On the thermodynamics of aggregation toward phosphorescent metallomesogens: From electronic tuning to supramolecular design

Two series with three Pt(II) complexes each (PtLPh‐n, PtLFpy‐n) bearing asymmetric tetradentate ligands as dianionic luminophores with variable alkyl chain lengths were synthesized. Hence, each ligand series is distinguished by one of its cyclometallating rings (phenyl vs. 2,6‐difluoropyrid‐3‐yl). Steady‐state and time‐resolved photoluminescence spectroscopic studies in diluted solutions at room temperature and in glassy matrices at 77 K show that the emissive state is mainly centered on the invariantly electron‐rich cyclometalated side while the second ring regulates the admixture of ligand‐centered and metal‐to‐ligand charge‐transfer character. Hence, the radiative rates can be controlled, as indicated by quantum‐mechanical calculations, which also explain the temperature‐dependent trend in the phosphorescence rate constants. Studies in condensed phases (single‐crystal X‐ray diffractometry, polarized optical microscopy, differential scanning calorimetry, steady‐state and time‐resolved photoluminescence micro(spectro)scopy) showed the development of a smectic A mesophase for the fluorinated species bearing the two longest alkyl chains. Nuclear magnetic resonance‐based studies on the thermodynamics of aggregation in solution confirm the marked enthalpic stabilization of aggregates mediated by the polar 2,6‐difluoropyrid‐3‐yl moiety (and to a lesser extent by dispersive forces between the alkyl chains). On the other hand, the negative entropy of aggregation is dominated by the restriction of degrees of freedom involving the peripheral alkyl moieties upon stacking, which becomes increasingly relevant for longer chains. All these factors control Pt···Pt coupling, a crucial interaction for the design of photofunctional mesogens based on Pt(II) complexes.


INTRODUCTION
The use of liquid crystals for the realization of displays (LCD) has evolved considerably since their introduction in the 1970s.The second revolution in the optoelectronic industry involved organic light-emitting diodes (OLEDs).Both technologies present advantages and drawbacks.For instance, LCDs have a longer durability, a lower fabrication cost, and a higher peak brightness; on the other hand, OLEDs have a higher efficiency in the black state, providing access to from the use of both singlet and triplet excitons. [3]However, Pt(II) presents a relevant advantage over Ir(III): Its planar coordination geometry vs. the octahedral geometry of Ir(III) provides an intrinsic molecular anisotropy that favors the formation of mesophases by intermolecular stacking. [4]7][8] Another consequence of the planar coordination geometry found in Pt(II) complexes is the well-known capability of Pt(II)-based species to form aggregates, both in solution and in the solid state.This phenomenon is mainly driven by van der Waals interactions, but dipolar coupling phenomena and hydrogen bonds also play a role. [9,10]When the distance between the Pt-atoms is approximately 3.5 Å or shorter (i.e., below the sum of their van der Waals radii), coupling among the 5d z 2 orbital lobes protruding out of the coordination plane becomes feasible.[16] Thus, the quantification of the intermolecular interactions is of great importance.In this frame, nuclear magnetic resonance (NMR) spectroscopy has shown to be a powerful tool to study the thermodynamics of aggregation equilibria in liquid solution.[19] In this work, two new series of asymmetric tetradentate ligand precursors (L Ph -n and L Fpy -n) were synthesized, along with their corresponding Pt(II) complexes having two different cyclometallating rings; within each of the series, three different alkyl chain lengths were probed.The synthesis of asymmetric CˆN*NˆC-coordinated yet non-mesogenic Pt(II) complexes was recently reported by Tunik et al., but without studying the effect of fluorination and chain length on the thermodynamics of aggregation while using orthogonal phenyl moieties at the bridging N-atom. [20]Herein, the photophysics of the six resulting complexes were studied by steady-state and time-resolved photoluminesce spectroscopy.To gain a deeper understanding of the excited-state properties, time-dependent density functional theory (TD-DFT) calculations were performed.Combination of the experimental and theoretical studies clearly demonstrate the effect of the two different cyclometallating rings on the excited state character.The electron-rich cyclometallating alkoxyphenyl-ring, which dominates the character of the highest occupied molecular orbital (HOMO), determines the energy of the mainly ligand-centered excited state (LC), together with the pyridine ring linked to it (where the lowest unoccupied molecular orbital (LUMO) is mainly located).On the other hand, the second cyclometallating moiety (phenyl vs. difluoropyridyl) dominates the admixture of metal-to-ligand charge-transfer character (more or less MLCT, respectively, depending on the concomitant availability of metallic electrons for the excited state).Thus, the variably spin-forbidden decay rates can be tuned by changing the degree of spin-orbit coupling depending on the relativistic metal perturbation.The temperaturedependent properties of these six compounds were studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and photoluminescence spectroscopy in condensed phases, showing mesomorphic properties for two of the complexes.Further quantitative aggregation studies in solution allowed us to understand the link between the emergence of mesomorphic properties and the aggregation equilibria in solution, which were correlated with molecular structure.The more polar 2,6-difluoropyrid-3-yl motif coupled with long alkyl chains favours stacking and enables the formation of mesophases, showing that both features are strongly linked: the marked enthalpic stabilization of aggregates is mainly mediated by the polar difluoropyridyl moiety and to a lesser extent by dispersive forces between the alkyl chains.On the other hand, the negative entropy of aggregation is dominated by the restriction of degrees of freedom involving the peripheral alkyl moieties upon stacking, which becomes increasingly relevant for longer chains.In this work, we intend to analyse these properties in an integrative context, demonstrating that they are crucial for the design of photofunctional mesogens based on photoactive metal complexes.

