Synthesis and characterization of Ln(BTFA)3DMPHEN complexes (Ln= Eu3+, Er3+, Ho3+, Tb3+, Tm3+,AND Yb3+, BTFA=4,4,4-triflouro-1-phenyl-1,3-butanedione,dmphen=4,7-dimethyl-1,10-phenathroline) and sol-gel materials for potential optical applications