  Ultrathin layered crystals of coordinated chromium(III) are promising not
only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR)
emitters owing to long-range spin correlation and efficient transition between
high and low-spin excited states of Cr3+ ions. In this study, we report on
dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic
emission bifurcates into fluorescence and phosphorescence depending on
thickness, temperature and defect density. In addition to the spectral
branching, the biexponential decay of PL transients, also affected by the three
factors, could be well described within a three-level kinetic model for
Cr(III). In essence, the PL bifurcations are governed by activated reverse
intersystem crossing from the low to high-spin states, and the transition
barrier becomes lower for thinner 2D samples because of surface-localized
defects. Our findings can be generalized to 2D solids of coordinated metals and
will be valuable in realizing novel magneto-optic functions and devices.
