A displaced and low-frequency vibration of phosphorescent state of trans-[Rh(ethylenediamine)2Cl2]PF6 in a range of 5–497 K

The lowest ${}^3{\mathrm{(d\pi -d\sigma }}^{*})$ excited states of both cis- and trans-isomers of $[{\mathrm{Rh(en)}}_{\mathrm{2}}{\mathrm{Cl}}_{\mathrm{2}}\mathrm{]X}$ (en=ethylenediamine; ${\mathrm{X=PF}}_6^{-},$ ${\mathrm{NO}}_3^{-})$ and the deuteriated crystal of trans-$[\mathrm{Rh(en}{\mathrm{-d}}_4{)}_2{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{]PF}}_6$ have been investigated in the solid state and in a wide temperature range of 5–497 K by means of emission spectra, lifetime and quantum yield measurements. Emission spectral simulation of trans-$[{\mathrm{Rh(en)}}_{\mathrm{2}}{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{]PF}}_6$ shows that the emission from the lowest ${}^3{\mathrm{(d\pi -d\sigma }}^{*})$ excited state exhibits a progression of a low-frequency metal-chloride stretching vibration (250 cm⁻¹) with a large Huang-Rhys factor (S) of 21 and a progression of a high-frequency N–H stretching vibration (3000 cm⁻¹). The increasing full-width at half maximum (2200 cm⁻¹→4400 cm⁻¹) with increasing temperature (77 K→468 K) is ascribed to hot bands from the excited levels of low-frequency vibration. The luminescence quantum yields of the crystal samples are determined to 0.0008 at 298 K and 0.003 at 80 K for trans-$[{\mathrm{Rh(en)}}_{\mathrm{2}}{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{]PF}}_6$ and 0.18 at 298 K and 0.40 at 80 K for trans-$[\mathrm{Rh(en}{\mathrm{-d}}_4{)}_2{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{]PF}}_6.$ From a combination of lifetime and emission quantum yield measurements, values for $k_r$ and $k_{\mathrm{nr}}$ have been obtained. The observed temperature dependence of nonradiative decay rates of trans-$[\mathrm{Rh(en}{\mathrm{-d}}_4{)}_2{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{]PF}}_6$ in a low-temperature region (<300 K) is possible to reconstitute by using the emission spectral fitting parameters and assuming nuclear tunneling mechanism. The temperature effect and deuteriation effect on the nonradiative rate definitively establishes that the dominant “accepting” modes in the nonradiative transition are a highly displaced $\text{(S=21)}$ vibrational mode of low-frequency Cl–Rh–Cl stretching and a weakly displaced $\text{(S=0.1)}$ vibrational mode of high-frequency N–D stretching. The nonradiative transition in a high-temperature region occurs via barrier passing along a displaced coordinate of Cl–Rh–Cl vibration with a pre-exponential factor of ${10}^{11}\hspace{0.17em}{\mathrm{s}}^{\text{−1}}$ and is relatively insensitive to the high-frequency vibrational mode. The crystal of cis-$[{\mathrm{Rh(en)}}_{\mathrm{2}}{\mathrm{Cl}}_{\mathrm{2}}{\mathrm{]NO}}_3$ shows a red shift of the emission peak energy and an increase in the full-width at half maximum with increasing temperature. The results of temperature-dependent decay and spectra of emission can be interpreted in terms of two ${}^3{\mathrm{(d\pi -d\sigma }}^{*})$ emitting states model.