Effect of alkyloxy substituents on mesomorphic and photophysical properties of star-shaped tristriazolotriazines

ABSTRACT Two series of tristriazolotriazine (TTT) derivatives with different length of aliphatic substituents have been synthesised in order to study the influence of peripheral substitution on mesomorphism and fluorescent properties of these star-shaped compounds. Homologues with methoxy and alkyloxy substituents (series 1) are non-mesogenic, in contrast to homologues with two alkyloxy substituents (series 2) exhibiting columnar mesophase. The results obtained by quantum-chemical calculations for the monomers and dimers of non-mesogenic TTT series and their comparison with truxenone derivatives having identical substituents made it possible to establish the reasons that deprive series 1 of mesogenity. Compounds of both TTT series possess good solubility in organic solvents and capable of specific interactions with solvents in the ground and excited states. They form fluorescenting floating layers and thin films. An analysis of the fluorescence spectra showed a high potential for their application, in particular, in the field of molecular sensing and precise non-invasive control. GRAPHICAL ABSTRACT

Tris [1,2,4]triazolo [1,3,5]triazine (TTT) derivatives represent a new subclass of conjugated star-shaped compounds [20][21][22][23].The nitrogen atoms in the π-conjugated core of a star-shaped molecule TTT enhance both electron-transport and luminescent properties, which is very important for practical applications [10,22].TTT derivatives can contain from three to six substituents attached to the small central core.Depending on the chemical nature, number, and length of the substituents, TTTs can show liquid crystalline properties.In [21][22][23] it was reported that hexasubstituted TTTs with aliphatic substituents of certain length (n ≥ 5) form a thermotropic columnar hexagonal mesophase of disordered type in a wide temperature range (~100°C).A decrease in the number of alkoxy substituents from six to three results in the disappearance of mesomorphic properties [22].
As regards the fluorescent properties of TTT derivatives and their practical application, previous studies were mainly concerned with their non-mesogenic derivatives [10,22].For example, a new emitter based on the TTT acceptor was reported in [10], which is characterised by thermally activated delayed fluorescence and exhibits emission in the blue spectrum region.In this case, the maximum external quantum efficiency reached 11%.Detailed studies of TTT derivatives with combined properties (both mesogenic and fluorescent) are presented in a minor number of publications [22,23].However, photophysical properties of TTTs in solutions are barely investigated.The latter is of paramount importance for proper distinguishing between intrinsic fluorescent properties, and the ones introduced by supramolecular organisation and heteromolecular interactions in thin films.
Since fluorescent materials are widely used in thin-film technologies, an important fundamental problem of great practical importance is the control and management of self-assembly of star-shaped compounds to ensure the optimal physicochemical properties of the resulting thinfilm materials [24][25][26].For the studying of such materials based on star-shaped compounds, Langmuir methods [25,27] or spin-coating [25] are usually used.Langmuir methods allow to obtain floating layers and subsequently thin films with the precise thickness at the molecular level [28][29][30].A serious problem that affects the physicochemical characteristics of thin films is the aggregation of molecules [31].Controlling the initial degree of surface coverage and the compression rate during formation of floating monolayer, it is possible to reduce aggregation processes in floating layers and, consequently, optimise the architecture of obtained thin films on solid substrates [32][33][34][35][36].The study of spin-coating films contributes to the understanding of the molecular self-assembly processes under conditions close to industrial ones.
The aim of this work is to establish the influence of the number and length of peripheral alkoxy substituents of two series of TTT derivatives on their mesogenity, fluorescent properties in organic solvents of different polarity and in thin films obtained by various methods (Figure 1).
Polarising optical microscopy was performed with the help of a Leitz Laborlux 12 Pol microscope equipped with a Mettler FP80 heating stage.Microphotographs of textures were obtained by digital photocamera DCM800.Phase transition temperatures and enthalpy changes (J*g −1 ) were determined with the help of differential scanning calorimeter (DSC 200 PC/1/M/H Phox) at a scanning rate of 10°C ⋅ min −1 .The differential scanning calorimeter system was calibrated using an indium standard under a nitrogen atmosphere.
Thin films were obtained in two ways: by the Langmuir-Schaefer method and by centrifugation (spincoating).Floating layers were formed from chloroform solutions (C = 8.2 × 10 −5 M) using an NT-MDT trough (Russia), an initial coverage degree of water surface c = 61%, which was calculated by the method published in [15].The compression rate was 55 cm 2 /min.The construction of molecular models and monomolecular layers (face-on arrangement) as well as the calculation of geometric characteristics were performed with the help of HyperChem 8.0 program using the molecular mechanics method (MM+), as it was previously described in [15].The monolayers were then placed on the surface of a preliminarily simulated volume of water.After optimisation by MM+, the models of monomolecular layers on the water surface were obtained.On the basis of these data, the model areas of the closest packing of TTT molecules at the air/water interface were calculated (Table 1).The calculated molecular areas of the closest packings were then compared with the areas obtained in the experiment.Based on these results, the floating layer structures were evaluated.The transfer of floating layers onto solid substrate was carried out by the Langmuir-Schaefer technique (horizontal lift), the number of transfers n = 1, 3, 5, 10 and 25 layers.
Spin-coated films were obtained on silicon or quartz glass substrates using Laurell WS-650MZ-23NPPB spin coater (USA) in a static operating mode, at substrate rotation speed of 1000 rpm, the concentration of the applied TTT chloroform solution was 10 −2 M. The films were dried in air.
The surface topology of thin films was studied by atomic force microscopy (AFM) in a semi-contact mode using a Solver 47 Pro microscope (NT-MDT, Russia).For AFM measurements, LS films were deposited on silicon substrates, spin-coated films -on glass substrate.
Photophysical properties of solutions in organic solvents and thin films were studied by the absorption and fluorescence spectroscopies.Absorption spectra were recorded in the wavelength range of 190-400 nm on an Aquilon SF-104 spectrophotometer controlled by the UV-Win 5.1.0software package.The measurement accuracy is ± 0.03 nm on the optical density scale and ± 0.05 nm on the wavelength scale.The measurements of solutions were carried out in a quartz cell with a lightabsorbing layer 10 mm thick.Thin-film materials were studied on solid substrates.Fluorescence spectra were obtained using a Cary Eclipse Varian-Agilent fluorescence spectrometer with the help of Cary Eclipse Scan Application 1.1 software.The interval of spectral measurements was 270-500 nm with an excitation wavelength of 260 nm and an excitation/detection slit width of 5/5 nm.The set temperature was maintained using a Peltier PTC-2 flow thermostat.
Quantum chemical calculations and geometry optimisation of monomers and dimers of truxenone and TTT derivatives were carried out by the DFT/B97D method with the basis 6-311++G** [44] using the Gaussian'09 program [45].The Grimme B97D hybrid exchange-correlation functional [46] takes into account dispersion interactions, which play an important role in modelling dimers.
Analytical data -see ESI.

