As a result of the Iranian takeover of the American Embassy on November 4, 1979, the United States and Iran severed diplomatic relations in April 1980. The United States and the Islamic Republic of Iran have had no formal diplomatic relationship since that date. Switzerland is the U.S. protecting power and provides limited consular services to U.S. citizens in Iran.
Iran has no embassy in Washington, D.C.
More information about the Islamic Republic of Iran is available from the Department of State and other sources, some of which are listed here: