Infrared rigidochromism: a new effect in the IR spectra of the excited states of coordination compounds

The magnitude of the shift in the ν(CO) IR spectrum of the MLCT excited state of [Re(CO)3Cl(bipy)](bipy = 2,2′-bipyridyl) in PrCN–EtCN solution, compared with the ground-state spectrum, decreases on cooling from fluid to glass; this ‘infrared rigidochromic’ effect is explained by the change in character of the MLCT state on glass formation.