We report a comprehensive investigation of thermal and photoinduced spin-crossover (SCO) in the Fe(II) complex [Fe(bppI)2](ClO4)2, combining single-crystal optical spectroscopy with variable-temperature single-crystal and powder X-ray diffraction. SCXRD at 100 (low-spin, LS) and 400 K (high-spin, HS) reveals a symmetry-retaining spin transition in the orthorhombic Pbcn space group, with no crystallographic phase change. Temperature-dependent absorption spectra exhibit a moderately sharp, scan-rate-independent spin transition (T1/2 = 335 K), indicating modest cooperativity and no kinetic trapping. Metal-to-ligand charge transfer (MLCT) bands, particularly the LS-state shoulder, offer a sensitive optical probe of spin-state evolution, with no spectral baseline shifts observed during thermal cycling. Efficient LS → HS photoexcitation (LIESST) at 632 nm, producing a long-lived HS state matching the thermal HS spectrum. Upon warming, a remarkable three-step T(LIESST) behavior emerges, marked by a plateau (incubation), abrupt domain-driven cooperative HS → LS transition, and gradual LS recovery. Notably, baseline shifts and interference fringes─absent in thermal transitions─reveal enhanced light-induced cooperativity via domain formation, strong domain–domain communication as a result of photoinduced structural reorganization or phase transition. Reverse-LIESST at 830 nm is inefficient (∼10%), limited by spectral overlap. Relaxation kinetics (57–70 K) display strong sigmoidal profiles characteristic of cooperative nucleation-and-growth dynamics modulated by structural heterogeneity, which is successfully reproduced by a combination of theoretical modeling within the mean-field and mechanoelastic frameworks. The mean-field model effectively describes high-temperature behavior through averaged interactions but fails at low temperatures due to local structural heterogeneities. In contrast, the mechanoelastic model, incorporating molecular-level stress and pressure effects, accurately captures the full temperature-dependent relaxation behavior. Light-induced interference fringes and baseline shifts suggest microstructural reorganizations not observed during thermal cycling. These findings highlight unusually complex structure–function relationships in SCO materials and challenge classical energy-gap-based models.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)