Luminescent rigidochromism of fac-[Re(CO)3(phen)(cis-bpe)]+ and its binuclear complex as photosensors

The photophysical properties of the fac-[Re(CO)3(phen)(L)]PF6 and [(phen)(CO)3Re(L)Re(CO)3(phen)](PF6)2 complexes, where L=cis-1,2-bis(4-pyridyl)ethylene (cis-bpe) and phen=1,10-phenanthroline, have been investigated. The cis isomer complexes, obtained photochemically from the corresponding non-emissive trans-bpe complexes, present an enhanced luminescence at room temperature. The broad emission profile is characteristic of the Re-based emissions from the lowest metal to ligand charge transfer state, 3MLCT. In a rigid medium, such as PMMA or glassy EPA at 77 K, the emission maxima of these complexes present hypsochromic shifts as the medium rigidity increases. Changes in emission properties (energy and lifetime of excited states) are discussed based on the luminescent rigidochromic effect, as the energy of MLCT excited states increases in rigid media. This effect is associated with the solvent dipole orientation response to the changes in electronic configuration between ground and excited states.