Construction of SCO-fluorescence bifunctional cobalt(II) complexes with anthracene-decorated terpy ligands

Abstract

The development of bifunctional materials integrating spin-crossover (SCO) and fluorescence properties has attracted increasing attention. However, no reports have thus far described Co(II)-based SCO systems exhibiting such dual functionality. In this study, we designed a terpyridine ligand incorporating an anthracene-decorated fluorophore, 4′-(anthracen-9-yl)-4,4″-dimethyl-2,2′:6′,2″-terpyridine (L), and synthesized three mononuclear Co(II) complexes: [Co(L)₂](BF₄)₂·Et₂O·0.8CH₃OH·H₂O (1), [Co(L)₂]Br₂·3DMF (2) and [Co(L)₂](ClO₄)₂·0.5H₂O (3). Magnetic susceptibility measurements revealed that all three complexes exhibit incomplete SCO behavior. Variable-temperature fluorescence spectroscopy demonstrated the coexistence of SCO and fluorescence in these systems. The intramolecular luminescence emission characteristics were further elucidated through variable-temperature UV-vis absorption spectroscopy and time-dependent density functional theory calculations. This work expands the system of bifunctional SCO-fluorescent materials and provides valuable insights into the design of multifunctional molecular systems.

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Introduction

Spin-crossover (SCO) materials, as the most widely studied switchable magnetic materials, can switch their spin states between high spin (HS) and low spin (LS) under external stimuli such as light, pressure, solvents, heat and magnetic fields.^1–3 This bistable state has potential applications in molecular switches, sensors, display devices and information storage.^4–8 Introducing fluorescence,^9,10 chirality,^11,12 ferroelectricity^13,14 and other properties into spin-crossover complexes can afford novel multifunctional spin-crossover materials. Luminescent SCO materials can adjust their luminescence through the spin state, and simultaneously detect the luminescence signal to monitor the changes in the spin state.¹⁵ In this vein, the development of novel SCO-fluorescence molecular materials is of essential significance for multi-channel switching, biosensing and spin-optical device applications.¹⁶

To date, achieving the integration of SCO with fluorescence while retaining the intrinsic characteristics of both properties remains a significant challenge, because the SCO property is highly sensitive to subtle structural perturbations and fluorescence is often severely quenched by the paramagnetic center. Recent studies suggest that the coupling mechanism in luminescent SCO complexes predominantly involves Förster resonance energy transfer (FRET), which requires spectral overlap between the emission band of the luminophore and the UV-vis absorption band of the metal ion, as well as an optimal spatial proximity to facilitate efficient energy transfer.¹⁷ However, emerging observations indicate that the excitation and emission processes in such systems are more complex than previously understood, due to hybridization between the metal center's 3d orbitals and the molecular orbitals of the luminophore.¹⁸ Recent theoretical studies have suggested that intramolecular luminescence involves metal-to-ligand charge transfer (MLCT) contributions, highlighting the intricate nature of these interactions.¹⁹ Nonetheless, detailed investigations into intramolecular luminescence processes within SCO systems remain limited.

