se selected from purine and pyrazolo 3,4-d!pyrimidine groups wherein the (CH2)q group is attached to the 7-position or 8 position of the purine and 3-position of the pyrazolo 3,4-d!pyrimidine groups and the R1 group is attached to the 9-position of the purine and to the 1-position of the pyrazolo 3,4-d!pyrimidine groups; Y is a functional linking group selected from a group consisting of --O--, --