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Structural patterns of several symmetric cyanines relating to trimethines (I), pentamethines (II), and heptamethines (III–V). While I-III exhibit an open methine chain, functionalization by bridged moieties results in either a planar (IV) or non-planar (V) structure. The terminal group A/A’ belongs to indolium, benzo[e]indolium, and benzo[c,d]indolium (Scheme 2). I–V comprise a further substituent in the meso-position.
