fin-de-siecle vs decadent

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

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

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

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

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

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