title: Novel phenanthroline ligands and their kinetically locked copper(I) complexes with unexpected photophysical properties.
abstract: The new, sterically encumbered phenanthroline ligands 1a,b, both characterized by the presence of bulky aryl substituents (3,5-di-tert-butyl-4-methoxyphenyl, 2,4,6-trimethylphenyl) in the 2,9-position, were prepared along with their homoleptic [Cu(1a,b)2]+ and heteroleptic complexes [Cu(1a,b)(phen)]+ (phen = parent 1,10-phenanthroline). Due to the pronounced steric shielding, particularly effective in ligand 1a, the formation of the homoleptic complex [Cu(1a)2]+ becomes very slow (5 days). Once formed, the homoleptic complexes [Cu(1a,b)2]+ do not exchange ligands even with phen added in excess because they are kinetically locked due to the large tert-butylphenyl substituents at the phenanthroline unit. The electronic absorption spectra of the homoleptic complexes [Cu(1a)2]+ and [Cu(1b)2]+ evidence a strongly different ground state geometry of the two compounds, the former being substantially more distorted. This trend is also observed in the excited-state geometry, as derived by emission spectra and lifetimes in CH2Cl2 solution. The less distorted [Cu(1b)2]+, compared to [Cu(1a)2]+, is characterized by a 15- and over 100-fold stronger emission at 298 and 77 K, respectively. Noticeably, the excited-state lifetime of [Cu(1a)2]+ in solution is unaffected by the presence of molecular oxygen and only slightly shortened in nucleophilic solvents. This unusual behavior supports the idea of a complex characterized by a "locked" coordination environment.
authors.affiliation: Center of Micro and Nanochemistry and Engineering, Organische Chemie I, Universität Siegen, Adolf-Reichwein-Strasse, D-57068 Siegen, Germany.
authors.forename: Venkateshwarlu
authors.initials: V
authors.lastname: Kalsani
authors.forename: Michael
authors.initials: M
authors.lastname: Schmittel
authors.forename: Andrea
authors.initials: A
authors.lastname: Listorti
authors.forename: Gianluca
authors.initials: G
authors.lastname: Accorsi
authors.forename: Nicola
authors.initials: N
authors.lastname: Armaroli
mesh_terms: D015392:Chemistry, Inorganic / Q000379:methods; D003300:Copper / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D010618:Phenanthrolines / Q000737:chemistry*; D010777:Photochemistry / Q000379:methods
chemical_list: D008024:Ligands; D010618:Phenanthrolines; D003300:Copper