Unveiling the Impact of Aggregation on Optical Anisotropy of Triazaacephenanthrylene Single Crystals. A Combined Quantum Crystallography and Conceptual Density Functional Theory Approach

Triazaacephenanthrylene (TAAP) triclinic single crystals show substantial optical anisotropy of absorption and fluorescence. The maximum effect can be correlated with the direction perpendicular to the plane of chromophores connected in a head-to-tail manner via weak dispersive interactions. This phenomenon is uncommon as usually the existence of postulated π···π interactions between the molecules forming dimers or stacks cause quenching of fluorescence. Herein we present a comprehensive study of inter- and intramolecular interactions in the crystal of TAAP enriched with the investigation of aromaticity. Our results show that intramolecular interactions stabilize the overall conformation of the molecule whereas dispersive forces determine the aggregation between TAAP molecules. In fact, there is no conventional π···π interaction between the molecules in the dimer. Instead, we observed a close contact between the lone pair of the bridgehead N10B atom and π-deficient pyrazine ring from an adjacent molecule. Optical anisotropy in TAAP crystals was directly correlated with the alignment of the molecular transition dipole moments caused by specific molecular self-assembly.


Table of Contents
Table S1 Details of data collection and refinement procedures S3 Figure S1 Centrosymmetric dimer of TAAP S4

Figure S2
Electrostatic potential plotted on electron density isosurfaces S4 Details of the multipolar refinement S8 Figure S3 Local coordinate system S8 Figure S4 Results of the residual density analysis S8 Table S4 Topological analysis of critical points for the selected bonds in TAAP S9 Table S5 Experimental topological analysis of bond critical points for weak interactions in TAAP S12

Figure S5
UV-vis spectra for solid state sample of TAAP S13 Measurements and calculations of refractive indices S13

Figure S2
Electrostatic potential plotted on 0.001 a.u. isosurface of electron density for a) isolated molecules of TAAP b) dimer of TAAP. Electrostatic potential for TAAP molecule in a crystal environment plotted on the 0.1eA -3 electron density isosurfaces data obtained from c) multipolar refinement on experimental electron density d) multipolar refinement on theoretical electron density.   Multipolar refinement was carried out in a step wise manner on an experimental data set obtained at 97K up to max angle of 51.27(Mo K radiation). Accurate positions and displacement parameters were obtained in high-order refinement (sin/ 0.7-2Å -1 ). Subsequently low-order refinement (sin/ 0-0.7Å -1 ) was carried out to receive accurate isotropic displacement parameters for hydrogen atoms. The hydrogen positions in the following refinements were fixed at the neutron distances of 1.092Å for C−H 1.083Å and for N−H 1.015 Å. The H-atom anisotropic displacement parameters were estimated using SHADE3* web server.

S5
Local coordinate system was placed on each atom ( Figure S2) and symmetry restrictions were applied in order to minimise the number of refined parameters and avoid overestimation (mZ for all non-H atoms except: N22 and C21, where no symmetry restrictions were applied, for N10B symmetry -6m2 was assumed).
The multipole expansion was truncated at the octapole level for the C, N and O atoms and at the dipole level for H atoms. The expansion and contraction parameters were taken from theory for carbon, nitrogen and oxygen atoms whereas for hydrogen atoms were fixed at the value of 1.20. The  and ' parameters were refined for carbon, nitrogen and oxygen atoms with constraints on ' being identical as . Multipolar refinement was also performed for electron density reconstructed from the theoretical structure factors obtained from ab-initio calculations.

Measurements and calculations of refractive indices
An attempt was made to measure refractive indices using immersion oil method. Three different oils were selected to examine the behavior of TAAP crystals under the polarizing microscope -bromoform (1.59), methyl iodide (1.74) and a saturated solution of Sulphur in methyl iodide (1.80). Unfortunately an accurate measurement was not possible due to slow but gradual solubility of TAAP in the selected high refractive index solutions. The closest match to one of three refractive indices was achieved in bromoform solution ( Figure  S6). TAAP crystals are triclinic thus biaxial with three different refractive indices not coinciding with crystallographic axes. The remaining two refractive indices were determined from birefringence measurements using Ehringhaus compensator and very thin plates of TAAP. Those results were confronted with calculations of refractive indices using modified the local field theory (LFT) according to the procedure described in Seidler, T.; Stadnicka, K.; Champagne, B. J. Chem. Theory Comput. 2014, 10 (5), 2114-2124. The positions of atoms were optimized at fixed cell parameters using Crystal17 at PBEsol0/def2-TZVP level of theory. Molecular properties were calculated at the dft b3lyp/6-311++G(d,p) level of theory.