formula a within a particular interior generation have the same chemical structure in some embodiments all of the molecular units of formula a in all interior generations have the same chemical structure in some embodiments 2 or more of the molecular units of formula a within a particular interior generation or between interior generations have differing chemical structures e g r 3 in one molecule differs from r 3 in 1 or more of the other formula a molecules either in the same interior generation or in a different interior generation in some embodiments the dendrimer comprises 1 generation thus the dendrimer contains 0 zero interior generations the dendrimer comprises an exterior layer that is attached to the outermost of the interior generations or to the core through the terminal residues if no interior generations are present the exterior layer comprises 2 or more molecular units independently having formula b wherein r 1 r 2 and r 3 carry the same definitions as in the formula a molecule and r 4 and r 5 are each h or r 4 r 5 and the nitrogen to which they are attached form a no 2 group in some embodiments r 4 and r 5 are each h in some embodiments r 4 r 5 and the nitrogen to which they are attached.