skip to main content

This content will become publicly available on June 27, 2023

Title: Correlation of solid-state order to optoelectronic behavior in heterocyclic oligomers
A longstanding challenge in the field of optoelectronic materials, the effects of solid-state arrangement and morphology are still a prominent factor associated with small-molecule and polymer-based device performance. Here, mixed heterocyclic aromatic oligomers containing thiophene, furan and pyrazine are prepared alongside their methylated congeners. Their solution and solid-phase properties were studied via spectroscopic, electrochemical and single-crystal X-ray diffraction (XRD) analysis. Comparative analysis between solid-state packing arrangements and photophysical properties revealed optical band gaps as low as 1.7 eV with Stokes-shifts up to 130 nm and quantum yields of 12%. Results of the study aid in further understanding the effects of molecular and solid-state arrangements that give rise to unique optical and photophysical properties critical to enhancing optoelectronic behavior.
Authors:
; ; ; ; ; ; ; ; ;
Award ID(s):
1664998 1652094
Publication Date:
NSF-PAR ID:
10345438
Journal Name:
CrystEngComm
Volume:
24
Issue:
25
Page Range or eLocation-ID:
4564 to 4572
ISSN:
1466-8033
Sponsoring Org:
National Science Foundation
More Like this
  1. Understanding the structural parameters that determine the extension of π-conjugation in 2-dimensions is key for controlling the optical, photophysical, and electronic properties of 2D-π-conjugated materials. In this article, three non-slanted H-mers including a donor–acceptor H-mer (H-mer-3) with an increase in dihedral angle (twist) between the strands and rungs are synthesized and studied. These non-slanted H-mers represent the repeat units of 2D-π-conjugated materials. H-mer-3, containing donor-strands and an acceptor-rung, is an unexplored donor–acceptor architecture in both slanted and non-slanted H-mers. The H-mers displayed both acid and base dependent optical properties. While the rungs have a little impact on the H-mer absorptionmore »spectra they play a key role in the emission and fluorescence lifetime. H-mer-3 ( i.e. , donor–acceptor H-mer) shows a higher Stokes shift and fluorescence lifetime than the other two H-mers. The twist and the presence of an electron deficient rung in H-mer-3 facilitated an intramolecular charge transfer in the excited state from the strands to the electron deficient rung, and therefore control over the H-mer emission properties. The lack of insulating pendant chains, reduced π–π interactions in thinfilms, and longer fluorescence lifetimes make these H-mers interesting candidates for various electronic and optoelectronic applications.« less
  2. A family of Zn 16 Ln(HA) 16 metallacrowns (MCs; Ln = Yb III , Er III , and Nd III ; HA = picoline- (picHA 2− ), pyrazine- (pyzHA 2− ), and quinaldine- (quinHA 2− ) hydroximates) with an ‘encapsulated sandwich’ structure possesses outstanding luminescence properties in the near-infrared (NIR) and suitability for cell imaging. Here, to decipher which parameters affect their functional and photophysical properties and how the nature of the hydroximate ligands can allow their fine tuning, we have completed this Zn 16 Ln(HA) 16 family by synthesizing MCs with two new ligands, naphthyridine- (napHA 2− ) andmore »quinoxaline- (quinoHA 2− ) hydroximates. Zn 16 Ln(napHA) 16 and Zn 16 Ln(quinoHA) 16 exhibit absorption bands extended into the visible range and efficiently sensitize the NIR emissions of Yb III , Er III , and Nd III upon excitation up to 630 nm. The energies of the lowest singlet (S 1 ), triplet (T 1 ) and intra-ligand charge transfer (ILCT) states have been determined. Ln III -centered total ( Q LLn) and intrinsic ( Q LnLn) quantum yields, sensitization efficiencies ( η sens ), observed ( τ obs ) and radiative ( τ rad ) luminescence lifetimes have been recorded and analyzed in the solid state and in CH 3 OH and CD 3 OD solutions for all Zn 16 Ln(HA) 16 . We found that, within the Zn 16 Ln(HA) 16 family, τ rad values are not constant for a particular Ln III . The close in energy positions of T 1 and ILCT states in Zn 16 Ln(picHA) 16 and Zn 16 Ln(quinHA) 16 are preferred for the sensitization of Ln III NIR emission and η sens values reach 100% for Nd III . Finally, the highest values of Q LLn are observed for Zn 16 Ln(quinHA) 16 in the solid state or in CD 3 OD solutions. With these data at hand, we are now capable of creating MCs with desired properties suitable for NIR optical imaging.« less
