Conformational Control of Oligo(p-phenyleneethynylene)s with Intrinsic Substituent Electronic Effects: Origin of the Twist in Pentiptycene-Containing Systems

A Molecular Rotor That Probes the Helical Inversion of Stiff-Stilbene

Probing the inversion kinetics of a molecular helix is inherently a challenging task. We demonstrate herein that a fast-rotating pentiptycene component could function as an external NMR probe to afford the kinetic information on the inversion of a neighboring helical stiff-stilbene unit.

Directed Polymorphism and Mechanofluorochromism of Conjugated Materials through Weak Non-Covalent Control

Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness "soft" non-covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet fluorescent crystal-to-crystal transitions occur for single crystals of CO2-1 and CO2-3. Finally, the PEs show reversible violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and tunable stimuli-responsive behavior in solids.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Spontaneous Symmetry Breaking Facilitates Metal-to-Ligand Charge Transfer: A Quantitative Two-Photon Absorption Study of Ferrocene-phenyleneethynylene Oligomers

Change of the permanent molecular electric dipole moment, Δμ, in a series of nominally centrosymmetric and noncentrosymmteric ferrocene-phenyleneethynylene oligomers was estimated by measuring the two-photon absorption cross-section spectra of the lower energy metal-to-ligand charge-transfer transitions using femtosecond nonlinear transmission method and was found to vary in the range up to 12 D, with the highest value corresponding to the most nonsymmetric system. Calculations of the Δμ performed by the TD-DFT method show quantitative agreement with the experimental values and reveal that facile rotation of the ferrocene moieties relative to the organic ligand breaks the ground-state inversion symmetry in the nominally symmetric structures.

Redox-Gated Tristable Molecular Brakes of Geared Rotation

p-Bis(arylcarbonyl)pentiptycenes 2 (aryl = 4-(trifluoromethyl)phenyl) and 3 (aryl = mesityl) have been prepared and investigated as redox-gated molecular rotors. For 2, rotations about the pentiptycene-carbonyl bond (the α rotation) and about the aryl-carbonyl bond (the β rotation) are independent, and the rotation barriers are 11.3 and 9.5 kcal mol^-1, respectively, at 298 K. In contrast, the α and β rotations in 3 are correlated (geared) in a 2-fold cogwheel pathway between the aryl and the pentiptycene groups with a much lower rotation barrier of 6.5 kcal mol^-1 at 298 K in spite of the bulkier aryl groups. Electrochemical reduction of the neutral forms led first to radical anions (2^•- and 3^•-) and then to a bis(radical anion) for 2^2- but a dianion for 3^2-. The redox operations switch the independent α and β rotations in 2 into a geared rotation in both 2^•- and 2^2- and result in a slow-fast-stop rotation mode for 2-2^•--2^2-. The two redox states 3^•- and 3^2- retain the geared α and β rotations and follow a fast-slow-stop mode for 3-3^•--3^2-. Both molecular systems mimic tristable molecular brakes and display 8-9 orders of magnitude difference in rotation rate through the redox switching.

Substituent Effects That Control Conjugated Oligomer Conformation through Non-covalent Interactions

Although understanding the conformations and arrangements of conjugated materials as solids is key to their prospective applications, predictive power over these structural factors remains elusive. In this work, substituent effects tune non-covalent interactions between side-chain fluorinated benzyl esters and main-chain terminal arenes, in turn controlling the conformations and interchromophore aggregation of three-ring phenylene-ethynylenes (PEs). Cofacial fluoroarene-arene (ArF-ArH) interactions cause twisting in the PE backbone, interrupting intramolecular conjugation as well as blocking chromophore aggregation, both of which prevent the typically observed bathochromic shift observed upon transitioning PEs from solution to solid. This work highlights two structural factors that determine whether the ArF-ArH interactions, and the resulting twisted, unaggregated chromophores, occur in these solids: (i) the electron-releasing characteristic of substituents on ArH, with more electron-releasing character favoring ArF-ArH interactions, and (ii) the fluorination pattern of the ArF ring, with 2,3,4,5,6-pentafluorophenyl favoring ArF-ArH interactions over 2,4,6-trifluorophenyl. These trends indicate that considerations of electrostatic complementarity, whether through a polar-π or substituent-substituent mechanism, can serve as an effective design principle in controlling the interaction strengths, and therefore the optoelectronic properties, of these molecules as solids.