Over the past decade, research on cooperative materials involving SCO and fluorescence has made significant progress. A variety of synthetic strategies have been established to construct luminescence-coupled SCO molecular systems, including the grafting of SCO units onto one-dimensional molecular chains, the design of two- and three-dimensional luminescent Hofmann-type frameworks, the substitution of mononuclear complexes, and the incorporation of luminescent rare-earth complexes.^20–30 Among the molecular SCO systems, mononuclear ones are particularly attractive due to their concise structures and synthesis routes, clear structure–property relationships, and the wealth of existing research examples. Triki et al. reported a mononuclear Fe^II complex exhibiting a correlated spin transition and strong fluorescence, whose coordination sphere is saturated by six phosphorescent ligands.³¹ Recently, Tang et al. reported a mononuclear complex [Fe(bpp-TPE)₂]·(ClO4)₂·H₂O·0.5CH₂Cl₂, which exhibits gradual SCO behavior and strong luminescence in the aggregated state.³² At the same time, Li et al. synthesized two mononuclear Fe^II complexes incorporating aggregation-induced emission (AIE)-active tetraphenylethene (TPE) luminophores.³³ Both complexes exhibited clear magnetic bistability alongside distinct fluorescence behavior. While a growing number of mononuclear Fe^II-based SCO complexes exhibiting luminescence-coupled behavior have been reported, studies explicitly addressing the underlying intramolecular energy transfer mechanisms remain limited.³⁴ Further exploration of these processes is essential for advancing the rational design of multifunctional SCO-luminescent systems. In comparison with Fe^II-based systems, Co^II complexes represent a less-studied yet equally promising SCO system. During the SCO process, Co^II complexes undergo smaller coordination bond length changes than Fe^II complexes (approximately 0.10 Å vs. 0.20 Å, respectively). This results in lower enthalpy and entropy changes (ΔH and ΔS), faster spin transition kinetics, and heightened sensitivity of the Co^II SCO process to external stimuli. Therefore, most Co^II complexes exhibit gradual SCO behavior. Moreover, the d–d transitions of Co^II and Fe^II and their UV-vis absorption bands are different. The luminescence coupling of Co^II SCO complexes may exhibit different properties from those of Fe^II SCO complexes. However, no Co^II complex exhibiting both SCO and fluorescence properties has been reported to date.

Co^II-2,2′;6′,2″-terpyridine (terpy) complexes and their derivatives are known to exhibit SCO behavior.^35,36 The SCO properties in these systems can be finely modulated by counterions, solvent molecules, and various non-covalent interactions.^37,38 In addition, the inherent structural flexibility of terpy ligands facilitates the design of supramolecular assemblies and coordination polymers with tunable spectral and magnetic characteristics, thereby enhancing physicochemical responsiveness and offering promising potential for applications in molecular switches, data storage devices, and sensing technologies. The SCO behavior of Co^II complexes can be further regulated through ligand modification and the selection of suitable anions and solvents.^39–41 In this work, we designed and synthesized an anthracene-functionalized terpy ligand, 4′-(anthracen-9-yl)-4,4″-dimethyl-2,2′:6′,2″-terpyridine (L). By coordinating a fluorophore-decorated terpy ligand with cobalt(II) center, we successfully obtained three complexes: [Co(L)₂](BF₄)₂·Et₂O·0.8CH₃OH·H₂O (1), [Co(L)₂]Br₂·3DMF (2) and [Co(L)₂](ClO₄)₂·0.5H₂O (3). A temperature-dependent structural, magnetic fluorescence spectra study demonstrated that all three complexes exhibit coexisting SCO and fluorescence properties.

Experimental

Materials and synthesis

The ligand 4′-(anthracen-9-yl)-4,4″-dimethyl-2,2′:6′,2″-terpyridine (L) was synthesized according to the procedure described in the literature.⁴² The synthesis details of ligand L are described in Scheme S1. The structure and purity of L were characterized by ¹H NMR (Fig. S1) and ESI-MS (m/z = 438.17).

Synthesis of [Co(L)₂](BF₄)₂·Et₂O·0.8CH₃OH·H₂O (1). To the solution of ligand L (0.2 mmol) in 10 mL dichloromethane, Co(BF₄)₂·6H₂O (0.1 mmol) in 5 mL methanol was added, the mixture was vigorously stirred for 30 min, the solution was filtered, and red flake-like crystals of complex 1 were obtained by diffusing diethyl ether into the dichloromethane solution that contains 1. Based on Co(BF₄)₂·6H₂O, its yield is about 60%. Anal. calcd (%) for 1 (C_66.8H_61.2B₂CoF₈N₆O_2.8): C, 65.48; H, 5.03; N, 6.86. Found: C, 65.74; H, 5.21; N, 6.58.

Synthesis of [Co(L)₂]Br₂·3DMF (2). To the solution of ligand L (0.2 mmol) in 20 mL N,N-dimethylformamide, CoBr₂ (0.1 mmol) in 1 mL methanol was added, the mixture was vigorously stirred for 30 min, the solution was filtered, and the bulky crystals of complex 2 were obtained by diffusing diethyl ether into the N,N-dimethylformamide (DMF) solution that contains 2. Based on CoBr₂, its yield is about 50%. Anal. calcd (%) for 2 (C₇₁H₆₇Br₂CoN₉O₃): C, 64.95; H, 5.14; N, 9.60. Found: C, 65.25; H, 4.95 N, 9.78.

