Sublimable Spin‐Crossover Complexes: From Spin‐State Switching to Molecular Devices

Abstract Spin‐crossover (SCO) active transition metal complexes are an important class of switchable molecular materials due to their bistable spin‐state switching characteristics at or around room temperature. Vacuum‐sublimable SCO complexes are a subclass of SCO complexes suitable for fabricating ultraclean spin‐switchable films desirable for applications, especially in molecular electronics/spintronics. Consequently, on‐surface SCO of thin‐films of sublimable SCO complexes have been studied employing spectroscopy and microscopy techniques, and results of fundamental and technological importance have been obtained. This Review provides complete coverage of advances made in the field of vacuum‐sublimable SCO complexes: progress made in the design and synthesis of sublimable functional SCO complexes, on‐surface SCO of molecular and multilayer thick films, and various molecular and thin‐film device architectures based on the sublimable SCO complexes.


Introduction
Switchable molecular materials are an important class of molecular materials with potential for applications in molecular electronics,s pintronics,i nformation storage,m icromechanics,a nd sensing. [1][2][3][4][5][6][7][8][9][10][11] In the emerging area of molecular spintronics,b oth the electronic charge and spin degrees of freedom are utilized to build device architectures with greater efficiency,b oth in terms of power and performance. [9,10,[12][13][14][15][16][17][18][19] Conventional inorganics such as metals and semiconductors are the commonly employed materials to fabricate spintronics architectures. [9,10,20] However,t he availability of only af ew inorganic materials suitable for applications and the difficulties to process them as tailor-made systems with additional functionalities and tunable quantum states necessitated the search for new materials. [21] Magnetic molecules,c omprising paramagnetic transition metal/lanthanide ions in an appropriate ligand field imposed by surrounding ligand(s), are promising molecular spintronic elements due to their quantized energy levels and the tunable nature of their magnetic characteristics. [16,[22][23][24][25][26][27] Spin-crossover (SCO) active first-row transition metal complexes with 3d n (n = 4-7) electronic configuration capableofundergoingreversible interconversion betweenlow-spin(LS)and high-spin(HS)states, or vice versa, area ni mportant classo fs witchablem olecular materials. Biandm ultistable SCOcomplexes showingabrupt andstepwise SCO behavior with thermal hysteresis (DT)a re suitable to develop molecular spintronics and memory architectures.The SCO systems are also proposed to be useful developing solvent sensors and micromechanical actuators. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] To realize SCO-based applications,t he spin-state switching behavior of the complexes must be studied in nanostructured environments,e specially in thin-film form. However,the transition from the bulk to nanometer (nm) regime results in the modulation of intermolecular interactions, thereby SCO in the nanometer regime differs from that in the bulk. [45] Thefabrication of SCO complexes as thin films on metallic surfaces creates an interfacial electronic structure (spinterface) due to the electronic coupling between mole-cules and surface states, [46][47][48][49] which alters the SCO behavior of the monoor few-layer films. [50,51] Drop-casting, spin-coating,l ithographic patterning, Langmuir-Blodgettery (LB), and vacuum sublimation techniques have been utilized to fabricate thin-film/nanostructured SCO materials. [52][53][54][55][56][57][58][59][60][61][62][63] Among the techniques,d irect vacuum sublimation of SCO complexes onto suitable surfaces emerged as ap referred method to obtain ultraclean and high-quality spin-switchable thin films. [64][65][66][67][68][69][70][71][72][73][74][75] Charge-neutral SCO complexes with ap ropensity to undergo sublimation are suitable candidates to prepare mono/multilayer thin spin-switchable films-results of fundamental and applied scientific importance,s uch as the elucidation of the electron-induced SCO [76] and memristance behavior at the single-molecule scale [77] have been reported. Despite the enormous application potential envisioned and significant advances made in terms of studying SCO in thin sublimed films,ad efinitive account covering SCO of sublimable SCO complexes in the bulk and the thin-film states, and device architectures based on SCO films is yet to be Spin-crossover (SCO) active transition metal complexes are an important class of switchablemolecular materials due to their bistable spin-state switchingc haracteristics at or around room temperature. Vacuum-sublimable SCO complexes are asubclass of SCO complexes suitable for fabricating ultraclean spin-switchable films desirable for applications,e specially in molecular electronics/spintronics.C onsequently,o n-surface SCO of thin-films of sublimable SCO complexes have been studied employing spectroscopyand microscopytechniques, and results of fundamental and technological importance have been obtained. This Review provides complete coverage of advances made in the field of vacuum-sublimable SCO complexes:p rogress made in the design and synthesis of sublimable functional SCO complexes,onsurface SCO of molecular and multilayer thickf ilms,and various molecular and thin-film device architectures based on the sublimable SCO complexes. presented in the form of areview.W ea im to bridge this gap by presenting an account detailing development made in the field of sublimable SCO materials.The emphasis is placed on the elucidation of physicochemical aspects governing the onsurface spin-state switching and device architectures based on thin vacuum-sublimed SCO films. Foraconceptual introduction of the SCO phenomenon, the reader is advised to consult the brief and concise overview presented by Gütlich and Garcia [78] and other informative reviews and books. [31,42,79] To aid the reader,t he most frequently used concepts are defined as follows:T he temperature at which equal proportions of LS and HS states coexist is denoted as T 1/2 .T he term "light-induced SCO" [80] is ag eneral term utilized to refer to light-induced excited spin-state trapping (LIESST) mediated LS!HS switching below 10 K. [40,41] Them etastable HS state produced via the LIESST effect is stable only up to at emperature denoted as T (LIESST) ,a bove which the HS system relaxes back to the corresponding LS state.T he analogous term "soft X-ray induced excited spin-state trapping" (SOXIESST) is aprocess in which soft X-ray irradiation mediates LS!HS switching at low temperatures. [81,82] TheSOXIESST effect is of importance when SCO of mono/multilayer films of SCO complexes is probed with X-ray absorption spectroscopy (XAS).

On-Surface SCO
Thes pin-state switching behavior of sublimable SCO complexes detailed in Section 2h as been probed on various surfaces at sub-monolayer to multilayer coverage,asdetailed below.

