Dicyanovinyl substituted push–pull chromophores: effects of central CC/phenyl spacers, crystal structures and application in hydrazine sensing

To discern the distinctive effect of the CC bond and the phenyl bridge on the photophysical and chemical properties of D–π–A molecular systems, three new dicyanovinyl substituted push–pull chromophores 1–3 containing either a CC bond or a phenyl ring as the central π-linker were synthesized by the Suzuki–Miyaura and Knoevenagel reactions. Together with the counterpart of 1 developed by Zhang's group (TPE-z), their optical properties and single crystal structures were systematically and comparatively investigated. Notably, the simple π-extension of a CC linker to a phenyl ring between electron donor and electron acceptor moieties could greatly affect the photophysical properties of chromophores, particularly leading to significant hypsochromic shifts in both absorption and emission spectra. Meanwhile, as a node of twisted sections in chromophores, the CC bond was demonstrated to play an important role in the nonradiative relaxation of excited states, as compared to the rigid phenyl spacer. Moreover, the three chromophores were found to display spectral responses to hydrazine with slightly different sensitivities, and visual detection of hydrazine in the gaseous state was achieved via using readily fabricated paper test strips.