In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (lambda(max) = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3c and P2(1)/c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0-15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc2(1)). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se...N'/O' secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional "brick-wall" packing pattern associated with the Cmc2(1) phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with sigma(RT) ranging from 10(-5) S cm(-1) for the P2(1)/c phase to 10(-3) S cm(-1) for the R3c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy E-act. For the R3c phase, band gap closure to yield an organic molecular metal with a sigma(RT) of similar to 10(2) S cm(-1) occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.