When were psychotropic drugs first developed and by whom? Was it a research accident or was it intentional?
Your question is actually more involved than you may think. First of all, it depends on how you define “psychotropic” and “drug” as well as whether you wish to include herbal remedies for mood problems. If you do, we can go back hundreds, if not thousands, of years. In his classic text, The Anatomy of Melancholy (1624), Robert Burton describes the use of herbs such as bugloss, melissa balm, and marigold, in the treatment of “melancholy.” (How effective they were I cannot say!) If you want to look at the era of “modern” psychopharmacology, we can answer your question a bit more precisely.
The antidepressant properties of both the tricyclics and the monoamine oxidase inhibitors were discovered by accident in the late 1950s; e.g., iproniazid was developed as an anti-tuberculosis drug, but the observation that it induced euphoria as a side effect led Dr. George Crane to conduct clinical trials on it (1957); Nathan Kline (1958) further demonstrated the antidepressant properties of MAOIs.
The antipsychotic, chlorpromazine (Thorazine) was initially synthesized as an antihistamine; its sedative properties led surgeon Henri Laborit, and later Delay and Deniker (1952) to administer it to patients with schizophrenia. Lithium’s mood-stabilizing properties were also discovered serendipitously by Dr. John Cade, in the late 1940′s. For a more detailed discussion of these historical issues, see the fine text by Janicak et al, Principles & Practice of Psychopharmacotherapy, Williams & Wilkins, 1993.
More recent development of psychotropic medications has been guided much more by known biochemical properties of effective agents, so that “designer” medications (such as risperidone, olanzapine, and others) are now the rule. You may want to see the book Psychopharmacology: The Fourth Generation of Progress (eds. Bloom & Kupfer) for a detailed discussion of this last topic.