[Fe(phen) 2 (NCS) 2 ]
Complex [Fe(phen) 2 (NCS) 2 ], showing T 1/2 = 176 Ki nt he bulk state,i st he first example of av acuum-sublimable SCO complex studied on as urface.A280 nm film of the complex on as ilicon (Si)s ubstrate underwent gradual SCO with T 1/2 % 175 K, which is comparable to the bulk T 1/2 % 176 K. However,t he 1K thermal hysteresis observed for the bulk sample has vanished in the thin film. [69] Theo ccurrence of SCO at the single-molecule level depends on the nature of molecule-surface interactions.A t4K, as pin-state coexistence was observed for as ub-monolayer film of [Fe(phen) 2 -(NCS) 2 ]o nC u (100). Thea ppearance of the complexes as two-lobed structures,inthe STM image,corresponding to the upward-pointing phenanthroline ligands,i ndicates chemisorption between the NCS groups and Cu(100) surface ( Figure 3c). [67] Theincreased (HS) and decreased (LS) spatial separation between the lobes facilitated identification of the spin-state of [Fe(phen) 2 (NCS) 2 ]a ts ub-monolayer coverage ( Figure 3a).
At 4K,t he spin-state co-existence is reduced for 1.3-4.4 monolayer (ML) films of [Fe(phen) 2 (NCS) 2 ]onCu (100). An increase in the HS fraction with increasing coverage is observed, as inferred from the XAS spectra in Figure 3e, which is contrary to the LS-state of the complex, at 4K,inthe bulk-state (Figures 1a and 3d). Due to the different orientation of [Fe(phen)(NCS) 2 ]i nt he second layer, relative to its orientation in the first layer on Cu(100) (Figure 3b,c), the topography of the complexes residing in the second layer is different, hindering the unambiguous assignment of the spinstate of the complex based on topography.T oe lucidate the spin-state of [Fe(phen)(NCS) 2 ]c omplexes residing in the second layer, differential conductance spectra (dI/dV)p rofiles of the reference HS complex in the first monolayer and complexes in the second layer were obtained by performing scanning tunneling spectroscopy (STS). Thes imilarity of the dI/dV profiles observed for the reference HS molecule and the molecules in the second layer unambiguously indicated the dominant HS character of the molecules in the second layer.   Table 1) comparable to those of the bulk samples. [71] To understand the role of surface states affecting SCO and coverage-dependent switching,amolecular level study was performed by depositing [Fe(H 2 B(pz) 2 [110] Electron-induced excited spin-state trapping (ELIESST) at low temperatures was demonstrated for [Fe(H 2 B(pz) 2 ) 2 -(phen)] in an STM-tip/[Fe(H 2 B(pz) 2 ) 2 (phen)] (2 layers)/Au-(111) junction. Thec omplexes in the second layer showed LS!HS switching upon application of av oltage pulse (V = 3V)a taremote place (Figure 4c). Ther everse HS!LS switching was induced by applying av oltage pulse of V = 1.8 V, by placing the STM tip right above af reshly switched HS molecule.T ransportation of STM-injected hot electrons by surface states of Au(111) across nm distances caused remote SCO of [Fe(H 2 B(pz) 2 ) 2 (phen)]. [76] To prevent surface-induced fragmentation, af unctional variant of [Fe(H 2 B(pz) 2 (111) with afraction of the complexes permanently trapped in the HS-state. [103] To avoid fragmentation, [Fe(H 2 B(pz) 2 ) 2 (phen)] was deposited on ap assive yet electrically conductive HOPG substrate.S TM analysis of the sub-monolayer film of [Fe-(H 2 B(pz) 2 ) 2 (phen)] revealed the intact molecular structure of the complex in direct contact with HOPG,a ss hown in Figure 5a.R emarkably,t he films underwent complete thermal and LIESST-mediated SCO at sub-monolayer coverage ( Figure 5b). [75] Therealization of RT SCO in surface-bound thin films is acriterion for the development of SCO-based applications.In the quest to achieve this,L D-LISC mediated spin-state modulation in a5nm film of [Fe(H 2 B(pz) 2 ) 2 (L 8 )] deposited on aA u(111) surface was studied. UV light (l = 285 nm) irradiation of the film at RT induced photocyclization of the open-ring HS isomer to the closed-ring LS isomer,t hereby effecting HS!LS switching in about 5% of the molecules. TheH Sm olecules in the as-sublimed film also showed thermal SCO;amore gradual and incomplete thermal SCO is observed, relative to the bulk phase,u pon cooling.T he LIESST-mediated LS!HS switching is observed at low temperatures for the 5nmfilm. [111] Thespin-state switching behavior of [Fe(H 2 B(pz) 2 ) 2 (bpy)] and its functional variants has also been studied on different substrates at varying coverages.AnSTM study elucidated the large-scale molecular organization ( Figure 6) of [Fe(H 2 B-(pz) 2 ) 2 (bpy)] in bilayer films,d riven by the p-p interaction between the bpy rings,o nA u (111). At 131 K, as pin-state coexistence with more dominant LS fraction (bright areas in Figure 6b,c) was observed in the bilayer films due to surfacemediated constraints imposed on the intermolecular interactions operating between the switching entities. [112] Density functional theory (DFT) studies of the [Fe(H 2 B(pz) 2 ) 2 (bpy)] bilayer on Au(111) revealed the three-and two-lobed structure of the LS and HS complexes,r espectively.T he DFT calculations also predicted alarger HOMO-LUMO gap (HOMO = highest occupied molecular orbital and LUMO = lowest occupied molecular orbital) for the LS complex than the HS complex. Conductance (dI/dV) spectra ( Figure 6c) measured above the three-lobed (bright) sites revealed al arger HOMO-LUMO gap than the gap observed above the two-lobed (dark depression) sites ( Figure 6c). Based on the DFT and STS studies,t he bright and dark areas in the bilayer film of [Fe(H 2 B(pz) 2 ) 2 (bpy)] on Au(111) were unambiguously identified as LS and HS sites,respectively.

