Balancing framework densification with charged, halogen-bonded-π-conjugated linkages: [PPh4]2{[E-TTF–I2][Re6Se8(CN)6]} versus [PPh4]2[EDT-TTF–I]2{[EDT-TTF–I][Re6Se8(CN)6]}
Hal⋯Hal interactions in a series of three isostructural salts of halogenated tetrathiafulvalenes. Contribution of the halogen atoms to the HOMO–HOMO overlap interactions