SUBPICOSECOND (1)MLCT-](5)T(2) INTERSYSTEM CROSSING OF LOW-SPIN POLYPYRIDYL FERROUS COMPLEXES

Two different DELTAS = 2 intersystem crossing processes, the 1MLCT --> 5T2 and 5T2 --> 1A1 conversions which follow the photoexcitation of low-spin Fe(II) complexes, are examined on the nanosecond, picosecond, and subpicosecond time scales. Aqueous solutions of thc complexes [Fe(tpen)](ClO4)2, [Fe(tptn)](ClO4)2, [Fe(t-tpchxn)](ClO4)2, [Fe(bpy)3](ClO4)2, [Fe-(phen)3](ClO4)2, and [Fe(terpy)2](ClO4)2 are studied, where in the first three complexes the hexadentate ligands are tetrakis(2-pyridylmethyl)ethylenediamine (tpen), tetrakis(2-pyridylmethyl)-1,3-propylenediamine (tptn), and tetrakis(2-pyridylmethyl)-trans-cyclohexane-1,2-diamine(t-tpchxn). The [Fe(tpen)]2+ complex is a spin-crossover complex with a 1A1 ground state and a thermally accessible 5T2 excited state. In H2O this complex exhibits an isobestic point in its electronic absorption spectrum at 302 +/- 2 nm. Picosecond laser-flash instrumentation is used with probe wavelengths from 266 to 460 nm to determine the nature of thc (18 +/- 2)-ns relaxation seen for [Fe(tpen)]2+. In the transient difference spectrum, there is a change from bleach to absorption response at approximately 300 nm. This definitively establishes the (18 +/- 2)-ns relaxation as being associated with the 5T2 --> 1A1 conversion. The related complex [Fe(tptn)]2+, which is all low-spin in solution throughout the temperature range examined, also shows a transient difference spectrum with an isosbestic at approximately 300 nm, suggesting that formation of 5T2 is a general feature. Laser instrumentation with a approximately 500-fs pulse (lambda(pump) = 314 nm) is used to measure the rate of formation of the 5T2 state following 1MLCT <-- 1A1 excitation of low-spin Fe(II) complexes. All of the above complexes were examined, and the observed kinetics were found to be independent of probe wavelength in the 385-510-nm range. A plot for each complex of DELTAOD versus time of data collected in increments of 167 fs over the first 10-15 ps following 1MLCT <-- 1A1 excitation clearly shows that the 5T2 state of these Fe(II) complexes is formed within approximately 700 fs. Intersystem crossing is at least as fast as internal conversion and vibrational cooling in these complexes. Transient absorption spectra (333-fs increments) suggest, furthermore, that vibrational cooling in the 5T2 excited state is complete within approximately 2-3 ps. Thc reasons for the surprisingly fast (< approximately 700 fs) MLCT --> 5T2 intersystem crossing where DELTAS = 2 are discussed.