The invention relates to cationic direct dyes of the formula I (1), wherein m and n are independently from each other 0 or 1, D is an aromatic or a cationic heteroaromatic group, which are further specified, K is an aromatic or heteroaromatic group, E.sub.1 and E.sub.2 are independently from each other CH or N, An is an anion and a is a number from 1 to 6 and R.sub.1 and R.sub.2 are independently from each other and further specified. The compounds show washfastness at hair-dying. ##STR00001##

The invention relates to cationic direct dyes of the formula I (1), wherein m and n are independently from each other 0 or 1, D is an aromatic or a cationic heteroaromatic group, which are further specified, K is an aromatic or heteroaromatic group, E.sub.1 and E.sub.2 are independently from each other CH or N, An is an anion and a is a number from 1 to 6 and R.sub.1 and R.sub.2 are independently from each other and further specified. The compounds show washfastness at hair-dying.

##STR00001##

Provided are a compound that generates an SHG signal but has suppressed generation of a TPF signal. Also provided is a cell examination method using the same. A compound for generating a second harmonic of light, the compound being an azobenzene derivative of formula (1), or a salt thereof. (R1 and R2 independently represent alkyl groups having 6-12 carbon atoms, and R3 to R18 independently represent substituents selected from hydrogen, halogens, alkyl groups, alkoxy groups, aryl groups, amino groups, hydroxyl groups, nitro groups, and cyano groups, but R5 and R6, R9 and R10, R13 and R14, and R17 and R18 may combine together and form a ring structure having 5-7 carbon atoms. X represents N+R19R20R21, a sulfonyl group, a carboxyl group, or an OR group. Here, R19, R20, and R21 independently represent straight-chain or branched alkyl groups having 1-5 carbon atoms, and OR represents a monovalent polyalkylene oxide group terminated by an alkoxy group. a is 0 or 1, b is 0 or 1, and n is an integer of 1-10.)