In some preferred embodiments of compounds of formula I, at least two of Ru-Rz are H, the remainder being substituents other than H. In particularly preferred embodiments at least two of Ru-Rz are H, the remainder being substituents other than H; and Y1 and Y2 are both O. In especially preferred embodiments, embodiments at least two of Ru-Rz are H, the remainder being substituents other than H; Y1