Halochromic Molecules. Spectrophotometric Investigation of the Acidobasic Behaviour of Substituted 6,6‐Diphenyl‐6H‐chromeno[4, 3‐b]indolizines and their Aza‐analoguesSubstituted 6,6‐diphenyl‐6H‐chromeno[4,3‐b]indolizines form ring‐opened, intensely coloured (diphenyl)‐indolizinylcarbenium salts with acid. UV/VIS spectra of the coloured salts are discussed on the basis of VEPPPCI calculations. The ϵpH* curves in buffered MeOH/H2O solutions and the pK* values are determined. The title compounds undergo very complex