Pre-Turkic Azari
The current Turkic Azeri language spoken in Azerbaijan began replacing the old Iranic language only with the advent of the Safavid dynasty's rule in Persia, when hundreds of thousands of Kizilbash (Shia) Turcomans from Anatolia arrived into Azerbaijan, being forced out by the Ottoman Sultan Selim I with more to follow. Earlier, many Turkic speaking nomads had chosen the green pastures of Azerbaijan, Aran and Shrivan for their settlement as early as the advent of the Seljuqs. However, they only filled in the pasturelands while the farmlands, villages and the cities remained Iranic in language and culture. The linguistic conversion of Azerbaijan went hand in hand with the conversion of the Azeris into Shiism. By the late 1800s, the Turkification of Azerbaijan was near completion with the old Iranic speakers found solely in tiny isolated recesses of the mountains or other remote areas (such as Harzand, Galin Ghuya, Shahrud villages in Khalkhal and Anarjan). The city of Tabriz---capital of Azerbaijan, however, maintained a number of distinctly Old Azari-speaking neighborhoods well into the Qajar period of the Persian history. The poet Ruhi Onarjani still composes an entire compendium in Old (Iranic) Azari language in the 19th century.
It seems the nail was driving into the coffin of the old language in Tabriz by the selection of that city as a second capital of Persia/Iran in the course of the 19th century where the Qajar crown prince, Muzaffar al-Din (later, the Shah) resided for nearly 50 years. Muzaffar al-Din used Turkic Azeri as the sole language of his court and himself could barely speak Persian upon assuming the throne in 1892.
Read more about this topic: Old Azari Language