Search for ${\chi }_{c1}\to {\pi }^{+}{\pi }^{-}{\eta }_c$ via $\psi (3686)\to \gamma {\chi }_{c1}$

Utilizing $(2712.4\pm 14.3)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector at the BEPCII collider, we search for the hadronic transition process ${\chi }_{c1}\to {\pi }^{+}{\pi }^{-}{\eta }_c$ following the decay $\psi (3686)\to \gamma {\chi }_{c1}$. No significant signal is observed, and an upper limit of $\mathcal{B}({\chi }_{c1}\to {\pi }^{+}{\pi }^{-}{\eta }_c)$ is determined to be $3.1times{10}^{-4}$~at 90% confidence level, which is one order of magnitude more stringent than the previous measurement.