decadent vs fin-de-siecle

decadent is a noun but fin-de-siecle is not a noun.

Word	Noun	Adjective	Verb	Adverb
decadent	Yes	Yes	No	No
fin-de-siecle	No	Yes	No	No

As adjectives, decadent and fin-de-siecle are synonyms defined as:

decadent and fin-de-siecle: marked by excessive self-indulgence and moral decay

decadent (noun)	fin-de-siecle (noun)
a person who has fallen into a decadent state (morally or artistically)

decadent (adjective)	fin-de-siecle (adjective)
marked by excessive self-indulgence and moral decay	relating to or characteristic of the end of a century (especially the end of the 19th century)
luxuriously self-indulgent	marked by excessive self-indulgence and moral decay