Chiroptical Properties of Indolenine Squaraines with a Stereogenic Center at Close Proximity

Inter- and intra-stacking-influenced charge hopping interactions in core-substituted squaraine dye derivatives: a computational investigation

The stacking arrangement and associated interactions within the crystal packing of ten core-substituted squaraine (CSQ) dyes, which were symmetric dicyanovinyl (DCV) acceptor group-functionalized squaraine derivatives with distinct electron-rich side donor moieties containing various substituents, were assessed using computational methods. Out of the three sets, two contained indolenine and quinolinium donor moieties as side substitutions. In the first set, the halogen substituents on indolenine donor moieties (such as -Cl, -Br, and -I) exhibited monoclinic polymorphism, while the -F substituted one exhibited triclinic polymorphism. Triclinic and orthorhombic polymorphs are known for the absence of any substituents. Second set of two CSQ dyes has quinolinium donor units that differ in length of alkyl chain substitution found in monoclinic polymorphism. The third set of CSQ contains monoclinic polymorphism in which one indolenine CSQ contains a chiral center while the other one is with benzothiazole ring. Herein, we investigated the impact of the DCV acceptor moiety present at the central C4 ring of squaraine dyes and the functionalization of side donor groups on the inter- and intra-stacking patterns during crystal packing to establish a charge transfer protocol. Theoretical calculations, including geometrical analysis, internal reorganization energy, charge-transfer integral, drift mobility and interaction energy, along with a complete investigation of the Hirshfeld surface analysis, were performed. The distinctive geometrical changes upon electron gain and loss were observed at different moieties, i.e. side aromatic rings and the central C4 ring, respectively, of the respective CSQs. Inter- or intra-stacking cofacial interactions between side-substituted donor rings offered electron transfer character, while the participation of slip-stacked central C4 rings provided a hole-transfer character to all CSQs. Although the very meager participation of the DCV group in the inter- and intra-stacking interactions of all motifs was studied, indirect influence was noted through the geometrical impacts. Such packing effects provide insights into inter- and intra-stacking interactions, possibly guiding experimentalists' ideas for shaping and using promising squaraine derivatives in advanced functional materials.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies g_abs of up to 2.6 × 10^-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Axially chiral indolenine derived chromophore dimers and their chiroptical absorption and emission properties

Yamamoto homocoupling of two chiral oxindoles led to the atropo-diastereoselective formation of an axially chiral oxindole dimer. This building block served as the starting material for the syntheses of axially chiral squaraine and merocyanine chromophore dimers. These dimers show pronounced chiroptical properties, this is, outstandingly high ECD signals (Δε up to ca. 1500 M⁻¹ cm⁻¹) as a couplet with positive Cotton effect for the P-configuration around the biaryl axis and a negative Cotton effect for the M-configuration. All investigated dimers also exhibit pronounced circularly polarised emission with anisotropy values of ca. 10⁻³ cgs. Time-dependent density functional calculations were used to analyse the three contributions (local one electron, electric–magnetic coupling, and exciton coupling) to the rotational strength applying the Rosenfeld equation to excitonically coupled chromophores. While the exciton coupling term proves to be the dominant one, the electric–magnetic coupling possesses the same sign and adds significantly to the total rotational strength owing to a favourable geometric arrangement of the two chromophores within the dimer.

From an axially chiral oxindole, squaraine and merocyanine chromophore dimers with pronounced chiroptical properties were prepared.