Features of phase behaviour
The study of phase behaviour by POM and DSC methods showed that all synthesised homologues of series 1 are non-mesogenic (Table 2, Figure S1).
An increase in the alkoxy substituent length leads to a decrease in melting points from 130.9°C for 1a to 106.0°C for 1c.According to DSC data (see Figure S1), compounds 1a and 1c exhibit crystalline polymorphism during the first heating cycle, while homologue 1b has only one crystalline phase.It should be noted that compounds 1a-c undergo supercooling, i.e. after first melting, on cooling from the isotropic state to room temperature, no crystallisation was observed.
The homologues of series 2 are mesogenic showing columnar disordered phase (Col hd ) with fan-like and mosaic texture (Figure 2).The type of mesophase was established by the authors of [21].The data on the phase transition temperatures of compounds 2a-c (Table 2) are in good agreement with those presented in [37][38][39][40].
The presented data can be interpreted from the standpoint of the microsegregation concept of the  central hydrophilic core and hydrophobic periphery of disc-like molecules [18].However, when explaining experimental data and a priori predicting the possible mesomorphism formation, it is also necessary to consider such factors as the distribution of electron density over the volume of central core, the steric effect, as well as specific and non-specific interactions.
This remark is clearly confirmed by the observed presence of mesomorphism in truxene 11a,b and truxenone 12a,b derivatives containing methoxy groups, and in contrast, the absence of mesomorphism in analogues TTT derivatives ( 1a -c).
The lack of mesomorphism in TTT derivatives of series 1 can be explained by the smaller conjugation area all over the TTT core, in contrast to truxenes 11а,b and truxenones 12a,b.Presumably, the phenyl rings, deployed relatively to the TTT core plane, break the conjugation and introduce a steric hindrance approaching the molecules to each other.

