Pressure-induced tuning of fluorescence to phosphorescence in [Cr(urea- h4) 6] (ClO 4) 3 and [Cr(urea- d4) 6] (ClO 4) 3

Abstract

In [Cr(urea- h 4) 6] (ClO 4) 3 the lowest sublevel of 4T 2g (t 2 2ge 1 g) lies Δ E = 90 ± 10 cm −1 above the lower 2E g(t 3 2g) sublevel. At 1.3 K one observes a fine-structured phosphorescence. With temperature increase, for example to 120 K, the quartet is thermally occupied and one obtains a broad-band fluorescence. Pressure application at the same temperature results in a blue-shift of 4T 2g relative to 2E g. This reduces the thermal repopulation of the quartet. Thus it is possible to induce a fine-structured phosphorescence by pressure. The emission intensity of the origin(s) corresponding to 2E g increases by a factor of about 10 2 with pressure application up to 20 kbar. [Cr(urea- d 4) 6] (ClO 4) 3 exhibits somewhat different properties. Δ E is larger (140 ± 10 cm −1) and the observed effects are less distinct. The origin lines corresponding to 2E g exhibit linear shifts of Δ ν /Δ p = −(5.5 ± 0.5) cm −1/kbar (protonated compound) and Δ ν /Δ p = −(5.3 ± 0.5) cm −1/kbar (deuterated compound). Vibrational frequencies, determined from vibronic satellites and IR spectra, are given for both compounds. Most vibronic satellites in the phosphorescence spectra are blue-shifted with pressure relative to the origins.