Synthesis of the complexes
Scheme 1 depicts the synthetic route employed to prepare the two families of complexes.Intermediate 1 was obtained by a base-promoted nucleophilic aromatic substitution on 2,6dibromopyridine employing 2-amino-6-bromopyridine.The subsequent alkylation of 1 with different alkyl-bromides yields compounds 2-n with diverse chain lengths (n = 6, 12, and 18).From the Suzuki-Miyaura cross-coupling of 2-n with 4-hydroxyphenylboronic acid, the key intermediates 3-n were attained.The 3-n compounds were obtained as main products by working with 1.2 equivalents of the boronic acid.Based on our previous work, we took advantage of the free phenol moiety to provide diverse alkyl chain lengths employing a straightforward Williamson's ether synthesis, thus yielding compounds 4-n. [21]Finally, a second Suzuki-Miyaura cross-coupling reaction between 4-n and the selected boronic acid provided the intended asymmetric ligands L Fpy -n and L Ph -n (n = 6, 12, and 18).The Pt(II) complexes PtL Fpy -n and PtL Ph -n (n = 6, 12, and 18) were synthesized by well-established cycloplatination conditions employing the corresponding ligand and K 2 PtCl 4 in glacial acetic acid at 130 • C. Detailed experimental procedures, as well as the exact mass (EM) spectrometry data and 2D-NMR spectra including the unambiguous signal assignment for all ligand precursors and their complexes, can be found in the Supporting Information (Figures S1-S114).

