lkyl or an aryl group, or ???(CH???CH)m???R4 wherein R4 is an aryl or heterocyclic group; m is 1 or 2; Y is selected from the group consisting of S, O, Se, ???C(R1)2, and ???NR1; X is a divalent linking group selected from a group consisting of substituted or unsubstituted methylenes, (???CR5R7???)n and [(???CR5R7)n???X1???(CR5R7???)P] wherein R5 or R7 are independently hydrogen, alkyl, substitute