Polycyclic aromatic hydrocarbons (PAHs) have emerged as a promising class of organic molecules with the potential for unconventional electronic and magnetic devices. Among them, triangulene is particularly noteworthy, as it features the smallest nanographene with a triplet ground state, which has sparked great research interest since its first synthesis. Its intriguing metal-free magnetism and captivating non-Kekulé structure, where all of the 22 π-electrons are impossible to concomitantly pair into a closed-shell structure, offer immense prospects for the development of next-generation molecular spintronic materials, such as spin filters and qubits for quantum computing. Triangulene has been proposed as a prototype for studying other PAHs, which can be doped, substituted, or incorporated into more complex architectures. Recent developments in the synthesis of extended structures made of triangulenes have led to the observation of many exotic phenomena, such as Dirac cones, collective spin excitation, and Haldane phase. Despite the successful realization of triangulene monomers, dimers, nano-stars, quantum rings, and 1D polymers, as well as its larger homologues such as [4]-, [5]-, and [6]-triangulene, the chemical versatility of triangulene and its derivatives remains understudied, especially for those with heteroatoms. For hydrocarbons with a bipartite honeycomb lattice, the wellestablished Ovchinnikov rule, i.e. Lieb's theorem, is commonly applied to predict the spin-polarized ground states of the non-Kekulé molecules. The total spin of the system emerges from half-filling of the degenerate highest occupied molecular orbitals (HOMOs) according to Hund's rule, and the multiplicity is given by S = (NA -NB)/2, where NA and NB are the numbers of sites belonging to the two honeycomb sublattices. Therefore, a different number of sites in the sublattices is crucial to achieve molecules with high-spin ground states. However, introducing heteroatoms, such as O, N and B, can break the condition of Lieb's theorem and thus go beyond the predictive capabilities of Ovchinnikov's rule. This may open doors for realizing unconventional metal-free magnetism. For example, nitrogen-doped triangulene, or aza-triangulene, has been observed to undergo Jahn-Teller distortion, lowering the symmetry from the intuitive D3h to C2v. Despite triangulene homologues, achieving a high-spin ground state and ferromagnetic (FM) interactions in nanographenes remains a significant challenge. While single-bond linked PAHs can be easily constructed via the Yamamoto or Ullmann coupling reactions and show great potential for building extended periodic structures, covalently-bonded triangulene derivatives, such as triangulene dimers, nano-stars, and linear polymers, currently only exhibit antiferromagnetic (AFM) interactions and open-shell singlet (OSS) ground states.