Schoenberg was a pioneer of atonal music, which rejects traditional tonality and harmony. His music often features unprepared dissonances, which are jarring clashes of notes that don't resolve in a traditional way. He believed these dissonances could effectively capture the emotional intensity and complexity of modern poetry.
One of Schoenberg's most famous works is his "Pierrot Lunaire", a song cycle for voice and chamber ensemble set to poems by the Belgian symbolist poet Albert Giraud. Schoenberg's use of Sprechgesang (speech-singing), along with the stark, dissonant music, perfectly captures the surreal and often disturbing nature of the poems.
However, it's important to note that other composers have also used dissonances in their settings of poetry. For example, Alban Berg also used dissonances in his operas, particularly in his setting of "Wozzeck", which is based on a play by Georg Büchner.
Therefore, while Schoenberg is known for his use of unprepared dissonances in setting poetry, it's not exclusive to him. Other composers have also explored this technique to create unique and powerful musical interpretations of poetic texts.