Cross-Conjugated Cruciform Fluorophores

Abstract

In optoelectronic devices, chromophores can be designed at the molecular level to create materials with properties desired for advanced applications. Organic fluorophores in particular can be constructed with macroscopic properties that arise from two distinct contributions: (i) the collective impact of the molecular backbone and substituents and (ii) the connectivity within the molecule (that is, the spatial molecular architecture). Accordingly, the exploration of novel conjugated architectures is a productive area of current research. Different two-dimensional, "X-shaped" conjugated materials have been synthesized for a variety of applications. They include spiro compounds, paracyclophanes, swivel-type dimers, bisoxazole-derived cruciforms, tetraethynylethenes, and tetrasubstituted tolanes. A subset of these compounds are constructed from two "perpendicular" pi-conjugated linear arms connected through a central aromatic core; examples of these include tetrakis(arylethynyl)benzenes, tetrakis(styryl)benzenes, and tetrasubstituted thiophenes. In this Account, we evaluate 1,4-distyryl-2,5-bis(arylethynyl)benzenes or cruciforms (XFs). Electronic substitution of this "X-shaped" cross-conjugated scaffold tunes both the energy levels of the frontier molecular orbitals (FMOs) and their spatial distribution in XFs. The resulting fluorophores exhibit FMO separation, imbuing XFs with unusual yet desirable properties for sensory applications. Using model analytes, we examine how the underlying FMO arrangement and the nature of analyte interaction elicit observable responses. These studies provide a foundation for accessing functional responsive ratiometric cores, demonstrating the importance and unique potential of FMO-separated fluorophores. We also highlight the essential contribution of serendipity in materials development. Moving beyond one-dimensional molecular wire-type fluorophores to two-dimensional "X-shaped" materials provides access to materials with unexpected and exciting properties. XFs represent such novel conjugated architectures, and their successful development has frequently has hinged on inspiration from structural components and principles developed in diverse research areas.

Effects of electronegative substitution on the optical and electronic properties of acenes and diazaacenes

“…When the calculated transition energy for 16c is compared to that for the parent 11c′ (R = H, X = CN), there is a decrease of almost 0.7 eV. This effect is less observed in acenes that have more congruent frontier molecular orbitals 19 . When comparing the tetracene 16a with cyano-substituted 16c, there also occurs a lowering of the computed transition energy; however, the shift of 0.55 eV is smaller than the effect observed when going from 11a′ to 11c′.…”

“…4b). A disjoint orbital structure 19 , that is, spatially separated frontier molecular orbitals results. The disjointing effect is further amplified by electronegative substituents and decreases the transition energies for diazaacenes.…”

“…Chlorination lowers both HOMO and LUMO, but with the former effect insufficient to preclude hole injection, whereas the latter effect facilitates electron injection; however, the optical gap does not change to any appreciable extent 15 . The tetrachloro substitution only led to a slight shift in the absorption maximum going from 643 nm for 5 16 to 654 nm for 6 in solution, suggesting that chlorine substitution had comparable impact on both the ground and first excited states [17][18][19] . Lowering the absorption energies can be useful in the context of organic solar cells when looking for acene derivatives showing absorption into the near-infrared, as observed in Anthony's hexacenes and heptacenes 20,21 .…”

Tuning the Direction of Intramolecular Charge Transfer and the Nature of the Fluorescent State in a T-Shaped Molecular Dyad

“…11 For example, D−A π-conjugated rods are of considerable importance for nanoscience and nanotechnology because they represent elementary functional building blocks showing interesting optical and electronic properties. 42 In that connection, molecular cruciforms [8][9][10][11][12][13][14][15]43,44 and T-shaped systems 35,45 having ICT transitions were investigated as fluorescence polarity indicators or metal ion sensors. Moreover, dual fluorescence was observed in several D−A systems owing to the interplay between locally excited (LE) and charge transfer (CT) states.…”

Acetylene‐Expanded Dendralene Segments with Exotopic Phosphaalkene Units

“…[21,22] In the case of oligo-phenylacetylenes and -vinylenes, the constituting arenes can formally be replaced relatively easily by other heteroaromatic units such as pyridines, thiophenes, and others. [1,2,11,23] The introduction of heteroatoms in the form of higher alkenes as surrogates of traditional alkenes has been explored to a much lesser extent, despite the advantages in terms of lower HOMO-LUMO gaps and polarizability that are offered by this strategy. [24][25][26] The first oligo-phenylethylene analogues that feature disilenes, [27][28][29] phosphaalkenes, [30,31] and diphosphenes [32,33] instead of classical C=C bonds have emerged in the literature only recently.…”