Researchers were searching for drugs to treat schizophrenia when they came across imipramine, which alleviated the symptoms of depression, although it was not effective against schizophrenia. It became the first of a class of drugs, all sharing a similar molecular structure, called:

Answer: Tricyclics

Explanation:

The chemical compounds which are known to have three analogous interconnected rings of the atoms. Several compounds are known to have this tricyclic structure, but in discipline such as pharmacology, this particular terminology has both traditional and contemporary meaning and thus further reserved in order to describe the hetero-cyclic drugs.