Crowded Cu(I) complexes involving benzo[h]quinoline: pi-stacking effects and long-lived excited states

The Friedlander condensation was employed to synthesize two series of 3,3 ' -polymethylene bridged ligands, L, based on 2-(2 ' -pyridyl)-benzo[h]quino line and 2,2 ' -bibenzo[h]quinoline (BHQ) along with the fully aromatic nap htho[1,2-b]-1,10-phenanthroline. Complexes [Cu(L)(2)](+) were prepared as t heir perchlorate or hexafluorophosphate salts. The solution state structure s were analyzed by NMR and shielding effects reflected significant interlig and pi -stacking interaction in the complexes. Solid-state structures of th e complexes where L = 3,3 ' -tetramethylene-2,2 ' -bibenzo[h]quinoline or n aphtho[1,2-b]-1,10-phenanthroline were determined by X-ray analysis. The te tramethylene bridged complex showed a highly distorted coordination geometr y with the BHQ rings of opposing ligands pi -stacked at a interplanar dista nce of about 3.37 Angstrom. Complexes of the BHQ series showed a pronounced MLCT absorption maximum which shifted bathochromically from 496 to 610 nm as the 3,3 ' -bridge decreased from 4 to 2 carbons. The BHQ complexes lumin esced strongly in CH2Cl2 solution and the tetramethylene-bridged system sho wed the longest yet recorded excited-state lifetime for a copper MLCT excit ed state, tau = 5.3 mus and Phi = 0.10.