Modulating the spin-flip rates and emission energies through ligand design in chromium(iii) molecular rubies

Three homoleptic spin-flip (SF) emitters, namely [Cr(Mebipzp)₂]³⁺ (1), [Cr(IMebipzp)₂]³⁺ (2) and [Cr(bip*)₂]³⁺ (3), have been successfully synthesized and characterized. The weak distortion compared to a perfect octahedron imparts favourable structural properties to the three complexes, which display spin-flip (SF) luminescence at approximately 740 nm with quantum yields in the range of 9-11% for 1 and 2 in deaerated acetonitrile solutions at 25 °C. Time-resolved luminescence and transient UV-vis absorption experiments unveiled lifetimes for the lowest-lying ²MC (metal-centered) of 1.5 ms for 1 and 350 μs for 2. The incorporation of iodine atoms onto the ligand scaffold in 2 accelerates the ²MC → ⁴A₂ relaxation process through simultaneous enhancements in the radiative and non-radiative rate constants. In agreement, the experimentally calculated absorption oscillator strength for the ²MC ← ⁴A₂ transition amounts to 9.8 × 10^-7 and 2.5 × 10^-6 for 1 and 2, respectively. The 2.5 factor enhancement observed in the iodine derivative indicates a higher spin-flip transition probability, translating into higher values of radiative rate constant (k _rad). Interestingly, in compound 3, the substitution of the distal methyl-pyrazole with indazole rings causes an important bathochromic shift of the SF emission energy to 12 000 cm^-1 (830 nm). Likely, the extended π-system and the more covalent bond character induced by the indazole decrease the interelectronic repulsion further stabilizing the SF excited states. The recorded excited state lifetime of 111 μs in 3 remains among the longest for a molecular ruby emitting beyond 800 nm. These discoveries signify an underexplored avenue for modifying deactivation pathways and emission energy while retaining high quantum yields and long-lived excited states in molecular rubies.