Polymorphic chiral squaraine crystallites in textured thin films

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.

Structural Disorder as the Origin of Optical Properties and Spectral Dynamics in Squaraine Nano-Aggregates

In contrast to regular J- and H-aggregates, thin film squaraine aggregates usually have broad absorption spectra containing both J-and H-like features, which are favorable for organic photovoltaics. Despite being successfully applied in organic photovoltaics for years, a clear interpretation of these optical properties by relating them to specific excited states and an underlying aggregate structure has not been made. In this work, by static and transient absorption spectroscopy on aggregated n-butyl anilino squaraines, we provide evidence that both the red- and blue-shifted peaks can be explained by assuming an ensemble of aggregates with intermolecular dipole-dipole resonance interactions and structural disorder deriving from the four different nearest neighbor alignments─in sharp contrast to previous association of the peaks with intermolecular charge-transfer interactions. In our model, the next-nearest neighbor dipole-dipole interactions may be negative or positive, which leads to the occurrence of J- and H-like features in the absorption spectrum. Upon femtosecond pulse excitation of the aggregated sample, a transient absorption spectrum deviating from the absorbance spectrum emerges. The deviation finds its origin in the excitation of two-exciton states by the probe pulse. The lifetime of the exciton is confirmed by the band integral dynamics, featuring a single-exponential decay with a lifetime of 205 ps. Our results disclose the aggregated structure and the origin of red- and blue-shifted peaks and explain the absence of photoluminescence in squaraine thin films. Our findings underline the important role of structural disorder of molecular aggregates for photovoltaic applications.

Template and Temperature-Controlled Polymorph Formation in Squaraine Thin Films

Controlling the polymorph formation in organic semiconductor thin films by the choice of processing parameters is a key factor for targeted device performance. Small molecular semiconductors such as the prototypical anilino squaraine compound with branched butyl chains as terminal functionalization (SQIB) allow both solution and vapor phase deposition methods. SQIB has been considered for various photovoltaic applications mainly as amorphous isotropic thin films due to its broad absorption within the visible to deep-red spectral range. The two known crystalline polymorphs adopting a monoclinic and orthorhombic crystal phase show characteristic Frenkel excitonic spectral signatures of overall H-type and J-type aggregates, respectively, with additional pronounced Davydov splitting. This gives a recognizable polarized optical response of crystalline thin films suitable for identification of the polymorphs. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spin-casted thin films depending on subsequent thermal annealing. By contrast, upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene, and gold, the polymorph expression depends basically on the choice of growth substrate. The same pronounced out-of-plane orientation is adopted in all crystalline cases, but with a surface templated in-plane alignment in case of crystalline substrates. Strikingly, the amorphous isotropic thin films obtained by vapor deposition cannot be crystallized by thermal postannealing, which is a key feature for the spin-casted thin films, here monitored by polarized in situ microscopy. Combining X-ray diffraction, atomic force microscopy, ellipsometry, and polarized spectro-microscopy, we identify the processing-dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.

Engineering of a kinetically driven phase of phenoxazine by surface crystallisation

Surface crystallisation yields an unknown polymorph of the phenoxazine molecule. Tuning the crystallisation conditions causes a defined crystal growth of either the thermodynamically stable phase or the kinetic phase observed exclusively within thin films.

A computational probe granting insight into intra and inter-stacking interactions in squaraine dye derivatives

In order to decipher structure function relationships regarding the optoelectronic properties of organic functional materials, two sets of anilinosquaraine dye derivatives were analysed computationally. 2,4-Bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine exhibits orthorhombic and monoclinic polymorphs (motif-1 and motif-2) in bulk and spin coated films, respectively, and concomitant J- and H-aggregate spectral features. In the second set, in spite of variation in the n-alkyl chain of the anilinosquaraine derivatives (n-propyl vs. n-butyl; motif-3 and motif-4), both acquired triclinic morphology and intermolecular charge transfer had been identified as the origin of their panchromaticity. The differences between these motifs were closely inspected at the theoretical level including geometrical parameters, internal reorganization energies, charge transfer integrals, drift mobilities, interaction energies and lattice energies along with a comprehensive Hirshfeld surface analysis. A correlation was established between C⋯C type intra-stack interactions with the observed red-shifted absorption patterns in the order motif-1 > motif-3 > motif-4. Hydrophobic interactions, π-π interactions and hydrogen bonds were found to influence the crystal packing patterns and concomitantly the overall molecule planarity and thereby the extent of π-bond delocalization/resonance. Insights into intra-stack and inter-stack interactions based on crystal packing effects are provided, which support and potentially guide the experimentalists' key ideas for shaping and encompassing the applications of promising squaraine derivative materials.

Polymorphism in Squaraine Dye Aggregates by Self-Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J-Aggregate Nanosheets

A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.