The optical properties of both isomers, p-ICz-PI and o-ICz-PI were measured in three solvents of varying polarity: methylcyclohexane (MCH), toluene, and dichloromethane (DCM), (Figure ). p-ICz-PI has a distinct vibronic structured absorption band between 360-420 nm in all solvents and a molar extinction coefficient of 37925 Lmol⁻¹cm⁻¹ in DCM at 400 nm (Figure and Supplementary Fig. ). In contrast, the absorption of o-ICz-PI in this region is significantly weaker (7320 Lmol⁻¹cm⁻¹) and the first peak in the absorption band is shifted to ~330 nm (Figure and Supplementary Fig. ). This signifies a fundamentally different ground state electronic character between the two isomers, a trend observed in different solvents (Figure and Supplementary Fig. ). The p-ICz-PI absorption band redshifts by only 20 nm in DCM, indicating a weak charge-transfer contribution to this otherwise strongly local transition. The reduced intensity of these transitions in o-ICz-PI implies stronger CT character, confirmed by the density difference plots for each molecule shown in the (Supplementary Information, Section 5). For comparison, Cz-PI, (Figure ), the twisted D-A analogue of ICz-PI has a strong absorption at 320-340 nm as seen in o-ICz-PI, which is attributed to the carbazole/indolocarbazole unit, and a second band at 400 nm with a longer red absorption tail (> 425 nm) than p-ICz-PI, characteristic of a typical twisted D-A direct charge-transfer absorption (Figure and Supplementary Fig. ). This confirms that in p-ICz-PI, the 360-420 nm band is predominantly a local π-π* transition with a weak CT character, whereas in o-ICz-PI the CT character is far stronger. The absorption of unsubstituted PI occurs at much higher energy, ca. 3.81 eV (325 nm). Upon excitation at 355 nm in MCH, p-ICz-PI exhibits a mirror-image emission band with small Stokes shift (11 nm), characteristic of a strongly local π-π* transition. Increasing the solvent polarity induces spectral broadening, larger Stokes shifts (28 nm in toluene and 56 nm in dichloromethane) and loss of vibronic structure, indicating increasing charge-transfer character. However, the excited state lifetime of p-ICz-PI does not change substantially, suggesting a weakly mixed LE/CT excited character is retained even in DCM (avg. = 5.6 ns) and acetonitrile (avg. = 8.9 ns) as compared to MCH (avg. = 6.1 ns), see Supplementary Fig. , Supplementary Table . ). In DCM, Cz-PI, has a poor PLQY in air, Φ ~0.04; however, it increases substantially to Φ = 0.39 (0.61) ± 0.05, when measured in toluene. While the increase upon degassing is often attributed to the prevention of triplet state quenching, we note that singlet state quenching by oxygen also occurs when lifetimes are long. Finally, the two ICz-PI isomers also have different phosphorescence spectra, Figure (green trace). p-ICz-PI is rather structureless with onset at 2.64 eV (469 nm) whereas o-ICz-PI and Cz-PI are similar, having vibronically structured phosphorescence, onset at 2.70 eV (461 nm). The vibronic structure indicates that the lowest triplet state has LE character, effectively residing on the carbazole moiety. For p-ICz-PI the lowest excited state triplet state appears to be more delocalised in character.