Supramolecular steric hindrance effect on morphologies and photophysical behaviors of spirocyclic aromatic hydrocarbon nanocrystals

Multiple aggregates from multiple polymorphs: structural and mechanistic insight into organic dye aggregates

This case study provides evidence for the appearance of multiple aggregation forms of a single organic dye, arising from its packing polymorphs in the solid state. Each aggregate can be spectroscopically matched to one polymorph, acquiring nanoscopic structural information even in the absence of conventional H- or J-type aggregation spectral features. The conversion from one polymorphic aggregate to another supports the action of Ostwald's rule of stages in organic aggregates suspended in solution. Mechanistically, dye molecules from one aggregate dissociate then renucleate the more stable aggregate form, the first demonstration for an aggregation-induced emission-active organic dye.

Phenyl-pyta-tricarbonylrhenium(I) complexes: combining a simplified structure and steric hindrance to modulate the photoluminescence properties

Strongly luminescent tricarbonylrhenium(I) complexes are promising candidates in the field of optical materials. In this study, three new complexes bearing a 3-(2-pyridyl)-1,2,4-triazole (pyta) bidentate ligand with an appended phenyl group were obtained in very good yields owing to an optimized synthetic procedure. The first member of this series, i.e. complex 1, was compared with the previously studied complex RePBO to understand the influence of the fluorescent benzoxazole unit grafted on the phenyl ring. Then, to gauge the effect of steric hindrance on the luminescence properties, the phenyl group of complex 1 was substituted in the para position by a bulky tert-butyl group or an adamantyl moiety, affording complexes 2 and 3, respectively. The results of theoretical calculations indicated that these complexes were quite similar from an electronic point of view, as evidenced by the electrochemical study. In dichloromethane solution, under excitation in the UV range, all the complexes emitted weak phosphorescence in the red region. In the solid state, they could be excited in the blue region of the visible spectrum and they emitted strong yellow light. The photoluminescence quantum yield was markedly increased with raising the size of the substituent, passing from 0.42 for 1 to 0.59 for 3. The latter complex also exhibited clear waveguiding properties, unprecedented for rhenium complexes. From this point of view, these easy-synthesized and spectroscopically attractive complexes constitute a new generation of emitters for use in imaging applications and functional materials. However, the comparison with RePBO showed that the presence of the benzoxazole group leads to unsurpassed mechanoresponsive luminescence (MRL) properties, due to the involvement of a unique photophysical mechanism that takes place only in this type of complex.

Energetic Descriptors of Steric Hindrance in Real Space: An Improved IQA Picture*

Steric hindrance (SH) plays a central role in the modern chemical narrative, lying at the core of chemical intuition. As it however happens with many successful chemical concepts, SH lacks an underlying physically sound root, and multiple mutually inconsistent approximations have been devised to relate this fuzzy concept to computationally derivable descriptors. We here argue that being SH related to spatial as well as energetic features of interacting systems, SH can be properly handled if we chose a real space energetic stance like the Interacting Quantum Atoms (IQA) approach. Drawing on previous work by Popelier and coworkers (ChemistryOpen 8, 560, 2019) we build an energetic estimator of SH, referred to as E_ST . We show that the rise in the self-energy of a fragment that accompanies steric congestion is a faithful proxy for the chemist's SH concept if we remove the effect of charge transfer. This can be done rigorously, and the E_ST here defined provides correct sterics even for hydrogen atoms, where the plain use of deformation energies leads to non-chemical results. The applicability of E_ST is validated in several chemical scenarios, going from atomic compressions to archetypal S_N2 reactions. E_ST is shown to be a robust steric hindrance descriptor.

Enhanced emission in organic nanocrystals via asymmetrical design of spirocyclic aromatic hydrocarbons

Two spirocyclic aromatic hydrocarbon derivatives were prepared to clarify the molecular geometry effects on the regulation of the crystalline morphologies and photophysical behaviors of organic nanocrystals. Due to the different structural symmetry of a spiro-center, distinguishing nanocrystal morphologies with unique crystallization-enhanced/quenched emission was achieved.

Spray coated perylenebisimide/polymer film with controllable molecular aggregation state and emission properties

Dye doped organic thin films with controllable molecular aggregation and emission properties are of broad interest to the scientific community owing to their large number of potential applications in physics, chemistry, and materials science. Here, a spray coating method was used to prepare perylenebisimides (PBI) doped polymer films. In this study, the effects of the dye concentration, polymer matrix, solvent, and casting process on the optical properties of the resulting films were studied. The aggregation of the PBI into monomer, dimer, and oligomer forms, was rapidly and simply controlled based on the concentration dependence of PBI. The molecular aggregation mechanism in the film forming process for PBI doped polystyrene (PS) was further analyzed by computer simulations. The blends rapidly reached their lowest Gibbs free energy owing to the "frozen" polymer chains and confinement of PBI, molecules with different aggregation states. Therefore, the PBIs/PS films prepared under different conditions had different fluorescent lifetimes and absolute quantum yields. Narrow emission, amplified spontaneous emission (ASE) and random lasing (RL) were observed in PBI doped PS films when photo-pumped at 532 nm in transmittance and waveguide modes, respectively. A lower ASE and RL threshold were obtained for PS films doped with monomeric PBI than those in other aggregation states. Moreover, the solvent use in film deposition greatly influenced the emission properties of the PS films by altering their microstructures. These results indicate potential applications for spray coated dye/polymer films in organic solid-state lasers.

Symmetric Fluoroborate and its Boron Modification: Crystal and Electronic Structures

Four boron-carrying molecules were synthesized and purified. These were found to be (a) relatively neutral with respect to the parent BF derivative and (b) functionalized by donor–acceptor groups resulting in a charge transfer within the molecule. The study discusses the steric effect and the influence of the substitution of the side rings on the surroundings of the boron atom. Electronic structures were characterized by real-space bonding indicators. Hirshfeld surface and energy frameworks tools were applied to examine the crystal packing features.

Optimization of aggregation-induced phosphorescence enhancement in mononuclear tricarbonyl rhenium(i) complexes: the influence of steric hindrance and isomerism

In order to improve the remarkable performance of a mononuclear tricarbonyl rhenium(i) complex (ReL1) that exhibits rare aggregation-induced phosphorescence enhancement (AIPE) behavior, two new complexes (ReL3 and ReL4) were prepared and investigated. They incorporate a 2-pyridyl-1,2,4-triazole (pyta) ligand connected to a 2-phenylbenzoxazole (PBO) moiety. Complex ReL3 differs from ReL1 by the presence of a bulky tert-butyl substituent, and ReL4 is an isomer where the PBO group is linked to the pyta ligand by its phenyl group. Theoretical calculations were in congruence with electrochemical and spectroscopic properties in solutions. Both new compounds exhibited strong AIPE and much better solid-state emission efficiency than ReL1, with photoluminescence quantum yields up to 55% for ReL4. Crystallographic data indicate that this increase in emission efficiency is due to optimum packing that prevents quenching. This work shows that minor structural changes may have major effects upon the solid-state spectroscopic properties and it provides a rational basis for accessing AIPE-active strongly emissive rhenium(i) complexes.