Photophysical characterization
The photophysical characterization of the complexes was performed in diluted dichloromethane (DCM) solutions (approximately c = 10 The ultraviolet-visible absorption spectra between 250 and 300 nm present bands that can be assigned to spin-allowed transitions into 1 LC states.On the other hand, the bands between 300 and 450 nm have their origin in transitions into mixed 1 LC/ 1 MLCT states (Figure S115 and Table S1).[24] The photoluminescence emission spectra of the six complexes show maxima at around 515-520 nm with a vibrational shoulder at 550 nm.This characteristic profile arises from 3 MP-LC states.The emission maxima of the PtL Ph -n complexes are only slightly blue-shifted if compared with the PtL Fpy -n species, and the photoluminescence spectra at 77 K appear blue-shifted with respect to r.t.measurements; this results from the lack of solvent reorganization around the excited molecules, which in turn reduces the MLCT character of the emissive state.Interestingly, the emission maxima at 77 K are identical for both series, with maxima at 500 nm and shoulders at 540 nm.This means that the emissive state is mainly dominated by the alkoxyphenyl motif rather than by the phenyl vs. 2,6-difluoropyrid-3-yl moieties.(TD-)DFT calculations reproduced this same trend, with adiabatic transition wavelengths between the T 1 and S 0 states of 511 nm for PtL Ph and 517 nm for PtL Fpy at 298.15 K in DCM.Theoretical calculations of de-aromatization upon excitation using the harmonic oscillator measure of aromaticity (HOMA) analysis of the four aryl rings support the assignment of the π-HOMO orbital to the alkoxyphenyl moiety, and the π-LUMO orbital to the pyridinic ring directly bound to the former (Tables S1  and S2, and Figures S139). [25,26]able 1 summarizes the photophysical properties of the six complexes.The long-lived excited states are quickly )  , using Φ L = 0.02 and Φ L = 0 as limits.d Biexponential decay; amplitude-weighted average lifetimes ( av ) are reported and used to estimate average rate constants, as suggested by Engelborghs et al. [ 32].
[30] All the compounds have significantly higher Φ L at r.t.than previous Pt(II) complexes with tetradentate luminophores reported by our group (Figures S117-S134). [13,21,24]This outcome probably stems from the replacement of the orthogonal-to-plane phenyl ring on the bridging N-atom by a slim alkyl moiety; hence, it reduces the non-radiative deactivation of the excited state via molecular distortion caused by repulsion between the Hatoms of the phenyl group at the bridge and at the pyridine rings of the luminophore. [24]t 77 K, all the complexes have unitary Φ L within the experimental uncertainty, which can be ascribed to the inaccessible thermal population of dissociative metal-centered states (MC, which would provide radiationless deactivation channels by conical intersections with the ground state).Interestingly, the PtL Ph -n complexes do not show a significant change of photoluminescence lifetime between r.t. and 77 K measurements; however, the PtL Fpy -n series possesses lifetimes that are increased approximately by 30 % upon cooling.The radiative rates (k r ) for PtL Fpy -n are lower in glassy matrices than in solution, due to a reduced MLCT character in the excited state that consequently restricts the participation of the heavy-metal atom, thus decreasing the k r . [31]However, the PtL Ph -n series does not follow this expected trend and shows a paradoxical rise of k r upon cooling.
