The molecular and electronic structure of dipiperidinosquaraine

Dipiperidinosquaraine ( 9 ) crystallizes in the orthorhombic system, space group Pnab , with four molecules in a cell of dimensions a = 7.1798(9), b = 9.5362(8), and c = 18.788(2) Å. The molecule is located over a center of symmetry, so the four‐membered ring (C – C = 1.456(1) Å av) is exactly planar; the nitrogen and oxygen atoms are nearly coplanar with the ring, with C O = 1.2396(7) and C N = 1.3112(7) Å. Both experimental and theoretical evidence indicates the presence of considerable C N and C O π‐bonding, but only a small amount of C C π‐bonding in the squaraine system. The calculations indicate the presence of strong 1,3‐repulsive interactions within the four‐membered ring. Ring puckering, which is a common mechanism to alleviate these repulsions, is prevented here by the extensive C N π‐bonding.