The
Contrary to the large-scale molecular organization observed for the bilayer film, non-uniform structural growth was observed for multilayer films thicker than 2nm, indicating the non-propagation of the higher-ordered structure associated with the bilayer film. Thet hicker 200 nm film of the complex showed comparable spin-state switching characteristics to the bulk phase indicating the weakening of the surface-induced constraints at nanometer distances from the interface. [65] Af acet not considered in the above studies is the enantiomeric nature (D or L)o ft he [Fe(H 2 B(pz) 2 ) 2 (phen)] and [Fe(H 2 B(pz) 2 ) 2 (bpy)] family of complexes ( Figure 7). A 77 KSTM study of abilayer island of [Fe(H 2 B(pz) 2 ) 2 (bpy)] on Au(111) revealed the arrangement of the complexes as isochiral rows (either D or L)i nt he top layer. However, the complexes in the adjacent rows are rotated 708 8 with respect to one another (colored triangles,F igure 7b), which produced oppositely handed isochiral rows,t hus leading to the overall racemic paving of the surface. [51] Theon-surface separation of the L and D enantiomers could be achieved by subliming the complexes on intrinsically chiral substrates such as Pt(643) [113] or Cu(874). [114] Thes elective absorption of molecular systems on chiral inorganic surfaces was previously reported. [115] Thep ossible resolution of the SCO-active racemic [Fe(H 2 B(pz) 2 ) 2 (phen)] and [Fe(H 2 B(pz) 2 ) 2 (bpy)] complex systems into their enantiomeric components may lead to novel chiroptical and magnetochiral spintronic components. [116,117] Cooperative intermolecular interactions facilitating the occurrence of abrupt and first-order SCO are at horoughly investigated phenomena in the bulk-state. [118] In aq uest to determine the size limit at which cooperativity becomes effective in thin films,t hermal and light-induced SCO of [Fe(H 2 B(pz) 2 ) 2 (bpy)] on HOPG with coverages ranging from 0.35(4) to 10(1) monolayers was studied. Thesub-monolayer to multilayer thick films showed complete thermal and lightinduced SCO (Figure 8a). Thetransition width of the thermal SCO curves decreased with increasing film thickness (Figure 8a,b), evidencing increasing cooperativity with increasing film thickness.Q uantitative estimation of the cooperativity was achieved by fitting the HS fraction (g HS )versus T curves shown in Figure 8a    c) Conductance map (V = À1V ,I = 1nA, and T = 131 K) of a20 20 nm bilayer film and d) the corresponding dI/dV curves measured either above bright (LS:r ed circles) or depressed parts (HS: blue squares). Reproduced with permission from ref. [112].C opyright (2013) American Chemical Society.
Here G is the interaction parameter, DH and DS are the enthalpy and entropy changes,r espectively,a ssociated with the SCO,a nd R is the gas constant. Negative values of G obtained for 0.35 ML and 0.69 ML films indicate noncooperative SCO behavior,whereas the positive value of G = 0.3(1) kJ mol À1 observed for the 2M Lf ilm (Figure 8b) indicates the onset of cooperative spin-state behavior starting from the second monolayer.Asubsequent increase of G with increasing film thickness indicates increased cooperativity in thicker films,w ith the 10 ML film showing G = 1.4 kJ mol À1 comparable to the G = 2.1 kJ mol À1 shown by the bulk phase. While the thermal SCO of HOPG-bound [Fe(H 2 B(pz) 2 ) 2 -(bpy)] is cooperative in nature,t he LIESST-mediated LS! HS transition of [Fe(H 2 B(pz) 2 ) 2 (bpy)] on HOPG is not cooperative and molecular in nature for all the film thicknesses.N ote,L IESST-induced LS!HS switching is also molecular in the bulk phase.The stability of the light-induced HS state of [Fe(H 2 B(pz) 2 ) 2 (bpy)] at sub-monolayer coverages on the HOPG surface was also studied in the temperature range of 8K-40 K. Remarkably,t he HS!LS relaxation behavior of the complexes in the sub-monolayer films exhibited as tretched exponential behavior, with the relaxation rate increasing with increasing temperature.Onthe other hand, the HS!LS spin relaxation in the bulk sample became pronounced when T is close to the T (LIESST) ; [97] that is,i nt he thermally activated regime as igmoidal relaxation behavior was observed. [119] Enhanced tunneling rates and ar educed energy barrier between the metastable HS and LS states caused the increased HS!LS relaxation rate,r elative to the bulk, observed for the sub-monolayer films on HOPG.
Thes pin-state switching properties of thin films of [Fe-(H 2 B(pz) 2 ) 2 (bpy)] were also studied on ferroelectric and magnetic oxide substrates.T he SCO of the 5nmf ilm of the complex was also studied on dielectric oxide surfaces-Al 2 O 3 and SiO 2 -to probe the role of electrostatic environment on determining the spin state of the complex. The1 0-25 ML thick film of the complex deposited on the organic copolymer ferroelectric polyvinylide fluoride-trifluoroethylene (PVDF-  Tr FE:  showed ferroelectric polarization dependence of the spin-state ( Figure 9). When deposited on the poled "up", that is,i nterfacial dipole pointing up,P VDF-TrFE substrate, the complex remained in the HS-state even around 100 K, which is well below the T 1/2 % 160 Kr eported for the bulk [Fe(H 2 B(pz) 2 ) 2 (bpy)].D eposition of the complex on the poled "down", that is,i nterfacial dipole pointing down, ferroelectric substrate locked the complex in the LS state, well above the bulk T 1/2, around 300 K. These results evidence the ferroelectric polarization mediated suppression of thermal SCO. [120] Subsequently,v oltage-controlled isothermal spin-state switching was demonstrated for the frozen LS [Fe(H 2 B(pz) 2 ) 2 (bpy)] films deposited on polyvinylide fluoride-hexafluoropropylene (PVDF-HFP) and croconic acid ferroelectric substrates. [121] On the other hand, temperatureinduced HS!LS switching of the 7nma nd 70 nm films of is not ferroelectric polarization dependent. However,t he efficiencyo fS OXIESST-mediated LS!HS switching at 3K is governed by the ferroelectric polarization of PMN-PT(011). When the ferroelectric polarization of the PMN-PT(011) points toward the surface,the SOXIESST mediated LS!HS switching proceeded with al arger magnitude than that of the opposite polarization. [122] Magnetic-field-mediated spin reversal in a1 0nmt hick film of [Fe(H 2 B(pz) 2 [123] Thespin-state of aseveral nm thick [Fe(H 2 B(pz) 2 ) 2 (bpy)] film is locked in the LS-state when it is deposited on SiO 2 / Al 2 O 3 dielectric substrate;s oft X-ray irradiation of the films induced LS!HS switching at RT. [124] The900 nm (DT = 15 K) and 300 nm (DT = 5K)f ilms of [Fe(H 2 B(pz) 2 ) 2 (bpy)] on Al 2 O 3 ,i nc ontrast to the bulk, showed hysteretic SCO mediated by [Fe(H 2 B(pz) 2 ) 2 (bpy)]/Al 2 O 3 interfacial interactions. [125] Theabove studies establish the importance of factors such as the nature of the surface,i nterfacial electronic structure, and intermolecular interactions in affecting the SCO characteristics of the sublimed thin-film architectures.H owever, direct control of the intermolecular interactions,agoverning factor in dictating SCO in the bulk phase,isarather difficult problem to tackle at the nanoscale,e specially on surfacebound thin films.Amolecular self-assembly strategy was invoked to copy the self-organization of the bulk phase in the thin-film state.   