The present invention is directed to compounds having the formula: In the formula, A2 represents N or C—X1a—R1 and A4 represents N or C—X2b—R2, provided that A2 and A4 are not both N. R1 and R2 independently represent H or a hydrocarbon group, provided that R1 and R2 are not both H. The hydrocarbon groups of R1 and R2 are preferably alkyl or alkenyl groups. More preferably, the hydrocarbon groups are alkyl groups. A1, A3, A6, and A8 independently represent CH or N. A5 and A9 independently represent CR3 or N, wherein R3 independently represents H, methyl, ethyl, or halo. A7 represents CR4 or N, wherein R4 represents H, methyl, ethyl, halo, nitro, hydroxy, amino, amido, or a methyl or ethyl group substituted independently with halo, nitro, hydroxy, amino, or amido. Not more than three of A5, A6, A7, A8, and A9 are N.