Synthesis of [Co(L)₂](ClO₄)₂·0.5H₂O (3). To the solution of ligand L (0.2 mmol) in 10 mL dichloromethane, Co(ClO₄)₂·6H₂O (0.05 mmol) in 5 mL methanol was added, the mixture was vigorously stirred for 30 min, the solution was filtered, and the bulky crystals of complex 3 could be obtained by slowly solvent evaporation at room temperature for 8–9 days. Based on Co(ClO₄)₂·6H₂O, its yield is about 70%. Anal. calcd (%) for 3 (C₆₂H₄₇Cl₂CoN₇O_8.5): C, 65.22; H, 4.15; N, 8.59. Found: C, 64.95; H, 4.33; N, 8.85.

Physical measurements

Single crystal-XRD data collection and structure refinement. The single crystal X-ray SC-XRD diffraction data of complexes 1, 2 and 3 at different temperatures were collected on a Bruker D8 Venture CMOS diffractometer (Mo-Kα radiation, λ = 0.71073 Å) using SMART and SAINT programs. Final unit cell parameters were based on all observed reflections from integration of all frame data. The structure was analyzed by using the intrinsic phasing through the ShelXT program implanted in the Olex2 software, and the data were refined by the full matrix least square method using the ShelXL program. The disorder modelling was adopted when analyzing the single crystal structures. For all complexes, all non-hydrogen atoms are treated with anisotropy, and hydrogen atoms were added theoretically and riding on the concerned atoms.

Thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) was performed using TG/DTA Q600 (Mettler Toledo) instruments in a nitrogen atmosphere at a warming rate of 10 K min⁻¹ from ambient temperature to 1073 K.

Powder X-ray diffraction (XRD). Powder X-ray diffraction (PXRD) data were obtained on a Bruker D8 Venture diffractometer (Cu-Kα radiation, λ = 0.154178 nm) in the range of 5° < 2θ < 50° at ambient temperature.

Magnetic measurements. Magnetic measurements were performed using the polycrystalline samples on Quantum Design PPMS-9. All dc susceptibilities were corrected for diamagnetic contribution from the sample holder and the molecule using Pascal's constants.

UV-vis absorption spectroscopy. UV-vis absorption spectra of solid samples were measured on a HITACHI HU4150 spectrophotometer (HITACHI Company, Tokyo, Japan) and cooled by liquid nitrogen within the temperature range of 80 to 400 K.

Luminescence spectroscopy. The luminescence spectra of the solid samples were collected using an Edinburgh FLS1000 fluorescence spectrophotometer (Edinburgh Company, Edinburgh, United Kingdom) equipped with a liquid-nitrogen-filled cryostat (Oxford).

Elemental analysis. Elemental analyses were performed on a Vario EL III element analyzer (Elementar Company, Hanau, Germany).

Theoretical calculation. All calculations were performed using the Gaussian 16 program package at the PBE0 level.⁴³ The Lanl2tz pseudopotential basis set was used for Co atoms,^44,45 and the 6-311g (d,p) Pople basis set was used for other atoms.⁴⁶ Energy calculation was based on the H-optimized crystalline geometries, and the keyword “stable = opt” was used to ensure the stability of ground state wavefunction. For both the LS state (spin multiplicity = 2) and the HS state (spin multiplicity = 4), the unrestricted determinants were used. The keyword “IOp(9/40 = 4)” was added in the TD-DFT calculation to output the excitation orbital pairs that configuration coefficient >0.0001 for hole–electron excitation analysis by Multiwfn. Absorption spectra simulation and hole/electron analysis were performed with Multiwfn.^47,48