Remarkably,t he complex self-organized as lamellar bilayer structures both in the bulk and sublimed films (Figure 10 b), as evidenced by the X-ray studies shown in Figure 10 c. Thebulk powder and 10 nm thin-film forms of the complex showed comparable SCO behavior (Figure 10 d) mediated by similar lamellar-bilayers-like self-assembly in both bulk and thin-film states.I mportantly,t he sublimation propensity of [Fe(H 2 B(pz) 2 ) 2 (L 7 )] demonstrates the possibility of subliming functional-group-appended SCO complexes, encouraging the study of functional SCO complexes on the surface for applications. [105] 3.3. Fe[HB (3,5- In the bulk-state,F e[HB(3,5-(Me) 2 pz) 3 ] 2 undergoes bistable SCO with T 1/2 % 190 Ka nd DT = 31 K. Unlike the previously discussed systems forming good-quality thin films,g rainy films were obtained upon sublimation of Fe-[HB(3,5-(Me) 2 pz) 3 ] 2 onto as urface,w ith roughness and crystallite size increasing with increasing film thickness. Incomplete SCO with the remnant and SCO-inactive metastable HS portion was observed for 8.02 mmand 130 nm films on quartz. Annealing of the films at 400 Kr esulted in nearcomplete spin-state switching with T 1/2 = 152 Ka nd DT = 17 K. [94] X-ray diffraction studies of the as-sublimed 8.02 mm film revealed peaks at 2q = 9.988 8 and 10.18 8 corresponding to the tetragonal and triclinic polymorphs,r espectively.T his indicates the coexistence of tetragonal and triclinic polymorphs in the as-sublimed films,i nc ontrast to the bulk sample that crystallizes in the triclinic space group.T hermal annealing of the films resulted in the disappearance of the peak (2q = 9.988 8)c orresponding to the tetragonal phase and retention of the peak (2q = 10.18 8)associated with the triclinic phase.Thus,the observed similarities in the SCO of thermally annealed films and bulk samples are due to the triclinic space group associated with the samples;t he coexistence of tetragonal and trigonal phases rendered SCO of the asprepared films incomplete.A570 nm thick film of Fe[HB-(3,5-(Me) 2 pz) 3 ] 2 on Si showed SCO with T 1/2 and DT in the range observed for the 130 nm post-annealed film on quartz. [66] In contrast to the near-complete SCO of the annealed films,asub-monolayer film of Fe[HB(3,5-(Me) 2 pz) 3 ] 2 in direct contact with Au(111) surface showed an incomplete SCO with T 1/2 % 154 Kand DT = 7K.Aspin-state coexistence,with onethird of the molecules in HS state,w as observed below 100 K. [82] (111). The S 1/2 superstructure is made of equal proportions of HS and LS molecules,i ndicating ap artial LIESST-mediated LS-HS switching.Whenthe blue-light illumination was stopped, the S 1/2 superstructure relaxed back to the S 1/3 superstructure with ar elaxation time of 131 AE 5min ( Figure 11). [126] Theb right HS molecules of [Fe[HB-(3,5-(Me) 2 pz) 3 ] 2 showed no Kondo effect at 4.6 K, indicating the weak coupling between the HS complex and the Au(111) substrate.A nu nambiguous spin-state identification was achieved by performing inelastic tunneling spectroscopy on the individual complex:L Sa nd HS molecules appeared as dark and bright spherical structures,r espectively.

[Fe(HB(trz) 3 ) 2 ]
Complex [Fe(HB(trz) 3 ) 2 ]w as studied as 20-200 nm thick films on fused silica, crystalline silicon (100), and polycrystalline Au substrates.T he as-prepared amorphous films crystallized upon water vapor annealing (relative humidity = 75-80 %) at RT.T he crystalline films showed thicknessindependent first-order SCO (T 1/2 = 338 K) as the bulk sample. [74] Thea s-prepared 100-200 nm thick films of the complex on fused silica substrate were solvent annealed to establish the nature of solvent in affecting the crystallinity, morphology,a nd SCO.W ater, diethyl ether, acetone,a nd ethanol solvents capable of accepting hydrogen bonds yielded highly oriented crystalline films,which showed an abrupt and complete SCO with T 1/2 = 336 K. Films annealed with dichloromethane (not ah ydrogen-bond acceptor) showed incomplete SCO due to the poor crystallinity of the film. Among the solvents studied, water yielded high-quality and continuous films.W ater vapor annealing of the films above and below 72 %r elative humidity yielded crystalline and semicrystalline films,respectively.Unlike the crystalline films, the semicrystalline films exhibited incomplete SCO.T he effect of thickness reduction on the SCO of microcrystalline thin films of [Fe(HB(trz) 3 ) 2 ], deposited on fused silica substrate,w as probed in the 200-45 nm range.A% 3K elevation of the T 1/2 value,t hat is,s tabilization of the LS phase,i so bserved upon thickness reduction. Thei ncrease in the surface energy of the HS-state relative to the LS state by 5mJm À2 caused the shift in the transition temperature in the 45 nm film. [73] Light irradiation of the SCO solids with femtosecond laser pulses leads to an out-of-equilibrium dynamical spin-state switching process,w hich involves as equence of three consecutive steps,namely,photo-, elastic,and thermal switching at distinct ps,n s, and mst imescales,r espectively. [88,127,128] Thep hoto-induced step involves ultrafast electronic and structural reorganization, that is,L S!HS switching and the associated volume change,a tt he molecular scale.T he consequent noninstantaneous elastic step causes al argescale volume change of the lattice due to strain propagation throughout the lattice.T he thermal switching process is triggered by heat diffusion, which increases the average temperature of the lattice.S ince both the strain wave propagation and heat diffusion take place at shorter timescales in nanoscale systems,s ize reduction is ap romising route to achieve ultrafast switching dynamics. [129] Further,on the bulk scale,the finite laser penetration depth (d = 50 mmat 570 nm) produces an inhomogeneous distribution of photoswitched molecules,which induce delayed dynamic responses. Theb ulk inhomogeneous effects can be suppressed in nanoscale objects and thin films with thicknesses less than the laser penetration depth. Moreover,t he nano-and microsecond dynamical responses associated with strain propagation and heat diffusion, respectively,c an be fine-tuned by varying the film thickness. [130] In this context, to understand the evolution of the dynamical properties of the light-induced spin-state switching in the nanometer