The selectivity and sensitivity of N,N'-bis(4-diethylaminosalicylidene)hydrazine (sensor 1) toward Cu2+ ion have been established both by distinct "naked-eye" color change as well as UV-vis spectrophotometry. Sensor 1 formed 1:1 complex with Cu2+ ion where the most stable s-trans form of the sensor is trapped in the high energy conformer. Single-crystal X-ray structure of sensor 1 supports its s-trans conformation. Theoretical calculations support well the experimental result.