Results and discussion

The crystal structures of complexes 1 (red), 2 (deep red) and 3 (deep red) at different temperatures were obtained through single crystal X-ray diffraction. Detailed crystallographic data are summarized in Tables S1–S3. Tables S4–S5 list the selected bond lengths and bond angles of complexes 1–3. Thermogravimetric analysis (TGA) of all complexes confirmed the presence of the solvent in the lattice (Fig. S2–S4). The phase purity of these complex samples was confirmed by powder X-ray diffraction (PXRD) patterns (Fig. S5–S7). Complex 1 crystallizes in the monoclinic space group P2₁/c (Table S1). The Co^II center is coordinated by six nitrogen atoms from two L ligands, forming a distorted octahedral CoN₆ coordination environment (Fig. 1a). Thermogravimetric analysis reveals the presence of co-crystallized solvent molecules in the lattice, which is consistent with the single crystal-XRD data (Fig. S2a). Upon temperature variation, changes in the Co–N bond lengths within the CoN₆ coordination environment were observed. A summary of the temperature-dependent selected bond lengths is provided in Table S4. At 120 K, the average Co–N bond length is 2.013 Å, consistent with an LS configuration of the Co^II center. Upon heating to 270 K, this value increased to 2.057 Å, indicating a thermally induced SCO to the HS state.^49,50 This spin-state transition is further supported by changes in the distortion of the CoN₆ octahedron. The distortion degree was evaluated by using two structural parameters: the octahedral distortion parameter Σ_Co, defined as the sum of the deviations of the 12 cis-N–Co–N angles from 90°,⁵¹ and the continuous shape measure (CShM), which quantifies the deviation from an ideal octahedral geometry (calculated using the program SHAPE 2.0).^52,53 As shown in Table S5, heating complex 1 from 120 to 270 K results in an increase in Σ_Co from 92.43 to 106.2, and in CShM from 2.521 to 3.176, indicating greater structural distortion in the HS state. At 120 K, complex 1 displays O–H⋯O hydrogen bonding interactions between ether and water molecules, as well as between methanol and water molecules. Additionally, a B–F⋯π interaction is observed between the tetrafluoroborate ion and the pyridine ring, as well as a C–H⋯F hydrogen bonding interaction between the tetrafluoroborate ion and the coordination complex (Fig. 1a). Adjacent complexes are also stabilized by C–H⋯π interactions (Fig. S8a). The dihedral angle of the anthracene moieties of adjacent complexes is 64.851°, and the distances of the C–H⋯π interactions are 3.794 Å and 3.584 Å, respectively. Upon increasing the temperature to 270 K, the C–H⋯π interactions between adjacent complexes become weaker at the elevated temperature (Fig. S8c). The dihedral angle of the anthracene moieties of adjacent complexes has increased to 71.068°, and the distance of the C–H⋯π interaction is 3.634 Å.

Fig. 1 Crystal structures of 1 (a), 2 (b) and 3 (c) at 120 K.

Complex 2 crystallizes in the same monoclinic space group P2₁/c (Table S2). Its structure closely resembles that of complex 1, featuring a [Co(L)₂]⁺ cation coordinated in a distorted CoN₆ octahedral geometry. However, complex 2 contains two bromide anions instead of tetrafluoroborate ions. Additionally, three DMF molecules are located in the lattice voids (Fig. 1b). At 120 K, the average Co–N bond length is 2.019 Å. Upon heating to 300 K, this value increases to 2.040 Å, indicating an incomplete SCO transition from the LS to HS state. Accompanying this spin transition, the octahedral distortion parameters Σ_Co and CShM increase from 96.949 and 2.592 at 120 K to 103.806 and 15.031 at 300 K, respectively (Table S5), indicating more pronounced structural distortion in the HS state. At 120 K, complex 2 exhibits C–H⋯Br hydrogen bonding interactions between the bromide counterions and the coordination complexes, C–H⋯O hydrogen bonding interactions between the DMF molecules and the coordination complexes, as well as C–H⋯π interactions involving the pyridine rings (Fig. 1b). C–H⋯π packing interactions are also observed between adjacent complexes (Fig. S9a). The anthracene moieties of adjacent complexes are parallel and are spaced far apart. At 300 K, complex 2 exhibits C–H⋯Br hydrogen bonding interactions between the bromide counterions and the coordination complexes and C–H⋯O hydrogen bonding interactions between the DMF molecules and the coordination complexes (Fig. S9b). The C–H⋯π interactions between adjacent complexes remain intact at the elevated temperature (Fig. S9c). The anthracene moieties of adjacent complexes remain parallel and are at a considerable distance from each other.