regime,RTspin-state switching dynamics in water solvent annealed thin films of [Fe(HB(trz) 3 ) 2 ]( 50 nm, 100 nm, and 150 nm) deposited on fused silica substrates were studied employing femtosecond pump-probe optical absorption spectroscopy.T he thin films were irradiated at l = 570 nm corresponding to the d-d transition band of the complex in the LS state.After irradiation (l = 570 nm) of the 100 nm thick film with af emtosecond laser pulse,t he time evolution of the relative change of the optical transmission (DT/T)i nt he charge-transfer region of the LS-state (l = 320 nm) on the ps timescale was probed at 293 K( LS) and 375 K( HS) (Figure 12 a). At 293 K, ap hoto-induced signal due to the light-induced LS!HS switching of asmall number of molecules was observed (the blue and green curves in Figure 12 a), whereas photo-irradiation of the same area of the film above the SCO transition temperature,3 75 K, produced no detectable photo-induced signal (the red curve in Figure 12  Probing of the photo-induced dynamics at RT in the 100 nm thick film on al onger 100 ps to ms temporal range evidenced different photoresponses (Figure 12 b). Forl aser pulse energies below 4 mJ, the HS complexes return to the LSstate through asingle-step relaxation process within 100 ns.A pronounced increase of the fraction of HS (Dn HS )complexes in the 20-40 ns time window was observed above athreshold excitation power of 6 mJ (Figure 12 b). Thetwo-step response was also observed in the 150 nm thick film, whereas such ar esponse was absent in the 50 nm thick film. Thep hotoresponse occurring in the ns regime is attributed to athermally activated LS!HS switching process,w hich delays the decay of the photo-induced HS species.O verall, as ub-picosecond intramolecular LS!HS photoswitching and athermally activated LS!HS switching in the ns regime is observed in the 150 nm and 100 nm films.E fficient heat transfer to the substrate in the 50 nm film prevented the occurrence of the second switching step and accelerated the recovery of the spin-state equilibrium, giving evidence of the size-reduction effects on the photo-induced dynamics of spin-state switching at the nanoscale,especially in thin-film architectures. [130]

Other Systems
Thec omplex [Fe(dpepd)(NCS) 2 ], deposited as as ubmonolayer (0.8 ML) film on HOPG,u nderwent gradual and fully reversible thermal SCO with T 1/2 = 235(6) K. On acomparative scale,the sub-monolayer film of the complex showed more gradual SCO with al ower transition temperature than the bulk phase,w hich exhibited SCO with T 1/2 = 251(3) K. Thes tudy was the first example showing the utility of ac arbon-based substrate in preserving the SCO behavior of molecules in direct contact with asubstrate. [90] A50nmfilm of [Fe(qnal) 2 ]o ng old showed temperature-dependent (T 1/2 = 210 K) and LIESST-a nd SOXIEEST-mediated SCO.T he SCO of [Fe(qnal) 2 ]inthin-film proceeded at alower temperature relative to the bulk. [72]  A40monolayer thick film of [Fe(pypyr(CF 3 ) 2 ) 2 (phen)] on layered metallic 1T-TiTe 2 showed T 1/2 = 330 K, which is significantly lower than the transition temperature (T 1/2 = 390 K) observed for the bulk complex. In contrast to the bulk complex (T LIESST % 2K), the 40 ML film showed lightinduced LS!HS switching below 100 K: the HS-state is more stable in the thin-film-state than in the bulk-state. [95] At submonolayer coverage,f ragmentation of [Fe(pypyr(CF 3 ) 2 ) 2 -(phen)] occurred on metallic substrates (Co/Cu(100), Au- (111), and graphene/Ni (111)). As ignificantly reduced fragmentation of the complex occurred on semiconducting (WSe 2 and HfS 2 :5 %) and semimetal (HOPG:2 %) substrates. Insertion of graphene as adecoupling layer between Ni (111) and [Fe(pypyr(CF 3 ) 2 ) 2 (phen)] reduced fragmentation:a bout 70 %o ft he complexes remained intact. Thes ub-monolayer films deposited on semiconducting and semimetal substrates showed light-induced LS!HS switching at temperatures below 100 K. Thea bove results demonstrate the role of the density of states near the Fermi level in determining the stability of [Fe(pypyr(CF 3 ) 2 ) 2 (phen)] in direct contact with asubstrate. [131] STM analysis of as ub-monolayer film of [Fe(pap) 2 ] + on Au (111) showed the presence of an intact complex cation, ClO 4 À anions,and fragments of [Fe(pap) 2 ] + ;attempted STMtip-induced switching of intact [Fe(pap) 2 ] + was not successful. [83] In short, at sub-monolayer coverage,t he coexistence of spin-states and the freezing of SCO were reported for complexes studied on metallic surfaces,e specially on Au- (111). More specifically,t he molecular structure of [Fe-(phen) 2 (NCS) 2 ]r emained intact on Au (111)  Theclose resemblance between the SCO of the bulk and multilayer films of most of the sublimable SCO complexes further demonstrates the role of the interfacial electronic structure in blocking SCO of the molecules in direct contact with the metallic substrates.T hus,i ti sc onclusive that the spin-state coexistence of the SCO complexes at sub-monolayer coverage on metallic surfaces primarily depends on the magnitude of the electronic coupling between the SCO molecule and the substrate and the stability of the complex under study.
Another important aspect is the lack of control associated with the thin-film growth leading to the difference in the intermolecular interactions in the thin-film state relative to the bulk state.S ince intermolecular interactions are ad ominant factor governing the nature of SCO,achieving the same molecular organization both in bulk and thin-film forms may manifest as acomparable SCO behavior.W ehave elucidated the utility of this concept by tethering the parent [Fe(H 2 B-(pz) 2 ) 2 (bpy)] with as elf-assembly-directinga lkyl chain substituent. Although the results are encouraging,t he presence of the insulating alkyl chain is ap otential hindrance to realizing device architecture.However,t his proof of concept could be extrapolated towards the design of device-suitable molecules such as hybrid sublimable SCO complexes tethered with organic semiconductors and self-assembly-directing groups.
From the molecular structure point of view,d esigning homoleptic complexes featuring bulky ligands with donor atoms less likely to interact with the surface may be astrategy to promote the spin-state switching of SCO complexes in direct contact with the surface.N ote the heteroleptic nature of the [Fe(H 2 B(pz) 2 ) 2 (bpy)] complex and the affinity of the bpy/phen ligand to Au (111), which facilitates fragmentation of the complex upon direct interaction with the surface,a nd the intact nature of the homoleptic [Fe[HB(3,5-(Me) 2 pz) 3 ] 2 ] complex on the Au(111) surface.C areful molecular design and the right choice of the substrate is the need of the hour to realize switchable devices based on SCO complexes.