For further evaluation of this effect and the experimental results in general, (TD-)DFT calculations were performed on two model complexes (PtL Ph 2 and PtL Fpy 2) bearing shorter ethyl-groups instead of hexyl-, dodecyl-or octadecyl-groups to save computational resources without affecting the character of the excited states.The band analysis of the simulated absorption spectra is in good agreement with experimental findings and supports the assignments described above.Spin-orbit coupling (SOC) was applied for calculations of emission properties, which introduces zero-field-splitting energy (ZFS) for triplet states yielding three thermally mixed yet non-degenerate states SOC 1-3 as a representation of T 1 .To account for that, vibrationally resolved emission spectra and phosphorescence rate constants (k P ) were obtained by Boltzmann-averaging contributions from the first three spinorbit-coupled states according to Equation (1) as suggested by Wang et al. [33] A detailed description of methods, band analysis and calculated spectra are provided in the SI (Figures S135-S139 and Tables S1 and S2).
The analysis of the T 1 -states' formal composition in terms of monoelectronic excitations from the ground state was performed using the inter-fragment charge-transfer analysis toolkit provided by the Multiwfn software package [34] and defining 5 fragments within the model complex (one for each aryl ring and one for the central platinum-atom).The excited state description explaining the emission properties was rationalized by the assignment of contributions from generalized monoelectronic excitations with LC-(ligandcentered), ILCT-(intra-ligand charge-transfer), MLCT-(metal-to-ligand charge-transfer), LMCT-(ligand-to-metal charge-transfer) and MC-type (metal-centered) character (Figure 2, left).It can be concluded that higher k r values observed for PtL Ph -n complexes compared to the PtL Fpy -n series at r.t. and 77 K originate from a difference in 3 MLCT character (17.9 % vs. 12.1 %, respectively, see Figure 2, right).Hence, the electron-poor 2,6-difluoropyrid-3-yl motif reduces the electron density at the Pt-atom, if compared with the plain phenyl ring, which consequently reduces the 3 MLCT contribution to the emissive state and introduces more 3 LC character instead (Figure 2).Thus, all spin-forbidden processes become slower (both radiative and radiationless deactivation paths).[37] It should be noted that vibronic transitions are typically analyzed by evaluating Herzberg-Teller-expanded Franck-Condon contributions (as derived from a Franck-Condon model by first-order Taylorexpansion). [38,39]For the fluorinated complex PtL Fpy , the Boltzmann-averaged k P over all three sub-states slightly overestimates this rate.However, in the case of PtL Ph , the Franck-Condon contribution to the phosphorescence transition is almost negligible, whereas the Herzberg-Teller part exceeds 90% for the two lowest-lying SOC states.Thus, the overestimation of k P is worse for PtL Ph .This can be partially overcome by using range-separated methods like CAM-B3LYP, which yields a better overall representation of the experimental values than with its B3LYP counterpart in terms of energy, kinetic rates, and temperature dependence (Figure S137).
The experimental k r values of the PtL Ph -n complexes are enhanced upon freezing, whereas the k r values of PtL Fpy -n complexes decrease at lower temperatures.In order to determine whether this is a matrix-related effect or an intrinsic property of the excited states, the temperature dependence of the computed phosphorescent rate constants, k P , was obtained by convoluting the Boltzmann distributions of the rates from each individual SOC-state as extrapolations from the calculated values at r.t. in DCM (Equation (1) and Figure 3). [39]Besides the quantitative differences between the experimental k r and the calculated k P values, the qualitative trends observed for k P as a function of temperature are in good agreement with the experimental tendency, rationalizing the seemingly counterintuitive behavior of PtL Ph -n.Interestingly, only Herzberg-Teller calculations (Figure S137) qualitatively reproduce the observed temperature dependence for PtL Ph , which arises when ZFS energies (especially between SOC 1 and SOC 2 (ΔE 2−1 ), are very small and the SOC 2 's k P exceeds the others).

