Temperature dependence of emission from bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I) ([Cu(dtbp)(2)](+)) as well as several other copper(I) bis(phenanthroline) complexes was measured between 190 and 300 K. An analysis based on a Boltzmann distribution model gives an energy gap between (MLCT)-M-1 and (MLCT)-M-3 excited states as 790 cm(-1) for [Cu(dtbp)(2)]SbF6. The contribution of the delayed fluorescence largely depends on the ligand substituents.