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 ms 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 (krad). 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 p-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 ms 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.