Condensed phase properties
Studies of the condensed phases of the complexes were performed by combining POM, DSC, and single-crystal X-ray diffractometry.
Single crystals for X-ray diffractometric analysis were obtained for PtL Fpy 6 by slow evaporation of a DCM/n-heptane solution.In the case of the other five complexes (those without F-atoms or with longer chains), only amorphous solids were retrieved.PtL Fpy 6 presents a monoclinic unit cell (Figure S140).The molecules are arranged in columns, in a head-to-tail fashion, with Pt-Pt distances of 6.058 and 6.563 Å, which are clearly beyond any significant Pt⋅⋅⋅Pt coupling (Figure 4, left).From this perspective, it is evident that the molecules form zig-zagpatterned sheets.Considering the structure within one of these sheets (Figure 4, right), it is possible to observe that the molecules are organized with their alkyl chains extended in order to fill the highest amount of space between the aromatic cores, [40] in contrast to comparable complexes where F-atoms support the formation of H-bonds. [41]y evaporation of THF/water and DCM/cyclohexane solutions, yellow crystals with yellowish emission and orange crystals with orange-reddish photoluminescence were obtained, respectively.Several crystallization attempts yielded polycrystalline samples, and no single crystal was obtained from these solvent mixtures.If compared with DCM/n-heptane, the red-shifted luminescence of the crystals from THF/water and DCM/cyclohexane mixtures and the shortening of the excited state lifetimes (Figure 5 and Figures S141-S143) are indicative of aggregates with shorter Pt-Pt contacts, if compared with decoupled monomeric species. [13,21,42]This shows that the aggregation with concomitant Pt-Pt interactions can be induced by slight changes in the polarity and/or nature of the solvent. [43]he complexes of the PtL Ph -n series did not show any mesogenic behavior and re-crystallized at 245  12 showed decomposition, which was confirmed by DSC (Figure S150 and Table S3).In contrast, PtL Fpy 6 melts from its crystalline form to give an isotropic liquid (Figure S147), while PtL Fpy 12 and PtL Fpy 18 displayed an enantiotropic phase behavior while showing the typical focal conic texture of a SmA phase (as determined by POM, see Figure 6).The transition temperatures are in line with those obtained by DSC (Figure S151).This finding agrees with the related calamitic Pt-mesogens reported by Kozhevnikov and Bruce et al. [44] In the case of PtL Fpy 12, the crystallization process is suppressed, and a transition into a glassy state is observed, which preserved the focal conic texture of the liquid crystalline phase.For both complexes, cold crystallization is observable via DSC at 108 • C for PtL Fpy 12 and at 102 • C for PtL Fpy 18 (Figure S151 and Table S3).We have previously demonstrated that the presence of 2,6-difluorophenyl and 2,6-difluoropyrid-3-yl rings as cyclometallating units increases the aggregation in Pt(II) complexes. [13,23]Here, the difluoropyridine ring seems to be a key factor for the introduction of liquid crystalline properties.47] The increased chain length broadens the temperature range of the mesophase by decreasing the crystallization temperatures of the compounds (Figure 7).Comparing the corresponding enthalpies of PtL Fpy 12 and PtL Fpy 18, the higher value observed for PtL Fpy 18 can be explained by considering that the transition corresponds to the melting of the alkyl chains.This finding is in concordance with the comparable clearing temperature and enthalpy observed for both compounds, which can also be associated with the lack of interactions between the cores of the complexes. [40]he effect of the temperature on the steady-state photoluminescence of the six complexes was studied in condensed phases.In one case, for the non-mesogenic complexes, the emission was measured at room temperature before and after heating them to reach the isotropic phase (Figure S152).In all cases, an enhanced red-shifted emission stemming from aggregated species was observed after heating the complexes.This can be explained considering that the aggregates explore further local minima corresponding to metastable phases: in the isotropic phase, the molecules have the necessary kinetic energy to move and form the aggregates with sizeable Pt-Pt interactions.F I G U R E 5 Photoluminescence spectra and micrographs of PtL Fpy 6 crystals obtained from DCM/n-heptane (green), THF/water (yellow), and DCM/cyclohexane (red) solutions; its molecular structure in the green-emitting crystals is also shown, as obtained from X-ray diffractometry (displacement ellipsoids are drawn at 50% probability, H atoms were omitted for clarity).The corresponding lifetime maps are found in the SI (Figures S142-S144).F I G U R E 7 Summary of the mesomorphic properties for the studied Pt(II) complexes, as determined by variable-temperature polarized optical microscopy (POM) imaging upon heating.The numbers correspond to the temperature range of the corresponding phases (complexes marked with an asterisk showed decomposition).
In the second case, the mesogenic complexes PtL Fyp 12 and PtL Fpy 18 show a totally different behavior (Figure S153).Both displayed a reddish luminescence already at room temperature, with emission maxima peaking at approximately 660 nm.The photoluminescence intensity increases drastically after the transition to the liquid crystalline phase and decreases again at higher temperatures, showing relative maxima at 115 • C and 120 • C, respectively.At 10 K above the intensity maxima, a red shift of 35-40 nm (0.1 eV) is also observed with a subsequent decrease in the emission intensity.The red shift implies that Pt-Pt interactions are favored at higher temperatures, as the stacks gain additional degrees of freedom when the side chains "melt" towards the liquid-crystalline phase The enhanced emission intensity after the transition to the mesophase can be attributed to the unrestricted formation of excimers. [48]The drop in the emission intensity at higher temperatures is a consequence of the thermal population of roto-vibrational states, which causes a higher k nr .