Device Architectures
Ever since the first proposal of am olecular rectifier by Aviram and Ratner in 1974, [132] the charge transport characteristics of molecular materials have been studied, aiming the bottom-up fabrication of molecular electronic devices. [13,[133][134][135][136] In al ater development, the report of giant magnetoresistance (GMR) [137,138] fueled the growth of molecular spintronics,w hich uses both charge and spin degrees of freedom to produce power-efficient devices.I nm olecular electronics and spintronics,t he charge and spin transport, respectively,c haracteristics of single or ensembles of molecules are studied by depositing them on suitable substrates. Such molecular deposition on metallic electrodes leads to the formation of ah ybrid spinterface.W hile the spin-transport characteristics upon electron injection from the bottom electrode are governed by the spinterface in spintronic junctions, [46,47,[139][140][141] the transport in molecular electronic junctions is determined by the orientation of frontier molecular orbitals with respect to the work function of the metallic electrode. [135,142] In spintronics junctions composed of magnetic molecules,t he molecules are sandwiched between electrodes,a nd the spin-transport characteristics are largely determined by the magnetic state of the molecule. [143][144][145][146][147] Organic radicals,s ingle-molecule magnets (SMMs), and SCO complexes are prime examples of magnetic molecules studied in spintronic junctions. [148][149][150] Theb istable SCO complexes are more advantageous for the fabrication of molecular electronic/spintronics devices due to their switchable magnetic characteristics effected by device-suitable electric field or light stimulus. [151][152][153][154][155][156][157] At the molecular scale, as pin-state change leads to ac hange in the electronic band gap,a nd the band gap is smaller in the HS-SCO complexes than in their LS counterparts.M oreover,t he relative orientation of the frontier molecular orbitals (FMOs) with respect to the work function of the electrode is also modified upon SCO,w hich in turn controls the charge-transport characteristics of the molecular and thin-film SCO junctions. These electronic effects occurring upon SCO manifest as conductance switching,e nabling the construction of SCObased switching and memory elements.A nother interesting aspect of the SCO-based metal/SCO molecule/metal junction is the ability of the HS-state to mediate spin-polarized transport upon charge injection from the nonmagnetic metallic electrode.T hus,t he HS molecule could act as aspin valve/filter-a fundamental spintronics element. [158][159][160] Apart from the electronics and spintronics applications,t he elastic nature of the SCO complexes is utilized to build nanoscale actuators and resonators. [11] Ac oncise outlook of device architectures based on sublimable SCO complexes is presented in the following sections.

Single-Molecule Devices
As ingle-molecule STM and STS study elucidated combined spin and conduction switching of [Fe(phen) 2 Figure 13 a.
Remarkably,the cyclic I(V)profiling resulted in hysteretic behavior at intermediate voltages.T he co-existence of spinstate-dependent conductance (resistance) variation along with hysteresis (memory) renders the system am emory resistor or memristor.T he observation of as table and reproducible I-V response over several cycles indicates the deterministic nature of the memristor. Thes ystem is addressed, that is,t he reading of the information stored in the memristor, by applying positive or negative bias pulses. [77] Another interesting study demonstrated memristive behavior of [Fe(pap) 2 ] + based on its robust, selective,a nd high-yield spin-state switching on aC u 2 N/Cu(100) surface. [161]

Thin-Film Devices
Probing the spin-state-dependent conductance of SCOactive thin films is necessary to realize large-area electronics/ spintronics elements.T he first-ever study in this direction reported electrical conductance characteristics of aA u/[Fe-(phen) 2 (NCS) 2 ]( 240 nm)/Auj unction. At RT,t he currentvoltage (I-V)characteristics of the junction, composed of HS [Fe(phen) 2 (NCS) 2 ]m olecules,i sO hmic at low bias voltages. Above 1.4 V, the conductance belongs to the space-chargelimited current (SCLC) regime with am obility (m) = 6.53 10 À6 cm 2 Vs À1 ,w hich is comparable to the mobility of small organic molecules.T he RT transport is mediated by the HSstate due to the close proximity of the HOMO level of HS [Fe(phen) 2 (NCS) 2 ]t ot he work function of Au (5.1 eV);t he temperature dependence of the electrical conductance of the film was not studied. [69] Theo ptoelectronic transport characteristics of 10 nm, 30 nm, and 100 nm films of [Fe(H 2 B(pz) 2 ) 2 (phen)] embedded between indium tin oxide (ITO) and aluminum (Al) electrodes were probed. Va riable-temperature I-V measurements of the ITO/[Fe(H 2 B(pz) 2 ) 2 (phen)]/Al junctions,i nt he absence of light irradiation, showed activationless tunneling conductivity for the 10 nm thick SCO film. On the other hand, the thicker 30 nm and 100 nm junctions exhibited diode-like

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Reviews 7514 www.angewandte.org rectifying characteristics and bulk-limited thermally activated currents.R emarkably,t he conductance of the 30 nm film at ac onstant bias of 5Vincreased with increasing temperature in the 100 Kt o2 93 Kr ange,w hich is ascribed to the occurrence of gradual thermal SCO in the junction. Visiblelight irradiation of the 10 nm junction at 5Kresulted in a7% reduction of current intensity mediated by the LIESSTinduced LS!HS switching of the film. Successive OFF-ON-OFF irradiation cycles at 5Kled to no change in the current intensity due to the persistence of the metastable HS state at 5K.H eating the device to 100 Kr estored the high-conductance-state due to the relaxation of the metastable HS-state to the LS-state. I-V measurements performed by irradiating the junctions at 100 K, far above the T (LIESST) = 46 Kof[Fe(H 2 B-(pz) 2 ) 2 (phen)],r esulted in an egligible change of the conductance:LIESST-induced conductance modulation at 5Kis elucidated. Note,a t5Kt he thin films of [Fe(H 2 B(pz) 2 ) 2 -(phen)] in the LS-state are more conductive than in the HSstate.S imilar light-induced conductance variations were observed for the thicker 30 nm and 100 nm films at 5K. [162] Thes pin-state dependence of the junction resistance in the thicker ITO/[Fe(H 2 B(pz) 2 ) 2 (phen)]/Al optoelectronic junctions at 5K was governed by the carrier hopping rates. Theh opping rate increased with increasing phonon frequencies.Onthe other hand, the LS!HS switching results in the decrease of vibrational frequencies due to the elongation of metal-ligand bonds.The reduced frequencies associated with the HS [Fe(H 2 B(pz) 2 ) 2 (phen)] films manifest as reduced charge carrier hopping rates;thereby decreased conductivity was observed upon LIESST-mediated LS!HS switching at low temperatures.Apoint noteworthy here is that the reduction in the HOMO-LUMO gap and the energetic proximity of the FMOs to the Fermi level of the electrodes seem to play no role in governing the transport properties of the ITO/[Fe(H 2 B(pz) 2 ) 2 (phen)]/Al optoelectronic junctions. In an utshell, the current switching in the thicker ITO/[Fe-(H 2 B(pz) 2 ) 2 (phen)]/Al optoelectronic junctions is not directly related to the change in electronic spin states,r ather the conductivity change is due to the coupling of electronic states with the phonon density of states,w hich affect the carrier hopping rates. [162] Thes pin-state switching mediated switching of the conductance mechanism, that is,f rom tunneling to hopping,a nd the more conductive nature of the HS [Fe(H 2 B(pz) 2 ) 2 (phen)] films,r elative to the LS complex, was established for the TS Au/[Fe(H 2 B(pz) 2 ) 2 (phen)]/EGaIn ( TS Au = template-stripped gold substrate,E GaIn = eutectic gallium-indium) junctions. [163] Thes imilar J versus T plots observed for the ITO/[Fe(H 2 B(pz) 2 ) 2 (phen)](30 nm)/Al and TS Au/[Fe(H 2 B-(pz) 2 ) 2 (phen)] (16.7 nm)/EGaIn junctions indicate higher conductance of the HS [Fe(H 2 B(pz) 2 ) 2 (phen)] films produced by temperature-induced SCO.Asimilar increase in conductivity upon LS!HS switching was observed for TS Au/[Fe-(HB(trz) 3 ) 2 ]( 6.7 nm)/EGaIn junction. [164] Thes ituation may be different for the HS films produced by light irradiation at low temperatures,asestablished for the ITO/[Fe(H 2 B(pz) 2 ) 2 -(phen)]/Al optoelectronic junctions.
