), respectively. Introducing various aldehyde monomers and combining them is thus an effective way to modulate the absorption properties of the resulting polymer or terpolymer, and the combination of BACDs and electron-deficient co-monomers lead to low-LUMO materials absorbing light across the whole visible range up to the NIR. Most rylene-based polymers and dyes typically undergo the formation of mono-anionic and bis-anionic species centered on BACD motifs during CV experiments when scanned toward negative potentials. Recently, we reported that ortho-pentannulated BACDs, large π-extended analogues of BACDs, exhibit an unusual electrochemical behaviour. In CV experiments, the first reduction event is split into two distinct waves. We demonstrated that these derivatives form strong intermolecular mixed-valence (MV) species through supramolecular dimerization of a radical anion and a neutral species, followed by π-dimers (π-DIM) formed via dimerization of two radical anions. These two electrochemical processes explain the splitting of the first reduction wave. Motivated by these findings, we investigated whether similar phenomena occur in BACD-based polymers. Detailed electrochemical studies were conducted on the polymer P(BACD-T). CV experiments (0.5 mM, 0.1 NBu₄PF₆ in CH₂Cl₂) revealed two distinct reduction events: a first reduction split into two peaks and a second, regular reduction (Figure ). Additionally, a scan-rate-dependent event emerged around -1.2 mV, which will be addressed later in the full interpretation of the CVs. The splitting of the first reduction wave, which was concentration-dependent, indicates intermolecular interactions. This can be attributed to the rapid formation of strong inter-chain MV complexes through interactions between reduced and neutral BACD fragments during partial reduction. As the potential decreases further, all BACD units along the polymer chain are reduced to their anionic forms, enabling the formation of π-DIM complexes between anionic diimides along the polymer chains. These processes were further elucidated using real-time spectroelectrochemistry (SEChem) under thinlayer cyclic voltammetry (TLCV) conditions (10 mV/s, Figure ). Under blocked-diffusion conditions, the absorption spectra of the solution were recorded in real time. Three distinct species were observed, as expected (Figures ), and isolated via principal component (PC) analysis, which extracted three unique optical signatures (Figures ). PC1 displayed a depletion of absorption around 400 nm, strong absorption centered at 620 nm with a shoulder around 580 nm, and a new band near 960 nm. A weak increase beyond 1350 nm suggests an absorption band extending beyond the detection limit (Figure , indicated by the *). PC2 showed similar depletion at 400 nm with absorption increases at 580 and 850 nm, alongside a broad NIR band peaking at 1400 nm. PC3 exhibited strong, broad absorption between 600 and 800 nm. Based on our prior studies, these signatures correspond to distinct stages of reduction: PC1 represents inter-chain MV complexes, PC2 corresponds to π-DIM species, and PC3 reflects bis-anionic diimides or fully reduced polymer chains (Figure ). PC1's broad absorption beyond 1400 nm is characteristic of the intervalence charge-transfer (IVCT) band in MV complexes. PC2's π-DIM complexes feature broad NIR absorption near 1400 nm and a new band at 850 nm. The absorption at 580 and 960 nm, shared by PC1 and PC2, arises from free monoreduced BACD fragments in equilibrium with MV and π-DIM species. The first reduction wave showed a splitting of ΔE = 0.185 V, yielding a high comproportionation constant 56 Kcom=1.5×10 3 , similar to ortho-pentannulated BACDs. Comparing the SEChem data of P(BACD-T) with that of the monomeric model compound 1,6/7-BACD-T₂ (Figure ) revealed a smaller splitting (ΔE = 0.130 V, Kcom=1.6×10 2 ), highlighting weaker intermolecular interactions in the monomer. This demonstrates that polymerization enhances intermolecular interactions by increasing conjugation and interaction sites. We previously found that phenyl-functionalized monomeric BACDs lacked significant CV splitting, despite concentration-dependent broadening and shouldering. The splitting observed in CH2Cl2 in 1,6/7-BACD-T₂ likely arises from N•••S chalcogen bonds, which planarize the molecule and enhance intermolecular interactions compared to phenyl-substituted BACDs. In the polymer, enhanced planarity and multiple interaction sites stabilize radical anions and facilitate MV and π-DIM formation. The second reduction step, corresponding to the reduction of mono-anionic chains into bis-anions, is observed around -0.95 V at a scan rate of 2000 mV/s but is significantly influenced by the limited solubility of the bis-anionic forms (Figure ). At slower scan rates, an adsorption peak emerges near -1.15 V at 50 mV/s, attributed to the reduction of bis-anions adsorbed on the electrode surface. Deconvoluted cyclic voltammograms confirm this interpretation, showing a pronounced asymmetry between the reduction and oxidation waves of the P²⁻/P •-redox couple (Figure ), indicative of non-ideal electrochemical behaviour. As expected, this adsorption process is amplified under diffusion-blocked conditions (Figure ) but does not lead to additional spectroscopic features beyond that characteristic of the bis-anionic forms. This electrochemical behaviour was successfully reproduced through simulations performed using DigiElch software (Figure and Figure ). To the best of our knowledge, this study provides the first characterization of inter-chain electronic interactions in reduced n-type polymers in solution. Such interactions are intrinsic to the BACD core, highlighting its potential as a building block for high-performance semiconductors. The strong interchain electron delocalization observed even in dilute solutions suggests the possibility of designing BACD-based polymers with enhanced charge transport properties. These findings are consistent with the known high-performance n-type polymer PNDI(2OD)₂T, whose efficiency derives from interchain charge hopping in the solid state. At last, one could argue that the observed splitting and the NIR signatures subsequently generated could be attributed to the formation of anionic polaronic and bipolaronic states. Such species are associated with the appearance of strongly absorbing broad transitions at low energy characterized by high absorption coefficients. However, the weak bands linked to a probably low absorption coefficient observed in the NIR window for P(BACD-T) do not match the characteristics of polaronic/bipolaronic species. The calculated LU-MOs centred on the BACD cores of P(BACD-T) further support this interpretation (Figure ).