Aggregation studies
In order to gain a deeper understanding of how the asymmetric ligand structure and the alkyl chains affect the intermolecular interactions within the supramolecular assemblies, we decided to study the aggregation equilibria of the complexes in liquid solutions.Hence, a fully quantitative analysis was achieved by concentration-and temperature-dependent 1 H-NMR spectroscopy.
The 1 H-NMR spectra of the Pt(II) complexes show that the chemical shift of different H-signals varies with the concentration and the temperature (Figure 8).Both the increase of concentration and the drop of temperature shift in the 1 H-NMR signals to higher fields.This means that within the aggregates, the annular current of the aromatic rings protects the nuclei of the H-atoms in their proximity; thus, the Hatoms closer to the luminophoric complex are more affected.This is particularly true for the aromatic signals and for those resulting from the H-atoms in the α-position of the alkyl chains at the N-bridge.
Due to the faster rate in the self-association reactions (if compared with the rate of nuclear spin decay in 1 H-NMR), the chemical shifts represent a time-average over all species in the sample, and it is possible from their values to determine the self-association equilibrium constants K.There are several models to estimate the K values.Hence, as it is not possible to guarantee the formation of dimers only (higher aggregates cannot be excluded), we herein resorted to an equal-constant model (EK-model), which assumes that K is independent of the number of molecules in the aggregate (i.e., without any significant (anti-)cooperativity). [49]The relation between the total monomer concentration C T and chemical shift δ in the EK-model is described by Equation (2), where δ M is the chemical shift for the monomer and δ Ag is the chemical shift of the aggregate for one particular nucleus. [50]δ M was estimated as the chemical shift at the lowest concentration with the highest temperature, and δ Ag as the chemical shift at the highest concentration with the lowest temperature used.
By plotting the term containing the chemical shifts (y) as a function of C T , K can be obtained from the slope.Hereafter, we favored the signal corresponding to the methylene group in the alpha position with respect to the N-bridge (Figure 9), as it provides the highest Δδ value among all the proton signals and consequently, the lowest relative experimental uncertainty.Since each concentration was measured at several temperatures, K was retrieved at different temperatures as well (Figures S154-S191).
The K values obtained (see Table 3) are comparable with those previously reported for other Pt(II) complexes. [10,51]wo particular trends can be observed from the K values: 1) the variable cyclometallating unit (in this case, 2,6-difluoropyrid-3-yl vs. phenyl) mainly defines the ranking of the equilibrium constant, with significantly higher K values for the fluorinated PtL Fpy -n complexes; 2) the increasing length of the alkyl chains length rises the K values as well.Interestingly, we reported before that the incorporation of hexyloxy groups instead of methoxy groups promotes an enhanced solubility of the complexes. [21]Herein, it becomes clear that further increments in the alkyl chain lengths favor the aggregation.This can be explained in terms of the variation in the layered molecular structure in the solid phase: shorter moieties did not result in the formation of alkyl chain layers (see PtL Fpy 6 crystals), which is related to a higher solubility.However, substitution with longer tails enables the formation of such layers (as observed in the PtL Fpy 12 and PtL Fpy 18 mesophases), which in turn enhances the cohesive energy in the solids and promotes aggregation with the increment of chain length. [52]This observation can be ascribed to increasing van der Waals interactions for progressively longer alkyl chains.
More information can be extracted from the experimental data by analyzing the effect of temperature on K. Using van´t Hoff's relation, Equation (3) is obtained.By plotting ln K as a function of 1/T, the enthalpy (ΔH) and the entropy (ΔS) F I G U R E 8 1 H-NMR spectra of PtL Fpy 6 at 300 K for different concentrations (top) and 1 H-NMR of PtL Fpy 6 as a 3.12 mM solution at different temperatures (bottom).change due to aggregation can be determined from the slope and the intercept of the linear regression, respectively (Figure S192). [50]

