Photochemistry and Photophysics of Coordination Compounds

42 N.A.P. Kane-Maguire
the time scale of nuclear motion, and have until recently been most frequently
associated with intramolecular processes such as vibrational equilibration
and conformational dynamics and with medium effects such as solvation
shell reorientation. The time frames involved are in the femtosecond to low
picosecond range, and because of the challenging experimental demands, the
field of ultrafast transition metal spectroscopy is still in its infancy [35–37].
As depicted earlier (Fig. 2), for the Franck–Condon excited level generated
by initial light absorption, evolution down to lower-energy excited states
involves processes such as IC, ISC, and vibrational relaxation (VR). Conventional
wisdom based on organic experience suggests that the rate constants
for these three processes would normally be in the order: kVR >kIC >
kISC [38]. However, the observation that Cr(III) 2 Eg → 4 A2g phosphorescence
yields and emission/reaction quenching ratios showed some dependence on
the wavelength chosen for 4 A2g → 4 T2g excitation led to the suggestion that
4 T2g → 2 Eg ISC may compete successfully with 4 T2g vibrational cooling [39–
43]. This possibility received support from picosecond pulse laser studies
that showed that the rise time for 2 Eg excited state transient absorption was
shorter than the instrument time response [12–14].
3.1
Ultrafast Dynamics of 2 Eg State Formation in Cr(acac)3
The first Cr(III) femtosecond spectroscopic study addressing the questions
raised above has been very recently reported in an elegant study by Juban
and McCusker for the test case of [Cr(acac)3] (where acac = acetylaceto-
Fig. 3 Simplified Jablonski state energy level diagram for [Cr(acac)3]