Indoloindolizines, as representatives of a new class of polycyclic heteroaromatic dyes, exhibit vivid colors and fluorescence across the entire visible spectrum (Figure , see also Section S3 for spectra of all obtained derivatives). The observed trends in spectral changes caused by structural modifications align closely with the design principles described above. Specifically, linear benzannulation of the indoloindolizine core at the 2,3-positions leads to a significant bathochromic shift of the lowest energy absorption, reaching up to 150 nm when comparing compound I1 to I9 (as shown in Figure ). This phenomenon is readily explained by Hückel's molecular orbital theory, which posits that expanding the conjugated π-system reduces the HOMO-LUMO energy gap. A recent analysis of COMPAS-2 database of over half a million molecules shows clearly that this trend is general for cata-condensed heteroaromatic polycyclic compounds. In contrast, adding phenyl rings at the 5,6 and 7,8 positions of indoloindolizine results in a notable hypsochromic shift up to 150 nm, when moving from compound I8a to I5 (Figure ). This effect is less intuitive since expanding the conjugated system typically reduces the HOMO-LUMO gap. Nonetheless, the extended Hückel molecular orbital theory accurately predicts this trend, showcasing its exceptional utility in molecular design (Figure ). Time-dependent density functional theory (TD-DFT) computations have confirmed that the lowest energy absorption bands in all studied compounds originate from transitions from HOMO to LUMO (Section S6). Additionally, visualization of frontier molecular orbitals shows a rather uniform distribution of electron density over the indoloindolizine core for both HOMO and LUMO, which indicates that these are π-π* transitions and the shift is not related to the presence of symmetry-forbidden transitions. Despite the good prediction of spectral trends, TD-DFT results did not explain the reason behind such behavior. To elucidate the foundation of the observed structure-property relationships, we conducted ring current analysis on the polycyclic cores of these compounds (detailed in Section S6). These calculations demonstrated that the addition of phenyl rings at the 2,3-positions integrates them into the global polycyclic aromatic ring current, akin to the benzannulation process observed in acenes. On the other hand, attaching phenyl rings to indoloindolizine at the 5,6 and/or 7,8 positions results in the formation of distinct localized ring current loops. Additionally, it separates the 6-membered ring of indolizine from the main polycyclic system's global ring current promoting an overall indol character. Consequently, this isolation gives rise to smaller aromatic subsystems (chromophores), which in turn leads to a hypsochromic shift in the electronic absorption spectra (Figure ). Fluorescence spectra of studied indoloindolizines follow similar trends to their absorption spectra (Figure , see also Section S3 for spectra of all derivatives). However, the vibronic fine structures of absorption bands are not perfectly reflected in the emission spectra of most derivatives. This suggests a difference in the vibronic coupling between ground and excited states. On the other side, the Stokes shift of most derivatives is relatively low (approx. 25 nm), which indicates high structural rigidity of the polycyclic aromatic core. Higher Stokes shift for I5 is presumably the result of its slightly bent structure caused by the steric clash between phenanthridine moiety and TMS-acetylene substituent. Fluorescence quantum yield varies significantly depending on the structure of the polycyclic core. Compound I8a and its derivatives, where one can identify only one Clar's sextet and one global delocalized aromatic ring current loop, exhibit low to very low quantum yields (Table ). On the other side, derivatives expanded on the 5,6 and 7,8 positions of indoloindolizine, with more localized ring current loops, show significantly higher quantum yields. Such change in photoluminescence quantum efficiency indicates the presence of non-radiative relaxation pathways in the former ones, which will be a topic of investigation in our future studies.