Complex 3 crystallizes in the triclinic space group P-1 (Table S3) and contains 0.5 water molecules per formula unit in the lattice voids (Fig. 1c). At 120 K, the average Co–N bond length is 2.013 Å, characteristic of the LS state of Co^II ions. Upon heating to 300 K, the bond length increases by 0.022 Å, consistent with a transition to the HS state. Structural distortion associated with the SCO is evident from the increase in Σ_Co and CShM values from 92.43 and 2.339 at 120 K to 102.52 and 2.831 at 300 K, respectively (Table S5), indicating more significant deviation from ideal octahedral geometry in the HS state. At 120 K, adjacent complexes are assembled via C–H⋯π and π⋯π stacking interactions (Fig. S10b). The anthracene moieties of adjacent complexes are parallel and are at a considerable distance from each other. At 300 K, the C–H⋯π and π⋯π interactions between neighboring complexes remain present at elevated temperature (Fig. S10c). The anthracene moieties of adjacent complexes remain parallel and are spaced far apart.

Variable-temperature direct current (dc) magnetic susceptibility measurements were performed at a scan rate of 2 K min⁻¹ under an applied magnetic field of 5000 Oe (Fig. 2). For complex 1, the χT value at 2 K is 0.61 cm³ mol⁻¹ K, slightly higher than the expected value for a Co^II ion in the LS state (S = 1/2).^40,54 As the temperature increased, the χT value gradually rose. Notably, the rate of increase slows down around 300 K, followed by a sharper rise beginning at 360 K, reaching a maximum of 2.36 cm³ mol⁻¹ K at 395 K. This value is lower than the theoretical χT value for a fully high-spin (HS) Co^II ion (S = 3/2),^40,54 indicating an incomplete SCO process. Complex 1 undergoes a two-step SCO within the temperature ranges of 2–301 K and 359–395 K. TGA analysis (Fig. S2a) suggests that the second-step transition may be associated with the loss of lattice solvent molecules. After desolvation, the χT value of complex 1 decreases gradually from 3.0 cm³ mol⁻¹ K at 395 K to 1.26 cm³ mol⁻¹ K at 2 K, indicating a slow and incomplete HS-to-LS transition. Measurements in heating mode confirm the absence of thermal hysteresis. Unfortunately, due to the poor crystallinity, accurate structural determination of the dehydrated phase was not possible. Complexes 2 and 3 exhibit similar gradual and incomplete SCO behavior. At 2 K, their χT values are 0.45 and 0.52 cm³ mol⁻¹ K, respectively, and then increase to 1.77 and 1.56 cm³ mol⁻¹ K at 395 K. No thermal hysteresis was observed upon cooling for either complex. Although complexes 1–3 share an identical cationic component, [Co(L)₂]²⁺, their differing counterions and lattice solvation states lead to distinct SCO behaviors. Interestingly, complexes 2 and 3 exhibit more similar SCO profiles and closely aligned transition temperatures. These findings highlight the significant influence of both counterions and lattice solvent molecules on the spin transition temperature and extent of spin-state conversion. Both factors are capable of modulating the local coordination environment of the Co^II center and altering intermolecular interactions within the lattice.^55,56

Fig. 2 Temperature dependence of the magnetic susceptibility for 1/desolvated 1/2/3 complexes in the solid state.

