  We have succeeded in synthesizing two types of new organic radical crystals
3-I-V [= 3- (3-iodophenyl)-1,5-diphenylverdazyl] and 3-Br-4-F-V [=
3-(3-bromo-4-fluorophenyl)-1,5- diphenylverdazyl]. Their crystal strucutures
are found to be isomorphous to that of previously reported spin ladder
3-Cl-4-F-V. Through the quantitative analysis of their molecular arrangements
and magnetic properties, we confirm that these materials form ferromagnetic
chain-based spin ladders with slightly modulated magnetic interactions. These
results present the first quantitative demonstration of the fine-tuning of
magnetic interactions in the molecular- based materials.
