  Combinatorial introduction of heteroatoms in the two-dimensional framework of
aromatic hydrocarbons opens up possibilities to design compound libraries
exhibiting desirable photovoltaic and photochemical properties. Exhaustive
enumeration and first-principles characterization of this chemical space
provide indispensable insights for rational compound design strategies. Here,
for the smallest seventy-seven Kekulean-benzenoid polycyclic systems, we reveal
combinatorial substitution of C atom pairs with the isosteric and isoelectronic
B, N pairs to result in 7,453,041,547,842 (7.4 tera) unique molecules. We
present comprehensive frequency distributions of this chemical space, analyze
trends and discuss a symmetry-controlled selectivity manifestable in synthesis
product-yield. Furthermore, by performing high-throughput ab initio density
functional theory calculations of over thirty-three thousand (33k)
representative molecules, we discuss quantitative trends in the structural
stability and inter-property relationships across heteroarenes. Our results
indicate a significant fraction of the 33k molecules to be electronically
active in the 1.5-2.5 eV region, encompassing the most intense region of the
solar spectrum, indicating their suitability as potential light-harvesting
molecular components in photo-catalyzed solar cells.