To investigate the possible interplay between SCO and fluorescence in these complexes, temperature-dependent solid-state fluorescence spectra were recorded for the free ligand L, desolvated 1, complexes 2 and 3. The fluorescence spectra of ligand L were measured over the temperature range of 80–400 K under excitation at 365 nm (Fig. 3a). As shown in Fig. 3e, Ligand L exhibited four emission peaks at 420, 441, 471, and 501 nm. As temperature increased, the intensities of all emission peaks decreased gradually, consistent with typical thermal quenching behavior. The fluorescence spectra of desolvated complex 1 were collected between 130 and 300 K using an excitation wavelength of 360 nm. Two broad emission peaks are observed at 436 and 466 nm (Fig. 3b), corresponding to blue shifts of approximately 5 nm relative to the 441 and 471 nm emissions of ligand L. The luminescence intensity of desolvated 1 decreased monotonically upon heating, again characteristic of conventional thermal quenching (Fig. 3f). For complex 2, fluorescence spectra were recorded over the temperature range of 130–350 K with excitation light at 365 nm. The emission profile consists of a broad peak centered at 474 nm and a shoulder at 445 nm (Fig. 3c). Compared with ligand L, these correspond to a tiny red shift. The maximum fluorescence intensities at these positions decreased slowly with increasing temperature, exhibiting plateaus across three temperature intervals: 200–220 K, 270–290 K, and 320–350 K (Fig. 3g). These plateaus suggest a balance between nonradiative relaxation and SCO-induced structural effects. Complex 3 was also investigated over the temperature range of 130–340 K under 365 nm excitation. Its emission spectrum features peaks at 420, 440, 456, 471, and 484 nm (Fig. 3d). Compared with ligand L, the peaks at 456 and 484 nm are newly emerged, likely originating from the metal–ligand interactions. As temperature increased, the emission intensities progressively declined (Fig. 3h), attributed primarily to enhanced nonradiative decay processes at elevated temperatures. The temperature-dependent luminescence behaviors observed in these three complexes confirm the coexistence of SCO and fluorescence properties. However, no abrupt changes or anomalies in fluorescence intensity were detected within the SCO temperature ranges, indicating that the two properties are not strongly coupled in these systems.

Fig. 3 Temperature-dependent luminescence emission spectra for L (a) (λ_ex = 365 nm), desolvated 1 (b) (λ_ex = 360 nm), 2 (c) (λ_ex = 365 nm) and 3 (d) (λ_ex = 365 nm) in the solid state. The χT and the luminescence emission intensity at different wavelengths as a function of temperature for L (e), desolvated 1 (f), 2 (g) and 3 (h).

To understand the absence of coupling between SCO and fluorescence in all three complexes, solid-state variable-temperature UV-vis–NIR absorption spectra of the free ligand L, desolvated complex 1, 2 and 3 were recorded (Fig. S11–S14). For desolvated complex 1, absorption bands are observed at 506 nm and 564 nm, corresponding to metal-to-ligand charge transfer (MLCT) transition from Co^II_LS to ligand L. A broad absorption feature spanning 652–875 nm might be attributed to LS Co^II d–d transitions (²E → ²T₁/²T₂).^57–59 As temperature increased, the intensities of both MLCT and d–d transitions associated with LS Co^II decreased, indicating a reduction in the LS population. Simultaneously, a new absorption band emerged in the range of 1064–1080 nm, characteristic of the d–d transition (⁴T_1g → ⁴T_2g) of HS Co^II.^57–59 These spectral changes align with the temperature-dependent magnetic susceptibility data. Similarly, for complex 2, the MLCT absorption bands at 512 and 553 nm diminished upon heating, accompanied by a reduction of d–d transition band between 611 and 885 nm for the LS Co^II. In contrast, the HS Co^II d–d transition band at 995–1076 nm became more pronounced at elevated temperatures. Complex 3 also displayed absorption features at 458, 513, and 562 nm, assigned to MLCT transitions of LS Co^II, along with a d–d transition band ranging from 636 to 887 nm. As the temperature increases, the intensities of all LS-related bands decreased, while a new absorption band emerges in the range of 1021–1098 nm, indicative of the HS Co^II d–d transition. The temperature-dependent UV-vis-NIR absorption spectra clearly confirm the occurrence of thermally induced SCO in all three complexes. However, the gradual nature of the spin-state interconversion and the broad transitions in the visible region may dampen the modulation of the excited states responsible for fluorescence. These factors likely account for the absence of SCO-fluorescence coupling.