Quantum chemical calculations of monomers and dimers of truxenone and tristriazolotriazine derivatives
An attempt to prove the appropriateness of our explanations about the reasons for the differences in the mesogenity of truxenones 12 and tristriazolotriazines 1 was carried out by quantum chemical calculations of monomers and dimers of both series.It is known that the length of substituents does not affect the geometric and electronic characteristics of the cores [52].Therefore, in order to reduce the modelling calculation time, the homologues with n = 6 were taken.
The core of truxenones 12 has a flat and rigid structure (Figure 4(a,c)).The internal rotation barriers of the substituents around C ar -O bonds are high.Therefore, a change in the monomer structure can only be associated with a change in the conformations of alkyl substituents.
Figure 5(a) shows the structure of the dimer 12-6, in which the distance between the cores is ~3.5 Å.This is equal to the π-π stacking distances observed in many discotic liquid crystal molecules [50].This stacking distance makes it possible to maintain good order within the columns and realise Col h phase during mesophase formation as it was observed for higher homologues of truxenone derivative 12 (see Figure 5).
Unlike truxenone 12-6, the monomer 1-6 can be considered non-rigid.As a result, 1-6 can exist in two conformations: I (Figure 4  transition states between them can be seen in Figure S3 (see ESI).
Thus, during the self-assembly of TTT 1-6 molecules in different conformational states, strong disorder arises (Figure 5(b)), which probably prevents both the formation of mesogenic columnar assemblies and crystallisation in the cooling mode from the isotropic state.

Photophysical properties of solutions of tristriazolotriazine derivatives
The analysis of literature data [21,[37][38][39][40] revealed that the photophysical characteristics of TTT derivatives have not been studied to a sufficient extent.Moreover, such data are unknown for the derivatives of series 1, which we have synthesised for the first time.The results of the solvent effect on the TTT absorption and luminescence maxima are reported in the literature for a limited number of solvents.From the published data, a weak negative solvatochromic effect is assumed: as the polarity of the solvent increases, the absorption maxima shift hypsochromically [40].Due to the great interest in the practical application of such compounds, one can find more often comparison of the spectral characteristics of solutions with the spectral properties of thin films.It is known that a bathochromic shift of the absorption band is observed in the solid phase/films and, as a rule, quite intense luminescence is observed.
Aryl substituents have a low rotational barrier relatively to the TTT core.However, the angle at which they are stabilised in the crystal structure can vary greatly depending on the substituent [37].The nature of the substitution of peripheral phenyl fragments can affect the distribution of electron density in the ground and excited states, and thus affect the solvatochromic response of the compound.
The solvatochromic characteristics of TTTs are unfairly neglected by researchers, although they are a source of valuable information about weak interactions that affect the molecular packing in thin-film materials obtained by spin-coating or Langmuir methods (Langmuir-Schaefer and Langmuir Blodgett technique).In addition, the potential of using TTTs as liquid-phase sensors cannot be revealed without thorough fundamental studies of the main reasons of their solvatochromic response.
The synthesised star-shaped TTT derivatives of series 1 and 2 have a good solubility in organic solvents and exhibit luminescent properties.When irradiated with UV light with a wavelength of 365 nm, both in solid form and in solutions, they show a blue-violet glow (Figure 6).
An analysis of the spectral characteristics in solvents that are not prone to specific interactions (such solvents as the ones having low values of hydrogendonor acidity/basicity and possessing no conjugated π-electron system) showed an insignificant dependence of the spectral properties on the nature of the TTT substituents (Tables S1 and S2 see ESI).
At the same time, in a number of solvents (methanol, ethanol, ethyl acetate, DMSO, DMF), the studied TTTs show strong changes in the spectral characteristics.The changes are associated with the redistribution of intensity between the main absorption maximum and the shoulder on the left slope of the absorption spectrum.Characteristic changes are detected for all studied compounds; however, with varying degrees of manifestation (Figure 7, Figure S4    Most likely, the observed changes are the result of a redistribution of intensity between different electronic transitions or a transformation in the vibronic structure of the spectra.Decreasing the concentration, heating the sample, and sonicating the solution do not cause a reverse redistribution of the intensity or the bands shift to the range characteristic of TTT derivatives in most solvents (300-310 nm).This fact also confirms that the above changes are due to the interactions of TTT derivatives with solvent (heteromolecular interactions).
The relative fluorescence quantum yield also strongly depends on the used solvent (Tables S1 and S2 see ESI).Quantum yield parameters are in agreement with several empirical solvent polarity scales.We approached our data with Catalan and Kamlet and Taft empirical solvent parameter scales and Onsager reaction field theory in different formalisms.Unfortunately, no reasonable correlations were recovered.This fact means complex nature of excited state properties in studied molecules.The total range of changes in the relative quantum yield is up to 20 times (between solutions in hexane and toluene).However, the type of dependencies is not described by empirical solvent polarity scales, which indicates the possibility of dynamic luminescence quenching processes and requires a separate study (Tables S1 and S2 see ESI).
Thus, the studied TTT derivatives (non-mesogenic of series 1 and mesogenic of series 2) are capable of specific interactions with solvents both in the ground state and in the excited state.This indicates a high potential for their application, in particular, in the field of molecular sensorics of solutions.

