The invention relates to a compound of formula Ia or Ib wherein R
1
represents —CO
2
R
3
, —COR
4
or —R
5
, wherein R
3
represents unsubstituted or substituted C
1
-C
6
alkyl, R
4
represents hydrogen, unsubstituted or substituted C
1
-C
6
alkyl, and R
5
represents hydrogen, unsubstituted or substituted C
1
-C
6
alkyl, R
2
represents —N
+
(R
6
)(R
7
)(R
8
)X
−
or a nitro group, wherein R
6
, R
7
, R
8
independently of each other represent unsubstituted or substituted C
1
-C
6
alkyl or unsubstituted or substituted —(CH
2
)n- with n=1 to 12 provided that at least two of the substituents R
6
R
7
R
8
are C
1
-C
6
alkyl, and X
−
represents a halide, sulphonate, unsubstituted or substituted acetate, sulphate, hydrogen sulphate, nitrate, perchlorate, or oxalate
本发明涉及公式Ia或Ib的化合物,其中R1代表—CO2R3,—COR4或—R5,其中R3代表未取代或取代的C1-C6烷基,R4代表氢,未取代或取代的C1-C6烷基,而R5代表氢,未取代或取代的C1-C6烷基,R2代表—N+(R6)(R7)(R8)X−或硝基,其中R6、R7、R8独立地代表未取代或取代的C1-C6烷基或未取代或取代的—(
CH2)n-,其中n=1至12,但至少有两个取代基R6R7R8为C1-C6烷基,而X−代表卤化物、
磺酸盐、未取代或取代的
乙酸盐、
硫酸盐、
氢硫酸盐、
硝酸盐、
高氯酸盐或
草酸盐。