Av oltage-controlled isothermal nonvolatile resistance change in [Fe(H 2 B(pz) 2 ) 2 (bpy)] films deposited on PVDF-HFP and croconic acid ferroelectric substrates was demonstrated as depicted in Figure 14. As discussed in Section 3.2 (Figure 9), the spin-state of [Fe(H 2 B(pz) 2 ) 2 (bpy)] is pinned in either LS-or HS-state depending on the direction of the ferroelectric polarization. TheH S[ Fe(H 2 B(pz) 2 ) 2 (bpy)] film on PVDF-PVP is more conductive than the LS film (Figure 14 a), and the conductance states of the film are retained even in the absence of applied voltage,e videncing the nonvolatile nature of the switching process.T he nonvolatile spin-state-dependant conductance switching process between two resistance states is also elucidated in a[ Fe(H 2 B(pz) 2 ) 2 -(bpy)] film on croconic acid as depicted in Figure 14 b; the resistance switching followed the ferroelectric switching of the croconic acid, conferring the film with bistable conductance switching characteristics. [121] Thet ransport studies described so far have relied on separate measurements to link electronic transport and the spin-state of the films.I naquest to capture the SCO-  originating due to the X-ray irradiation of the device is substracted from the total resistance change and the resultant resistance is observed to be evolving along the Fe L 3 edge,as shown in Figure 15 c(bottom panel). This supports the role of HS molecules in mediating transport. By comparing the material-centric experiments (Figure 15 b) and in operando device-centric measurements (Figure 15 c), aclear correlation between spin-state and device resistance is elucidated. [104] Te mperature-dependent I-V measurements of 200 nm thick films of [Fe(HB(pz) 3 ) 2 ]e mbedded between interdigitated gold electrodes revealed ag radual increase of the conductivity of the films in the first heating step until % 350 K. [124] Thecharge-hopping-mediated conductivity dropped above 350 Kd ue to the onset of LS!HS switching:t he LS-state is more conductive than the HS-state.F urther heating of the sample,i nt he first heating step,a bove 370 K resulted in ad rop of the conductivity due to the irreversible structural transformation from the metastable tetragonal to the stable and more insulating monoclinic form, which persisted in the subsequent I-V cycles.Aread-only memory (ROM) is fabricated exploiting the irreversible conductivity drop associated between the first and second cycles;i nformation is written by heating the film above 370 K, that is,by inducing the irreversible tetragonal-to-triclinic structural transition, and the reading process is carried out by measuring the resistivity change of the sample at RT. [92] In summary,t he electrical conductance studies of the sublimable complexes revealed that the HS films of [Fe-  3 ) 2 ]s tudied in optoelectronic (5 K) and thermal junctions,r espectively,c annot be explained based on FMO energies.Amechanistic pathway based on charge hopping,m ediated by vibrational frequency differences associated with the LS!HS switching, is invoked to explain the conductivity modulations associated with [Fe-(H 2 B(pz) 2 ) 2 (phen)] films at 5K.

