The present invention relates to azo dyes of formula ##STR00001##
wherein R.sub.1 denotes C.sub.1-C.sub.12 alkyl which is unsubstituted or substituted by one or more C.sub.1-C.sub.12 alkoxy groups, hydroxyl groups, amino groups, cyano groups or halogen atoms and which may be interrupted one or more times by the radical O, S, NR.sub.4, COO or OOC; R.sub.4 is hydrogen or C.sub.1-C.sub.12 alkyl; either R.sub.2 is cyano and R.sub.3 is halogen or R.sub.2 is halogen and R.sub.3 is cyano; and Ar represents a carbocyclic or heterocyclic aromatic radical, to the process for the preparation thereof, to mixtures containing said dyes and to the use thereof in dyeing or printing semi-synthetic and especially synthetic hydrophobic fiber materials, more especially textile materials.

A solvent medium for diazotization reaction includes a mixed solution of polyethylene glycol and water. The molecular weight of polyethylene glycol is 200-600, and the volume ratio of polyethylene glycol and water is 1:0.4-1.5. The mixed solution also includes hydrochloric acid or sulfuric acid, and the concentration of H.sup.+ in the mixed solution is 0.32-1.72 mol L.sup.1.

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.)

A solvent medium for diazotization reaction includes a mixed solution of polyethylene glycol and water. The molecular weight of polyethylene glycol is 200-600, and the volume ratio of polyethylene glycol and water is 1:0.4-1.5. The mixed solution also includes hydrochloric acid or sulfuric acid, and the concentration of H.sup.+ in the mixed solution is 0.32-1.72 mol L.sup.1.