Morphology and spectral characteristics of thin films
Atomic force microscopy (AFM) was used to reveal the influence patterns of molecular structure of the studied compounds on aggregation processes in thin films (Figures 8, 10).The remaining AFM images of LSfilms and the height distribution of 3D-aggregates of compounds 1a-c and 2a-c represented in Figures S5 and  S6, see ESI.
The study results of the surface relief showed that large single 3D aggregates are formed in the LS-films of compound 1a.Small values of films roughness coefficients indicate complete coverage of the substrate with these aggregates.An increase in the length of aliphatic substituents from n = 8 to n = 12 leads to a decrease in the parameters of the formed 3D aggregates.However, the substrate surface of the LS-film of the longest homologue 1c (n = 12) is covered incompletely, as it is evidenced by the high value of the roughness coefficient (Table 3).For the homologue 1c, an increase in the number of transfers from 1 to 5 layers leads to an increase in aggregation and a decrease in the roughness coefficient by a maximum factor of 53.5.This indicates a high coverage of the substrate surface with aggregates.
For the series 2, the presence of alkoxy substituents of the equal length in the 3,4-position leads to the formation of larger 3D aggregates in comparison with the aggregates in films of compounds series 1.An increase of transfer from 1 to 5 layers promotes an increase in aggregation and a decrease in the roughness coefficient, which indicates a high filling of the substrate surface with aggregates (Table 3).Thus, it has been shown that for the series 1, the elongation of alkoxy substituents decreases the size of aggregates in the formed film.For the homologue 1c (n = 12), the height and diameter of aggregates in the film decrease by 10.37 nm and 59.2 nm, respectively, as compared to the homologue 1a (n = 8).For the series 2, a decrease in the diameter of aggregates by 118 nm is also observed compared to compounds of series 1.However, no pattern of change in the height of aggregates was established (Figure 9, also in Figures S7 and S8, see ESI).
The study of the surface relief of thin films obtained by spin-coating method (SC-films) showed that compounds 1c and 2c strongly aggregate (Figure 10).The aggregates sizes of SC-film of nonmesogenic compound 1c were estimated: the  diameter of the 3D aggregates is 667 nm, the height of aggregates is 97.65 nm, and the roughness coefficient is 27.81.The film surface relief of SCfilm of mesogenic compound 2c is more uniform: the aggregate height decreases by 80.7 nm, but the diameter increases by 667 nm, and the roughness coefficient is 2.67.

Photophysical properties of tristriazolotriazine derivatives thin films
Spectral studies of thin films of compounds 1а-с and 2ас were carried out with the aim to determine the effect of intermolecular interactions on photophysical characteristics of the studied chromophores.The results were compared with chloroform solutions, since the films were formed from this solvent (Figure 11).In the absorption spectra of chloroform solutions of 1a-c and 2a-c, the band at 311 nm was observed, which is associated with transitions in the conjugated system of TTT derivatives (Figure 11(a)).

Absorption of thin films
The type of absorption spectra of SC-films of compounds 1c and 2c (Figure 11(b)) are similar to their spectra in chloroform solutions.However, the band of non-mesogenic 1c is bathochromically shifted (by 16 nm) and has less diffuse character than the band of mesogenic 2c.Moreover, in contrast to solution, the band of 1c has two quite distinct shoulders -on the right and left slopes, which indicate the formation of various types of aggregates.
The common feature of absorption spectra of LS-films is the broadening of the main absorption band at 311 nm and its insignificant bathochromic shifts up to 4 nm (for 1a-c) and up to 10 nm (for 2a-c) (Figure 11(c,d)).The number of transfers (from 1 to 25 layers) does not affect the structure of spectra to a significant extent.It should be noted that in all spectra there is a shoulder on the right slope of the band (Figure 11(c,d)), which is more structured in non-mesogenic 1.Such kind of spectra suggests insignificant aggregation, which is confirmed by the AFM data (Table 3).To verify this suggestion, on example of compounds 1c and 2c, the dependence of the optical density of the films on the number of transfers was plotted (Figure 12, Figure S9, see ESI).It can be seen that the deposition of more layers leads to increase in the film absorption intensity.The obtained dependences are described by linear equations with correlation coefficients not lower than 0.99, confirming the production of LSfilms of a constant architecture with insignificant content of 3D aggregates.