SCO-Based Microelectromechanical Systems
At ypical micro-or nanoelectromechanicals ystem (MEMS or NEMS) device comprises electrical and mechanical components integrated into asilicon chip.The mechanical components such as microsensors respond to the external conditions,and the microelectronics components process the information received from the mechanical components and effect the desired change to the external environment. The sensitive nature of SCO complexes to external stimulus,such as light, temperature,o rp ressure,r ender them suitable to function as sensing and actuating elements in MEMS and NEMS devices.M oreover,t he molecular nature of the SCO complexes is advantageous for the development of more efficient and smaller NEMS devices.Aprototypical SCObased MEMS device is composed of an SCO thin film coated on af reestanding Si cantilever integrated with magnetic actuation and piezoresistive detection components (Figure 16 a). In response to the external stimulus,t he film undergoes spin-state switching,which effects ac hange in the unit cell parameters and the associated lattice-volume expansion/compression. Theelastic modulations taking place in the crystal lattice of an SCO complex couple with the underlying Si cantilever and the cantilever is actuated. Ar eversible change of the resonance frequency of the Si cantilever is also effected, when measured in dynamic mode,bysuch coupling, as depicted in Figure 16 b. [11] A2 00 nm film of [Fe(H 2 B(pz) 2 ) 2 (phen)] coated on aS i cantilever (500 mml ength, 120 mmw idth, and 20 mmt hickness), which was integrated with am agnetic actuationpizeoresistive detection system, acted as an actuator in ap rototypical SCO-based MEMS device.T he mechanical coupling between the Si cantilever and the softening of the crystal lattice,b rought in by the light-induced LS!HS switching of the film at 10 K, is observed as as hift in the resonance frequency (Df r ) = À0.52 Hz;t hat is, f r decreased upon LS!HS switching. [165] In as tep towards realistic technological applications,abistable MEMS operating under ambient conditions is demonstrated. TheM EMS device is produced by coating a1 40 nm thick film of [Fe(HB(trz) 3

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Reviews 7516 www.angewandte.org ap iezoresistive detection system. Thet emperature-induced LS!HS switching of the film produced ar eversible upward bending of the cantilever in the static mode.I nt he dynamic mode,that is,when the mechanical device was actuated at its resonance frequency,ar esonance frequency( f r )d ecrease of about 66 Hz was measured as shown in Figure 16 b. [166] Note the remarkable increase in the Df r observed for the smaller device based on [Fe(HB(trz) 3 ) 2 ]r elative to the larger [Fe-(H 2 B(pz) 2 ) 2 (phen)]-based device,i ndicating the utility of the sublimable SCO complexes to fabricate more efficient NEMS devices.A na ttempt to correlate the mechanical properties and the crystal structure of [Fe(HB(trz) 3 ) 2 ]r evealed anisotropic variation in the unit cell parameters,c ausing an anisotropic variation of the elastic modulus of the complex upon LS!HS switching. [167] AL S !HS switching mediated resonance frequencyd ecrease is also observed in aM EMS device made of an organic microelectromechanical resonator coated with [Fe(H 2 B(pz) 2 ) 2 (dmbpy)] (dmbpy = 4,4'-dimethyl-2,2'-bipyridine). Theutility of the MEMS device as alight and temperature detector and as an onvolatile memory was demonstrated. [168]

Summary and Outlook
Ad etailed account of advances made in the field of sublimable SCO complexes is presented. TheS CO of the several hundred nm thick films of the complexes on arange of substrates resembled the bulk SCO behavior,i ndicating the dominant role of intermolecular interactions in determining the SCO characteristics in thick films.T he increase in the cooperativity with increasing film thickness and bulk-like SCO observed for [Fe(H 2 B(pz) 2 ) 2 (phen)] on HOPG is at estimony to the role of intermolecular interactions in determining SCO parameters in thick films,and an indication of the weakening of interfacial interactions with increasing distance from the interfacial region. On the other hand, SCO of the sub-monolayer and monolayer films of the complexes deposited on metallic substrates is blocked either due to fragmentation of the complexes or due to the interfacial constrains.C omplete SCO at the sub-monolayer scale is achieved for complexes deposited on passive HOPG;h owever,this result needs to be substantiated by studying agreater number of SCO complexes at sub-monolayer coverage on HOPG.D espite much progress made in terms of achieving SCO at the molecular scale and in thick films,the realization of as ublimable SCO complex capable of undergoing abrupt and hysteretic SCO at or around RT both in the bulk and thinfilm states remains elusive:s ystematic investigations are necessary to obtain thin bistable films.T ransport characteristics of the SCO complexes from the single-molecule level to the thin-film level evidenced higher conductance values associated with the HS complexes relative to their LS counterparts.T he relatively low conductance of the HS [Fe(H 2 B(pz) 2 ) 2 (phen)] in optoelectronic junctions at 10 Ki s ar emarkable exception. Thev ariation in conductivity upon spin-state switching is utilized to fabricate memory architectures.
Despite much progress made and device architectures fabricated, areas that need more attention still remain. The chemical synthesis of designer functional sublimable SCO complexes is still in its infancy, and more systematic efforts are warranted in this direction. In this regard, it is desirable to design SCO complexes with an additional physical property, thus functional-SCO complexes,w hich could be sublimed onto as uitable substrate with preservation of switching characteristics.F unctional-SCO complexes tailored with organic semiconductors, [169] photoluminescent substituents, [170][171][172][173] and self-assembly-inducing moieties [105,174] are proposed. Thehigh resistance of organic molecules is abottleneck for the realization of active molecular electronic components for applications.T he discovery of conducting polymers and developments made in the field of organic semiconductors led to molecular materials with applications in organic electronics and photovoltaics. [175][176][177][178] Herein, we propose to study the spin-state switching characteristics of sublimable complexes tethered with organic semiconductors both in the bulk and thin-film states to obtain spin-statedependent synergistic conductance modulation. By studying the spin-state switching characteristics of luminescent SCO complexes,m agneto-optical correlations,t hat is,t he synergistic coupling between luminescence and spin-state,could be established, which may facilitate the optical addressing of SCO. [170,173] On the other hand, appending SCO moieties with self-assembly-inducing substituents may facilitate the fabrication of spin-state-tunable nanoarchitectures via sublimation, which may enable the establishment of thickness and self-assembly dependence of SCO at the nanoscale,a s established for [Fe(H 2 B(pz) 2 ) 2 (phen)] on HOPG. [75] Chiral resolution of SCO complexes in the bulk and thin-film states may result in novel magneto-optical hybrids.
From the interfacial regime,t he realization of spin-state switchable films of molecular thickness/nanostructures on magnetically "active" ferromagnetic surfaces may facilitate the observation of bistable spinterface based on the presence (ON) or absence (OFF) of magnetic coupling between the HS or LS SCO center and the ferromagnetic substrate,r espectively.N ovel magneto-electric hybrids taking advantage of the device-suitable magnetic properties of SCO complexes and superior electrical and optical characteristics of the graphene could also be realized. [151,158,179] In this context, the self-assembly of sublimable SCO complexes on ag raphene surface could be explored;t he observation of efficient spinstate switching at the single-molecule level on HOPG encourages further investigation in this direction. Thep ossibility of doping graphene,a nalogous to the surface chemical doping of graphene, [180,181] by an SCO complex may facilitate the reversible modulation of charge carriers," switchabledoping,"i ng raphene depending upon the spin-state of an SCO complex, ap rerequisite for practical realization of graphene-based electronics.S of ar, the SCO of sublimable complexes has been studied mostly on metallic and HOPG substrates.Areasonable continuation in this direction would be probing the SCO of sublimable complexes on 2D materials,f or example,M oS 2 and CrI 3 ,w ith intriguing magnetic, optical, and electrical characteristics. [182][183][184][185][186][187][188][189] Overall, agreat deal of progress has been made in terms of studying on-surface switching characteristics of sublimable SCO complexes and the fabrication of device architectures showing spin-state-dependent conductivity modulation. More systematic and relentless efforts may lead to the fundamental understanding of spin-state switching process in the nanostructured environments along with the realization of futuristic molecular electronics and spintronics devices based on functional vacuum-sublimable SCO complexes.