wherein when R4 is a substituted aryl group the substituted aryl group is a phenyl having at least one substituent, a naphthyl having at least one substituent, a biphenyl having at least one substituent, a diphenylmethyl having at least one substituent, or a benzophenone having at least one substituent, wherein when R4 is a substituted aryl group the substituted aryl group of R4 has at least two s