Fluorescence of thin films
The fluorescent characteristics of thin films of series 1 and 2 also have pronounced dissimilarities, which differ little from the electronic absorption spectra in their regularities (Figure 11).
It is important to note that the fluorescent spectra of films of series 1 have an obvious structuring, therefore we can suppose that these films are highly ordered.The presence of individual transitions (shoulders) in the spectrum structure can be associated with the degeneracy of the electron-vibrational levels (or with a decrease in the number of allowed transitions) during the formation of aggregative structures.
It is worth to note that the compounds of series 1 are non-mesogenic, which allows us to assume that forces driving aggregation processes in these compounds upon their transfer from the interface may differ from forces driving the columnar mesophase formation of compounds of series 2. This assumption is additionally confirmed by differences in fluorescence spectra of LS-films and SC-films, considering that the SC-films are an intermediate case between monomolecular layers and bulk systems by their structure.In this case, we see a gradual decrease in the structuring of the fluorescence spectra going from to SC-films.
This indicates the possibility to control fluorescence spectra of films by varying the methods of preparation thin-film materials.

Conclusion
Based on the synthesis and study of two series of starshaped tristriazolotriazine derivatives, differing in the length of alkyloxy substituents, the following main conclusions should be noted: • replacement (shortening) of three out of six long alkyloxy substituents by methoxy groups leads to the loss of mesogenity.According to quantum chemical calculations, this is due to the appearance of different molecular conformers with close electronic energy and low transition barrier (only 0.11 kCal/mol) in non-mesogenic representatives.
During the self-assembly, various conformers prevent both columnar mesogenic packing and crystallisation upon cooling; • in organic solvents, the absorption and fluorescence spectra of all studied compounds are affected not so much by the molecular structure of tristriazolotriazine derivative as by the type of the solvent used.Important whether the solvent has the ability to specific (heteromolecular) interactions or has not.The variation range in the values of quantum yields of the same compound in these different types of solvents can reach up to 20 times; • fluorescence spectra of thin films based on the studied non-mesogenic tristriazolotriazine derivatives are more structured than those based on mesogenic ones.Most clearly, this trend is manifested for the films prepared by the Langmuir-Schaefer method.
The data obtained in this study indicate possible diversities of approaches to controlling the properties and using both mesogenic and non-mesogenic star-shaped tristriazolotriazine derivatives.These compounds can be applied to create thin-film materials for photovoltaics/ optoelectronics, fluorescent sensorics in solutions and at the phase boundary, as well as for non-invasive control in biomedical research.

K 2 CO 3 (
60 mmol) and 2-butanone (150 ml) was refluxed for 25 h under vigorous stirring.After the completion of the alkylation reaction, the reaction mixture was cooled to room temperature.The precipitate was filtered off and washed with hot 2-butanone.The solvent was removed under reduced pressure and the crude product recrystallised from ethanol.Analytical data -see electronic supporting information (ESI).
(b)) and II (Figure S2(a), ESI), as well as in the form of regioisomer III (Figure S2(b), ESI).These three structures I, II and III are distinguished by the turning of the phenyl groups relatively to the small rigid core (Figure S2, ESI).Calculations show that the conformer I has a symmetry close to C 3 and almost identical torsion angles N-C-CPh 1 -CPh 2 of three phenyl fragments.Conformer II has one of the torsion angles of the opposite sign.The electronic energy of conformers I and II differs by only 0.11 kcal/mol (Figure S3, ESI).In the transition state (TS) between these two conformers, the selected phenyl fragment lies in the plane of the rigid core, and the transition barrier is only 0.51 kcal/ mol, which is comparable to the thermal energy RT at T ~ 260 K.It means that both conformers I and II can exist in crystalline state, as can the regioisomer III (Figure S2, ESI), which has a lower energy than the conformers.The relative energies of structures I, II, and III, as well as the

Figure 9 .
Figure 9. Aggregate sizes obtained by AFM for the LS-films with one layer transfer.

Figure 12 .
Figure 12. (Colour online) Dependence of optical density of the LS-film (at a wavelength of 316 nm) on the number of transfers (n = 1-25) for compounds 1c and 2c.

Table 1 .
Areas of molecules in the closest face-on model packing.

Table 2 .
Phase behaviour of series 1 and 2.

Table 3 .
Surface morphology of LS-films.