The phase behaviour and spin states of [Fe(NCS)₂(dpq)₂] (1; dpq = dipyrido[3,2-f:2′,3′-h]quinoxaline), [Fe(NCS)₂(dppz)₂] (2; dppz = dipyrido[3,2-a:2′3′-c]phenazine) and [Fe(NCS)₂(dppn)₂] (3; dppn = dipyrido[3,2-a:2′3′-c]benzophenazine) have been investigated. Solvent-free 1 and 2 are isostructural and low-spin in the crystalline state, in contrast to previously published 2·py (py = pyridine) which exhibits a hysteretic spin-crossover (SCO) transition near 140 K. The inactivity of 1 and 2 towards SCO may relate to their more crowded intermolecular lattice environment, particularly two very short intermolecular anion⋯π contacts involving the NCS⁻ ligands. Two solvate phases of 1 are also described, including 1·2py which undergoes gradual SCO with T_½ca. 188 K. Bulk samples of 2 and 3 are predominantly low-spin and isostructural with the crystals of 2 by powder diffraction, but bulk samples of 1 contain an extra phase that exhibits hysteretic SCO, but was not crystallographically characterised. Crystal structures of low-spin [Fe(dppz)₃][ClO₄]₂ (4) and a solvate of [Fe(dppn)₃][BF₄]₂ (5) are also described, which are the first homoleptic complexes of these ligands to be crystallographically characterised.