed for both 6a and 6b (Fig. -25). Illustratively, each redox state of 6b displays characteristic UV-vis absorption signature. The absorption of radical cation 6b• + is characterized by the appearance of redshifted transitions (λ = 750 nm, Fig. ), which are characteristic of a π-delocalized radical. Upon further oxidation toward the dicationic form, this low energy absorption disappears and a new transition rises at 398-414 nm (Fig. ). Vertical TD-DFT calculations for the 6b• + and 6b 2+ species respectively yield the lowest absorption band at 707 nm (f=0.803) and 399 nm (f=1.908) which is obviously consistent with the measurements. The EDD plots for these transitions are available in the ESI (Fig. ). The initial spectrum of 6b was recovered upon restoring the starting potential (Fig. ), demonstrating the reversibility of the process. A similar behavior is observed for 6a (Fig. ). EPR spectrum (g = 2.001) was recorded in DCM at 77 K after chemical oxidation of 6b with nitrosonium hexafluorophosphate (Fig. ) confirming the formation of an organic radical. The structure of the spectrum is reproduced considering the coupling of the unpaired electron with the two equivalent phosphorus atoms (the four equivalent protons only lead to unresolved broad signals) (Fig. ). This evidences that the radical is delocalized on the entire π-system. Indeed, the computed spin density displayed in Fig. , shows a symmetric distribution delocalized over the two λ 5 -phosphinine groups. As evidenced during the optical/redox study, 6a and 6b are intrinsically fluorescent and they can be reversibly oxidized giving rise to stable radicals. To take advantage of this specific property, we investigated the redox modulation of the luminescence of 6b in DCM solution in a homemade electrofluorochromic set up (see the ESI). As illustrated in Fig. , the application of + 400 mV vs Pt allowed to switch off the red luminescence of 6b. The luminescence was then restored upon reversal of the potential (-500 mV vs Pt). To illustrate the reversibility of the process, the luminescence of 6b was successfully turned off and on (see Fig. and Fig. ) upon applying a sequence of oxidation (+500 mV vs Pt) and reduction (-500 mV vs Pt). These preliminary results indicate that λ 5 -phosphinines can be viewed not only as an emerging class of fluorophores usable in OLEDs/lasers, but can also likely find applications related to their appealing redox properties, as illustrated by these electrofluorochromic properties. In addition, it extends the scope of Weitz-type organophosphorus derivatives by showing that they can operate under oxidative quenching.