To gain deeper insight into the charge/energy transfer processes underlying the SCO and optical properties of the complexes, time-dependent density functional theory (TD-DFT) calculations were performed on both LS and HS states. All calculations were carried out under vacuum conditions, and the results were analyzed using the Multiwfn program.^47,48 The structures corresponding to the LS and HS states of complex 1 were optimized respectively. For complex 1, spin density plots reveal that in both LS and HS states, the majority of spin density is localized on the Co center (Fig. S15). Mulliken population analysis yields spin populations of 0.93881 (LS) and 2.74892 (HS), which closely approximate the theoretical expectations of 1 and 3, respectively (Tables S6 and S7).^60,61 TD-DFT calculations were used to simulate the vertical electronic excitations. To ensure the calculated absorption wavelengths cover the visible spectral region, 102 lowest excited states for the LS structure and 72 lowest excited states for the HS structure were computed. The simulated absorption spectra of both LS and HS states (Fig. 4a), along with transitions exhibiting oscillator strengths greater than 0.02 (Tables S8–S11), indicate that both spin states possess multiple excited states within the 200–500 nm range. The most intense transitions for LS and HS states are observed at 385 nm (excited states 34 and 26, respectively). The dominant absorption bands for both spin states lie between 250 and 350 nm, with the HS state exhibiting a reduced absorption intensity near the main peak compared to the LS state. Notably, the LS state and the HS both display a shoulder at 393 nm. To further analyse the nature of the excitations, hole–electron distribution analysis was conducted for excited states with oscillator strengths greater than 0.03 (Fig. 5a and b). For the LS state of complex 1, excited state 34 primarily involves a π–π* transition within the anthracene moiety of ligand L. In contrast, the HS state exhibits a weakened π–π* transition and features additional contributions from metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT). Mulliken population analysis of the excited state density change reveals that Co contributes 4.56% to the electron distribution and 1.20% to the hole distribution in the HS state (Table S13), with a negative integral of the electron density difference on Co, confirming the MLCT characteristic. The other several LS excited states with high oscillator strength (Fig. S16) lacks an HS counterpart with comparable oscillator strength (>0.03), indicating it may correspond to a symmetry-forbidden or less probable transition in the HS state. The HS state of complex 1 shows three strongly allowed transitions (state 26, 24 and 13, oscillator strength 0.16450, 13015 and 0.0915), and the metal contribution to this transition is significantly lower than in the LS state. These findings suggest that MLCT and LLCT are predominantly associated with the HS state, leading to greater nonradiative losses and consequently weaker fluorescence.

Fig. 4 (a) The TD-DFT calculated UV-vis spectra of complex 1 in the LS and HS states. The TD-DFT calculated UV-vis spectra of complexes 2 (b) and 3 (c) in the LS states (the calculation stops at some energy and no calculation is performed for shorter wavelengths).

Fig. 5 The hole (blue)–electron (red) distribution caused by excited state 34 in LS (a) state and excited state 26 in HS (b) state for complex 1. The hole (blue)–electron (red) distribution caused by excited state 64 in the LS state for complex 2 (c) and excited state 68 in the LS state for complex 3 (d).

Based on the optimized LS structure, TD-DFT calculations were performed to examine the electronic excitation behavior at interplanar angles of 30° and 90° between the anthracene moieties and the chelating π-backbone. The spin-density maps reveal that in both configurations, the majority of the spin density remains localized on the Co center (Fig. S18). For each LS configuration of complex 1, 102 excited states were computed, showing that both spin states exhibit multiple electronic transitions within the 200–500 nm region (Fig. S19). The most intense absorptions are predicted at 464 nm and 388 nm for the 30° and 90° configurations, corresponding to excited states 17 and 32, respectively. When the interplanar angle is 30°, the dominant absorption band spans 375–650 nm with a shoulder at 320 nm; at 90°, the principal absorption lies between 250 and 350 nm with a shoulder around 383 nm. Hole–electron analysis further reveals that for the 30° configuration, excited state 17 involves a combination of ligand-to-ligand charge transfer (LLCT), metal-to-ligand charge transfer (MLCT), and intraligand π–π* transitions within the anthracene unit (Fig. S20a). In contrast, for the 90° configuration, excited state 32 is dominated by intraligand π–π* transitions within the anthracene moiety (Fig. S20b). These results indicate that at 30°, the spatial orientation allows efficient charge transfer between the anthracene and the chelating π-backbone, leading to charge-transfer-dominated excitations, whereas at 90°, this interaction is suppressed, and the excitation becomes primarily π–π*.

