Pressure-induced tuning of fluorescence to phosphorescence in [Cr(urea-h₄)₆](ClO₄)₃ and [Cr(urea-d₄)₆](ClO₄)₃

Abstract

In [Cr(urea-h₄)₆](ClO₄)₃ the lowest sublevel of 4T2g(t2g2 eg1) lies ΔE=90±10 cm⁻¹ above the lower 2Eg(t2g2) 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 4T2g relative to 2Eg. 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 2Eg increases by a factor of about 10² with pressure application up to 20 kbar. [Cr(urea-d₄)₆](CIO₄)₃ exhibits somewhat different properties. ΔE is larger (140±10 cm⁻¹) and the observed effects are less distinct. The origin lines corresponding to 2Eg exhibit linear shifts of Δν/Δp= -(5.5±0.5) cm⁻¹/kbar (protonated compound) and Δν/Δp= -(5.3±O.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.