| abstract |
The invention relates to materials of the formula I: in which R₁, R₂, and R₃ can each independently be the same or different and represent hydrogen atom, halogen atom, hydroxyl group, nitro group, cyano group or monovalent, optionally substituted organic radical and R₄ represents hydrogen atom, halogen atom, nitro group, cyano group or monovalent, optionally substituted organic radical and R₅ can be the same or different and means halogen atom, hydroxyl group, nitro group, cyano group or monovalent, optionally substituted organic radical. |