A new rhodamine derivative,N-(3-carboxy)acryloyl rhodamine B hydrazide(CARB),has been synthesized,and its unusual spectroscopic reaction with Cu2+ has been investigated.The derivative exhibits a rapid and reversible non-fluorescent absorption upon coordination to Cu2+,which is a rather unusual phenomenon for rhodamine B derivatives.Stoichiometric measurements using the Job’s method and the molar ratio method reveal that one CARB molecule combines two Cu2+ ions,and the two Cu2+ ions play different roles:one opens the spirocyclic structure and the other quenches the fluorescence of the xanthene moiety.This reaction mechanism is supported by a comparative study on the model compound N-acryloyl rhodamine B hydrazide as well as by the density functional theory calculations.Furthermore,the absorption response of CARB is highly selective for Cu2+ over other common ions,which implies that CARB may be used as a colorimetric probe for the rapid visual detection of Cu2+. A new rhodamine derivative, N- (3-carboxy) acryloyl rhodamine B hydrazide (CARB), has been synthesized, and its unusual spectroscopic reaction with Cu2 + has been investigated. The derivative exhibits a rapid and reversible non-fluorescent absorption upon coordination to Cu2 + , which is a rather unusual phenomenon for rhodamine B derivatives. Stoichiometric measurements using the Job’s method and the molar ratio method reveal that one CARB molecule combines two Cu2 + ions, and the two Cu2 + ions play different roles: one opens the spirocyclic structure and the other quenches the fluorescence of the xanthene moiety. This reaction mechanism is supported by a comparative study on the model compound N-acryloyl rhodamine B hydrazide as well as by the density functional theory calculations. Still further, the absorption response of CARB is highly selective for Cu2 + over other common ions, which implies that CARB may be used as a colorimetric probe for the rapid visual detection of Cu2 +.