provided that when R1 and R4 taken together form optionally substituted butadienylene, R1 is a group represented by the formula: ???Y1???Y2???Y3???Ra wherein Y1 is single bond; Y8 is optionally substituted alkylene; Y2 is ???NRb???C(???O)???, ???C(???O)???NRb???; Ra is optionally substituted carbocyclic group, optionally substituted heterocyclic group; and Rb is hydrogen or optionally substituted