STERIC INFLUENCES ON THE PHOTOLUMINESCENCE FROM COPPER(I) PHENANTHROLINES IN RIGID MEDIA

Data for a series of copper(1) phenanthrolines in an alcohol glass at 90 K have revealed that substituents in the 2,9-positions of the ligand have a pronounced influence on the energy, intensity, and lifetime of the photoluminescence. All of the complexes exhibit intense metal-to-ligand charge-transfer absorption bands of similar energy in the visible region. However, the absorption maxima and, to an even greater extent, the emission maxima shift to higher energy as bulkier substituents are introduced. At the same time, the emission quantum yields and lifetimes increase. The results can be understood in terms of contrasting geometric preferences of the ground and excited states involved. Whereas the d10 ground state tends toward idealized D2d symmetry, the d9 excited state favors a D2 symmetry. Bulkier ligands inhibit geometric relaxation within the excited state and give rise to higher energy excited states. Consequently, nonradiative decay is less efficient in these systems, and the quantum yields and excited-state lifetimes are enhanced.