TA B L E 3
The negative values (Table 4) indicate that the aggregation in the DCM solution is an enthalpy-driven process.
Replacement of the phenyl ring by 2,6-difluoropyrid-3-yl has a significant effect (by a factor of roughly two) for both thermodynamic magnitudes.Notably, this change can be related to the higher dipolar moment in PtL Fpy -n as compared to PtL Ph -n complexes, which lock the head-to-tail dimers more tightly than the phenyl-moiety.Thus, stronger dipolar interactions produce a higher stabilization of the aggregates and more negative ΔH values.The clearly more negative ΔS values for the fluorinated ligand, on the other hand, can be  ascribed to a stronger restriction of the degrees of freedom in the tighter aggregates, supported by the larger dipolar moments.The more pronounced loss of entropy observed for the longer chains can be interpreted as a progressively growing number of roto-vibrational degrees of freedom that become constrained upon aggregation.

CONCLUSIONS
In conclusion, we herein report a series of six phosphorescent Pt(II) complexes including two asymmetric coordination compounds with liquid crystalline behavior.The emission profiles in diluted liquid solutions and frozen glassy matrices show that, independently of the asymmetric cyclometallating unit, the emissive state is mainly centered on the ring with the highest-lying occupied orbital.Incorporation of the 2,6-difluoropyrid-3-yl motif increases the 3 LC and reduces the 3 MLCT character of the excited state, which causes a reduction of the k r .Theoretical calculations allowed us to interpret the trends observed for the radiative rates at variable temperatures while estimating k P values as Boltzmann-weighted averages of the SOC components.Additionally, the incorporation of the 2,6-difluoropyrid-3-yl motif enables the formation of SmA mesophases, as a result of the increased dipole moment.A second consequence of the higher polarity is the drastic increase in the aggregation constants observed for the complexes in liquid DCM solutions.The growing chain length further boosts the aggregation constant, due to enhanced van der Waals interactions within the lamellar structures.The photoluminescence of Pt Fpy 6 crystals can be attributed to variable Pt-Pt distances: Crys-tallization from DCM/n-heptane mixtures, where n-heptane molecules can interact with the hexyloxy chains, gives samples without sizeable Pt⋅⋅⋅Pt, as the alkyl chains have no interactions between them and fill the empty space between the aromatic cores.Crystallization from a polar mixture (such as THF/water) or from a mixture without a linear aliphatic solvent (such as DCM/cyclohexane) yields crystals with emission from aggregates, probably due to a lamellar organization of the molecules, which are similar to the smectic phases.Altogether, we conclude that two main factors favoring aggregation are needed to obtain liquid crystalline phases: a high dipole moment in the molecular core to support enthalpy-driven π-π stacking and the lateral interaction within them inside the layers, as well as alkyl chains that are long enough to allow the micro-segregation of the two molecular blocks within the mesophase (with sufficiently strong van der Waals interactions to compensate the loss of entropy upon aggregation).Finally, the combination of the temperature responsiveness on both the phase transition and the emission intensity for the two metallomesogens could open a door to the design of luminescent thermo-sensors based on liquid-crystalline matrices.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

1
Raw time-resolved photoluminescence decays are found in the SI (FiguresS118-S135).a Monoexponential decay.b The upper limit for k r was estimated with k r =  L  av , using Φ L = 0.02.c The intervals for k nr were estimated with k nr = 1− L  av

3
Temperature dependence of the calculated phosphorescent rate constant (k P ) for PtL Ph (red) and PtL Fpy (blue).

F I G U R E 4
Molecular packing in crystals of PtL Fpy 6 viewed along the c axis, showing the columnar arrangement (left) and the view along the a axis (right).Displacement ellipsoids are drawn at 50% probability; H atoms were omitted for clarity.The corresponding file, namely CCDC-2219870, contains the supplementary crystallographic data.

F I G U R E 6
Polarized optical microscopy (POM) images upon heating of PtL Fpy 12 at 145 • C (left), and PtL Fpy 18 at 152 • C (right) using crossed polarizers and showing the focal conic structure of the SmA phase.

A
C K N O W L E D G M E N T S M.E.G.S. gratefully acknowledges a doctoral fellowship from the Deutscher Akademischer Austauschdienst (DAAD).The authors gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) ─ Project-ID 433682494 -SFB 1459 Intelligent Matter.C.A.S. gratefully acknowledges the generous financial support for the acquisition of an "Integrated Confocal Luminescence Spectrometer with Spatiotemporal Resolution and Multiphoton Excitation" (DFG/Land NRW: INST 211/915-1 FUGG; DFG EXC1003: "Berufungsmittel").

exemplarily shows the spectra of PtL Ph 12 and PtL Fpy 12, as the main luminophore is essentially invari- ant within each family. For both PtL Ph -n and PtL Fpy -n
−5M) at room temperature (r.t.) and in

Ph 12 PtL Ph 18 PtL Fpy 6 P t L Fpy 12 PtL Fpy 18
Experimental absorption spectra (A), normalized emission spectra at 298 K (B) and at 77 K (C) of PtL Ph 12 (solid red lines) or PtL Fpy 12 (solid blue lines), as well as theoretical absorption (D) and emission spectra corrected for adiabatic transition maxima at 298 K (E) or 77 K (F) of PtL Ph 2 (dashed red lines) and of PtL Fpy 2 (dashed blue lines).TA B L E 1 Photophysical properties of the complexes in different conditions.Air-equilibrated DCM solution at r.t.
Experimental average radiative rate constants and calculated phosphorescence rates.
a Average experimental radiative rate of the three complexes of each series.b Boltzmann-average of the three SOC rate constants.
• C for PtL Ph 6, at 235 • C for PtL Ph 12, and at 215 • C for PtL Ph 18, as determined by variable-temperature POM imaging (Figures S144-S146).PtL Ph 6 and PtL Ph Aggregation equilibrium constants at different temperatures.The missing values could not be measured due to the precipitation of the complexes.Uncertainties were obtained from the linear adjustment (see SI). a van´t Hoff plot for PtL Ph 12 (red) and PtL Fpy 12 (blue).Aggregation enthalpies and aggregation entropies.