A compound represented by the following general formula (I) or a salt thereof: ##STR00001## wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 independently represent 2-quinolyl group which may be substituted, 1-isoquinolyl group which may be substituted, or 3-isoquinolyl group which may be substituted; m, n, p, and q independently represent 1 or 2; and L represents a single bond, or an alkylene group having 1 to 5 carbon atoms wherein said alkylene group may contain one or more heteroatoms in the main chain, and may have one or more substituents on the main chain, which is useful as a fluorescent zinc ion sensor.