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

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 of 13+ 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-lying 4MC 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.


Figure S1
1 H NMR spectrum for the ligand btmp S2

Figure S2
13 C NMR spectrum for the ligand btmp S2

Figure S3
ESI mass spectrum for the complex 1 3+ S3

Figure S4
UV-Visible absorption spectrum recorded for an MeCN solution of btmp S3

Figure S6
Excitation spectrum recorded for an MeCN solution of 1 3+ S4

Figure S9
Spectral output profile of 23W compact fluorescent lamp (CFL) source S6

Figure S14
Steady-state fluorescence spectrum recorded for an MeCN solution of 1 3+ S8

Figure S15
Selected FLUPS kinetic and spectral data from measurement cycles 1-4 S8

Figure S16
Calculated spin density plots for optimised quartet and doublet states of 1 3+ S9

Table S1
Summarised results from quantum chemical calculations S9 TD-DFT calculations of quartet-quartet vertical excitations S10

Table S2
Selected calculated molecular orbitals for the quartet ground state S11 Optimised quartet state geometry for 1 3+ S13 Optimised doublet state geometry for 1 3+ S15 Optimised quintet state geometry for 1 2+ S17 Optimised triplet state geometry for 1 2+ S19

Figure S3
High

Figure S4
UV-Visible absorption spectrum recorded for an MeCN solution of the ligand btmp.

Figure S5
Cyclic voltammogram recorded at 100 mVs -1 for a 1.5 mmol dm -3 solution of 1 3+ in MeCN containing 0.2 mol dm -3 n NBu4PF6. (Features marked * were also present in preliminary voltammograms recorded for the electrolyte-containing solvent in the absence of 1 3+ ).

Figure S6
UV-Visible electronic absorption spectrum (black) and excitation spectrum for luminescence at λem = 760 nm (green) recorded for an aerated MeCN solution of 1 3+ .

Figure S7
UV-Visible electronic absorption spectra recorded for an MeCN solution of 1 3+ , stored in the dark, over 0-72 hours.

Figure S8
UV-Visible electronic absorption spectra recorded for an aqueous solution of 1 3+ , stored in the dark, over 0-72 hours.

Figure S9
Output profile of the 23W compact fluorescent lamp (CFL) irradiation source employed for photostability experiments.

Figure S11
Photoluminescence spectra recorded for a room temperature aerated MeCN solution of 1 3+ during irradiation with a 23W CFL (λex = 410 nm).

Figure S15
a: Kinetic decay traces at 550 nm obtained from each of the four FLUPS measurement cycles. b: Spectral data recorded at -500 fs (solid lines) and 140 fs (symbols) time delay for each of the four FLUPS measurement cycles. (These plots are presented to demonstrate the reproducibility of the FLUPS signal between cycles) a b

Figure S16
Calculated spin density plots for the optimized lowest energy quartet (a) and doublet (b) states of 1 3+ (Results obtained at the uB3LYP*/6-311G(d) level of theory in acetonitrile).

Table S1
Results from quantum chemical uDFT calculations for 1 3+ carried out at the B3LYP*/6-311G(d) level of theory in an implicit acetonitrile solvent. For each state and geometry combination, the results include the total energies (Etot) in Hartrees, the energy relative to the calculated overall 4 Cr(III) ground state (Erel) in eV, as well as the calculated Mulliken Spin Density (SD) on the metal centre.