Abstract Protolytic interactions in the series of prospective fluorescent ratiometric wide-range pH indicators — structurally rigid analogs of 2,6-distyrylpyridine — (3E,5E)-3,5-dibenzylidene-8-phenyl-1,2,3,5,6,7-hexahydrodicyclopentano[b,e]pyridine — were investigated. The pyridine nitrogen atom basicity in these compounds is significantly lower in comparison with that of unsubstituted heterocycle and 2,6-distyrylpyridine. The photochemical E→Z photoisomerization and the side benzene rings substituents influence on the acid-base equilibria were studied. The complex multi-stage mechanism of the acid-base interactions of the polysubstituted compounds was elucidated. The most significant spectral effects were typical to the N,N-dialkylamino substituted compounds of the investigated series. The widest pH sensitivity interval covering nearly the full range of 0–14 units was demonstrated for compounds with both proton donor and proton acceptor substituents.