Fluorescent probe

A method for designing a fluorescent probe which is represented by the following formula (I) following formula (I):

• wherein,
• R1 and R2 each independently represents hydrogen atom, or a substituent for trapping proton, a metal ion, or an active oxygen species, provided that both of R1 and R2 do not simultaneously represent hydrogen atoms, or R1 and R2 may combine to each other to form a ring structure for trapping proton, a metal ion, or an active oxygen species;
• R3 is a lower alkyl group or a lower alkoxy group;
• R4 and R5 each independently represents hydrogen atom or a halogen atom;
• R6 represents hydrogen atom, an alkylcarbonyl group, or an alkylcarbonyloxymethyl group), which comprises selecting, as a combination of R1, R2, and R3, the combination which provides:
• (1) substantially high electron density of the benzene ring to which said groups bind so that the compound represented by the formula (I) is substantially no fluorescent before the trapping of proton, a metal ion, or an active oxygen species, and
• (2) substantial reduced electron density of the benzene ring to which said groups bind so that a compound after the trapping, which is derived from the compound represented by the formula (I), is substantially highly fluorescent after the trapping of proton, a metal ion, or an active oxygen species.