  3. Two-dimensional semiconductors (2DSCs) are attractive for a variety of optoelectronic and catalytic applications due to their ability to be fabricated as wide-area, monolayer-thick films and their unique optical and electronic properties which emerge at this scale. One important class of 2DSCs are the transition metal dichalcogenides (TMDs), which are of particular interest as absorbing layers in ultrathin optoelectronic devices. While TMDs are known to exhibit excellent photovoltaic properties at the bulk level, it is not yet clear how carriers are transported in these materials at thicknesses approaching the monolayer limit, where distinct changes in band structure and the nature ofmore »photogenerated carriers occur. Here, it is demonstrated that electrochemical microscopy techniques can be employed as powerful tools for visualizing these processes in 2DSCs, even within individual monolayers. Carrier generation-tip collection scanning electrochemical cell microscopy (CG-TC SECCM), which utilizes spatially-offset optical and pipet-based electrochemical probes to locally generate and detect photogenerated carriers, was applied to visualize carrier generation and transport within well-defined n-WSe 2 samples prepared via mechanical exfoliation. Data from these experiments directly reveal how carrier transport varies within complex 2DSC structures as layer thicknesses approach the monolayer limit. These results not only provide valuable new insights into carrier transport within monolayer TMD materials, but also demonstrate electrochemical imaging to be a powerful, yet underutilized approach for visualizing solid-state processes in semiconducting materials.« less
  4. UV-irradiation of assembled urea-tethered triphenylamine dimers results in the formation of persistent radicals, whereas radicals generated in solution are reactive and quickly degrade. In the solid-state, high quantities of radicals (approximately 1 in 150 molecules) are formed with a half-life of one week with no significant change in the single crystal X-ray diffraction. Remarkably, after decay, re-irradiation of the solid sample regenerates the radicals to their original concentration. The photophysics upon radical generation are also altered. Both the absorption and emission are significantly quenched without external oxidation likely due to the delocalization of the radicals within the crystals. The factorsmore »that influence radical stability and generation are correlated to the rigid supramolecular framework formed by the urea tether of the triphenylamine dimer. Electrochemical evidence demonstrates that these compounds can be oxidized in solution at 1.0 V vs. SCE to generate radical cations, whose EPR spectra were compared with spectra of the solid-state photogenerated radicals. Additionally, these compounds display changes in emission due to solvent effects from fluorescence to phosphorescence. Understanding how solid-state assembly alters the photophysical properties of triphenylamines could lead to further applications of these compounds for magnetic and conductive materials.« less
  5. J-dimer emission is an emergent property that occurs when pairs of ground state fluorophores associate, typically in a dilute solution medium. The resulting fluorescence is shifted with respect to the monomer. J-dimer emission, however, has never been observed in concentrated dispersions or in the solid state. We posited that multivariate (MTV) MOFs with double interwoven structures would help to isolate these dimers within their crystalline matrix. Using this strategy, J-aggregate density was controlled during crystallization by following a substitutional solid solution approach. Here, we identified the presence of J-dimers over the entire composition range for interwoven PIZOF-2 / NNU-28 structuresmore »with variable amounts of a diethynyl-anthracene aggregate-forming link. We produced bulk crystals that systematically shifted their fluorescence from green to red with lifetimes (up to 13 ns) and quantum yields (up to 76%) characteristic of π–π stacked aggregates. Photophysical studies also revealed an equilibrium constant of dimerization, K D = 1.5 ± 0.3 M −1 , enabling the first thermodynamic quantification of link–link interactions that occur during MOF assembly. Our findings elucidate the role that supramolecular effects play during crystallization of MTV MOFs, opening pathways for the preparation of solid-state materials with solution-like properties by design.« less