Materiais moleculares funcionais contendo n-heterociclos - capes

Table 1Thermal and optical properties of compounds 3a–eThermal propertiesOptical propertiesTransitions/1C a T g /1C b T dec. /1C c CompoundsSol. d Film Sol. d FilmeF Fl E g /eV fAbs. l max /nm Fl. l max /nm3a C 278.0 I — 400 290 290 360 363 0.25 3.93b C 129.9 I 55.4 445 294 288 362 366 0.33 3.93c C 93.8 I 46.7 437 294 287 362 368 0.33 3.93d C 86.6 I 70.3 441 294 286 362 371 0.34 3.93e C 92.2 (19.3) Col h 207.6 (6.8) I — 436 311 316 387 388 0.47 3.5a First heating scan (DSC). C: crystal; Col h : hexagonal columnar phase; I: isotropic liquid. (DH/kJ mol1 ). b Second heating scan (DSC). c Onsetof decomposition under N 2 (TGA). d CHCl 3 solution (10 5 mol L 1 ). e Relative to the PBD (F Fl = 0.83) 7 . f Determined from absorption spectraof the films.clearly in a non-planar conformation with respect to thetriazine core. The mean plane angles for the twist of the ringsB, C and D from the central ring A are 19.3(1), 28.7(1) and61.8(1)1, respectively. Molecular packing is mainly due tovan der Waals interactions, which is in agreement with thesoftness of the crystals, and also due to dipolar interactions(calculated dipole moment is 2.0 D, with a large component(1.8 D) perpendicular to the tristriazolotriazine core). Molecularp-stacking is sterically inhibited in the solid phase, whichimproves the potential for these materials to be applied asgood solid state emitters, since the formation of excimers viap-stacking is known to suppress the emission in solids. 8B3LYP/6-311G(d,p) calculations 9 (see ESIw) for compound3a corroborated the observed non-planar conformations andthe structural parameters were in excellent agreement with thecrystallographic data. Conformational analysis of one phenylgroup yielded 11.5 kJ mol1 for the barrier to internal rotationof a single phenyl group. This value is quite small, thusallowing a conformational equilibrium to be attained quicklyat room temperature and, thus, a symmetric structure can beexpected in solution in the NMR time scale. In fact, symmetricstructures in the 1 H, 13 C and 15 N NMR spectra of the finalcompounds were found.The thermal and optical properties of compounds 3a–e aresummarized in Table 1. They are thermally stable compoundsas shown by their high decomposition temperatures (4400 1C)according to TGA measurements. Except for 3a, these compoundsdo not show tendencies towards crystallization, whichis further evidence of non-aggregation in the solid state. Aftermelting, compounds 3a–d do not crystallize upon cooling andthe obtained transparent films remain stable for months withoutany crystallization. Upon heating, the second DSC scan ofthese compounds reveals only one glass transition temperature(see ESIw). It was observed that increasing the length of thealiphatic chains leads to a decrease in the melting point andimproves the film forming behavior. Compound 3e, with sixperipheral aliphatic chains, is an interesting discotic liquidcrystal exhibiting a very wide hexagonal columnar phase(Col h ) from 92.2 to 207.6 1C. The mesophase was assignedas Col h on the basis of the typical pseudo focal conic texturesobserved by polarized optical microscopy on cooling (Fig. 2)and confirmed by X-ray diffraction (see ESIw). The cell parametera was calculated to be 30.4 A˚ , smaller than the van derWaals diameter of the molecule (43.0 A˚ ) in the most extendedconformation. This indicates either interdigitation or partialfolding of the chains. The core–core mean distance was foundto be 3.5 A˚ , suitable for p-stacking. In addition, similarities inthe optical textures were observed at temperatures above andbelow the melting point, indicating the retention of a columnarstructure in the low temperature phase.As a consequence of this amorphous behavior, stable spincoatedfilms are easily obtained on quartz plates from theirrespective CHCl 3 solutions. UV and fluorescence spectra werethen measured in solution and in solid films (Fig. 3).The maximum absorption wavelengths for compounds 3a–ein solution and in the film are quite similar, peaking at around290–311 nm (e B 50 000 L mol1 cm 1 ) and 286–316 nm,respectively. These are attributed to the p–p* transitions in theheteroaromatic portion of the molecule due to the high molarabsorption coefficient.The INDO/S-CIS calculated electronic spectrum 10 for a nonplanarconformation of compound 3a is in better agreementwith the experimental data than that calculated for the planarconformation (see ESIw). The band at 290 nm involves twotransitions (calculated at 295 nm), each one involving mainlyHOMO–LUMO and HOMO–LUMO+1 orbitals. The nearesttransitions to this main band occur at B 280 nm with very lowoscillator strength values and involve very high energy unoccupiedmolecular orbitals. Thus, the observed band has only twocontributions whose molecular orbitals are illustrated in Fig. 4.These compounds show strong blue fluorescence in solutionwith maximum emission wavelengths between 360 and 387 nmand fluorescence quantum yields (F Fl ) varying from 25 to 47%(Table 1). They also showed strong fluorescence in the solid phase,exhibiting emission maxima at wavelengths between 363 and390 nm, with very small red-shifts compared to those in solution.Fig. 2 Polarizing optical photomicrographs of pseudo focal conictexture of Col h for 3e obtained at 205 1C (left) and 202 1C (right) oncooling.This journal is c The Royal Society of Chemistry 2008 Chem. Commun., 2008, 5134–5136 | 5135