Direct Determination of the Rate of Intersystem Crossing in a Near-IR Luminescent Cr(III) Triazolyl Complex

Abstract

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 solutionstable complex [Cr(btmp) 2 ] 3+ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) (1 3+ ), which displays near-infrared (NIR) luminescence at 760 nm (τ = 13.7 μs, ϕ = 0.1%) in fluid solution. The excited-state properties of 1 3+ are probed in detail through a combination of ultrafast transient absorption (TA) and femtosecond-topicosecond 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-lying 4 MC state therefore allows us to assign a value of (823 fs) −1 to 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.

Electronic Structure and Excited-State Dynamics of the NIR-II Emissive Molybdenum(III) Analogue to the Molecular Ruby

“… Hence, we assign this process to the transition from the initially excited 4 LMCT state to the spin-flip state ( 2 T 2 / 2 T 1 / 2 E) with an upper limit for the time constant for intersystem crossing (ISC) τ ISC of 2.5 ps. This is in line with a recent study on a chromium­(III) complex, where it was not possible to isolate the ISC process with TA spectroscopy alone …”

“…This is in line with a recent study on a chromium(III) complex, where it was not possible to isolate the ISC process with TA spectroscopy alone. 108 Next, the bands sharpen and shift to higher energy with a time constant of 6.2 ps, which is consistent with internal conversion and vibrational cooling within the doublet excited states 2 T 1 / 2 E (Figure S58). After 17 ps, the shape of the TA spectra does not change significantly anymore (Figure S58) and the 2 T 1 / 2 E states decay to the 4 A 2 ground state with a time constant of τ = 1.4 ns.…”

A Near-Infrared Luminescent Cr(III) N-Heterocyclic Carbene Complex

“…The torsion angle between the central pyridyl and flanking imidazolylidene moieties is 2.8°, revealing the essentially planar nature of the ligand, with the lack of conformational flexibility preventing helical wrapping around the Cr(III) center, as observed for [Cr(ddpd) 2 ] 3+ 7 and [Cr(btmp) 2 ] 3+ 12 for example. The Cr−N(pyridyl) bond length of 2.020(4) Å is typical for Cr(III) complexes,7,10,12 with the Cr−C(carbene) lengths being only slightly longer at ca. 2.1 Å.…”

“…Consequently, diversification of ligand design and the introduction of new, ideally strong-field, donor moieties to Cr­(III) coordination chemistry is an essential tool in achieving control over the energy of luminescence. We have recently reported the luminescent chromium­(III) triazolyl complex [Cr­(btmp) 2 ] 3+ [btmp = 2,6- bis (4-phenyl-1,2,3-triazol-1-ylmethyl)­pyridine], although phosphorescence falls within the more typical spectral region at λ em = 760 nm . A series of complexes featuring 1,8-( bis -oxazolyl)­carbazolide ligands were reported to emit over the range 813–845 nm in fluid solution, whereas the complex [Cr­(bpi) 2 ] 3+ [bpi = 1,3- bis (2′-pyridylimino)­isoindoline] displays weak room temperature (r.t.) luminescence centered at 950 nm .…”

Time-resolved radioluminescence of the Cu(i) cluster Cu4I62−. Different responses to photo, X-ray, β-ray and α-particle excitation

“…The strictly exponential PL decays imply internal conversion/intersystem crossing (IC/ISC) to the emissive states on a much faster time frame, consistent with sub-ns IC/ISC rates found with other metal complexes. 22…”