Solvent effects on the spectroscopic and photophysical properties of the trans-(1,4,8,11-Tetraazacyclotetradecane)diisothiocyanatochromium(III) ion, trans-[Cr(cyclam)(NCS)2]+

The spectroscopy and photophysics of trans-[Cr(cyclam)(NCS)(2)](+) (where cyclam is 1,4,8,1 1-tetraazacyclotetradecane) were studied in a range of solvents. The cyclam NH stretching vibration [nu(NH)] wavenumber correlates with the Gutmann donor number, whereas the thiocyanate CN stretching vibration [nu(CN)] wavenumber correlates with the Snyder solvent strength (P') scale. These results signify that there is a difference in the solvent interactions with the two types of ligands. The energy of the ligand-to-metal charge transfer absorption maximum between 310 and 320 nm and the energy of the spin-forbidden (doublet-quartet) absorption and emission bands above 700 nm correlate with the nu(CN) wavenumber. This establishes the dominant role of solvent effects at the NCS- ligand in "tuning" the energy of these spectroscopic features. Quantum yields phi(rx) for photosubstitution are < 0.02 at 54 degrees C and < 0.002 at 22 degrees C, demonstrating that photochemical reaction is a very minor pathway. The effects of solvent and temperature on the nonradiative decay of the doublet excited-state were investigated by observing the time-resolved phosphorescence between 700 and 750 nm. Below 30 degrees C, the lifetimes are relatively temperature-independent, whereas at higher temperatures, a strong Arrhenius-type dependence is observed. Values for the preexponential factor (A) and the activation energy (E-a) are solvent-dependent and follow a Barclay-Butler-type correlation. These observations are consistent with a dominant back-intersystem crossing pathway for nonradiative decay in the higher-temperature region. From trends observed between In(A) and the nu(CN) frequency, it appears that solvent effects at the thiocyanate ligand play a dominant role in influencing the rate of nonradiative decay in the high-temperature region.