For complex 2, spin density analysis confirms that the unpaired electron is largely localized on the Co atom (Fig. S21). TD-DFT calculations of the 202 lowest excited states of the LS state reveal numerous electronic transitions in the 200–400 nm region. The principal transition associated with the anthracene unit, excited state 64, occurs at 374 nm. The calculated absorption spectrum (Fig. 4b) features strong peaks between 250 and 340 nm, with a shoulder at 363 nm. Hole–electron analysis shows that excited state 64 involves π–π* transitions within the anthracene ring, charge transfer from the anthracene to the pyridine ring, and seldom ligand-to-metal charge transfer to the Co^II center (Fig. 5c). For complex 3, spin density for the LS state is similarly concentrated on the Co center (Fig. S22), with a calculated spin population of 0.92, consistent with an LS configuration (Table S26).^60,61 A total of 153 excited states were computed for the LS state, and transitions with oscillator strengths greater than 0.005 were analyzed (Tables S27–S28). The main absorption features appear between 250 and 450 nm, with the strongest transition (excited state 68) located at 382 nm (Fig. 4c). Hole–electron analysis of excited state 68 reveals similar excitation behavior to complex 2, involving π–π* transitions within the anthracene moiety, charge transfer to the pyridine ring, and seldom LMCT to Co(II) (Fig. 5d). Additional excited states with oscillator strengths above 0.1 were also analyzed to confirm this pattern (Fig. S23). DFT and TD-DFT calculations combined with hole–electron analysis reveal that the LS states of all three complexes primarily involve intramolecular π–π* transitions and LMCT, while the HS state of complex 1 features additional MLCT and LLCT. These HS transitions are energetically less favorable and more prone to nonradiative relaxation, providing a plausible explanation for the weaker luminescence observed in the HS state and the lack of SCO–fluorescence coupling.

Conclusions

In summary, a fluorophore-functionalized terpyridine ligand (L) and its corresponding three cobalt(II) spin-crossover (SCO) complexes, 1, 2, and 3, were successfully synthesized and systematically characterized. Variable-temperature photoluminescence measurements revealed that all three complexes exhibit the coexistence of spin-crossover behavior and fluorescence emission in the solid state. Hole–electron distribution analysis indicates that the electronic transitions from the Co^II_HS center to the ligand (MLCT) are present, while the characteristic intramolecular π–π* transitions are significantly weakened compared to the LS state. This reduction in π–π* transition contribution likely accounts for the gradual quenching of luminescence with increasing temperature. This study represents the rare example of Co^II-based SCO complexes exhibiting intrinsic fluorescence, providing new insights into the design of multifunctional molecular systems. Future work will focus on constructing Co(II) SCO systems with enhanced coupling between SCO and luminescence processes, aiming to achieve cooperative or switchable photophysical responses.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI) and can be obtained from the authors. Supplementary information: CIF files, and structure information in detail. See DOI: https://doi.org/10.1039/d5nj03386a.

CCDC 2451729 (for 1 at 120 K), 2451737 (for 1 at 270 K), 2451738 (for 2 at 120 K), 2451741 (for 2 at 300 K), 2451742 (for 3 at 120 K), and 2451752 (for 3 at 300 K) contain the supplementary crystallographic data for this paper.^62a–f

Acknowledgements

This work was partly supported by the NSFC (Grants 22222103, 22025101, 22173015, and 22103009), the Fundamental Research Funds for the Central Universities (DUT22LAB606), the Liaoning Binhai Laboratory (LBLE-2023-02), and “Excellence Co-innovation Program” International Exchange Fund Project (DUTIO-ZG-202505). We would like to thank Dr Jingyi Xiao and Dr Qiang Liu at the Instrumental Analysis center of Dalian University of Technology for their assistance with magnetic measurements.

Notes and references

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