Primary Photoprocesses in Transition Metal Complexes

Forster, Leslie S.

doi:10.1002/9780470133460.ch4

Cited by 11 publications

(1 citation statement)

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“…The photochemical and photophysical behavior of Cr(III) complexes have intrigued researchers for several decades. − These systems show several characteristics that make them particularly attractive for investigating photochemical and photophysical kinetics. They are thermally inert in solution but possess appreciable quantum yields for photosubstitution upon visible irradiation.…”

Section: Introductionmentioning

confidence: 99%

Solvent Effects on the Spectroscopic and Photophysical Properties of thetrans-(1,4,8,11-Tetraazacyclotetradecane)diisothiocyanatochromium(III) Ion,trans-[Cr(cyclam)(NCS)₂]⁺

2007

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The spectroscopy and photophysics of trans-[Cr(cyclam)(NCS)2]+ (where cyclam is 1,4,8,11-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 phirx 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 (Ea) 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 ln(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.

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