A detailed understanding of the dynamics of photoinduced
processes
occurring in the electronic excited state is essential in informing
the rational design of photoactive transition-metal complexes. Here,
the rate of intersystem crossing in a Cr(III)-centered spin-flip emitter
is directly determined through the use of ultrafast broadband fluorescence
upconversion spectroscopy (FLUPS). In this contribution, we combine
1,2,3-triazole-based ligands with a Cr(III) center and report the
solution-stable complex [Cr(btmp)2]3+(btmp
= 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine)
(13+), which displays near-infrared
(NIR) luminescence at 760 nm (τ = 13.7 μs, ϕ = 0.1%)
in fluid solution. The excited-state properties of13+are probed in detail through a combination
of ultrafast transient absorption (TA) and femtosecond-to-picosecond
FLUPS. Although TA spectroscopy allows us to observe the evolution
of phosphorescent excited states within the doublet manifold, more
significantly and for the first time for a complex of Cr(III), we
utilize FLUPS to capture the short-lived fluorescence from initially
populated quartet excited states immediately prior to the intersystem
crossing process. The decay of fluorescence from the low-lying4MC state therefore allows us to assign a value of (823 fs)−1to the rate of intersystem crossing. Importantly,
the sensitivity of FLUPS to only luminescent states allows us to disentangle
the rate of intersystem crossing from other closely associated excited-state
events, something which has not been possible in the spectroscopic
studies previously reported for luminescent Cr(III) systems.